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CBD-108. Air Conditioning Processes

Originally published December 1968.

Control of the temperature and humidity of room air is one of the important aspects of conditioning for human comfort (CBD 102). Heat and moisture must be supplied to or removed from the air of a space to maintain the desired conditions. This normally involves provision of an air stream at a higher or lower temperature and humidity than that of the space to the degree needed to balance the heat and moisture loads in the room (CBD 106). A knowledge of the processes by which the treatment of air is carried out is essential for an understanding of the capabilities and limitations of an air conditioning system. The problems or expense involved in achieving certain conditions are important considerations in the selection and specification of the air conditioning requirements for a building, and are ultimately related to the design of the building enclosure. It is the purpose of this Digest, therefore, to examine some of the processes involved in air treatment.

The properties of air that are manipulated by air conditioning equipment are temperature, moisture content and enthalpy or heat content. These three properties are interrelated, so that any two completely define the state point or condition of the air.

A psychrometric chart is a graphical representation of all possible conditions within the range for which the chart is constructed. One design of such a chart in skeleton form is shown in Figure 1 to 4. The horizontal scale is air temperature or dry-bulb temperature and the vertical scale is moisture content expressed in pounds of water per pound of air. Vertical lines are, therefore, constant temperature lines, and horizontal lines are constant moisture content lines. The curved line on the left is the saturation line or 100 per cent relative humidity line, which represents the maximum amount of moisture that can be held at the various temperatures and is a boundary of the chart. The temperatures at points along this line are referred to as saturation, or dew-point, temperatures. Other degrees of saturation can also be shown, as indicated by the 50 per cent relative humidity line.

The heat content of moist air represents the heat content of both the air and the water vapour it contains. Thus the total heat content can be increased by raising either the temperature or the moisture content. A constant heat content line is, therefore, one of increasing temperature and decreasing moisture content or of increasing moisture content and decreasing temperature. Constant heat content lines are straight lines sloping downward to the right.

The chart is an excellent tool for visualizing air conditioning processes and can also be used for calculations. The problem to be solved is usually one of maintaining a particular condition, such as A in Figure 1. If a room loses heat and moisture, air must be supplied to the room at some condition such as B having a higher temperature and higher moisture content than that in the room. If the room gains heat and moisture, air must be supplied at some condition such as C having a lower temperature and moisture content. Some of the processes by which conditions such as B and C can be achieved are relatively simple while others are rather involved.

Figure 1.  Heating, cooling, and mixing processes.

Heating air from condition D to condition E in Figure 1 involves only an increase in heat content and no change in moisture content. On the chart this is represented by a constant moisture content line from D to E. Similarly, cooling from E to D is represented by removing heat, with no change in moisture content. Cooling beyond point D can proceed without moisture removal until the saturation line is reached; but any further cooling must proceed along the saturation line to some point F, with a resulting decrease in moisture content.

Air Mixing

Two streams of air at different temperature and humidity conditions are often mixed to produce one air stream with the desired supply conditions. The temperature of the mixture will be the weighted mean temperature of the two original streams, and the moisture content (lb of water per lb of dry air), their weighted mean moisture content. With the mixing technique, outside air and air returned from the space can often be used to provide the desired supply condition. This is illustrated in Figure 1. Room air with temperature and moisture content conditions at A is mixed with outside air at condition G. The condition of the mixture is some point C on a straight line joining G and A. The location of C on the line depends on the proportions of the mixture. The supply conditions can be adjusted by adjusting the proportions of each. Two separately conditioned air streams are sometimes mixed to achieve the desired supply condition.

Cooling and Humidifying

Cooling and humidifying can be accomplished by spraying water into the air. Such a process is called adiabatic, in as much as no heat is added or extracted. It is also known as evaporative cooling. When an unsaturated stream of air is passed through a recirculated water spray, water is evaporated; if the spray is adequate, the air will be saturated. With no heat added in the process, the heat required to evaporate the water can only come from the air, thus reducing air temperature while increasing moisture content. The total heat content of the moist air remains unchanged.

This process is shown as line HI in Figure 2, where air, initially at condition H, is treated in an adiabatic spray. As no heat is added while the moisture content of the air is increased, the process must proceed along a line of constant heat content and will reach condition I at saturation. The temperature achieved at saturation, therefore, depends only on the initial total heat content and this, in turn, depends on the initial dry-bulb temperature and moisture content. This temperature is referred to as the thermodynamic wet-bulb temperature and is approximately equal to the temperature of the wetted wick of a wet-bulb thermometer. Lines of constant wet-bulb temperature can be regarded as lines of constant heat content.

Figure 2.  The evaporative process.

If the spray were inadequate, the process might proceed only to some unsaturated condition J on the same wet-bulb line. This would produce the same effect as conditioning only part of the air stream to condition I and then mixing it with air at the initial condition H. Such a process would result if part of the air were allowed to bypass the spray.

The exit condition of the air can be controlled if the wet-bulb temperature of the entering air is adjusted by heating or cooling, or preferably by mixing with outside air. Heating air above condition X or cooling it below condition Y will produce the required entering wet-bulb temperature. Air at either X or Y can also be brought to condition I by passing it through a sprayed coil or a spray maintained at temperature I by heating or cooling.

The application of the adiabatic spray process to air conditioning is limited by the outside air wet-bulb temperatures and the space air conditions required: If the need is for space conditions of 75°F and 50 per cent relative humidity, corresponding to a saturation temperature of 55°F, evaporative cooling will not be adequate during the summer in most parts of Canada because outside wet-bulb temperatures exceed this value for a significant part of the time. In industrial and commercial applications where higher humidities and temperatures can be tolerated for part of the time, and where large air quantities are acceptable, outside air treated in an adiabatic spray may be supplied to the space to relieve high temperature conditions during summer months.

The evaporative cooling process is used extensively for cooling water. Both the water temperature and the air dry-bulb temperature approach the wet-bulb temperature. This is the process used in cooling towers.

Heating and Humidifying

Heating and humidifying, a common winter requirement, involves the supply of heat to raise the temperature of the air and its associated water vapour plus addition of heat to evaporate the added moisture. Certain psychrometric conditions must also be fulfilled in order to introduce the moisture into the air. This is best illustrated by an example.

If air at condition K (Figure 3) is to be heated and humidified to condition L: (h_L - h_K) Btu of heat and (m_L - m_K) lb of water must be added for each pound of air.

Figure 3.  Heating and humidifying process.

There are several ways in which this can be accomplished. The lowest temperature at which air will hold the desired moisture quantity is the dew-point or saturation temperature corresponding to condition L and the lowest heat content possible for this condition is h_O. The process can be carried out by treating air in a spray heated to t_O where the air will be heated and saturated at t_O; it can then be heated without moisture addition to condition L. Alternatively, air can be heated to M where its wet-bulb temperature is t_O, then treated in an adiabatic spray to cool and saturate it at t_O, followed by heating to L. A third alternative is to heat air to N where the wet-bulb temperature is equal to the wet-bulb desired for condition L, then add adiabatically the exact amount of water to increase the moisture content and decrease the dry-bulb temperature to reach condition L.

If air is humidified with steam, it must first be heated so that its heat content plus the heat content of the steam exceeds h_O. Usually only one heating coil is used, in which case the air is heated to some condition P, so that the heat content at P plus the heat added by the steam produces the required heat content and temperature at L.

Cooling and Dehumidifying

Cooling and dehumidifying are also common requirements in air conditioning. When air passes through a heat exchanger or coil whose temperature is below the dew-point temperature of the air, moisture condenses and the dry-bulb temperature of the air is reduced. The heat extraction involves both the heat content of the air and the latent heat of the water vapour condensed.

The condition of air leaving a cooling coil depends on the coil design. If a water spray is added to the coil or if a cooled water spray is used, the exit air will for practical purposes be saturated at the spray-water temperature, provided only that the spray is adequate. An example is shown in Figure 4 where air at condition Q is cooled and dehumidified to condition R by passing through a sprayed coil or water spray maintained at the temperature R. The reduction in temperature, moisture content, and heat content is apparent.

Figure 4.  Cooling and dehumidifying process.

Cases often arise where dehumidifying requirements result in an air temperature too low for direct introduction into the room. Heating along a constant moisture content line to some condition S is required. This situation usually arises when both temperature and humidity in the space must be controlled and the dehumidifying load demands a saturated stream. The temperature and moisture content in a saturated stream cannot be controlled independently, however. For a given cooling load and a given dehumidifying load, there is only one possible air quantity and only one possible saturated temperature that will provide the correct deficiency in heat and the correct deficiency in moisture to maintain the desired temperature and humidity in the space.

If the heat and moisture loads change, but change in the same proportion, the air quantity can be changed or the supply temperature can be changed (within narrow limits) to maintain the desired room conditions. But if the ratio of the heat and moisture loads changes, it is no longer possible to maintain both the desired temperature and humidity. When close humidity control is required, therefore, it is usually necessary to operate the system so as to regulate the humidity level but overcool and then reheat for temperature control, as in Figure 4.

For most comfort conditioning applications close humidity control is not usually necessary and the system can be operated in such a way that space temperature is controlled and humidity allowed to vary; this avoids the requirement for reheat and results in a considerable saving in energy. In the design of the system the air quantity and the saturated temperature are usually selected so as to maintain some desired space condition at the design cooling and dehumidifying loads. At part loads the temperature is controlled and the humidity allowed to vary above and below the desired value. When both temperature and humidity must be under positive control, the complexity of the system and its controls, and thus the costs, are increased.

Conclusion

In this Digest, it has been possible to describe only in general terms the processes involved in treating air for comfort air conditioning. To assess the suitability of a given air-conditioning system for a specific building, a detailed quantitative analysis is necessary. It is only at this point that the full implications of the building and its loads become apparent.

The psychrometric chart provides a graphical representation of the properties of moist air and a means of illustrating the nature of the processes involved. It is necessary to understand these processes in order to appreciate the general capabilities and limitations of different systems, and to recognize the implications of the room conditions that are specified. An arbitrary selection of a specific temperature and humidity, without reference to the possible range of conditions that will adequately satisfy the requirements of the occupancy, may lead to unnecessarily complex and costly systems.