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  1. #1
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    Condenser Pressure Control



    Hello,

    So I'm designing an industrial refrigeration system using a screw compressor, around 90 ton capacity, with R134a. Evaporator pressure around 46 psia which has to be maintained due to temperature restrictions. Compressor discharge around 149 psia steady. The condenser is running on cooling water, which at it's warmest is 79F and at it's coldest can be 61F. I set it up with a fairly warm condensing temp of 105F with a saturated liquid outlet. The compressor only has a suction side slider. There is a hot gas bypass line in place already that can be modified if need be.
    My original plan was to throttle the condenser cooling water for turndown conditions in the loop, to maintain steady condenser pressure. A 3:1 turndown is possible in the loop in terms of tonnage, and the loop must be able to go to complete idle, which is mainly what the hot gas bypass is for. It might sound unusual, but the end user is requesting that they use more water than I designed it for, thereby lowering the condenser pressure. Since there is no secondary slider, the refrigerant just expands down to the condenser pressure when it exits the screw. The oil separator has a minimum of 110 psia to maintain good oil separation.
    Based on the amount of water the end user is proposing to use, the pressure could end up below 110 psia, and they refuse to turn the water flow down even in load turn down conditions. Do I have any options to try and maintain condenser pressure despite the excessive cooling water? I'm sort of on the fence with the idea that my expansion valve can be throttled to backpressure my condenser. I'm not entirely sure if that idea would work, I'm leaning towards no. I'm trying to maintain the 46 psia evaporator pressure constant no matter what the load. I've got a fairly large receiver below the condenser, so I doubt I'd ever flood the condenser.
    I've over thought it all too much, and need a fresh perspective on if this can be done, or maybe it's commonplace in industry already. I understand most of the basics of refrigeration, but this one threw me for a loop. I'm trying to think of ways to maintain condenser pressure in these turn down cases where the pressure might drop below 110 psia. I'd be glad to answer any questions or clarify things I didn't explain right.

    Thanks



  2. #2
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    Re: Condenser Pressure Control

    Actually just found a pretty helpful article on hold back valves. This might be the solution to my issue. It's sort of exactly what I was looking for. The was on the right track, trying to backpressure the condenser, but I couldn't quite put the pieces together that would produce that result. Guess I found it.

  3. #3
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    Re: Condenser Pressure Control

    What is the method of liquid control is supplying the evaporator?

    A couple of thoughts other than head pressure control valves:

    Three port valve on the water side, to maintain line flow without lowering head pressure excessively.

    PM type valve between oil separator and condenser inlet to maintain correct operating pressure in the separator, and let the head pressure fall to whatever the water temperature permits - provided this doesn't interfere with your liquid metering device.

  4. #4
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    Re: Condenser Pressure Control

    PM valve on the compressor discharge would be most efficient, it also prevents the oil from rushing out into the system if the compressor trips.

    other than that, frequency drive on the condenser pump, or a pressure regulated valve on the water outlet from the condenser.
    -Cheers-

    Tycho

  5. #5
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    Brian_UK is online now Moderator I am starting to push the Mods: of RE Site Moderator : and general nice guy
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    Re: Condenser Pressure Control

    If the customer wants a specific water flow along their pipe then allow that but take the quantity that you need for your condenser from their flow pipe. Pump it around the condenser and return it to their pipeline.

    Is there some reason why they are insisting on having a different flow rate than that required by the machine?
    Brian - Newton Abbot, Devon, UK
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