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  Heat Load Estimation
In warm countries the primary aim of Air-conditioning is to bring the temperature down within the conditioned space. We know that temperature can be brought down by removing heat, and Air-conditioning systems are employed to pump out this heat from within the space.

It is important to select the right Air-conditioning equipment to do the job. A system that is too large for the requirement would not only cost more, but also be a waste of capacity. On the other hand, a system with low capacity would not be able to satisfy the comfort needs of the occupants. Further, being of low capacity, the system would have to run for a longer time thereby being prone to abnormal wear and tear.

It stands to reason therefore, that in order to select the equipment of the right capacity, one must know the quantity of heat that is to be removed from the conditioned space. This ‘quantity of heat’ is calculated using certain formulae and this process is referred to as Heat Load Estimation.

The heat within the space comes from various sources both external and internal. The sun brings in external heat into the space through the walls, roof and glazing. Fresh air brought into the conditioned space from outside, contributes substantially to the heat load. The internal heat comes from electrical equipment, machinery, lighting and from the occupants themselves.
Heat Load from equipment
  Type of work environment   Watts/ft2   Btu/ft2  
  General office with few typewriters, computers & electrical items.   0.25 to 1.00   1 to 4  
  Offices where most workers have personal computers   1.00 to 3.00   3 to 10  
  Rooms with main frame computers   15.00 to 50.00   50 to 175  
  Laboratories   5 to 20   15 to 70  
  Manufacturing plants   5 to 45   15 to 150  
Humans dissipate heat into the space and their perspiration adds to the humidity and therefore to the latent heat. The quantum of heat added by the occupants depends on their level of activity. People at rest will contribute less heat load than those doing more physical activity.
Heat gain from occupants
  Activity   Sensible Heat Btu/person   Latent Heat Btu/person  
  Seated at theatre   225   105  
  Seated, very light work   245   155  
  Moderately active office work   250   200  
  Standing light work, retail store   250   200  
  Light bench work, factory   275   475  
  Walking, 3 mph, factory   375   625  
  Heavy machine work   635   965  
  Indoor stadium/Gymnasium   710   1090  
Calculating heat loads has evolved over the years to become very precise. An Air-conditioning engineer relies on certain checklist like forms or special computer software to estimate the heat load.