Tandem Compressors

[clydemule]

Does anyone have experience hooking 2 Discus compressors in Tandem?

Copeland says you must use and oil separator for oil equalization, but I have heard others say that for only 2 compressors an equalizer line between the 2 crankcases is OK, and that for 3 or more you need the separators.

Any opinions of electrical vs. mechanical level control?


Medium Temp application(20 FSST/120F SCT), 404a, POE, Air cooled.

Thanks.

clyde

[R12rules]

Smart & Final had that in their warehouse stores. Jus like you described.

[gm-1978]

DOESN'T HURT TO HAVE A OIL SEP WITH A OIL RESERVOIR AND FLOATS.I HAVE EXPERIENCED OIL EQUILIZING LINES, JUST QUITE DONT MAKE IT.
DEPENDING ON YOUR APPLICATION AND THE WAY THE STORE OR WAREHOUSE IS PIPED, SIZING UP A OIL LEVEL CONTROL SYSTEMS
MAKES MORE SENSE TO ME.

GOOD NIGHT!

[clydemule]

Actually, this is for a packaged glycol chiller for a brewery. The compressors are side by side, about 10" between them. Suction comes from a brazed plate evaporator, through a accumulator, then splits to each compressor. Suction line is maybe 36" total.

Condenser sits right behind the compressors.


I agree with the "can't hurt", but I always try to do without if it doesn't hurt reliability. A lot of times any extra part you add causes as many problems as it solves.

This chiller is going to be installed near my shop, so I am willing to build the chiller without it and monitor the operation. If I have oil problems, I think I will add them.

One of the things that I don't like is the recommendation to connect the crankcases thorugh the 1/4" npt fill port. Seems to small. I would rather pull of the blanked out sight glasses and use a 1 7/8" BC flange, like you find on oil level controls.

Does anybody know someone who makes the flange that can be brazed to 7/8" or 1 1/8" pipe? I can machine one myself out of steel or copper, but would rather buy one.

Thanks,

Clyde

[condenseddave]

You're looking at building up a miniature parallel rack, not running compressors in tandem.

Tandem compressors share a common suction endbell.

One suction service valve, two motors, pumps, and separate discharge valves.

Kinda rare to see tandems anymore. It was NOT a good thing to have a valve plate go on one of those.

Those plates are available from a Copeland wholesaler.

Get yourself an AC&R Helical oil separator, oil reservoir, a Sporlan OCV and two Sporlan oil level regulators, and you'll be set to go.

[cynic]

this would be a tandem A/C not refrigeration though

[refer dude 2479]

Originally posted by clydemule
.

One of the things that I don't like is the recommendation to connect the crankcases thorugh the 1/4" npt fill port. Seems to small. I would rather pull of the blanked out sight glasses and use a 1 7/8" BC flange, like you find on oil level controls.

Does anybody know someone who makes the flange that can be brazed to 7/8" or 1 1/8" pipe? I can machine one myself out of steel or copper, but would rather buy one.

Thanks,

Clyde

Your gut feeling is correct. DO NOT try to equilize the oil with the plug on the crankcase. You need to equilize through the blanked off oil sight glass port. The purpose for the equilizer line is two fold. First to insure that the two crankcases are at the same pressure so oil will run normally through the check valve between the motor chamber and the oil sump. Second you need a connection between crankcases that will establish and equal oil level between the two compressors. If you have access to installation literature for the Carrier 5H sewries compressors there is a good description on crankcase equilization. They actually recommed two lines. One vapor only equilization, and the second for establishing the common oil level. Hussmann did this for years on thier direct drive compressor systems.

Copeland offers the pipe flange connection. Try to get a copy of Copeland Application Bulletin AE-1235-R1. It is a one pager but gives the basics that Copeland recommends.

[condenseddave]

That's a true tandem that cynic posted a picture of.

Don't recall ever seeing one do any duty other than AC. I cna't imagine that the tandem would be a good medium/low temp unit...

[ballvalve]

Just a side note. That picture cynic posted sends chills down my spine. When those things work OK everything is OK. But if you have a burnout in one compressor its bad, very bad, especially if one comp continues to run and evenly distibute the acid. I have seen this and if i ever see it again it will be too soon. Cheapo customer did not want to replace both compressors as we advised. Stupid us replaced one. Stupid us were back in no time for another burnout. Even after oil changes, suction and liquid drier changes. My theory is that the second compressor was damaged during the original burnout.

[Dowadudda]

I've worked on tandems in older stores. PIA. Old equipment and we would have oil trips. It was equalized like you say through the oil plug. That don't do squat. All that does is equalize crankase pressure and not oil column. True equalization takes place when your actually physically equalizing your oil such as a 1 1/8 line between the two oil sight glasses. Carriers older crap for their a/c chillers did this almost exclusively it seemed on the equipment I worked on. Anyhow. Tandems work great with a seprator. They work awesome with it.

The problem with not having a separator on something like that is because those pumps aren't always fully loaded up and they both don't get to drink the same amount of oil, especially so if there only sucking from that small 1/2 line between the plugs. Add a separator and be done with it.

[NedFlanders]

Originally posted by condenseddave
That's a true tandem that cynic posted a picture of.

Don't recall ever seeing one do any duty other than AC. I cna't imagine that the tandem would be a good medium/low temp unit...

I seen motor rooms with nothing but tandems . When I walk into these , it's like walking back in time. I 've done my share of comp changes and it's always fun.

I really don't know what Copeland was thinking when they made that design. I cannot see ANYthing good about it except you are saving on the oil separator,resv and floats.

(Some engineer at Copeland with a white lab coat and glasses)
"Hey I got a great idea , lets bolt 2 very heavy awkward compressers back to back. Then we'll put boxes in front of the oil pumps with all the power in there . Then we'll pipe around it with rigid pipe. Ok , this is the best part, to change one , any time a compresser needs to be changed , the whole system will be dead for that time . Oh yes , this is great too , if you bump the windings on the case as you are swingin' it back in , throw that compresser away. That way I can retire sooner because sales will be up."

(Some fat guy with a cigar in his mouth replies) "BRILLIANT,
rush it into production ! Before you do that , make sure the sevice valves don't hold ."

[refer dude 2479]

Originally posted by NedFlanders

Originally posted by condenseddave
That's a true tandem that cynic posted a picture of.

Don't recall ever seeing one do any duty other than AC. I cna't imagine that the tandem would be a good medium/low temp unit...

(Some fat guy with a cigar in his mouth replies) "BRILLIANT,
rush it into production ! Before you do that , make sure the sevice valves don't hold ."

Yep I think I remember reading in a trade journal years ago about that very design and sales meeting. My experiance with these pieces of crap was in a store in a little town named Burke in northern Idaho. They had 4 of these thing made by the Weber Refrigeration Co. Weber went out of business years ago.

This store featured some other "inovations" as well. The store A/C was a water coil in an air handler mounter in the back room. The source of cold water was the South fork of the Coeur D'Alene river. When it got to hot in the store the t-stat would start a pump and pump water from the river to the coil and then back to the river.

Store heat was another piece of art. Air cooled condenser for refrigeration was mounted on it's side in mechanical room. Normal run would suck air through room thru cond and out side wall of room thru fans. When store needed heat all condenser fans would reverse direction and thru some dampered ducting draw air from the back room of the store. Air would be blown into mech room and through another dampered duct that fed the air into the back room at the inlet of the air handler (Same one with the water coil.) When the system was really working good in the dead of winter the store would stay at about 50 degrees, which felt pretty warm compared to the -10 to 0 degrees outside. These were big belt drive fans too. 48 inch.

Saw way too much of this stuff up there could not take it and moved back to sunny Cal.

[frozensolid]

I can see it now. Manager; why is my a/c not working? Tech; I'm not sure, but I did find a trout in your coil.