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walk in cooler txv question

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I have a question regarding sizing a txv for a walk in cooler that is to maintain 38-40 degrees box temperature. When selecting the temperature I assume I should us +25? 25 degree evaporator then would result in 40 degree air? We do mostly HVAC, and very little refrigeration that is why I am asking. Also I assume that even if a two evaporator box has the same thermostat that we still need solenoid valves up stream of each TXV. Thanks to all

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  • motoguy128
    motoguy128 Member Posts: 393
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    I don’t have a lot of experience with commercial refrigeration, but thought the target coil temp was higher so you avoid a frozen coil/defrost. I would think 35f was the target and those units use very high constant flow rates and a oversized evaporator to create low approaches.  
    zepfan
  • pecmsg
    pecmsg Member Posts: 4,883
    edited December 2020
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    Size depends on the BTU/h of the evaporator(s), What %RH do you want to maintain, and product temperature, Refrigerant.

    I prefer 1 solenoid valve if possible.
    zepfan
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 15,622
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    @motoguy128

    1 solenoid valve is fine if both coils are on -off at the same time.

    You pick TXvs on temp of the evaporator so 25 deg sounds correct. But you pick the size of the TXVs on the pressure differential. So figure out your condensing temp and convert that to pressure. Subtract from that the suction temp converted to pressure and also the tonnage of the condensing unit (at the pressures you are running at) devided by 2 if you have two evaps the same size. Use a txv for each coil

    Air conditiong stuff is based on nominal tonnage. Refrigeratin stuff is different
    zepfan
  • zepfan
    zepfan Member Posts: 398
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    Thanks to all that responded to this. I had another question regarding defrost on this medium temperature walk in cooler. If the evaporator is listed as “air defrost” with no time clock or heater thermostat, will the unit have a defrost cycle? And if so how would that work? I assume that it needs defrost. Right now on the evaporator we have the solenoid valve, TXV, A19 thermostat, and a pressure control in the condenser. Thanks and Happy holidays to all
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 15,622
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    It may or may not need much defrost. Depends on the humidity in your area.

    As you know water freezes at 32 but a coil with air moving across it at 25 usually will not freeze, but your on the border line.but if the humidity is high it could.

    What you can do is set the temp control at 35ish and see what happens.

    If you need defrost

    I would use a time clock to shut down the solenoid valve if need be a few times a day. Probably 3-4 20 min defrost/24 hourday if needed.

    Temp control should control the solenoid and the condensing unit runs on the pressure control for pump down
    zepfan
  • pecmsg
    pecmsg Member Posts: 4,883
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    It’s called a off cycle defrost. When the air sensing stat is satisfied the fans continue to run.  The problem is if ice or when ice forms it only gets worse. 
    As a back up I use an A12-701 or 700coil sensing control. This does not allow the system to run until the coil reaches 38 -41*F. Every off cycle is a complete defrost. 
    zepfan
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 15,622
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    @zepfan

    You can use the control @pecmsg mentioned with the correct time clock. You set the clock for a 20 min defrost with temperature termination. If defrosted in less than 20 min the temp control terminates the deffrost sooner if the coil is defrosted
    zepfan
  • pecmsg
    pecmsg Member Posts: 4,883
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    None of my Walk In Coolers have a time clock. 
    zepfan