COMPRESSED AIR SYSTEMS

As an Engineman, you should have a thorough knowledge of air compressors. After studying the information in this chapter, you should understand that compressed air serves many different purposes aboard ship. These purposes include, but are not limited to, the operation of pneumatic tools and equipment, prairie-masker systems, diesel engine starting and speed control systems, and propulsion control systems. In this chapter, we will discuss how compressed air is supplied to the various systems by high-, medium-, or low-pressure air compressors, depending on the needs of the ship. After studying the information in this chapter, you should be able to identify the common types of air compressors used in Navy ships in terms of their design and classification, purpose, function, principles of operation, and associated equipment. You should also be able to identify the components of a compressed air system. You should be able to recognize how compressed air is prepared and delivered for use aboard ship. You should also be able to identify some of the operational and safety procedures you must use when you are working with compressed air systems.

AIR COMPRESSORS

The compressor is the heart of any compressed air system. It takes in atmospheric air, compresses it to the pressure desired, 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 in use on Navy ships will be discussed in this chapter.

Before we describe the various types of air compressors used on Navy ships, lets talk about the composition of air and some of the things air may contain. This discussion should help you to 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 earths atmosphere), air is in a gaseous form. The earths atmosphere, of course, also contains varying amounts of water. Depending upon 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 will expand when it is heated. Consequently, the heating of air will cause a given amount of air 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 air to cool and to reach the dew point, any water vapor in the air will condense into a liquid state (water). In other words, one method of drying air out is to cool it until it reaches the dew point.

In addition to 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 will 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 will pass directly through the filter material and will be compressed with the air. When air is compressed, it becomes very hot. As we mentioned earlier, hot air is capable of holding great amounts of water. The water is removed as the compressed air is routed through 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 there is a requirement to provide clean dry air that is suitable for pneumatic control and other ship-board systems, 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. Several of the more common devices used to remove water and oil vapor from the air-stream will be explained later in this chapter.