Systems using thermostatic expansion valves use basically one of two types of valves: internally equalized and externally equalized. The two types of expansion valves are similar, but not interchangeable, both types of expansion valves are Installed in the system to lower the pressure before the refrigerant enters the evaporator. The reduction in pressure is accomplished simply by passing the refrigerant through a small hole (orifice), but the opening and closing of the orifice must be controlled to compensate for changes in pressure and temperature, the temperature of refrigerant leaving the evaporator is sensed by a thermal bulb and capillary tube which moves the valve seat via a diaphragm and actuating pins. Internally equalized expansion valves permit refrigerant pressure from the outlet side of the orifice to pass through an internal passage and push against the underside of the diaphragm.
Externally equalized expansion valves have a line connected to the outlet side of the evaporator and refrigerant pressure passes through this line to push against the underside of the diaphragm.
The thermal expansion valve is a precision device designed to regulate the rate of refrigerant liquid flow into the evaporator in the exact proportion to the rate of evaporation of the refrigerant liquid in the evaporator. The amount of refrigerant gas leaving the evaporator can be regulated since the thermal expansion valve responds to (1) the temperature of the refrigerant gas leaving the evaporator and (2) the pressure In the evaporator. This controlled flow prevents the return of refrigerant liquid to the Compressor. The thermal expansion Valve controls the flow of gas by maintaining a predetermined super heat. Three forces which govern the thermal expansion valve’s operation are (1) the power element and remote bulb pressure (P1), (2) the evaporator pressure (P2), and (3) the superheat spring equivalent pressure (P3). We are concerned here with the single outlet type of thermal expansion valve and will discuss it under two headings: (1) a valve with an internal equalizer feature, and (2) a valve with an external equalizer feature.
Opposed to this opening force on the underneath side of the diaphragm and acting in the closing direction are two forces: (1) the force exerted by the evaporator pressure and (2) that exerted by the superheat spring. In the first condition, the valve will assume a stable control position when these three Forces are in balance (that is, when P1 = P2 + P3). In the next step, the temperature of the refrigerant gas at the evaporator outlet (remote bulb location) increases above the saturation temperature corresponding to the evaporator pressure as it becomes superheated. The pressure thus generated in the remote bulb, due to this higher temperature, Increases above the combined pressures of the evaporator pressure and the superheat spring (P1 greater than P2 + P3) And causes the valve pin to move in an opening direction. Conversely, as the temperature of the refrigerant gas leaving the evaporator decreases, the pressure in the remote bulb and Power assembly also decreases and the combine evaporator and Spring pressure cause the valve pin to move in a closing Direction (P1 less than P2 + P3).
For example, when the evaporator is operating with R12 at a Temperature of 40EF or a pressure of 37 Psig and the refrigerant gas leaving the evaporator at the remote bulb Location is 50EF a condition of 10EF superheat exists.
Since the remote bulb and power assembly are charged with the same refrigerant as that used in the system (R12), its Pressure (P1) will follow its saturation pressure temperature characteristics. With the liquid in the remote bulb at 50ºF the pressure inside the remote bulb and power assembly will be 46.7 Psig acting in an opening direction.
Beneath the diaphragm and action a closing direction is the evaporator pressure (P2) of 37 Psig and the spring pressure (P3) for a 10EF superheat setting of 9.7 Psig (37 + 9.7 = 46.7) making A total of 46.7 Psig. The valve is in balance, 46.7 Psig Above the diaphragm and 46.7 Psig below the diaphragm. changes in load, increasing the superheat, will cause the thermal expansion valve pin to move in an opening direction. Conversely, a change, decreasing the superheat, will cause the thermal valve pin to move in a closing direction.
For example, an evaporator is fed by a thermal expansion valve with an internal equalizer, where a sizeable pressure drop of 10 Psig is present. The pressure at point "c" is 27 Psig or 10 Psig lower than at the valve outlet, point "a"; however, the pressure of 37 Psig at point "a" is the pressure acting on the lower side of the diaphragm in a closing Direction. With the valve spring set at a compression equivalent to 10EF superheat or a pressure of 9.7 Psig, the required pressure above the diaphragm to equalize the forces is (37 + 9.7) or 46.7 Psig. This pressure corresponds to a saturation temperature of 50EF. It is evident that the refrigerant temperature at point "c" must be 50EF if the valve is to be in equilibrium. Since the pressure at this point is only 27 Psig and the corresponding saturation temperature is 28EF a superheat of 50EF - 28EF or 22 Degrees is required to open the valve. This increase in superheat, from 10 to 22 degrees make it necessary to use more of the evaporator surface to produce this higher superheated refrigerant gas. Therefore, the amount of evaporator surface available for absorption of latent heat of vaporization of the refrigerant is reduced; the evaporator is starved before the required superheat is reached. Since the pressure drop across the evaporator, which caused this high superheat condition, increases with the load because of friction, this "restriction" or "starving" effect is increased when the demand on the thermal valve capacity is greatest.
In order to compensate for an excessive pressure drop through an evaporator, the thermal expansion valve must be of the external equalizer type, with the equalizer line connected either into the evaporator at a point beyond the greatest pressure drop or into the suction line at a point on the compressor side of the remote bulb location.
In general and as a rule of thumb, the equalizer line should be connected to the suction line at the evaporator outlet. If the external equalizer type of thermal expansion valve is used, with the equalizer line connected to the suction line, the true evaporator outlet pressure is exerted beneath the thermal valve diaphragm, the operating pressures on the valve diaphragm are now free from any effect of the pressure drop through the evaporator, and the thermal valve will respond to the superheat of the refrigerant gas leaving the evaporator.
When the same conditions of pressure drop exists in a system with a thermal expansion valve which has the external equalizer feature, the same pressure drop still exists through the evaporator; however, the pressure under the diaphragm is now the same as the pressure at the end of the evaporator, point "c", or 27 Psig.
The required pressure above the diaphragm for equilibrium is 27 + 9.7 or 36 Psig. This pressure, 36.7 Psig, corresponds to a saturation temperature of 40EF and the superheat required is now (40EF - 28EF) 12 degrees. The use of an external equalizer has reduced the superheat from 22 to 12 degrees. Thus, the capacity of a system, having an evaporator with a sizable pressure drop, will be increased by the use of a thermal expansion valve with the external equalizer as compared to the use of an internally equalized valve. As pointed out Earlier the external equalizer line must be installed beyond the point of greatest pressure drop.
Troubleshooting expansion valves:
Low suction with high superheat:
Ice on the expansion valve:
Adjust superheat to manufacture’s specification, or follower the instructions supplied with the valve if available.
Power assembly has lost its charge:
Low suction pressure with low superheat:
There may be gas in the liquid line (from pressure drop or an Insufficient charge). There are additional possibilities but you will find that these are the major problems associated with low suction pressure, high and low superheat.
As the refrigerant moves along in the coil, the liquid boils off into a vapor, causing the amount of liquid present to decrease. All of the liquid is finally evaporated at point b because it has absorbed sufficient heat from the surrounding atmosphere to change the refrigerant liquid to a vapor. The Refrigerant gas continues along the coil and remains at the same pressure (37 Psig); however, its temperature increases due to continued absorption of heat from the surrounding atmosphere. When the gas reaches the end of the evaporator, (See point c) its temperature is 50EF. This refrigerant gas is now superheated and the amount of superheat is 10EF (50E - 40E). The degree to which the refrigerant gas is superheated depends on (1) the amount of refrigerant being fed to the evaporator by the thermal valve and (2) the heat Load to which the evaporator is exposed.
Adjustment of superheat:
This method is in error by the temperature equivalent of the pressure drop between the two points of temperature. Where the pressure drop between the evaporator inlet and Outlet is 1 Psig or less, the two temperature method will yield fairly accurate results. However, evaporator pressure drop is usually an unknown and will vary with the load.
For this reason, the two temperature method cannot be relied on for absolute superheat readings. It should be noted that the error in the two temperature method is negative and always Indicates a superheat lower than the actual figure. The other method commonly used to check superheat involves taking the temperature at the evaporator outlet and utilizing the Compressor suction pressure as the evaporator saturation pressure. The error here is obviously due to the pressure drop in the suction line between the evaporator outlet and the Compressor suction gauge. On packaged equipment and close coupled installations, the pressure drop and resulting error are usually small. However, on large built up systems or systems with long runs of suction line, considerable discrepancies will usually result.
Since estimates of suction line pressure drop are usually not accurate enough to give a true picture of the superheat, this method cannot be relied on for absolute values. It should be noted that the error in this instance will always be positive and the superheat resulting will be higher than the actual value. Restating the above, the only method of checking superheat that will yield an absolute value involves a pressure and temperature reading at the evaporator outlet. Other methods employed will yield a fictitious superheat that can prove misleading when used to analyze thermal valve performance.
By realizing the limitations of these approximate methods and the direction of the error, it is often possible to determine that the cause of a service call is due to the use of Improper methods of instrumentation rather than any malfunction.