cem 350 hvac air side systems 10-2016
TRANSCRIPT
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Air Conditioning & Refrigeration:
CEM 350 Facility Systems DesignAir-Side Systems
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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
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Fundamental HVAC Concepts
• Five subsystems or loops of HVAC• Air-side• Chilled water• Refrigeration equipment• Heat rejection• Controls
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33 oC, 28 oC
27 oC33 oC
25 oC
Conditioned space
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13 oC 13 oC
33 oC, 28 oC
27 oC33 oC
25 oC
Air side system
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13 oC 13 oC
12 oC
7 oC
33 oC, 28 oC
7 oC
12 oC
27 oC33 oC
25 oC
Chilled watersystem
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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
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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
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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
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Individual room air-conditioning system
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Primary air fan coil unit system
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Dual duct system
What type of air conditioning system it is?
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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
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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
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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
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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
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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)
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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
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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
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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
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Direct expansion (DX) coil
Water cooling coil
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Water heating coil
Steam heating coil
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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
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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)
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Low efficiency (panel-type) Medium efficiency (bag-type)
HEPA and ULPA filters Activated carbon filter
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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
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Steam grid humidifier
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Air washer
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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
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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 RRRR1111
21
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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
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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)
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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
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Fan system operating point & system effect
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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
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Fan modulation curve
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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
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Damper, inlet vanes & fan speed modulation
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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
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Supply and exhaust fans
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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
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Supply and relief fans
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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
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Supply and return fans
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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
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Control diagram of a VAV reheat system for year-round operation
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Year-round operationRegion I: Refrigeration/evaporative coolingRegion II: Free cooling & refrigerationRegion III: Free cooling, evaporative cooling & refrigerationRegion VI: Winter heating
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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
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Air System Basics
• Fan energy use• For each l/s air
supplied:
• For system using separate outdoor ventilation:
mf
sypsl
W
1000/
mf
sySO
pR
slW
1000
)1(/ ,
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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
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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
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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