# Cooling load for a whole building

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To determine the size of the necessary refrigeration plant the cooling load should be calculated at about 15.00 h sun-time for the entire building. The design refrigeration load is defined by the following equation:

 Qr = design refrigeration load KW <2S = sensible heat gain KW Qi = latent heat gain KW Qfa = fresh air load KW <2Sf = supply fan power KW (2sd = supply duct heat gain KW QIa = return air load (recirculation fan power and heat gain through extract Ventilated luminaires) KW Qrh = reheat load KW /p = factor to cover chilled water pump power and heat gain to pipes
 Qr = [2s + Cl + <2fa + Gsf + Gsd + Gra + 2rhl/p (8-1)

Provided that the type of air conditioning system used for the building can take advantage of the reduction in sensible heat gains, the calculation of Qs, at 15.00 h in July, will involve the use ot diversity factors for people, lights and business machines. People will be absent for various reasons and not all the lights will be on, or all the business machines in use. Diversity in the solar gain through glass will be naturally accounted for because windows on the easterly side of the building will be in shade at 15.00 h in July. Similarly, the part of the latent gain due to people will also be subject to the use of a diversity factor.

The fresh air load is maximised by the choice of 15.00 h in July. Supply fan power is not subject to diversity, unless a variable air volume system is used and then only if an efficient method of controlling fan capacity is employed. There is no contribution from heat gain to recirculated air ducts because the air within is at about 22°C. With a well-designed system there should be no reheat load at the time of greatest cooling load. The factor/p is used only if a chilled water system is adopted, as would be the case for larger installations. The contribution of pump power and heat gain to pipes is usually very small and/p could have a value of 1.01 or 1.02. In such a case it might as well be taken as unity, since the
calculation of the other elements in the load is not done with this degree of accuracy. The design engineer must exercise discretion.

Posted in Engineering Fifth Edition