Question on Expansion valves

  • Thread starter Buzzworks
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For example in typical window-type home air conditioning:

-If I remove the expansion valve and replace the valve with a small diameter tube to constrict the flow of gas so that the average gas pressure difference between the condenser and evaporator coils are the same as when you had expansion valve, will I get the same amount of cooling?

-Will I still get cooling effect at all?

-If answers are both yes, why not use small diameter tubes so it will be simpler and with no moving parts?
 

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  • #2
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bump...

Or perhaps I may have posted this on the wrong section. Should be on engineering?
 
  • #3
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I have experience with automotive air conditioning systems so I will speak to your question from that perspective

Automotive air conditioning systems use both types of systems depending on the vehicle. The small diameter tubes are called orifice tubes. Both systems work but there are design considerations for the rest of the system that prevent you from directly swapping one for the other.

In most orifice tube systems the compressor is cycled on and off based on low side pressure. In an R134a automotive system it would cycle on at about 40 psi and off at about 30. The reason for the lower pressure limit is to prevent the evaporator from getting so cold that water vapor freezes to it forming ice which would block air flow. In this type of system the accumulator must be on the low side of the system and serves a secondary function of separating liquid and gaseous refrigerant. If it were not there the compressor would be damaged by attempting to compress liquid refrigerant when it first engaged.

There are also a few systems out there that use an orifice tube with a variable displacement compressor to regulate low side pressure.

Orifice tube systems are cheaper to build but are less efficient because they allow the low side pressure to cycle over a much wider range then an expansion valve. In a time when EPA regulations are getting harder and harder to meet I suspect we'll be seeing fewer and fewer of these.
 
  • #4
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Thanks mrspeedybob! Never knew about Orifice tubes! I just thought it up as replacement to the expansion valves. Probably my ignorance to the term and to the part itself stems from the fact I haven't encountered the term yet in my research in A/C systems. It's always expansion valves!

Thanks for pointing out both will work in providing cooling.
 
  • #5
russ_watters
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-If I remove the expansion valve and replace the valve with a small diameter tube to constrict the flow of gas so that the average gas pressure difference between the condenser and evaporator coils are the same as when you had expansion valve, will I get the same amount of cooling?
Well, you're basically replacing the expansion valve with a less efficient one. If the delta-T is the same but there is a lot more pipe, there is a lot more friction and a lot more loss.
-If answers are both yes, why not use small diameter tubes so it will be simpler and with no moving parts?
I'm not quite following: the expansion valve has no moving parts in it.
 
  • #6
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Well, you're basically replacing the expansion valve with a less efficient one. If the delta-T is the same but there is a lot more pipe, there is a lot more friction and a lot more loss. I'm not quite following: the expansion valve has no moving parts in it.
Thanks Russ, now that you said that, I got thinking once more!

What if we don't use an orifice tube. What if we simply block the A/C coils at the same location you'd put an expansion valve or an orifice tube with a 2mm thick metal.

Then we make a pinhole in that metal. We'll call it 'pinhole restrictor'. Would that serve the purpose of an orifice tube of restricting gas flow to cause the pressure difference in the A/C coil? The pinhole will be of smaller area(less distance for the gases to traverse compared to tube), thus, causing less friction compared to the orifice tube.

Will it actually be more efficient than orifice tube and still provide the same cooling effect?
 

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