Transcript Slide 1

MECH3005 – Building Services
http://www.hku.hk/bse/mech3005/
Air Conditioning & Refrigeration:
Air-side Systems
Dr. Sam C M Hui
Department of Mechanical Engineering
The University of Hong Kong
E-mail: [email protected]
Contents
• Fundamental HVAC Concepts
• Air Handling Units
• Air System Basics
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Fan-duct systems
Fan modulation & combinations
Year-round operation & economizers
Fan energy use & outdoor air
Fundamental HVAC Concepts
• Five subsystems or loops of HVAC
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Air-side
Chilled water
Refrigeration equipment
Heat rejection
Controls
33 oC, 28 oC
33 oC
27 oC
25 oC
Conditioned space
33 oC, 28 oC
27 oC
33 oC
25 oC
13 oC
13 oC
Air side system
33 oC, 28 oC
Chilled water
system
27 oC
33 oC
12 oC
25 oC
12 oC
7
13 oC
13 oC
oC
7 oC
33 oC, 28 oC
Refrigeration
equipment
27 oC
33 oC
12 oC
25
oC
49 oC
12 oC
7
13 oC
10 oC
13 oC
oC
7 oC
3
oC
38 oC
33 oC, 28 oC
Heat rejection
27 oC
33 oC
12 oC
25
oC
7 oC
13 oC
13 oC
10 oC
12 oC
49 oC
7 oC
38 oC
3 oC
35 oC
35 oC
29 oC
29 oC
33 oC, 28 oC
Control Loop
27 oC
33 oC
12 oC
25
oC
7 oC
13 oC
13 oC
10 oC
12 oC
49 oC
7 oC
38 oC
3 oC
35 oC
35 oC
29 oC
29 oC
Individual room air-conditioning system
Primary air fan coil unit system
What type of air conditioning system it is?
Dual duct system
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:
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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:
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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
Direct expansion (DX) coil
Water cooling coil
Water heating coil
Steam heating coil
Air Handling Units
• Air filters
• Air cleaning and filtration
• Operating performance:
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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)
HEPA and ULPA filters
Low efficiency (panel-type)
Activated carbon filter
Medium efficiency (bag-type)
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
Steam grid humidifier
Air washer
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
o
rate V
2

p  R V
• Duct sections in series:
• Duct sections in parallel:
Rs  R1  R2    Rn
1
1
1
1


 
Rp
R1
R2
Rn
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
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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
Fan system operating point & system effect
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
Fan 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
Damper, inlet vanes & fan speed modulation
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
Supply and exhaust fans
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
Supply and relief fans
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
Supply and return fans
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
Control diagram of a VAV reheat system for year-round operation
Year-round operation
Region I: Refrigeration/evaporative cooling
Region II: Free cooling & refrigeration
Region III: Free cooling, evaporative cooling & refrigeration
Region 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:
psy
W

l / s 1000 f m
psy
W
 (1  RO,S )
l/s
1000 f m
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