What are Thermostatic Expansion Valve and its applications in refrigeration systems?
A thermostatic expansion valve ( TXV) is a device used to control the flow of liquid refrigerant entering the evaporator at a rate that matches the amount of refrigerant being boiled off in the evaporator. It also provides a pressure drop in the system, separating the high-pressure side of the system from the low-pressure side. This allows low-pressure refrigerant to absorb the heat.
Operation
The valve has 3 forces that act in combination
- Bulb Pressure P1
- Evaporator Pressure P2
- Spring Pressure P3
There are 2 closing forces P2 & P3 and one opening force P1. The valve in this figure is in equilibrium, P1 = P2+P3. If the evaporator pressure increases while the bulb pressure stayed that same the valve would close. P2 + P3 would be greater than P1. If the bulb pressure increases to the larger amount, the valve would open. P1 is greater than P2+P3.
As the load across evaporator increases the available refrigerant will boil off more rapidly. if it completely evaporated prior to exiting the evaporator, the vapor itself will continue to absorb heat. This heat is referred to as superheat.
Superheat is heat added to a substance above its saturation temperature. The bulb will sense this increase in temperature exiting the evaporator and increase the pressure on P1. P1 now being greater than P2+P3 will allow the valve to open allowing more refrigerant to enter the evaporator. Now that more refrigerant is being introduced into the evaporator there is more availability to absorb heat. If there is insufficient heat to boiling off all the refrigerant prior to it exiting the evaporator the temperature at the sensing bulb will decrease reducing the pressure at P1 and causing the valve to close. P1 is less than P2+ P3.
Superheat setting
Amount of superheat in the system is important. If it is insufficient the liquid refrigerant entering compressor washing out the oil and at worst case, causing hydraulic pressure thus breaking mechanical parts. If too much superheat in the refrigerant then valuable evaporator space wasted and possibly causing compressor overheating problems. From the factory TXV's come pre-calibrated at a static superheat setting 6F. Static superheat is the amount of superheat required to get the pin to begin travel, any increase of superheat over this point is referred to a gradient. The manufactures often calibrate for 4 to 6F of gradient superheat. The combination of this 2 means that at 10 to 12F total superheat the valve will be opened to it's full rated capacity. Note that this superheat setting is set at the exiting of the evaporator and not at the compressor. Therefor 15F superheat reading at the compressor is normal.
Externally equalized
Externally equalized TXV's must be used when the pressure drop in the evaporator exceeds
- 3F for Air Conditioning
- 2F for Refrigeration
- 1F for Low Temp applications
Valve Hunting
One of the major problems with single ported valves is that they are susceptible to valve hunting. Valve hunting is defined as the alternate overfeeding and starving of refrigerant flow to the evaporator. This can be noticed by the constant movement you may see in your refrigerant gauges. Hunting can be reduced by either relocating the sensing bulb to a better location or by purchasing a valve designed for reduced hunting.
Balanced Port Valves
Balanced port valves are designed for systems that experience fluctuating pressures. On conventional TXV's when the pressure drop across them changes so does the amount of superheat. Potentially you could have either flooding or overheating of the compressor due to this reason. The balanced port cancels this effect allowing proper superheat to be maintained.
Double Ported Balanced Port valves
When dealing with systems that have light load conditions, double ported valves should be used these valves have 2 ports for varying capacities and can run down to 15% of their rated capacity.
