19A1. Heat gains inside a submarine. With all the discussed facts in mind, the general picture of air-conditioning in a submarine may now be considered. Let it be assumed that the air in question contains enough oxygen for the needs of all the occupants within the given space. A supply of oxygen compressed in cylinders is carried on board submarines to be released into the air if this need arises.

There is continuous production of heat, given off to the air by hot engines, storage batteries, galley stove, electric lights, electric heaters, other devices, and human occupants.

19A2. Moisture gains inside a submarine. There is also continuous production of moisture, given off to the air by evaporation from four main sources: 1) storage batteries, 2) cooking; 3) human occupants, and 4) the bilges. This production of moisture averages about 1000 pounds of water per day under ordinary conditions.

19A3. Elimination of heat and moisture. If the submarine is running on the surface, it is an easy matter to discharge this excess heat and moisture outboard. But if the submarine is submerged, it cannot be discharged outboard, and must be eliminated during recirculation of the air.

19A4. Transfer of heat between submarine and ocean. A submarine is practically all metal, and metal is an excellent conductor of heat. In addition, the surface area of a submarine is fairly large. Moreover, the ocean is filled with convection currents, either natural currents or currents caused by the vessel's motion. These facts combine to make the heat transfer between submarine and ocean an active process. Even when the temperature difference between the vessel and the ocean is small, the total heat transfer is considerable because of the large contact area.

19A5. Loss of heat from ship to ocean. If

  the submarine is warmer than the ocean water, the interior heat of the vessel gradually passes through the shell into the water. The temperature of the air inside drops and, when it reaches the dewpoint, the water vapor in the air begins to condense on every available surface. For this reason, the interior surfaces of a submarine are coated, wherever practicable, with cork paint, to prevent or reduce this condensation to a minimum. The beginning of the condensation, however, depends upon the dewpoint, and this can be controlled by air-conditioning. Therefore, air-conditioning is just as essential for this purpose as for the comfort of the crew. It is always advisable to lower the dewpoint before a dive, if possible.

19A6. Gain of heat from ocean to ship. On the other hand, if the ocean is warmer than the submarine, there is a passage of heat from the water into the ship., However small this may be, it adds to the interior heat. The same result occurs if the loss of heat from this vessel to the ocean is less than the interior heat production. In hot summer weather, especially in tropical and subtropical regions, the air temperature in a submarine may rise to fairly high levels.

19A7. Refrigeration capacity of a submarine. In a submarine, only a few cubic feet of space are available for air-conditioning machinery, and the cooling capacity is of necessity limited. It should be clearly understood that the purpose of the air-conditioning system is not to cool the submarine as a whole.

19A8. Humidity in a submarine. However, there is room enough for sufficient air-conditioning machinery to control the dewpoint, that is, to set it at any desired temperature. Control of the dewpoint means control of the relative humidity, and it is the relative humidity, more than the mere temperature of


the air, that causes discomfort. With a build up of a thousand pounds of moisture per day   in the air of a submarine, relative humidity becomes a factor of major importance.
19B1. Lowering the dewpoint. The dewpoint of a sample of air is the temperature at which that air is saturated with moisture. If the temperature of the air is then further lowered, some of the moisture must condense out.

Suppose that the air in a room is so high in relative humidity as to be uncomfortable. The method of reducing this relative humidity is as follows:

The air in the room is drawn by fans into and through the ducts to the air-conditioning evaporator. There, in passing over the cooling coils, its temperature drops below its dewpoint and part of its water vapor condenses out, inside the evaporation cabinet. The condensed water is not permitted to get back into the air, but is drained off into a tank. The air, now lower in moisture content and slightly lower in temperature also, continues its flow through the ducts and is finally blown out into the room again. This conditioned air mixes with the moisture and warmer air still in the room, resulting in an over-all condition that is drier than the original unconditioned air.

19B2. Heat action in the evaporator. When some of the moisture in the air is condensed out of it in the evaporator, the condensation is caused by the passage of latent heat from the water vapor in the air to the refrigerant inside the cooling coils. This removal of latent heat reduces the total heat of the humidity of the air. Refer again to the psychrometric chart. While the lowering of the dry-bulb temperature by itself means an increase in relative humidity, this increase resulting from the lowering of -total heat, or wet-bulb temperature, is always less in the range of high air temperatures within which high relative humidity is uncomfortable. The resultant relative humidity is therefore always decreased.

19B3. Importance of dewpoint. It becomes evident that in air-conditioning the significant viewpoint is not the dry-bulb or ordinary air temperature, but the wet-bulb temperature, or, preferably, the dewpoint. The latter is preferable, because the dewpoint is the temperature

  at which a given sample of air holds all the water vapor it can. If the temperature is lowered when the air is at its dewpoint, water must condense out of that air.

When air is at its dewpoint, the dry-bulb and wet-bulb temperatures are both the same as the dewpoint temperature, the air is saturated, and its relative humidity is 100 percent. The dewpoint is a factor through which all relationships are correctly seen, and by the use of which the operation of air-conditioning becomes simple.

19B4. Two actions on the air. The whole subject is perhaps none too easy to grasp at first glance, so it may be well to draw special attention to a point that is usually overlooked, but which is really the vital matter in the operation of air-conditioning. It should be clearly realized that the air is being separately acted on in two different places where the conditions are quite different. One place is in the room itself; the other is in the evaporator casing.

The room is the place a person occupies and sees, and it is where he feels the satisfactory or unsatisfactory quality of the air. But the conditioning of the pair does not take place in the room. The actual conditioning of the air takes place inside the evaporator casing. The environment in the two places is very different. While in the room, the water vapor of the air is usually warm and, to a certain extent, free and expansive; but when it passes along with the air through the ducts to the evaporator, it suddenly is confined in a small cabinet where it is cold. So it compresses together and forms liquid drops (see Sections 16A7 and 16A8).

19B5. Summary of air-conditioning principles. In a room, the air continuously gains heat and moisture. In an evaporator, the air is quickly and considerably cooled and loses moisture.

To be sure, when the air returns from the evaporator to the room, it may lower the temperature therein by a few degrees, but


the essential requirement is either 1) the reduction of relative humidity, or 2) the prevention of condensation in the room.

In the first case, the relative humidity in the room is reduced by causing condensation in the evaporator, that is, by cooling the air to below its dewpoint in the evaporator.

In the second case, condensation in the room is prevented by lowering the dewpoint of the air in the room, and this again is done by cooling the air, and hence removing some of the moisture by condensation, in the evaporator.

  When a submarine loses heat to the ocean, the temperature of the air inside begins to drop. If the dewpoint of the air is high, it is not long before the temperature of the air drops to the dewpoint, and condensation in the compartment begins. This condensation appears as a film of liquid water or droplets on all available cool surfaces. The interior surfaces begin to sweat; and that means, always, a potential danger from short circuits or grounds in electric systems. Remember that the dewpoint is the temperature at which the water vapor in the air begins to condense.
19C1. Various climatic conditions encountered. A submarine may be on patrol in tropical, arctic, or temperate waters. The atmospheric conditions and water temperatures vary greatly in these different regions, and require different operation of the air-conditioning system.

19C2. General rule for air-conditioning. However, it is possible to set up a general rule, as follows: Operate the fans or blowers at such speed that the air leaving the outlet sides of the evaporators is always below the dewpoint; and adjust the louvers to give the best distribution of air throughout the vessel.

The adjustment of the louvers for best air distribution naturally varies in different classes of submarines, and even in different vessels of the same class. Practical experience in each submarine must dictate the fine points of louver adjustment. If they are opened and closed frequently in accordance with individual caprice, proper air-conditioning of the vessel cannot result.

19C3. Air-conditioning in temperate climate. The handling of air-conditioning in temperate climates is comparatively easy. It usually requires attention only to the general rule stated in Section 19C2. The blowers need not be operated at full speed, and may often be operated at minimum speed. The louvers may be partly closed.

It is important that the operator should frequently note the suction pressure and suction temperature at the compressors, in order to keep the system operating at its most

  efficient rate. When the air-conditioning compressors operate at 33 psi suction pressure and 117 psi discharge pressure, the compressors are at their best. These pressures can be obtained by adjusting the speed of the fan, the compressor, or both. For example, if the fan is running at 3/4 speed and the compressor at slow speed, with a suction pressure rising to 45 psi, the compressor should be speeded up until the pressure drops to around 33 psi. The fan should not be slowed down until the compressor is running at full speed, and the suction pressure is still up around 45 to 50 psi. Then the fan can be reduced in speed. It should not, however, be reduced below 1/4 speed on the rheostat; otherwise proper ventilation will be lost.

19C4. Air-conditioning in arctic climate. In arctic regions and in cold winter weather in temperate regions, the main problem is the prevention of condensation within the submarine. Reference to the psychrometric chart (Figure 16-1) shows that at lower temperatures, the dry-bulb, wet-bulb, and dewpoint temperatures are closer together than they are at higher temperatures. This means that a drop of fewer degrees causes condensation. Much closer attention to the temperatures, therefore, is required in cold weather.

A usual condition is a low dry-bulb temperature in the vessel with a wet-bulb temperature close to it. The shell of the vessel and other metal parts are usually colder than the dewpoint by only a degree or two. Moisture condenses on such surfaces, or, in other words,


the vessel sweats, necessitating the use of the air-conditioning system to dry it out.

Under these conditions, if an attempt is made to dry the vessel by operating the compressors at low speed and the blowers at 1/2 speed, the suction pressure will be about 18 to 20 psi. If this pressure is continued for a considerable time, the coils accumulate a coating of frost inside, and eventually become so plugged up as to prevent air from passing through. To prevent this, operate the compressors at slow speed, and the fans at full speed. The suction pressure then rises to 25 psi or more, and the frosting stops, while the drying of the vessel continues.

The proper method of starting the air conditioning system, when the injection water is at low temperature, is to operate with manual control instead of automatic. If the system is run on automatic control, it short cycles, because of low head pressure; that is, the suction pressure drops to 20 psi and the compressor stops on the low-pressure cutout. In a short time, the suction pressure builds up to 40 psi, the compressor starts, and runs a few minutes; then the suction pressure drops back to 20 psi, and the low-pressure cutout stops the compressor, alternating or short cycling this way every few minutes. This causes overheating of the starter and in due time burns out the motor. In operating manually, run the air-conditioning compressors for ten minutes, with the condenser water discharge valve closed, or until a head pressure of about 100 psi has been obtained. The reason for this is that there must be a difference of 60 psi between the high- and low-pressure sides to operate an expansion valve. The compressors can then be switched to automatic operation.

19C5. Air-conditioning in the tropics. In hot climates, the air-conditioning system usually must be operated at maximum capacity. In general, the best procedure is to operate the compressors at full speed and the blowers at such speed that the temperature of the air from the discharge side of the evaporators is below the dewpoint. The conditions of high air temperature in the tropics are naturally adverse to effective air-conditioning, especially

  when the large area of open water surfaces in the bilges is considered. The plant must be run so as to obtain maximum comfort and protection of equipment against moisture, both of which are obtained by abstracting the largest possible quantity of water vapor from the air. This condensing or wringing process is accomplished by always maintaining the temperature of the discharge air from the evaporators below the dewpoint.

Recent experiments on the ventilation of submarines in the tropics have shown that improved results are obtained, at least on some types of vessels, by ventilation in any of the following ways:

1. Run the supply blowers at highest speed allowable without overloading the motors.

2. Run the air-conditioning compressors at highest speed allowable without overloading the motors.

3. Shut off the supply outlets to the control room and conning tower when the upper conning tower hatch is open; close the door between the control room and the forward battery compartment.

4. Close all exhaust terminals from the forward torpedo room to the forward engine room while charging batteries, except the galley range terminal when the range is being used.

5. Remove the air spreader plates over the wardroom supply outlet on all Portsmouth submarines of the SS228 class and up. These spreader plates force air out into the passage way and allow no circulation of air in the wardroom.

6. Bypass all back-pressure regulators on both air-conditioning coils.

7. Cause the thermostat cutout to be wide open at all times in hot climates.

8. Use the normal fuel oil tank outboard of the forward battery compartment early in patrol in order to have sea water, which has a higher heat conductivity constant than oil, next to the pressure hull.

9. Shut off as many lights forward as possible. Use a fluorescent tube if available. A 20-watt tube furnishes twice the light with only about one-tenth the amount of heat as a 50-watt electric light bulb. The forward battery compartment has three times as many


lights on at all times as the after battery compartment.

10. Leave the air-conditioning condenser circulating water suction and discharge valves wide open.

11. Close the supply outlets to the engine rooms. Make no attempt to cool the engine room with the ventilation system.

12. Do not use the present system for exhausting air from the after part of the submarine when submerged. Close all engine room doors and the after engine room exhaust bulkhead flapper; with the supply blower, take a suction from the maneuvering room and

  engine rooms through the engine air induction lines in the superstructure, by way of the main induction valve aft of the conning tower. One ship reports air as being cooler throughout since using this system.

13. Run only one conning tower air-conditioning coil at a time.

14. Secure the conning tower air-conditioning coil when the upper conning tower hatch is open.

15. Rig all reserve fuel oil tanks to the main ballast tanks as soon as fuel is used. On diving, a considerable amount of heat is taken from the pressure hull by water filling the tanks.


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