Latent Heat
The sensible heat of a substance is associated with a sensible change in temperature.
In contrast, the latent heat of a substance is always involved with a change in state of a substance, such as from ice to water and from water to steam or water vapor.
Latent heat is very important in HVAC calculations and design, because the total heat content of air almost always contains some water in the form of vapor. The concept of latent heat may be clarified by consideration of the changes of state of water.
When heat is added to ice, the temperature rises until the ice reaches its melting point. Then, the ice continues to absorb heat without a change in temperature until a required amount of heat is absorbed per pound of ice, at which point it begins melting to form liquid water. The reverse is also true: if the liquid is cooled to the freezing point, this same quantity of heat must be removed to cause the liquid water to change to the solid (ice) state. This heat is called the latent heat of fusion for water. It is equal to 144 Btu and will convert 1 lb of ice at 32F to 1 lb of water at 32F. Thus,
Latent heat of fusion for water = 144 Btu/ lb
If the pound of water is heated further, say to 212F, then an additional 180 Btu of heat must be added to effect the 180F sensible change in temperature. At this temperature, any further addition of heat will not increase the temperature of the water beyond 212F. With the continued application of heat, the water experiences violent agitation, called boiling. The boiling temperature of water is 212F at atmospheric pressure.
With continued heating, the boiling water absorbs 970 Btu for each pound of water without a change in temperature and completely changes its state from liquid at 212F to water vapor, or steam, at 212F. Therefore, at 212F, Conversely, when steam at 212F is cooled or condensed to a liquid at 212F, 970 Btu per pound of steam (water) must be removed. This heat removal and change of state is called condensation.
When a body of water is permitted to evaporate into the air at normal atmospheric pressure, 29.92 in of mercury, a small portion of the body of water evaporates from the water surface at temperatures below the boiling point. The latent heat of vaporization is supplied by the body of water and the air, and hence both become cooler. The amount of vapor formed and that absorbed by the air above the water surface depends on the capacity of the air to retain water at the existing temperature and the amount of water vapor already in the air.
Table 13.2 lists the latent heat of vaporization of water for various air temperatures and normal atmospheric pressure. More extensive tables of thermodynamic properties of air, water, and steam are given in the ASHRAE Handbook Fundamentals, American Society of Heating, Refrigerating and Air-Conditioning Engineers.
Enthalpy
Enthalpy is a measure of the total heat (sensible and latent) in a substance and is equivalent to the sum of its internal energy plus its ability or capacity to perform work, or PV/J, where P is the pressure of the substance, V its volume, and J its mechanical equivalent of heat. Specific enthalpy is the heat per unit of weight, Btu/ lb, and is the property used on psychrometric charts and in HVAC calculations.
The specific enthalpy of dry air ha is taken as zero at 0F. At higher temperatures, ha is equal to the product of the specific heat, about 0.24, multiplied by the temperature, F. (See Table 13.2.)
The specific enthalpy of saturated air hs , which includes the latent heat of vaporization of the water vapor, is indicated in Table 13.2. The specific enthalpy of the water vapor or moisture at the air temperature may also be obtained from Table 13.2 by subtracting ha from hs .
Table 13.2 also lists the humidity ratio of the air at saturation for various temperatures (weight, lb, of water vapor in saturated air per pound of dry air). In addition, the specific enthalpy of saturated water vapor hg , Btu/ lb, is given in Table 13.2 and represents the sum of the latent heat of vaporization and the specific enthalpy of water at various temperatures.
The specific enthalpy of unsaturated air is equal to the sensible heat of dry air at the existing temperature, with the sensible heat at 0F taken as zero, plus the product of the humidity ratio of the unsaturated air and hg for the existing temperature.
Cooling by Evaporation
Evaporation of water requires a supply of heat. If there is no external source of heat, and evaporation occurs, then the water itself must provide the necessary heat of vaporization. In other words, a portion of the sensible heat in the liquid will be converted into the latent heat of vaporization. As a result, the temperature of the liquid remaining will drop. Since no external heat is added or removed by this process of evaporation, it is called adiabatic cooling.
Human beings are also cooled adiabatically by evaporation of perspiration from skin surfaces. Similarly, in hot climates with relatively dry air, air conditioning is provided by the vaporation of water into air. And refrigeration is also accomplished by the evaporation of a refrigerant.
Heating by Condensation
Many thermal processes occur without addition or subtraction of heat from the process. Under these conditions, the process is called adiabatic.
When a volume of moist air is cooled, a point will be reached at which further cooling cannot occur without reaching a fully saturated condition, that is, 100% saturation or 100% relative humidity. With continued cooling, some of the moisture condenses and appears as a liquid. The temperature at which condensation occurs is called the dew point temperature. If no heat is removed by the condensation, then the latent heat of vaporization of the water vapor will be converted to sensible heat in the air, with a resultant rise in temperature.
Thus, an increase in temperature is often accomplished by the formation of fog, and when rain or snow begins to fall, there will usually be an increase in temperature of the air.
