Book - School of Science and Technology

Chapter 18Calculations for air-conditioning designIt is now proposed to consider the fundamental principles underlying the design of an airconditioningsystem. These principles are the same no matter what particular form thesystem may take, but the degree of accuracy necessary to be achieved will depend uponthe application and the sophistication of the controls to be provided. For instance, amicrochip manufacturing plant may require minimum tolerance in conditions, whereasless strict limits would be acceptable for comfort conditioning in the case of a departmentstore.First to be considered here is the general case, as applied to a central air-conditioningsystem for a single large space, and this is followed by some notes on how these generalprinciples may be applied to certain of the specific types of apparatus already discussed.Design data have been built up around each of the particular forms of equipmentmentioned and it would be beyond the scope of this book to explore each one in detail.Heat gainsThe various factors which contribute to the heat gains and losses which occur in aconditioned space have been outlined in Chapters 2 and 3. When designing an airconditioningsystem the principal concern is directed towards heat gains, especially duringthe summer months, although the same system will most probably provide a heatingservice in mid-seasons and winter also. The reason for this approach is that heat gainspresent more searching demands than do heat losses.Sensible heat gainsThe quantity of conditioned air which must be provided to combat sensible heat gains isdirectly proportional to the difference between the supply air temperature and that to bemaintained in the space. The temperature rise which may be permitted will probably belimited to 6 or 8 K owing to the difficulty of mixing cool entering air with warmer roomair without producing draughts. The mass of air flow required to maintain a desired roomtemperature is thus arrived at very simply by use of an expression similar to that noted inChapter 13, where:M ˆ H/(ct)