Expansion Tubes Vs. Orifice Tubes

Most people think that when testing an air conditioner, if the high pressure is low and the low pressure is high, then the compressor is bad – end of story. When you replace the compressor, as a best practice, you should also replace the expansion valve. Now, when the system starts back up, the AC will be fixed. There you go, it needed a new compressor.

The reality isn’t that simple. It isn’t always the compressor. Sometimes the compressor is fine (or at least has some life left in it) and the expansion valve is stuck open. However, if you have an AC system with an Orifice Tube, the compressor is always the case.

 

How can that be?

Some “expansion valves” aren’t valves. They are a fixed size hole, properly called an Orifice Tube. The hole creates a pressure drop from the high side of the air conditioning to the low side.

Some are more complicated. These are called Thermostatic Expansion Valve (TXV) or a True Expansion Valve. The TXV hole gets smaller and larger as necessary. TXV are almost always installed in any vehicle with multiple evaporators.

 

So, what does a TXV do?

As Freon flows through the evaporator, it flows from a liquid to a gas. If everything is working correctly, the TXV(through some tricky chemistry) will open just enough so that the Freon coming out of the evaporator is just barely all gas.

For the air conditioner to work as well as it can, the evaporator should be as full of liquid as possible without letting the Freon leave the evaporator as a liquid. That produces the best heat transfer and makes the car as cold as possible. Liquid is, of course, much better at transferring heat than a gas.

If it’s opened too little, the high side will be super high, and the low side will be super low:

  • The evaporator will freeze up.
  • The compressor won’t be able to pump much fluid (since it’s working so hard pumping against a high pressure drop)
  • Flow of Freon will reduce (because the pump can’t pump)
  • Less flow means 70 or 80% of the evaporator will be all gas and not transfer much heat.

If it’s open too much: the low side will be high and high side will be low (just like a bad compressor with an orifice tube, as mentioned earlier). Also:

  • The evaporator will be too warm (since Freon needs the pressure drop to get cold)

As you can see it’s a careful balance. The valve is constantly moving to make sure the flow of Freon is just right. It also makes for the coldest AC system.

 

How can you tell them apart?

Orifice valves are really cheap. Generally, a few dollars or less. Expansion valves are larger, generally look like a metal block, and cost $25 or more.

Figure 1: orifice tube

Figure 2: a true Expansion Valve

 

How do you know if the TXV is working?

You can measure the “super heat”. The Super Heat is a temperature measurement of the gas leaving the evaporator past the “Evaporator Saturation Temperature”.

Saturation Temperature: The temperature of the Freon boils given the “low side” pressure. This is generally listed on the gauge. On the gauge below, 100 psi (most outer set of numbers) correlates to a saturation temperature of 30F° (most inner set of numbers). THE PICTURE BELOW IS NOT A R134A GAUGE SET, SO YOUR GAUGES WILL SHOW DIFFERENT NUMBERS.

Figure 3: low side gauge with pressures and saturation temperatures. This is a home AC gauge, so your gauges will have different numbers on them. The relationship between pressure and saturation temperature is refrigerant dependent.

Here’s how you measure Super Heat

  1. Look at the low side gauge. Read the saturation temperature from the R134a scale.
  2. Measure the actual temperature on the outside of the line leaving the evaporator. The best way is to use a wired sensor. Cable tie the sensor to the line, then wrap a rag around the outside of the sensor (this is to make sure it’s measuring the line temperature and not the outside air). Wait a minute or two. The rag should cover at least an inch in both directions. If you’re using a shop rag, it should be wrapped a few times.
  3. Subtract the number from #2 from #1. If the temperature difference is less than about 7F°, you have a TXV problem.

 

Why do TXVs’ fail?

They generally get stuck open because of debris. The debris often comes from a failing compressor that sends metal shards in the condenser then into the TXV. That isn’t always the case.

 

So, what you should do if the TXV is stuck open?

It’s hard to know why the TXV stuck open. If you replace just the valve, another bit of metal could come out of the condenser and go right back into the TXV. On the other hand, it could also last for a long time. It’s hard to say.

If you take apart the TXV and see debris, the safest plan is the replace the TXV, condenser, and compressor. If the valve is clean, it may have been just a bad TXV and the customer may be okay for a while.

Generally, you can leave this up to the customer. The way to be sure that it won’t happen again is to replace the condenser, compressor, TXV, and filter (drier). However, that can be expensive.

 

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