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Custom-built system -- how much refrigerant PART 2

SpinRite on Thu July 30, 2009 7:31 PM User is offline

Year: 94
Make: Holiday Rambler
Model: Navigator
Engine Size: 8.3L Dsl
Refrigerant Type: R-134a
Ambient Temp: 86
Pressure Low: 20
Pressure High: 220
Country of Origin: United States

This is the second half of a story about a motorhome system. You can read Part 1 HERE.

Well, here we are, six weeks later: New compressor and receiver-drier. Condenser, evaporator, TXV, and hoses were all flushed and dried.

Put 8 oz of PAG-100 in the R-D. Drained shipping oil from compressor, added about 4 oz. PAG-100, turned over a couple dozen revolutions. Added remainder of PAG oil into the discharge hose. Total oil in system is 16oz. Pumped it down to a hard vacuum, no leakage after two days.

Put in 4 cans of r134a, as liquid through the high side port, then started it up. At idle, ~900rpm at the compressor, low=25, high=150. Gradually added another can, checking pressures and duct air temp, without much happening until I got it all in.

Now have about 60oz total.

Ambient is 86, humidity very high.
At 900rpm, low=35, high=180
At 1500rpm, low=18-20, high=200

I can't feel the input side of the evaporator, since the TWX is right on the front of the evaporator, inside the airbox, but the line coming out is cool. If it will help any, I could remove the airbox cover and get at the short tubing between the TWX and evaporator to check the temp.

The air coming out of the ducts is cool, but not cold. (There is no hot water going into the heater.)

A little more refrigerant? Any ideas?

HVargas on Thu July 30, 2009 7:41 PM User is offlineView users profile

I would suggest more. Your low side is awfully low at 1500rpm.

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TRB on Thu July 30, 2009 8:39 PM User is offlineView users profile

Not seeing the complete set up, that sure seems like a lot of oil. This system have dual evaps?

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bohica2xo on Thu July 30, 2009 10:22 PM User is offline

60 ounces of refrigerant is a start on something with lines like that - especially if the high side line is -6 or larger... Put some more refrigerant in.

Tim - that would be a lot of oil if it did not have 30 feet of suction line on it.

B.

-------------------------
"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

SpinRite on Thu July 30, 2009 10:50 PM User is offline

It only has one evaporator, but as bohica2xo says, the it's a looooong way from the front to the back.
The compressor, condenser, R-D are in back, the evaporator is in the very front.
Probably a bit over 35-feet, since overall vehicle length is 38 feet.




Also, here is the condenser, it's behind those slots at the lower rear corner in the picture above.
It has two sixteen-inch fans, which I had to replace. The left one was just removed in this picture, and you can see the tube-and-fin condenser. The tubing looked to be about 3/8-inch -- overall size is about 18x36.



Edited: Thu July 30, 2009 at 11:45 PM by SpinRite

TRB on Fri July 31, 2009 10:02 AM User is offlineView users profile

I was not sure on the hose length for this app.

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When considering your next auto A/C purchase, please consider the site that supports you: ACkits.com
Contact: ACKits.com

NickD on Fri July 31, 2009 2:22 PM User is offline

Assume both condenser fans are back in place and running, at 85*F, shoot for 26-27 PSI on the low side watching that high side for around 250 PSI. Mixing up you with Marvin, can you find a sight glass anywhere? Nice to know you are getting liquid instead of foam. Using a 30# cylinder is so much nicer and a lot cheaper in the long run with less possibility of putting air in the system.

SpinRite on Fri July 31, 2009 5:13 PM User is offline

There is no sight glass.
Service ports are right at the compressor connections.
Evaporator is about 8x16 inches.



Watching the pressures, I have now added 2 more cans -- for a total of 84oz in this system!
(But, may have lost an oz. or two during can changes and hose purges.)

At 1500rpm, LO=30, HI=250, plenty of condensate from evap drain and on suction hose.
Door and windows open, 95F and very high humidity, dash air 66F.

Closed door and windows, let it run for awhile(~20min), parked in shade
Inside temp dropped to 84F, vent temps 64F

Now pressures have dropped down to LO=18-20, HI=220 -- pretty much where they were yesterday -- and 24 oz. ago!

If it was overfilled, wouldn't high side go way up?

NickD on Fri July 31, 2009 5:54 PM User is offline

Overcharging, can't help but go way up on the high side. Now you have me wondering if you have a leak.

bohica2xo on Fri July 31, 2009 7:45 PM User is offline

Spin:

Your 84 ounce charge looks good. The full load test is right where it should be, especially with high humidity. The high side ratio is less than 2.7:1, and the low side looks good for that system.

When testing a system like that you need to consider the 10 square feet of additional evaporator called the suction line. The 30 psi low side reflects this.

The pressures dropped as the heat load dropped. If you were to test with a hot coach again the results would be the same. Hopefully you put some dye in the system, so if it does leak you have a head start on finding it.

Write that 84 ounce number in the owners's manual, it is a good number. Like I said in the other thread, start @ 60 to 70 ounces before testing on that type of system. You are not overcharged.

B.

-------------------------
"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

SpinRite on Fri July 31, 2009 10:31 PM User is offline

Yes, I do have dye in the system.

Any thoughts about the duct temps -- mid 60's...?
Is that "normal"?

bohica2xo on Sat August 01, 2009 12:39 AM User is offline

"Normal" is a relative term on a beast like that.

You started with high humidity, high temperature air - in a heat soaked vehicle. Your evaporator had an unlimited supply of work, and you have a compressor that is not as large as the OEM unit in a '70's vintage GM sedan.

That compressor does not reach full output until someplace over 3,000 shaft rpm. If it is driven 2:1 off of that diesel, you are close to full output at 1500 rpm high idle. Look at the drive ratios, I find many diesels with less than optimal drives on A/C compressors...

So you have less compressor capacity than a 1970 Buick Skylark, to cool a space you could park two of the buicks in. A suction line with 10 square feet of surface cooling the pavement. The evaporator is managing a 30f drop @ several hundred CFM, and will eventually pull everything down. You saw a 10 degree delta on cabin temps, which included all of the heat soaked interior components. Imagine putting a window shaker in an apartment - how long does it take to cool?

Eventually, you will see lower vent temps. The pressures were dropping off, and you might be able to take even a bit more charge. If you can borrow an electronic sightglass, you can look at the liquid line just before the TXV. I usually do this to determine final charge.

B.

-------------------------
"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

SpinRite on Sat August 01, 2009 2:55 AM User is offline

Thanks for putting it in perspective.
I don't know what an electronic sightglass is, unless it's an ultrasonic stethoscope.
(In a former life, I could have borrowed one, but those days are passed.)

NickD on Sat August 01, 2009 7:14 AM User is offline

Seems bit undersized to me, you have 128 Sq In, evaporator in a 38 footer, I have a 144 Sq In in a 27 footer. While charging with 30 psi in the hot sun and doors opened, would have suspected in the shade, your TXV would have closed a tad increasing rather decreasing low side pressures. A long liquid line isn't bad and attempts to extend the side of the condenser, but that suction line has to be well insulated.

Didn't catch which compressor you are using, mine came with the largest York they made, forgot the size of it. Friend had an electronic sight glass, suppose to measure the pulsations in the liquid line and amplify them, this was some time ago, but we could do a better job of feeling the pulsations with our hands. Idea is to charge until those pulsations disappear.

My girls force me to switch on the generator after the home sits in the hot sun, run it for about ten minutes then switch it off, this was on a 90*F day, then wife complains it's too cold after driving for about 20 minutes. Don't think they would be happy with a 66*F vent temperature and don't feel you are happy with it either. What are the inlet and outlet temperatures of your evaporator tubes after you run it for twenty minutes in the shade?

bohica2xo on Sun August 02, 2009 3:17 AM User is offline

Spin:

My electronic sightglass uses two ultrasonic transducers clamped on to the line near the TXV. It has always worked well for me. Snap On charged me a bloody fortune for it years ago...


A few years ago we were using class A slide outs for field offices on remote projects. When it hits 115f you have to run the APU, both roof mount units AND the dash A/C to hold things down in a 34 footer. At least the engines never overheated when we were doing that. Sure burned a lot of fuel though.

Driving the coach may have an effect on condensor airflow as well. take it for a highway spin, and see if it is comfortable to drive - this is the main use for the dash A/C anyway.

B.

-------------------------
"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

NickD on Sun August 02, 2009 7:33 AM User is offline

Where is the condenser mounted on this home? It is a pusher. Prices of these things sure came down, especially when the price of gas was over four bucks a gallon. I looked at some pushers and even a 78 GMC, but the complexity and parts costs of these things scared me. Our dealer had brand new ones selling for $350K, but looking at used ones, like two years old, could suffer over $250K depreication. Wife liked a 454 powered 40 footer with a master suite and a laundry room, but dealer said to only expect about 5-6 mpg, for just the two of us, a 27 footer was plenty. Had six brand new tires on it and three brand new batteries, so didn't think $4,000 was too bad, but put in another $2,000 worth of parts plus lots of labor.

But we are having second thoughts about it, state parks are charging 30-40 bucks a night now plus have to make reservations a year in advanced, private parks are like staying in a Wal-Mart parking lot. But we did have fun with it staying at Veterans Park over the 4th in Milwaukee and was nice to have a clean restroom and plenty of good food to eat. We also enjoyed our honeymoon we couldn't take until we were married for two years due to immigration and other matters. Was concerned a couple of months ago about my overhead AC, but is meeting full performance specs, main problem was the sun hitting that huge piece of glass up front. It also nice to have that generator in case we lose power, can keep the furnace, sump pump, and refrigerator working. Had that out too for cleaning and a rebuild, starts the instant you touch the button.

SpinRite on Wed August 04, 2010 12:41 AM User is offline

I know people seldom come back after getting their problem fixed, so I wanted to stop in and say "Thanks" to all the kind folks who helped me with my RV's a/c system repair.

I was visiting my son in San Diego a couple weeks ago, and passed through Phoenix on my way home to North Carolina. (It was a Sunday, so I didn't stop by the shop to say "hello" in person.)

I sure was glad to have some relief from the Phoenix heat. It was 114 outside, but we were able to maintain 84 inside. (Not too bad considering we were surrounded by a lot of non-thermopane glass.

You guys are great!


Edited: Wed August 04, 2010 at 12:42 AM by SpinRite

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