capacity control

[gbfromsd]

Got a customer with several restaurants. Undercounter reachin cabinets and drawers used to be standalone, a few years ago all were put on remotes. Problem I have is where 1 CU is feeding 3 or 4 evaps, no capacity control. Coils experience frequent icing problems, compressor failures every 3-4 yrs due to high superheat during low load conditions. These are pump down systems.

After talking with a couple suppliers, there doesnt seem to be an easy solution to provide some means of capacity control - or specifically to provide for the low loads these compressors are likely to see. HG bypassing is not possible as the linesets in some cases are nearly 100feet long, and in some stores run under the floors.

If I can just figure a setup, I can sell two dozen of em.
I'm thinking some form of evap pressure control coupled with a liquid injection?? Or maybe I should just take the
pump down systems out and toss em in the trash. Willing to consider all options as long as I can ensure good oil return and superheat control. Corporate requires box temps at 38-40 and NEVER over 40.
What's out there I should be looking at?

g

[clydemule]

Can't you do HGBP by bypassing from the discharge into the suction and using a desuperheating expansion valve? This would be done at the CU.

Not the best solution in terms of oil control though.

What controls each load, liquid line solenoid valves?

I you were to use EPR's you still would have to do hot gas or something wouldn't you?

Clyde

[toptech]

Originally posted by gbfromsd
Got a customer with several restaurants. Undercounter reachin cabinets and drawers used to be standalone, a few years ago all were put on remotes. Problem I have is where 1 CU is feeding 3 or 4 evaps, no capacity control. Coils experience frequent icing problems, compressor failures every 3-4 yrs due to high superheat during low load conditions. These are pump down systems.

After talking with a couple suppliers, there doesnt seem to be an easy solution to provide some means of capacity control - or specifically to provide for the low loads these compressors are likely to see. HG bypassing is not possible as the linesets in some cases are nearly 100feet long, and in some stores run under the floors.

If I can just figure a setup, I can sell two dozen of em.
I'm thinking some form of evap pressure control coupled with a liquid injection?? Or maybe I should just take the
pump down systems out and toss em in the trash. Willing to consider all options as long as I can ensure good oil return and superheat control. Corporate requires box temps at 38-40 and NEVER over 40.
What's out there I should be looking at?

g
How do you mean " high super heat during low load conditions"? Do you mean the compreesor overheats during a lengthy pump down?
I assume it takes an in ordinate amount of time to pump these babies down. Is there so much refrigerant that they sometimes can't pump down?
Perhaps electronic stats and solenoids @ each evap will do the trick. 6 F off cycle, no pump down. I'd install an accumulator @ the condensing unit and or HGBV to desuper heat the liquid on start up and dump the pump down.
Anywho, this is a starting point for a flood of ideas sure to follow.
Bernie

[jeffcolledge]

How about removing all the solenoids and installing EPR valves in every evap. coil. This should maintain box temp and prevent freeze-ups.

[R12rules]

I think we need some more info here.


If there are oil return problems and you already have pump down in affect ... hmmm.... sounds like there may be piping issues there.

Are these little pots or semi-hermetics?

Are the CU's located higher, lower or level to the evaporator coils?

What's the ambient at the CU's? Is it Arizona dry heat over a hundred 24/7 ??? Or is it California dry heat over a hundred .... 10 hours a day?

Or .... is there humidity involved?

Something to think about is those units will not be returning oil all that well during peak summer season simply due to lost capacity cause of the extreme heat. Maybe one of your "To-Do's" could be to include a cooling affect for your CU's ..... as part of the overall project there.


I assume these units are all air cooled and the condensors are in good shape.

What freon are they running on?

What is the condition of the insulation on the linesets?

Do any of these linesets pass thru a super hot environment ... like the attic?

[gbfromsd]

There are no oil return problems, only that whatever solution I look to - it must consider oil return.

The CU's are all R22 medium temp Tecumseh or Cope
semi-hermetics. They are in a utility closet with
a supply fan set to turn on at 80* ambient. Being close to the coast, these CU's should never see more than 90* even in the hottest of summers.

Though the configuration is typical of most stores - let me provide one individual system as a starting point.

This particular system serves 2 reach-in cabinets. 1 cabinet has a 1700btu OmniTemp OTM17, 1 cabinet has a
1200btu OmniTemp OTM12. It also serves a 4 drawer undercounter reachin with 2each OTM12's for a total of
5300btu's. As part of a recent overhaul, the condensing unit and lineset were replaced. The original lineset was
found to be leaking and ran under the flooring. The new
lineset in part runs through the attic, and in part is
hung along the ceiling through the dishwashing area.
The condensing unit is at 5ft above ground while the coils
are all 2ft off the floor. The length of lineset is 90ft
with a 10ft rise from the coils to the ceiling. Traps
as needed were made at the bottoms and the suction runs
downhill to the compressor.

Controls as installed are: A419's and/or mercury bulb switches (omnitemp stock) with LL solenoids and TXV's. The
CU pumps down on a LP switch.

When only one coil is calling (1200btu load on a 6000btu
3/4HP) the suction pressure typically runs 22psi with
a superheat approaching 60degrees. With all coils in operation, the suction is a more reasonable 38psi with a
58* return temp and a compressor discharge of 190.

The old compressor did, in fact, overheat as evidenced by the brown label on the compressor casing. I figure this
setup is good for 3 or 4 years when the compressor again
burns up. In the meantime, the coils run the ragged edge
of icing up and/or barely holding box temp at 38*.

[Freezeking2000]

I would just let it go and let the refrigeration gods take care of it!

Honestly i would just install a 5 minute delay on break timer on compressor contactor and add defrost time until they dont ice up any more!

The 5 minute delay will help by allowing maybe another cabinet or two to call before the comp comes back on.

Unles the custome wants to go for all mew units!

[icemeister]

Originally posted by gbfromsd
The old compressor did, in fact, overheat as evidenced by the brown label on the compressor casing. I figure this setup is good for 3 or 4 years when the compressor again burns up. In the meantime, the coils run the ragged edge of icing up and/or barely holding box temp at 38*.
Hell, True units do this all the time. ;) :D

What you're trying to avoid is premature compressor failure, coil icing and temperature control issues. Multiplexing these boxes together on one small condensing unit leaves no other practical choice but to accept compromises.

The best scenario I can see here is essentially what you've got with individual T'stat/solenoid temp control with the addition of a common defrost clock to bring all loads on at once three or four times a day to not only keep the coils cleared but to help drive the oil back to the compressor. I'm assuming also that these evaps have TXVs and not cap tubes. :eek:

So the customer has saved half the bucks installing a single 3/4 HP unit instead of a bunch of little 1/4 HP systems that might last twice as long but without all the hassles in between.

[gbfromsd]

Ice, you bonked it right on it's noggin.
All these boxes used to be stand alones a few years
ago with their attendant problems.
Currently defrost times are 12x20 - that's an
awful lot. My old boss, a man of 20yrs, figured
that's what was needed but I'm open to other opinions.
Still . . . wish there were an off the shelf widget valve or framastat box I could slap on it.

[Freezeking2000]

I usally run 4 (per 24 hours)at 40 or 50 mins on the line units. The duration must be suffecient to clear the coils even if the door was open for 6 hours.

[gbfromsd]

Aaah, I'm seeing the light. After reading a bit I'm leaning towards putting an EPR on the common suction line. I can do this because all of the boxes run at the same temp anyways. Then adjusting the defrost times to cause all the coils to call at once several times a day. I think visiting the TXV's would be good too.

[Freezeking2000]

i think your compressor will not like an epr too much, the suction will dropp so much if only one or two coils are calling that the comp superheat will go wild!

I have done the epr thing only of i get frozen coils after increasing the defrost duration