The Professor: TXV Dos and Don’ts
The Externally Equalized Thermostatic Expansion Valve
The thermostatic expansion valve (TXV or TEV) is a metering device located between the liquid line and the evaporator. The TXV is a dynamic valve that constantly modulates and meters just enough refrigerant into the evaporator to satisfy all heat loadings on the evaporator. The basic TXV will control a set amount of evaporator superheat under varying heat loads.
The main functions of the TXV are to:
• Provide a constant amount of evaporator superheat under varying evaporator heat load conditions, provided the range and capacity of the valve is not exceeded;
• Keep the entire evaporator full of refrigerant under all heat load conditions; and
• Prevent flood back of refrigerant to the compressor under varying evaporator heat loadings.
What TXVs do not do:
• Control evaporator pressure;
• Cycle the compressor;
• Control running time; or
• Control box temperature.
Remember, the main function of a TXV is to meter the right amount of refrigerant into the evaporator coil under all loading conditions. By doing this, the valve controls a set amount of evaporator superheat to maintain active evaporators and keep refrigeration and air conditioning systems running safe and efficient.
Pressures acting on a TXV are:
• Remote bulb pressure (opening force);
• Spring pressure (closing force); and
• Evaporator pressure (closing force).
Remote Bulb Pressure
In an exploded diagram of a TXV (Figure 1), notice that the pressure from the volatile fluid in the remote bulb acts on the top side of the flexible bellows through the connecting capillary tube. This causes the valve to open as remote bulb pressure increases. This changing pressure in the remote bulb is caused from the evaporator outlet temperature getting hotter or colder from varying heat loads on the evaporator which momentarily affect evaporator superheat.
The higher the superheat, the more fluid that will vaporize in the remote bulb, causing higher pressures. The less the superheat, the more condensing of vapor that will take place in the remote bulb, thus decreasing its pressure and closing the valve.
As soon as the TXV realizes that the evaporator superheat is changing, it will react and either open or close, putting more or less refrigerant into the evaporator from changing remote bulb pressures. This maintains a set amount of evaporator superheat.
The remote bulb is clamped firmly to the suction line for good thermal contact (Figure 2). The remote bulb is the TXV’s feedback mechanism that gives the valve an indication of the evaporator superheat’s value. Remote bulbs come in a variety of charges for special applications. Please consult the TXV manufacturer’s manual for specific information on remote bulb charges.
The spring pressure acts in the same direction on the bellows as the evaporator pressure. It is a closing force. When the spring pressure and the evaporator pressure equal the remote bulb pressure, the valve is said to be in dynamic balance and will neither open nor close until the evaporator heat loading changes. If the evaporator pressure rises or falls, the valve tends to open or close. Also, if the remote bulb pressure changes from changing superheat, the valve will open or close.
Spring pressure is preset in most valves by the OEM and set for a predetermined amount of evaporator superheat (usually between 7-10°F). This spring is often referred to as the superheat spring, since it is the only way a technician can change the amount of superheat in a TXV system.
Increasing the spring tension by turning the spring adjustment clockwise will tighten the spring. This, in turn, will give the TXV more closing force and cause the superheat setting to increase. This will give the system more operating evaporator superheat.
Please note that there are some TXVs on the market whose superheat springs are nonadjustable and come with a set amount of superheat that cannot be changed by a technician. These TXVs are usually used in applications where superheat settings are critical for proper performance, such as some ice makers.
As heat loads on the evaporator change in time, so does the vaporization rate of the refrigerant in the evaporator. Higher heat loads cause higher rates of vaporization of the refrigerant in the evaporator, which will increase evaporator pressure. Lower heat loads on the evaporator will decrease the rate of vaporization of the refrigerant and decrease evaporator pressure.
Evaporator pressure acts on the underside of the flexible bellows of the TXV. This will cause the valve to close as the evaporator pressure increases. In fact, through a drilled passageway, recognized as the internal equalizer, evaporator pressure from the evaporator’s coils’ entrance acts on the underside of the bellows. This evaporator pressure opposes the remote bulb’s pressure.
External Equalized TXVs
There are instances when a larger system may have a large evaporator. Large evaporators have more pressure drop associated with them because of the length of piping and more U-bends. These evaporators will need to use an externally equalized TXV to compensate for this pressure drop and still fill out the evaporator with the proper set amount of superheat.
Externally equalized valves sense evaporator pressure at the evaporator outlet, not the evaporator inlet (Figures 2 and 3). Now the evaporator pressure will be sensed very close to the remote bulb location. This will compensate for any pressure drops throughout the length of the evaporator.
By sensing this smaller evaporator outlet pressure, there will be less pressure on the bottom of the TXV’s bellows than if evaporator inlet pressure was sensed as in an internally equalized TXV. This lower pressure will be a smaller closing force, thus the valve will remain more open and fill out more of the evaporator by compensating for the pressure drop through the evaporator.
The external equalizer does not get rid of pressure drop in an evaporator, it only compensates for pressure drop by sensing evaporator outlet pressure instead of evaporator inlet pressure.
An external equalized TXV is required whenever the pressure drop in the evaporator exceeds:
• 3 psig (air conditioning applications);
• 2 psig (commercial refrigeration applications); or
• 1 psig (low-temperature applications).
A distributor at the outlet of the TXV that feeds the inlet of the evaporator will decrease the pressure drop through the evaporator. The distributor (shown in Figures 3 and 4) divides the evaporator into two parallel evaporators. Instead of having one large evaporator with all the tubing in series with one another, the evaporator now becomes two coils in parallel with one another.
In conclusion, externally equalized TXVs used in conjunction with a distributor will make the evaporator more efficient and more active by having the proper amount of evaporator superheats and minimum amounts of pressure drop.
Publication date: 12/2/2013