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How do you calculate refrigerant volume?

vwlimo on Tue August 19, 2014 9:04 AM User is offline

I have a custom built VW Beetle limo that I am installing AC into. The system is custom from all sorts of random pieces. How do I figure out the optimal amount of R134A to charge it with. Also how much oil?

Edited: Tue August 19, 2014 at 9:05 AM by vwlimo

Cussboy on Tue August 19, 2014 10:55 PM User is offline

I'd contact Gilmore or IceAC in San Diego, they know systems for old VWs, maybe hey could help.

mk378 on Wed August 20, 2014 8:35 AM User is offline

What are the parts? Multiple evaporators? Old rear engine Beetle would involve long lines from the front to the back, which hold up a lot of refrigerant but the amount can be calculated. Does the compressor have an oil sump like a V7, A6 etc?

Edited: Wed August 20, 2014 at 8:36 AM by mk378

vwlimo on Thu August 21, 2014 8:14 AM User is offline

I'm using a sanden 508 compressor. CN 20001 condenser and an evaporator from a Mercedes 500. The evaporator is behind the back seat so the lines aren't that long. Compressor to condenser is about 4.5'. Condenser to evaporator 2.5'. Evaporator to compressor 3'.

NickD on Sat August 23, 2014 2:42 PM User is offline

Goal it to obtain solid refrigerant in the high side line when the ambient temperature is above 85*F, at sight glass in the high line helps with this.

Also to observe low and high side pressures to make sure they are well within limits.

vwlimo on Sun August 24, 2014 3:04 PM User is offline

So I started charging the system today to see what would happen. After one 12oz can the pressures were reading 20 psi low and 175 high. As soon as i connected the second can the low side pressure shot up to the 75-80psi range. I shut off the valve and it settled in at 30 psi. The high side was reading 295psi. And the vent air temps got down to 78*F. The outside temps were 90*F. And to top it off the compressor made a waaaaah sound when i revved the engine above 2500 rpm. I'm sure that it needs more refrigerant but don't know why the pressures are so high. Any clues?

NickD on Sun August 24, 2014 3:25 PM User is offline

Assume you drew a high vacuum, and purged the air out of the lines before charging.

mk378 on Sun August 24, 2014 5:19 PM User is offline

If you did take the air out so the refrigerant is not contaminated, this means your condenser performance is inadequate. It's been a while since I saw an original Beetle, but it seems that a rear-mounted condenser would not be practical. Where did you mount the condenser? Does it have good fans pulling from a source of outside temperature air (not hot air from the engine)?

Excessive high side pressure and overall poor performance will also result from using way too much oil.

Edited: Sun August 24, 2014 at 5:22 PM by mk378

vwlimo on Sun August 24, 2014 5:45 PM User is offline

I did evacuate the system. The condenser is mounted under the transaxle with a fan drawing air down through it. The condenser is a smaller universal one and my evaporator is quite large . The compressor came pre filled with oil. I didn't measure the quantity. Could the high rpm noise be from excessive oil our not enough refrigerant?

mk378 on Sun August 24, 2014 6:32 PM User is offline

Noise is probably the belt slipping. The old fashioned v-belts can't take a lot of torque, and the compressor gets harder to turn the higher the pressure goes.

NickD on Mon August 25, 2014 5:41 AM User is offline

Basic AC principles, the condenser can't ever be made large enough, then the type, tube and fin or parallel flow. As an old time engineer, still feel the field of thermodynamics is an art, even with computer modeling, just too darn many variables to consider. So you really don't know until you try something new and make precise measurements. Even though it may seem perfectly logical.

We use to have an AC engineer on this board, AC-Detroit was his handle, would tell you the same thing. Ha, back to the drawing board.

vwlimo on Mon August 25, 2014 10:21 AM User is offline

Running a serpentine belt so that's not it, the noise is not squealing enough to be a belt. The clutch maybe? Yes back to the drawing board.

HECAT on Mon August 25, 2014 1:21 PM User is offline

I have seen it done at the system manufacturers facility. They model the entire system including the proper position of all components and line sets in relation to each other. They measure pressures and temperatures at many many many (did I say many) points and vary charge by grams to finally determine the manufacturers recommended charge by weight. They are also looking for a target return oil flow to the compressor, so this is also being evaluated and eventually determine during these tests also. That is how it is done.

Doing this testing, measuring, and calculating on your hybrid system will take a lot of trial and error; and may require the sacrifice of a few oil starved compressors to get it sorted out. Remember that too much oil coats the heat exchanger internals and will effect the thermal transfer. But once you get your "thousand points" of measuring devices hooked up and talking to each other thru your custom software, it will all make sense to you then.

Unfortunately, it probably would have been better to go with a complete add-on system from a manufacturer that could provide such engineering data for you. I think it is going to be easier for you to get to the "good enough" refrigerant charge, than it is to determine the right amount of oil to "glide" (return to compressor) properly.

Good Luck.


HECAT: You support the Forum when you consider for your a/c parts.


NickD on Tue August 26, 2014 9:20 AM User is offline

Seen guys go nuts buying used rear powered diesel motorhomes trying to figure out how much oil to add. So chose a front engine gas powered one with a York compressor. Getting lazy in my old age. All I had to so was to look at the sight glass and even had a refrigerant sight glass on the receiver. But still hooked up gauges.

This two cycle engine approach to compressor lubrication made life a lot more complicated, even more so by using PAG that is highly hygroscopic in nature. General strictly a ballpark figure, can vary from about 2-5 ounces per pound of refrigerant. Just too many variables, if the compressor as some kind of sump in it, how much will settle in the accumulator or receive, or in low spots.

Practically all other refrigeration and HVAC systems have a sump in the compressor and only recirculating refrigerant making life a lot simpler. And even automotive started off this way, But the bean counters are trying to save a couple of cents. We pay the price for this, sooner or later, mostly later.

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