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Thread: HG bypass

  1. #1
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    HG bypass

    i service a SS water chiller and all summer long they have be saying that the water temps have been a bit higher than normal. i figured it was due to the extreme heat in the midwest

    today i looked things over and noticed that there is no control for the hot gas solinoid it just on all the time. there is a bypass regulator on this machine. so its not so bad but i thought thats kind of a waste of btu's

    getting to my question, can i control this solinoid with a pressure switch? or was it designed to bleed HG into the evap for a reason...

    when bypass is not needed the hot gas in the line may condense and then i have a water hammer effect on the line...?

  2. #2
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    Why control it with a pressure switch? Most of the time its used to control evaporated temp based on load. A hot gas bypass open all the time would severely lower capacity.

    Sent from my ERIS using Tapatalk 2

  3. #3
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    Mixing oil and fire with a big spoon.
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    the hot gas bypass valve should only be opened when the evaporator/suction pressure drop below the acceptable limit (application dependant but usually used to keep the pressure above freezing but not always)). the solenoid valve is used to isolate the hot gas line during pump down. hot gas line sizing and regulator/valve sizing and placement are very important things to consider if you want it to work properly.
    When I am late for work, I usually make up for it by leaving early.

  4. #4
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    evansville indiana
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    There has to be something to energize the solenoid ... Look at the electrical schematic .

    If it's energized all the time then I would expect to see some type of adjustable regulator.

    In other words the controller may energize as needed for capacity control or may energize in a run condition and a regulator takes over .

    Hot gas is NOT normally used for suction pressure control ...it is generally for unloading IE capacity control ....yes there are other uses but I am responding based on my interpretation of your post
    mikeacman

  5. #5
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    Quote Originally Posted by jdblack View Post
    Why control it with a pressure switch? Most of the time its used to control evaporated temp based on load. A hot gas bypass open all the time would severely lower capacity.

    Sent from my ERIS using Tapatalk 2
    what he said +1
    mikeacman

  6. #6
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    Typical HGBP operation has the solenoid on at all times. From a Liebert Crac unit to a Carrier Chiller.
    Thst solenoid needs to be closed only on pump down.

    It should have a hot gas pressure regulator down stream of the solenoid valve.

    For typical comfort cooling applications its set to start openining and put a false load on the evaporater. Its just a capacity control.

    For R22 you would want to set it for 58 psi and 410A set for 102 psi.

    If its adding Hot gas to your evaporater check the setting on the valve or confirm your'e not low on gas or have a restriction on that circuit.....or both..

  7. #7
    Join Date
    Oct 2001
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    6,966
    Trane runs the HGBP thru a control solinoid and a pressure regualator allen keyed,or hat adjusting nut.then at 55PSI for 22 is should start to bleed discharge into the evap to keep the machine on..basically your nearing setpoint shut dow.the hgbp keeps the machine on till the actual load increases is the regulator mounted with the solinoid.basicly HGBP should be active only on staging down cause setpoint is being reached
    "when in doubt...jump it out" http://www.youtube.com/watch?v=U1qEZHhJubY

  8. #8

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    You May As Well Check The Solenoid Valve Mechanical Mechanism, Because Sometimes The Electric Coil May Be Good, But The Valve Won''t Work.

  9. #9
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    Quote Originally Posted by wadesquad View Post
    i service a SS water chiller and all summer long they have be saying that the water temps have been a bit higher than normal. i figured it was due to the extreme heat in the midwest

    today i looked things over and noticed that there is no control for the hot gas solinoid it just on all the time. there is a bypass regulator on this machine. so its not so bad but i thought thats kind of a waste of btu's

    getting to my question, can i control this solinoid with a pressure switch? or was it designed to bleed HG into the evap for a reason...

    when bypass is not needed the hot gas in the line may condense and then i have a water hammer effect on the line...?
    Normally a hot gas bypass is used for compressor capacity control when unloading is not available, or hot gas bypass is used with a compressor which has capacity control and its last step of unloading does not accurately control evaporator pressure.

    A hot gas bypass such as a Sporlan ADRHE-6 is factory set to begin to open @ 60 PSIG. As the the saturation temperature drops below this setting the valve will modulate to open, and would be at its rated capacity @28 degrees saturation temperature or 52 PSIG. This equates to approximately 8 PSI from the point of fully closed to its rated capacity or fully open.

    It will normally be up to the person who sets up the control to establish a good set point but in any case should not be set below 32 degrees saturation temperature...

    In any case it is normally required that some sort of control such as a solenoid valve be in line before the hot gas bypass. Depending on the unit requirements and control strategy that is necessary to obtain a pump down effect at start up or pump out at end of cycle.

    As for trouble shooting the system this can be done very easily by closing the hot gas valve 100% so that actual operating conditions can be established. By doing this you will see how the unit is really performing. Actual operating pressures will be visible and you can identify problems such as low charge, superheat settings, obstructions in liquid filter/driers and so forth.

    After this the valve is reset to what ever position you feel is correct. 59 PSI as an example.

    This is just the minimum to consider. The application and design of piping is also very important, in addition to oil return, and overheating.

    Hope this helps

    _______________
    Understand the symptoms and you will find a solution.

  10. #10
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    Aug 2012
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    That's the right concept.

  11. #11
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    Feb 2009
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    Ft Madison IA
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    those are all great responses,

    first of all this is a retrofit newer condenser on an old evap and old controls

    the hg solinoid is energized at the same time as the LL solinoid so it is hot gas all the time but i uses the regulator as a load control.

    they make bread so on some days they use more ice water than other days (more or less mixers) so when all the mixers are on the load is high and the relulator is only alowing a small amout of HG to pass. on other days there are less mixers so more HG is needed.

    there is an estimated 200 feet from condenser to evap. the HG solinoid is @ the evap. my question is : if i control that solinoid with a pressure switch. on days that the HG is not really needed will there be 200 feet of condesed HG liquid that will slug and hammer the pipe as the solinoid opens and closes?

    im sure i am over thinking this but i want to be sure.

    thanx for all the replys

  12. #12
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    Upstate SC
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    Your hot gas is probably not so hot after 200ft of pipe

    Sent from my ERIS using Tapatalk 2

  13. #13
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    Jul 2012
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    Western KY
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    With most of the mixers running there should be no bypass taking place, I would think. If you have most or all the mixers up and running and are still bypassing red flags should be going up. If this is the case disconnect the HGB solenoid with all or most mixers running, heavy load, and check you pressures. If below 58 on R22 start looking for that problem, fouled tubes, low charge, bad HGB regulator, etc. I would not change control of solenoid leave it energized with the LLS. IMHO.

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