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  1. #1

    Confused Compressor has high head pressure immediately after startup

    Hello,

    I hope I am posting this in the correct category. I operate a chiller at an outdoor ice skating rink. The system is seven years old and has operated fairly well in that time.

    Over the past few weeks it would have an occasional shutdown due to high head pressure. I called my local technician to help with this problem, and we came to a solution that seemed to work well. However, over the last weekend one of my compressors shut down due to high head pressure, and now will not run for more than a few seconds (5-10) before shutting down again.

    At first I suspected a faulty pressure sensor, but we checked the pressure with gauges, and the gas side of the compressor spikes wildly and never settles. We checked all the valves in the system and everything appears to be functioning normally. The condenser fans are running normally and all system voltages are normal. That being said, it is entirely possible we have overlooked something.

    Having exhausted all other ideas we set the superheat controller to hold the expansion valve at 10% for 30 seconds during startup (normally set to 50% for 5 seconds). As expected the pressure drops until the valve opens. Then it rises fairly quickly until emergency shutdown (< 30 seconds). At this point we have run out of ideas.

    My setup:

    Bitzer 6F50 compressor
    Emerson ECE-X33 superheat control system, EX6 valve
    CAREL pCO3 PLC
    R-507 refrigerant

  2. #2
    Join Date
    Jun 2013
    Location
    Austin, TX
    Posts
    641
    Assuming no unauthorized service has been done, make sure the condenser is clean.

  3. #3
    Join Date
    Aug 2009
    Location
    Prattville, Alabama
    Posts
    2,106
    Also, make sure that a free wheeling condenser fan, rotating backwards, hasn't been mistaken as running normally. I've seen that mistake made more than once. Even by people who should know better...

  4. #4
    Join Date
    Apr 2009
    Location
    Columbus,Ohio
    Posts
    20
    Check expansion valve for proper operation

  5. #5
    Thank you for the responses.

    The condenser is clean and clear. I thought perhaps ice had formed inside the cabinet due to recent cold weather, but I checked and it is clear.

    The fans are operating normally and spinning the right direction. I had a person manually engage the fans while I was near to check for slipping/other issues, and they are both operating well and spinning the right direction. The cabinet itself is sealed and closed (it's a walk-in style condenser cabinet).

    As a test I manually engaged both the primary and secondary fans before starting up (the secondary fan is set to engage at 280 pounds pressure (my technician's recommended pressure)) and still had high pressure.

    The expansion valve appears to be operating normally. I normally have it set to the factory setting of 50% at startup for 5 seconds before switching to automatic mode, but while diagnosing this problem I have it set to 10% for 30 seconds. I can hear it moving after switching to automatic, and there is a lot of hunting. All four of my units do that for a minute or so at startup before stabilizing. This unit does not have a chance to stabilize before shutdown.

    I would suspect an overcharge of refrigerant, but we have not added any refrigerant or oil in five years, and the system has run well until now.

    Just for the record:

    This unit is used seasonally from November through March. It is offline from April through October. We take oil samples each year before startup for testing. I check oil levels about once every two weeks and perform other routine maintenance on the manufacturer's recommended schedule. Any repairs or maintenance that affect the operation of the compressor units and their related parts is done only by our certified technician. In this situation even he is stumped.

  6. #6
    Join Date
    Jan 2011
    Posts
    1,191
    Is this a dual circuited unit? If so, what is happening with the other compressor? I have seen (on many occasions) where the refrigerant from one circuit made it's way into the other circuit through faulty gaskets. One compressor would go off on high discharge pressure and the other would cycle on low refrigerant pressure.

  7. #7
    Join Date
    Aug 2006
    Location
    Mixing oil and fire with a big spoon.
    Posts
    4,443
    What is the High Pressure Cutout Setpoint?

  8. #8
    The cutout point is normally set to 350 lbs. My technician had me raise the point to 400 lbs temporarily, just to see if it would cough up a hairball and stabilize. I will not be leaving it there once this situation is resolved.

  9. #9
    These are four isolated circuits. The only point of contact is in the chiller barrel, and even then they have independent pipes. I have not noticed any changes in the other three cabinets.

    I'm glad they're independent, actually. At the beginning of this season I had a burnout in the #3 cabinet (bad power phase + faulty overload sensor = bad things), and the repairs would have been exponentially higher if they were dual-circuited.

  10. #10
    Join Date
    Aug 2006
    Location
    Mixing oil and fire with a big spoon.
    Posts
    4,443
    So, did you ever check the %RLA at start up to see if it is starting loaded?

  11. #11
    Join Date
    Dec 2010
    Location
    Central US
    Posts
    427
    When you refer to % EXV is that 10% open or 90% open, and when you release control it closes or is there another electric valve you are referring to IE Hot gas bypass or unloader etc. And what superheat are you actually running compared to what the controller is trying to maintain. Could simply be a faulty saturated suction temp sensor thinking the evap is flooded. Need to check sensor with actual measured. The more info the better, like what exactly are you working on M# manufacture, possibly a log sheet. Just my two cents worth.

  12. #12
    Join Date
    Jan 2011
    Posts
    1,191
    Quote Originally Posted by crashputer
    The only point of contact is in the chiller barrel, and even then they have independent pipes.
    True, but what separates the circuits are gaskets...the only thing separating the refrigerant circuits from each other. If the gaskets fail then the circuits can "swap" refrigerant.

  13. #13
    Quote Originally Posted by KnewYork View Post
    True, but what separates the circuits are gaskets...the only thing separating the refrigerant circuits from each other. If the gaskets fail then the circuits can "swap" refrigerant.
    Hmm... I haven't had any noticeable drop in pressure in the other compressors, but while checking all systems the other day I noticed that the pressures on them were topping out around 250. I wasn't overly-concerned because the ambient temperature was low. However, last year I had to add refrigerant to two of the compressors, and my technician spent a long time with his leak detector and was unable to find anything. I'll investigate this a little more. This repair sounds like it's needlessly complicated?

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