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Old 03-24-2012, 04:51 PM   #15
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Quote:
Originally Posted by Bug512 View Post
I was going to say the same thing. You can not measure out refrigerant when you have a chiller or air handling unit that holds several hundred pounds.


I need gages, thermometers and a temperature and pressure chart.

Quote:
Originally Posted by TXiceman View Post
Bug512...I hear you. I also work on systems holding over 20,000# of refrigerant.

But these are flooded evaporators and you charge by liquid level.

Ken
You guys are kind of making my point. Different systems require different service procedures.

Sorry but I just couldn't let it the subject lie. Got to kick this sleeping dog one more time.

I understand what you are saying about charging big chiller systems with gauges thermometers and charts. But I think you will agree there are significant differences between charging those refrigerating / chiller systems and an automotive system. When charging very small refrigerant - A/C systems, donít you have to be quite precise to charge them correctly?

Arenít we are talking apples and oranges here?

Consider this, you have to charge this system that uses a compressor that constantly changes speed between 1500 and 5000 rpm. Now an evaporator fan that has three or four different speeds, and corresponding evaporation rates, and a condenser that can have an air flow equivalent to 25 miles per hour to . . . well letís just say up to 80 mph, and the different condensation rates. Now these three conditions can vary independently of one another. Think motor home climbing a long hill, engine screaming and a compressor speed of 5k rpm and an condenser air flow equal to about 25mph) Now that same motor home is going down the other side at 30 mph and a compressor speed of 2500 rpm. And it has to cool in both instances. Oh and humidity that can range from 10% to 99%. Now it has to work tomorrow too and all these variables will change. Also keep in mind that some systems have a fixed orifice tube for restriction and not a nice thermostatic expansion that will control the superheat across the evaporator. Now at what speed do you run the engine and what chart do you use to determine the correct pressures and temperatures to determine the correct charge? Can you see the problem with gauges, temperatures and charts. Gauge and temperature readings for automotive situations are merely guidelines. The high side pressure can vary from 120psi to 280psi on the same system in different situations different days, shoot even on the same day at the same engine speed.

Oh and one more thing. R134a don't act like R12, well execpt that it gets cold. R12 is quick to respond to temp/press changes. R134a is not. R134a is like 90wt gear oil compared to R12. R134a moves slow and changes state slow. (in comparison to R12) That is why a retrofit from R12 to R134a calls for 10% less refrigerant charge and sometimes a larger condenser. Also R134a never completely mixes with the oil like R12 does. If you haven't worked much wtih R134a, lets just say it is the most wierd stuff I ever worked with. There is more wierd about R134a but this will do for now.

Hypothetical scenario:

Charge a system with gauges, here in Virginia today with 85 degrees, our lovely humidity, and it works fine. Now head to Florida and it gets a little weak because the humidity is higher there. Then maybe we head to central Texas and it works fine again. Then we head into the desert and it starts to fail miserably because it now it is way over charged due to the lack of humidity. Frustrated because you just had it fixed in Virginia, you take it to a shop and they cannot determine the problem. The pressures are good but the hot side is only slightly warm and the cold side is warm as well. Shoot, must be air (non-condensables) in the system. (with R134a pressures donít climb with an overcharge like they do with R12, it is very hard to determine an overcharge with R134a), so they recover the refrigerant, pull a vacuum, and then fill it with gages and a thermometer stuck in the dash vent. Now, vacations over and your back in Virginia and the thing wonít cool again! Because it is now undercharged.

The only way to get the right amount of reserve into the accumulator or receiver dryer is by weight. Oh, and donít tell me to use the sight glass on the receiver dryer. It donít work with R134a. R134a never completely emulsifies with the refrigerant oil and therefore when you see droplets of oil going by in the receiver dryer sight glass, looks just like vapor bubbles, and leads to an over charge. Once we even tried the fancy Snap-On electronic bubble sensor tool, to see if it could tell oil from vapor, and that sensed the oil bubbles going past.

I was a trainer for Caterpillar for over 20 years and each spring for 6 weeks of classes, taught A/C class on systems using R134a refrigerant. I fought with a new bunch of guys and a couple of gals, who wanted to show me they could charge R134a with gauges and a thermometer. You can get close but itís not right. Here is my main reasoning. 20 years as a trainer, times 6 classes each year, times 10 students in each class. On the last day of class I challenged them, two students working together, each pair had 30 minutes, starting from a fully evacuated and vacuumed system, to fill the system within 10% of the proper weight of refrigerant) This was not to pass the class, just a demonstration. Thatís about 600 tests and not once could any of them get within even 20%, and most were at least 30% overcharged. (we were charging a training aid with a 2lb receiver dryer and took exactly 4lb. of R134a ) There were three conclusions that came out of all this silliness. Charging by weight is more accurate, and second it is quicker, and third the system will cool really well under all conditions when filled by weight.

That's my tale and I'm sticking to it. Please no hard feeling. Use all this, don't use it that is fine.
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Old 03-24-2012, 07:16 PM   #16
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RikersRanch, I have charged many a small system with with gauges and superheat and never a problem. The point is if the tech understands the basics of the Rankin Cycle, he can diagnose and charge most mechanical systems with a little thought.

Yes, weight charge will get you right every time on identical systems...but only if you pull the entire remaining charge and weigh in the new charge. Also, the weight charge is designed for the one design condition where is it right-on. Now you get a little off design and that is no longer the optimal charge.

So if you charge in by gauges and superheat, and you do it right, it is correct for that particular operating condition. Again, you get off that specific point and the charge is no longer optimized.

I am not arguing with you, I am just explaining why I am right.

Ken
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Old 03-26-2012, 08:21 PM   #17
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I agree

Quote:
Originally Posted by TXiceman View Post
So if you charge in by gauges and superheat, and you do it right, it is correct for that particular operating condition. Again, you get off that specific point and the charge is no longer optimized.

I am not arguing with you, I am just explaining why I am right.

Ken
I cannot argue with that. Ken, Believe me, I do understand the procedure. I just wish we could sit and chat for a couple of hours and I could pick your brain for a while. I know (knew) just enough to help my techs get the job done.

I am a wrench at heart and not an A/C wrench. I pulled wrenches on Caterpillar heavy equipment for over 20 years. They pulled me off the shop floor and put me in the training department and after 5 or 6 years made a trainer out of me. but I'm still just a wrench at heart. I learned all I could to help my guys get systems working but I'll admit i still had a lot I didn't have the specifics on. I was restricted to teaching the Caterpillar way and i guess i drank too much Cat cool-aid. I Retired from Cat in 2007 to take care of my disabled wife. She was diagnosed with MS that year. So I've been away from this for a while.
A/C was one of my most fun classes so I guess I got a little carried away here.

Thanks for the follow up. Wayne
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Old 03-26-2012, 11:50 PM   #18
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Quote:
Originally Posted by KIX View Post
An A/C tech can determine the correct quantity by gauge pressures.
To check performance use two thermometers.
Place one in the air supply and one in the air return.
Give a few minutes to settle out and you should see a difference on 20 degrees plus or minus a couple.
Checke air flow meaning clean the condenser coil on the roof and clean or change the air filter. It's a good idea to vacuum the evap coils or if practical clean them with a non rinse coils cleaner.

To check the charge using pressures.

1. Hook up the guages.
2. Turn the unit on and drop the t-stat. It is best that the unit run for at
least 7 minutes to allow the system to "balance".
3. Take the out side ambient temp.
4. Add 32 to the temp. This temp will correspond to the pressure you
should have on the high side. The guages have the pressures on the on
the outer edge and temps on the inner scales. These are color coded.
R-134A light blue, R-22 green, R-410 pink, R 404A orange.
A pressure/temp. chart can be had online or at a hvac parts house if
your refrigerant is not on the guages.
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Old 03-26-2012, 11:55 PM   #19
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I remember once using a slide chart for 410.
You set it for the outdoor ambient, off coil temp. and then factored in s/h some how but it's been a long time.
I mostly use 410 A now and do well enough with the old ambient plus 32 method.
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Old 03-27-2012, 12:01 AM   #20
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Quote:
Originally Posted by TXiceman View Post
RikersRanch, I have charged many a small system with with gauges and superheat and never a problem. The point is if the tech understands the basics of the Rankin Cycle, he can diagnose and charge most mechanical systems with a little thought.

Yes, weight charge will get you right every time on identical systems...but only if you pull the entire remaining charge and weigh in the new charge. Also, the weight charge is designed for the one design condition where is it right-on. Now you get a little off design and that is no longer the optimal charge.

So if you charge in by gauges and superheat, and you do it right, it is correct for that particular operating condition. Again, you get off that specific point and the charge is no longer optimized.

I am not arguing with you, I am just explaining why I am right.

Ken
The later thing is sub cooling. You might say it's the inverse of s/h should be 18-25 degrees. I run with a group that weighs in for a lot of small appliances. Problem is they keep forgetting to allow for what's in the hose.
On a 4 pound charge you've got some wiggle room but on less than about a pound and a half a few ounces means a lot.
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