Cooler expansion valve. In this article, we are going to look at the thermal expansion valve of a chiller. We are going to look at the three most common types of expansion valves found on typical modern commercial chillers, namely the pilot-operated thermal expansion valve. The thermostatic expansion valve, as well as the electronic expansion valve.
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The expansion valve controls the amount of refrigerant that flows between the condenser and the evaporator. The valve responds to overheating of the suction line entering the compressor. The amount of refrigerant flowing through the evaporator will vary to accommodate the change in cooling load. The evaporator needs a certain amount of liquid refrigerant for sufficient heat exchange to take place. If there is a large amount of superheat in the suction line, there is a lot of steam going into the compressor. If so, more liquid refrigerant can be allowed to enter the evaporator from the condenser.
The expansion valve maintains this balance between the correct amount of liquid refrigerant and the correct amount of superheat.
The balance must be right. You just want steam to get into the compressor. If liquid refrigerant enters the compressor, it will seriously damage the compressor because liquids cannot be compressed.
pilot operated regulator
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| Pilot-operated-expansion-valve- jcool |
Above is an image of a real world piloted regulator. The refrigerant will flow in liquid form from the condenser, through the standpipe, to the expansion valve, and then to the evaporator. The large block attached to the bottom of the white pipe is known as the king valve. This can be used to isolate the capacitor.
You will notice there is another much smaller Thermostatic Expansion Valve pictured. This feeds the main regulator pilot, the flow in this smaller regulator is what the main regulator controls. A capillary tube is connected to the pilot regulator, goes up and connects to a heat bulb. Which is placed in the suction line of the compressor.
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| Pilot-operated-expansion-valve-chiller-side-view- jcool |
Side view of a water-cooled chiller model. We can see the path of the refrigerant from the condenser through the main expansion valve and to the evaporator. The pilot valve is also connected to the condenser and feeds the main expansion valve and is controlled by the heat bulb in the suction line.
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| thermal bulb jcool |
The heat bulb will be located in the suction line and will look similar to the image above. A capillary tube will pass between the bulb and the valve head. The bulb is filled with coolant. As superheat increases, it causes heat transfer from the suction line to the bulb.
As the bulb heats up, the coolant inside begins to boil and expand. As it expands, it pushes the refrigerant through the capillary tube, causing the pressure in the capillary tube to increase. This pressure pushes all the way down the capillary tube to the top of the pilot valve. This pressure pushes up on the valve, the pressure on the valve causes it to close or open, and we use it to control the flow of refrigerant through the cooler.
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| Illustration-of-chiller-pilot-expansion-valve- jcool |
Looking at the drawing of the system. We can see that as the pressure in the capillary tube increases, the pressure will drop and force the thermostatic pilot valve to open. Refrigerant will flow through it and lift the main expansion valve open. Then the refrigerant will flow from the condenser to the evaporator.
thermostatic expansion valve
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| Thermostatic-expansion-valve-chiller jcool |
The next expansion valve that we are going to look at is the thermostatic expansion valve. These are quite common in air coolers that have a smaller cooling capacity. These perform the same function as the previous regulator, just that they work a little differently.
Notice on the valve that there is a thin capillary tube coming out of the top of the valve (valve is on the side) that goes down the suction line to measure and react to superheat. This particular valve also has a thicker equalizer tube coming out of the side of the valve, but that's a different topic that we'll cover later.
This type is basically what we saw used as a pilot operated valve in the previous type of expansion valve, except that it is used to directly control the chiller. There is no "main regulator", it is the main valve in this design.
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| Thermostatic-expansion-valve-illustration- jcool |
With this design, we use a heat bulb to measure the superheat temperature in the cooler suction line. When the bulb heats up, the refrigerant inside boils, creating pressure in the capillary tube. Pressure moves down the capillary and pushes down on the valve head. When the header is pushed down, it opens the valve to allow more refrigerant to flow into the evaporator. When the superheat decreases, the pressure also decreases, and therefore the valve begins to close.
electronic expansion valve
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| Electronic-expansion-valve-chiller- jcool |
The third regulator that we are going to look at is the electronic regulator. This type of regulator offers the best performance because the valve uses a stepper motor in the body. The stepper motor precisely controls the positioning of the valve, which allows a very exact and precise flow of refrigerant.
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| Electronic-expansion-valve-illustration jcool |
It can be seen in the illustration above. The stepper motor on the far left. In the center is a tree that is attached to the spire. The needle pushes against a seat in the valve. The stepper motor pushes and pulls the shaft back and forth. As the shaft moves, it opens or locks the valve seat to allow or restrict refrigerant flow from the condenser to the evaporator. The controller measures the pressure and temperature of the refrigerant to determine how much to open/close.
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