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dash air service

June 20, 2014, 09:05:16 am

Dash air system.
I was wondering;
Why do we suck a vacuum prior to service?
I assume it is more to rid the system of moisture than to leak check the system.
Was also wondering as to why not purge the system with dry nitrogen to remove moisture and use the dry nitrogen to pressurize the system for leak check?
It is easier to find a pressure leak that a vacuum leak.
then suck a vacuum or service
Just asking for my edification.

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Re: dash air service

Reply #1 – June 20, 2014, 09:13:12 am

The system needs to be evacuated so that it can be filled with the proper amount of refrigerant. We don't want anything other than refrigerant in the system.

Did I miss something else in your question?

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Re: dash air service

Reply #2 – June 20, 2014, 11:02:50 am

Bill, Evacuation of the system removes water that accumulates and also will tell you of any leaks if system will not hold vacuum.  Absolutely essential if system is to operate at peak.
Gary B

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Reply #3 – June 20, 2014, 12:32:00 pm

A quick answer is that vacuum is used to remove all the moisture from the system. Blowing it out with nitrogen would not get every last drop.

Pulling a vacuum boils the water--causing the water to expand into a vapor, take up more volume, and get sucked out by the vacuum pump. The boiling point of water depends on the absolute pressure surrounding it. At very low pressures (like in a refrigerant system with a vacuum pump connected), the water will boil at room temp.

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Re: dash air service

Reply #4 – June 20, 2014, 12:50:57 pm

Bill, both Gary, David, and krush are spot on.  To expand a bit:  I believe your approach would work just fine; but to know that a system is really "tight enough" for and automotive system one should make it prove that it can hold a vacuum overnight.....quite a wait. Whereas, pulling a vacuum (good pumps will pull a leak-free system down to 15 or 20 microns of Hg rather quickly) boils off moisture very, very  fast..... and, as I am sure you know (a) moisture is very undesirable in any mechanical refrigeration system + (b) air is a very "poor refrigerant".  BTW- "perfectionists" used to triple evacuate systems when recharging; i.e., they evacuate, then introduce enough refrigerant to bring the system up to 0 psig (1 atmosphere) and then evacuate again.  After the third evacuation, the system is ready to be charged.  This may not be permitted nowadays though.

After opening/repairing a system one normally pulls a vacuum then adds enough refrigerant to leak check it under the (considerable) refrigerant vapor pressure.  If no leaks.....complete the charging process.  The specs for halogen leak detectors used in the field usually claim they can detect a leak as small as .05 oz ..... per year! 

SOooo your method would work fine, I believe; however, the methods "normally" used  would be much faster ..... and perhaps, more fool proof regarding moisture removal ?  Just my $.02

Ed no-Foretravel-yet S.  Hands-on HVAC business owner ..... in my previous life.

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Re: dash air service

Reply #5 – June 20, 2014, 04:16:21 pm

I did not mean to leave the nitrogen in the system, Only implied that it would be easier to find a "pressure leak" than a "vacuum leak".
It could be done with compressed air also, but that would leave a lot of moisture in the system to be evacuated, where dry nitrogen has very little moisture.

bill

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Re: dash air service

Reply #6 – June 20, 2014, 04:21:56 pm

P.S.
The specs for halogen leak detectors used in the field usually claim they can detect a leak as small as .05 oz ..... per year! 
very difficult to get leak detector  in the trough from rear ( compressor ) to the front of motorhome.

Long hose runs in airplanes and motorhomes \!!

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Re: dash air service

Reply #7 – June 20, 2014, 05:22:18 pm

Have to say I've wondered about this also.  I can see that the vacuum will cause the water to boil off, but what I don't see is how that vapor is going to magically find its way to the vacuum pump through 75' of line, and then exhausted out.  Seems like the old days of "sweeping" the lines with some gas probably rendered a better final result. 

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Re: dash air service

Reply #8 – June 22, 2014, 12:00:42 am

What temps and operating pressures do ya'll see on the low/high side at a fast idle for the dash air? Also, any clue where the low pressure cut-off switch is? There appears to be a high pressure switch on the compressor and another switch on the reciever after the condenser

On the same topic, I just recharged my dash A/C today. I do quite a bit of HVAC work and have all the tools, including the scale. But, I admit in this case I didn't use a scale as I was just going to "top off".

As found, the compressor was running and the return line was cool. Somewhat cool air blew out of the ducts as well.

Well, I started adding refrigerant and the temps came down...eventually to a nice 45deg. Funny this was, as I added, the compressor began to cycle. I guess it turns out that it was so low on charge that the TXV was wide open and none of the refrigerant was actually being condensed to liquid.

I continued to charge and pressure came to be around what I would expect. The high side seems a little high, running around 300-350. It'll drop down if I spray some water on the condenser. I'm going to use some coil cleaner on it.

Inside, the dash is blowing nice and cold. No cycling is really happening on HI fan...some cycling on LOW. But the cyling on/off is slow. Off for nearly 30-60seconds or more.

I still think I need to add a bit more as the low side seems a bit low, but I want to keep the high side pressures in check and make sure the condenser is clean. Charging with a TXV is a little more tricky as it tries to keep a constant superheat. This compressor doesn't appear to be variable displacement, but I could be wrong.......

I know the "proper" way is to charge by weight. But there's a lot of variables involved and weight isn't always perfect either. Sight glasses are usually the best, but I didn't see any (and it's tough with 134a anyway).

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Re: dash air service

Reply #9 – June 22, 2014, 12:28:21 am

Krush, We also don't see AC sight glass any more. What do you mean by 'and it's tough with 134a anyway'? I know it worked with R12, but does 134a not bubble? We miss the sight glass, too.

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Re: dash air service

Reply #10 – June 22, 2014, 09:01:43 am

I think they stop putting sight glasses in r134a systems because.......well, I don't really know the exact answer! There had to be some reason.

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Re: dash air service

Reply #11 – June 22, 2014, 09:15:35 pm

On a r134a system if you charge until the sight glass is clear then the system is overcharged. Weight and gauge readings are the best way.

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