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Thermal Expansion Valve Function and Debug

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Thermal Expansion Valve Function

Thermal expansion valve (short called “TXV” ) is one basic system components for refrigeration cycle. His functions as follow:

1. Throttle and pressure reduction

Throttle and reduce pressure  of normal temperature and high pressure liquid refrigerant after condensation in the condenser, adjust the refrigerant which flows through the evaporator, and control the overheat at the evaporator outlet. Overheat = air outlet temperature – evaporation temperature.

2. Regulate flow

According to the temperature signal obtained by the temperature sensor, TXV can automatically adjust the refrigerant flow entering the evaporator to adapt the changes of refrigeration load.

TXV Features

3. Prevent abnormal overheating

The expansion valve adjusts the flow rate to make the evaporator keep a certain overheat, ensure the effective use of the evaporator, meanwhile avoid liquid refrigerant entering the compressor to cause liquid strike; at the same time, it can prevent abnormal overheating.

TXV Classification and Installation

TXV can divide into two types: internal balanced and external balanced.

Internal balanced TXV

Temperature sensor pressure = spring pressure + evaporator inlet pressure. Valve diaphragm pressure comes from evaporator inlet.

External balanced TXV

temperature sensor pressure = spring pressure + evaporator outlet pressure. Valve diaphragm pressure comes from evaporator outlet, always used in systems with large evaporator resistance.

TXV Structure

Install temperature sensor onto the horizontal air-outlet pipe at the evaporator outlet, 1.5m away from the compressor suction port. Temperature sensor mustn’t place on the pipeline with accumulated liquid. And wrap it tightly against the pipe, meanwhile clean the oxide scale at the contact in order to expose the metal natural color.

When air-outlet pipe diameter is <25mm, the temperature sensor can be tied at the pipe top; when diameter is >25mm, it can be tied at 45° on the lower side of the air-outlet pipe to prevent oil accumulation at the pipe bottom from affecting temperature sensor performance properly.

temperature sensor installation

Temperature Sensor Installation

TXV Debug

Must debug TXV under normal refrigeration operation condition. If the flow rate is too large, turn the debugging screw half a turn clockwise. Otherwise, turn the debugging screw counter-clockwise, also half a turn. Don’t turn the debugging screw more than half a turn at one time.

There must be an interval of more than 15 minutes between each debug.

TXV overheat should be 5~8℃. If not, adjust it.

TXV Debug Example

Fault: A constant temperature and humidity air conditioner is running with both compressors, air outlet is 22.5℃, air inlet is 16.8℃, its cooling performance is not good. Freon was sufficient, and filter not blocked.

Inspection: 2 compressors’ air outlet was overheated and temperature at the expansion valve outlet was low. The evaporator outlet temperature was 18℃, and air outlet pressure was 3.2kg/cm2 (the corresponding temperature was -5℃, overheat was 23℃), which was obviously deviated from the normal overheat. Reason is the expansion valve wasn’t open enough.

Debug: After correct debug, the evaporator outlet temperature was 12℃, air outlet pressure was 4.8kg/cm2, the corresponding temperature was 4.5℃, and the overheat was 7.5℃.

The specific data before and after the on-site debug are as follows:

ItemEvaporator outlet Temp (°C)Compressor air outlet pressure (kg/cm2)Ralated temp for Compressor air outlet pressure(°C)Overheat(°C)Overheat meet the request or not
Manufacturer's debug standard value<144.5~62.5~115~8Yes
(#1 compressor) before debug
21.03.2-5.026.0No
(#1 compressor) after debug12.35.05.87.6Yes
(#2 compressor) before debug
20.54.00.020.5No
(#2 compressor) after debug13.95.26.07.9Yes

In this debug, the optimal overheat was 7.6℃ and 7.9℃ respectively.

ItemAir conditioning air outletAir conditioning air outletAir conditioning air inletAir conditioning air inletTemp gap between air inlet and outlet
UnitTemp (°C)Humidity (%)Temp (°C)Humidity (%)(°C)
Before debug22.554.816.875.45.7
After debug22.554.814.384.38.2

 

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Vic Cheung

Hi, I'm Vic! Start working in HVACR field since 2008, our main products included: air conditioning, cold room refrigeration system, water chiller, ice machine, commercial dehydrator, dehumidifier, heat pump, electric motor, and all accessories. Since joined China Speedway Group in 2012, I worked in here more than 10 years, as the general manager of export department, we have established 50+ agent in different countries and areas.
As our experience and strong technical support, we can solve your issue once you have, and our chief engineer has worked in HVACR field since 1997. I am so proud of our knowlege is more and more popular not only for engineer, and for HVACR business new comer, DIY lovers. Hope you are enjoying our articles, if any question or comments just welcome to send me marketing at cn-beyond.com

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