The production of excess heat in a system will cause problems. Normally, matter expands when heated. This is the principle of thermal expansion. The linear dimensions increase, as does the volume. Removing heat from a substance causes it to contract in linear dimensions and in volume. This is the principle of the liquid in a glass thermometer.
Temperature is the measure of hotness or coldness on a definite scale. Every substance has temperature.
Molecules are always in motion. They move faster with a temperature increase, and more slowly with a temperature decrease. In theory, molecules stop moving at the lowest temperature possible. This temperature is called absolute zero. It is approximately -460°F (-273°C).
The amount of heat in a substance is directly related to the amount of molecular motion. The absence of heat would occur only at absolute zero. Above that temperature, there is molecular motion. The amount of molecular motion corresponds to the amount of heat.
The addition of heat causes a temperature increase. The removal of heat causes a temperature decrease. This is true except when matter is going through a change of state.
Heat is often confused with temperature. Temperature is the measurement of heat intensity. It is not a direct measure of heat content. Heat content is not dependent on temperature. Heat content depends on the type of material, the volume of the material, and the amount of heat that has been put into or taken from the material. For example, one cup of coffee at 200ºF (93.3°C) contains less heat than one gallon of coffee at 200ºF (93.3°C). The cup at 200ºF (93.3°C) can also contain less heat than the gallon at a lower temperature of 180ºF (82.2°C).
Fig. 2-8 Thermometer and pressure gage. (Marsh)
Specific Heat
Every substance has a characteristic called specific heat. This is the measure of the temperature change in a substance when a given amount of heat is applied to it. One Btu (British thermal unit) is the amount of heat required to raise 1 lb of water 1ºF at 39º F. With a few exceptions, such as ammonia gas and helium, all substances require less heat per pound than water to
raise the temperature 1° F.
Thus, the specific heat scale is based on water, which has a specific heat of 1.0. The specific heat of aluminum is 0.2. This means that 0.2 Btu will raise the temperature of 1 lb of aluminum by 1° F. One Btu will raise the temperature of 5 lb of aluminum by 1° F, or of 1 lb, 5° F.
Heat Content
Every substance theoretically contains an amount of heat equal to the heat energy required to raise its temperature from absolute zero to its temperature at a given time. This is referred to as heat content, which consists of sensible heat and latent heat. Sensible heat can be felt because it changes the temperature of the substance. Latent heat, which is not felt, is seen as it changes the state of substance from solid to liquid or liquid to gas.
Sensible Heat
Heat that changes the temperature of a substance, with- out changing its state, when added or removed is called sensible heat. Its effect can be measured with a thermometer in degrees as the difference in temperatures of a substance (Delta T, or DT).
If the weight and specific heat of a medium are known, the amount of heat added or removed in Btu can be computed by multiplying the sensible change (DT) by the weight of the medium and by its specific heat. Thus, the amount of heat required for raising the temperature of one gallon of water (8.34 pounds) from 140ºF to 160ºF is:
Sensible heat = DT ´ weight ´ specific heat =
(160 – 140) ´ 8.34 ´ 1 = 20 ´ 8.34 = 166.8 Btu
Latent Heat
The heat required to change the state of a substance without changing its temperature is called latent heat, or hidden heat. Theoretically, any substance can be a gas, liquid, or solid, depending on its temperature and pressure. It takes heat to change a substance from a solid to a liquid or, from a liquid to a gas.
For example, it takes 144 Btu of latent heat to change 1 lb of ice at 32ºF to 1 lb of water at 32ºF. It takes 180 Btu of sensible heat to raise the temperature of 1 lb of water 180ºF from 32ºF to 212ºF. It takes 970 Btu of latent heat to change 1 lb of water to steam at 212ºF. When the opposite change is affected, equal amounts of heat are taken out or given up by the sub- stance.
This exchange of heat, or the capability of a medium, such as water to take and give up heat, is the basis for most of the heating and air-conditioning industry. Most of the functions of the industry are concerned with adding or removing heat at a central point and distributing the heated or cooled medium through- out a structure to warm or cool the space.
Other Sources of Heat
Other heat in buildings comes principally from four sources: electrical energy, the sun, outdoor air temperatures and the building’s occupants. Every kilowatt of electrical energy in use produces 3413 Btu/h, whether it is used in lights, the heating elements of kitchen ranges, toasters, or irons.
The sun is a source of heat. At noon, a square foot of surface directly facing the sun may receive 300 Btu/h, on a clear day. When outdoor air temperatures exceed the indoor space temperature. The outdoors become a source of heat. The amount of heat communicated depends on the size and number of windows, among other factors.
The occupants of a building are a source of heat, since body temperatures are higher than normal room temperatures. An individual, seated and at rest, will give off about 400 Btu/h in a 74°F (23.3°C) room. If the person becomes active, this amount of heat may be increased two or three times, depending upon the activity involved. Some of this heat is sensible heat, which the body gives off by convection and radiation. The remainder is latent heat, resulting from the evaporation of visible or invisible perspiration. The sensible heat increases the temperature of the room. The latent heat increases the humidity. Both add to the total heat in the room.
.
