The power input to the compressor is converted to heat in the compression process. This heat is removed by a cooling system for two primary reasons: (1) to prevent the compressed air and various compressor parts from reaching excessively high temperatures, and (2) to improve the efficiency of multistage compressors by increasing the density of air between stages of compressions.
In reciprocating compressors, the compression cylinders are cooled by fresh water or seawater, which is circulated through cooling water passages in the cylinder block. When removable cylinder
Figure 14-8.-Lubricating oil system of a low-pressure air compressor.
liners or cylinder sleeves are used, the cylinder block may incorporate wells or bores for the liners so that the cooling water does not come into direct contact with the cylinder liners (dry liners). In another design, the liner may be held in shoulders with O-ring seals within the cylinder block so that the cylinder liners are “wetted” by the cooling water (wet liners). Cylinder jackets are fitted with handholes and covers so that the water spaces may be inspected and cleaned. On many compressors, water passes directly through the joint between the cylinder and the head. On such designs, extreme care must be taken so that the joint is properly gasketed to prevent leakage. If allowed to continue, water leakage would cause corrosion problems or more severe damage.
In addition to cylinder cooling, each stage of a reciprocating compressor has an air cooler in which the discharge air is cooled before it enters the next stage. The coolers are usually of the shell and tube design. Compressed air is directed through the tubes of the cooler, with the cooling water flowing through the shell and over the tubes. On some compressors, this design may be reversed on the low-pressure stages (first and second) so that the cooling water flows through the tubes and the air through the shell. The coolers between stages are called INTERCOOLERS. The last cooler is the AFTERCOOLER. (Intercoolers and aftercoolers will be discussed in greater detail later in this chapter.)
You may encounter several different types of cooling systems, depending, of course, on the ship to which you are assigned. On older air compressors, cooling is provided by a seawater system that serves the compression cylinders as well as the intercoolers and after-cooler. (See fig. 14-9.) In these systems, the seawater flows essentially in a series arrange-ment- first through the intercoolers and after-cooler, and then through the compression cylinders. This process ensures that the air entering the cylinders is always cooler than the valve chambers and cylinders. Therefore, moisture from condensation is minimized. However, with seawater temperatures substantially lower than 85°F, condensation may occur within the cylinders and discharge valve chambers (or cylinder heads) on the compression stroke. Also,
Figure 14-9.-Seawater cooling (open) system of a multistage air compressor.
because of the compact design of some air compressors, low seawater temperatures can cause the compressor frame and oil sump to reach temperatures that are sufficiently low to cause condensation within the oil sump. This con-dition can cause rapid water buildup and subsequent bearing failure. For protection against the overcooling of compressors that are cooled entirely by seawater, it is recom-mended that cooling water be throttled. This action will reduce the cooling effect in the compression cylinders. However, excessive re-duction in cooling water flow can result in hot spots in the cylinder areas where flow under normal conditions becomes marginal. In this regard, you must follow the recom-mendations in the NAVSEA technical manual for any specific compressor.
The majority of compressors, including oil-free compressors on surface ships, employ seawater cooling systems for the intercoolers and aftercoolers and a secondary freshwater system for cylinder cooling. (See fig. 14-10.) The closed freshwater cooling system consists of a pump, a surge tank, a thermostatic valve, and a heat exchanger. The pressurized coolant is moved from the surge tank by the water pump and is directed to each cylinder assembly. A high-water-temperature shutdown switch downstream from the cylinder assemblies monitors the coolant temperature. Coolant at the thermostatic valve flows either directly to the cylinders or through the heat exchanger if the cylinder water requires cool-ing. The design provides a constant rate of flow and thermostatic temperature control in the cylinder cooling system. This type of control ensures uniform operating condi-ions for the compression stages and helps the system avoid the harmful excessive cool-ing of cylinders. As we mentioned earlier, excessive cooling causes condensation on cylinder walls-a condition that results in early cat-astrophic seal failure.
The intercoolers and aftercoolers act to remove heat that is generated whenever air is compressed. They also cause any water vapor that may be present in the airstream
Figure 14-10.-Cooling water flow diagram showing seawater and freshwater systems.
to condense into a liquid. Figure 14-11 is a diagram of a basic cooler and separator unit. Intercoolers and aftercoolers are normally fitted with moisture separators. Moisture sep-arators, which come in a variety of designs, serve to remove the condensed moisture and oil vapor from the airstream. The liquid is removed by centrifugal force, impact, or sudden changes in velocity of the airstream. Notice that the condensate collects in the lower section of the outlet header. Drains on each separator serve to remove the water and oil. The condensate must be drained at regular intervals to prevent carryover into the next stage of compression. When the condensate accumulates at low points, it may cause water hammer or freezing and bursting of pipes in exposed locations. It may also cause faulty operation of pneumatic tools and diesel engine air start systems, and possible damage to electrical apparatus when air is used for cleaning. The removal of heat is also necessary for efficient compression. During compression, the temperature of the air increases. As we explained earlier, heated air expands to a larger volume. The larger volume of air requires a corresponding increase of work to compress it. Multistaging, therefore, with interstage cooling of the air, reduces the power requirement for a given capacity.
Interstage cooling reduces the maximum temperature in each cylinder. This temperature reduction, of course, reduces the amount of heat that must be removed by the water jacket at the cylinder. Figure 14-12 illustrates the pressures and temperatures through a four-stage compressor. The intercoolers and the aftercoolers (on the output of the final stage) are of the same general construction. The exception is that the after-coolers are designed to withstand a higher work-ing pressure than that of the intercoolers. Both intercoolers and aftercoolers are generally fitted with relief valves on both the air and the water sides. The relief valves must be set according to PMS.
In all compressors fitted with thermostatically controlled freshwater cooling systems, such as the one shown in figure 14-10, the possibility of over-cooling the cylinders occurs only when the ther-mostat is malfunctioning. The air cannot be cooled too much in the intercoolers and aftercoolers. There are no detrimental effects for low air discharge temperatures from the coolers. The more the air is cooled in the intercoolers, the lower the brake horsepower will be. When more water is condensed within the cooler, fewer chances exist for the water to condense during the subsequent compression stroke.
Oil coolers may be of the coil type, shell and tube type, or a variety of commercial designs. Although external oil coolers are generally used, some compressors are fitted with a base-type oil cooler. In this design, cooling water circulates through a coil placed in the oil sump. On most compressors, the circulating water system is arranged so that the amount of cooling water passing through the oil cooler can be regulated without disturbing the quantity of water passing through the cylinder jackets, intercoolers, or after-coolers. Thermometers are fitted to the circulating water inlet and outlet connections, the intake
Figure 14-11.-Basic cooler and separator.
Figure 14-12.-Pressure and temperature resulting from multistaging and interstage cooling.
and discharge of each stage of compression, the final air discharge, and the oil sump.