Psychrometry
The measurement and determination of atmospheric conditions, particularly relating to the water vapor or moisture content in dry air, is an important branch of physics known as psychrometry. (Some psychrometric terms and conditions have already been presented in this article. Many others remain to be considered.)
An ideal gas follows certain established laws of physics. The mixture of water vapor and dry air behaves at normal atmospheric temperatures and pressures almost as an ideal gas. As an example, air temperatures, volumes, and pressures may be calculated by use of Eq. (13.8), Pv = RT.
Daltons law also applies. It states:
When two or more gases occupy a common space or container, each gas will fill the volume just as if the other gas or gases were not present. Daltons law also requires:
1. That each gas in a mixture occupy the same volume or space and also be at the same temperature as each other gas in the mixture.
2. That the total weight of the gases in the mixture equal the sum of the individual weights of the gases.
3. That the pressure of a mixture of several gases equal the sum of the pressures that each gas would exert if it existed alone in the volume enclosing the mixture.
4. That the total enthalpy of the mixture of gases equals the sum of the enthalpies of each gas.
An excellent example of the application of Daltons law of partial pressures is the use of a liquid barometer to indicate atmospheric pressure. The barometer level indicates the sum of the partial pressure of water vapor and the partial pressure of the air.
Partial pressures of air and water vapor are of great importance in psychrometry and are used to calculate the degree of saturation of the air or relative humidity at a specific dry-bulb temperature.
Relative Humidity and Specific Humidity
Relative humidity is sometimes defined by the use of mole fractions, a difficult definition for psychrometric use. Hence, a more usable definition is desired. For this purpose, relative humidity may be closely determined by the ratio of the partial pressure of the water vapor in the air to the saturation pressure of water vapor at the same temperature, usually expressed as a percentage.
Thus, dry air is indicated as 0% relative humidity and fully saturated air is termed 100% relative humidity.
Computation of relative humidity by use of humidity ratios is also often done, but with somewhat less accuracy. Humidity ratio, or specific humidity Wa, at a specific temperature is the weight, lb, of water vapor in air per pound of dry air.
If Ws represents the humidity ratio of saturated air at the same temperature (Table 13.2), then relative humidity can be calculated approximately from the equation
It is difficult to use this equation, however, because of the difficulty in measuring the partial pressures with special scientific equipment that is required and rarely available outside of research laboratories. Therefore, it is common practice to utilize simpler types of equipment in the field. These will provide direct readings that can be converted into humidity ratios or relative humidity.
A simple and commonly used device is the wet- and dry-bulb thermometer. This device is a packaged assembly consisting of both thermometers and a sock with scales. It is called a sling psychrometer. Both thermometers are identical, except that the wet-bulb thermometer is fitted with a wick-type sock over the bulb. The sock is wet with water, and the device is rapidly spun or rotated in the air. As the water in the sock evaporates, a drop in temperature occurs in the remaining water in the sock, and also in the wet-bulb thermometer. When there is no further temperature reduction and the temperature remains constant, the reading is called the wet-bulb temperature. The other thermometer will simultaneously read the dry-bulb temperature.
A difference between the two thermometer readings always exists when the air is less than saturated, at or less than 100% relative humidity. Inspection of a psychrometric
chart will indicate that the wet-bulb and dry-bulb temperatures are identical only at fully saturated conditions, that is, at 100% relative humidity. Commercial psychrometers usually include appropriate charts or tables that indicate the relative humidity for a wide range of specific wet- and dry-bulb temperature readings.
These tables are also found in books on psychrometry and HVAC books and publications.
Dew-Point Temperatures
Dew is the condensation of water vapor. It is most easily recognized by the presence of droplets in warm weather on grass, trees, automobiles, and many other outdoor surfaces in the early morning. Dew is formed during the night as the air temperature drops, and the air reaches a temperature at which it is saturated with moisture. This is the dew-point temperature. It is also equal to both the wet-bulb temperature and dry-bulb temperature. At the dew-point temperature, the air is fully saturated, that is, at 100% relative humidity. With any further cooling or drop in temperature, condensation begins and continues with any further reduction in temperature. The amount of moisture condensed is the excess moisture that the air cannot hold at saturation at the lowered temperature. The condensation forms drops of water, frequently referred to as dew.
Dew-point temperature, thus, is the temperature at which condensation of water vapor begins for any specific condition of humidity and pressure as the air temperature is reduced.
The dew-point temperature can be calculated, when the relative humidity is known, by use of Eq. (13.13) and Table 13.2. For the temperature of the unsaturated air, the humidity ratio at saturation is determined from Table 13.2. The product of the humidity ratio and the relative humidity equals the humidity ratio for the dewpoint temperature, which also can be determined from Table 13.2. As an example, to determine the dew-point temperature of air at 90F and 50% relative humidity, reference to Table 13.2 indicates a humidity ratio at saturation of 0.0312 at 90F.
Multiplication by 0.50 yields a humidity ratio of 0.0156. By interpolation in Table 13.2 between humidity ratios at saturation temperatures of 65 and 70F, the dewpoint temperature is found to be 69.6F.
A simpler way to determine the dew-point temperature and many other properties of air-vapor mixtures is to use a psychrometric chart. This chart graphically relates dry-bulb, wet-bulb, and dew-point temperatures to relative humidity, humidity ratio, and specific volume of air. Psychrometric charts are often provided in books on psychrometrics and HVAC handbooks.
Refrigeration Ton
A ton of refrigeration is a common term used in air conditioning to designate the cooling rate of air-conditioning equipment. A ton of refrigeration indicates the ability of an evaporator to remove 200 Btu/min or 12,000 Btu/hr. The concept is a carry-over from the days of icemaking and was based on the concept that 200 Btu/ min had to be removed from 32F water to produce 1 ton of ice at 32F in 24 hr.
Hence,
