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  1. #1

    Trane CVHF 650 Short Cycling

    Hi all -

    I'm trying to troubleshoot this machine in a new (circa 2009) university campus office/lab building. The chiller seems to be oversized - it generally runs at between 15 and 30% of load. A previous thread seemed to have a similar problem, but no resolution:

    http://hvac-talk.com/vbb/showthread.php?t=399122

    The behavior we are seeing is that the chiller goes into a short cycling mode at low load - only at night. The machine will oscillate between off and on for hours on end - a particularly bad bout saw 24 cycles in 10 hours. This can't be good for the motor. I suspect the cycling repeats due to the differential to start and stop settings, which are +/- 5*. The chiller setpoint is 50*, and inlet temps vary between 55 and 60. However, it's not clear what exactly pushes the machine into the short cycling state in the first place - shouldn't the chiller be able to operate all the way down to 10% load or so?

    The thread mentioned above does propose the solution of lowering the chilled water setpoint by 2-3*, but I'd rather avoid that solution and figure out what in the controls might cause this in the first place.

    I should mention that there is a pump in series with the chiller inlet (constant speed, not on a VFD), and the building management system aims to keep the differential pressure across this pump at 32 PSI by using a PID loop to actuate a bypass valve that sits in parallel to the chiller, which shunts chilled water back to the inlet and reduces the delta-T of the chiller. Does this make sense?

    I'm an electrical guy by training (see above, "shunt", "series", "parallel", etc.), so I've probably left out some important details. Please let me know how I can help you help me.

    Thanks.

    -jay

  2. #2
    SFTB is offline Bad email address, please contact Admin to fix
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    I saw a similar problem at a building with a over sized chiller. From what I could tell the primary (building) loop only had 2-way valves and the VFD would ramp down to maintain the differential (about 16 PSID). It mostly would happen at night and low loads that if the valves closed out in the loop the GPM returning back to the secondary (chiller) loop would not be close to enough to match the chiller GPM. Chiller loop would cool below setpoint and shut off. Since the chiller loop was rather small then the warm return would mix in and chiller would be back on in several minutes. I feel the best solution here was to have 3-way valves so the return water would be cooler for a longer period of time. The only other solution I could think of was some kind of huge chilled water storage tank but that sound like more of a process application. The engineers solution was to raise the differential to start to 10 deg. (max. setting) but that only eliminated about 5 starts a day. Sometimes we saw 45 start a day on a 275 ton machine. I wish it was my building because I would have added some kind of bypasses on the end of the loop runs to try to mimc 3-way valves. Unfortunately the building pump would speed up and for goodness sakes that wasn't efficient, but I guess starting and stopping a chiller when we didn't need to was. Sorry about the long winded message. Could your chiller just unoccupy if the load isn't there? 10% capacity is asking alot for a single circuit machine whether low or high pressure. I did just see a building with a 90 ton scroll chiller for nights and low ambients.

  3. #3
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    I would drop setpoint to 45* and change diff to start to 10*. No, this is not good for the machine. It will be a short lived life for the gal for sure.

    My main advice would be to call in some professional help to iron this situation out, el pronto. The equipment owners pocket book will fair better to pay for a solution than overhauling the chiller.
    It's hard to stop a Trane. but I have made one helluva living keeping them going.

  4. #4
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    I agree with Southern Mech. You have big problems in your future if you don't address the lack of load or flow that is causing the machine to cycle like that.

    I don't mean to sound dismissive or anything, but the setup you're describing sounds very strange to me. It seems like maybe you're missing something there?
    "There is no greater inequality than the equal treatment of unequals."

    -Thomas Jefferson

  5. #5
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    your ''shunt loop'' as you describe sounds like a veriprime system, as valves close in buildinfg the veriprime opens to allow enough gpm through chiller to keep on line.

    I know you question the drop from 50 to 45* water temp setpoint but any supply water temp 50* and over I consider loosing the building. I run no higher than 45* water temp on any of my sites, most at 41*. This would add much load to you the chiller, you are not gaining any energy efficiency with that high water temp, as of now your killing it with start up in rush amperage. I would look for some design criteria for this chiller to compare.
    It's hard to stop a Trane. but I have made one helluva living keeping them going.

  6. #6
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    I had an account with an 800T CVHE, UCP2, with a hot gas bypass on it. Call the factory and see if they still offer it. I had another account with a 450T screw chiller used only for reheat. The first year it had 20,000 starts. Still running 5yrs later. Funny what some engineers will design.

  7. #7
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    10% load is rather low load for a centrifugal chiller, and could cause more problem with oil return. As mentioned low your set point to 45 and see what this does. All that stopping and starting does put a strain on your motor.

  8. #8
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    It does sound like you have a variable primary system. I'd do as the others suggested and drop your setpoint down to at least 45. I'd also try and see if you can find a way to unoccupy the chiller in the evening. If there isn't someone in the building or some process that needs cooling 24 / 7 I think that would be the best way to go.

  9. #9
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    I don't think it's variable primary flow, I think he just has a constant flow primary loop with two-way chilled water valves and a bypass that opens when the pump discharge pressure rises due to those two-way valves closing.

    If that's the case, variable primary flow would clearly be the way to go as it wouldn't take much on the mechanical side to convert the plant over, provided his chiller controls and bas can handle it.

    Like the other folks said, lower the chilled water setpoint to 44 degrees F or so. Put a load on the machine when it's occupied and shut it down at night if you can. 50 degrees F is too high to do any useful dehumidification on the air side, anyway.
    "There is no greater inequality than the equal treatment of unequals."

    -Thomas Jefferson

  10. #10
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    If you have preheat coils on the AHU's, try increasing the preheat temp. This will give you some false load, (use more energy) but it will save the thrust bearings and motor, due to excessive starts, till you get the issue resolved.

  11. #11
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    Quote Originally Posted by jkt7 View Post
    shouldn't the chiller be able to operate all the way down to 10% load or so?
    I'm pretty sure centrifugal chillers only run between 40-100%. If a chiller runs unloaded, even at 40% for an extended period of time you could have oil loss issues.

    Unless you have a chiller with VFD, then you can unload as low as 15%.

  12. #12
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    Quote Originally Posted by TheChillerMan View Post
    Unless you have a chiller with VFD, then you can unload as low as 15%.
    Some Carriers with the double inductor modification can run 10% all day with no oil return problems.

  13. #13
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    Way to many unknowns? Pump in series with chiller? chilled water side or condenser side? Draw a pic of cond, evap piping, pumps and valves then post it. Show diff press location. Is this primary secondary or just primary? "MOST" engineers dont design variable flow through the chiller that drop below minimum flow factory recomendations. May be something simple like a STRAINER!!!!!!! Just a thought.

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