What is HVAC Cooling Load Calculations ?

Cooling load calculation is normally made to size HVAC systems and their components. In principle, the loads are calculated to maintain the indoor design conditions.

In cooling load calculation, there are four heat flow terms:

1-Cooling load:
Is the total heat required to be removed from the space in order to bring it at the desired temperature by the A.C. equipments. The purpose of estimation of cooling load is to determine the size of the A.C. equipment.

2-Heat gain:
Is a simultaneous summation of all external heat flows plus the heat generated inside the building (heat gain )

3-Space heat extraction rate:
Is usually the same as the space cooling load but with an assumption that the space temperature remain constant.

4-Cooling coil load:
Is the summation of all the cooling loads of the various space served by the equipment plus any loads external to the spaces such as duct heat gain, duct leakage, fan heat.

Components of cooling load:
The components of cooling load are classified into (external sources) which are the heat gain sources coming from outside and (internal sources) which are the heat gain sources generated inside the conditioned space.

There is another classification for the cooling load components which are: sensible & latent heat gain:

A-Sensible heat gain:
The sensible heat gain is occur when there are a direct addition of heat to the enclosed space which caused by any or all of the following sources:
1- Conduction through exterior walls, floors, ceilings, doors & windows due to temperature difference.
2- The heat received from solar radiation

  • Heat transmitted directly through glass of windows or doors.
  • Heat absorbed by wall & roofs exposed to solar radiation.
3- Heat conducted from unconditioned places in the same building.
4- Heat given from light, motors, machinery, cooking operation ,etc.
5- Heat received from the occupants.
6- Heat carried by infiltrating air.
7- Heat gain through the walls of ducts carrying conditioned air through unconditioned spaces.
8- Heat gain from the fan work.

B-Latent heat gain:
Which is occur due to addition of water vapor to the air of enclosed space. The latent heat gain may occur due to any or all of the following sources :

1- Heat gain due to moisture in the outside air entering by infiltration.
2- Heat gain due to vaporization of moisture from occupants.
3- Heat gain due to vaporization of moisture from any process such as cooking of food.
4- Heat gain due to moisture passing directly into the conditioned space through permeable walls or from outside where the water vapor pressure is higher.

Design conditions:
The amount of cooling heat has to be accomplished to keep building comfortable depends on the desired conditions indoor & on the outdoor conditions in a given day.

Indoor design conditions:
For most of the comfort systems, the recommended indoor conditions:

1- Summer: 23 to 26 Co DBT & 50% RH

2- Winter : 21 to 23 Co DBT & 20-30% RH

Outdoor design conditions:
Outdoor design conditions are determined from published data for the specific location.

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