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  1. #1
    Join Date
    Dec 2007
    Location
    Northern Indiana
    Posts
    82

    Liquid Line/Suction Line Heat Exchanger purpose?

    Hello all,

    I have a question about a liquid line/ suction line heat exchanger.
    I ran across my first one the other day in a walkin freezer.

    What is the purpose of it besides raising my SH back to the compressor? Will it lower my head pressure also? I really just don't understand what it does.

    If someone can explain it to me. I'll ask another question about the freezer in question that I am having problems with.
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  2. #2
    Join Date
    Feb 2011
    Location
    Georgia
    Posts
    274
    Keeps the liquid from flashing off too early by not letting it absorb any heat

  3. #3
    Join Date
    Jul 2005
    Location
    Southern California
    Posts
    596
    It does a few things.

    1. Helps ensure a full column of liquid at the TXV by preventing flash gas by subcooling the liquid
    2. Increases system performance by lowering liquid temp at TXV
    3. Raises the vapor temp to help evaporate any droplets of liquid not boiled off completely in the evap
    Knowledge is knowing that a tomato is a fruit. Wisdom is knowing not to put it in a fruit salad.

  4. #4
    Join Date
    Feb 2010
    Location
    Calgary, Alberta
    Posts
    1,151
    It will increase/maintain subcooling in the liquid line to ensure a solid column of liquid to the metering device.
    Also helps prevent flood back to the compressor, at the expense of increasing vapor volume and dropping suction pressure.
    They marginally reduce system efficiency in order to increase "safety".

  5. #5
    Join Date
    Jan 2013
    Location
    florida
    Posts
    771
    bally uses them for boxes with short line runs

  6. #6
    Join Date
    Dec 2007
    Location
    Northern Indiana
    Posts
    82
    As stated earlier this is being used on a small walkin freezer. 404a. Line set is maybe 20ft. Condensing unit is on a mezzanine right above. This freezer has been shut down for about 3 months. BTW. I work for a school corporation and we shut down refrigeration during summer break to save energy. There is always a problem child that doesn't want to pull down to temp.

    Went to start up and unit was short cycling. Going out on low cut out. Took a peek at sg and saw nothing. Installed gauges and pressures were very low considering 83degree box temps.

    Was getting late in the day so I grabbed some gas and cleared the sg. Just to get it to pull down so I could check it the next day. Box only pulled down to 40*.

    Ok first. Will the exchanger give me a false reading in my sg? My reasoning is if the gas is being cooled wont it show me a full glass even though line is not full? I still think I'm low on charge SH is still high at evap and compressor. But won't it be hard to get 20-30* at comp. with the exchanger fighting me?
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  7. #7
    Join Date
    Jan 2010
    Posts
    6,777
    Your site glass should be full regardless of the heat exchanger. The only time you might not have a full glass, is when the freezer box is at temp. It could look like a flowing river and still have the correct charge. Fill the glass and look for your evaporator superheat to be in the 4-6* range. If you can not maintain at least 20* superheat at the compressor, then raise the superheat at the evaporator.

  8. #8
    Join Date
    Dec 2011
    Location
    New Zealand
    Posts
    2,272
    There is quite a bit crap written.
    LT compressors generally can not have more than 10C (16f) superheat at the compressor, you can check this with this against the manufactures working envelope.
    The liquid/suction line heat exchanger, can increase the system performance, and is related to the specifics of the system (refrigerant and compression ratio), If the TXV bulb is fitted after the SLHE, then it can increase the capacity of the evap by approx 10%.
    The heat exchanger also coverts most of the gained superheat into useful superheat not non useful superheat.
    Yes it does help in ensuring a liquid head at the TXV inlet.
    if the evap is a high pressure drop then some liquid sub cooling can reduce this drop increasing the LMTD, basically lifting the outlet suction pressure slightly.
    They are not used as often as they should because of the associated cost.
    Your sight glass is an aid to charging.
    freezer room are normally designed to maintain a temp, so pull down can take quite some time (excessive load)
    You also may have an MOP valve, which restricts the refrigerant mass flow, to ensure head pressure does not rise excessively. This can give false indications especially very high suction superheat on start up. (you see this more often just after a defrost)
    As a side note, it is quite common that freezer rooms that are insulated with expanded polystyrene, can absorb quite a bit of water vapour (which turns to ice) during it normal working life. So by turning it off, the ice will melt. So when you turn it on back on you tend to find a reasonble pull down time to -6C, it then stops there for quite a while as the water re-freezes (remember water SHC 1BTU/Lb/f, verses latent heat of water/ice which is 144Btu/lb) this can add quite a load to the refrig system.
    Last edited by barbar; 08-18-2013 at 08:39 AM.

  9. #9
    Join Date
    Dec 2007
    Location
    Northern Indiana
    Posts
    82
    Quote Originally Posted by barbar View Post
    There is quite a bit crap written.
    LT compressors generally can not have more than 10C (16f) superheat at the compressor, you can check this with this against the manufactures working envelope.
    The liquid/suction line heat exchanger, can increase the system performance, and is related to the specifics of the system (refrigerant and compression ratio), If the TXV bulb is fitted after the SLHE, then it can increase the capacity of the evap by approx 10%.
    The heat exchanger also coverts most of the gained superheat into useful superheat not non useful superheat.
    Yes it does help in ensuring a liquid head at the TXV inlet.
    if the evap is a high pressure drop then some liquid sub cooling can reduce this drop increasing the LMTD, basically lifting the outlet suction pressure slightly.
    They are not used as often as they should because of the associated cost.
    Your sight glass is an aid to charging.
    freezer room are normally designed to maintain a temp, so pull down can take quite some time (excessive load)
    You also may have an MOP valve, which restricts the refrigerant mass flow, to ensure head pressure does not rise excessively. This can give false indications especially very high suction superheat on start up. (you see this more often just after a defrost)
    As a side note, it is quite common that freezer rooms that are insulated with expanded polystyrene, can absorb quite a bit of water vapour (which turns to ice) during it normal working life. So by turning it off, the ice will melt. So when you turn it on back on you tend to find a reasonble pull down time to -6C, it then stops there for quite a while as the water re-freezes (remember water SHC 1BTU/Lb/f, verses latent heat of water/ice which is 144Btu/lb) this can add quite a load to the refrig system.
    Wow Barbar! You just caused high SH to my brain and we had meltdown.

    The SLHE is actually located after the txv bulb. Within about 6"of it though. What will that cause? Unit has been running a 0* the last few years when I have restarted it.

    Not to change subject but do any of you have an opinion on shutting down equipment for extended periods of time and then restarting them. Is there a procedure that should be taken when shutting them off for the summer and then restarting them? Just curious.

    I just shut the breakers off to compressor, fans, door heaters and lights. Prop the door open and put a pedestal fan in the doorway to keep from molding till it dries out. On startup I do pretty much just the opposite.
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  10. #10
    Join Date
    Jun 2013
    Location
    N. Canada
    Posts
    404
    Quote Originally Posted by VTP99 View Post
    Your site glass should be full regardless of the heat exchanger. The only time you might not have a full glass, is when the freezer box is at temp. It could look like a flowing river and still have the correct charge. Fill the glass and look for your evaporator superheat to be in the 4-6* range. If you can not maintain at least 20* superheat at the compressor, then raise the superheat at the evaporator.
    Biggest error in refrigeration... pissing around with SH adjustment in a previously working, 40deg above target temp, probable ZP element TXV.

    Ck for restriction, confirm element is controlling, but stay the heck away from the adjustment when at +40f !!

  11. #11
    Join Date
    Jun 2013
    Location
    N. Canada
    Posts
    404

    Seasonal shutdowns

    Ptech, we do lots of seasonal shutdowns -fur farm, hunting lodges, wild game processors.
    The "breaker off" is not a good shut off procedure! Turn the stat up to initiate a full pumpdown. We then close the LL valve.

    Seasonal compressors MUST be equipped with CCH. With stat set hi, units are powered 24hr before turn on.
    Problem is refrigerants' great affinity for oil...it will find a way of migrating into the compressor & there go the valves on cold startup.

  12. #12
    Join Date
    May 2003
    Location
    Utah
    Posts
    742
    There is no free lunch...

    Lowering the liquid temperature will result in less flashing through the TEV, meaning there is a higher percentage of liquid in the liquid/vapor mixture at the outlet of the TEV. The higher the percentage of liquid is, the less the mass flow rate needs to be...increasing efficiency and reducing compressor run time.

    When the suction vapor temperature is increased, it becomes less dense. Compressor capacity is decreased as the vapor density (lb/cu ft) decreases. The compressor is a volume pump, so while it's pumping the same volume, pumping the less dense vapor reduces the mass flow rate.

    Liquid to suction heat exchangers reduce liquid temperature, but at the expense of higher suction vapor temperature. Any increase in efficiency gained by the lower liquid temperature is offset by the decrease in compressor capacity from the higher suction vapor temperature.

    Classic case of robbing Peter to pay Paul.

    Primary purpose of the SLUE is to insure vapor free liquid at the TEV inlet. Secondary benefit is that it can eliminate flooding in some cases.

  13. #13
    Join Date
    Jan 2010
    Posts
    6,777
    Quote Originally Posted by IRBH View Post
    Biggest error in refrigeration... pissing around with SH adjustment in a previously working, 40deg above target temp, probable ZP element TXV.

    Ck for restriction, confirm element is controlling, but stay the heck away from the adjustment when at +40f !!
    Blah Blah Blah... Maybe you should reread my post. Never said he should adjust the superheat unless he did not have 20* at the compressor. I said he should look for a superheat of 4-6* at the evaporator. How would he know otherwise if he is getting complete efficiency out of his evaporator ? Also in case you didn't know a ZP element has noting to do with the evaporator superheat. Don't underestimate the OP knowledge.

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