Air Conditioning

In some types of buildings, ventilation may not be adequate to maintain the optimum temperature. The amount of heat produced by the animals, the processing, outside temperatures and so on, may exceed the ability of the ventilation system to maintain the desired temperature. In these situations, air conditioners are used.

Historically many different systems were used to condition the air. Cooling with ice, pumping well water through a radiator, evaporative cooling, or drawing air through underground tubes has all been used. The effectiveness of these methods was limited. The use of ice will produce large amounts of cooling, but the resulting water must be disposed of, and a continuous supply of ice must be available. The amount of cooling available from well water is limited, and evaporative cooling is effective only if the outside air has a low relative humidity.

Today, the term air conditioning refers to conditioning air through the use of mechanical refrigeration. Refrigeration is the process of transferring heat from one substance to another; and for air conditioning, refrigeration moves heat from the air inside a structure to the air outside the structure. For this to occur, the air that is to be cooled must come in contact with a material at a temperature lower than that of the inside air. When this happens, heat from the air will flow into the colder material. Some air conditioners use a chilled metal surface to provide a cold mass, whereas others use a spray of chilled water. The heat is transferred by a substance circulating between the inside air and the outside air. In smaller units, the transfer substance will be a gas; for larger systems, the substance may be a liquid. Figure 23.7 is an illustration of the basic components of a mechanical refrigeration system.

FIGURE 23.7. Mechanical refrigeration system.

In a mechanical refrigeration system, heat is moved by alternately compressing, liquefying, expanding, and evaporating a refrigerant, commonly Freon. The compressor increases the pressure and the temperature of the refrigerant as it compresses the gas. As the refrigerant passes through the condenser, the heat absorbed by the refrigerant as it passed through the evaporator, and the heat produced by compression, is transferred into the air. Thus, the condenser will be located where outside air moving across it absorbs the heat causing the refrigerant to liquefy. As the liquid refrigerant flows through the expansion valve and into the evaporator, the pressure drops and the refrigerant absorb heat from the surrounding air. The evaporator will be located inside an air duct or other location where air can pass through it and be cooled, causing the refrigerant to be vaporized. The hot, low pressure gas flows back to the compressor and the cycle begins again. The expansion valve and a thermal bulb regulate the flow of the refrigerant to produce the desired evaporator temperature.

It is common for mechanical refrigeration air conditioning to lower the temperature 20 to 30 degrees. Often this results in an air temperature that is below the dew point. This is the reason building and automobile air conditioners produce water. This can be shown on a psychrometric chart.

Problem: How many pounds of water will be removed per pound of dry air when air at 95°Fdb and 70% rh is cooled to 75°Fdb?

Solution: The solution is mapped out in Figure 23.8. Locate the initial state point on a psychrometric chart. Air conditioning lowers the dry-bulb temperature, so move left to 75 degrees. As the air is cooled, the relative humidity increases and in this example the saturation point is reached before 75 degrees. In this situation, follow the saturation line down to 75 degrees. This is the second state point. The amount of water removed is the difference in the humidity ratio between state point one and state point two.

lb H2O lb H2O lb H2O lb H2O

lb dry air lb dry air lb dry air lb dry air

Evaporative cooling is still useful for reducing the temperature of air in climates where the relative humidity is low. An evaporative cooler pulls outside air through a wet pad of porous material. As the air passes through the pad the temperature of the air causes water to evaporate, lowering the temperature, and raising the relative humidity. Figure 23.9 illustrates the components of a window or portable unit. The larger systems used in greenhouses and other structures have similar components. The principles of evaporative cooling can be demonstrated using a psychrometric chart.

Problem: Determine the amount of reduction in the temperature of air that is at 100°Fdb and 40% rh after it passes through an evaporative pad and becomes 90% saturated.

Solution: Figure 23.10 shows the solution mapped out on the psychrometric chart. Start by locating state point number one. In evaporative cooling, water is converted from liquid to a vapor. This requires heat. The heat for evaporation is drawn from

Pressure Enthalpy Diagram Carbon Dioxide

Dry buib temperature

Figure 23.8. Solution for air conditioning problem.

Dry buib temperature

Figure 23.8. Solution for air conditioning problem.

Water reservoir FIGURE 23.9. Evaporator cooler.

the air, which lowers the dry-bulb temperature. Lowering the dry-bulb temperature is represented by moving horizontally to the left from state point number 1. But evaporative cooling also adds water vapor to the air. Adding water vapor to the air is represented by moving vertically up the chart from state point number 2. The result is a vector problem. The best estimate of the direction to move using the chart is to follow the wet-bulb line to the left and up until the second state point is reached, 90% relative humidity. In this example, evaporative cooling lowers the temperature from 100°Fdb to 81°Fdb, a difference of 19°F.

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