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A brief presentation on HVAC cooling load calculations
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H V A CHeatingVentilatingAir Conditioning
Outline of Presentation Introduction Cooling Load Calculation
Step 1. Air Conditioning Design
Step 2. Basic Concept
Step 3. Outdoor Design Conditions
Step 4. Indoor Design Criteria
Step 5. Cooling Load Principles
Step 6. Computer Software's
Air Conditioning Design
Cooling load calculation for individual areas are required to
determine the peak air conditioning loads and the required supply air
for each area or zone of the building that is being designed.
Air Conditioning Design
Cooling load calculation should be made using data found in the ASHRAE Handbooks and by using format shown in the Design and Calculation topic.
The following information are which will make to generate the cooling load computations.
Orientation and location of the building, Types of operation. General construction of the space or building such as walls, roof, floor,
ceiling, etc. Door / Window types and quantities Machinery used or intended to be used within the space Lighting layouts Occupancy, that is the number of people and their degree of activity
within the given space Code and Standard requirements that deal with the cooling and heating
system for the building or space involved
Basic Concepts Thermal load
– The amount of heat that must be added or removed from the space to maintain the proper temperature in the space
When thermal loads push conditions outsider of the comfort range, HVAC systems are used to bring the thermal conditions back to comfort conditions
Basic Concepts Heat transfer mechanism
– Conduction– Convection– Radiation
Thermal properties of building materials– Overall thermal transmittance (U-value)– Thermal conductivity– Thermal capacity (specific heat)
Outdoor Design Conditions
They are used to calculate design space loads
Climatic design information– General info: e.g. latitude, longitude, altitude, atm. pressure– Outdoor design conditions
• Derived from statistical analysis of weather data• Typical data can be found in handbooks/databooks, such as
ASHRAE Fundamentals Handbooks
Outdoor Design Conditions
Climatic design conditions from ASHRAE– Previous data & method (before 1997)
• For Summer (Jun. to Sep.) & Winter (Dec, Jan, Feb)• Based on 1%, 2.5% & 5% nos. hours of occurrence
– New method (ASHRAE Fundamentals 2001):• Based on annual percentiles and cumulative frequency
of occurrence, e.g. 0.4%, 1%, 2%
Outdoor Design Conditions
Climatic design conditions (ASHRAE 2001):– Heating and wind design conditions
• Heating dry-bulb (DB) temp.• Extreme wind speed• Coldest month wind speed (WS) & mean coincident dry-
bulb temp. (MDB)• Mean wind speed (MWS) & prevailing wind direction
(PWD) to DB• Average of annual extreme max. & min. DB temp. &
standard deviations
Outdoor Design Conditions
Climatic design conditions (ASHRAE):– Cooling and dehumidification design conditions
• Cooling DB/MWB: Dry-bulb temp. (DB) + Mean coincident wet-bulb temp. (MWB)
• Evaporation WB/MDB: Web-bulb temp. (WB) + Mean coincident dry-bulb temp. (MDB)
• Dehumidification DP/MDB and HR: Dew-point temp. (DP) + MDB + Humidity ratio (HR)
• Mean daily (diurnal) range of dry-bulb temp.
Indoor Design CriteriaIndoor Design Criteria
Basic design parameters: (for thermal comfort)– Air temp. & air movement
• Typical: summer 24-26 oC; winter 21-23 oC• Air velocity: summer < 0.25 m/s; winter < 0.15 m/s
– Relative humidity• Summer: 40-50% (preferred), 30-65 (tolerable)• Winter: 25-30% (with humidifier); not specified (w/o
humidifier)– See also ASHRAE Standard 55-2004
• ASHRAE comfort zone
(*Source: ASHRAE Standard 55-2004)
Indoor Design CriteriaIndoor Design Criteria
Indoor air quality:– Air contaminants
• e.g. particulates, VOC, radon, bioeffluents– Outdoor ventilation rate provided
• ASHRAE Standard 62-2001– Air cleanliness (e.g. for processing)
Other design parameters:– Sound level– Pressure differential between the space & surroundings
(e.g. +ve to prevent infiltration)
Cooling Load PrinciplesCooling Load Principles
Cooling Load Principles
Terminology:– Space – a volume w/o a partition, or a partitioned room, or
group of rooms– Room – an enclosed space (a single load)– Zone – a space, or several rooms, or units of space having
some sort of coincident loads or similar operating characteristics
Cooling Load Principles
• Cooling load calculations• To determine volume flow rate of air system• To size the coil and HVAC&R equipment• To provide info for energy calculations/analysis
• Two categories:• External loads• Internal loads
Cooling Load Principles
• External loads• Heat gain through exterior walls and roofs• Solar heat gain through fenestrations (windows)• Conductive heat gain through fenestrations• Heat gain through partitions & interior doors• Infiltration of outdoor air
Convective and radiative heat in a conditioned space
Cooling Load Principles
• Internal loads• People• Electric lights• Equipment and appliances
• Sensible & latent cooling loads• Convert instantaneous heat gain into cooling load• Which components have only sensible loads?
[Source: ASHRAE Fundamentals Handbook 2001]
Cooling Load Principles
Instantaneous heat gain vs space cooling loads– They are NOT the same
Effect of heat storage– Night shutdown period
• HVAC is switched off. What happens to the space?
– Cool-down or warm-up period• When HVAC system begins to operate
– Conditioning period• Space air temperature within the limits
Thermal Storage Effect in Cooling Load from Lights
Cooling Load Principles
Load profile– Shows the variation of space load– Such as 24-hr cycle– What factors will affect load profile?– Useful for operation & energy analysis
Peak load and block load– Peak load = max. cooling load– Block load = sum of zone loads at a specific time
Block load and thermal zoning
Cooling Load Principles
Moisture transfer– Two paths:
• Moisture migrates in building envelope• Air leakage (infiltration or exfiltration)
– If slight RH variation is acceptable, then storage effect of moisture can be ignored• Latent heat gain = latent cooling load (instantaneously)
Cooling Load Components
• Cooling coil load consists of:• Space cooling load (sensible & latent)• Supply system heat gain (fan + air duct)• Return system heat gain (plenum + fan + air duct)• Load due to outdoor ventilation rates (or
ventilation load)
Cooling load
Cooling coil load
Cooling Load Components
• Space cooling load• To determine supply air flow rate & size of air
system, ducts, terminals, diffusers• It is a component of cooling coil load• Infiltration heat gain is an instant. cooling load
• Cooling coil load• To determine the size of cooling coil &
refrigeration system• Ventilation load is a coil load
“Carrier” E20-II Computer Program
Zone or Space
Input Sheet
System Input
Sheet
“Carrier” E20-II Computer Program
Zone Input Sheet :
Zone or Space Name General Zone Data Lighting Load Other Electrical Load People Miscellaneous Loads Infiltration Slab
Wall & Roof Data Wall & Roof Exposure Glass & External Shade Data Glass Exposure Partition Data
“Carrier” E20-II Computer Program
System Input Sheet : General System Data Sizing Specification Data Fan Data Thermostat Setpoint Factors Return Air Plenum Data System Arrangement Zone or Space Selection
Coefficient of Heat TransmissionU - value
I. Exterior Wall:
Resistance
“R” value
1. Outside Surface, @ 7.5 MPH wind - summer = 0.25
2. Cement Plaster, 3/4” thick = 0.15
3. Concrete block wall, 8” thick = 2.55
4. Cement Plaster, 3/4” thick = 0.15
5. Inside Surface, Still Air = 0.68
Total R-value = 3.78 Hr-Ft²-ºF/BTU
U-value = 1 / total R = 1 / 3.78 = 0.264 BTU / Hr - Ft² - ºF
Coefficient of Heat TransmissionU - value
II. Partition Wall (Type - 1):
Resistance
“R” value
1. Inside Surface, Still Air = 0.68
2. Cement Plaster, 3/4” thick = 0.15
3. Concrete block wall, 6” thick = 1.80
4. Cement Plaster, 3/4” thick = 0.15
5. Inside Surface, Still Air = 0.68
Total R-value = 3.46 Hr-Ft²-ºF/BTU
U-value = 1 / total R = 1 / 3.46 = 0.0289 BTU / Hr - Ft² - ºF
Coefficient of Heat TransmissionU - value
III. Partition Wall (Type - 2):
Resistance
“R” value
1. Inside Surface, Still Air = 0.68
2. Cement Plaster, 3/4” thick = 0.15
3. Concrete block wall, 6” thick = 1.80
4. Cement Mortar, 3/4” thick = 0.077
5. Ceramic Tile = 0.05
6. Inside Surface, Still Air = 0.68
Total R-value = 3.427 Hr-Ft²-ºF/BTU
U-value = 1 / total R = 1 / 3.427 = 0.29 BTU / Hr - Ft² - ºF
Coefficient of Heat TransmissionU - value
IV. Roof: Resistance
“R” value
1. Outside Surface, @ 7.5 MPH wind - summer = 0.25
2. Finish Floor Tile = 0.05
3. Cement Mortar, 1”thick, 120 ld./cu.ft. density = 0.10
4. Felt Water Proofing Membrane = 0.06
5. Glass Fiber organic bonded, 2” thick = 8.00
6. Roof Slab, 6” thick, 140 ld/Ft³ density = 0.51
7. Non-reflective air space = 1.00
8. Ceiling tile gypsum board, 5/8” thick = 0.56
9. Inside Surface, Still Air = 0.92
Total R-value = 11.45 Hr-Ft²-ºF/BTU
U-value = 1 / total R = 1 / 11.45 = 0.0873 BTU / Hr - Ft² - ºF
Coefficient of Heat TransmissionU - valueV. Glass:
Type: Clear Glass Single, 1/4” thick in aluminum framing
U = 1.04 BTU / Hr - Ft² - ºF
SC = 0.95 (Shading Coefficient factor)
VI. Wood Door:Type: Hollow flush door, 1-3/4” thick
U = 0.46 BTU / Hr - Ft² - ºF
VII. Steel Door:
Type: Solid fire rated with insulation, 1-3/4” thick
U = 0.29 BTU / Hr - Ft² - ºF
References
ASHRAE Handbook Fundamentals 2001– Chapter 26 – Ventilation and Infiltration– Chapter 27 – Climatic Design Information– Chapter 28 – Residential Cooling and
Heating Load Calculations– Chapter 29 – Nonresidential Cooling and
Heating Load Calculations– Chapter 31 – Energy Estimation and
Modeling Methods
References
Air Conditioning Principles and Systems by Edward G. Pita