Almost all working and living spaces on newer ships are air conditioned. The equipment used on these ships was carefully tested to see which types would best dehumidify and cool ship compartments. Two basic types of equipment have been found most effective and are now in general use. They are chilled water circulating systems and self-contained air conditioners.

CHILLED WATER CIRCULATING SYSTEMS

TWO basic types of chilled water air-conditioning systems are now in use. They are a vapor compression unit and a lithium bromide absorption unit. In the vapor compression unit, the primary refrigerant cools the secondary refrigerant (chilled water) that is used to cool the spaces. This type uses the vapor compression cycle and R-11 or R-114 as the primary refrigerant. The type of primary refrigerant depends on the size and type of compressor. The lithium bromide unit operates on the absorption cycle and uses water as the primary refrigerant. Lithium bromide is used as an absorbent.

Vapor compression plants are used in most ships. However, lithium bromide plants are used in submarines because they require no compression, which means a quieter operation.

Vapor Compression Units

The vapor compression chilled water circulating system differs from a refrigerant circulating (direct expansion) air-conditioning system. In vapor compression chilled water circulating systems, the air is conditioned by using a secondary refrigerant (chilled water) that is circulated to the various cooling coils. Heat from the air-conditioned space is absorbed by the circulating chilled water. Heat is then removed from the water by the primary refrigerant system in the water chiller. In large ton vapor compression systems, the compressor is a centrifugal type that uses R-11 or R-114 as the primary refrigerant.

The operating cycle of the centrifugal refrigeration plant fig. 10-18 is basically the same as other refrigeration plants except for the method of compression. The refrigerant gas is pressurized in the centrifugal turbocompressor. This then is discharged into the condenser where it is condensed by circulating seawater flowing through the condenser tubes. The condensed liquid refrigerant drains to the bottom of the condenser into a float chamber. When the refrigerant level is high enough, a float-operated valve opens. (NOTE: In some R-11 units, an orifice is installed instead of a float valve.) This allows the liquid high-pressure refrigerant to spray out into the water chiller (evaporator). Water to be chilled flows through the tubes of the water chiller. As the refrigerant from the condenser sprays out over the tubes, the water within the tubes is chilled or cooled due to the vaporization of the liquid refrigerant. Then, the vaporized refrigerant reenters the suction side of the compressor to start the cycle again.

Figure 10-18.-Vapor compressor (centrifugal) unit.

The load on the air-conditioning plant is determined by the desired chilled water temperature. The compressor load is changed by either an increased or decreased demand of the chilled water temperature. Upon demand, the load is changed by the use of adjustable prerotation vanes. The vanes are located on the suction side of the compressor. The vanes act as dampers to increase or decrease the flow of refrigerant vapor into the suction of the compressor. This throttling action at the compressor suction allows an increase or decrease of the capacity of the compressor without changing the compressor speed.

Figure 10-19 shows a centrifugal compressor with the inlet piping removed. Note that the prerotation vanes are in the fully open position. The vane position is normally controlled automatically through an electropneumatic control system. The control system senses and maintains the chilled water outlet temperature of the chiller at a preset value by varying the position of the vanes.

In some plants, the electric motor used In some plants, the electric motoerrive the compressor is hermetically sealed and is cooled by a flow of refrigerant through it. The compressor is lubricated by a force-feed lubrication system. This system normally consists of an auxiliary oil pump, an attached oil pump (integral with compressor), an oil cooler, and a set of oil filters. The auxiliary oil pump is used for starting and securing the plant.

Several automatic controls are built into the centrifugal compressor control system. These devices increase the self-operating ability of the plant by automatically shutting down the compressor if a hazardous condition develops. Some of these conditions are high condenser pressure, low compressor lube oil pressure, seawater loss to the condenser, loss of chilled water, low refrigerant temperature, low chilled water temperature, and high discharge temperature.

Figure l0-19.-Suction end of a centrifugal compressor showing prerotation vanes.

An oil heater keeps the oil warm in the oil sump of the compressor during plant shutdown. If the oil is not kept heated, it absorbs large amounts of refrigerant. This results in excessive oil foaming when the unit is started. The heaters in most plants are connected so that they are automatically turned on when the compressor is off, and off when the compressor is on..

Figure 10-20 shows a centrifugal compressor air-conditioning unit. This particular plant has a 150-ton capacity and uses R-114 as the refrigerant. The gauges and controls for the plant are on the other side of the unit.

Lithium Bromide Absorption Unit

Water is used as a refrigerant in the lithium bromide absorption cycle. The absorption system differs from the compression-type refrigeration machines. The absorption cycle uses heat energy instead of mechanical energy to cause the change in conditions necessary for a complete refrigeration cycle. In other words, the compressor is replaced by steam heat.

The following are the two principles that form the basis for the lithium bromide absorption refrigeration cycle:

1. Lithium bromide has the ability to absorb large quantities of water vapor.

2. When under a high vacuum, water boils (vaporizes) at a low temperature and, in doing so, absorbs heat.

To understand the lithium bromide absorption cycle, follow along on figure 10-21 during as you read the following explanation. Notice that the EVAPORATOR and ABSORBER sections are in a common shell. The sections are separated by the refrigerant tray and baffles. This shell is under a high vacuum of about 29.8 in.Hg. Water boils at 35F (1.7C) at this pressure. (Note that this is only 3F above the freezing point of water.) The refrigerant pump circulates the refrigerant (water) through the evaporator. The water is sprayed out over 88the chilled water tubes through a spray header. This causes the water to vaporize (or flash) more readily. As the water vaporizes around the chilled water tubes, it removes heat from the circulating chilled water. The water vapor is floating about in the evaporator/absorber shell. Now, the absorber comes into play.

Lithium bromide solution is sprayed out from a spray header in the absorber. The absorber pump provides the driving head for the spray. As the lithium bromide solution is sprayed out, it absorbs the water vapor, which is in the shell from the evaporation process. As the lithium bromide absorbs more and more water vapor, its ability to absorb decreases. This is

Figure 10-20.-R-114 centrifugal air-conditioning plant.

Figure 10-21.-Basic absorption cycle.

known as a WEAK solution. Here, in the generator section of the plant, the weak solution is rejuvenated for reuse as a STRONG solution. The generator pump pumps the weak solution from the weak solution section of the absorber up to the generator.

In the generator, the weak lithium bromide solution is sprayed out over steam tubes that heat the solution and drive the water vapor out of the solution. The strong solution thus produced flows back into the absorber for reuse. The water vapor driven out of the solution flows from the generator into the condenser where it is condensed by circulating seawater for reuse as a refrigerant. The condensed vapor flows into the evaporator and down to the refrigerant tray.

A regenerative heat exchanger is provided in the system for the lithium bromide solution. The weak solution must be heated to drive out the water vapor; the strong solution must be cooled to absorb water vapor.

The regenerative heat exchanger aids in this process by cooling the strong solution and preheating the weak solution in the cycle.

Seawater (condensing) flow is provided through the absorber section. It cools the strong solution returning from the generator and removes the heat produced as the lithium bromide solution absorbs the water vapor. The outlet seawater from the absorber is the inlet water for the condenser.

The absorber pump and the generator pump are driven by a common electric motor. Therefore, the two pumps are referred to cumulatively as the absorber/generator pump.

A purge system (not shown) consists of a pump, an eductor, and a purge tank. The system is provided with the lithium bromide absorption system to keep air and noncondensables out of the evaporator/absorber shell.

The maintenance of the high vacuum within the shell is important to the proper operation of the plant.

Fan-Coil Assemblies

Fan-coil assemblies (fig. 10-22) use chilled water to air-condition spaces. These assemblies are known as spot coolers. The chilled water is piped through the cooling coils of the units, and a fan forces air over the coils. Note the chilled water connections, the vent cock at the top, and the condensate collection tray at the bottom of the unit.

The condensate collection tray collects the moisture condensed out of the air. The condensate is generally piped to the bilge or a waste water drain system. It is important that the drain for the collection tray be kept clear. If the condensate cannot drain out of the tray, it collects and evaporates, leaving impurities that can rapidly cause the tray to corrode.