producing and maintaining vacuum

Art Montemayor Introduction to Vacuum Systems August 21, 2002Rev: 1(02-16-04)

of 25 FileName: document.xlsWorksheet: Scope of Work

Workbook Scope:The purpose of this workbook is to provide informative engineering experience and guidelines to allow the successful design and operation of a process plant vacuum system. This also involves:

1.0 Selection of vacuum-producing equipment;2.0 Operation of vacuum-producing equipment; and,3.0 Trouble-shooting of vacuum-producing equipment operating at 0.04 inches of mercury or

higher. Vacuum operation lower than 0.04 inches of mercury is considered a special and unique vacuum design and requires additional expertise.

SAFETYVacuum equipment either must be capable of withstanding the maximum possible pressure to which it might be subjected in case of a malfunction, or it should be protected by a rupture disc or buckling pin. For example, a cast iron steam jet ejector using 125 psig steam as motive force and built with a 25 psig maximum allowable working pressure (MAWP) on the body (or the same steam jet connected to a 25 psig MAWP vessel) should have a rupture disk or buckling pin installed to prevent the 125 psig steam pressure from beingapplied to either the jet ejector's body or the process vessel.

GENERAL ENGINEERING CONSIDERATIONSInitially, the required process vacuum must be established at the startup of the related equipment.This is called the "pump-down" period, and the vacuum system must be able to evacuate the connected equipment in the required amount of time.After the process has been analyzed properly to determine the vacuum required, it then becomes necessary to investigate the process material(s) and to determine the amount or rate of gases generated during process that ultimately have to be handled by the vacuum-pumping equipment. Corrosive vapors and suspended solid materials then must be identified to determine what materials of construction will be necessary to obtain reasonable life expectncy from the vacuum-producing device.

Analysis of the process equipment then must be made to determine a reasonable air leakage value. Number and type of packing glands, valves, sight glasses, etc, all must be taken into consideration and due allowances made. By estimating a reasonable air leakage of equipment and the other noncondensables from the process, it is possible to determine the required capacity of the vacuum-producing equipment. However, it is very important to determine the saturation factor (i.e., every pound of air and other noncondensables carry with them some quantity of process vapor according to the law of partial pressures

Needless to say, all equipment connected to a vacuum-producing device should be 100% vacuum rated.

- as expounded in the well-known Dalton's Law.)



DEFINITION OF TERMSAbsolute Pressure This is a pressure value measured from absolute zero pressure, i.e., from an absolute

vacuum base starting point. Note: often some scientists and engineers are delinquent in using the term "vacuum" when they mean a negative value from the base point of atmospheric pressure. This is often results in confusion and misunderstanding. Absolute pressure is recommended usage because it is precisely that - absolute.

Static Pressure This is a pressure value measured within the gas medium such that no effect fromthe velocity of the gaseous medium is reflected on the measurement; i.e., the velocity

Suction Pressure This is the absolute static pressure prevailing at the suction of the vacuum-producing equipment expressed in pascals, psia, torr, or in inches, millimeters, or microns of mercury.

Discharge Pressure This is the absolute static pressure prevailing at the discharge of the vacuum-producing equipment expressed in pascals, psia, inches of mercury, or mm Hg (torr).

Support Pressure This is the maximum discharge pressure against which a stream jet can operate stably.

Absolute Temperature The temperature above absolute zero, expressed in degrees Rankine or Kelvin.

Suction Temperature The temperature of the gas at the suction of the vacuum-producing equipment.

Stable Operation The mechanical operation of the vacuum producing equipment without violent fluctuation of the suction pressure.

Capacity

capacity is expressed as a volume rate of flow at the specified suction pressure and suction temperature (usually Acfm of dry air).

Dry Air This is atmospheric air at normal room temperature, devoid of any water vapor contentthat is usually found in atmospheric air. The very small amount of water vapor in it is considered insignificant and is ignored. For example, the weight of water vapor in

of water per pound of dry air.

Equivalent Air This is the calculated weight rate of air in lbs/hr that is equivalent to the weight rate of gas handled by the vacuum-producing equipment at the suction conditions.

VACUUM PRESSURE MEASUREMENT UNITS

1 Atmosphere = 14.696 psia= 101.325 pascals

= 760 mm mercury= 760 torr= 760,000 microns mercury

= 29.94

1 Bar = 14.5 psia

head effect (v2/2g - where v = fluid velocity, ft/sec; g = 32.2.ft/sec2) is not measured.

The capacity of steam jet ejectors is expressed as a weight rate of flow of a specific

gas [usually as lb/hr of dry air at 21 oC (70 oF)]. For mechanical vacuum pumps,

atmospheric air at 50% relative humidity and 25 oC (77 oF) temperature is 0.010 pound

inches mercury absolute (@ 32 oF)



PUMP-DOWN TIME

Plant equipment (especially distillation columns) are fixed assets that must be employed to the maximum in order to maximize the return on their investment. The column start-up time should be as short as possible and requires large vacuum rates that are in far excess of the designed, steady-state value that the system usually runs at. You should decide on the maximum time allowable for start-up and make sure you have sufficient vacuum rate to evacuate the system in that period. For this, you must have accurate volumetric estimates of all the system components, including all connected piping.

The following equations (which neglect air seepage into the system) may be used to estimate the evacuation time for a system to be reduced in pressure if the system is initially found filled with atmospheric air.

For vacuum systems using positive-displacement, mechanical vacuum "Pumps":

For Steam vacuum jets:

where,

V =cfm = Mechanical pump capacity, Acfm

Initial pressure, inches Hg absolute

Final pressure, inches Hg absolute

DA =

(1) Calculate the time to evacuate a 3,14029.9 inches Hg absolute, down to 1.0 inch of Hg absolute by using a

1,000 Acfm positive displacement vacuum pump.

Solution: 11.7 minutes

(2) Calculate the time to evacuate a 1,000jet ejector used is 40.0 lb/hr of dry air.

Solution: 55.0 minutes

TEvac = The approximate time to evacuate the system from P1 to P2, minutes

System volume, Ft3

P1 =

P2 =

Steam jet design capacity, lbs dry air/hr @ 70 oF

Calculation examples: (Key in input into YELLOW cells and answer is in RED)

ft3 chamber from atmospheric pressure,

TEvac =

Ft3 system if the design capacityof the steam

TEvac =

T Evac=(1 .1 )( Vcfm )Ln(P1

P2)

T Evac=(2.2 )( VDA )

Art Montemayor Calculating Vacuum Requirements August 21, 2002Rev: 1(02-16-04)

of 25 FileName: document.xlsWorkSheet: Air Seepage

DETERMINATION OF AIR SEEPAGE INTO VACUUM EQUIPMENT

Determination of the required vacuum capacity for a given system should include the following:

1) Atmospheric air seepage into the system through joints and seals;2) Gases generated by, or inherent to, the process;3) Liquid impurities whose vapor pressures are higher than the vacuum setting;4) Maximimum pump-down time required for startup of the system.

The amount of air leakage (actually seepage) into a system is a function of design, operation, and maintenance. Energy conservation and environmental emission controls dictate that air seepage be minimized and kept to the lowest practical levels. Once air is introduced into a vacuum system, its negative effects are compounded downstream.

all process fluid should be evacuated from the subject equipment and the system should be subjected to a vacuum level within the range of 1 to 10 inches Hg absolute. Once the vacuum level is attained, the vacuum-producing equipment should be quickly and positively isolated from the system. The time that it takes the system to reach a given rise in pressure is measured as accurately as possible. As an example,measure the time required to achieve a pressure rise of 2 inches Hg. The system pressure should not be allowed to increase the system pressure above 15 inches Hg during the test. To calculate the estimated air seepage rate into the system, use the following relationship:

where,

V =

Final system pressure, inches Hg abs

Initial system pressure, inches Hg abst = time required for the system pressure increase, minutes

Calculation example: (Key in input into YELLOW cells and answer is in RED)

A system has 3005.0 inches Hg abs to 7.0 inches Hg absolute pressure in

31.0 minutes.

The air seepage rate is = 2.9 lbs/hr

The air seepage into a process can originate from various sources. Leaks can occur through gasketed joints;through holes, cracks, or other flaws in the base material of construction; through permeation; and throughflaws in the welded vessel joints. Most of these sources are related to the pressure or concentration differential existing between the vacuum-producing equipment and the surrounding atmosphere. Leakageallowances should always be estimated on the conservative side (allowing for an excess vacuum capacity) inorder to ensure that there will be sufficient capacity for a variety of conditions that may arise.Air seepage can be estimated for new or existing systems by using the following empirical relationships:

One way to identify air seepage into existing equipment is to employ a field pressure drop test. For this test,

System volume, ft3

P2 =

P1 =

ft3 of internal volume and increases from

Air Seepage , lb/hr=(0 .15 ) (V ) (P2−P1)

t



First: Estimate the air seepage due to metal porosities and weld line cracks and flaws with one of the following three equations…

For a design vacuum pressure (P) less than 0.4 inches Hg abs, use

For a design vacuum pressure (P) within the range of 0.4 to 4.0 inches Hg abs, use

For a design vacuum pressure (P) greater than 4.0 inches Hg abs, use

Second: Estimate the air seepage due to the various system components using one of the following threeequations. Determine the value of the specific heat leak, Q, from the subsequent Table below.

For a design vacuum pressure (P) less than 0.4 inches Hg abs, use

For a design vacuum pressure (P) within the range of 0.4 to 4.0 inches Hg abs, use

For a design vacuum pressure (P) greater than 4.0 inches Hg abs, use

Note: D = the nominal diameter, in inches, of a sealed joint

W 1 , lbs /hr=(0 . 0781 ) P0. 34 V 0. 6

W 1 , lbs /hr=(0 . 0742 ) P0. 26 V 0 . 6

W 1 , lbs /hr=(0 . 106 ) V 0 .6

W 2 , lbs /hr=3 π D Q P0 .34

W 2 , lbs /hr=2 . 78 π D Q P0 . 26

W 2 , lbs/hr=3 .98 π D Q



Table of Specific Air Seepage rates for vacuum system components

System Component

Static SealsO-ring construction 0.0020Conventional gasket seals 0.0050Thermally cycled static seals

Temperature < 100 oC (212 oF) 0.0050

0.0180

0.0320

Motion (Rotary) Seals0-ring construction 0.1000Mechanical seals 0.1000Conventional packing 0.2500

Threaded Connections 0.0150Access Ports 0.0200Viewing Windows 0.0150Globe Valves, diameter < 2 inches 0.2400

values and the answer will be the total air seepage rate in lb/hr.

Q

lb/hr-in.

100 oC < Temperature > 200 oC (392 oF)

Temperature >200 oC

Third: To obtain the total estimated air seepage, add the W1 estimate to the sum of all the estimated W2



The air seepage into a process can originate from various sources. Leaks can occur through gasketed joints;

allowances should always be estimated on the conservative side (allowing for an excess vacuum capacity) in

equipment is to employ a field pressure drop test. For this test,

Art Montemayor Calculating Vacuum Requirements August 21, 2002Rev: 0

of 25 FileName: document.xlsWorkSheet: Process-Generated Gas

Process Gases and Non-Condensables Generated within, or inherent to, Vacuum Systems

Process gases such as water vapor, alcohol, oxygen, hydrogen sulfide, carbon dioxide, ammonia, chlorine, sulfur dioxide, sulfur trioxide, carbon monoxide, the nitrogen oxides, etc. are often found to be generated within a process operation - especially one involving sufficient heat to cause decomposition of some of the liquid components (i.e., the vacuum distillation of crude nitric acid I did at the DuPont Plant in Victoria, TX).Determination of the quantities and rates involved is necessary since either they have to be handled by the related vacuum system just as if they were so much more seepage air or they must be removed by condensation or by chemical scrubbing done subsequently downstream. Familiarity and/or intimate knowledge of the chemistry involved in the process is required in the absence of actual laboratory determinations. Those with a command of Dalton's Law of Partial Pressures will appreciate the fact that anyliquid components with vapor pressures in excess of the system vacuum at the operating temperature will emit their flash vapors in accordance with Dalton's Law.

In some cases, the amount of dissolved gases or generated gases will be found to be quite large and may even be more than the amount of seepage air. In many cases, however, these gases are very small in quantity and can be considered as additional air leakage. In other cases, these process gases can be considered as nil or neglected altogether.

If there is a possibility of removing process gases selectively by employment of condensation, scrubbing, adsorption, or even possibly a reaction, this should always be considered first before discarding the option.When a condensable process gas temperature is higher than that of the available condensing or scrubbing media, it always requires less energy to remove the gas as a liquid with a barometric leg (or pump) rather than to "pump" it as a gas. The size of the vacuum-producing equipment and its energy consumption can be reduced economically any time that water vapor or process gases can be condensed or scrubbed ahead of the vacuum-producing equipment.

However attractive the incentives to remove process gases may appear, they inherently carry the inevitableengineering trade-off. Any process or technique applied to removing these gases will introduce a pressure drop immediately prior to the vacuum-producing equipment and this directly increases the size and/or complexity of the subject equipment - and consequently the total capital cost. Nevertheless, such is the attractiveness of the incentives to remove the process gases that it is always in the interests of the design engineer to carefully analyze the available options and their merits. Frequently, due to strict emission laws and potential hazardous conditions, there is no choice and it is a practical decision to install such equipment directly rather than waste irrelevant time and effort by subjecting it to economic analysis and justification.

Art Montemayor Calculating Vacuum Requirements August 21, 2002Rev: 0

of 25 FileName: document.xlsWorkSheet: Process-Generated Gas

liquid components (i.e., the vacuum distillation of crude nitric acid I did at the DuPont Plant in Victoria, TX).

even be more than the amount of seepage air. In many cases, however, these gases are very small in quantity

Art Montemayor Example Vacuum Capacity Calculation August 21, 2002Rev: 0

of 25 FileName: document.xlsWorkSheet: Sizing Example Problem

TYPICAL PROCESS VACUUM SIZING APPLICATION PROBLEM

A Process Reactor with 2:1 ellipsoidal heads is fitted with a packed,

2-inch diameter agitator shaft, a 24-inch filling port, a 12-inch diameterviewing window, and a 6-inch threaded outlet connection. The reaction takes place at 3.5 inches Hg abs and requires the removal of watervapor at a rate of 2,000 lb/hr. It is required that the design vacuum be achieved within 10 minutes after the reactor has been charged and

Calculate the required size of

a) A mechanical, positive displacement pump;b) A steam jet ejector

Engineering Solution:

Assume that the reactor ellipsoidal heads have a 2" straight flange.The volume of each ellipsoidal head is calculated by using my Vessel Volumes Workbook:

Pump-down Capacity:

Volume of two ellipsodial heads = 56.55

Cylindrical vessel volume = 263.89

Total vessel internal volume = 320.45

Initial vessel pressure = 29.94 inches Hg absolute (vessel is filled at atm. Press.)Final vessel pressure = 3.5 inches Hg absolute

Required Pump-down time = 10.0 minutes

Required Mechanical pump capacity = 75.7 Acfm (refer to Work Scope Worksheet)

Required Steam jet ejector capacity = 70.5

Air Seepage Calculation:For estimating the amount of air seepage due to metal porosity, use the following equation as found in the Air Seepage Worksheet:

where,P = 3.5 inches Hg absolute

V =therefore,

3.3 lbs/hr of air seepage through welding cracks and flaws

sealed. Cooling water is supplied at 20 oC.

Ft3 (refer to attached Worksheet, Ellipsoidal Head volume)

Ft3

Ft3

lbs dry air/hr @ 70 oF (refer to Work Scope Worksheet)

Total vessel volume, Ft3 (see above calculation)

W1 =

72" ID 108" S/S

W 1 , lbs /hr=(0 . 0742 ) P0. 26 V 0 . 6



In order to estimate the air seepage through the various system components, the following equation from the Air Seepage Worksheet is used:

The various components' contributions in seepage are detailed:

1.571.510.570.283.93

15.1 lbs/hr of air seepage through the system's components

18.4 lbs/hr of air seepage

By applying the Universal Gas Law, volumetric equivalent capacity for the total Seepage Air is:

where,Z = Compressibility Factor, assumed as 1.0n = 0.01 lb moles of air/min

R = 10.73

T = 581P = 1.72 psia

V = 38 Acfm

Process Gas Calculations:The water vapor generated and evacuated at the vacuum conditions behaves as if it were a process gas. This is thus because it's original state is at the reaction temperature. When a vacuum is pulled over the liquid water, a portion of the fluid is vaporized as the system pressure is lowered below the fluid's vapor pressure.Although the reaction temperature is not given in the problem statement, it must be at least as high as the corresponding saturation temperature of water at the given vacuum of 3.5 inches Hg absolute (1.719 psia).

The Thermodynamic properties of saturated water vapor are taken from the NIST website and are as tabulated:

119.89 1.69 0.0049137 203.51120.11 1.70 0.0049410 202.39120.32 1.71 0.0049684 201.27120.53 1.72 0.0049957 200.17120.74 1.73 0.0050230 199.09

Water vapor evacuation capacity required = 6,672

Seepage from the agitator shaft seal = p (2) (0.25) =Seepage from the filling port gasket = p (24) (0.02) =

Seepage from the viewing window gasket = p (12) (0.015) =Seepage from the threaded outlet = p (6) (0.015) =

Total S of p D Q effect =

W2 =

Total Air Seepage = W1 + W2 =

psia - Ft3/lbmol - oRoR

Temperature (oF)

Pressure (psia)

Density (lbm/ft3)

Volume (ft3/lbm)

Actual Ft3/min (Acfm)

W 2 , lbs /hr=2 . 78 π D Q P0 . 26

V=ZnRTP

This is the Specific Volume involved

Go to: http://webbook.nist.gov/chemistry/fluid/for themodata on water & other fluids.

Art Montemayor

A91

Go to: http://webbook.nist.gov/chemistry/fluid/ for themodata on water & other fluids. Art Montemayor



To convert any gas or vapor to an equivalent air weight requires a conversion factor equal to the square root of the molecular weight ratio. As an example, the 2,000 lb/hr of water vapor is equivalent to:

therefore,

Equivalent Air Load = 2,539 lb/hr of dry air

Equivalent Air Load=2 ,000√ MW of airMW of water

Art Montemayor Vent Condenser Calculations September 30, 2002Rev: 1(02-16-04)

of 25 FileName: document.xlsWorkSheet: Vent Condenser Application

VENT CONDENSER APPLICATIONS

During the design phase of certain vacuum applications, it is worthwhile to consider the possibility of employinga vent condenser on the downstream side of the main condenser (if one is dealing with a distillation column) or by itself just previous to the entrance to the vacuum-producing equipment (if one is dealing with pure evacuation of a vessel or system - as in a batch reactor that has to be evacuated prior to the transfer of its reaction products).

Often, the gaseous or vapor products that inevitably form part of the evacuated vapor stream are considered valuable compounds (such as solvents and light components - acetone, methanol, Carbon Tetrachloride, etc) or they are classified as undesirable atmospheric emissions (such as solvents, methanol, ethanol, etc.).

The utilization of a vent condenser in a Vacuum application allows the removal and/or recovery of the condensable components in the evacuated gaseous stream. Of course, in order to be effective, the vent condenser should have a coolant (usually on the tube side) that has a temperature well below the condensing temperature of the condensable components.

Other favorable and positive characteristics of a vent condenser in such applications are:1. The capacity of the vacuum-producing equipment is reduced as well as its capital cost;2. The required capacity of the downstream disposal facilities (such as vent headers, flare

systems, incinerators, etc.) and their related capital cost is also reduced.

Such are the attactive features of a vent condenser application that it is expedient for a design engineer to evaluate the potential results and benefits of a vent condenser. Vent condensers can be installed In series or they can be installed in inter-stage vacuum applications where deeper vacuum levels have to be attained by using multi-stage equipment.

Although the benefits of a vent condenser are immediately apparent, there are trade-offs that must also be considered during their evaluation. A higher pressure drop is the main nemesis in a vacuum system. Special care and expertise is required to design and fabricate the vacuum (usually shellside) side of a vent condenser - as is also the case for a main overheads condenser in a distillation application. Often, the presure drop anticipated is so critical and controlling that it is left to experienced and recognized experts in the field to design the Main and Vent Condensers - and sometimes the entire vacuum-producing package.

As an example of judicious Vent Condenser application, consider one on the vacuum system described previously in the Sizing Example WorkSheet:

Vent Condenser

18.4 lb/h air +2,000 lb/h water

20 oC CWS

to vacuum-producing equipment

CWR

liquid water



In order to find the volumetric flow rate of the gases exiting the vent condenser, assume that the outlet gas

22 0.38367 0.001214 823.6423 0.40771 0.001286 777.6624 0.43305 0.001361 734.5825 0.45976 0.001441 694.226 0.48789 0.001524 656.3227 0.51751 0.001611 620.79

The above table identifies the saturated water vapor pressure as 0.95 inches Hg absolute (0.45976 psia)

Since the vapor stream is a binary of (water + air), by Dalton's Law the partial pressure of the air is:

Air partial pressure = (3.5 - 0.95) = 3.04 inches Hg absolute

By applying the Universal Gas Law, volumetric equivalent capacity for the total Seepage Air is:

where,Z = Compressibility Factor, assumed as 1.0n = 0.01 lb moles of air/min

R = 10.73

T = 537P = 1.72 psia

V = 35 Acfm

This volumetric flowrate also includes the evacuated water vapor; by Dalton's Law, the water vapor also

equipment is:

Water Vapor evacuated = 0.051059 lb/min

= 3.06 lb/hr

The equivalent dry air load for this quantity of water vapor = 3.89 lb/hr

Note the tremendous load difference in the vacuum-producing equipment when a vent condenser is used.

will approach the inlet cooling water by 5 oC (9 oF) so the gas to the vacuum-producing equipment will be

at 25 oC.

The FREE NIST databank (at: http://webbook.nist.gov/chemistry/fluid/) for water vapor yields:Temperature

(oC)Pressure

(psia)Density (lbm/ft3)

Volume (ft3/lbm)

This pressure is also the partial pressure of water vapor in the system at 25 oC.

psia - Ft3/lbmol - oRoR

occupies this volumetric flowrate. From the above NIST data, the specific volume of water at 25 oC and

0.95 inches Hg is 694.2 Ft3/lb. Therefore, the water vapor being evacuated by the vacuum-producing

The water vapor load has been reduced by (2,000 - 3.06) = 1,997 lb/hr. (99.8 % of the total)

V=ZnRTP



Often, the gaseous or vapor products that inevitably form part of the evacuated vapor stream are considered

etc) or they are classified as undesirable atmospheric emissions (such as solvents, methanol, ethanol, etc.).

Art Montemayor Vacuum Specifications December 12, 2002Rev: 0

of 25 FileName: document.xlsWorkSheet: Equipment Sizing

Sizing Specification for a Mechanical Vacuum Pump

1.

Capacity = 75.7 Acfm (Refer to Sizing Example WorkSheet)Vacuum pressure = 3.5 inches Hg absolute (Refer to Sizing Example WorkSheet)

Process Temperature = 25 (Assume this as ambient, since not stated)

2. The capacity required for evacuating process gases and air seepage is as follows:

Capacity = 6,710 AcfmVacuum pressure = 3.5 inches Hg absolute (Refer to Sizing Example WorkSheet)

Process Temperature = 49 (Refer to Sizing Example WorkSheet)

Capacity = 35 AcfmVacuum pressure = 0.95 inches Hg absolute (Refer to Vent Condenser Application WorkSheet)

Process Temperature = 25 (Refer to Vent Condenser Application WorkSheet)

Sizing Specification for a Steam Vacuum Jet Ejector

1.

Capacity = 70.5 Lbs Steam/hr (Refer to Sizing Example WorkSheet)Vacuum pressure = 3.5 inches Hg absolute (Refer to Sizing Example WorkSheet)

Process Temperature = 25 (Assume this as ambient, since not stated)

2. The capacity required for evacuating process gases and air seepage is as follows:

Capacity = 6,710 Acfm

= 2,557Vacuum pressure = 3.5 inches Hg absolute (Refer to Sizing Example WorkSheet)

Process Temperature = 49 (Refer to Sizing Example WorkSheet)

Capacity = 35 Acfm

= 21Vacuum pressure = 0.95 inches Hg absolute (Refer to Vent Condenser Application WorkSheet)

Process Temperature = 25 (Refer to Vent Condenser Application WorkSheet)

The capacity required for pump-down of the system is as follows:

oC

(air seepage + water vapor, without a vent condenser)

oC

(air seepage + water vapor, with a vent condenser)

oC

The capacity required for pump-down of the system is as follows:

oC

(air seepage + water vapor, without a vent condenser)

lb/hr of dry air @ 70 oF

oC

(air seepage + water vapor, with a vent condenser)

lb/hr of dry air @ 70 oF

oC

Art Montemayor Vacuum Specifications December 12, 2002Rev: 0

of 25 FileName: document.xlsWorkSheet: Equipment Sizing

Important observations to consider:a. Note that the pump-down capacity is based on evacuating the reactor within the time span of 10 minutes.

The calculated required pump capacity of 75.7 Acfm is a conservative value, since the calculation does not take into consideration the net reactor volume after the vessel is charged. In other words, the time required for reaching the design vacuum will be less due to the fact that the net volumeto evacuate is less than the total vessel volume. Since the problem does not state the volume of the liquid reactants, the net reactor volume cannot be deduced nor calculated for a more accurate value.

b. Note that the employment of a vent condenser prior to the vacuum-producing equipment reduces the amount of required capacity by 99.5 %. However, this dramatic reduction in equipment capacity is obtained at the expense of a lower vacuum requirement due to the vapor pressure that must be maintained at the vent condenser outlet in order to assure condensation and subsequent liquid

pressure drop through the vent condenser's process side be no greater than (3.5 - 0.95) = 2.55 inches Hg.If the pressure drop through the vent condenser is greater than this value, the ultimate vacuum level in the reactor will be higher; i.e., the vacuum effect will be less (or the pressure will be more positive).This relatively small amount of pressure drop (1.25 psi) is a very stringent design specification for a shell-and-tube vent condenser and an experienced and recognized fabricator is usually called upon to design this type of special equipment. Proprietary, special design baffles are often used in this type of application.

c. The 10 minutes of pump-down time using a mechanical pump requires a capacity for 75.7 Acfm. If a longer pump-down time can be tolerated, a smaller mechanical pump can be used by also employing a vent condenser. For example, the 35 Acfm capacity required with a vent condenser can be

30 minutes instead.By using a vent condenser, the capacity has been greatly reduced, but the pump-down time has been

d. The 10 minutes of pump-down time using a steam jet ejector requires a capacity for 70.5lb dry air/hr. If a longer pump-down time can be tolerated, a smaller ejector can be used by alsoemploying a vent condenser. For example, the 21 lb/hr of dry air required with a vent condenser can be

29 minutes instead.Again, take note that the time elapsed for pump-down has almost tripled due to the lower vacuum that has to be produced at the ejector inlet due to the vent condenser's pressure drop.

e. There is not much one can do about non-condensable process gases in a vacuum operation. These fluids impose a vacuum capacity requirement that can only be resolved with equally-sized vacuum producing equipment. However, where condensable process vapors are involved, the utilization of vent condensers is a conventional and natural technique to reduce process emissions and energy requirements.

separation at the 25 oC outlet temperature. This physical example of Dalton's Law requires that the

serviced by using a nominal 40 Acfm machine that would take

tripled in duration.

serviced by using a nominal 24 lb dry air/hr unit that would take

Art Montemayor Designing Vacuum Lines December 26, 2002Rev: 0

of 25 FileName: document.xlsWorkSheet: Vacuum Line Sizing

As has been mentioned in the Vent Condenser Application WorkSheet, pressure drops introduced in the trajectory of a vacuum system cause additional work requirements on the vacuum-producing equipment.flow. In other words, the ultimate vacuum-producing equipment must be specified for a higher vacuum in order to produce the desired end result further upstream, within the process.

Typical equipment employed and causing pressure drops in a vacuum system are:

1. Piping, fittings;2. Valves;3. Condensers, heat exchangers;4. Traps, separators;5. Scrubbers, filters;6. Low piping points or piping "traps".

Piping should be designed with as short a process run as is feasible within the guidelines and scope of the equipment layout and process requirements. All piping should be positively sloped to avoid any liquid accumulation in the run. Strategically-located condensate drop-legs should be in incorporated - especially inthe main header runs. Branch tees should be avoided as much as possible and all elbows should be minimal long-radius design - with piping bends used as a preference. Characteristically, process vacuum piping (especially headers) will be fabricated as "Big-Bore" piping (pipe with an O.D. greater than 24"). This size and type of pipe is normally installed with mitered elbows (minimal 3-cut) to reduce presssure drop. Big Bore piping requires special supports and expansion capability design; sometimes it may have to be re-inforced for vacuum rating. The discharge piping of vacuum-producing equipment should be liberally sized to reduce pressure drop to a minimum because the equipment's performance is adversely affected by a high discharge pressue - especially steam jet ejectors.

Valves should be held to a minimal quantity. Only block valves are employed as line valves; there is no throttling required in the main piping. Vacuum flow control is done in the smallest size piping - usually in the process area. Any Ball or Gate Valve employed as a block should be a "Full-bore" type.

Condensers are always specially designed for vacuum service, due to the pressure drop constraints. This means that the shellside is normally used for the vacuum side, since this is the side that offers the lowest pressure drop available. As a result of this, baffle design and phase separation become key design criteria in order to keep the pressure drop as low as possible while maintaining the required heat transfer. Condensing under a vacuum is an industrial specialty and fabricators usually have proprietary techniques and know-how that enables them to design, fabricate and warrant the operation of successful equipment.Young or inexperienced engineers with less than 10 to 15 years design and operating experience are advised to distance themselves from this specialty design area and leave it to the experts who will warrant their design and equipment supply.



Traps, separators, scrubbers, and filters should be kept out of a vacuum system if this action can be justified.However, many times, because of process characteristics and needs, this is not possible. Nevertheless, the best advice for a successful vacuum system is to keep the process SIMPLE, with a minimal of hardware and controls.

Recommended Design vapor velocities for vacuum lines are as follows:

System Process PressureTorr (mm Hg)

Absolute Vacuum Ft/sec Ft/sec

755 - 760 0 - 5 300 275725 - 755 5 - 35 250 225685 - 725 35 - 75 200 175380 - 685 75 - 380 150 150

0 - 380 380 - 760 150 150

Maximum Allowable Velocity

Desired Operating Velocity



accumulation in the run. Strategically-located condensate drop-legs should be in incorporated - especially inthe main header runs. Branch tees should be avoided as much as possible and all elbows should be minimal

Young or inexperienced engineers with less than 10 to 15 years design and operating experience are advised



Traps, separators, scrubbers, and filters should be kept out of a vacuum system if this action can be justified.

best advice for a successful vacuum system is to keep the process SIMPLE, with a minimal of hardware and

Art Montemayor Vacuum-Producing Equipment December 27, 2002Rev: 0

of 25 FileName: document.xlsWorkSheet: Equipment Types

The vacuum-producing equipment related to the information in this workbook are the following types:

Vacuum Range Capacity, Absolute PressureEquipment Type Inches Hg Torr Psi

1. Liquid Piston Ring Rotary Pump 0.05 to 29.94 1.27 to 760 0.024 to 14.72. Positive Displacement Pumps 0.05 to 29.94 1.27 to 760 0.024 to 14.73. Steam Jet Ejectors 0.05 to 29.94 1.27 to 760 0.024 to 14.74. Liquid Jet Ejectors 2.0 to 29.94 50.77 to 760 0.98 to 14.75. Positive Displacement Blowers 2.0 to 29.94 50.77 to 760 0.98 to 14.7

Liquid Piston Ring Rotary PumpsThese machines originated in a USA patent and were originally called "Nash" or "Nash HyTor" pumps.The concept is unique in that they employ a liquid piston rotating in an oval chamber as a displacementmedium for the gases being evacuated. The liquid can be any fluid that has an adequate vapor pressureat the operating temperature and is compatible with the process gases. Some typical liquids used as

contributed to the eradication of Mercury as the seal piston fluid. The concept, besides being unique, is very efficient and trouble-free in operation. Since there is no metal-to-metal contact or wear, the machine is relatively free of noise and maintenance. Because the seal piston fluid is always in direct contact with the gases being evacuated, the fluid's vapor pressure has a direct effect upon the capacity of the liquid ring pump.As the operating temperature of the pump increases (due to the compression ratio and the evacuated gases), the fluid's vapor pressure increases also -- until it reaches a point where it vaporizes so fast that it "vapor locks" the pump. The vacuum will not be maintained and the pump fluid must be either cooled or changed to a compatible one with less vapor pressure. For vacuums lower than 5 inches Hg absolute, these types of vacuum pumps have to be multi-staged. These positive-displacement pumps are more efficient than equivalent capacity steam jet ejectors and can easily handle so-called "dirty gases". In fact, their characteristic liquid piston acts as a liquid scrubber and collects the dirt in gases. This characteristic can be a positive feature in applications where the seal fluid is recycled, filtered, and cooled through an external heat exchanger and filter in order to minimize pollution-abatement problems. Although relatively expensive, these machines traditionally give excellent service and reliability with minimal emission problems.

Positive Displacement PumpsThe word "pump" as used for some vacuum-producing equipment is a misnomer that unfortunately has gotten entrenched in the industry's nomenclature. Pumps handle liquids; they cannot handle gases or vapors. Blowers and compressors handle gases and vapors. As long as the engineer understands this unfortunate use of an erroneous label, he can design around it.

the sealing piston are: Dowtherm A, ethylene glycol, sulfuric acid and water. In fact, in the past it was quite normal to employ Mercury as the seal piston. Recent environmental and health concerns have



This category of vacuum pumps covers a wide range of positive-displacement devices from reciprocating pistons through sliding rotary vanes and rotating lobes. Most of these types of devices depend on very close clearances between metallic or solid parts and, as a result, they require oil for lubrication and sealing. This oil is subject to being separated and filtered back into the pumps process by way of special-designed equipment and often is a process and maintenance problem.

Although they are limited to clean, dry gases and vapors, the type that employs a rolling-type of internal contact are better suited for limited solids' loadings than the type that uses a sliding-type of contact. The necessity of lube oil also causes concern when the gases handled can react or form gels, gums, or other semi-solid compounds with the lube oil. Nevertheless, this type of device is the most efficient single vacuum-producing equipment available at present. Although the capacity is limited to the device's displacement, this shortcoming can be overcome by using rotary blowers as a first stage of vacuum. Thiscombination of equipment has the maximum thermal efficiency for vacuums less than 8 inches Hg absolute.

Steam Jet EjectorsThese simple, venturi-type devices were once the "King of the Hill" with regards to vacuum-producing applications in the chemical processing industry. However, their demise began with the ever-increasing

century they were considered as the least desirable of the vacuum-producing devices because of increasing fuel (and consequently, steam) costs and their emission characteristics.

Nevertheless, they are probably the lowest capital cost equipment for this service, very simple to operateand essentially maintenance-free. They literally have "no moving parts". For vacuum applications below6 inches Hg absolute, they are operated in stages connected in tandem -with water-cooled condensers in-between vacuum stages. Below 0.2 inches Hg absolute, the intercondensers cannot be used with steam jets and the various stages must be direct connected.

Liquid Jet EjectorsThese devices are very similar to the Steam Jet Ejector except that they use a high pressure liquid insteadof steam as the motive force producing the vacuum. They have a limited operating range but are more efficient than the steam jet variety while sharing their low cost and simplicity. One serious draw-back maybe potential pollution problems if once-through water is used as the motive force fluid; however, this may be avoided if the water is recirculated or if a liquid compatible with the process gases is used instead.

emission restrictions imposed by EPA, local, and state regulatory agencies. By the end of the 20 th



Positive Displacement BlowersThese devices were first introduced in the USA as the Roots-Connersville blower. They utilize two intermeshing rotors (usually with a figure "8" profile) that are maintained with a very close mechanical tolerance and are driven by external timing gears. Within their operating range they are more efficient thanliquid ring pumps and at vacuum higher than 8 inches Hg absolute, they are the most efficient vacuum-producing equipment available. However, they also have their drawbacks, or trade-offs.

Although they are often referred-to as "pumps", they are positive displacement machines and cannot tolerate liquid entrainment into their chambers. Consequently, they can only be applied on dry, clean gases or vapors. Additionally, they are extremely noisy and for an acceptable OSHA noise limit to be met, they often require mufflers. These mufflers deter and negate a lot of the energy and evacuating efficiency that these blowers offer under a basically plain design devoid of extra equipment.

Centrifugal-type blowers, because of their inherent suction characteristics are incapable of generating a conventional vacuum and, as such, are never applied to this unit operation.

Art Montemayor Ellipsoidal Head Volume September 12, 1997Rev 1(01/19/00)

of 25 Electronic FileName: document.xlsWorkSheet: Ellipsoidal Head Volume

I. D., inches Vol. Gallons12 0.9818 3.3124 7.8330 15.3036 26.4442 41.9948 62.6754 89.2360 122.4166 162.9272 211.5278 268.9384 335.8990 413.1296 501.38

102 601.39108 713.88114 839.59120 979.26126 1,133.61132 1,303.39138 1,489.33144 1,692.16150 1,912.61156 2,151.43162 2,409.34168 2,687.08174 2,985.39180 3,304.99186 3,646.63192 4,011.04198 4,398.95 Ellipsoidal Head Inside Diameter = 72 inches204 4,811.09

210 5,248.21 Volume of Single Ellipsodial Head = 211.52 Gallons

216 5,711.03 = 28.28222 6,200.29228 6,716.73234 7,261.07240 7,834.06

Ft3

0 50 100 150 200 250 300

0.00

1,000.00

2,000.00

3,000.00

4,000.00

5,000.00

6,000.00

7,000.00

8,000.00

9,000.00

f(x) = 0.000567137020246092 x^2.99984269005698R² = 0.999999987165382

2:1 Ellipsoidal Head Volume

Inside Diameter, Ft

Vo

lum

e,

Cu

Ft

Reference: Trinity Industries, Inc.Head DivisionNavasota, TXProduct & ServicesCatalog # 7962M (1996)

producing and maintaining vacuum

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