mech3005 – building services dr. sam c m hui department of mechanical engineering the university...
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MECH3005 – Building Serviceshttp://www.hku.hk/bse/mech3005/
Dr. Sam C M HuiDepartment of Mechanical Engineering
The University of Hong KongE-mail: [email protected]
Air-side Systems
Air Conditioning & Refrigeration:
Contents
• Fundamental HVAC Concepts
• Air Handling Units
• Air System Basics• Fan-duct systems• Fan modulation & combinations• Year-round operation & economizers• Fan energy use & outdoor air
Fundamental HVAC Concepts
• Five subsystems or loops of HVAC• Air-side• Chilled water• Refrigeration equipment• Heat rejection• Controls
13 oC 13 oC
12 oC
7 oC 3 oC38 oC
49 oC10 oC
33 oC, 28 oC
7 oC
12 oC
27 oC33 oC
25 oC
Refrigerationequipment
13 oC 13 oC
12 oC
7 oC 3 oC38 oC
49 oC10 oC
35 oC
29 oC
33 oC, 28 oC
7 oC
12 oC
29 oC
35 oC
27 oC33 oC
25 oC
Heat rejection
13 oC 13 oC
12 oC
7 oC 3 oC38 oC
49 oC10 oC
35 oC
29 oC
33 oC, 28 oC
7 oC
12 oC
29 oC
35 oC
27 oC33 oC
25 oC
Control Loop
Air Handling Units
• Terminal unit or device• Such as fan coil units, VAV boxes
• Air handling unit (AHU)• Primary equipment of the air system• Handle & condition the air, control it to a required
state, and transport it• Basic components:
• Supply fan, water cooling coil, filters, mixing box, dampers, controls & outer casing
• A return or relief fan is optional, so as a humidifier
Air Handling Units
• Types of AHUs:• Horizontal or vertical• Draw-through or blow through• Factory-fabricated and field built-up• Rooftop and indoor• Make-up (primary) air • Recirculating
Air Handling Units
• Package units• A self-contained air conditioner• Equipped with a DX (direct expansion) coil• “Air handler”: the portion inside that handles air
• Rooftop package units• Gas heating / electric cooling• Electric heating / electric cooling• Heat pump system
Air Handling Units
• Indoor package units• Factory-assembly unit installed in a fan room or
machinery room• Floor-mounted or ceiling-mounted• Cooling, cooling/heating, heat pump• Heat rejection:
• Connected to an air-cooled condenser, OR• Water-cooled condenser
• Split package units• Indoor air handler + outdoor condensing unit
Air Handling Units
• Rating conditions & minimum performance• ARI and ASHRAE standards• Rating indices:
• Energy efficiency ratio (EER)
• Seasonal EER
• Integrated part-load value (IPLV)
• Heating seasonal performance factor (HSPF)
Air Handling Units
• Coils• Indirect contact heat exchangers• Heat transfer between air flowing over the coil and
water, refrigerant, steam or brine insider the coil• Fins: extended (secondary) surfaces
• Fin spacing and density
• Water circuits• Number of water flow passages
Air Handling Units
• Direct expansion (DX) coil• Refrigerant is fed (e.g. R-22 and R-134a)• Air and refrigerant flow:
• Usually counterflow and cross flow
• Typical evaporating temperature = 3-10 oC• Condensate drain pan (to collect condensation)• Performance factors:
• Face velocity, heat transfer coefficients, air-side pressure drop, physical size
Air Handling Units
• Water cooling coils - dry-wet coil• Chilled water flowing at 4-10 oC• Brine or glycol-water at 1-4 oC• Temperature rise (typical) = 7-14 oC
• Water cooling coils - dry coil• Sensible cooling (dry); no condensation• Poorer heat transfer coefficient
• Steam heating coil
Air Handling Units
• Air filters• Air cleaning and filtration• Operating performance:
• Efficiency or effectiveness of dust removal
• Dust holding capacity
• Initial & final pressure drop
• Service life
• Types: low-, medium-, and high-efficiency filters + carbon activated filters
Air Handling Units
• Test methods of air filters• Weight arrestance test
• For low-efficiency air filters
• Atmospheric dust spot efficiency• For medium-efficiency air filters
• DOP (dioctyl phthalate) penetration and efficiency test
• For high-efficiency air filters• HEPA (high-efficiency particle air)
• ULPA (ultra-low penetration air)
Low efficiency (panel-type) Medium efficiency (bag-type)
HEPA and ULPA filters Activated carbon filter
Air Handling Units
• Humidifiers• Steam humidifiers• Air washers
• Selection of AHUs• Face velocity < 3 m/s to prevent entrained droplets• Proper size of the equipment (energy efficiency)• Medium-efficiency filter >50% dust spot eff.:
improve IAQ, prevent smudging & discoloring• Air economizer: save energy
Air-side Components
• EcoAdvisor: Energy Trainer for Energy Managers: HAVC Module 1 [AV 697 E19]• HVAC components
• Distribution equipment• Air-side components
• Dampers, filters, coils, fans, ducts and plenums, terminal units, diffusers, humidifiers, dehumidifiers
Air System Basics
• Fan-duct systems• Flow resistance R, pressure drop Δp and volume flow
rate V
• Duct sections in series:
• Duct sections in parallel:
2VRp o
ns RRRR 21
np RRRR
1111
21
Air System Basics
• Fan-duct systems• Terminology
• Primary air (conditioned air or makeup air)
• Secondary air (induced space air, plenum air, or recirculating air)
• Transfer air (indoor air that moves from an adjacent area)
• System curve: volume flow vs pressure loss• System operating point
Air System Basics
• Fan Laws• Speed (n)
• Volume flow (V)
• Total pressure loss (Δp )
• Air density (ρ)
• For air systems that are geometrically & dynamically similar: (D = impeller diameter)
Air System Basics
• System effect Δpts
• Its additional total pressure loss caused by uneven or non-uniform velocity profile at the fan inlet, or at duct fittings after fan outlet
• Due to the actual inlet and outlet connections as compared with the total pressure loss of the fan test unit during laboratory ratings
Inlet Outlet
Air System Basics
• Modulation of air systems• Constant volume system
• Volume flow rate remains constant; supply temperature is raised during part load
• Variable-air-volume (VAV) system• Volume flow rate is reduced to match part load
operation
• Modulation curve
Air System Basics
• Fan modulation methods• Damper (vary the opening of the air flow passage)
• Waste energy
• Inlet vanes (opening & angle of inlet vanes)• Low cost; less efficient than following types
• Inlet cone (peripheral area of fan impeller)• Inexpensive; for backward curved centrifugal fan
• Blade pitch (blade angle of axial fan)• Fan speed (using adjustable frequency drives)
• Most energy-efficient; but more expensive
Air System Basics
• Fan combinations in AHUs• Supply and exhaust fan/barometric damper
• Used when no return duct or low return pressure loss
• Barometric relief damper: to prevent excessive high space pressure
• Suitable for systems w/ no air economizer mode & a low pressure drop in return system
Air System Basics
• Fan combinations in AHUs• Supply and relief fan
• Relief fan to prevent excessive high space pressure
• Usually operated in air economizer mode
• Outdoor air damper is fully opened; recirculating damper closed
• Suitable when pressure drop of return system < 75 Pa, or there is a considerable pressure drop in relief or exhaust flow passage
Air System Basics
• Fan combinations in AHUs• Supply and return fan
• Return fan at the upstream of the junction of return, recirculating and exhaust flow passage (ru)
• Has similar characteristics as supply+relief fan, but a higher total pressure at junction “ru”
• Careful selection of return fan is needed to avoid excessive pressure in the duct system
• Suitable when pressure drop of return system > 150 Pa, or those requiring a negative space pressure
Air System Basics
• Year-round operation of a VAV system• Region I: Refrigeration/evaporative cooling
• Enthalpy of outdoor > that of recirculating air
• Region II: Free cooling & refrigeration• Enthalpy of outdoor ≤ that of recirculating air
• Region III: Free cooling, evaporative cooling & refrigeration
• Enthalpy of outdoor ≤ that of recirculating air
• Region VI: Winter heating
Year-round operationRegion I: Refrigeration/evaporative coolingRegion II: Free cooling & refrigerationRegion III: Free cooling, evaporative cooling & refrigerationRegion VI: Winter heating
Air System Basics
• Economizer• A device consisting of dampers and control that
uses free cooling capacity of either outdoor air or evaporatively cooled water from cooling tower instead of mechanical refrigeration
• Air economizer:• Enthalpy-based
• Temperature-based
• Water economizer
Air System Basics
• Fan energy use• For each l/s air
supplied:
• For system using separate outdoor ventilation:
mf
syp
sl
W
1000/
mf
sySO
pR
sl
W
1000
)1(/ ,
Air System Basics
• Outdoor ventilation air supply• Aims
• Provide acceptable indoor air quality
• Achieve energy efficiency in the system
• ASHRAE Standard 62-2001 (IAQ standard)• Demand-based outdoor ventilation air control
using CO2 as an indicator
• CO2 concentration < 800-1,000 ppm
• A specific indoor air contaminant can also be used
Air System Basics
• Outdoor ventilation air supply• Minimum outdoor air control
• Type I: uses a CO2 or mixed gas sensor + DDC controller to control the volume flow rate of outdoor air
• The best one but can be expensive
• Type II: uses a CO2 or mixed gas sensor + DDC controller to control the ratio of the openings between outdoor and recirculating dampers
• Suitable for VAV system
Air System Basics
• Outdoor ventilation air supply• Minimum outdoor air control (cont’d)
• Type III: uses a flow sensor or a pressure sensor + DDC controller to control the dampers to provide nearly constant volume outdoor air intake
• More complicated & may cause energy waste
• Type IV: adjust the opening of outdoor damper manually to provide constant volume outdoor air
• Mainly used for constant-volume systems