R134a pressures - central HVAC vs automotive

[tinmanSC]

Is there anyone here that works with central HVAC systems and can shed some light on an issue I'm having. I been involved in a conversation over at the Clublexus forms. It has been stated that R134a (and other refrigerants) use the same pressure no matter the cooling system (refrigerators, automotive A/C, central A/C). However the pressure charts that he pulled out are far different than every source I can find for low and high side pressure in cars. He says that on a 90 degree day, high side pressure should be around 100-115 psi, but all my sources and the factory service manual for my Lexus GS300 says it should be 200-215. Does anyone know for sure if automotive A/C systems are higher pressure for the same type of refrigerant? If so, can you site a source?

Please keep in mind that this question assumes that a central system is using R134a.

 

[mechtech2]

High side at 100 lbs?
That would be the cold REST pressure at about 90F.
Running, figure 200 or so @ 90F.

 

[meangreen01]

To answer your question fully would require getting into some depth on a number of issues. What is it you're converting?

134a isn't used in central A/C cooling. It has uses in commercial refrigeration(i.e. the beer cooler at your quickie mart). As far as refrigerant pressure being the same throughout all types of cooling systems? That's not correct. System design and components would have varying pressures to achieve different results. That is also why there are 100's of different refrigerants on the market to allow the use of the one that fits the application the best.

As mechtech noted. 100 psi @90 degrees is about what you'd see when the system is off.

 

[440Magnum]

I've never heard of a central system that uses R-134a. R134a, like R-12 before it, is more suited for smaller systems and refrigerators.

That aside, the actual pressures can vary from system to system even with the same refrigerant. The low-side pressure in HVAC is more or less fixed by the boiling pressure of the refrigerant at just above 32F evaporator temp, so its close to the same across different systems. Now obviously if you're talking about a freezer instead of HVAC, the low-side pressure can be much different because you aren't tied to having the coil above freezing... in fact you WANT it lower.

But back to comparing HVAC systems: the high-side pressure is more of a design choice and is affected by how much condensor area can be used and how much air can be shoved across it.

 

[tinmanSC]

This is the chart that he pulls out to show what pressure should be with the system in operation. Is this pressure at rest? That is, i assume, the pressure required to keep this stuff liquid at the prescribed temperature?
http://assets.fluke.com/appnotes/electricalpower/1265816_.pdf

 

[Chris142]

Automotive R134a high side pressure needs to be around 2.2x ambient temp for it to work right. Low side will vary from 20 to 40 or so.

One would assume that a cooler that uses R134a would be around the same pressures.

 

[meangreen01]

Originally Posted By: tinmanSC
This is the chart that he pulls out to show what pressure should be with the system in operation. Is this pressure at rest? That is, i assume, the pressure required to keep this stuff liquid at the prescribed temperature?
http://assets.fluke.com/appnotes/electricalpower/1265816_.pdf


That is approximate pressures at rest.

One of the basic tenants of refrigerant theory is that at any given temperature the refrigerant will be a certain pressure. At rest, the system is in equilibrium, so at 90F both of your gauges will read ~104 psi. When the system is on though you have to take into account the compressor is compressing the refrigerant against a metering device. That's how you get the "low side" and "high side of the system".

What was it again you're retrofitting?

 

[punisher]

Originally Posted By: meangreen01

That is approximate pressures at rest.


Technically, yes, but.......... There is no way to know the amount of refrigerant in a static system as some of the refrigerant will be in a liquid state for most temperatures. On a given room temp system, static pressure is the same for a given refrigerant, no matter the charge if any of the refrigerant is a liquid (vapor pressure and all).

Will post more later, gotta pick the wife up from work.