Results 1 to 13 of 13

Thread: Subcooling

  1. #1
    terryw's Avatar
    terryw Guest

    Subcooling



    Hi All:

    I just started a new refrigeration rack today and was wondering about a few things.

    It is R404A, MT & LT with one condenser. Heat reclaim for the LT and water heaters for the MT.

    The system does not use a "receiver" but uses the receiver as an expansion tank in the summer.

    Here's my dilemma:

    Today I was 55* outside. The expansion tank is empty. I was told that the expansion tank would only fill when my subcooling reached greater than 14.5*

    I don't think it will ever make 15* subcooling. Also, could I turn the condenser fans off, raise the head, and get 15* subcooling on a 55* day?

    I was only getting 3* or 4* degrees subcooling today.

    So does condenser size determine subcooling if the system is charged correctly and everything is within parameters?

    In other words, if I am charged correctly, will outside ambient affect my subcooling and if so why?

    Thanks;

    terryw



  2. #2
    Join Date
    Mar 2006
    Location
    Merate (LC) - Italy
    Age
    52
    Posts
    2,554
    Rep Power
    24

    Re: Subcooling

    I don't think you are charged correctly, because subcooling is inversely proportional to ambient temperature, provided fans are turning at constant speed. Liquid will subcool more and more as ambient temperature cools.
    So, if you have a right SC with 55° outside, than it will not be subcooled enough at 100°.
    I would suggest to use a fan speed regulator (to keep SC as constant as possible) or test the refrigerant charge.

  3. #3
    Join Date
    Oct 2003
    Location
    Belgium
    Age
    62
    Posts
    5,645
    Rep Power
    34

    Re: Subcooling

    Quote Originally Posted by terryw View Post
    The system does not use a "receiver" but uses the receiver as an expansion tank in the summer.
    I don't see your setup.
    Aren't you speaking about a bottom fed receiver which isn't an expansion tank in the exact meaning of an expansion tank?
    It's better to keep your mouth shut and give the impression that you're stupid than to open it and remove all doubt.

  4. #4
    Join Date
    Jun 2002
    Location
    UK
    Posts
    633
    Rep Power
    23

    Re: Subcooling

    Hi terryw

    This sounds like a similar operating principle to the Tyler Enviroguard pack.

    Regards. Mark

  5. #5
    terryw's Avatar
    terryw Guest

    Talking Re: Subcooling

    Quote Originally Posted by MarkFiddy View Post
    Hi terryw

    This sounds like a similar operating principle to the Tyler Enviroguard pack.

    Regards. Mark
    Yes, it is a Tyler with Enviroguard. I was afraid of mentioning names.

    After thinking about this some, I realized that sub cooling liquid will not be below ambient temperature. So, if the subcooling is designed for 10* at 100* ambient, then 15* subcooling would not be reachable at 55* because the condenser is outside the design conditions.

    I am under the impression that sub cooling is set with certain design parameters, one of them being a 100* ambient, and anything outside that parameter voids the sub cooling. In other words, it is built into the condenser during manufacturing, and can't be adjusted or changed like superheat can.

    Basically, this is what happens.
    1. In the summer, if all condenser fans are on, then the Enviroguard, if energized, will let excess refrigerant into the receiver.
    2. When the Enviroguard denergizes, the condenser goes back to normal operation and the receiver is pumped out into the suction header.
    3. In the winter, the condenser is basically flooded and the receiver stays empty.
    4. There are no head pressure regulating valve on the condenser for winter control.

    So, I still say that "subcooling" is a designed product, and you may charge the system by subcooling, it does not mean that you can "change" the subcooling.
    The system will not be charged correctly, but when the charge is correct, the subcooling will be correct, due to the manufacturing process.

    When I size a condenser, I size by heat rejection of BTU's not by the amount of subcooling I want.

    Comments welcome;
    Thanks;
    terryw

  6. #6
    Frostman's Avatar
    Frostman Guest

    Re: Subcooling

    There is one point which is confusing me.
    Both Liquid and vapor exist in the receiver, the temperature in the receiver should be saturated at that pressure, if the system doesn't have any other uncondensabel gas.
    If the liquid from the receiver is subcooled, thermodynamically the subcooled liquid will completly fill with the receiver. Then NO vapor space in the receiver tank.
    But from my experience, the liquid is subcooled and the receiver is not full. Anybody can help me to explain this?

  7. #7
    Join Date
    Jun 2001
    Location
    N.Ireland
    Age
    50
    Posts
    1,632
    Rep Power
    24

    Re: Subcooling

    Hi Guys
    correct me if I am wrong

    In the summer we need all or most of our condensing surface area to acheive decent condensing temperatures.

    In the winter we can use the condenser surface area to give us an extra increase of COP when not much is running.

    The excess liquid not being used in the winter time is backed up into the condenser, keeps the condensing pressure up to give good expansion valve performance and adds a good deal of subcooling.

    The rest is just controls and charge

    Now what way does the enviro guard tell the differance between winter and summer and what happens between seasons

    To answer another question the liquid in the receiver will be pressured slightly higher than the liquid line with a hot gas feed to stop the liquid flashing off

    Am I correct guys

    Kind Regards Andy
    If you can't fix it leave it that no one else will:rolleyes:

  8. #8
    Join Date
    Jan 2007
    Location
    USA
    Age
    58
    Posts
    145
    Rep Power
    18

    Re: Subcooling

    I work on enviroguard systems. You do have mechanical subcooling,yes. What type of controller are you using? What is the set up on the condenser? Is the condenser air cooled /evap? You should easily see 55 degree liquid on a cool day. Do youhave a liquid pressure regulating valve? Do you have enough refrigerant in both systems, +35and your lt and mt? The amount of liquid refrigerant is controlled by the spr valve. You can find out operation on tylers website. Good luck.

  9. #9
    Join Date
    Nov 2006
    Location
    UK
    Age
    63
    Posts
    395
    Rep Power
    18

    Re: Subcooling

    I will try and find the manual on the enviroguard system but as my memory serves there is a small tube leading from a phile on the inlet to air on of the condenser which monitors the temperature the other end connects to a valve in the line into the storage vessel from the liquid line. The valve compares the air on temp with the pressure in the liquid line and decides whether to let liquid into the storage vessel or not dependant on head pressure against ambiant, liquid is always flowing from the storage vessel into the suction header through an orifice. The condenser fans run all the time. As far as I can remember it is not subcooling control in the way we usually understand it. It sounds like you may be short of charge slightly. The first time I came across one of these was on a Tesco roof at 3oclock on a snowy sunday morning trying to work out why the thing I thought was a receiver was empty. I will try to find the manual in the morning.

    Ian

  10. #10
    Join Date
    Apr 2001
    Location
    New Port Richey, Florida - USA
    Age
    78
    Posts
    5,072
    Rep Power
    34

    Re: Subcooling

    Quote Originally Posted by Frostman View Post
    There is one point which is confusing me.
    Both Liquid and vapor exist in the receiver, the temperature in the receiver should be saturated at that pressure, if the system doesn't have any other uncondensabel gas.
    If the liquid from the receiver is subcooled, thermodynamically the subcooled liquid will completly fill with the receiver. Then NO vapor space in the receiver tank.
    But from my experience, the liquid is subcooled and the receiver is not full. Anybody can help me to explain this?
    Imagine a half full container of refrigerant sitting on a block of ice, with a heating pad on top.

    The liquid on the bottom is subcooled.

    The vapor at the top is superheated.

    Saturation exists where the vapor is in direct physical contact with the liquid.

  11. #11
    Join Date
    Oct 2001
    Location
    Nottingham UK
    Posts
    5,733
    Rep Power
    51

    Re: Subcooling

    Hi Gary - nice to see you back posting

  12. #12
    Join Date
    Apr 2001
    Location
    New Port Richey, Florida - USA
    Age
    78
    Posts
    5,072
    Rep Power
    34

    Re: Subcooling

    Hi Frank... nice to be back.

    I am getting close to retirement, so you may be seeing more of me in the near future.

  13. #13
    Join Date
    Jul 2001
    Location
    Sydney, Australia
    Posts
    764
    Rep Power
    25

    Re: Subcooling

    SPR will open up on a differential between discharge pressure and the sat cond temp for the refrig charge at that ambient temp (uses a bulb with liquid/vapour charge sitting under condenser sensing SCT and acting on top of SPR valve) - this differential is adjustable. To set up you need design SCT, condenser design TD and a good thermometer.
    The charge in the SPR valve is the same as a bottle of refrigerant - will change with ambient. Get system running with a fullish charge and charged SPR bulb, fans should be running flat out (till the PD causes the TXV's to starve, then raise a little - usually around 10 to 11 bar HP is as far as you can go without resetting txv's with sporlan balanced port).

    Find condenser design TD - say 10K
    Add safety for fouling of condenser - say 5K
    Total max TD - 15K
    Find design ambient - say 40C
    Max Cond Temp - 40C + 15K = 55C
    Max Cond Pressure - 55C on R404a = 355psi
    Cond Pressure equal to ambient - 40C on R404a = 250psi
    SPR valve differential - 355 - 250 = 105psi

    So, at 105 psi above corresponding ambient pressure the SPR valve will start to bypass into receiver, you can fake this differential to set the valve, its only pressure.

    Target bypass is 105psi
    Say its a 30C ambient
    Corresponding pressure is 190psi on r404a
    SPR bypass at 105psi + 190psi = 295psi

    Wind in SPR until no refrig visible bleeding thru
    Shut off fans to raise HP slowly (not all, 1 by 1)
    When HP raises to 295psi (@30C ambient) adjust SPR (wind in) until refrig bleeds thru sightglass.

    HP should slowly begin to stabilise/drop towards target differential (190psi).
    Turn on fans
    SPR is set!

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •