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  1. #1
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    +4C walk-in coldbox temp control problem



    Hi,

    Wonder if someone could help me with this problem. We have a 90sf2 +4C coldbox. 10' ceiling height. This box has 3/4 ton of refrigeration. During our temperature mapping of the box (we are in biotech and this mapping is a requirement for validation), we saw several areas inside the box would dip below-1C. This is really bad for our products and render the cold box unseable. The mfg is saying that the evap split is designed for 10C and that the box is too much refrigeration capacity. Are there any options to reduce capacity without having to replace the evap/condenser? or just control the discharge air from the evap to say 2C-8C instead.? I was told that the orifice can not be changed.

    Also, we think part of the undershot (setpoint is at 5C, liquid solenoid shutoff at 4C, compressor continue to run for 25 sec before shutting off.) was also due to the liquid solenoid is 25 ft away from the evap. Is there merit in this thought.? We calculated that about 20% of the refrigerant is in the line between the solenoid and the evap.

    Thank you all for your input.



  2. #2
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    Re: +4C walk-in coldbox temp control problem

    hi pdm,
    you say its -1c in some parts of the cold room , so up the controle stat < where is the stat sensor?<
    you say the fridge unit is 3/4 of a ton
    but you dont say how big your cold room is?
    your unit would run for 25 seconds where ever the solinoid valve was , and when its pumped down there is only gas vapour in the 25ft of liquid line,
    going back to you saying its -1c in parts of the cold room , is your evap partially blocked or is the stored biotec blocking the air flow?
    waiting to hear your reply, sedgy,

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    Re: +4C walk-in coldbox temp control problem

    Hi there,

    When pumping down, evaporating temperature drops and discharge air drops too. In one case I had 3°C temperature difference than the thermostat setting during the pump-down.
    If the solenoid valve is far away then pump-down takes longer.

    Check if this temperature drop happens during the pump-down.

    If NOT then your evaporating temperature is below 0°C and therefore, the discharge air is also below 0°C.

    Cheers
    Even Einstein Asked Questions

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    Re: +4C walk-in coldbox temp control problem

    Please supply us with technical info about the refrigerant control components such as TXV - make and model, evaporator coil - space between fins, fans - push or pull, type of temp control, place of sensing bulb, evaporating temp at set point and......pictures.

    The board of experts will love to assist you.

    And of course, welcome.

  5. #5
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    Re: +4C walk-in coldbox temp control problem

    What is the humidity requirement?

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    Re: +4C walk-in coldbox temp control problem

    Quote Originally Posted by pdn6420 View Post
    Hi,

    Wonder if someone could help me with this problem. We have a 90sf2 +4C coldbox. 10' ceiling height. This box has 3/4 ton of refrigeration. During our temperature mapping of the box (we are in biotech and this mapping is a requirement for validation), we saw several areas inside the box would dip below-1C. This is really bad for our products and render the cold box unseable. The mfg is saying that the evap split is designed for 10C and that the box is too much refrigeration capacity. Are there any options to reduce capacity without having to replace the evap/condenser? or just control the discharge air from the evap to say 2C-8C instead.? I was told that the orifice can not be changed.

    Also, we think part of the undershot (setpoint is at 5C, liquid solenoid shutoff at 4C, compressor continue to run for 25 sec before shutting off.) was also due to the liquid solenoid is 25 ft away from the evap. Is there merit in this thought.? We calculated that about 20% of the refrigerant is in the line between the solenoid and the evap.

    Thank you all for your input.
    Hi
    you should check the calibration of the controller sensor and only then continue,this will eliminate a wild goose chase,
    it would also be very helpful if you could elaborate on size of room type of evaporator ,type of gas and evaporating pressures,
    good luck and as said before welcome
    THE DEFINITION OF A SMART PERSON IS ONE WHOM LEARNS FROM HIS MISTAKES!!!

  7. #7
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    Re: +4C walk-in coldbox temp control problem

    Hello again,

    Thank you everyone for your inputs. Let me add more details to this question. The cold box is 90 square feet, the internal height of the box is 10'. The refrigeration system is sized for 3/4 ton. There is an RTD mounted 12" from the back of the evaporator. this RTD is the control sensor. It has been calibrated. The liquid solenoid valve is actually on the roof (about 25ft away from the evaporator). Thus when the valve is off, there is still liquid in this line. I was told by the mfg that this does not cause the temperature to overshot, however, they had calculated the mass in this line which is roughly 15-20% of full charged.

    When we map the room, we use type T thermocouples at each corner of the rooms and at 5 feet elevation. The control system is set at 5C with a +/-1C deadband. At 6C, we saw the liquid line solenoid come on then the compressor. Within 4-5 min, we see that the liquid line solenoid shutoff, temp is at or below 4C, then 25 sec later the compressor turn off. The spots that read -1C are mid height at 5 feet opposite from the evaporator and around the peripheral walls.
    The mfg had checked pressure, TXV and maintain that the refrigeration system is working. It is simply too large for the load. The temp mapping is done with no load pattern, and shelvings are against the walls on the perimeter not impeding circulation. We were also told that the delta T for the evap is around 10C, and we would need a split of 5C to maintain temp 2C or higher.

    I will send pictures tomorrow if that would help.

    TIA.

    Pdn

  8. #8
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    Re: +4C walk-in coldbox temp control problem

    Hello again,

    Just realize I did not answer all questions.

    1. There is no humidity requirements.
    2. We took temp reading and the evap temperature exit air is below 0C.
    3. I will get details specs and post.

    Regards,

    Pdn

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    Re: +4C walk-in coldbox temp control problem

    I would suggest an EPR valve in the suction line. Adjust this to maintain an evap air out low limit of 2C.

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