THERMOSTATIC EXPANSION VALVES
THERMOSTATIC EXPANSION VALVES
The thermostatic expansion valve is essentially a reducing valve between the high-pressure side and the low-pressure side of the system. The valve is designed to proportion the rate at which the refrigerant enters the cooling coil to the rate of evaporation of the liquid refrigerant in the coil; the amount depends, of course, on the amount of heat being removed from the refrigerated space.
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 this temperature difference does not exist 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. The major trouble can usually be traced to moisture or dirt collecting at the valve seat and orifice.
Testing and Adjustment
The thermostatic expansion valves used in most shipboard systems can be adjusted by means of a gear and screw arrangement to maintain a superheat ranging from about 4°F to 12°F at the cooling coil outlet. The proper superheat adjustment varies with the design and service operating conditions of the valve and the design of the particular plant. Increased spring pressure increases the degree of superheat at the coil outlet.
Decreased spring pressure decreases the degree of superheat at the coil outlet.
Some thermostatic expansion valves have a fixed (nonadjustable) superheat. These valves are used primarily in self-contained equipment where the piping configuration and evaporating conditions are constant. If expansion valves are adjusted to give a high superheat at the coil outlet or if the valve is stuck shut, the amount of refrigerant admitted to the cooling coil will be reduced. With an insufficient amount of refrigerant, the coil will be “starved” and will operate at a reduced capacity. Also, the velocity of the refrigerant through the coil may not be adequate to carry oil through the coil. This robs the compressor crankcase and provides a condition where slugs of lubricating oil may be drawn back into the compressor. If the expansion valve is adjusted for too low a degree of superheat or if the valve is stuck open, liquid refrigerant may flood from the cooling coils back into the compressor. When liquid refrigerant collects at a low point in the suction line or coil and is drawn back into the compressor intermittently in slugs, there is 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) normally are adjusted for higher superheat than the expansion valves for cold storage refrigeration and ship’s service store equipment (low-temperature installations).
You may not be able to adjust expansion valves to the desired settings, or you may suspect that the expansion valve assembly is defective and requires replacement. In either case, you should make appropriate tests. First you should be sure that the liquid strainers are clean, that the solenoid valves are operative, and that the system is sufficiently charged with refrigerant.
The major pieces of equipment required for expansion valve tests is as follows:
l 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 does not have to be the same as that used in the thermal element of the valve being tested.
l A high-pressure and a low-pressure gauge. The low-pressure gauge should be accurate and in good condition so that the pointer does not have any appreciable lost motion. The high-pressure gauge, while not absolutely necessary, will be useful in showing the pressure on the inlet side of the valve. Refrigeration plants are provided with suitable replacement and test pressure gauges.
The procedure for testing is as follows:
1. Connect the valve inlet to the gas supply with the high-pressure gauge attached to indicate the gas pressure to the valve. Connect the low-pressure gauge loosely to the expansion valve outlet. The reason the low-pressure gauge is connected loosely is to allow a small amount of leakage through the connection.
2. Insert the expansion valve thermal element in a bath of crushed ice. Do NOT attempt to perform this test with a container full of water in which a small amount of crushed ice is floating.
3. Open the valve on either 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 gauge connected in the line to the valve inlet.
4. The expansion valve can now be adjusted. If you want to adjust for 10°F superheat, the pressure on the outlet gauge should be 22.5 psig. This is equivalent to an R-12 evaporating temperature of 22°F. Since the ice maintains the bulb at 32°F. the valve adjustment is for 10°F 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. There must be a small amount of leakage through the low-pressure gauge 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 gauge needle should not jump more than 1 psi.
6. Now tighten the low-pressure gauge connection to stop the leakage at the joint and 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. But with a leaking valve, the pressure will build up rapidly until it equals the inlet pressure. With externally equalized valves, the equalizer line must be connected to the piping from the valve outlet to the test gauge to obtain an accurate superheat setting.
7. Again loosen the gauge to permit leakage at the gauge connection. Remove the thermal element, or control bulb, from the crushed ice. Warm it with your hands or place it in water that is at room temperature. When this is done, 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 readings showing on both gauges, the valve can be tested to determine if the body joints or the bellows leak This can be done by using a halide leak detector. When you perform this test, it is important that the body of the valve have a fairly high pressure applied to it. In addition, the gauges and other fittings should be made up tightly at the joints to eliminate leakage at these points.
Replacement of Valves
If the expansion valve is defective, it must be replaced. Most valves used on naval ships have replaceable assemblies. Sometimes it is possible to replace a faulty power element or some other part of the valve without having to replace the entire assembly. When replacement of an expansion valve is necessary, you must replace the unit with a valve of the same capacity and type.
