A/C Recharge (without knowing proper quantity)

[DrBobD]

Birken Vogt, I know there is no actual answer to this question but could you elaborate on how sub cooling may help to charge a system or test to see if the system is fully charged. Obviously I have heard of sub cooling, don't completely understand it 100% but have a good knowledge of the principle. I know that it would help to diagnose problems on the high side but how do I determine the proper charge from that I don't understand. How do I know if the system is fully charged?

Please expand further so some of us may understand this better.
Thanks

 

[Birken Vogt]

I don't have the answer either but I will do my best. I don't think anybody really has a full grasp on its exact effect on the system, or if they do, they don't work for truck or equipment manufacturers because we all know that they continue to this day to turn out trucks and equipment that do not cool very well right out of the factory. It is frustrating.

The answer you want is that the compressor compresses the refrigerant and discharges it to the top of condenser. The top rows of the condenser cool the hot gas down to the temperature where it starts to condense. (Condensing happens at a higher pressure when the temperature is higher, therefore the compressor is there to increase the temperature so that the refrigerant will condense at the elevated temperarures present.) There it begins to condense on the metal walls of the condenser as it works its way down. This is happening at a theoretically constant pressure and temperature, since, whether you have one pint or one gallon of liquid in the condenser at a given pressure, the temperature remains the same. It is just that more and more of the vapor is condensing to liquid as it works its way down, giving up heat as it goes, and that is what the fan and surface of the condenser is for. If the quantity of charge is sufficient, the last, bottom few rows of the condenser will be full of solid liquid. This is where subcooling comes in. This liquid is still at a somewhat elevated temperature and the last few turns are still giving up heat, so the temperature of this liquid begins to drop below the condensing temperature mentioned above before it exits the condenser rows. Measuring subcooling at the condenser outlet will give you some idea of how many rows of the condenser are full of liquid.

This must be done very accurately with a probe made for the purpose and surrounded with an insulating compound. It would be even better to put multiple probes on the last few rows of the condenser to know where subcooling starts (the liquid level) but we are getting into engineering now instead of field service, and in real life it isn't that clear cut anyway.

In the old days we used to use a glass window on the receiver/dryer and charge to solid liquid but some manufacturers say that with R134a, the window may never go solid anymore, and they are usually hard to see anyway.

The bottom line answer is, who knows? Like anything, they make it work good enough to get it off the showroom floor, and patch it up good enough to make you go away, and after 1 year, it is up to you to try and keep it working.

 

[TomA]

None of my Cats is new enough to have cabs or AC but with this 105° heat we have been having it is good to have some of the cars or trucks AC working. My 89 Jetta diesel has a sight glass where the high side line exits the condenser. It was very easy to tell when it was charged. 00 Subaru is real finikey as to charge. Compressor will overheat and kick out. I have found if I keep rpm over 2000 and rolling down the highway it will keep going.

 

[FSERVICE]

something to keep in mind is sub cooling ONLY works on TEV systems!!!!!

 

[willie59]

I have to admit fellas, I'm way out of my expertise with A/C systems, still a learning novice myself, but I've come to the conclusion that AC is an exact science that at times can still remain simple. All sorts of thermodynamic principles apply, but at the same time, as TD25C stated, stick a thermometer in the vent and when you see 50F, stop adding refrigerant, and as Birken Vogt added "resist the temptation to add more". I can attest to this, my 2008 Peterbilt, accumulator system with no sight glass, thought I'd add a little more R134A, had a negative effect, no longer making cold air, only cool. Released some of the pressure/refrigerant and cold air comes back. Hmm, there is an exact science to add refrigerant until it hits a certain level of cold air and resist to adding more. I'm intrigued by the concept of FSERVICE of doing the vac draw down, then adding liquid to the high side until pressure/flow is stabilized, then proceed with start up and finish refrigerant charge as needed, this would speed things up a bit, but I'm still in the learning phase, especially when it comes to troubleshooting problems with working systems. Very good knowledge at this thread.

 

[Birken Vogt]

something to keep in mind is sub cooling ONLY works on TEV systems!!!!!

If you had a CCOT system with no data, how would you go about determining charge level? If I understand correctly you would like the evaporator to be full of liquid. Do you want the suction accumulator to be full or empty, and how would you go about finding the level? I know you want to keep liquid out of the compressor like always.

When I worked for the fire department we had E-350 cutaway chassis ambulances with a box module on the back, which had the stock orifice tube on the cab and a TXV on the back A/C. They worked great, both sides. It always struck me as funny but it worked.

 

[DrBobD]

Just adding a little info in to Birken's comment. I do work for the ambulance services here in Ontario and have for the last 18 years. Ford actually makes a ambulance package that has the a/c lines run to the bottom of the cutaway chassis just behind the drives seat. They have done this since 1994. The odd thing is that the cooling problem is usually a complaint in the front of the vehicle that is controlled by the orifice tube not the rear that is TXV controlled. When the system get's low, 90% of the time it is a complaint about the front A/C.

Just wanted to through that out there, it’s a little off topic.

 

[BlazinSS934]

I have some HVAC experience under my belt. If you learn and understand exactly how Subcooling and Superheat works you can tackle any refrigeration problem with a fluke clamp on thermocouple and a set of gauges.


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[BlazinSS934]

Forgot to add this link this is for R22 and R410A refrigerants I can't find the one for automotive systems http://www.hvacsalesandsupply.com/Linked Documents/Tech Tips/Proper System Charging.pdf


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[AndrewC]

Sounds like Ill have to do a bit more reading. Blazin you said to use clamp on temp probes, will a temp gun work instead of buying probes, or is not not accurate enough?

 

[Birken Vogt]

I believe it is very important that you use probes made for the purpose, and that they be installed properly. It is kind of a fine, detailed measurement that you take.

The page on fixed orifice vs txv applies to home/office a/c and not necessarily automotive. The principles are the same but the details on how and what the measurements are are different. For instance, the suction accumulator is used on car fixed orifice systems, but not on home/office systems. I think this is to prevent liquid slugging of the comressor under varying load and compressor speeds. How and where to measure superheat in this type of system is not clear to me.

There is very little info out there on technician's theory of automotive a/c systems, mostly just "vacuum and charge per nameplate" which is, of course, only good if the nameplate can be read, and the system has not been piggybacked as in the ambulance, and the nameplate was correct in the first place (I have seen that too)

 

[da'yoop]

This is great guys, please keep the information coming. It is very much appreciated.

I do use a micron gauge on about 50% of the units I work on but here is the problem. I do field service which is 1 to 3 hours away. I advise the customer that the system should be drawn down to 500 but in most cases they let me run the pump for 1/2 an hour. Obviously they don't want to pay me for standing around. I do recommend that when the unit is down for service etc.. to have it done properly. Whether I see it back or not time will tell. The other 50% of the calls are local and the unit is shut down for a period of time so it can be done properly. On these jobs I can let the pump run longer. I use my good equipment on these, Robinair 6 cfm dual stage pump and yellow jacket brute 2 gauges with a 3/8 vacuum line but it still takes a long time to reach 500 microns. For example I had a unit with a leaking fitting so I replaced the fitting and the receiver dryer. Checked for other leaks with nitrogen, everything was OK. The system had not worked for a few weeks so I let low pressure nitrogen flow from the high pressure side to the low side to help dry the system. It took 4 1/4 hours to get the system to 500 microns. Any advice on how to draw the system down faster. My next route was to remove the shredder valves to speed the process.



Thanks, Guys

You should ALWAYS remove the Schrader valves when evacuating a system. This will speed up the process for you. Another thing you can do is to triple evacuate. To do that, let the vac pump run until you see the needle isn't moving much lower, then isolate the pump and let refrigerant flow into the system until you get about 1 or 2 pounds of pressure (just enough to get above 0) repeat this procedure two more times and try get the vacuum as low as you can without the customer going ballistic about you standing around. 500 microns is the goal but sometimes (like you said) you just don't have the time. Don't add too much refrigerant between vacuums or when you open the system to the pump, it will blow the oil out of the vac pump. Hope this helps.

 

[AndrewC]

Im a little confused, when I fill the system I evacuate and then check for leaks with the vacuum gauge and fill using the vacuum in the system. I cant understand why you would remove the shreader valves unless you re-vacuum after you evacuate.

 

[FSERVICE]

You should ALWAYS remove the Schrader valves when evacuating a system. This will speed up the process for you. Another thing you can do is to triple evacuate. To do that, let the vac pump run until you see the needle isn't moving much lower, then isolate the pump and let refrigerant flow into the system until you get about 1 or 2 pounds of pressure (just enough to get above 0) repeat this procedure two more times and try get the vacuum as low as you can without the customer going ballistic about you standing around. 500 microns is the goal but sometimes (like you said) you just don't have the time. Don't add too much refrigerant between vacuums or when you open the system to the pump, it will blow the oil out of the vac pump. Hope this helps.

u should know that it is a violation of the EPAs montreal protocol to vent Freon into the atmosphere!!!! that practice is very outdated & illegal!! just remove Schrader valves from system & use a good pump. tell the customer they get what they pay for!!!

 

[FSERVICE]

Im a little confused, when I fill the system I evacuate and then check for leaks with the vacuum gauge and fill using the vacuum in the system. I cant understand why you would remove the shreader valves unless you re-vacuum after you evacuate.

there is a tool that allows the valve to be removed & installed while the system is under pressure/vaccumn!!

 

[Birken Vogt]

He is doing it that way to use a little refrigerant to displace any air that may be trapped in the system a little quicker than vacuuming it forever and waiting for the air to make its way to the pump.

It would also work ok if you were to blow the lines out with refrigerant instead of vacuuming assuming the dryer was dry. But this isn't legal or cheap to do.

 

[da'yoop]

u should know that it is a violation of the EPAs montreal protocol to vent Freon into the atmosphere!!!! that practice is very outdated & illegal!! just remove Schrader valves from system & use a good pump. tell the customer they get what they pay for!!!

You are right........I should have said to use nitrogen and only bring it up to 0 psi.

 

[AndrewC]

The only tool that I have seen is for hvac applications not auto, could you point me in a direction to find it, thanks.

 

[Mjrdude1]

In field service of equipment A/C, our foreman won't allow for that much downtime of the machine, they would rather it run with the windows open. lol

We recover and evacuate the system with the vacuum pump on it for no more than 15 minutes, then recharge.(this is just for minor repairs, not component replacements btw) The methods may not be perfect, but then again this is being done in the 'field', and it gets the machine up and running quickly with a happy operator. The shop jobs get more attention of course.

Field servicing of heavy equipment is a touchy subject with most companies, downtime on the machine is critical as it costs more than the repair normally.

Our field trucks use an air powered vacuum pump that runs off the compressed air system, we got them from Bumper to Bumper auto parts.

 

[FSERVICE]

In field service of equipment A/C, our foreman won't allow for that much downtime of the machine, they would rather it run with the windows open. lol

We recover and evacuate the system with the vacuum pump on it for no more than 15 minutes, then recharge.(this is just for minor repairs, not component replacements btw) The methods may not be perfect, but then again this is being done in the 'field', and it gets the machine up and running quickly with a happy operator. The shop jobs get more attention of course.

Field servicing of heavy equipment is a touchy subject with most companies, downtime on the machine is critical as it costs more than the repair normally.

Our field trucks use an air powered vacuum pump that runs off the compressed air system, we got them from Bumper to Bumper auto parts.

IF u cant make time to fix it right,( my thoughts are wait till you can) BUT then I open that can of worms!! So get a big CFM pump from a HVACR company itll pull down faster. will run on 110volts off genset/welder they are pricey but will make the boss happy that the equipment is up & running.. mine is made by JB & is 8CFM!!!