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Old 07-23-2005, 07:14 AM
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A/C question: Fixed orifice vs. expansion valve?

What are the advantages/disadvantages of fixed orifice AC systems versus expansion valve ones?
Old 07-23-2005, 07:58 AM
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One possibility...

A fixed orifice is probably the simpler, more economical choice. An expansion valve is usually adjustable, if necessary. Some expansion valves may require some logic controls to regulate their set point. I'd WAG an expansion valve is more common for commercial applications than residential.

While my HVAC knowledge here is limited, at chemical plants a fixed orifice is often used as a cheaper solution to control flow and pressure (since they're related) than placing a valve in the line...unless more control is desired.
- Jinx
Old 07-23-2005, 02:02 PM
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In automotive systems, typically an orifice tube is used in conjunction with compressor cycling (turning the compressor off and back on as needed to achieve the proper cooling). Expansion valves typically are used with the compressor always running. My understanding is that orifice tube systems are less expensive to manufacture and install, and help improve vehicle fuel mileage. Expansion valve systems probably provide more precise control, but practically speaking most people would never perceive the difference.
Old 07-23-2005, 04:31 PM
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When it comes to auto systems, I agree with Gary T with one addition, Working on lots of cars from one car maker, I see both systems. IMHO an expansion valve system is a little more efficent and gives better cooling.
Old 07-25-2005, 04:51 AM
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Thanks for the answers. I have some more A/C related questions.

-Can I fill R12 systems with R134a? I know that the connectors are different, but there are R134a to R12 adaptors available on the market.

-Any good tips on finding leaks? It is tough to find small leaks even with flourescent dye.

-Is 30 minutes of vacuum enough for finding leaks?

(I have a Facom AC.222)
Old 07-25-2005, 02:34 PM
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-Can I fill R12 systems with R134a? I know that the connectors are different, but there are R134a to R12 adaptors available on the market.

There's not a "one size fits all" answer to this one. Generally, if you get ALL the R-12 out of the system, you can add R-134a and the proper oil and have a functioning system. However...

--The amount of R-134a needed will typically be 80-90% of the R-12 charge. It's best to determine on a case-by-case basis while monitoring system pressures and output air temperature.

--The cooling may not be as good as with R-12. This is especially likely on systems with a condenser that is too small for efficient operation with R-134a.

--There should be a condenser fan that runs whenever the compressor clutch is engaged. The car may or may not already have this.

--On older systems, the hoses may leak R-134a. The cure is to replace them with newer design hoses.

--To be legal, the service fittings should be changed to the R-134a type and a high-pressure cut-out switch should be installed (if the car didn't come with one). The latter helps protect against blowing a "fuse plug" and losing all the refrigerant.

-Any good tips on finding leaks? It is tough to find small leaks even with flourescent dye.

A high-quality electronic leak detector is the industry standard, sometimes in conjunction with dye. I don't know of other methods that are reliable, particularly with small leaks.

-Is 30 minutes of vacuum enough for finding leaks?

It's enough to stabilize the system pressure (vacuum) in order to see if there are leaks, provided you use a gauge set (that doesn't leak itself). Vacuum won't help you find leaks, unless they're really big and obvious, in which case two minutes of evacuation would suffice. Thirty minutes of vacuum is the standard for removing air, old refrigerant, and moisture.
Old 07-25-2005, 02:39 PM
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Clarification on that last item -- evacuate for 30 minutes, close the gauge valves, tap the low pressure gauge to make sure the needle doesn't stick, wait at least 5 minutes, tap the gauge again. If the needle is reading less vacuum, there is leakage. For good measure, give it 15 minutes if you suspect small leaks. If the vacuum reading holds with NO dropping of the needle position, it's almost certainly leak free. I say "almost" because there are a few leakage situations that won't be addressed by this test (leaks pressure but holds vacuum, leaks only under high operation pressure -- both rare cases).
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