A building gains heat from a number of sources. To keep internal environmental conditions at a comfortable level, we have to cool the building. To design a system, which will do so, require calculating so-called cooling loads. Such systems are referred to as air conditioning systems though incorrectly as these systems ‘condition’ the air to a comfortable level which can be either cooling or heating it. However, in this section reference to air conditioning system will mean cooling the air.
In this section, we will describe how to determine the cooling loads. To do so, we will discuss how a building gains heat and what temperatures a building can reach during summer time if no cooling takes place.
Heat gains in buildings
The sources which contribute to a building’s heat gain include: solar radiations; internal heat gains; and ventilation and infiltration. We will discuss these below.
When there are no clouds, we experience sunshine or heat through solar radiation. The amount of heat that reaches us is called the solar irradiance. This heat which strikes the buildings at a certain angle and intensity depending upon which part of the earth you live in. This heat is transferred inside through walls, roof and windows. As these elements of a building are made up of different materials, the heat which actually gets into the buildings through these is not the same.
There are many ways to calculate the heat gain of a building through solar radiations. This includes some reference tables, guidelines and computer software.
Internal heat gains
This is the most important factor contributing to the heat gains. Consider a room full of people with lots of computers, printers, fax machines, etc. These all generate heat thereby increasing the internal temperature and heat gain of the building. The floor and wall finishes and the type of furnishings/furniture also contribute significantly.
Hence, how much a building gain heat because of its environment depends upon number and age of people, activity they are doing, internal finishes and machinery and equipment.
Ventilation and infiltration
When you open a window or door in a room to have some fresh air, you are allowing the movement of air into and outside the building. This is what we can call as ventilation. On the other hand, air can also leak into the buildings through cracks and where doors and windows are not adequately sealed. This air leakage is called infiltration.
Depending upon the difference between outside and inside temperatures, ventilation and/or infiltration can have a significant contribution to the overall heat gain by a building.
Total heat gain
Total heat gain or the cooling load of a building is the sum of all the individual heat gains.
Total heat gain = S Heat gains through [solar radiations (glass, walls, roof) + internal + ventilation/infiltration]
Peak Summertime Temperatures
As discussed earlier, the internal temperature of a building can increase to point where it becomes uncomfortable. As human beings perceive comfort differently, it is difficult to fix a value of comfortable temperature. However, a temperature in the range of 24°C to 27°C is generally considered as comfortable.
A building with heat gains in excess of these temperatures needs to be cooled. To ascertain whether a building requires cooling or not, we can calculate Peak Summertime Temperatures.
To do so, we can monitor a building for 24 hours to find the mean temperature. This will include all the sources as described earlier. We also look into the variation around this mean temperature called a ‘swing’. We should also consider some heat loss from the building.
The final temperature is calculated by balancing the heat gains and heat lost.
The publications listed contain a more detailed explanation of heat gains and cooling loads. You are strongly advised to read these before attempting the tasks.
Some useful websites and video resources are listed with self explanatory titles which will help you understand the concepts. These include some useful calculators.