AIR CONDITIONING

Air conditioning is a field of engineering that deals with the design, construction, operation, and maintenance of equipment used to establish and maintain desirable indoor air conditions. It is used to maintain the environment of an enclosure at any required temperature, humidity, and air quality. Simply stated, air conditioning involves the cooling, heating, dehumidifying, ventilating, and cleaning of air. This chapter deals with that process.

THE PURPOSES OF AIR CONDITIONING AND RELATED FACTORS

One of the chief purposes of air conditioning aboard ship is to keep the crew comfortable, alert, and physically fit. None of us can long maintain a high level of efficiency under adverse environmental conditions. We have to maintain a variety of compartments at a prescribed temperature with proper circulation. They must have proper moisture content, the correct proportion of oxygen, and an acceptable level of air contamination (dust, airborne dirt, and so forth). The comfort and fitness of the crew is only one purpose of air conditioning. Mechanical cooling or ventilation must also be provided in ammunition spaces to prevent deterioration of ammunition components and in gas storage spaces to prevent excessive pressure buildup in the metal bottles and contamination of the space by gas leakage. Finally, we must provide cooling and ventilation in electrical/electronic equipment spaces. This is done to maintain the ambient temperature and humidity as specified for the equipment.

Proper air conditioning must consider the humidity, total heat of the air, temperature, body heat balance, effect of airflow, and physical sensation of comfort.

HUMIDITY

Humidity is the vapor content of the atmosphere; it has a great influence on human comfort. The common expression It isnt the heat, its the humidity, indicates discomfort produced by moisture-laden air in hot weather. Extremely low moisture content also has undesirable effects on the human body. The measurement and control of moisture in the air is an important phase of air conditioning. To understand this phase, you should become familiar with the meaning of saturated air, absolute and specific humidity, and relative humidity.

Saturated Air

Air can hold varying amounts of water vapor. It depends on the temperature of the air at a given atmospheric pressure. As the temperature rises, the amount of moisture the air can hold increases (assuming no change in atmospheric pressure). But for every temperature there is a definite limit to the amount of moisture the air can hold. When air contains the maximum amount of moisture it can hold at a specific temperature and pressure, it is said to be saturated.

The temperature at which air becomes saturated with water vapor is called the DEW POINT. If the temperature of air falls below its dew point, some of the vapor in the air must condense to water. An example is the dew that appears on decks and bulkheads in the early morning. This normally happens when there is a drop in temperature. Another is the sweating of cold water pipes as water vapor from the relatively warm air condenses on the cold surface of the pipes.

Absolute and Specific Humidity

The amount of water vapor in the air is ex-pressed in terms of the weight of the moisture. The weight is usually given in grains (7,000 grains = 1 pound). ABSOLUTE HUMIDITY is the weight in grains of water vapor per cubic foot of air. SPECIFIC HUMIDITY is the weight in grains of water vapor per pound of air. (The weight of water vapor refers only to moisture that may be present in the liquid state, such as rain or dew.)