capacity requirement

[sajicb]

Many places the capacity of air conditioning required for a room is specified in terms of "ton per area".Why is it so?.Is it not logical to specify the capacity in terms of ton per room volume?.
(like 1 ton/ 40 cubic metre )

[herefishy]

It is a "rule of thumb" that seems to be in the ballpark for most applications (for which it references). Generally there is a considerable safety factor in such estimations, and is a good starting point for checking what calculated requirement may be.

I take the "rule of thumb" and use it as a reference to evaluate the my own requirement calculations, however I do not spec equipment by these generalities. If you're in the feild, and have some concerns regarding an application, such information is useful to be versed in to evaluate system design on the spot.

[Argus]

Another reason that apeals to my cynical nature is the fact that determining the cost per square foot or metre or whatever measure you favour makes it easy for bean counters to understand.

I watched a so-called designer apply air conditioners to an open plan store once by drawing 3 metre radius circles all over a floor plan. Each one was a 10 kW unit and each unit cost.......
Some of us still do it properly.
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video reviews

[BritCit_Juve]

The primary reason that floor area is used rather than volume is basically to make the calculation easier. An asumption is made that room heights are fairly regular 2.7 to 3m) so can be ignored. The remaining variables are room length and width.

I agree that these should only ever be used as a budget calculation as they ignore the building type and load, erring on the excess side in most cases and therefore an oversized system.

I believe that Toshiba base their budget calcs on 40w/m3.

[frank]

In the CIBSE guide the "rule of thumb" heat gains are stated @ 125w/m2 floor space. This covers all heat gains within the space, occupancy (1 person per 10m2), lighting, power, infiltration, solar etc. etc.

As it states - these are only "rule of thumb" and are not a substitute for detailed heat gain calcs.