Original message
| Chuck78
 |  "5 ton condensor on roof, rise/orphice size?"
, posted Tue 19 Sep 16:40  
Today, I had to change out a bad Copeland compressor on a unit that was around 3 years old I believe. The furnace is located in the basement of the building, and the condensor is located on the rooftop. This is I believe a 4 story building, and I would estimate the lineset to be at least 60 feet long, maybe 65 feet? roughly. Most of that lineset is vertical. I am unsure of wether or not there were any traps installed in the suction line to prevent oil migration, but the big issue here is sizing.
It is a 5 ton condensor and matching coil, and I believe a 120,000 BTU Trane Furnace. Condenser is a Gibson (Westinghouse).
I got the thing charged as best as I could, and had to leave due to dentist appt. I need to investigate this further, because I think the old compressor went bad due to being overcharged. I suspected the outdoor fan motor going bad, so I replaced that as well. Filter clean, and I believe the maintenence guy replaced the indoor blower motor recently.
The problem - my suction pressure was WAY too high, and I had to overcharge it to keep the coil from freezing according to my guages, due to suction pressure being too low.
Now I have no schraeder access port on the coil in the basement, but measuring my superheat at condensor, it came out to 15, but my subcooling was way high. I suspect now that from measuring superheat on the roof, with 55-60' of vertical lineset down to coil, that my reading was innaccurate. Also, my boss points out that even if my suction pressure reading was that low way up on the roof, I probably am running a much higher suction pressure at the coil, so even though I read it as 33 degrees on roof, he tells me he bets the actual coil temperature is okay and not risking freezing up.
I believe I read somewhere that when you have so much rise to the lineset between the coil and condensor, you must change the orphice size accordingly. Not sure what I recall the sizing would be, but from my problem, I would assume that with that tall of a lineset, if this is the answer to the problem, then I must need to change the fixed orphice out to a size or two larger, correct?
Being a 5 ton unit (biggest Nordyne offers, I beleive), I wonder how much larger they will go in orphice sizes beyond the standard 5 ton size?
Am I right on track here? And to properly charge this system, is it necessary for me to install an access fitting on the suction line at the coil exit?
Thanks guys, hope for a quick reply so I can impress the boss tomorrow. And pardon for my lack of formal training, I will be taking classes this fall.
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Replies:
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| Dog Biscuit
| "Re(1):5 ton condensor on roof, rise/orphice s" , posted Fri 29 Sep 20:26
The author at this website recommends adding five degrees of sub-cooling for every thirty feet of liquid-line lift. The oil-return is very important.
http://www.refrigtech.com/charge.html
To err is human to repair divine
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| | Dog Biscuit
| "Re(2):5 ton condensor on roof, rise/orphice s" , posted Fri 29 Sep 20:47
I guess "Better late than never" applies here!
Varify this:
If the density (WEIGHT/VOLUME as a liquid) of
R-22 compares to water, a 60 feet column of water
would generate 30PSI at the TXV, based on every
two feet of vertical generates one PSI of pressure.
Could that 30PSI additional pressure be part of the
problem?
To err is human to repair divine
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| chiller guy
| "Re(1):5 ton condensor on roof, rise/orphice s" , posted Wed 20 Sep 10:18
My concern here is not oil migration, but oil return. A 60-65 foot lift can be a problem if not piped and sized properly. Check the line sizing for that installation with the manufacturer (unless you are good at piping design). The compressor may have been lost due to lack of lubrication (oil is downstairs) or liquid slugging (if it came back all at once).
All refrigerants are safe - All refrigerant are dangerous. The difference is YOU !!!
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| | Chuck78
| "Re(2):5 ton condensor on roof, rise/orphice s" , posted Wed 20 Sep 20:00
The suction line is sized at 1-1/8", should be plenty of suction line for a 5 ton I would imagine. The lineset is buried behind walls, not sure if any traps were piped into it or not.
Also, the 10 SEER unit was not available now, but I switched to a 12 SEER compressor to avoid having to replace the entire unit. the 12 SEER I would have thought would have been added help for the extreme lift situation. The pressures were still really off. I swear I read somewhere about having to change the orphice size when long vertical runs were in the lineset. Looking for someone to confirm this, so that I could fix this problem. It is a scroll compressor, so I figured it could handle it for the time being until I was able to get back and figure out the problem, but the unit is currently overcharged to keep pressures up. I had a hard time when charging it to get the pressure out of the 50's, which is where I left it, just above 32 degrees on my suction guage. Suction line at coil measured 42 degrees.
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| | Chuck78
| "Re(3):5 ton condensor on roof, rise/orphice s" , posted Wed 20 Sep 20:05
Now with that much lift in the lineset, is my boss correct in saying that my pressures at the rooftop unit are going to be different than the pressures at the coil in the basement? If that is the case, and my suction is reading, say, 55 on condensor, which is say 33 degrees, and I have 41 degrees suction line temp @ coil, can I attempt to charge it based on my temperature readings down there? But if my high side is say 270 (way too high), I am sure it is only going to be higher at the coil, bringing up my first point, should I be switching to a larger orfice in the coil?
Your comment about the oil is bothering me. Compressor could get hit with a big slug of oil (after being starved) or liquid if there are no traps in the lineset at that lift height... and if the oil is all in the coil, big problem, no wonder it only lasted 3 years.
Any other thoughts?
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| | chiller guy
| "Re(4):5 ton condensor on roof, rise/orphice s" , posted Thu 21 Sep 08:16
Doing a quick check with my handy-dandy line sizing calculatior, and using R-22 I see that using an 1 1/8 suction line gives you about 850 FPM velocity. Good piping design calls for a min of 1000 FPM in the vertical rise for oil return. Also the piping design should be based on a 3 lb pressure drop in the suction line. It appears you have much more than that. All of these numbers can be fudged a bit depending on conditions.
And yes, your boss is correct in that the suction pressure at the coil outlet will be higher than at the inlet of the compressor. But it they should be within a few pounds of each other. You may have an evaporator fouled (full of oil). You need to get some pressure taps at the evap so you can see what is going on down there.
All refrigerants are safe - All refrigerant are dangerous. The difference is YOU !!!
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