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  1. #1
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    Dec 2008
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    Suction superheat is abnormal,Defective TXV?

    I'm maintainencing a 540 ton Mcquay centrifugal chiller. Model is WSC 087. The suction superheat can not be controlled properly. When the chiller run at higher capacity (for example,70%), suction superheat is about 1 ℃, subcooling is about 4.5 ℃ When chiller run at lower capacity (<50%), suction superheat increases to 6 ℃, subcooling is 2.5 ℃. According to manufacture data, the rated suction superheat at different capacities (from 10% to 100% load condition) is 0.6 ℃. Is there some problem with TXV? BTW, why is the rated suction superheat (0.6 ℃) so low for flooded evaporator? what's impact of high suction superheat on system? How to quantify this impact? COP decrease? Loss of refrigerant effect?

  2. #2
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    The boiling action of refgt in a flooded evap is hard to describe if you've never seen it. It sounds like at lower loads you're not covering the top 2-3 rows of tubes, or there is a large amount of dead space between the top row and the mist eliminators (if Mc is still using them) due to tube bundle size. More likely the former. At 50% and 70% are you measuring FLA or actual capacity? Are your flows correct per design? As the load drops on any flooded chiller, the boiling action is going to lessen and have a tendency to do what you're describing under certain conditions. 6* does sound like a little much, and maybe someone that is intimately familiar with the McQuay controls can shed some more light, but it doesn't sound like enough to be detrimental. Efficiency drops as the machine unloads, naturally.

    Suction superheat is normally in the 1-2 degree range on flooded eqpt 'cause that's the nature of the beast - if your tubes are covered, there's really nothing left to superheat with, no source of heat to add sensible to the refgt other than convective heat thru the shells. I'd be more concerned about whether my waterside TD matches my machine capacity at any given point. You can get gray hair hunting 1* or 2* of superheat on a 550TR flooded evap. Also see what your discharge superheat is doing while all this is going on.

  3. #3
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    Dec 2008
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    32
    Thank you Klove, 50% and 60% is based on the RLA of the unit. There's no water flow meter on the unit, so I can not get the actual capacity. I also found that the waterside TD does not match with the capacity. I calculated the capacity based on an assumed fixed chilled water flow and measured TD, and found there's severious capacity loss compared with the manufacture data. If the TXV is malfunctional, the ref in the evap will not be enough to cover the tube. Can this lead to the capacity loss?

    Quote Originally Posted by klove View Post
    The boiling action of refgt in a flooded evap is hard to describe if you've never seen it. It sounds like at lower loads you're not covering the top 2-3 rows of tubes, or there is a large amount of dead space between the top row and the mist eliminators (if Mc is still using them) due to tube bundle size. More likely the former. At 50% and 70% are you measuring FLA or actual capacity? Are your flows correct per design? As the load drops on any flooded chiller, the boiling action is going to lessen and have a tendency to do what you're describing under certain conditions. 6* does sound like a little much, and maybe someone that is intimately familiar with the McQuay controls can shed some more light, but it doesn't sound like enough to be detrimental. Efficiency drops as the machine unloads, naturally.

    Suction superheat is normally in the 1-2 degree range on flooded eqpt 'cause that's the nature of the beast - if your tubes are covered, there's really nothing left to superheat with, no source of heat to add sensible to the refgt other than convective heat thru the shells. I'd be more concerned about whether my waterside TD matches my machine capacity at any given point. You can get gray hair hunting 1* or 2* of superheat on a 550TR flooded evap. Also see what your discharge superheat is doing while all this is going on.

  4. #4
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    It will lead to capacity loss, but before you can determine that sort of thing, it's imperative that you have the design data for the machine. E.G.: High water flow = low temp differential, even if the machine is working as advertised from a capacity standpoint. Some things can make you think capacity loss when in fact that's not an issue. The place to start is with the distributor or the factory, and the machine model and serial. Get the actual design machine specs, or everything you do will be based on a swag. There's enough Kentucky windage that has to be used at low loads because capacity and RLA/FLA don't work on a straight line but on a curve, so to get reasonably good data to go by for troubleshooting, you have to know what it's supposed to do at full load and interpolate at the lower loads (unless you can get the part load testing points, assuming they even exist). I'm just speculating, but your whole issue could be caused by control imposed limits to refgt level in the evap at low load conditions, and there may not be any problem at all. Again, that's speculation at this point. You also need to post the full model/serial and the type of control panel you have on this thread, along with a full set of readings on water temps and refgt side, so that all who respond will know what they're talking about. Some of Mc's guys are members here, but they need to know what you have to be able to tell you what to look at.

    You seem to be bent on a TXV problem, and that may be what you have, but I would advise to stop leaning that way until you have some better data. Floodeds do some odd things at low loads depending on a lot of factors, and if your superheat is dropping with higher capacities on the machine, that doesn't sound like a TXV issue.

  5. #5
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    Dec 2008
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    Thanks very much for your reply. Here is the detailed information of the unit:

    Unit code: WSC087MAV508/E3012-BE-2*A/C2612-CLYY-2*AYYY/R134-BAABR

    Lower load condition:

    COMPRLA%: 50%
    cond water inlet temp: 24.39℃
    cond water outlet temp: 26.67℃
    condensing temp: 27.67℃
    cond liquid line temp: 25.17
    comp discharge temp: 44.28℃
    evap water inlet temp: 8.28℃
    evap water outlet temp: 7℃
    evaporating temp: 3.89℃
    suction temp: 10.06℃

    Higher load condition:


    COMPRLA%: 80%
    cond water inlet temp: 26.28℃
    cond water outlet temp: 30.28℃
    condensing temp: 32.33℃
    cond liquid line temp: 27.28℃
    comp discharge temp: 41.56℃
    evap water inlet temp: 9.89℃
    evap water outlet temp: 7.06℃
    evaporating temp: 2.61℃
    suction temp: 3.06℃

    The designed cond water flow rate is: 104.3 l/s
    The designed cond water flow rate is: 91.6 l/s

    So the suction superheat is very low at higher load condition. I know there should be some increase when the chiller run at lower load, but the variation of this machine is so big. Does this make sense?

    Quote Originally Posted by klove View Post
    It will lead to capacity loss, but before you can determine that sort of thing, it's imperative that you have the design data for the machine. E.G.: High water flow = low temp differential, even if the machine is working as advertised from a capacity standpoint. Some things can make you think capacity loss when in fact that's not an issue. The place to start is with the distributor or the factory, and the machine model and serial. Get the actual design machine specs, or everything you do will be based on a swag. There's enough Kentucky windage that has to be used at low loads because capacity and RLA/FLA don't work on a straight line but on a curve, so to get reasonably good data to go by for troubleshooting, you have to know what it's supposed to do at full load and interpolate at the lower loads (unless you can get the part load testing points, assuming they even exist). I'm just speculating, but your whole issue could be caused by control imposed limits to refgt level in the evap at low load conditions, and there may not be any problem at all. Again, that's speculation at this point. You also need to post the full model/serial and the type of control panel you have on this thread, along with a full set of readings on water temps and refgt side, so that all who respond will know what they're talking about. Some of Mc's guys are members here, but they need to know what you have to be able to tell you what to look at.

    You seem to be bent on a TXV problem, and that may be what you have, but I would advise to stop leaning that way until you have some better data. Floodeds do some odd things at low loads depending on a lot of factors, and if your superheat is dropping with higher capacities on the machine, that doesn't sound like a TXV issue.

  6. #6
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    Quote Originally Posted by Karl2008 View Post
    Thanks very much for your reply. Here is the detailed information of the unit:

    Unit code: WSC087MAV508/E3012-BE-2*A/C2612-CLYY-2*AYYY/R134-BAABR

    Lower load condition:

    COMPRLA%: 50%
    cond water inlet temp: 24.39℃
    cond water outlet temp: 26.67℃
    condensing temp: 27.67℃
    cond liquid line temp: 25.17
    comp discharge temp: 44.28℃
    evap water inlet temp: 8.28℃
    evap water outlet temp: 7℃
    evaporating temp: 3.89℃
    suction temp: 10.06℃

    Higher load condition:


    COMPRLA%: 80%
    cond water inlet temp: 26.28℃
    cond water outlet temp: 30.28℃
    condensing temp: 32.33℃
    cond liquid line temp: 27.28℃
    comp discharge temp: 41.56℃
    evap water inlet temp: 9.89℃
    evap water outlet temp: 7.06℃
    evaporating temp: 2.61℃
    suction temp: 3.06℃

    The designed cond water flow rate is: 104.3 l/s
    The designed cond water flow rate is: 91.6 l/s

    So the suction superheat is very low at higher load condition. I know there should be some increase when the chiller run at lower load, but the variation of this machine is so big. Does this make sense?
    I hate to do this to you, Karl, but you have to look at where I live. I can't convert metric in my head, and I don't have a conversion chart handy for temps and flows. If you could do that over to US, I'll be glad to give an opinion. Also, your two water flow rates are both saying condenser. I assume the lower one is for the evap? And what type refrigerant is being used? Have all your sensors been tested for accuracy, and does your %RLA actually match what is being drawn by the compressor motor?

  7. #7
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    Dec 2008
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    32
    I convert it as below:

    Lower load condition:

    COMPRLA%: 50%
    cond water inlet temp: 24.39℃ -75.90F
    cond water outlet temp: 26.67℃ -80.01F
    condensing temp: 27.67℃ -81.81F
    cond liquid line temp: 25.17 -77.31F
    comp discharge temp: 44.28℃ -111.7F
    evap water inlet temp: 8.28℃ -46.90F
    evap water outlet temp: 7℃ -44.60F
    evaporating temp: 3.89℃ -39F
    suction temp: 10.06℃ -50.11F

    Higher load condition:


    COMPRLA%: 80%
    cond water inlet temp: 26.28℃ -79.3F
    cond water outlet temp: 30.28℃ -86.5F
    condensing temp: 32.33℃ -90.19F
    cond liquid line temp: 27.28℃ -81.1F
    comp discharge temp: 41.56℃ -106.81F
    evap water inlet temp: 9.89℃ -49.8F
    evap water outlet temp: 7.06℃ -44.71F
    evaporating temp: 2.61℃ -36.7F
    suction temp: 3.06℃ -37.51F

    The designed cond water flow rate is: 104.3 l/s
    The designed evap water flow rate is: 91.6 l/s

    The refrigerant is R134A
    %RLA is drawn by the compressor.
    The sensor should be no problem.

    Quote Originally Posted by klove View Post
    I hate to do this to you, Karl, but you have to look at where I live. I can't convert metric in my head, and I don't have a conversion chart handy for temps and flows. If you could do that over to US, I'll be glad to give an opinion. Also, your two water flow rates are both saying condenser. I assume the lower one is for the evap? And what type refrigerant is being used? Have all your sensors been tested for accuracy, and does your %RLA actually match what is being drawn by the compressor motor?

  8. #8
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    Quote Originally Posted by Karl2008 View Post
    I convert it as below:

    Lower load condition:

    COMPRLA%: 50%
    cond water inlet temp: 24.39℃ -75.90F
    cond water outlet temp: 26.67℃ -80.01F
    condensing temp: 27.67℃ -81.81F
    cond liquid line temp: 25.17 -77.31F
    comp discharge temp: 44.28℃ -111.7F
    evap water inlet temp: 8.28℃ -46.90F
    evap water outlet temp: 7℃ -44.60F
    evaporating temp: 3.89℃ -39F
    suction temp: 10.06℃ -50.11F

    Higher load condition:


    COMPRLA%: 80%
    cond water inlet temp: 26.28℃ -79.3F
    cond water outlet temp: 30.28℃ -86.5F
    condensing temp: 32.33℃ -90.19F
    cond liquid line temp: 27.28℃ -81.1F
    comp discharge temp: 41.56℃ -106.81F
    evap water inlet temp: 9.89℃ -49.8F
    evap water outlet temp: 7.06℃ -44.71F
    evaporating temp: 2.61℃ -36.7F
    suction temp: 3.06℃ -37.51F

    The designed cond water flow rate is: 104.3 l/s
    The designed evap water flow rate is: 91.6 l/s

    The refrigerant is R134A
    %RLA is drawn by the compressor.
    The sensor should be no problem.
    Fouled or obstructed evaporator tubes is the first thing I'd look for based on your numbers. If it's a 2 pass evaporator, you may also have a divider plate gasket leaking a significant amount of flow past it back into the leaving water. Either of these two things, or you have a very small tube bundle by design and the numbers you have are normal (I wouldn't suspect this). What I'm looking at is the fact that your evap approach at low load is 6*F, which is high already. At 80%RLA the evap approach goes to 8*F, but the suction superheat drops off dramatically. The real puzzling thing is that your discharge superheat is no lower than it is. Almost as if you have a sensor reading incorrectly. I have had a machine or two that had fouled tubes and low refrigerant charge at the same time and gave exactly these symptoms.

    You've given the design flow rates - do you have a chart from Mcquay in an IOM that will give you the pressure drop at the correct flow - and do you have the design temperature differentials on the cond and evap?
    Last edited by klove; 02-08-2010 at 05:59 PM.

  9. #9
    Join Date
    Jul 2006
    Location
    The Great country of Texas
    Posts
    429
    As long as you have good subcooling, your discharge gas temp should be at 16 degrees at full load with the liquid injection turned off. With that said, your suction superheat will be around 1 - 2 degrees F, If not, look at the evap tubes or divider like Klove said.
    "I'm from Texas, what country are you from?"

  10. #10
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    Dec 2008
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    32
    Thank you guys for the reply. Yes I agree with you that the dirty evaporator maybe the major reason caused the capacity loss. What I really want to make sure is whether the TXV is bad. 0~6 ℃ change of suction superheat is so weird. What's the variation range is normal for a flooded evaporator under different load conditions. I think 0~3℃ does make sense. What's your experience on this? Thanks!

  11. #11
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    Quote Originally Posted by Karl2008 View Post
    Thank you guys for the reply. Yes I agree with you that the dirty evaporator maybe the major reason caused the capacity loss. What I really want to make sure is whether the TXV is bad. 0~6 ℃ change of suction superheat is so weird. What's the variation range is normal for a flooded evaporator under different load conditions. I think 0~3℃ does make sense. What's your experience on this? Thanks!

    The issue here is that the superheat is going down as you load. If the TXV were feeding too little and starving the evap, then the SH would be going up as you load and uncover the tubes. As has been said, a 6* variation is a bit much, but your conditions aren't necessarily adding up to a bad expansion valve.

  12. #12
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    The ideal superheat should be 0.6 *. How is the impact on the system if superheat increase to 6 *, can it be quantified?

    Quote Originally Posted by klove View Post
    The issue here is that the superheat is going down as you load. If the TXV were feeding too little and starving the evap, then the SH would be going up as you load and uncover the tubes. As has been said, a 6* variation is a bit much, but your conditions aren't necessarily adding up to a bad expansion valve.

  13. #13
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    Quote Originally Posted by Karl2008 View Post
    The ideal superheat should be 0.6 *. How is the impact on the system if superheat increase to 6 *, can it be quantified?
    You can plot it out on a p/h chart for R134A and do the calculations, but if you don't get your flows correct and make sure you have good heat transfer first, you're chasin' your tail. If your SH is going up due to uncovering tubes, then your efficiency is going down, if it's going up due to some other dynamic that unloading the machine is amplifying and the tubes are covered, then it's just going up. Ideal may be 0.6*, but I would worry a lot more about that as it loaded. And remember when you start chasing 6/10ths of 1 degree (especially on refrigerant gas temperature), it may be VERY hard to catch - that's a setpoint, and this is a 540TR chiller with boiling refrigerant in it. Things are rarely exact.

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