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13.5 HP glycol chiller pulls down loads but won't reach glycol setpoint

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  • 13.5 HP glycol chiller pulls down loads but won't reach glycol setpoint

    We have a 13.5 HP glycol chiller running a mix of 30 and 60 bbl tanks. This unit has been in operation for 4 years and has had zero issues to this point, routinely crashing tanks to 32 F in the heat of Florida summer. 3 hp product pump and 1.5 hp glycol pump, 200 gal reservoir filled well over the minimum.

    Issue is the chiller can drop a 30 bbl tank from 75 F down to 38-40 F in <12 hrs, but stops there. I do not believe it is a loop/piping issue. The glycol inside the unit hangs in the low-mid 30's and never reaches its 26 F setpoint, even with minimal load on the system. Have read a lot about flowing glycol too fast preventing proper heat exchange... But our tank farm has not really changed since we started having this problem, and we still are not reaching 26 F inside the chiller.

    Glycol concentration is at 35%, per manufacturer spec. Compressor was recently replaced, after days of testing and discussion between our HVAC techs and the manufacturer. After continuing to have issues (although not as bad as before -- the compressor was clearly dying), the TXV was replaced as well.

    On the refrigerant side, the suction pressure is somewhat low. And at the TXV's manufacturer-spec'd adjustment, we don't get sufficient cooling, i.e. we can barely pull down loads. Opening it up a touch helps, but too much and we slug refrigerant into the compressor. It seems to be a catch 22. What the manufacturer is specifying and what we then achieve do not line up.

    Wondering if we have managed to clog or foul the chiller's heat exchanger. Not sure how, with a Y-strainer on the glycol return, and not sure it makes sense with a decreased flow across it that we're able to crash tanks, yet not pull the glycol temp down...

  • #2
    Sounds like your TxV is not properly sized for the load or you do not have enough refrigerant. If your suction side pressure is too low, it means you are not getting enough expanded refrigerant thru the TxV, or your system is under charged. The reason you never hit your setpoint is likely due to the system hitting the low pressure cut-out and shutting the compressor off to prevent liquid return into it.

    If you had insufficient flow in the heat exchanger, you could see low suction side pressures if you had a static orifice, but with a TxV, it will adjust the refrigerant expansion to the temperature of the suction side coming out of the heat exchanger.

    You need a tech out who can put a set of gauges on it while it is under load, calculate all the pertinent details(superheat, subcooling) to determine the state of charge, and also the adequacy of the TxV. It is not enough just to put the manufacturer stated weight of refrigerant in, new parts, new plumbing all require a new charge calculation.

    Comment


    • #3
      Originally posted by fmeric View Post
      We have a 13.5 HP glycol chiller running a mix of 30 and 60 bbl tanks. This unit has been in operation for 4 years and has had zero issues to this point, routinely crashing tanks to 32 F in the heat of Florida summer. 3 hp product pump and 1.5 hp glycol pump, 200 gal reservoir filled well over the minimum.

      Issue is the chiller can drop a 30 bbl tank from 75 F down to 38-40 F in <12 hrs, but stops there. I do not believe it is a loop/piping issue. The glycol inside the unit hangs in the low-mid 30's and never reaches its 26 F setpoint, even with minimal load on the system. Have read a lot about flowing glycol too fast preventing proper heat exchange... But our tank farm has not really changed since we started having this problem, and we still are not reaching 26 F inside the chiller.

      Glycol concentration is at 35%, per manufacturer spec. Compressor was recently replaced, after days of testing and discussion between our HVAC techs and the manufacturer. After continuing to have issues (although not as bad as before -- the compressor was clearly dying), the TXV was replaced as well.

      On the refrigerant side, the suction pressure is somewhat low. And at the TXV's manufacturer-spec'd adjustment, we don't get sufficient cooling, i.e. we can barely pull down loads. Opening it up a touch helps, but too much and we slug refrigerant into the compressor. It seems to be a catch 22. What the manufacturer is specifying and what we then achieve do not line up.

      Wondering if we have managed to clog or foul the chiller's heat exchanger. Not sure how, with a Y-strainer on the glycol return, and not sure it makes sense with a decreased flow across it that we're able to crash tanks, yet not pull the glycol temp down...
      Based on the tone of this posting, the root problems may not have been seen or dealt with as yet.
      Any time a compressor fails, its the attending Technicians job to as best as possible, look at the broadest possible scope of what could have been major causitive factors in the failure.
      This is easier said than done in the volume driven cut corner world we are in nowadays.
      There are a major lot of systems out there at large that " seem " to run, but never really run correctly in the entire life of the system. Many more than what can be imagined. The ones that are really dialed in well and run like a top do exist, but are fewer and generally have some very careful hands on them.
      Questions to ask.
      What was the failure on the old compressor?
      Was it burned and were cleanup measures invoked?
      Why were they prompted to change the TXV?
      Is the charge fractionated?
      What were the ambient conditions, system pressures and running AMPS when the system worked correctly in the past?
      How do those compare?
      Is the operating sequence correct and smooth?
      Does the system pump down correctly?
      Is the inlet strainer on the NEW TXV Clean after small run time?
      What compressor superheat the OEM is calling for?
      How much has your total load " Creeped " up over time?

      35% Glycol is a bit weak for any system that is located outside due to the saturated temperature the evaporator runs at during certain operating conditions. This is especially true if the low ambient controls are not set up correctly which is common.
      Glycol concentration is best checked and verified by BRIX.
      Low Suction can be caused by a whole host of things, but can indicate under a certain set of conditions that you are underfeeding the evaporator if all other things are correct.
      Low suction in real terms also = low head within a relative range because you are underloading the system.
      What is the Compressor running current and system pressures at full load?
      If you hit any chiller of this type with 50-60F glycol, it should get your attention on running amps and system pressures.

      Wye Strainers belong on the Discharge side of the pump, not the return side. You do not want to restrict the inlet for any reason. You also need the strong line pressure to blow down the strainer of that type.
      When any HX is fouled on the water side, you will tend to not transfer heat to or from that fluid very well. IN that case the TXV will throttle because it is seeing an inadequate load condition.
      If it was that bad I would think you'd be tripping the manual Freeze-Pressure stat if you have one on board.
      I would think the OEM could advise you of what kind of TD to expect across the HX for a certain set of conditions.
      To determine the fluid pressure drop on the glycol side and see if you are getting design flow through would be a different matter.
      You need to break the problem down into component parts and address the obvious things first.
      Warren Turner
      Industrial Engineering Technician
      HVACR-Electrical Systems Specialist
      Moab Brewery
      The Thought Police are Attempting to Suppress Free Speech and Sugar coat everything. This is both Cowardice and Treason given to their own kind.

      Comment


      • #4
        Would first like to confirm if this was a system we (Pro) manufactured for you? I don't see any service records or see you in our customer directory.

        Could you supply some operating data from the system?

        (As much info as possible recorded at the same time)

        Suction Pressure:
        Refrigerant Temperature at Evaporator Outlet:
        Discharge Pressure:
        Glycol Temperature Evap Inlet:
        Glycol Temperature Evap Outlet:
        Compressor Amp Draw:

        few thoughts/comments I have:

        You will definitely have a hard time crashing your beer much colder than high 30's with glycol in the "low to mid 30's".

        That the chiller never reaches the setpoint temperature tells me that we are bringing a load back to the chiller- so I wouldn't suspect too much that we have a flow issue in the brewery.

        You mentioned not much has changed in the brewery recently, have you had a load calculation ran just to confirm that you aren't simply overloading the system? If you shut the process pump off, does the system quickly pull down to setpoint?

        For a complimentary Load Calc we have a brewery load survey form on our website at https://prochiller.com/fermented-cra.../#craftbrewery scroll down to resource section and select "Online Brewload Survey".

        Depending on the style of the evaporator, its possible you could have some fouling/blockage that would definitely cause low suction pressure and make TXV adjustment a challenge. Was the new TXV the same model as what was removed? The operating data will help us here.

        These can be frustrating ones to troubleshoot- and the fact that much of the country is sweltering hot right now (and most tech support teams are running ragged serving customer calls)- only makes it tougher.

        I hope you can get to the bottom of it quickly, good luck!

        Jim

        Jim VanderGiessen Jr
        Pro Chiller Systems
        jimvgjr at prorefrigeration.com

        Comment


        • #5
          I appreciate your responses to this thread.

          I am not an HVAC person but will answer based off what I've learned through this experience.

          starcat's questions:
          What was the failure on the old compressor?
          Internal valve failure, could not pull a vacuum when tested
          Was it burned and were cleanup measures invoked?
          Was not burned
          Why were they prompted to change the TXV?
          This was at the recommendation of the manufacturer after being given specs they requested under various load scenarios:
          With glycol at 30 F, 25 suction (mfr wants 40), 250 head, 24* superheat (mfr wants 15), 10* subcooling.

          Is the charge fractionated?
          Our techs evacuated the old refrigerant, replaced the filter dryer, and recharged with r404a this morning to eliminate this variable
          What were the ambient conditions, system pressures and running AMPS when the system worked correctly in the past?
          I will ask my tech if he has logs from past checks
          How do those compare?
          I know when we commissioned the chiller 4 years ago everything was in spec per mfr. I will need to see if they have continued records.
          Is the operating sequence correct and smooth?
          Yes, sounds good coming online and when pumping down when we do reach setpoint (basically removing all loads and waiting to come down to 26 F)
          Does the system pump down correctly?
          Yes
          Is the inlet strainer on the NEW TXV Clean after small run time?
          It is brazed in, assume we'd have to remove it to check this. The old valve was clean.
          What compressor superheat the OEM is calling for?
          15* with glycol running at 30 F
          How much has your total load " Creeped " up over time?
          We are almost exactly where we were this time last year. Wish that weren't the case!

          We are operating at 36% glycol mix per mfr. I discovered we were actually running on the low side about 33.5% for quite some time without issue, but have since corrected. This is via Brix reading on optical refrac and also SG hydrometer.

          Compressor running "full load" hard for me to understand this because it varies based on the glycol temp. The compressor RLA is 45A, thermal max 70A. High pressure cutout is set to 425.
          At 32 F, we saw 24 suction, 260 head, 34A draw.
          At 38 F, we saw 35 suction, 275 head, 39A draw.
          At 46 F, we saw 44 suction, 300 head, 44A draw.
          At 58 F (startup after compressor replacement) we saw 57 suction, 300 head, 48A draw.

          Y strainer is on the header return just before reservoir. We have not tripped the freeze pressure stat.

          One oddity I have noticed (which may just be my misunderstanding of the chiller's operation) is that the amp load seems intermittent. At times we will draw 41A While the glycol is in the low 30's, and the air coming off the condenser is nice and hot. A few hours later with a similar glycol temp, we'll be at 35A and the condenser is barely warm, and the temp pulls down much more slowly. I'm not sure if this is normal behavior, and concerns my techs because it seems to be a symptom. We are getting about a 2.5 degree temp drop across the heat exchanger with 23 PSI on the glycol inlet. I am waiting the mfr to calculate the pressure drop.

          Comment


          • #6
            Jim, thank you for your response as well. We are not one of your customers. If you still want to look at our numbers, I'd certainly be grateful.

            Yes we are having a hard time crashing tanks! Even with a few small loads on it (3-4 tanks in the upper 30's), it seems to hang around 32-35, which as you said isn't going to bring those tanks down at all.

            We have calculated that our BTU load requirement when crashing 60 bbls of liquid (68 to 32 F in 24 hrs), with 90 bbls active fermentation, plus 120 bbl holding, plus a 30 bbl CLT, is around 65k WITH a 15% efficiency loss/fudge factor. The chiller is rated for about 85k BTU at 26 F leaving temp. I wouldn't think that 3 tanks around 40 F would push that. Right now, if I crash a warm tank while trying to finish crashing those? Forget about it, glycol shoots into the 40's and those tanks all warm up while the chiller tries for the rest of the day to recover. The past 2 summers, we could crash 90 bbls overnight and hold our other tanks down around 35 no problem.

            With no loads and just the loop running, it will pull down at a rate of about 1 degree F per 2:30. This is a 150 gal reservoir plus the header, which I've calculated by pipe size and length holds about 80 gallons. I have yet to try without the process pump running to isolate the reservoir.

            New TXV identical to old one, sourced from mfr.

            I located an old piece of drop pipe from a CLT we recently repositioned, so this pipe saw a lot of glycol flow in 4 years. It has a white residue coating the walls, not unlike chalk dust, but a bit more film like. It is very thin, but it is there. Could this be enough to cause significant fouling issues in our heat exchanger? It does seem the issue came on rather suddenly and acutely, but when watching temps on large vessels, slow drift over time could be hard to notice. Our water is somewhat hard, but not that bad. We don't get much scaling anywhere in the brewery really.

            Cheers
            Eric

            Comment


            • #7
              Eric,

              Before I start, one quick thing about a comment in your post:

              "flowing glycol too fast preventing proper heat exchange."

              FYI, this doesn't happen. However it is a very common misconception. I have seen a lot of people making things worse by following this line of thinking.

              One thing really has caught my attention, and that is on your post where you stated the glycol temp, the suction, and the discharge, and the amps.

              You didn't put units on that, so am assuming you mean pressure when you say suction? The discharge or head is obviously pressure based on the numbers.

              If these are pressures, I would say you have a serious problem.

              Here is a table:
              Pressure/correlating saturated suction temp/DT

              DT= indicated glycol temp - SST

              24: -9F : 41F
              35: 2.4 F : 35.6 F
              44: 10 F : 36 F
              57: 21 F : 36F

              A 36F DT between saturated suction temp and your glycol temp is super high. And those low suction temps could be putting the compressor out of it's envelope and at best totally make it lose capacity. The DT should be 8-12-ish.

              You mentioned the inlet pressure was 23 psi. Not sure the routing of your chiller, but if the inlet hits the evaporator, then goes into the tank, the DP is really close to 23 psi cause the tank is 0 psi. You would subtract piping losses from the 23 but that is not going to be more than 4-5 (I hope!).

              A high flow rate across you evap would also cause high DP and low DT across the unit (you said 2.5 f) . But that could also indicate crap in your heat exchanger. Crap in your HXR would increase the pressure drop AND reduce it's thermal effectiveness. Reduction in effectiveness means you need a higher temp difference between the cooling medium (the evaporating refrigerant) and the glycol. Which you have. Reduction in effectiveness also means a lower temp difference across the heat exchanger on the glycol side. Which you have. As the heat exchanger is extracting less heat, your super heat will be low. Which you have.

              My hypothesis is there is crap in your evap. This is supported by your comments about white stuff in your piping as well. To get a superheat value that won't flood your compressor, it sounds like you are damn near cranking it closed or 1-2 turns near closed. That is putting your suction pressure in the toilet.

              The whole condenser gets hot and cold thing is probably that the TXV is open (but not much) for a while (hot condenser) them goes shut or more shut (cold condenser). You really aren't transferring much heat so then you aren't rejecting it either. It could also be that with those low suction temps and low flows, you are freezing up.

              If those are temps and not pressures, them I am way off!

              Other stuff to check:
              1. Are those glycol temps what is leaving the evaporator or just in the tank? They aren't always the same.

              2. Measure the discharge temp 6" or so from the compressor discharge. Low suction pressure and normal head increases the compression ratio. That leads to high discharge temps. That is a compressor killer. I don't have a chart in front of me right now to say what it should be. The right numbers could also come from the compressor MFG. We use R-410a which has higher discharge temps than 404a, and we will run 200-210 ish under normal brewery conditions. I am thinking 404a should be 175-ish (like I said pulling out of my butt at the moment). If you are north of 200F with 404a it would reinforce my hypothesis.

              3. If this the case, I would recommend a new evap. Guessing this is a brazed plate. You could dink around cleaning and backflushing, but if it is would you aren't going to get it to 100%. I would replace it. I don't know how hard that is to do on that unit. It shouldnt bee to terrible.

              I hope this is helpful.

              Jeff Johnson
              Johnson Thermal Systems
              208-453-1000

              Sent from my Pixel 2 XL using Tapatalk
              Last edited by Johnson_Thermal; 08-05-2018, 11:47 AM.
              Johnson Thermal Systems
              sales@johnsonthermal.com
              Johnsonthermal.com
              208.453.1000

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