Results 1 to 15 of 15
  1. #1
    Join Date
    Mar 2006
    Location
    Lancashire
    Posts
    1,869
    Rep Power
    21

    Minimum pressure to a txv



    Low ambient control needed

    Do TXVs have to have a minimum pressure at the inlet in order to work as designed?
    I understand by lowering head pressure then efficiency is increased and that capacity would be decreased but how low is too low a head pressure for the valve to work as designed and not cause problems.

    In the winter times I see so many units especially the air handler types with internal and external units that are running at a head pressure of 10-11 bar( fixed speed compressors)
    just basic units but because the indoor fan is a fixed speed fan it just runs at a constant speed and can't regulate the HP

    The evap coils are always freezing up even *though the return air from the building onto the coil is usually 21-22degrees.

    Gas charges are ok subcooling is around 6-8k and superheat is always being controlled at 6k and no moisture present in the system, other than the coils freezing up the systems heat and cool fine.

    Other than the humidity factor all I can think is the low head pressure could be affecting the txvs but not too sure.
    Can any one shed any light on this for me please.


    The difference between genius and stupidity is that genius has its limits.

    Marc

  2. #2
    Join Date
    Jul 2008
    Location
    England
    Age
    32
    Posts
    31
    Rep Power
    0

    Re: Minimum pressure to a txv

    I have the exact same problem on my AHU. Both are in a food production area and my initial thought of the freezing evap was when they wash down between product changes the steam created would be drawn across the coil and cause it to freeze. However after monitoring it i am satisfied it wasn't the cause.

    I am also suffering with low head pressure due to the weather in the UK right now

  3. #3
    Join Date
    Jul 2010
    Location
    manchester
    Posts
    212
    Rep Power
    9

    Re: Minimum pressure to a txv

    your only as good as your last job

  4. #4
    Join Date
    Sep 2007
    Location
    Somerset
    Age
    63
    Posts
    4,467
    Rep Power
    38

    Re: Minimum pressure to a txv

    Hi Marc.
    Yours is a good post especially as you point out at this time of year the lower head pressures become an issue.
    I completely concur!
    In my experience if the discharge pressure is dropped (For what ever reason) too much.
    Some styles of expansion valve close down or more commonly don't open fast enough too prevent a LP shutdown.

    Some manufacturers are now starting to have software settings to accommodate Low ambient conditions and fan speed control.

    We have a site where we cover part of the condenser to accommodate the lower ambients experienced.
    That in itself is another story.

    But as is your original question, yes you do need a certain amount of "lift" across the Exp valve.
    For it to work correctly.
    Some of the earlier Danfoss valves on adapkool needed something like 100psi diff for them to operate correctly.

    You will find it difficult to find this written down anywhere though!

    Good Luck Grizzly

  5. #5
    Join Date
    Mar 2006
    Location
    Lancashire
    Posts
    1,869
    Rep Power
    21

    Re: Minimum pressure to a txv

    Quote Originally Posted by Grizzly View Post
    We have a site where we cover part of the condenser to accommodate the lower ambients experienced.
    That in itself is another story.
    We have exactly the same problem on a few of our sites as well and have resorted in blocking the condenser like you described.
    I've worked on an Airedale condenser recently that seemed to regulate the HP by use of the evaporator fan so was thinking that is how some units work in the winter but i'm still not sure about that.
    The difference between genius and stupidity is that genius has its limits.

    Marc

  6. #6
    Join Date
    Sep 2007
    Location
    Somerset
    Age
    63
    Posts
    4,467
    Rep Power
    38

    Re: Minimum pressure to a txv

    Hi Marc.
    Some of the sites I look after have tier chillers running all year round despite the ambient being well below their design parameters.
    Extreme I know but as they say "That's Life"!
    I would be interested to know which Airedale that was as they have 2 different selectable control strategies.
    Add this to their free cooling units and it can get interesting.
    No disrespect to Airedale, theirs is some good kit, Only even theirs don't like all ambient conditions.

    Grizzly

  7. #7
    Brian_UK's Avatar
    Brian_UK is online now Moderator I am starting to push the Mods: of RE Site Moderator : and general nice guy
    Join Date
    Mar 2001
    Location
    Dorset
    Age
    69
    Posts
    10,377
    Rep Power
    49

    Re: Minimum pressure to a txv

    Sporlan always recommended a liquid line temperature of 100F, 38C, at the TEV inlet. They also provided balanced port valves for floating head systems.

    http://www.sporlanonline.com/10-9_32011.pdf

    There are lots of ways around the problem as long as there is enough money.
    Brian - Newton Abbot, Devon, UK
    Retired March 2015
    Please support http://the100project.net/Home

  8. #8
    Join Date
    May 2007
    Location
    Auckland
    Age
    67
    Posts
    3,362
    Rep Power
    31

    Re: Minimum pressure to a txv

    Hi Mark.
    a txv is a pressure related variable control devise, designed to deliver a variable volume of liquid at a varaible pressure differential between high and low sides. Change one variable and flow drops or increases. Low ambients is the killer.

  9. #9
    Join Date
    Aug 2009
    Location
    New Zealand
    Age
    53
    Posts
    2,556
    Rep Power
    20

    Re: Minimum pressure to a txv

    This is more complicated than it first looks, firstly by dropping your head pressure, you increase the possible net cooling effect, which mean that the TD across the coil will increase to meet "equilibrium" (lower SST) also you are likely to find that the air on has a lower RH (fresh air make up has lower amount of humidity), this will also increase TD.
    If your maintaining a constant suction superheat, then the valve is controlling so is doing what it should and as yet is not being effected by the lower liquid pressure. When you loose control your SH (increasing superheat), then you do not have enough pressure to drive the correct amount refrigerant through the valve.
    You could look at reducing your superheat setting, to make greater use of your coil, this should lift the SST slightly.
    Just one thing is the superheat constant even with a coil free of ice!

  10. #10
    Join Date
    Mar 2006
    Location
    Lancashire
    Posts
    1,869
    Rep Power
    21

    Re: Minimum pressure to a txv

    Quote Originally Posted by Grizzly View Post
    Hi Marc.
    Some of the sites I look after have tier chillers running all year round despite the ambient being well below their design parameters.
    Extreme I know but as they say "That's Life"!
    I would be interested to know which Airedale that was as they have 2 different selectable control strategies.
    Add this to their free cooling units and it can get interesting.
    No disrespect to Airedale, theirs is some good kit, Only even theirs don't like all ambient conditions.

    Grizzly
    Hi Grizzly,
    Tier units? I'm not familiar with those, I take it they are heat pump chillers?

    I think the Airedale units were CUHS 6.0, the ones where the fan discharges the air upwards.

    I noticed when it was in heat mode and the outdoor fan speed controller was in auto that it the fan as
    The fan speed increased suction pressure increased but also discharge pressure increased and the
    Opposite if the fan slowed down.
    I would have expected the fan to run flat out in order to evaporate all the refrigerant in the coil but
    It didn't- it looked like it was maintaining head pressure.
    The difference between genius and stupidity is that genius has its limits.

    Marc

  11. #11
    Join Date
    Mar 2006
    Location
    Lancashire
    Posts
    1,869
    Rep Power
    21

    Re: Minimum pressure to a txv

    Quote Originally Posted by Brian_UK View Post
    Sporlan always recommended a liquid line temperature of 100F, 38C, at the TEV inlet. They also provided balanced port valves for floating head systems.

    http://www.sporlanonline.com/10-9_32011.pdf

    There are lots of ways around the problem as long as there is enough money.
    Hi Brian, thanks for the link, I'll have a read through that later.

    As you've said I always thought that minimum temp to a TXV should be 38-42C but I couldn't find any
    References to this when I've been searching.
    The difference between genius and stupidity is that genius has its limits.

    Marc

  12. #12
    Join Date
    Mar 2006
    Location
    Lancashire
    Posts
    1,869
    Rep Power
    21

    Re: Minimum pressure to a txv

    Quote Originally Posted by Magoo View Post
    Hi Mark.
    a txv is a pressure related variable control devise, designed to deliver a variable volume of liquid at a varaible pressure differential between high and low sides. Change one variable and flow drops or increases. Low ambients is the killer.
    If low ambients cause so many problems then why don't the manufacturers design their systems to
    Combat this.
    On a few systems I've worked on they have electric heaters which on the trend are labeled as frost heaters. I always set these to come on at 10C which is the maximum I can set them to but to be honest they don't increase the head pressure that much.
    The difference between genius and stupidity is that genius has its limits.

    Marc

  13. #13
    Join Date
    Mar 2006
    Location
    Lancashire
    Posts
    1,869
    Rep Power
    21

    Re: Minimum pressure to a txv

    Quote Originally Posted by mad fridgie View Post
    This is more complicated than it first looks, firstly by dropping your head pressure, you increase the possible net cooling effect, which mean that the TD across the coil will increase to meet "equilibrium" (lower SST) also you are likely to find that the air on has a lower RH (fresh air make up has lower amount of humidity), this will also increase TD.
    If your maintaining a constant suction superheat, then the valve is controlling so is doing what it should and as yet is not being effected by the lower liquid pressure. When you loose control your SH (increasing superheat), then you do not have enough pressure to drive the correct amount refrigerant through the valve.
    You could look at reducing your superheat setting, to make greater use of your coil, this should lift the SST slightly.
    Just one thing is the superheat constant even with a coil free of ice!


    Yes I suppose i could try to open the valve slightly to get the SST above 0DegC.
    Not all the systems I work on have accumulators though so I wouldn't be able to try this approach on some systems.
    On the AHU types I work on, the manufacturers have been back to fit defrost stats to switch the 4 way valves when the coils begin to I've up but to me this is masking over a problem i.e why the coils are freezing up in the first place.

    Yes the superheat seems constant when the coil is free of ice, why do you ask?
    The difference between genius and stupidity is that genius has its limits.

    Marc

  14. #14
    Join Date
    Aug 2009
    Location
    New Zealand
    Age
    53
    Posts
    2,556
    Rep Power
    20

    Re: Minimum pressure to a txv

    If superheat is constant across the whole range, then your pressure is not effecting the TEV performance.
    The coils are freezing because you have more duty from from your refrigeration and less load, (lower moisture removal) The combination will will allow the suction to drop well below 0C, frost will build, whence ice is built the suction just continues to fall making more and more ice. Id SST were to remain the same, you would get around 15-20% more cooling, but your indoor coil is the limiting factor, so evap temp must drop, this does cause balance out some of the gains made, by dropping the head pressure, so you could be getting around 8% actual increase. You need to check your dew point of the air entering the indoor, I suspect this lower than design.
    If this is a major, you could look at fan speed, or sporlan head control system or just a simple hot gas injection to keep the evap above freezing.

  15. #15
    Join Date
    Mar 2006
    Location
    Lancashire
    Posts
    1,869
    Rep Power
    21

    Re: Minimum pressure to a txv

    Thanks MF, that makes perfect sense.
    Next time I'm at one of these units I'll take some better readings including dew point temp and see if that is the case, I'll also keep a closer eye on the SH to see if it stable.
    The difference between genius and stupidity is that genius has its limits.

    Marc

Posting Permissions

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