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  1. #1
    Join Date
    Mar 2008
    Location
    Montreal
    Posts
    106

    Hermetic compressor pumping or not?

    During a rooftop unit maintenance yesterday the restaurant owner asked if i could check 2 prep tables refrigerators that were not cold enough(2 evaps on one condensing with LL solenoids valves mounted in pump down). I checked pressures at condensing unit(205/68); I checked the superheat 12 inches before the compressor suction rotolock valve and it was way too high; around 40; with both lls opened; subcooling-13f; took the amperage 6.2 out of a 7.25rla(i dont have the sst/amps chart). I had no access to the fridges because the kitchen area was crowded with people working. I decided to check if the compressor was pumping by attaching a gauge to the service port of the suction rotolock valve and frontseating the valve to see if the compressor pulls down to 5 psi. Not sure if i am right on this.... The compressor pulled from 70 to 40psi rapidly but never went below 40psi. Is it the compression ratio that is important or all hermetic compressors should be able to pull safely at 5 psi??? Tecumseh told me that if there is at least 100 psi between suction and discharge the comp is ok; how would this work with a pumped down system?
    Learn as if you were to live together and live as if you were to die tomorrow. -Gandhi

  2. #2
    Join Date
    Mar 2008
    Location
    Montreal
    Posts
    106
    Air cooled tecumseh with receiver and 2 txv at each evap; aka9427exa R22; ambient 95f boxes target temp=35f

  3. #3
    Join Date
    Jan 2010
    Posts
    6,320
    [QUOTE=refmet;16084621] The compressor pulled from 70 to 40psi rapidly but never went below 40psi.

    Looks like you will be installing a compressor there soon. Unless the service valve leaks.

  4. #4
    Join Date
    Jun 2013
    Location
    N. Canada
    Posts
    402
    From the rooftop, it does not look good for the compressor.
    Only managing 40f at the evap. and 8f cond TD.
    Do you remember if herm. was super hot?

  5. #5
    Join Date
    Jun 2009
    Location
    DFW, TX
    Posts
    673
    Your problem is that you have a 1/3hp compressor running two prep tables. Go back with a 3/4hp condensing unit and watch it rock 'n roll.

  6. #6
    Join Date
    Jul 2011
    Location
    Lehigh Valley, PA
    Posts
    447
    How big are these tables? A single Trualsen 10' prep table uses a 3/4hp alone. That aka is only good for 2700 btu @ 20F evap. Like Trip10 said...

  7. #7
    Join Date
    Jun 2013
    Location
    N. Canada
    Posts
    402
    Quote Originally Posted by Capz View Post
    How big are these tables? A single Trualsen 10' prep table uses a 3/4hp alone. That aka is only good for 2700 btu @ 20F evap. Like Trip10 said...
    My read is "Prep table refrigerators" not "refrigerated prep table".
    The 2 half-height units if stacked would nicely total a full-sized fridge, thus 2700btu @ -20f.

    Always have to ask "did it ever work to satisfaction with this equipment and these conditions"?

    Ref guys probably know that multi-load selective run systems face the problem of insufficient suction cooling return to compressor.
    The 3/4hp recommended might not have lasted out the week!

  8. #8
    Join Date
    Mar 2008
    Location
    Montreal
    Posts
    106
    It is prep tables refrigerators and combined together they are almost the size of a 2 door reach in fridge. System has been running at 35f for 7 years. Compressor is hot because it nevrr stops as the lps for pump down is setted to 15psi and compressor does not pull below 40 psi. The superheat (40) was taken at compressor as i had no access to the evaps during lunch hours. I spoke with tecumseh again and they assure me that it could be normal that this comp would not pull below 40 psi. They just wanna see an increase in pressure of at least 100 psi. How could this be? I thought i had a worn out valve or rings. Does this comp has internal by pass valve?

  9. #9
    Join Date
    Sep 2002
    Posts
    12,549
    That compressor should pull down into a vacuum in seconds. Think about what you are doing: front seating the suction service valve on the compressor results in Zero refrigerant getting to the piston(s) - you are sensing suction pressure on the compressor side of a closed suction valve - so of course it should immediately pump down to nothing.

    Unless the compressor is defective and is not pumping. If it's an iron body I would pull the head and look at the valve plate. In fact; I would get the valve plate first so I didn't have to do it twice. If it a welded compressor - take a new one the next time you go there.

    PHM
    -------






    Quote Originally Posted by refmet View Post
    During a rooftop unit maintenance yesterday the restaurant owner asked if i could check 2 prep tables refrigerators that were not cold enough(2 evaps on one condensing with LL solenoids valves mounted in pump down). I checked pressures at condensing unit(205/68); I checked the superheat 12 inches before the compressor suction rotolock valve and it was way too high; around 40; with both lls opened; subcooling-13f; took the amperage 6.2 out of a 7.25rla(i dont have the sst/amps chart). I had no access to the fridges because the kitchen area was crowded with people working. I decided to check if the compressor was pumping by attaching a gauge to the service port of the suction rotolock valve and frontseating the valve to see if the compressor pulls down to 5 psi. Not sure if i am right on this.... The compressor pulled from 70 to 40psi rapidly but never went below 40psi. Is it the compression ratio that is important or all hermetic compressors should be able to pull safely at 5 psi??? Tecumseh told me that if there is at least 100 psi between suction and discharge the comp is ok; how would this work with a pumped down system?
    PHM
    --------
    The conventional view serves to protect us from the painful job of thinking.

  10. #10
    Join Date
    Mar 2008
    Location
    Montreal
    Posts
    106
    Exactly what i thought but Martin Rex at Tecumseh tech support says otherwise; he says that the increase from 68 to 210 is enough to say that this comp is working good mechanically. He says many models of fractionnal tecumseh compressor won't pull down to vacuum or even 0 psi. He says 40 psi suction with frontseated suction valve and 210 discharge does not mean comp has failed! Why would they say such thing? I will quote a new replacement compressor but i would just like to know for future reference how do you guys do it to check if a comp is pumping properly?

  11. #11
    Join Date
    Sep 2002
    Posts
    12,549
    So what he is saying that the 'slippage' in the compressor is so great that it cannot pump down below a 40 lb. back pressure? Look; I am not TPCo - but I sure do find that hard to believe. Once the gas is shoved past the discharge valve on the upstroke - how does it get back to the suction side? Are there no rings on the piston? How does a positive displacement compressor Not pump at nearly 100% ?

    Anybody?

    PHM
    ------





    Quote Originally Posted by refmet View Post
    Exactly what i thought but Martin Rex at Tecumseh tech support says otherwise; he says that the increase from 68 to 210 is enough to say that this comp is working good mechanically. He says many models of fractionnal tecumseh compressor won't pull down to vacuum or even 0 psi. He says 40 psi suction with frontseated suction valve and 210 discharge does not mean comp has failed! Why would they say such thing? I will quote a new replacement compressor but i would just like to know for future reference how do you guys do it to check if a comp is pumping properly?
    PHM
    --------
    The conventional view serves to protect us from the painful job of thinking.

  12. #12
    Join Date
    Oct 2009
    Location
    Raleigh, NC
    Posts
    799
    This is a pump-down unit? So, isn't the pressure control set to cut out way below 40psig? That is 18F, which is what I would expect your evaps to be running with the thermostat near satisfying.

    I think you have a bad compressor.

  13. #13
    Join Date
    Oct 2009
    Location
    Raleigh, NC
    Posts
    799
    Quote Originally Posted by Poodle Head Mikey View Post
    So what he is saying that the 'slippage' in the compressor is so great that it cannot pump down below a 40 lb. back pressure? Look; I am not TPCo - but I sure do find that hard to believe. Once the gas is shoved past the discharge valve on the upstroke - how does it get back to the suction side? Are there no rings on the piston? How does a positive displacement compressor Not pump at nearly 100% ?

    Anybody?

    PHM
    ------
    I think with some compressors that have cylinders with very small displacements, there comes a time when the gas the compressor is processing is thin and at a low pressure, then the gas in the cylinder is not dense enough to actually open the discharge valve. The gas just expands and compresses with the piston but never reaches a pressure high enough to open the valve.

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