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  1. #1
    Join Date
    Feb 2015
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    Low suction pressure - checked all I can think of

    Short history. I'm a marine engineer, have my epa cert., been working with hvac/refrigeration for 6 years. It's a big part of our job, maintenance and troubleshooting, no design.

    One unit onboard my ship cools 4 offices. 11F SH, 8F SC, unit starts okay with decent suction pressure around 5 bar once a bank of cylinders unloads. BOCK 6 piston/3 head compressor. Cooling from return temp 70F to 67F. As the space cools, suction pressure drops to 4 bar and below. LP cutout set at 3.5 Even with superheat around 10F my suction lines/compressor start to frost due to low suction pressure. Eventually if i try to cool the space below 67F suction pressure will become too low. This BOCK compressor motor is suction gas cooled.

    Compressor - in good condition, checked heads
    Suction filter - clean
    Drier - no temp drop, moisture indicator OK
    Checked TXV - keeps superheat w/i range after adjustment
    EVAP - blown with nitrogen to check for oil - none found. Coils are clean. No damper setting changed in recent history. Air flow is stock, but I want to check blower motor speed
    Refrigerant charge - Due to SH being good I have only added to increase the subcooling a bit as it started off very low. Still, this does not help while TXV is throttling to keep 10F SH.

    Is it possible that this system can be that oversized during cold winter months? There was trouble with the unit last summer, I found the TEV out of wack during inspection and set the spring to stock measurements. Unit is 404a.

  2. #2
    Join Date
    May 2014
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    Bay Area California
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    Right now, I'd probably start looking at the airflow. Its a bit hard to tell once the compressor starts unloading; can you load it up and measure the difference between the supply and return air?

    With R-404a used for air conditioning on a fully loaded compressor, and a 70*F return air, you'd be looking at about a 70 psi or slightly above suction pressure (~5 bar), and about a 50*F supply air. Under normal conditions. Weight and available space can be a big factor in marine applications, so sometimes things are plumbed a bit differently, thereby giving you slightly different numbers.

  3. #3
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    Feb 2015
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    Quote Originally Posted by BBeerme View Post
    Right now, I'd probably start looking at the airflow. Its a bit hard to tell once the compressor starts unloading; can you load it up and measure the difference between the supply and return air?

    With R-404a used for air conditioning on a fully loaded compressor, and a 70*F return air, you'd be looking at about a 70 psi or slightly above suction pressure (~5 bar), and about a 50*F supply air. Under normal conditions. Weight and available space can be a big factor in marine applications, so sometimes things are plumbed a bit differently, thereby giving you slightly different numbers.
    Yes I can load it up but it makes the situation worse w/ the low suction pressure. It does fully load up on start up, soon thereafter is will unload and be steady around 5 bar until the return air begins to drop in temp.
    Another tid bit about this unit. The water cooled condenser flow is throttled about 90%. This is the only way to keep any head pressure on the system. It seems to me like this whole unit was an afterthought during the shipyard and people on here have had trouble with it for a long time.

  4. #4
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    As the spaces cool, you are running out of heat to raise the suction pressure. As the return air temp drops, so will the evap saturation temp, and therefore, the suction.

    Comfort cooling should be set to 68° F as a minimum value. Below 68, and you are starting into an area for which comfort cooling is not designed.
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  5. #5
    Join Date
    May 2014
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    Bay Area California
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    Mis-matched components would answer it. An evaporator too small for the capacity of the compressor. Which would also mean there would probably not be enough air flow also.

    The reason I wanted you to load it up would be to see if the suction stayed relative to the return air temp. The evap should stay about 40 degrees colder than the return air (fully loaded and with the proper air flow), somewhat less if there are high humidity's. And you should get about 20 degrees difference between the supply and return air.

    The variance in the numbers will guide you to the problem. I'll be away from electricity for the next three days, but I'll be interested in any progress you make, and will check in when I get back.

    By the way, they make valves that will automatically control the proper amount of water to keep your head just right.


    Quote Originally Posted by flani33 View Post
    Yes I can load it up but it makes the situation worse w/ the low suction pressure. It does fully load up on start up, soon thereafter is will unload and be steady around 5 bar until the return air begins to drop in temp.
    Another tid bit about this unit. The water cooled condenser flow is throttled about 90%. This is the only way to keep any head pressure on the system. It seems to me like this whole unit was an afterthought during the shipyard and people on here have had trouble with it for a long time.

  6. #6
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    May 2014
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    If the return air is 70*F, and everything is properly matched, a compressor should easily be able to unload. And in doing so, the suction would increase, not continue to decrease.


    Quote Originally Posted by timebuilder View Post
    As the spaces cool, you are running out of heat to raise the suction pressure. As the return air temp drops, so will the evap saturation temp, and therefore, the suction.

    Comfort cooling should be set to 68° F as a minimum value. Below 68, and you are starting into an area for which comfort cooling is not designed.

  7. #7
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    Jun 2013
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    I believe the system is oversized.
    Shipboard offices surely are not huge?

    So a 6 cylinder any brand compressor is too much for four offices.

    5 bar sounds fine for suction pressure, and it's hauling down the temp to below-comfort level

    Add a few more refrigerated spaces & you'll be fine.

  8. #8
    Join Date
    Nov 2011
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    USA
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    It seems to me like this whole unit was an afterthought during the shipyard and people on here have had trouble with it for a long time.
    ..maybe since Day 1?

    The numbers you posted are textbook, low evaporator load symptoms, as previously suggested. Instead of looking for a "failure", investigate the possibilities of "mis-design": too much compressor, not enough evaporator/blower, inadequate (undersized) duct.

  9. #9
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    Jun 2014
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    Where is the condenser? What is the ambient temperature of air at condenser? 404A is best for refrigeration. It does not perform well at extreme temperatures. If the condenser is outside and it is cold, you most likely need head pressure controls with a receiver. The reciever would be to hold extra winter charge that is used to flood the condensor keeping head pressure up. Only subcooling should be used for charging. As you stated, if txv is working it will maintain the superheat. Disclaimer, I have not worked on ships.

  10. #10
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    Jun 2014
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    Missouri
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    I agree with Saturatedpsi. Check the capacity of the compressor and evaporator, they should match. Check capacity of the system to the load conditions it is serving.

  11. #11
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    Quote Originally Posted by BBeerme View Post
    If the return air is 70*F, and everything is properly matched, a compressor should easily be able to unload. And in doing so, the suction would increase, not continue to decrease.
    Sounds like he has only 33% unloading, since there's only suction cooling.

    Wouldn't take long to be back into the lo suction zone.

  12. #12
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    Jun 2013
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    Quote "What is the ambient temperature of air at condenser?*"

    Ship, it's water cooled.

  13. #13
    Join Date
    Feb 2015
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    Supply water to condenser is at a temperature controlled 87F. The entire unit is indoors at a temp of around 75. This will vary a bit as there are other HVAC systems in this room and it's only minimally cooled.

    I appreciate all the feedback. I was, and still am leaning towards the low load situation causing such a low suction pressure as I can not find anything else wrong with the system.
    This bock compressor has the ability to unload 4/6 cylinders but, (i will double check) one unloader is strictly for starting and the other is for capacity control.

    One the system warms up, as in the first few minutes it kicks on and there is some heat in the EVAP it runs well and will almost stay loaded.

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