cem 350 hvac air side systems 10-2016
TRANSCRIPT
Air Conditioning & Refrigeration:
CEM 350 Facility Systems DesignAir-Side Systems
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
33 oC, 28 oC
27 oC33 oC
25 oC
Conditioned space
13 oC 13 oC
33 oC, 28 oC
27 oC33 oC
25 oC
Air side system
13 oC 13 oC
12 oC
7 oC
33 oC, 28 oC
7 oC
12 oC
27 oC33 oC
25 oC
Chilled watersystem
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
Individual room air-conditioning system
Primary air fan coil unit system
Dual duct system
What type of air conditioning system it is?
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
Direct expansion (DX) coil
Water cooling coil
Water heating coil
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
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
rate V
• Duct sections in series:• Duct sections in parallel:
2VRp o
ns RRRR 21
np RRRR1111
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
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 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
sypsl
W
1000/
mf
sySO
pR
slW
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