The thermostatic expansion valve (TXV) is a metering device designed to regulate the flow of liquid refrigerant into the evaporator at a rate equal to the evaporation of the liquid in the evaporator. The TXV responds to both the temperature and the pressure of the refrigerant exiting the evaporator to maintain a constant superheat at the evaporator outlet. Positive superheat at the evaporator outlet in the suction line will help ensure that only refrigerant vapor returns to the compressor.
Here are some tips to ensure maximum TXV performance.
When mounting the bulb, locate it as close to the evaporator outlet as possible.
Bulb Mounting/Location Tips
Extreme care should be taken when mounting the thermal bulb of the TXV, since this is crucial to proper operation of the valve. Too often, this function is rushed and not done correctly. The results can be erratic valve operation and unnecessary valve adjustments along the way. During installation of the system, create a straight, clean piping surface for mounting of the thermal bulb. Locate the thermal bulb as close to the evaporator outlet as possible (generally 3 to 10 inches). Using the factory authorized and supplied metal clamp, secure the thermal bulb tightly. You should not be able to move the bulb around on the suction line after this process. Using the metal clamp supplied will also increase the thermal conductivity to the bulb. The thermal bulb itself is not position sensitive in relation to its process tube. Avoid mounting the bulb on couplings or other brazed joints. Thermal conductivity is decreased through pipe fittings. Avoid mounting the bulb on vertical lines. Be sure to mount the bulb away from any traps. On suction lines that are 5/8-inch O.D. or smaller, the bulb should be installed on top or on the side (preferably at the 3 o'clock position). On suction lines that are 7/8-inch O.D. or larger, the bulb should be installed closer to the bottom side of the pipe (preferably at the 4 o'clock or 8 o'clock position). On systems that have multiple evaporators, the bulb must be mounted on the evaporator for which it controls. Fully insulate the thermal bulb regardless of the refrigerant or application. This ensures that the bulb responds to the evaporator suction gas temperature and not to any other outside influences.
When brazing, do not disassemble the TXV valve for installation. Instead, wrap the valve body with a wet rag or use a good thermal conductive paste.
Brazing Tips Limit body temperature to 250 degrees F (120 degrees C) due to internal seals used for balanced port construction. Always direct the flame away from the valve body. Do not disassemble the TXV valve for installation. Instead, wrap the valve body with a wet rag or use a good thermal conductive paste or other substance on the market today to limit heat to the valve body. For copper to copper connections, use any of the commonly used types of solders such as Sil-Fos, Easy-Flo, Phos-Copper, Stay Brite, or equivalents. If soldering valve directly to a brass distributor, use appropriate solders such as Easy-Flo or Stay Brite.
Valve Orientation Tips Parker TXV valves are not position sensitive; however, optimal operation is achieved with the power element UP to avoid oil accumulation near diaphragm cavity.
External Equalizer Use Tips Use external equalized valves for evaporator/distributor pressure drops over 3 psi. The external equalizer line can be mounted upstream or downstream of the thermal bulb for Parker valves due to balanced port construction. Keep the external equalizer connection toward the top of the suction line to prevent oil draining into the valve.
When making superheat adjustments, turn the stem no more than 1/4 to 1/2 rotation at one time.
Superheat Adjustment Tips Most expansion valves have an adjustment stem to vary the superheat. This is sometimes necessary because of the varied evaporators and load conditions any particular model valve may be used in. When making superheat adjustments, turn the stem no more than 1/4 to 1/2 rotation at one time. Let the system operate after an adjustment at least 15 minutes before checking the actual evaporator superheat. Superheat degrees/turn vary per refrigerant and evaporator temperature, approximately 5 psig/turn (2-4 degrees). Turning the adjustment stem clockwise increases superheat/decreases refrigerant flow. Turning the adjustment stem counterclockwise decreases superheat/increases refrigerant flow.
Troubleshooting Tips The chart at right provides some tips for troubleshooting TXV problems.
Steve Maxson is a supermarket specialist for Parker Hannifin Corp., Climate & Industrial Controls Group, Cleveland. For more TXV information, go to www.parker.com/rac. Maxson can be contacted at firstname.lastname@example.org.
Publication date: 05/02/2005