Ships without central air conditioning may use self-contained air-conditioning units. Naval Sea Systems Command (NAVSEASYSCOM) approval is required. A self-contained air-conditioning unit is simply the type of air conditioner you see installed in the windows of many homes. All that is required for installation is to mount the proper brackets for the unit case and provide electrical power.

These units use nonaccessible hermetically sealed compressors (motor and compressor are contained in a welded steel shell). For this reason, shipboard maintenance of the motor-compressor unit is impractical. The thermal expansion valve used in these units is preset and nonadjustable. However, a thermostat and fan speed control are normally provided for comfort adjustment.

AIR COMPRESSORS

The air compressor is the heart of any compressed air system. It takes in atmospheric air, compresses it to the desired pressure, and moves it into supply lines or into storage tanks for later use. Air compressors come in different designs and configurations and have different methods of compression. Some of the most common types used on gas turbine ships are discussed in this chapter.

Before describing the various types of air compressors, you need to know about the composition of air and some of the things air may contain. This discussion should help you understand why air compressors have special features that prevent water, dirt, and oil vapor from getting into compressed air piping systems.

Air is mostly composed of nitrogen and oxygen. At atmospheric pressure (within the range of temperatures for the earth's atmosphere), air is in a gaseous form. The

Figure 10-22.-Fan-coil assembly.

earth's atmosphere also contains varying amounts of water. Depending on weather conditions, water will appear in a variety of forms, such as rain (liquid water), snow crystals, ice (solid water), and vapor. Vapor is composed of tiny drops of water that are light enough to stay airborne. Clouds are an example of the existence of water vapor.

Since air is a gas, it expands when heated. Consequently, heating air causes a given amount of air2routed through to expand, take up more space (volume), and hold more water vapor. When a given amount of air at a given temperature and pressure is no longer able to soak up water vapor, the air is saturated, and the humidity is 100 percent.

When air cools, its density increases; however, its volume and ability to hold water decrease, When temperature and pressure conditions cause the air to cool and to reach the dew point, any water vapor in the air condenses into a liquid state (water). In other words, one method of drying air out is to cool it until it reaches the dew point.

Besides nitrogen, oxygen, and water vapor, air contains particles of dust and dirt that are so tiny and lightweight that they remain suspended in the air. You may wonder how the composition of air directly affects the work of an air compressor. Although one cubic foot of air will not hold a tremendous amount of water or dirt, you should realize that air compressors have capacities that are rated in hundreds of standard cubic feet per minute (cfm). This is a very high rate of flow. When a high flow rate of dirty, moisture-laden air is allowed to enter and pass through an air compressor, the result is rapid wear of the seals and load-bearing parts, internal corrosion (rust), and early failure of the unit. The reliability and useful life of any air compressor is extended by the installation of filters. Filters remove most of the dirt and dust from the air before it enters the equipment. On the other hand, most of the water vapor in the air at the intake passes directly through the filter material and is compressed with the air. When air is compressed, it becomes very hot. As you know, hot air is capable of holding great amounts of water. The water is removed as the compressed air is routed through the coolers. The coolers remove the heat from the airstream and cause some of the water vapor to condense into liquid (condensate). The condensate must be periodically drained from the compressor.

Although the coolers will remove some of the water from the air, simple cooling between the stages of compression (intercooling) and cooling of the airstream after it leaves the compressor (aftercooling) will not make the air dry. When clean dry air suitable for pneumatic control and other shipboard systems are required air from the compressor is routed through air-drying units. Many air-drying units are capable of removing enough water vapor from the airstream to cause the dew point to be as low as -60F. Some of the more common devices used to remove water vapor from the airstream, such as dehydrators, are discussed later in this chapter.