When the thermostatic expansion valve is operating properly, the temperature at the outlet side of the valve is much lower than that at the inlet side. If there is no such temperature difference when the system is in operation, the valve seat is probably dirty and clogged with foreign matter.

Once a valve is properly adjusted, further adjustment should not be necessary. Any major trouble can usually be traced to moisture or dirt collecting at the valve seat and at the orifice.

TESTING AND ADJUSTMENT

Thermostatic expansion valves used in most shipboard systems can be adjusted by means of a gear and screw arrangement, (superheat to adjustment), maintain a superheat ranging approximately from 4 to 12 F at the cooling coil outlet. The proper superheat adjustment varies, of course, with the design and the service operating conditions of the valve, and the design of a particular plant. Increased spring pressure increases the degree of superheat at the coil outlet; decreased pressure on the other hand, has the opposite effect. Many thermostatic expansion valves are initially adjusted by the manufacturer to maintain a predetermined degree of superheat, and no provisions are made for further adjustments in service.

When the expansion valves are adjusted to give a high degree of superheat at the coil outlet, or when a valve is stuck shut, the amount of refrigerant admitted to the cooling coil is reduced. With an insufficient amount of refrigerant, the coil is starved and operates at a reduced capacity. Compressor lubricating oil carried with the refrigerant tends to collect at the bottom of the cooling coils, thus robbing the compressor crankcase, and providing a condition whereby slugs of lubricating oil are drawn back to the compressor.

If an expansion valve is adjusted for too low a degree of superheat, or if the valve is stuck open, the liquid refrigerant may flood from the cooling coils back to the compressor. Should the liquid refrigerant collect at a low point in the suction line or coil, and be drawn back to the compressor intermittently in slugs, there will be danger of injury to the moving parts of the compressor.

In general, the expansion valves for air conditioning and water cooling plants (high temperature installations) are adjusted for higher superheat than the expansion valves for cold storage refrigeration and ships service store equipment (low temperature installations). If it is impossible to adjust expansion valves to the desired settings, or if it is suspected that the expansion valve assembly is defective and re-quires replacement, make appropriate tests. (First make sure that the liquid strainers are clean, that the solenoid valves are operative, and that the system is sufficiently charged with refrigerant.)

The major equipment required for expansion valve tests is as follows:

1. A service drum of R-12, or a supply of clean dry air at 70 to 100 psig. The service drum is used to supply gas under pressure. The gas used does not have to be the same as that employed in the thermal element of the valve be-ing tested.

2. A high pressure and a low pressure gage. The low pressure gage should be accurate and in good condition so that the pointer does not have any appreciable lost motion. The high pressure gage, while not absolutely necessary, is useful in showing the pressure on the inlet side of the valve.

Normally, refrigeration plants are provided with suitable replacement and test pressure gages. The procedure for testing is as follows:

1. Connect the valve inlet to the gas supply with the high pressure gage attached to indicate the gas pressure to the valve, and the low pressure gage loosely connected to the expansion valve outlet. The low pressure gage must be connected up loosely so as to provide a small amount of leakage through the connection.

2. Insert the expansion valve thermal element in a bath of crushed ice. Do not perform this test with a container full of water in which only a small amount of crushed ice is floating.

3. Open the valve on the service drum or in the air supply line. Make certain that the gas supply is sufficient to build up the pressure to at least 70 psi on the high pressure gage.

4. Adjust the expansion valve, if it is desired to adjust for 10F superheat, the pressure on the outlet gage should be 22.5 psig. This pressure is equivalent to the pressure of an R-12 evaporating temperature of 22F. Since the ice maintains the bulb at 32F, the valve adjustment is for 10F superheat (difference between 32 and 22). For a 5 F superheat adjustment, the valve should be adjusted to give a pressure of approximately 26.1 psig. Allow for a small amount of leakage through the low pressure gage connection while this adjustment is being made.

5. To determine if the valve operates smoothly, tap the valve body lightly with a small weight. The low pressure gage needle should not jump more than 1 psi.

6. Tighten the low pressure gage connection and stop the leakage at the joints. Determine if the expansion valve seats tightly. If the valve is in good condition, the pressure will increase a few pounds and then either stop or build up very slowly. If the valve is leaking, the pressure will build up rapidly until it equals the inlet pressure.

7. Again loosen the gage to permit leakage at the gage connections. Remove the thermal element, or control bulb, from the crushed ice and warm it with the hand or place it in water that is at room temperature. The pressure should increase rapidly, showing that the power element has not lost its charge. If there is no increase in pressure, the power element is dead.

8. With high pressure showing on both gages as outlined above, the valve can be tested to determine whether the body joints or the bellows leak. This can be done by using a halide leak detector.

When performing this test, it is important that the body of the valve have a fairly high pressure applied to it. In addition, the gages and other fittings should be made up tightly at the joints so as to eliminate leakage at these points.