HVAC Equipment Presentation
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Transcript of HVAC Equipment Presentation
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HVAC Equipment
MAE 406
Energy Conservation in IndustryStephen Terry
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HVAC Fundamentals
HVAC stands for Heating, Ventilation, and Air
Conditioning.
Sometimes see written as HVAC-R, which
includes Refrigeration HVAC important in residential, commercial, and
industrial plants
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Focus in this Course
Weve already looked at using boilers to provide
steam which is used by many large facilities,
including NCSU, for heating.
This lecture and the next will focus mostly onspace air conditioning and refrigeration.
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Method of Providing Cooling
Heat naturally moves from hot to cold,
Refrigeration systems must move heat from a cold
space to a hot space
This is accomplished using a refrigeration cyclemanipulating the pressure and temperature of a
refrigerant
This process also requires energy heat pump
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Refrigeration Cycle
hFrom Heating, Ventilating, and Air Conditioning Analysis andDesign, McQuiston and Parker, 3rd edition
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Cycle Efficiency MAE302
Depends on condenser and evaporator
temperatures & compressor isentropic efficiency
Usually discuss efficiency as COP or in kW/ton
Efficiency (COP) = Desired Output / Required Input= Qevaporator / Wcompressor
= Qevaporator/ (Qcondenser Qevaporator)
Note: 1 ton = 12,000 BTU/hr
1 kWh = 3,413 BTU
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Typical Operating Temperatures
Evaporator Temperature: between 35-45F
Operating colder than this can cause freezing /
frost on coils defrost cycle
Condenser Temperature: 80-90F for water cooled applications
110-140F for air cooled applicationsWhy?
Compressor isentropic efficiency, ~70%
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Types of Refrigeration Units
Air cooled DX unit
Uses air-cooled condenser
Usually cools air directly
by venting return / outside
air through ductwork
Packaged and split system
varieties.
Efficiency 1.2-1.4 kW/ton
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Advantages / Disadvantages of DX Unit
Advantages
Flexible system, can grow
with plant or building
Self-contained system
Relatively low first cost
Disadvantages
Efficiency is poor
Maintenance can be
expensive with many units
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Types of Refrigeration Units
Central Water Chiller
Refrigeration cycle cools
water that circulates
around building to AHUs
Chiller can be water or
air-cooled
Requires piping and
pumps
Efficiency 0.6 kW/ton to
1.0 kW/ton
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Advantages / Disadvantages of Chiller
Disadvantages
Requires cooling tower
and pumps / pipes
Must operate at lowest
temperature required for
entire system
Must maintain chilled
water system and coolingtower water system with
chemicals
Advantages
Great energy efficiency
Central unit reduces
maintenance costs
Can provide space
conditioning and process
cooling
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Refrigerants
R-134a and R-22 are commonly used today
Older chillers use R-11/R-12 Ozone depleters
Since R-11 not made any more, older chillers must
be retrofitted if plants supply of refrigerant runsout.
Food manufacturers will often use NH3 in a
double stage system.
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Double Stage System
Useful for largesystems with two
zones
Ammonia ischeap&efficient,
but is toxic!
Condenser good
for heat recovery
to cleaning water.
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Cooling Towers
Most often used to provide condenser for chillers.
Can also be used to cool process equipment,
especially air compressors.
Water is sprayed / falls through a media while airis drawn through.
Evaporation of water represents most of
refrigeration effect
Outlet temperature depends on wet-bulb temp.
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Cooling Towers
Efficiency: 0.05 kW/ton
Typical temperatures:
90 F in / 70-80 F out
Fans usually are 2-50 hp
Can use multiple cells
Two speed motors /
VSDs
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Absorption Chillers
Uses low pressure steam to generate chilled water!
Chilled water temps: 40-50 F
Refrigerant is a Li-Br salt. Steam is used to
concentrate salt like a compressor is used tocompress vapor
Efficiencies: 1-2 Btu cooling / Btu of steam
Requires a large cooling tower Systems are 2x the cost of regular chiller, so it is
not economical unless steam is free
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Air Distribution System Single Zone
Rooftop Unit
Most common type of system seen in smaller
industrial plants
Utilizes an air cooled direct expansion unit to
provide cold refrigerant to the cooling coil System may have multiple compressors to stage
cooling
May have a gas pack or steam coil for heating
Often utilizes an economizer with relief venting
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Single Zone Roof-top Unit Schematicfilters
Return air from
room
Heating Coil or
Gas PackEconomizerdamper
Condenser
Compressors
fan
Cooling
Coil
Supply air to
room
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Economizers
Compares outdoor air temperature / enthalpy to
indoor conditions. Three modes of operation:
Free cooling outside air temperature less than supply
air temperature use mixture of outdoor air and return
air, deactivate compressors
Economizer cooling outdoor air cooler than indoor,
but above supply air temperature. Use all outdoor air
and cool it to supply temperature
No savings outdoor air hot and system uses all
return air (except minimum outside air)
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Outdoor Air Requirements
Must supply minimum outside air to space to
remove CO2 and contaminants, especially in
newer buildings.
Can utilize some air-to-air heat recovery in coldclimates
ASHRAE standard is 15 cfm per person
Can be varied for classrooms or auditoriums if a
CO2 sensor is used to determine fresh air required.
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Multiple Zones
Dual Deck System
Utilizes ductwork with hot air and ductwork with cold
air, which is mixed in proportion to maintain conditions
Energy inefficient since hot and cold air are mixed.
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Multiple Zones
Variable air volume (VAV)
Utilizes sensors to vary flow of air and therefore
cooling through unit. Can use electric strip heater for
heating purposes
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Psychrometrics
Used to determine actual cooling / heating
requirements, considering both temperature and
humidity.
Refer to psychrometric chart in thermodynamicsbook or ASHRAE handbook
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Definitions Sensible Heat
Sensible heating heating or cooling that is done
to change temperature only, no change in
moisture. Example:
Determine sensible heat to raise the temperature of 1,000 lb/hr of air from
70 to 110 F.
Sensible Heat = 1,000 lb/hr x 0.24 BTU/lb- F x (110-70 F)= 9,600 BTU/hr
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Latent Heat
Latent heat is heat added in the form of moisture.
Evaporative cooling is an example water
sprayed into dry air causes liquid to evaporate to
vapor, removing heat in the process. Moisture content of air specified by dew-point
temperature or by specific humidity. Lines of
constant dew-point are horizontal on chart
Note that relative humidity is not horizontal
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Enthalpy
A measure of sensible and latent heat
Is a true measure of energy required to change
state.
Can be used to determine actual cooling loads,including latent heat removal.
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Example
Find the heat removed from 10,000 lb/hr (2,222
cfm) of air starting at 95F and 75F dew-point to
55F and saturated.Solution:
h 95/75 = 44 BTU/lb
h 55/55 = 23 BTU/lb
Q removed = 10,000 lb/hr x (44 23 BTU/lb)
= 210,000 BTU/hr
= 17.5 tons of cooling
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Air Washer Systems
Uses a water spray to spray cold chilled water into
an air stream.
Air is first saturated along wet-bulb line (constant h)
Once saturation is reached, saturated air is cooled by
conduction to exit temperature
Air is usually dehumidified in process
Direct contact heat exchanger
infinite surface area Must make provision for cleaning out dirt and grime
from chilled water