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  1. #1
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    EPRs on a mid temp multi evaporator system

    Morning all.

    Been a while since I was last on here and my refrigeration knowledge is somewhat rusty due to being out of the trade for a few years.

    Anyway I'm planning on using a single condenser unit for 4 medium temperature displays for meat (-2 to +2) adjacent to each other in the same room of equal temperatures with their own controllers, and a separate line set from each unit to a common manifold of a suitable size of maybe 2' long.

    My thoughts are...

    1. Use EPRs on every evaporator at the end of each low side line set close to the common manifold, and individual pressure switches to maintain a constant pressure within each leg when the compressor is running to serve only one display. So that when all the evaporators are satisfied the EPRs will prevent the other pressure switches from prematurely opening and closing in the other low sides, should one or more evaporator pressure start to rise. Causing them to over cycle and freeze up and erroneously not hold temperature.

    2. Fit a liquid and suction line sol valve controlled from an individual pressure switch, and use the pressure switch to also energise a contactor as well as the sol valve to switch a variable voltage from a transformer to the compressor. Then its volumetric duty is closely matched to the load and it doesn't over suck and virtually pump down the line sets of the other evaporators when only one display calls for cooling.

    3. Maybe a combination of both the above methods.


    I used to fit and modify systems like this but I have almost forgotten due to being rusty.

    Any further thoughts would be appreciated.
    You don't get owt for nowt.

    Cheers ah kid

  2. #2
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    With 4 coils and 4 EPR's you should be able to control the pump on pressure control only.

  3. #3
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    The EPR valves will help maintain a stable normal evaporator pressure at the display cases which will tend to maximize their "on time" thus minimizing low loads at the compressor. The individual thermostatic control is also best used as a low temperature limit control for each case and the individual liquid solenoids can do double duty for off-cyle air defrost. Finally, I would prefer to see all four cases defrost simultaneously since it will minimize the compressor's low load run time.

    That's all well and good for the cases, but the compressor cooling will need to be addressed. A hot gas bypass with a bypass capacity of perhaps 50% of the compressor capacity would be ideal to help keep the compressor suction up during low loads, but it will need to have a desuperheating valve with a temperature pilot control installed to maintain control of the compressor discharge gas temperature to around 95-100ーC. Normally, because of its affect on oil return, I don't like using a HGBP at the compressor instead of injecting it after the TEV, but in this case with multiple evaporator and only partial unloading I don't see any problems with oil return.

  4. #4
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    Thread Starter
    Thank's for the replies lads, I was planning on relying on the air on temp above 0 deg C as a defrost method.

    To be honest oil return is something that I overlooked so I'm glad that I asked, I'll consider using an oil collector because I don't think that I have ever used desuperheating methods before. If I remember correctly the last time I did one like this, I used a 12" fan controlled from a 80 deg C button stat to cool the compressor when it was running on low load, and reduced the voltage when only one evaporator P.S. was calling to cool so the compressor could match the load duty.

    I don't think the fan ever run though as it was a similar set up but with two evaporators and they always called to cool simultaneously within about five minutes of each other. I didn't consider the oil return neither but as the compressor voltage was reduced by half when one evap called, it probably didn't starve the compressor of oil to a level of risk.

    I forgot to mention that the equipment is second hand and the compressor is hermetic around 8 kw duty, so this is why I'm considering EPRs and reducing its voltage from 60 volts steps per 25 % load increments to compensate the effect of an over sized compressor. It came out of a butchers shop and its going into another butchers, the evaps were simply branched off a common line set with no EPRs, only separate sol valves and controllers.

    The displays must have fully pumped down into a vacuum when they were satisfied and I知 suspecting that it probably had temperature problems and over cycled the compressor causing it to trip the klixon. I've tested the compressor and the insulation is healthy but I think I値l replace the klixon, I just hope that mechanically it痴 as healthy.

    The reason why I'm considering voltage control over VVF control as a means of matching the compressor load duty is down to reliability and cost, and I want to keep simple with the minimum of electronics. I know that VVFs are common with aircon splits and larger industrial applications, but their budgets are higher than what they are for a butchers shop. Also after being reminded about oil return, the varying capacity may even reduce oil migration into the high side with little return. I知 also considering a low head pressure bypass as a preventive measure to be on the safe side when its on partial load.

    Thanks again lads and lasses.
    You don't get owt for nowt.

    Cheers ah kid

  5. #5
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    Quote Originally Posted by chilliwilly View Post
    Thank's for the replies lads, I was planning on relying on the air on temp above 0 deg C as a defrost method.

    To be honest oil return is something that I overlooked so I'm glad that I asked, I'll consider using an oil collector because I don't think that I have ever used desuperheating methods before. If I remember correctly the last time I did one like this, I used a 12" fan controlled from a 80 deg C button stat to cool the compressor when it was running on low load, and reduced the voltage when only one evaporator P.S. was calling to cool so the compressor could match the load duty.

    I don't think the fan ever run though as it was a similar set up but with two evaporators and they always called to cool simultaneously within about five minutes of each other. I didn't consider the oil return neither but as the compressor voltage was reduced by half when one evap called, it probably didn't starve the compressor of oil to a level of risk.

    I forgot to mention that the equipment is second hand and the compressor is hermetic around 8 kw duty, so this is why I'm considering EPRs and reducing its voltage from 60 volts steps per 25 % load increments to compensate the effect of an over sized compressor. It came out of a butchers shop and its going into another butchers, the evaps were simply branched off a common line set with no EPRs, only separate sol valves and controllers.

    The displays must have fully pumped down into a vacuum when they were satisfied and I知 suspecting that it probably had temperature problems and over cycled the compressor causing it to trip the klixon. I've tested the compressor and the insulation is healthy but I think I値l replace the klixon, I just hope that mechanically it痴 as healthy.

    The reason why I'm considering voltage control over VVF control as a means of matching the compressor load duty is down to reliability and cost, and I want to keep simple with the minimum of electronics. I know that VVFs are common with aircon splits and larger industrial applications, but their budgets are higher than what they are for a butchers shop. Also after being reminded about oil return, the varying capacity may even reduce oil migration into the high side with little return. I知 also considering a low head pressure bypass as a preventive measure to be on the safe side when its on partial load.

    Thanks again lads and lasses.
    I suggested the hot gas bypass as a means of unloading with a non-unloading compressor. Of course, with a variable speed compressor it's not needed.

    Oil return is still a concern, so you may want to consider a timed ramp up to full load every hour or so like the mini-split inverter systems do.

  6. #6
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    What about using a double riser oil trap system for oil return?

    I was thinking maybe 4 x 3/8" suction lines from EPRs branching in to a 3/4" common suction line maintaining a level, then forming a trap by splitting and branching out with a 3/8" for the low load riser and a 1/2" for the remaining load riser. I know that this is something that is more associated with multi low and mid temp applications on a common condenser, but I can't see any reason why it wouldn't work on a multi mid temp application.

    At least if only one evaporator is calling for cooling for a short time or any of the remaining only ever simultaneously call, at least there will be sufficient oil returned back to the compressor to equalise what enters into the high side.

    I suppose the cost associated with time taken to fabricate a double riser might be on par with using a high side oil collector.
    You don't get owt for nowt.

    Cheers ah kid

  7. #7
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    Those pipe sizes are quite small for meat display cases. What are the loads we're looking at here?

    If they are indeed those sizes, it's unlikely there would be a need for a double riser setup as a single reduced riser would usually suffice without an excessive pressure drop. As the piping sizes get larger the velocity needed to get oil to go vertically up increases as well.

    Tecumseh's refrigerant pipe sizing program has a handy riser sizing function: http://boxload.tecumseh.com/RefLineSizing.aspx

  8. #8
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    Thread Starter
    The compressor is around 8 kw and the evaporators are about 1.5-2 kw each, at this stage I'm just putting some initial ideas together before I fine tune any selection criteria and actually start the job.

    I'm just concerned that the oil won't return unless at least 2 evaporators call, and if the compressor doesn't unload then it will be an over sized compressor. Hence me considering varying the capacity of the compressor and the double riser.

    The pipe sizes are what the stubs on the evaporators are, so they're just what I'm going by. I didn't actually take the system out and it may have had another load connected to the compressor.

    The engineer that took the system out has been sacked by the owner of the butchers shop for over charging one of his other systems without cleaning the condenser, resulting it tripping on high pressure and not holding.

    So there could be another cabinet that's been left in the other shop, but looking at the full picture, it does need capacity control.
    You don't get owt for nowt.

    Cheers ah kid

  9. #9
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    Thread Starter
    Thank's for the link.
    You don't get owt for nowt.

    Cheers ah kid

  10. #10
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    OK, 8 KW to a metrically challenged Yank, would be about 27000 Btuh. Using the Tecumseh program for R449A at +20ーF evap temp I get 7/8" OD as a good size for a 20 ft riser with only .6 psi pressure drop and yet it will work for loads as low as 7000 Btuh or approximately 2 KW.

    This was based a chart published in ASHRAE Handbook- 2006 Refrigeration section 2 table 19 which shows the minimum capacity required for oil return up a suction riser at various conditions for both R134A and R22. I used the R22 velocity for R449A and arrived at the closest pipe size from that. I'm sure it's not all that precise, but close enough.

    So it looks like you could use a single riser for this, especially if you were to figure that most often there will be at least two evaporators calling.

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