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15
INTERCONNECTION OF SYSTEMS
 
A. RELATION OF CAPACITIES
 
15A1. Necessity for compressors of different capacities. The air-conditioning system and the refrigeration system aboard a submarine are designed as two separate and distinct systems. Each is capable of performing its task independent of the other. However, in practice it is desirable that these two systems be interconnected so that the air-conditioning compressor can serve as a standby for the refrigeration system. This insures continuous operation of a compressor in the refrigerating system; otherwise, spoilage of the foodstuffs stored in the refrigerating rooms might result.

In earlier references and explanations within this text, the rated capacity of the refrigeration system compressor has been given as 1/2 refrigeration ton, while the rated capacity of each of the air-conditioning system compressors is given as 4 refrigeration tons. This difference in rated capacity of the two units results from the fact that the air-conditioning system performs a greater amount of work than does the refrigeration system.

The refrigeration system must perform sufficient work to remove heat from the comparatively small space of the cool room and refrigerating room, with the minor addition of the ice tuber.- Since these two rooms are thoroughly insulated, little or no heat enters them from the outside. The only source of heat inside the rooms is from the stowed foodstuffs and from persons entering for supplies. On the other hand, the air-conditioning system must remove heat generated throughout the vessel, that is, heat that passes into the air of the vessel from the crew, engines, cooking, batteries, electric light bulbs, equipment, and at times from the surrounding water outside the hull.

These requirements determine the work load of each system. This work load expressed in refrigeration tons determines the capacity of the compressor needed for each system.

15A2. Relation of capacities. The capacity

  of the compressor is the amount of work, expressed in refrigeration tons, that a compressor is capable of doing under a given set of operating conditions. A change in the operating conditions causes a corresponding change in the rated capacity of the compressor. Therefore, the relation between the capacity of a compressor on a refrigeration system and a compressor on an air-conditioning system is a comparison of the operating conditions of evaporator temperature, the speed of the compressor, and the temperature of the cooling medium for each system, and not a comparison of the compressor or its maximum work load under optimum conditions.

The misunderstanding of this relationship has often given rise to the question: Is there a difference between a ton of air-conditioning and a ton of refrigeration? The cause of this question is the apparent increase in the capacity in refrigeration tons developed by a compressor on an air-conditioning unit over the capacity developed by the same compressor when operating on a refrigerating unit. It would appear that there were a difference between a refrigeration ton and an air-conditioning ton. Actually there is no difference and the term air-conditioning ton is not in common usage. The basic rating of a refrigeration or air-conditioning system in refrigeration tons is a measure of the heat-removing capacity of the system and determines the rate at which heat is removed from a substance in a refrigerator and the rate at which heat and humidity are removed from air. However, a compressor rated at 2.95 refrigeration tons when operated at -5 degrees F evaporator temperature, running at 600 rpm with the same temperature of cooling water in the condenser in both cases, has a rating of 8.348 refrigeration tons if it is operated at a 35 degrees F evaporator temperature (see the following tables). Thus the rating of a compressor may vary with the evaporator temperature; also the rating of a compressor

 
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may vary with the speed of the compressor and the temperature of the cooling water flowing through the condenser.

The operating temperature of the evaporator (suction pressure) has the greatest effect on the number of refrigeration tons that the compressor may develop. Refer to the table of properties of saturated Freon 12 vapor, Section 5D1. This table illustrates why the capacity of a compressor varies with the evaporator temperature (suction pressure), with constant compressor speed and condensing temperature. For example, at 0 degrees F the density of 1 pound of Freon 12 vapor is 0.6109 and at 40 degrees F the density is 1.263. In other words, the compressor handles approximately twice the gas at 40 degrees F that it does at 0 degrees F. Therefore, more refrigeration tons are developed at the higher evaporator temperature (suction pressure). The following tables illustrate this fact and also the effect of varying temperatures of the cooling water.

The figures given in the tables are obtained from engineering tests and computations.

The functional reason for this variation in capacity is that the capacity of a refrigeration machine is influenced by the operating pressures. The capacity increases as the suction pressure increases. The capacity decreases as the discharge pressure increases. The power

  requirement also increases under these same pressure conditions, although, as seen in the table for the 4 x 4 inch compressor, the relative power per refrigeration ton decreases.

Since Freon 12 has a positive pressure temperature relationship, it may also correctly be said that the capacity increases as the evaporator temperature increases. Tables such as the previous ones are given in terms of the evaporator temperature, rather than suction pressure, because the temperature of the space cooled by the evaporator is the primary consideration.

The table does not give decrease of capacity against increase of discharge pressure, but against increase of condenser water temperature. The discharge pressure depends upon the temperature of the condensing water, as stated in Section 6B5. The condensing water temperature is of primary interest because it varies according to geographical location and season of the year. Therefore, although increase in discharge pressure decreases the capacity, it may be said that the capacity decreases as the condenser water temperature increases.

The discharge pressure also varies according to the speed of the compressor, but the usual practice is to operate the compressor at a relatively constant speed.

 
COMPARISON OF REFRIGERATION AND AIR-CONDITIONING COMPRESSOR RATINGS
CHARACTERISTICREFRIGERATIONAIR-CONDITIONING
Bore2 5/8 inches4 inches
Stroke2 1/2 inches4 inches
Cylinders2.2.
Displacement27.10100.50
Rpm600. 600.
TR at -5 degrees F evap. temp.0.7422.95
TR at 35 degrees F evap. temp.2.178.348
Bhp at 5 degrees F evap. temp.1.896.94
Bhp at 35 degrees F evap. temp.2.428.95
 
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VARIATION IN COMPRESSOR RATINGS IN TR AND BHP
FOR 4 X 4 INCH, 2-CYLINDER COMPRESSOR
Evaporator
Temperature
Condensing Water Temperature in Degrees Fahrenheit
90 deg. 95 deg. 100 deg. 105 deg. 110 deg.
TrBhp TrBhp TrBhp TrBhp TrBhp
-20 deg. 2.2 5.9 2.1 6.0 2.0 6.1 1.8 6.2 1.7 6.3
-15 deg. 2.6 6.3 2.4 6.4 2.3 6.5 2.1 6.7 2.0 6.8
-10 deg. 3.0 6.7 2.9 6.8 2.1 7.0 2.5 7.1 2.4 7.2
-5 deg. 3.5 7.0 3.4 7.2 3.2 7.3 3.0 7.5 2.8 7.7
0 deg. 4.2 7.5 4.0 7.6 3.8 7.8 3.5 8.0 3.3 8.2
5 deg. 4.8 7.8 4.6 8.0 4.4 8.2 4.1 8.4 3.9 8.6
10 deg. 5.6 8,0 5.3 8.3 5.0 8.5 4.8 8.7 4.5 8.9
15 deg. 6.3 8.3 6.0 8.6 5.7 8.8 5.4 9.1 5.1 9.3
20 deg. 7.1 8;5 6.7 8.8 6.4 9.1 6.1 9.4 5.7 9.7
25 deg. 7.9 8.6 7.5 9.0 7.2 9.3 6.8 9.7 6.5 10.0
30 deg. 8.8 8.7 8.4 9.1 8.0 9.5 7.6 9.9 7.3 10.3
35 deg. 9.8 8.8 9.4 9.2 9.0 9.6 8.6 10.1 8.2 10.5
40 deg. 10.9 8.7 10.5 9.2 10.1 9.7 9.7 10.2 9.3 10.7
45 deg. 12.2 8.7 11.7 9.2 11.3 9.8 10.8 10.3 10.4 10.9
Ratings at 650 rpm. To obtain capacity rating of any intermediate speed proportion directly to compressor rpm.
 
B. INTERCONNECTION OF SYSTEMS
 
15B1. Cross-connection of air-conditioning and refrigeration systems. In an emergency necessitating the securing of the refrigerating compressor, it is possible to cross-connect at least one of the air-conditioning compressors, condensers, and receivers to the refrigerating system evaporator, and maintain the desired temperatures in the refrigeration rooms. On some classes of vessels, either of the air-conditioning compressors may be cross connected to the refrigeration system; on other classes only the No. 1 air-conditioning compressor can be used. As the arrangement and location of valves and lines vary on each installation, no detailed description can be given here. It is never necessary or desirable   to cross-connect the refrigerating compressor to the air-conditioning evaporators.

In cross-connecting the air-conditioning compressor to the refrigerating evaporator, several major adjustments must be made on all installations. Reset the low-pressure cutout switch on the air-conditioning compressor so that it does not stop the compressor until the suction pressure drops down to 2 psi. Normally this cutout is set to stop the compressor when the suction pressure on the air-conditioning evaporator reaches 32 psi. If the compressor is to operate both the air-conditioning system and the refrigerating system, the bypass around the suction pressure regulating valve should be closed, the stops opened, and

 
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the valve cut into the system. With the suction regulating valve in operation, a 32-psi suction pressure in the air-conditioning evaporator is maintained while the compressor is operating at the lower suction pressure necessary on the refrigerating system. The operation of the two systems in this manner is desirable because the capacity of the air-conditioning compressor is much more than is needed to maintain the refrigerating rooms at the desired temperatures.

Another point that must be checked is that electric current is supplied to the thermostatic control circuit on the refrigeration system; otherwise, the solenoid valves remain closed and no refrigerant flows through the system. On some vessels, the thermostatic control circuits are energized from the 110-volt lighting circuit. In this case, the main switch to the refrigeration compressor may be pulled and the thermostatic control circuits energized through the refrigerating control panel. When the main switch is pulled on the refrigerating compressor, it interrupts the supply of current to the thermostatic control circuits. In this case, the following procedure should be followed:

  Leave in the main switch supplying current to the refrigerating compressor. With a wooden pencil or other insulated material, lift the overload relay cutout located on the bot tom left side of the half-ton compressor control panel, to the OFF position, making sure that the overload relay cutout stays up. Then turn the selector switch on the half-ton system to either MANUAL or AUTOMATIC. This insures a supply of current to the thermostatic controls.

15B2. Cross-connection of air-conditioning systems. On some classes of vessels, it is possible to operate the No. 1 compressor and condenser on the No. 2 evaporator, and vice versa. The air-conditioning systems on the 300 class submarines are cross-connected by the compressor discharge lines and the high-pressure liquid lines only. There is no cross-connection between the suction lines. Because of this arrangement, in the air-conditioning system the No. 1 compressor can be connected to the No. 2 condenser, and the No. 2 compressor can be connected to the No. 1 condenser. But the No. 1 compressor cannot be connected to the No. 2 evaporator, or the No. 2 compressor to the No. 1 evaporator.

 
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