COOLING LOAD CALCULATION METHOD
For a thorough calculation of the zones and whole-building loads, one of the following three methods should be employed:
a. Transfer Function Method (TFM):
This is the most complex of the methods proposed by ASHRAE and requires the use of a computer program or advanced spreadsheet.
b. Cooling Load Temperature Differential/Cooling Load Factors (CLTD/CLF):
This method is derived from the TFM method and uses tabulated data to simplify the calculation process. The method can be fairly easily transferred into simple spreadsheet programs but has some limitations due to the use of tabulated data.
c. Total Equivalent Temperature Differential/Time-Averaging (TETD/TA):
This was the preferred method for hand or simple spreadsheet calculation before the introduction of the CLTD/CLF method.
These Factors Define the Heat gain or heat loss through the building.
1) Weather
2) Building Material ( Wall, Roof ,Ceiling & Partition - U value)
3) Orientation of the building.
4) Floor Height.
5) Infiltration.
6) Occupancy.
7) Activity & other Equipment inside the building.
8) Lighting Load.
9) Home appliances.
In Air conditioning design , these are the Heat flow rates are contribute in system.
a. Space heat gain.
b. Space cooling load.
c. Space heat extraction.
d. Cooling Load.
Space heat gain
This instantaneous rate of heat gain is the rate at which heat enters into and/or is generated within a space at a given instant.
Heat gain is classified by:
a. Solar radiation through transparent surfaces such as windows
b. Heat conduction through exterior walls and roofs
c. Heat conduction through interior partitions, ceilings and floors
d. Heat generated within the space by occupants, lights, appliances, equipment and processes
e. Loads as a result of ventilation and infiltration of outdoor air
f. Other miscellaneous heat gains
Sensible heat
Heat which a substance absorbs, and while its temperature goes up, the substance does not change state. Sensible heat gain is directly added to the conditioned space by conduction, convection, and/or radiation. Note that the sensible heat gain entering the conditioned space does not equal the sensible cooling load during the same time interval because of the stored heat in the building envelope. Only the convective heat becomes cooling load instantaneous.
Sensible heat generated by
a. Heat transmitted thru floors, ceilings, walls
b. Occupant’s body heat
c. Appliance & Light heat
d. Solar Heat gain thru glass
e. Infiltration of outside air
f. Air introduced by Ventilation
Latent Heat Loads
Latent heat gain occurs when moisture is added to the space either from internal sources (e.g. vapor emitted by occupants and equipment) or from outdoor air as a result of infiltration or ventilation to maintain proper indoor air quality
Latent heat load is total of
a. Moisture-laden outside air form Infiltration & Ventilation
b. Occupant Respiration & Activities
c. Moisture from Equipment & Appliances
Space Heat Gain V/s Cooling Load (Heat Storage Effect):
The heat received from the heat sources (conduction, convection, solar radiation, lightning, people, equipment, etc...) does not go immediately to heating the room air.
Only some portion of it is absorbed by the air in the conditioned space instantaneously leading to a minute change in its temperature.
Most of the radiation heat especially from sun, lighting, people is first absorbed by the internal surfaces, which include ceiling, floor, internal walls, furniture etc.
Due to the large but finite thermal capacity of the roof, floor, walls etc., their temperature increases slowly due to absorption of radiant heat.
The radiant portion introduces a time lag and also a decrement factor depending upon the dynamic characteristics of the surfaces.
Due to the time lag, the effect of radiation will be felt even when the source of radiation, in this case the sun is removed.
Space Cooling V/s Cooling Load (Coil)
Space cooling is the rate at which heat must be removed from the spaces to maintain air temperature at a constant value. Cooling load, on the other hand, is the rate at which energy is removed at the cooling coil that serves one or more conditioned spaces in any central air conditioning system. It is equal to the instantaneous sum of the space cooling loads for all spaces served by the system plus any additional load imposed on the system external to the conditioned spaces items such as fan energy, fan location, duct heat gain, duct leakage, heat extraction lighting systems and type of return air systems all affect component sizing.
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