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Engineering GuidCritical Environment

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Each different room type in the hospitalenvironment requires a unique and strategicapproach to air distribution and temperaturecontrol. Past and ongoing industry researchcontinues to impact the thinking of HVACengineers involved in the design of healthcare facilities or the development ofgoverning standards. This section will helpexplain some of the key North Americancodes and standards for health care facilitydesign as they relate to each space type,as well as the logic behind these designguidelines. Following these design standards,air distribution strategies are presented forpatient care areas, waiting rooms, operatingrooms, and hospital pharmacies and labs.

The many design examples included in thischapter should serve to further clarify the keypoints presented in each section.

The health care facility includes a numberof different spaces, all with unique HVACrequirements. Well designed hospital HVACsystems should support asepsis controlwhile also taking advantage of energy savingtechnologies and strategies. Understandingthe needs and goals of each space as well asthe regulations that govern their design areimportant first steps toward building a highperforming facility.

The following chapter will discuss a numberof key areas in the hospital and providerecommendation on available HVACtechnology and how it can be applied.

Examples throughout the chapter give moreclarity on practical application of the conceptsand standards presented.

Patient care areas in a health care facilityare carefully controlled environmentsdesigned to provide contaminant controlwithout sacrificing the comfort of patientsor other occupants. Various hospital roomsfall into the Patient Care category, includingstandard patient rooms, isolation rooms, andburn center intensive care units.These spacesare similar in that they are all used for patienttreatment and/or recovery, however, each hasunique design characteristics which clearlydistinguish one type from another. With adifferent purpose for each room type, thestandards and general HVAC considerations

that dictate the design of the local airdistribution systems are also different.

Standard Patient Rooms

The majority of patient care areas in a hospitalare standard patient rooms. As such, thedesign of the air distribution systems in theserooms can greatly affect the performance andcost of operating the overall HVAC system.Standard patient rooms can be either singleor multiple patient spaces and are typicallydedicated to the care of individuals withoutserious health risk. These rooms will oftenhave an exterior wall and window which canhave a significant impact on room air patternsin certain climatic regions.

Introduction to Patient Care Areas

PATIENT ROOM

NeutralPressure Space

NegativePressure Space

Corridor

The variability in occupancy levels, solar/conduction loads through exterior walls, andequipment loads will generally facilitate theneed for reheat, VAV control, supplementalroom heating or cooling, exterior shading,or any combination of these features. Ifinsulating liners are used for supply terminalboxes, no fibers should be exposed to theair stream, with closed-cell foam liners thepreferred option. Venturi valves are becomingmore popular for return/exhaust applications

in patient rooms as they can provide accurateflow control without the need for cross-flowsensors. Frequent linen changes in patientrooms can result in high levels of airbornelint which may interfere with the functionof the cross-flow sensors typical to mostterminal boxes. Another common designpractice is to locate a radiant panel above thewindow on an exterior wall.The radiant panelserves to moderate the temperature aroundthe exterior wall as to prevent unfavorableinfluence over the room air pattern.

ASHRAE suggests the use of eithergroup A (horizontal throw) or group E(vertical throw) diffusers in standard patientrooms, however, the former is far morecommon for these applications. Use of

displacement ventilation products in patientrooms is also gaining traction as researchsurrounding the contaminant controlbenefits of single-pass DV air flow continuesto justify support for the technology.

Supply and exhaust diffusers should beselected and located in such a way asto promote the movement of air fromclean to less clean areas and also preventuncomfortable drafts. Mixing type, groupA diffusers will typically be located directlyabove the patient bed with exhaust outletslocated near the door to the corridor or toiletroom. Attention should be given to ceiling-mounted obstructions that may interfere

with the discharge pattern of horizontal throwdiffusers. Lift rails or curtains may redirecthigh velocity supply air directly at thepatient if positioned too close to the diffuser.Exhaust outlets can be located at a low level,but eggcrate style ceiling returns are morecommon and preferred.

Isolation Rooms

Isolation rooms can be separated into twomain categories: Airborne Infection Isolation

(AII) and Protective Environment (PE)rooms. As the names would suggest, theserooms have different functions. AII roomsare designed for patients with serious andcontagious conditions (e.g. Tuberculosis),while PE rooms exist to protect patientswith weakened immune systems or someform of impairment to their natural defensesystem. AII rooms are designed with theprimary purpose of protecting hospitaloccupants (other than the patient) fromairborne infection, while PE rooms shouldprotect the more vulnerable patients (e.g.bone marrow transplant patients) fromairborne contaminants present in thehospital environment.

Airborne Infection Isolation

(AII) RoomsThe general layout and some designconditions of an AII room will be similar to thatof a standard patient room. The possibilityof exterior walls and windows, an attachedtoilet room, and similar equipment loadsare a few commonalities between thesespaces. Yet the AII room serves a differentpurpose and consequently has a number ofkey differences with respect to standards andair distribution equipment.

The minimum total air-change rate for an AIIroom is typically 12 ach, but this may varydepending on the applicable code.

Figure 1: Typical patient room

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All Metric dimensions ( ) are soft conversion. Copyright Price Industries Limited 2011.

Imperial dimensions are converted to metric and rounded to the nearest millimeter.

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An Airborne Infection Isolation (AII)room must maintain a negative pressuredifferential relative to the hospital corridor.Toachieve an acceptable pressure differential itis usually necessary to supply approximately20% less air to the room than is exhausted.The actual air flow offset will need to bedetermined at the time of commissioning.Furthermore, all air exhausted from theAII room, or associated toilet room andanteroom, should be exhausted directlyoutdoors without mixing with the exhaustair from any other spaces.

HEPA filtration of the AII exhaust air isrequired in cases where the air is notdischarged clear of building openings or

mixes with exhaust air from non-AII spacesprior to being discharged outside.

Most standards do not require anterooms forAII rooms, however, most modern medicalfacilities utilize them. An AII anteroomshould have a negative pressure differentialrelative to the corridor, and a positivepressure differential relative to the AII room.The minimum relative pressure differentialbetween these spaces will depend on thejurisdict ion and applicable standards.This pressurization scheme supports themovement of air and contaminants fromclean (the corridor) to less clean (AII room)spaces, reducing the potential for furtherspread of infection. AII rooms also requirea permanently installed device to monitor

the differential pressure between the roomand adjacent areas (ASHRAE Standard 170-2008; CSA Z317.2-10). Most modern healthcare facilities achieve this level of monitoringby using a room pressure monitor with abuilt-in pressure sensor. Typically, the roompressure monitor is only used to detect anddisplay differential pressure between theroom and corridor, however, some modelsalso include an integral controller for supplyand exhaust valve control. Audible alarmsare often used in conjunction with the visualsignal displayed on the screen of the roompressure monitor.

The relatively high air-change raterequirements for AII rooms typically meanadditional supply air is not necessary to

meet cooling loads; however, VAV controlis still preferable in AII rooms for a numberof reasons:

Adjustment of supply/exhaust air ow to

maintain room pressure differential.

Airborne infection isolation rooms

should include provisions for standardpatient care when AII precautions are notrequired (ASHRAE Standard 170-2008).

The number of air-changes may be

reduced when the room is unoccupiedprovided that pressure relationshipswith adjacent spaces are maintained atthe reduced air-change rate (ASHRAEStandard 170-2008; CSA Z317.2-10).

A typical VAV configuration for an AII roomis shown in Figure 2. Venturi valve exhaustunits are sometimes preferred in areaswhere airborne lint might be a concern.

The current ASHRAE Standard 170 suggests

that group A (horizontal throw - mixingdiffuser) or group E (vertical throw) diffusersare suitable for AII room applications. TheCSA (2010) recommends non-aspiratingdiffusers (i.e. laminar flow diffusers). Onechallenge is supplying enough air while alsominimizing airborne particle entrainment.Toaddress this challenge, radial flow diffusersare a good alternative. Radial flow diffusersare high capacity outlets with short throwsand one or two way discharge patterns.The units are also available with adjustablepattern controllers, but designers shouldexercise caution when selecting thisoption, particularly if pattern controllerscan be adjusted without removing the face.

Cleaning staff may inadvertently mopattern controllers, compromising the safof healthy room occupants.

The position of the radial flow diffuseequally as important as the selection of

diffuser itself.The diffuser should be instato promote the movement of air toward patient and away from the room entranWith this in mind, the supply diffuser shobe located near the room door. The exhagrille, should be ceiling-mounted direabove the patient bed (ASHRAE Stand170-2008) or wall-mounted at a low lenear the head of the patient bed (CZ317.2-10) (ASHRAE Standard 170-200unless it can be demonstrated that supositions are impractical.The exhaust grishould be sized to achieve a core velocityapproximately 500 fpm in order to reaa desirable noise and pressure drop lev

Anteroom

TU

TU

TU

CV

VV

SupplyDu

ct

ExhaustDuct

Exhaust

Supply

Supply

Exhaust

Exhaust

Figure 2: Airborne infection isolation (AII) venturi valve setup

1 way RFDAnteroom

1 way RFD

Exhaust

Exhaust

ExhaustCorridor

Figure 3: Airborne infection isolation (AII) supply and exhaust outlets

Copyright Price Industries Limited 2011. All Metric dimensions ( ) are soft conversion.


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Protective Environment (PE) Rooms

PE rooms, while also considered isolationrooms, are essentially used for theopposite purpose to that of an AirborneInfection Isolation (AII) room. Patients inthese rooms have a high susceptibilityto infection and need greater protectionthan the average hospital occupant toavoid further health complications. Thesepatients include, but arent limited to, burnpatients, bone marrow or organ transplantpatients, leukemia, and AIDS patients.

Most codes require a minimum total air-change rate of 12 ach for a PE room. Theexhaust rate should be approximately 20%less than the supply to achieve a suitablepositive pressure differential relative tothe corridor. The actual exhaust air flowrate will need to be determined at the timeof commissioning. Air from a PE roomdoes not need to be exhausted directly tothe outdoors as it should with an AII room,however, recirculating room HVAC unitsor finned tube elements are not permittedby most codes. Some standards will allowrecirculation devices equipped with HEPAfilters (ASHRAE Standard 170-2008).

A PE room does not require an anteroomunless the PE room also requires airborneinfection isolation control (see nextsection). Standard PE rooms shouldexhibit a positive pressure differentialrelative to the corridor, toilet room, orany other adjacent spaces to prevent theinfiltration of contaminants. PE rooms alsorequire a permanently installed device toconstantly monitor and provide a visualoutput of the differential pressure betweenthe room and adjacent areas.

PE rooms can be used for standard patientcare, but air flow parameters cannot bereversed for the purposes of switchingbetween PE and AII room function.

Current standards suggest group E,non-aspirating supply diffusers, shouldbe used for PE room applications. Thiswould include laminar flow diffuserswith average air velocities just below thediffuser face of 30 fpm.The same challenge

exists in PE rooms with supplying highvolumes of air while also minimizingairborne particle entrainment. Radial flowdiffusers are better suited to address thisissue than laminar flow diffusers. Radialflow diffusers are not yet categorized byASHRAE standards, but there is strongprecedent for their use in such criticalapplications and they typically require lessceiling space for equivalent air flow rates.Both radial and laminar flow diffusers areavailable with integral HEPA filters.

Standards require the supply diffuser tobe located directly above the patient bedwith the exhaust outlet positioned near

the door to the corridor or anteroom. Theintent of this relative outlet location is toestablish a vertical downward wash ofclean air through the breathing zone of thepatient before the air passes through therest of the room.The style of exhaust grillewill depend on the installation location.Louvered bar grilles are typically usedfor low-level applications and eggcratefor ceiling applications. Either grille typeshould be sized based on a core velocityof 500 fpm for optimal pressure drop andnoise levels.

PE ROOM

NegativePressure Space

Corridor

PositivePressure Space

TU

CV

Return

VV

Supply

SupplyDuct

ExhaustDuct

Exhaust

Figure 4: PE room air flow direction

Figure 5: PE room venturi valve setup

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BURN CENTER ICU

Corridor

LFDLFD

Radiant Panel

Figure 6: PE room with airborne infection isolation (All)

Figure 7: Burn center ICU equipment layout

Airborne Infection Isolation (AII) inProtective Environment Rooms

In situations where a patient is bothcontagious and highly susceptible tofurther infection, the PE room design ismodified to include airborne infectionisolation.

An anteroom is required for these spaces.Pressure in the anteroom relative tothe PE room will generally be positive,allowing staff to gown and mask insidethe anteroom with less serious risk.Also, positive pressure in the anteroomwill reduce the probability of airbornecontaminants from the corridor enteringthe space occupied by the patient.

Unlike the standard PE room, a PE roomwith AII precautions should be undernegative pressure relative to adjacentspaces (other than the toilet room) toprevent exfiltration of contaminants intoother occupied zones. Supply and returnoutlet selection and location should matchstandard PE room applications.

Burn Center Intensive Care Units(ICU)

Burn center ICUs have a number of uniquechallenges. These spaces are generallykept at higher relative humidity levels toprevent excess moisture loss from woundsand the associated complications. Draftrepresents another major consideration.

In any application, steps are taken toreduce the possibility and inconvenienceof draft in an occupied space, but draftin a burn center can result in severe painfor patients and must therefore be morecarefully avoided. The last significantcomfort related design criterion is theneed for rapid temperature adjustmentaround the patient bed. During a wounddressing change it is preferable to raisethe temperature around the patient from10 F to 15 F. This reduces the T betweenthe air and wound temperature, thuscreating a more comfortable conditionfor the patient.

A burn center ICU will typically have apositive differential pressure relative to

adjacent spaces but this is not a requirementin all jurisdictions. Recirculating roomHVAC units are not permitted but airfiltration requirements are equivalent tothat of a standard patient room.

Group E, non-aspirating diffusers (i.e.laminar flow diffusers) with HEPA filtrationshould be used in burn center ICU patient

rooms for their low discharge velocity andentrainment characteristics. Buoyancyeffects can be an issue when heating withlaminar flow diffusers, which can result inthe clean air not reaching the patient. Toaddress this situation, radiant panels areoften used to provide heat near the patientbed without significantly adjusting thesupply air temperature. Exhaust outletswill typically be louvered bar grilleslocated at a low level near the room doorto minimize mixing in the occupied space.

Negative pressurerelative to anteroom

Negative pressurerelative to PE room

Anteroom

Corridor

Positive

pressure

relative

to

corridor



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Waiting and examination rooms involve anumber of unknowns. In most cases, thepatients in these rooms have yet to bediagnosed and the contagion risk must stillbe determined. The goal of the ventilationsystem in these spaces should be to providea comfortable environment while alsoreducing the probability that infection willspread between occupants.

A waiting room is generally a patient holdingarea for one or more examination rooms.Waiting rooms are sized to allow for at leasttwo chairs for each associated examinationor treatment room. Depending on the size ofthe facility, waiting rooms can be quite largewith a wide range of potential occupancy

levels. Single bed examination roomsshould have a minimum clear floor area of120 ft2(FGI, 2010). For both spaces, ASHRAErecommends the use of either group A(horizontal throw) or group E (vertical throw)supply air outlets. Of the two, group A outletsare the most typical for these spaces.

One of the more common group A diffusersused in waiting rooms and examinationrooms is the square plaque diffuser (Figure10). These diffusers produce a 360 patternand rapidly mix supply air with room airproviding a high level of comfort and reducedrisk of draft in the occupied zone.The flat faceand removable center plaque make it easyto clean and sterilize.

Another group A outlet suitable for theseapplications, especially larger waitingrooms, is the louvered face directional ceilingdiffuser (Figure 11). Similar to the squareplaque diffuser, this outlet has a removablecore and provides thorough mixing of roomair. Louvered face type diffusers have longthrows, and are further differentiated by thevariety of available core styles and dischargepatterns to suit different room layouts.

Other alternatives include displacementventilation (DV) or active chilled beams (ACB).Although less common, these technologiesare gaining traction in the North Americanmarket. In the case of ACBs, some codesallow the air volume induced through thebeam to be considered part of the total roomair-change rate. With typical induction ratiosnear 5:1, ACBs are often capable of satisfyingtotal air-change requirements while onlysupplying the minimum outdoor air volumerequirement directly to the diffuser.

When locating supply air outlets in thewaiting room, care must be taken toensure there are no uncomfortable draftsin the occupied zone. Diffuser selection andlocation should be such that the velocity ofair inside the occupied zone does not exceed50 fpm.

Emergency or chest X-ray (respiratory)waiting rooms should have a negativepressure differential relative to adjacentspaces; most codes also require total room

Figure 8: Waiting room

Figure 9:Single bed examination room

EXAM ROOM

WAITING ROOM

air-change rates of 12 ach. Examinationrooms will most often be maintained at aneutral pressure relative to adjacent spaces,but minimum air-change rates will varysignificantly from one region to another.

Exhaust air outlets in these spaces aretypically located in the ceiling, away from

Figure 11: Louvered face supply diffuserFigure 10:Square plaque diffuser

the supply outlets, with an eggcrate typegrille representing a common selection. Thesize of the exhaust grilles should be basedon a core velocity of approximately 500 fpm.Selection at 500 fpm will generally result inan acceptable pressure drop and noise level.

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Waiting and Examination Rooms



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Example 1 - Patient Room with Mixing Diffusers

The patient room used for the following example is a single bed space designed for a maximum of five occupants (patient, medstaff and visitors). The room includes overhead lighting, one television and medical monitoring equipment. The control temperature the space is 75 F. The patient room is 14 ft x 18 ft with an attached 5 ft x 7 ft toilet room. Both spaces have a common 9 ft ceiling heigThere is one exterior wall and window.

Patient Room Design Criteria (ASHRAE Standard 170-2008):

6 ach minimum (total, based on supply volume)

Neutral room pressure relative to corridor

Toilet Room Design Criteria (ASHRAE Standard 170-2008):

10 ach minimum (total, exhaust only)

Negative room pressure relative to patient room

Space Considerations

Some of the assumptions made for this space are as follows:

Supply air temperature is 55 F

Specific heat of dry air, cp, is 0.24 Btu/lbF

Density of dry air, , is 0.075 lb/ft3

PATIENT ROOM

Corridor

14 ft

18 ft

5 ft

7 ft

Patient Room Loads

Heat SourceDesign

Conditions (Btu/h)Typical Daytime Conditions (Btu/h

Patient 160 160

Medical Staff/Visitors (4x) 1000 250

Television 500 500

Medical Equipment 500 250

Overhead Lighting 900 500

Solar/Conduction (ext. wall) 2750 1400

Total 5810 3060



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Calculating the supply air flow rate to satisfy the design load:

Calculating the supply air flow to satisfy the required air-change rate:

Calculating the reheat threshold:

The required air flow rate to satisfy the maximum cooling load (269 cfm) is greater than the minimum total air-change requirementbased on code (227 cfm). As such, the supply air flow will range between 227 cfm and 269 cfm during occupied periods and reheatwill be necessary to prevent overcooling whenever room loads are below 4903 Btu/h.

The typical daytime cooling load in this patient room is well below the design load for the space. Calculating the typical supply airtemperature:


Supply Air Outlet Sizing & Selection

Standard patient rooms are typicallyoccupied by patients without highlycontagious conditions or abnormalsusceptibility to infection. As such, controlof airborne contamination in these spacesis of somewhat less priority than it wouldbe in isolation or operating rooms, allowinghorizontal throw (mixing) diffusers to be asafe and practical alternative.

A number of different mixing diffusers can beused in standard patient rooms. As with anydiffuser selection, the primary considerations

for a patient room would be throw, noise,pressure drop and architectural appeal.Acceptable throw, noise and pressure droplevels can often be achieved with properdiffuser sizing; whereas architectural appealis primarily based on designer preferenceand industry norms. Another considerationthat applies to this example has to do withthe asymmetry of the room. Since the floorarea of this room is not square, a modularstyle diffuser with flexible discharge patternoptions is most suitable. Louvered facedirectional diffusers offer this flexibility andare popular for patient rooms.

4 Way Louvered Diffuser 3 Way Louvered Diffuser

The placement of a louvered face directionaldiffuser will almost entirely dictate the styleselected. A common location for this type ofdiffuser in a patient room would be directlyabove the patient bed near the head wall.This placement requires the selection of a3 way discharge diffuser, as shown in theimage above.


To reduce the chance of draft in this patientroom, the 50 fpm terminal velocity of thedischarge air should not enter the occupiedzone (i.e. the discharge supply air shoulddecelerate to 50 fpm or less before enteringthe occupied zone). Unlike in a waiting room,there will typically not be occupants in the1 ft perimeter volume of the patient room.

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9 ft

9 ft 9 ft

18 ft

14ft

9 ft

Occupied Zone 6 ft

12 in 12 in

With this in mind, the occupied zone for thisexample will be the volume of the roomfrom the finished floor to 6 ft above the floor,excluding the 1 ft perimeter volume alongeach wall. Based on this definition of occupiedzone, the required throw to a 50 fpm terminalvelocity can be approximated by adding thedistance from the diffuser to the wall and thedistance from the ceiling to the finished floor.

Given the dimensions of this patient room(18 ft x 14 ft x 9 ft) and the placement of thesupply diffuser within the room (above thepatient bed, against the head wall), the targetthrow characteristics from the diffuser wouldbe 18 ft out either side of the diffuser and 23ft in the direction of the foot wall.

Noise and total pressure drop are theother selection parameters that should beconsidered when choosing an air outlet.The target noise range for a patient room isbetween NC 25 and NC 35 (AHRI Standard885-2008). Since cataloged diffuser NCvalues typically account for only the diffusergenerated noise and not other duct relatednoise, the conservative approach is toselect an air outlet near the low end of thetarget range. Please refer to the AcousticalConsiderations in Health Care Spaces of thePrice Engineers Handbook for more detailon noise calculations.

Performance Data - 3 Way Louvered Diffuser - Rectangular Neck

Duct size, in.

Neck Velocity, fpm 300 400 500

Velocity Pressure, in. w.g. 0.006 0.01 0.016

Total Pressure, in. w.g. 0.036 0.065 0.099

9 x 9Duct

Area0.56 ft2

Total cfm 169 225 282

NC - - 21

A B A B A B

cfm/side 42 63 56 84 71 106

throw, ft 6-9-16 7-11-18 8-12-18 10-15-21 10-15-21 12-16-23

From the performance data for the Price AMD with a 9 in. x 9 in. duct size (shown above), the 3A outlet will provide a 3 way throcombination close to the target for this example. From the catalog data, the 50 fpm terminal velocity throw is 18 ft out each side and ft out the end of the diffuser (based on 225 cfm). When cooling loads demand air-change rates in excess of the minimum required code (227 cfm), the throw is still close to the desired range. This diffuser selection also provides a noise level below the target of NCas well as a relatively low pressure drop.

In rooms with multi-level ceilings, privacy curtains or other ceiling mounted obstructions, it should be verified that air flow will not prematurely redirected toward the occupied zone.

With the diffuser requirements known, an appropriate air outlet can be selected from catalog data:



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Return Grille Sizing & Selection

Patient rooms are typically kept at a neutralor slight positive pressure relative to thecorridor. For this example a neutral pressuredifferential relative to the corridor will beused, and the exhaust air flow will matchthat of the supply. The toilet room will havea negative pressure differential relative tothe patient room, with an exhaust rate of10 ach and no dedicated supply air outlet.

The first step in the process of selecting anappropriate return grille is to choose thelocation. Return grille location will dictatethe type of grille selected and will affectroom air flow patterns.

Since the supply diffuser selected for thisexample is a horizontal throw air outlet,the room air will be completely mixed.When this is the case, it is generally mostpractical to have a ceiling mounted returngrille that will not be inadvertently blockedby furniture or other obstructions. Partiallyblocked returns will lead to higher noiseand pressure drop levels than planned forduring design. The return grille should alsobe located away from the supply diffuser tominimize the tendency for fresh supply air tobe exhausted before conditioning the space.With the supply diffuser above the headof the patient bed, a suitable and commonplacement for the return grille is near thepatient room door. A ceiling mounted,eggcrate grille is an economical selectionthat generally has attractive noise andpressure drop characteristics.

Since this patient room will be kept at neutral pressure relative to the corridor, thesame volume of air will be exhausted from the total space as is supplied. Therefore,the total exhaust air flow from the patient room will range between 227 cfm and269 cfm, depending on room loads.The toilet room will have a constant volume exhaust rateof 10 ach which must be accounted for in the total exhaust rate from the patient room itself.

The exhaust rate inside the patient room (excluding the toilet room exhaust rate) willtherefore range between 174 cfm and 216 cfm. It is important that the toilet room exhaustis accounted for in this calculation. If the toilet room exhaust rate was ignored, the neteffect would be a negative pressure differential in the patient room relative to the corridorand an increase of infiltration air into the space.

When selecting a return grille from catalog data there will be an inverse relationship

between the grille size and the noise and pressure drop generated by that grille. The returngrille should be large enough that pressure drop and noise are at reasonable levels.

Suitable eggcrate grilles can be selected using catalog data:

Performance Data - 80 Core Eggcrate Exhaust Grille

Core Area, ft Nominal Size

Neck Velocity, fpm 600 700 800 1000

Velocity Pressure, in. w.g. .022 .031 .040 .062

Negative Ps, in. w.g. .047 .066 .085 .132

0.188 x 4 cfm 108 126 144 180

6 x 6 NC - - - 22

0.2210 x 4 cfm 132 154 176 220

7 x 6 NC - - - 23

0.2612 x 4 cfm 156 182 208 260

8 x 6 NC - - 15 24

0.3014 x 4 cfm 180 210 240 300

NC - - 16 24

A number of options exist for the return grille selection. From the table above, any return grille with a core area of 0.22, 0.26, or 0.30 ft2would be appropriate for the exhaust outlet near the patient room door. All provide relatively low noise and pressure drop levels at theexhaust flow rates calculated in this example. At a constant flow rate of 53 cfm, the toilet room exhaust flow rate is easily satisfied with a0.15 ft2core area eggcrate return grille. Positioning of the exhaust grille in the toilet room ceiling will have little impact on its effectiveness.

PATIENT ROOM

Corridor

EggcrateExhaust Grille

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Displacement Ventilation (DV) Design Procedure

DV in Patient Rooms

The following step-by-step design procedure is offered as a simplified approach to determine the ventilation rate and supply air temperatfor typical displacement ventilation applications. The procedures presented are based on the findings of ASHRAE RP-949 (Chen et 1999) and the procedure outlined in the ASHRAE Design Guide, and incorporate the requirements of ASHRAE Standard 170-2008. Nthat local codes may have different requirements.

For a complete explanation and derivation of the assumptions and equations used to develop this procedure, please refer to ASHRStandard 129-1997. The design procedure applies to typical North American health care spaces, such as patient and waiting rooms. Thprocedures should be used with care when applied to large spaces such as theaters or atria; a computational fluid dynamic analy(CFD) of large spaces is recommended to optimize the air supply volume.

Only the sensible loads should be used for the calculations. These calculations are only for determining the air flow requirementsmaintain the set-point in the space; the total building load remains the same as with a mixing system.

Step 1: Determine the Summer Cooling Load

Use a cooling load program or the ASHRAE manual method to determine the design cooling load of the space in the summer. If possib

assume a 1.1F/ft [2 K/m] vertical temperature gradient in the space for the computer simulation, as the room air temperature is nuniform with displacement ventilation. Itemize the cooling load into the following categories:

The occupants, desk lamps and equipment, qoe(Btu/h [W])

The overhead lighting, ql(Btu/h [W])

The heat conduction through the room envelope and transmitted solar radiation, qex(Btu/h [W])

Step 2: Determine the Cooling Load Ventilation Flow Rate, QDV

The flow rate required for summer cooling, using standard air, is:

Step 3: Determine the Minimum Flow Rate

Ventilation of health care spaces is typically regulated by code. ASHRAE (2008) and CSA (2010) define an air-change rate which is usto determine the minimum air flow rate:

Step 4: Determine Supply Air Flow Rate

Choose the greater of the required flow rate for summer cooling and the required ventilation rate as the design flow rate of the supply

Step 5: Determine Supply Air Temperature

The supply air temperature can be determined from the ASHRAE Design Guide equations and simplified to:

Step 6: Determine Exhaust Air Temperature

The exhaust air temperature can be determined by the following method:

Step 7: Selection of Diffusers

The goal is to maximize comfort in the space and minimize the quantity of diffusers. At a maximum, ASHRAE suggests a 40 fpm fvelocity, but this value may increase or decrease depending on the space and comfort requirements. A CFD simulation can validate tdesign and is recommended for larger spaces.

Example 2 - Energy SavingsDisplacement Ventilation vs. Overhead Mixing



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Three patient rooms supplied by the same AHU all have exterior walls and windows facing different directions. The cooling load in eachroom varies significantly due to the large differences in solar heat gain. The geometry of the three rooms is identical, with a floor areaof 250 ft2and 9 ft ceilings. The control temperature for each room is 75 F.


Some of the assumptions made for the space are as follows:

The specic heat of dry air, cp= 0.24 Btu/lbF

Density of dry air,= 0.075 lb/ft3

The instantaneous cooling loads in each room are broken down as follows:



Mixing Ventilation Calculations

Assumptions:

Supply air temperature from AHU after dehumidication is 55 F

Calculating the supply air flow rate to meet code (ASHRAE Standard 170-2008):

Calculating the supply air temperature to each room:

Calculating the instantaneous reheat requirements:

To warm the supply air from 55 F to 57 F:

Similarly,

Design Considerations Room #1 Room #2 Room #3

Occupants & Equipment qoe 1300 Btu/h 1000 Btu/h 1100 Btu/h

Overhead Lighting ql 400 Btu/h 700 Btu/h 700 Btu/h

Exterior Load qex 2700 Btu/h 700 Btu/h 1400 Btu/hTotal qT 4400 Btu/h 2400 Btu/h 3200 Btu/h

tsupply(room#2) 65 F


qreheat(room#2) 2430 Btu/h


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Displacement Ventilation Calculations

Assumptions:

Temperature difference from head to feet for sedentary occupants (thf ) is 3.6 F Supply air temperature from AHU after dehumidication and energy recovery is 65 F

Calculating the displacement air flow requirement for each room:

Using the procedure described in example 16.1 of the Price Engineers HVAC Handbook:

Proposed changes to ASHRAE Standard 170-2008 would see the minimum total air change rate for single bed patient rooms appliedthe occupied zone (0 - 6 ft) only when low-level displacement ventilation is used.

Calculating the supply air flow rate to meet code assuming six air changes in the occupied zone:

Calculating the supply air temperature to each room:

Calculating the instantaneous reheat requirements for the room:

To warm the supply air from 65 F to 66 F:

Similarly,

Qcooling(room#2) 133 cfm

Qcooling(room#3) 174 cfm



qreheat(room#2) 0 Btu/hqreheat(room#3) 0 Btu/h



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Note the significant reduction in reheat relative to the mixing ventilation system. The above calculations for reheat with a displacementventilation system are based on recent changes to the definition of minimum total air change rate in ASHRAE Standard 170-2008 forsingle bed patient rooms.

Recalculating the supply air temperature assuming no change to the definition of minimum total air change rate for single bed patientrooms (i.e. current ASHRAE Standard 170-2008 requirements and Qmin= 225 cfm):

Similarly,

The narrow range of supply air temperatures combined with the superior energy recovery potential of displacement ventilation systemsallows for the dramatic reheat savings demonstrated by this example.





System Type Room #1 Room #2 Room #3Total Energy

used for Reheat

Mixing (6 ach min.) 486 Btu/h 2430 Btu/h 1701 Btu/h 4617 Btu/h

Displacement (4 ach min.) 260 Btu/h 0 0 260 Btu/hDisplacement (6 ach min.) 260 Btu/h 729 Btu/h 486 Btu/h 1475 Btu/h


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Research Highlight 1 - Displacement Ventilation in Patient Rooms

Guity, Gulick & Marmion (2009) conducted research to answer a number of practical questions related to the use of displacemventilation (DV) in single bed patient rooms. The research compared low sidewall DV at 4 air changes per hour (ach) with overheventilation (OHV) at 6 ach (current accepted minimum) in order to identify the impact of the following conditions and design paramete

Hot summer conditions (including solar gain)

Supplemental cooling

Cold winter conditions

Supplemental heating

Impact of air diffuser and grille locations

Movement

Coughing

Metrics

Two main sets of metrics were chosen to compare performance; the first to evaluate thermal comfort and the other to evaluate ventilateffectiveness/contaminant removal.

Predicted percentage dissatisfied (PPD) was the metric used to compare performance from a thermal comfort perspective. PPD w

empirically derived from human responses to test conditions where individuals reported their level of comfort. The metric was then binto the numerical model to describe a set of room conditions in terms of the percentage of occupants who are likely to be dissatisfiwith those conditions.

Four indices were used to evaluate the results from a contaminant removal perspective; they fall into two main categories, as follow

Ventilation Effectiveness (VE), or the ability of a ventilation system to remove internally generated pollutants from a building, zoor space. Ventilation effectiveness (as defined by ASHRAE Standard 62.1-2007) is calculated for the caregiver, visitor, and whole roo

Air-change Effectiveness (ACE), or the ability of a ventilation system to distribute ventilation air to a building, zone, or space. Ais defined by ASHRAE Standard 129-1997.

Results

The test results showed that DV at 4 ach performed equally or better than OHV at 6 ach for thermal comfort, ventilation effectiveneand contaminant concentration. The figures below show the extent to which contaminants spread in the DV and OHV scenarios. The air flow pattern results in a more contained contaminant concentration at a high level.

Case Description Weather Load Air Changes/HourSupply Temp.,

F [C]

1DV Limit

w/Solar LoadSummer, 105 F Standard 4

Adjustable,

60 F Min

2 Overhead Winter, -10 F Reduced 6Adjustable,

105 F Max

3DV w/RadiantHeating Panel

Winter, -10 F Reduced 4 67.1 F

4DV w/BaseboardHeater

Winter, -10 F Reduced 4 67.1 F

5DV w/RadiantCooling Panel

Summer, 105 F Standard 4 67.1 F

6 Overhead Summer, 105 F Standard 6Adjustable,

55 F Min

7 DV w/Solar Load Summer, 105 F Standard 4 60 F

8DV w/HighSupply Temp.

Winter, -10 F Reduced 4 87 F

Standard Load = Load from patient, caregiver, TV, and equipment.

Reduced Load = Load from patient and caregiver only.



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Case

PPD

Patient Area

Visitor Area

VE - Caregiver

3.6 ft

5.6 ft

VE - Visitor

3.6 ft

5.6 ft

VE - Whole Room

3.6 ft

5.6 ft

ACE

3.6 ft

5.6 ft

18.61

8.91

1.22

1.05

1.45

1.38

1.31

1.18

0.95

0.91

27.06

21.10

0.99

0.87

1.12

1.09

1.04

1.00

0.84

0.82

35.89

9.84

1.67

1.48

1.39

1.37

1.71

1.63

1.60

1.18

47.05

8.37

3.24

2.58

2.53

2.37

3.07

2.67

0.85

0.71

55.90

6.64

0.97

0.80

1.19

1.19

1.01

0.90

0.92

0.91

65.75

14.99

1.03

0.94

1.09

1.08

1.06

1.03

0.75

0.74

712.53

11.57

1.65

1.35

1.91

1.60

1.77

1.53

1.08

1.28

88.17

8.74

0.80

0.76

0.94

0.94

0.81

0.79

0.79

0.80

Discussion

The research concluded that DV at 4 achperformed equally or better than OHV at 6 achfor thermal comfort, ventilation effectivenessand contaminant concentration, and that theperformance of DV is dependent on several

integrated elements of the air delivery androom exhaust air system design.

Impact of Hot Summer Conditions andSupplemental Cooling

Increases in direct solar gains had asignificant impact on both thermal comfortand ventilation effectiveness. Thermalcomfort proved to be the limiting factoras ventilation effectiveness requirementswere always satisfied if thermal comfortwas maintained.

Lowering the DV supply air temperatureto 60 F (case 7) was sufficient to maintainthermal comfort, but too low to achieve airtemperature stratification requirements(per ASHRAE Standard 55-2004). Supplying

67 F air in combination with a supplementalcooling strategy (case 5) allowed bothrequirements of the standard to be satisfiedsimultaneously.

Room thermal gains and losses must becontrolled if the performance of the DVsystem is to be maintained:

Facades should be designed to minimize

thermal gains and losses in order toprevent warm and cold surfaceswarmsurfaces may cause fresh DV air toprematurely rise, while cold surfaces mayimpede stratification in areas of the roomwithout warm objects (i.e. equipment oroccupants) to drive air motion.

Solar shading devices should be installed

to minimize/eliminate direct solar gains.Floor surfaces warmed by direct solargains can act as thermal hot spots,

causing most of the displacement supplyair to short circuit at the breathing level.

Lighting and medical equipment loads

should be minimized to keep coolingloads at acceptable levels and allow moreDV air flow to be driven by occupants.

Impact of Cold Winter Conditions andSupplemental Heating Modes

Due to the same buoyancy principles thatallow DV to work in cooling mode, supplyair that is warmer than the room air willimmediately rise as it enters the space (case8).The warmer supply air cannot drop towardthe floor to form stratified layers. The lowerventilation effectiveness and longer mean

age of air in the occupied region indicate areduction in performance of the DV systemin terms of indoor air quality. Using eitherbaseboard heating or ceiling mounted

radiant panels while supplying air at67 F improved ventilation effectiveness andallowed thermal comfort to be maintained(case 3 and 4).

Impact of Movement on DisplacementVentilation

The study demonstrated that movingobjects can carry contaminants in theirwake. The movements can cause swingsin the contaminant concentration in thebreathing level of sitting and standingpositions for 10 to 90 seconds. Since thevariation lasted for 90 seconds or less, itwould not likely change the exposure riskfor occupants.

Displacement Ventilation at 4 ach Overhead Ventilation at 6 ach

10

Concentration1[ppm]

5

0

10

Concentration1[ppm]

5

0

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In most cases, the DV system with 4 achprovided better air quality than the OHVsystem with 6 ach. Around the bed regionor close to the contaminant source, theDV system had a higher contaminantconcentration than the OHV system.However, in most parts of the room, theconcentration was lower with the DV system.

Impact of Coughing on DisplacementVentilation

Around the bed, where a caregiver is mostlikely to stand, the DV case showed a highercontaminant concentration for a short periodof time after a coughing incident; while atthe end of the simulation period (5 minutes),the concentration was lower than observedwith the OHV system. The reason for thisresult is because displacement ventilationconfines or contains the concentration

Hospital Operating Rooms

around the patient, while OHV, given itsmixing nature and higher air-change rate,is able to better dilute the contaminantlevels around the bed. Consistent withthe above discussion, DV delivers a betterconcentration in the visitor area, as it isfurther away from the patient. When lookingat the room as a whole, the results achievedby the two ventilation systems in terms ofresponse to coughing were comparable.

Impact of Air Diffuser and GrilleLocations

The placement of the DV supply air diffuseris not critical, but should be coordinatedwith the room design. The diffuser shouldbe located at a low level in a location thatwill not be blocked with solid furnituresuch as a storage cabinet. DV supply airis not discharged in a particular direction

or pattern and will navigate arouobstructions, unless they are completblocking the outlet.

Related research by Yin et al., (20demonstrated that the toilet room transgrille should be located at a high levPerformance of the system can be negativaffected if DV supply air is allowedshort circuit through an undercut or lolevel transfer grille to the toilet room. Tproblem is the result of a negative pressdifferential in the toilet room relative to patient room itself. A low-level transfer poto the toilet room will pull a disproportionamount of air into the toilet room when toilet room door is closed. The negat

pressure differential is effectively lost wthe toilet room door open and will not havsignificant impact on system performan

Operating rooms are among the mostunique spaces in any hospital. The patientswho occupy operating rooms typicallyundergo invasive procedures that willexpose internal tissue to room air. It is notuncommon for these patients to alreadyhave weakened immune defenses, and thephysical interference with their organs and

systems (skin, blood flow, body temperature,etc.) can make them even more susceptibleto infection. The air distribution system inan operating room (OR) can either reduceor promote the frequency of surgical siteinfections, depending on the design.

Mixing type air distribution systems are notsuitable for hospital operating rooms. Inaddition to uniform temperature distributionfrom floor to ceiling, a well-designed mixingsystem will produce an even distribution ofcontaminants in the air, increasing the risk ofinfection during surgical procedures.

In an OR, control of airborne contaminantsand comfort are both major considerations.The three primary sources of airborneparticulates are ventilation, infiltration

and occupants. The particulate level ofventilation air is controlled using highefficiency filters, while space contaminationthrough infiltration is minimized bymaintaining a positive pressure differentialbetween the OR and adjacent areas of thehospital. Consequently, these means ofspace contamination represent less of aconcern than the presence of the patientand surgical team.

The largest source of airborne contaminationin most modern operating rooms (andmost challenging to control) is the surgicalteam and patient. Scrubbing and gowning

tactics used by surgical teams help tominimize the amount of airborne particlesreleased during a procedure, but they donot eliminate them completely. Also, withoperating rooms maintaining a positivepressure differential with respect to adjacentareas, there will inherently be recirculatingair (and contaminants) inside the room at all

times.The goal is to control and isolate thesecontaminants in such a way as to minimizetheir time in the surgical zone. The OR airdistribution system is the means by whichthis source of contamination is controlled,and it involves three main components.The first is dilution. Diluting airbornecontaminants to an acceptable level has ledto air supply exchange rates much in excessof those typically required for thermalcontrol. These increased air exchange ratescan lead to thermal discomfort due todrafts and the air distribution system musttherefore be capable of introducing a largevolume of supply air without compromisingcomfort in the occupied zone. The secondand third requirements of the air distribution

system are to remove particulates from thesurgical zone and to reduce or eliminate thetendency for those particulates to reenterthe clean air stream over the patient. AnOR environment should be comfortable foroccupants without contributing to the riskfor surgical wound infection. Achieving thisgoal from an air distribution perspectiveinvolves control of a number of factors.

Design Considerations

The face velocity of non-aspirating diffusersover the surgical table should not exceed 35fpm (Memarzadeh & Manning, 2002) as toavoid high velocity air near the patient. Highvelocity air in the surgical zone can have a

number of negative consequences:

1. Elevated rate of skin particle erosionsurgical team members (Cook & Int-H2009).

2. Overcooling of the patient, resultin hypothermic complications (KuSessler & Lenhardt, 1996).

3. Uncomfortable drafts.4. Entrainment of contaminated air.

Operating rooms typically require a positpressure differential relative to corridand other adjacent spaces. This is achievby supplying more air to the room this exhausted. The actual air flow offbetween supply and exhaust is dependon the target differential pressure aleakage from the room envelope acannot be determined in advance of rocommissioning. However, a 20% offsetypically required to maintain a reasonapressure differential.

All operating rooms should have individtemperature control and a device to mon

differential pressure between the room aadjacent spaces. Each category of OR whave different equipment needs (thermloads), as well as different conditrequirements with respect to air patteand temperature.

Cooling Loads

In most cases, the total air-change rrequired by code will be sufficientmeet OR cooling loads at the supply temperature range required by the surgteam. However, for some procedurincluding cardiac or transplant surgeit is necessary improved thermal contincreased cooling capacity, or both.



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Occupancy loads present an issue similar tolighting loads. Operating rooms will oftenhave periods of high population density atcertain times during a procedure, and theHVAC system should be capable of handlingthese peak loads with the ability to adjustfor reduced occupancy levels.

Equipment loads often account for themajority of heat generation in the OR.Cooling load requirements should typicallybe based on the equipment manufacturersBtu ratings, however, caution shouldbe taken when using these ratings incalculations. For example, a blood pressuremeter will have relatively constant powerconsumption whereas the peak draw of an

X-ray machine will only occur during X-rayexposure lasting a fraction of a second.

Types Of Operating Room AirDistribution Systems

There are two ventilation systems commonlyaccepted for use in hospital operatingrooms today: laminar diffuser systems andair curtain systems. Both systems have beenwidely used in all types of operating roomsand are described in more detail here.

Laminar Diffuser Systems

Laminar diffuser systems were developed tocontrol airborne contamination in operatingrooms by providing a downward wash ofclean supply air at relatively low velocity.

The most effective laminar diffuser systemswould see the entire ceiling filled withlaminar flow diffusers and all air exhaustingthrough floor grilles. By covering the entireceiling with diffusers, room conditionswould be close to isothermal, reducingthe chance of supply air acceleration dueto temperature gradients. The practice ofcovering the entire ceiling in diffusers isnot only impractical for an operating room,but the supply air volumes would be well inexcess of code requirements.

Reducing the size of the laminar diffuserarray opens space for other ceiling-mountedequipment (lights, booms, gas columns,etc.). At minimum, laminar flow diffusersshould cover 70% of the ceiling area directlyabove the area defined by the surgical tableand a 12 in. offset (ASHRAE Standard 170-2008). This minimum requirement willusually not satisfy minimum room air-change requirements and additional supplydiffusers beyond the primary diffuserarray area are most often necessary. Thisminimum requirement is not common toall jurisdictions and should be verified priorto design.

Figure 12: Laminar flow system with full ceiling coverage

Figure 14: Laminar flow air pattern

Figure 13: Laminar flow diffuser

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Although laminar flow diffusers areconsidered by many to be non-aspiratingair outlets, some entrainment of room airstill occurs within the first 3 to 6 in. belowthe diffuser face. The holes in the perforatedface act as individual air jets, causing air toaccelerate as it passes through the smallerfree area. This is why many codes refer tothe average velocity below the face of thediffuser. The average velocity near the faceof a laminar flow diffuser is based on the airflow rate per nominal face area, not actualair velocity. Once through the perforatedface, the air jets will expand, coalesce anddecelerate. By the time the air mass is morethan 6 in. from the diffuser face, the air

velocity profile will be more consistent andthe actual velocity of the supply air will bemuch closer to the average velocity. Whenthe actual air velocity is in the 25 to 35 cfm/ft2range there is minimal entrainment ofroom air.

The supply air temperature in most operatingrooms is 5 F to 10 F below the room set-point. The cooler supply will have a greaterdensity than the surrounding room air, andwill therefore have the tendency to acceleratetoward the surgical table. The warmer roomair will also transfer heat to the boundarylayer of the laminar air flow, causing it tobecome more buoyant. The result of thisthermal interaction causes the air in thecenter of the supply air column to accelerate

toward the surgical zone at a higher rate thanthe air around its perimeter (i.e. boundarylayer).The relatively high velocity will pull thecolumn boundary inward, creating a taperedcolumn of air. This tapering will occur undercooling conditions, regardless of the numberof diffusers in the array. Depending on themagnitude of this effect, the surgical teammay not get washed by the clean supply air,leading to discomfort and contaminationissues. Extending the laminar diffuser arraybeyond the footprint of the surgical table willtypically address this issue.

Air filtration in a laminar diffuser system canbe accomplished in one of two ways. Typicalpractice is to use HEPA filters in either anupstream filter bank or directly in the laminar

flow diffusers themselves. When multipleoperating spaces are supplied through acommon system, it is often most economicalto use the HEPA filter bank approach.Additionally, with the HEPA filters located ina bank upstream of the operating room, filterservice and maintenance can be performedwithout entering the sterile environment ofthe operating room.

Supply diffusers with integral room-sidereplaceable HEPA filters offer ease ofaccessibility through the diffuser face forfilter service and replacement, but they mustbe accessed from inside the sterile operationroom. With this arrangement it may be

necessary for the operating room to be re-sterilized each time the filters are accessed.In some applications, a shut-off damperis installed in the branch duct feeding thelaminar diffuser.This allows for the air supplyto one diffuser to be shut off during filterchange-out while maintaining the supply ofair through the remaining laminar devices.

Air Curtain Systems

An air curtain system combines a laminardiffuser array above the surgical table witha four-sided linear slot diffuser system. Thefunction of the linear slot diffusers is tocreate a barrier of air between recirculatingcontaminants in the perimeter of the roomand the surgical zone. An air curtain systemtypically use less ceiling space (particularlyabove the surgical zone) to introduce thesame volume of air into the operating room.

The air curtain is created using speciallydesigned linear slot diffusers on each ofthe four sides around the surgical table.The linear slot diffusers are installed in the

ceiling with a minimum 3 ft offset betwethe surgical table and the inside of the linslot diffusers. This gives the surgical sspace to move around the table withentering the protective, air curtain field. Tlinear slot diffusers typically feature fixor adjustable deflection blades to dischasupply air at an angle of between 5 and from vertical, away from the table.

The linear slot diffusers which form thecurtain typically have two slots, creata thick, uniform curtain of air around surgical area. The air curtain presentbarrier of high velocity clean air betweenlaminar flow diffusers and any particulawhich may be recirculating in the rooparticularly near the ceiling level whlaminar diffuser are most likely to entrroom air. The air curtain entrains roomand any particulates in its outer boundlayer, carrying them down and away frthe surgical area toward the low-leexhaust grilles. The air curtain shouldsized to deliver between 25 and 45 cfm

Figure 16: Air curtain systems Figure 17: Air curtain systems

Figure 15: Air curtain systems air pattern

Laminar Flow Diffuser Linear Slot DiffuserLinear Slot Diffuser

Return GrilleReturn Grille

OR Work Area

Air

Curta

inAirC

urta

in

Module Length = L

ModuleWidth

W

Plan View

Surgical Zone



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At flow rates below 25 cfm/ft, the air curtainmay not properly isolate the interior laminarflow diffusers, increasing the possibilityof surgical zone contamination due toentrainment of the recirculating room air.In contrast, air flow exceeding 45 cfm/ft willincrease the potential for re-entrainmentof particulates and debris that may havesettled on the floor.

The purpose of an air curtain system goesbeyond simply creating a barrier betweenthe surgical zone and the perimeter areaof the operating room. The air curtain alsoserves to control air velocity at the operatingtable level. This system characteristic isof utmost importance given the existing

research that suggests high velocity air atthe surgical table can increase the risk ofsurgical site infection.

As the relatively lower velocity air exitsthe laminar diffusers above the patient,the higher velocity air from the linear slotdiffusers will induce (pull) the laminar flowoutward. The laminar flow will expand tofill the zone enclosed by the air curtain,mitigating the tendency for the cold airmass to accelerate as described earlier.The net result is the ability to maintain airvelocities at the operating table close to, oreven slower than, those at the diffuser face.

Air curtain systems allow slightly lessflexibility with regard to filter locationscompared with all laminar systems. Thelinear slot diffuser plenums, which make upthe four-sided air curtain, are too narrowto effectively incorporate an integralhigh efficiency filter without resulting insignificant and undesirable pressure drop.The laminar flow diffusers above the surgicaltable may still include integral filters, but allair supplied to the linear slot diffusers mustbe filtered upstream of the system.

No official guideline exists to define thedivision of total supply air between the linearslot diffusers and laminar flow diffusers ofan air curtain system. However a commonmethod is to supply 60% to 75% of the totalsupply air through the linear slot diffuserswith the remaining volume suppliedthrough the laminar diffusers. Since thesurgical zone served by the laminar flowdiffusers is typically less than 25% of thetotal operating room area, the net result is ahigher air-change rate within the air curtainthan the room average. The result is fasterdilution and removal of particulates at thesurgical table.

Selection of the air curtain system must takeinto account the standard air distributiondesign parameters such as sound, pressuredrop and comfort, plus the additional issueof particulate control.

The module size of the air curtain is governedby four factors:

Size of the area to be contained;recommended practice is to take the sizeof the surgical table plus a 3 ft perimeterwork area for the surgical team.

The total air volume required by the room toachieve the desired number of air-changes.It may be necessary in the case of very largeoperating rooms to use longer linear slotdiffusers (i.e. larger module size) than is

dictated by the surgical work area in orderto meet the air-change requirements of theoperating room.

Allowances and concessions may have to bemade for other ceiling-mounted equipmentsuch as I.V. tracks, surgical lights, gascolumns, general lighting, etc.

The shape of the operating room mayrestrict the space in one direction or theother. It is recommended to keep at least3 ft between the linear slot diffusers andthe OR walls.

Air Curtain Types

There are two types of air curtain systemscommonly used today: the modular andthe continuous plenum. The selection ofone over the other is typically associatedwith the available space for facilitating ductconnections.

Modular Plenum Air Curtain

The modular plenum air curtain systemhas four independent plenums. Since theplenums are independent, each one must be

ducted separately. The quantity and/or sizeof the inlets are dependent on the lengthof the linear slot. For slot lengths of up to120 in., a single inlet located close to thecenter of the plenum is sufficient to provideequal air flow along the entire length ofthe slot. For lengths greater than 120 in.,multiple inlets are usually required. Multipleinlets should be equally spaced along theentire diffuser length to support air flowequalization along the full slot length. Inletsare typically rectangular and sized for aninlet velocity of approximately 500 fpm.

Figure 18: Modular air curtain room-side and plan view

Plan View - Plenum

Module Length = L

ModuleWidth=W

Reflected Ceiling Layout

Module Length = L

ModuleWidth=W

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The modular plenum style offers thefollowing advantages:

The corners of the modular plenums overlap,resulting in an almost continuous air curtainwith only small gaps at the corners. Thisminimizes the potential for particulates tomigrate inside the air curtain and into thelaminar air flowing over the patient.

Fewer field connections. Since each diffusersection is independent there is no need toconnect plenums with corner elbows.

With inlet connections on all sides of theair system, it is easier to achieve a uniformair distribution along the entire length ofthe linear slot diffusers, resulting in a more

effective barrier.The perceived disadvantage to the modularsystem is the quantity of inlets. With eachdiffuser section separately ducted, ceilingspace can become congested.

Continuous Plenum Air Curtain

The continuous plenum air curtain systemhas one common ring plenum with eachside connected by flanged elbows abovethe ceiling. Since all plenum sections areconnected, it is possible to use fewer inlets,however, it is still beneficial to have oneinlet on each side of the air curtain to helpequalize air flow. The minimum numberof inlets recommended for this system istwo, with the inlets located as far apart aspossible to support effective equalizationaround the entire air curtain. As with themodular systems, these inlets should besized for an inlet velocity of approximately500 fpm.

Advantages of the continuous systemsinclude:

Fewer inlets, resulting in less ductwork

and connections.

Perceived disadvantages include:

With fewer inlets and the same air volume,

ductwork to each inlet will be larger.

Continuous plenum air curtains have four

elbows which require field connection andsealing.

The active slots do not overlap in the

corners, resulting in larger gaps in the airbarrier than with modular systems.

Return Grilles

Ideally, there should be four low-level returngrilles, centered in each wall, or mountedin each corner of the room (Memarzadeh& Manning, 2002). Since space is usuallyat a premium in operating rooms, there isnot always sufficient room to include fourreturns. In this case, the next best option isto use two return grilles located as far apartas possible.

Figure 19: Continuous air curtain room-side and plan view

Reflected Ceiling Layout

Module Length = L

ModuleWidth=W

Plan View - Plenum

Module Length = L

ModuleWidth=W

Return grilles are typically located at alow level, with the bottom of the grilleinstalled approximately 8 in. above the floor(ASHRAE Standard 170-2008). This locationis beneficial for ease of cleaning as wellas for removal of heavier-than-air gases,a category which includes most medicalgases (i.e. CO2, N2O, O2).

Return grilles are most often constructedof stainless steel, typically for strength anddurability properties as opposed to the

need for corrosion resistance. Since thesegrilles are typically located at a low level, thepotential is high for impact and damage bycleaning staff or mobile equipment.

Return grilles are sized to return less air thanis supplied in order to maintain a positivepressure in the operating room. Once thetotal return air flow has been determined,the air flow per grille should be calculatedwith grilles sized based on a core velocityof approximately 500 fpm. Selecting returnsat 500 fpm will provide desirable noise andpressure drop levels.



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Example 3 - Operating Room Air Distribution


The operating room (OR) used in the following example includes an anesthesia machine, two LCD monitors, two surgical lights andoverhead lighting. The OR was designed for a maximum of seven occupants (patient, surgical team and support staff) and has one 2ft x 6.5 ft surgical table in the center of the room. Control temperature for the space is 68 F and the room dimensions are 22 ft x 22 ftwith a 12 ft ceiling height.

Operating Room Design Criteria (ASHRAE Standard 170-2008)

20 ach minimum (total, based on supply volume)

Positive room pressure relative to corridor

Target HVAC system noise range 25 to 35 NC (AHRI Standard 885-2008)


Some of the assumptions made for this space are as follows:

Supply air temperature is 55 F

Specic heat of dry air (cp) is 0.24 Btu/lbF Density of dry air, is 0.075 lb/ft3

Operating Room Loads

Heat Source Design Conditions (Btu/h)

Patient 160

Surgical Team (4) 1200

Support Staff (2) 600

Anesthesia equipment 900

LCD monitors 850

Surgical lights 1500

Overhead lighting 2400

Total 7610

Air Flow Rate Calculations:

Calculating the supply air flow rate to satisfy the design load:

For rooms with a positive pressure differential relative to adjacent zones, the minimum air-change rate is based on the supply air volume.Calculating the supply air flow to satisfy the required air-change rate:

Less air must be exhausted from the operating room than is supplied in order to maintain the required positive differential pressure. Theactual air flow offset will need to be determined at the time of commissioning, but a 20% offset will be used for this example. Calculatingthe exhaust air flow rate to provide the desired positive differential pressurization:

The minimum total supply air flow rate for this OR is 20 ach or 1936 cfm (ASHRAE Standard 170-2008). The air flow rate required tosatisfy the design load conditions is only 542 cfm, well below the ASHRAE specified minimum. As a result, there will be constant volumesupply and exhaust air flow to and from this OR and reheat will be required at all times to prevent overcooling.

Calculating the minimum supply air temperature:

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The difference between the supply airtemperature and the room temperature(T) must be monitored to ensure thereis no significant acceleration of supplyair directly over the patient and surgicalteam. As T values increase, so do thechances of acceleration of laminar air flowover the patient. Air-change rates can bereduced when the room is unoccupied,however, pressure relationships should bemaintained.

Choosing a System

The two widely accepted air distributionsystems for operating rooms are the laminardiffuser system and the air curtain system.No scientific, comparative performance testresults exist for which each of these systemswas included. As such, there are conflictingopinions on which system is more effectivein controlling airborne contamination.

Standards and guidelines that govern thedesign of operating room air distributionsystems vary from one to the next in terms ofrequirements (i.e. air-change rates, diffuserquantity/type/location, etc.). ASHRAEStandard 170 is currently the primarydocument used for OR air distributiondesign in the US and many countriesabroad. For this reason, it will be used todictate the design of the OR air distributionsystems detailed in this example.

The key details from ASHRAE Standard 170-

2008 that relate to the design of the OR airdistribution system are summarized below:

Room should be maintained at a positive

pressure relative to all adjoining spaces.

The primary diffuser array above the

operating table should satisfy thefollowing conditions:

Diffusers should be non-aspirating

with unidirectional, downward air flowat an average velocity of 25 to 35 cfm/ft2at the diffuser face (i.e. laminar flowdiffusers).

Cover 70% of the ceiling area directly

above the area defined by the surgicaltable plus a 12 in. offset on all sides.

Diffusers outside the primary diffuser

array area should discharge air verticallydownward.

OR should have at least two low-level

(8 in. above floor) sidewall return orexhaust grilles spaced as far apart aspossible.

This example will detail how each systemshould be designed based on this criteria.

Laminar Diffuser Array

Laminar diffuser array systems use laminarflow diffusers concentrated above thesurgical table to deliver 100% of the supplyair to the operating room. ASHRAE definesthe primary diffuser array area as the ceiling

12 in.

12 in.

Primary diffuser array area

4 ft2.5 ft

4 ft

8 ft 8 ft30% of primary diffuser array areaused for non-diffuser application

area directly above the operating table plusa 12 in. offset on all sides of the table. Upto 30% of this primary diffuser array areamay be used for non-diffuser equipment(i.e. lights, booms, etc.).

The average discharge velocity from thelaminar flow diffusers should be between25 and 35 cfm/ft2(Memarzadeh & Manning,2002). For this example, a target of 30 cfm/

ft2

will be used.Calculating the required laminar flowdiffuser face area:

There are a wide range of laminar flowdiffuser sizes available, as well as customsizes. A combination of diffusers must bearrayed above the surgical table, ensuring nomore than 30% of the primary diffuser arrayarea is used for non-diffuser equipment.

There is no correct way to arrange laminar diffusers and the placement is ofpartially dictated by the location of otceiling mounted equipment. To reduce acceleration potential of supply air othe surgical zone, it is preferable to hgaps between diffusers when practicSpacing diffusers is often a challenge wlaminar diffuser systems due to the latotal diffuser face area. The diffuser farea requirement for this example cansatisfied with eight 24 in. x 48 in. lamiflow diffusers arranged around the surglight boom, as in the image below.

The inlet neck selection has negligible impon the performance of these diffusers30 cfm/ft2, each laminar flow diffuser wsupply approximately 242 cfm and eithe10 in. or 12 in. inlet neck diameter will resin suitable pressure drop levels and nolevels below our target range of 25 to 35



E-2


24/37



Performance Data - Laminar Flow Diffuser

24 in. x 48 in. Panel - 10 in. Round Inlet

cfm Ps, in. w.g. Pt, in. w.g. NC

160 0.01 0.02 -

240 0.02 0.04 -

320 0.04 0.06 -

400 0.07 0.10 17

480 0.09 0.14 23

Air Curtain System

An air curtain system consists of aprimary laminar diffuser array above theoperating table and a four-sided linear slotdiffuser system surrounding the surgicalzone. The linear slot diffusers must bespecifically engineered for OR applications,

as conventional slot diffusers are notmanufactured to accommodate thoroughsterilization, nor is the air pattern theyproduce appropriate for protection of thesurgical zone.

The laminar flow diffusers are locateddirectly above the surgical table, while thelinear slot diffusers surround the operatingtable and a perimeter work area for thesurgical team. It is also recommended thatthe linear slot diffusers are installed at least1 ft from the laminar flow diffusers to reducethe potential for entrainment of laminar airflow into the higher velocity discharge ofthe linear slots. The target supply rate fora linear slot diffuser system should beanywhere between 25 and 45 cfm/ft. Flow

rates below this range will not create aneffective curtain, while higher flow ratesmay stir particles that are settled on thefloor. Typically 60 to 75% of the total supplyair volume will be through the linear slotdiffusers with the balance supplied throughthe laminar diffuser array.

The first step in the design of the air curtainsystem is to establish the size and layoutof the primary laminar diffuser array toensure ASHRAE requirements are satisfied.The recommended practice is to keep thesize of the primary diffuser array to theminimum ASHRAE requirement and deliverthe balance of the supply air through the

Performance Data - Laminar Flow Diffuser

24 in. x 48 in. Panel - 12 in. Round Inlet

cfm Ps, in. w.g. Pt, in. w.g. NC

160 - 0.01 -

240 0.02 0.03 -

320 0.04 0.05 -

400 0.06 0.07 -

480 0.08 0.11 16

linear slot diffusers. Following this process will keep more of the prime ceiling area abovethe surgical zone available for non-diffuser equipment while also minimizing the potentialfor acceleration of supply air over the patient and surgical team.

Determine the Size and Quantity of Laminar Flow Diffusers

Based on the assumption that the operating table is 2 ft x 6.5 ft, the required diffuser facearea for the primary diffuser array should be approximately 24 ft 2.

Operating table size = 2 ft x 6.5 ft

Primary diffuser array area = [(2 ft + 2 ft) x (6.5 ft + 2 ft)] x (1 0.3) = 23.8 ft 2 24 ft2

At the recommended 30 cfm/ft2average discharge velocity through the diffuser face area,the total supply air volume through the primary diffuser array will be 720 cfm.The selectionand arrangement of laminar flow diffusers for the primary diffuser array will depend inpart on the location of other ceiling mounted equipment. Since the surgical light boom ismounted in the center of the room above the operating table, the primary diffuser arraymust be positioned around this obstruction.

Four 24 in. x 36 in. laminar flow diffusers positioned as shown below will provide sufficientcoverage of the primary diffuser array area without interfering with the surgical light boom.

2.5 ft3 ft 3 ft

4 ft 4 ft30% of primary diffuser array areaused for non-diffuser application

The following performance table is for a typical laminar flow diffuser (Price LFD):

E-24





25/37


12.5 ft

8 ft

Determine the Size of the Linear SlotAir Curtain

The linear slot diffuser layout is defined indifferent ways by different manufacturers.The inside dimensions of the linear slotdiffuser layout are often used to describethe system. As a starting point, the linearslot diffusers will be sized to enclose theoperating table plus a 3 ft perimeter. This3 ft perimeter will allow the surgical teamto move around the patient and work indifferent positions without passing throughor standing directly beneath the air curtain.It should also be verified that the linearslot diffusers are at least 12 in. from anylaminar flow diffuser as noted earlier in this

example.

Based on the 2 ft x 6.5 ft surgical table, theinside dimensions of the linear slot diffusersystem will be 8 ft x 12.5 ft. The ceilingplan should be referenced to ensure thelinear slot diffusers do not interfere withany other ceiling mounted equipment. Itshould also be noted that the 3 ft perimeterwork space around the operating table is ageneral guideline. Depending on the type ofoperating room, the associated number andposition of surgeons, and space restrictions,the perimeter may be more or less than this3 ft value. The linear slot diffuser systemshould be adjusted accordingly.

There are three options to consider ifinterference exists between the linear slotdiffusers and other equipment, in order ofpreference:

Reposition the other ceiling mountedequipment typically not done

Increase the size of the air curtain untilinterference with other equipment nolonger exists

Split the air curtain around the other ceilingmounted equipment

The third option is the least desirable as it willcreate a gap in the air curtain where airbornecontaminants could enter the surgical zone.For this example it is assumed that thereis no interference between the linear slotdiffuser system and other ceiling mountedequipment. The inside dimensions of thelinear slot diffuser system will thereforeremain 8 ft x 12.5 ft.

The total linear slot length can beapproximated by adding these twodimensions and doubling that value. Theactual linear slot length will depend on thewidth of the linear slot face (will dependon manufacturer) and the type of systemused (i.e. modular or continuous). Theapproximate total linear slot length will be:

(2)(8 ft + 12.5 ft) = 41 ft


Performance Data - HORD - Linear Slot Diffuser

cfm/ft 25 30 35 40 45

7 ft - 6 ft

NC - - - 19 24

Throw* 0-1-4 1-2-6 1-2-6 1-3-7 2-4-7

Ps, in. w.g. 0.048 0.070 0.054 0.070 0.088

8 ft - 6 ft

NC - - 16 21 25

Throw* 1-1-5 1-2-6 1-2-7 1-3-7 2-4-8

Ps, in. w.g. 0.051 0.074 0.061 0.080 0.101

10 ft

NC - - 18 23 28

Throw* 1-1-6 1-2-7 1-3-7 2-4-8 2-5-8

Ps, in. w.g. 0.049 0.071 0.065 0.084 0.107

12 ft

NC - 15 21 26 31

Throw* 1-2-7 1-2-8 1-3-8 2-4-9 2-5-9

Ps, in. w.g. 0.066 0.095 0.066 0.086 0.109

14 ft

NC - 18 23 28 33

Throw* 1-2-7 1-3-8 2-4-9 2-5-10 3-6-10

Ps, in.

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