THERMOSTATIC EXPANSION VALVE EQUALIZERS

1. ELEMENT CHARGES

2. EQUALIZERS

3. HUNTING

The external equalizer consists of a line connecting the evaporator outlet to the area in the valve under the diaphragm.

The internal equalizer consists of a passage way in the valve from the valve outlet to the lower side of the power element.

Another way to describe the difference between the two is that the internal equalizer provides a closing force based on the pressure of the refrigerant vapor at the evaporator INLET whereas the external equalizer provides a closing force based on the refrigerant vapor at the evaporator OUTLET.

THERMOSTATIC EXPANSION VALVE CROSS CHARGES

The element charge (i.e., the fluid in the power element of a TX valve) is usually one of three types:

1. GAS CHARGE

2. LIQUID CHARGE

3. CROSS CHARGE

CROSS CHARGES use a refrigerant in the thermostatic element that is different from the refrigerant used in the refrigeration system. By using a different refrigerant, the relationship between the forces due to the evaporator pressure and suction pressure can be modified. Cross charges are often introduced so that the bulb pressure does not rise as rapidly as the pressure of the refrigerant in the system.

Cross charges have advantages similar to those of gas charges (flood-back protection during shut-down and startup), and at the same time, they are suitable for low-temperature work. Cross charges also help to reduce hunting.

THERMOSTATIC EXPANSION VALVE LIQUID CHARGES

The element charge (i.e., the fluid in the power element of a TX valve) is usually one of three types:

1. GAS CHARGE

2. LIQUID CHARGE

LIQUID CHARGES, like gas charges, use the same refrigerant in the element as is used in the refrigeration system. The difference between the liquid and gas charges is that the liquid charge has a volume such that there will always be some liquid in the bulb regardless of the amount of superheat in the suction line.

Because liquid is always present in the bulb, the thermostatic element always has control of the opening force on the valve. Therefore, unlike the gas charge, the liquid charge can contribute to flood-back during startup.

Furthermore, liquid charges tend to cause more hunting than other charges (hunting is presented later in this module). Also, liquid charges should be avoided in low-temperature systems.

THERMOSTATIC EXPANSION VALVE ADJUSTMENT CONCERNS

It is very rare that a superheat adjustment setting needs to be made in an existing system. If an adjustment is made, keep two things in mind:

1. Count the number of turns of the valve stem so that you can return the TXV valve to its original state if you don’t get the desired results.

2. After making any superheat adjustments, always observe system operation for several minutes (15-20) to insure you get the desired results.

PROPER THERMOSTATIC EXPANSION VALVE TECHNIQUES

If the superheat were too high, we could manually adjust the valve by turning the valve stem counter-clockwise (as viewed from the bottom), which would loosen the spring tension.

This would allow the bulb pressure to overcome the evaporator and spring pressure causing the valve to open, which would feed more refrigerant into the evaporator, thus lowering the superheat

CALCULATING PROPER SUPERHEAT ADJUSTMENT

Now, with less refrigerant in the evaporator, the vapor leaving the evaporator will be superheated. If, in this example, the superheated vapor leaving the evaporator rises to 50°F (10° superheat), then the temperature of the charge in the bulb will also rise to 50°F.

At a temperature of 50°F, the charge has a pressure (pushing on the diaphragm) of 47 psig.

We now see that the 10° superheat created a pressure differential between the charge and the evaporator equal to:

charge 47 psig

evaporator -37 psig

10 psig

THERMOSTATIC EXPANSION VALVE POWER ELEMENT

The power element (consisting of the bulb, capillary tube, and power head) contains a charge which is usually a refrigerant - the same refrigerant as that running in the system.

The refrigerant in the bulb is a saturated vapor. Because the bulb is strapped to the suction line, the temperature of the charge is equal to the suction line temperature. As the suction line temperature increases so does the temperature of the charge.