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  1. #1

    Low Suction Compressor #1 StulZ VHS

    Hello, I've been racking my brain trying to figure this one out. Last night I went on an emergency service call for a huge data center thats got a 6 of these floor mounted VHS Stulz units. Anyway There was one all the way in the back both circuits frozen from the spreader tubes of the txv's all the way back to the compressors (copeland Scrolls). Of course I disabled the compressors and let the blower run to thaw the coil out before I took any pressures and tems. To be noted Circuit #1 actually had a frozen TXV body where as circuit 2 just had fronzen speader tubes. Anyway after defrosting I noticed that circuit number 1 had a much lower suction (52psi) than circuit 2(71 psi). Both circuits were maintaing about 16 degrees super heat and circuit number one had 27 degrees subcooling, circuit 2 17. I think the low pressure in circuit one is causing low refrigerant Temp, even though maintaining superhead and eventually is frosting up the coil, inevitably affecting circuit 2. Once I got it going again I had 55 degree discharge air and 82 return air. Checked filters and even made sure the re-heat slab coil on top of the A-coil was free of debree. Poor airflow under floor due to cables on top of cables in the cold Isle. TXV on circuit one was recently replaced as well and so was drier. checked for restriction at drier, found none. Can't figure out why the nearly 20 psi difference in Compressor number on suction. Any help would be appreciated.

  2. #2
    Sounds like the 20psi difference is coming from whoever installed the TXV and didn't properly adjust along with that high subcooling. Is this unit condenser an air cooled or water cooled? What where your head pressures? Was the unit properly charged after TXV change with proper load? Is 82F return normal load or just due to this unit offline? When you tested unit after ice thaw, were ckt 1 or 2 sight glasses flashing (bubbling)?

  3. #3
    The units been iced up before at the txv, hence the original technician who did the job, rebuilt the txv and replaced the drier. I kind of ruled out the txv since it was maintaing a good super heat. The sight glass on both circuits were clear and the head pressure on the circuit one was 255 with a 165 degree discharge line temp. It has a water cooled condener with 3 way valve, also has a slab coil on top of the A coil, I guess for free coolingn in the winter. According to the customer, around 82 degrees is normal return for that unit being that its in the part of the data center that they've added a bunch of extra racks, too many in my opinion for the units to keep up with. The tech who did the job prior to me is pretty competent I trust his judgement, or at least the charge. I like to see around 20 degrees subcooling, but it is a self contained unit with no long L line run. But like I said, the unit was deffinetly maintaing good superheat at the actual Evap outlet. Like I said, I was thinking perhaps high static under the floor at the outlet of the blower because of all the cables, limiting mass airflow across the coil and perhaps causing a lower evap pressure...but why only on circuit 1?

  4. #4
    Join Date
    Jul 2011
    Location
    Columbus, Ohio
    Posts
    136

    Low air flow

    A 27 degree delta T across the evaporator is a problem. Air flow sounds low, check the fan amperage and compare to the other units that are running ok pressures. Depending on the coil arrangement, air flow distrabution, ect, the number one compressor may be getting less air than number two.

  5. #5
    Join Date
    Nov 2006
    Location
    Southeastern Pa
    Posts
    17,001
    My two cents is low heat load due to poor airflow. I would also want to see subcool a little lower, unless the manu says otherwise.
    [Avatar photo from a Florida training accident. Everyone walked away.]
    2 Tim 3:16-17

    RSES CMS, HVAC Electrical Specialist

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  6. #6
    Join Date
    Nov 2011
    Location
    British Columbia
    Posts
    48
    I think circuit 1 is short-charged pal. The tech before you replaced txv and drier... ok, but only charges refrigerant until the site glass clears. Resulting short-charge raises the evap outlet temp and drives the txv wide open. By chance, superheat is what you might expect. 3-way condenser valve has likely modulated down to maintain head pressure - thus showing you a good reading, even good sub-cooling and a clear sight glass.

    Frost at the txv is occuring because of the high pressure drop accross it. Saturated vapour is unable to make it to the evap coil (system isn't full yet) and is flashing immediately at the txv outlet.

    If you can, check with the last tech on this – how much was charged in? Does the nameplate specify an amount (non split-system)? Then go from there. Otherwise if it was my job, I'd pull the charge, pressure test, evaccuate, and recharge to nameplate value (one circuit on at a time).

    Given your description, a short-charge must be ruled out first. It's the most likely condition to have changed recently. A short-charge on circuit 1 also rationalizes it's impairment vs circuit 2 – CFM and RA temp is the same to each.

  7. #7
    Thanks so much for all of your suggestions and theories. I agree with Synergy that perhaps is Low heat load across the evap coil due to poor air flow, but It just seems like it would affect both circuits. I dont know exactly how the circuits run through the evap coil and which one is exposed to more air than the other, Its a good I idea to compare and contrast blower amps. And SpaceMan, I thought about the idea the unit could be low, but with that superheat and subcooling I ruled that out. After the coil thawed I monitored both txv's and no frosting occured, and yes 27 degrees subcooling is a bit high. Also the last charge was weighed in, it was written on the name plate although I've forgotten how much. And to be remembered it froze up before all the initial work was done. So the tech that did the work likely weighed in the charge according to what was on the name plate, But still I'll ask him. Something else! didn't even pay attention to it much, but the unit is agains a wall that has like 2 slot diffusers above it, certainly affecting the return air temp and probably accounting for the abnormal delta across the coil. I dont even know of those are active. And even if they were, some of the other units have the same diffusers above them. Now that I look back thats another thing I need to check.

  8. #8
    Join Date
    Nov 2006
    Location
    Southeastern Pa
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    In order for the low charge idea to hold water, the tech would have had to mistakenly thought the problem was the txv, and after the repair, load in an insufficient charge a second time. I think the chance of that is remote. Its not impossible, by any means.

    I like the idea of eliminating this possibility by removing the charge, pulling a deep vac, and charging by nameplate. Then, when the same problem remains, you know a low charge is not the problem.
    [Avatar photo from a Florida training accident. Everyone walked away.]
    2 Tim 3:16-17

    RSES CMS, HVAC Electrical Specialist

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  9. #9
    Join Date
    Sep 2002
    Posts
    2,639
    What is the humidity in the room?

  10. #10
    Low airflow would cause and can be verified by amps vs nameplate FLA. If that was an issue, i would think you would of heard of hot spots in the room by now. You said this unit has a free cooling coil. Most free cooling valves are set to open at 45F water temp. Most water cooled condenser are designed for 80-85F inlet water and the head pressure regulating valves are set for this. Does this unit have manual adjustment or electronic adjust water reg valves? The 255 psi is a little high. What is the entering water temp to the condenser?

  11. #11
    Okay, So I went back out there tonight, same deal!!! arrgh! This time, I really took my time and recorded as much data as I could. Compared blower amps, compressor amps and supply and return glycol temps with the other three units in the room as well. So here's what I found: Pretty much everything matched up with the other units as far as all amp draws. FLA nameplate amps for Blower was 10.2 and unit was at 8.3 avg. leg. Compressor amps for circuit#1 14.2 FLA 19.5
    S.H. on circuit 2 was 19 degrees, subcooling was 32 degrees!! Suction pressure 54 Discharge was 266. Tried close off the water regulating valve and bring the head pressure up so that it could adversely affect the low side pressure but that didn't work. With head pressure near 290psi I was only getting 56psi on the low side. Interesting with just Circuit #1 running, the pressures mimicked circuit #2 But as soo as circuit 2 ran, it adversely affected circuit 1. With the door to the compressor cabnet cracked (because of taking measurements with gauges and temp probes) even with both stages running I was able to get the low side of circuit #1 up to 61 psi.Hot Air bypassing the filters and free-cooling slab coil brought the pressures slightly up, because the air is still coming in above the coil (obviously a downflow unit). So it ended up getting left that way. Why circuit #2 txv body was frozen and not circuit 1...I can't say honestly. The door will be modified to allow more of that hot return air to bypass that free cooling coil...which is obviously in itself a sort of airflow restriction. Also with the modification to the door, a filter will be installed. A Meeting with the customer again took place and poor airflow conditions under the floor was discussed. They are supposed to be overhauling that whole data center...we'll see.

  12. #12
    Sorry!! Circuit #1 had the subcooling of 32degrees and a superheat of 10 degrees, Circuit 2 had the 19 degree superheat.

  13. #13
    Join Date
    Nov 2006
    Location
    Southeastern Pa
    Posts
    17,001
    Looks like circuit one was overcharged in an attempt to raise the suction pressure, which is low due to a heat transfer issue.

    Post back and let us know what the final fix is. One thing I'd look for is a pressure drop across the coils, especially that slab coil. Reding the data center sounds like a long term fix, but I'd see what can be done now, short of leaving a panel open. Can a portion of air return be increased near the unit?
    Last edited by timebuilder; 11-23-2011 at 07:30 AM.
    [Avatar photo from a Florida training accident. Everyone walked away.]
    2 Tim 3:16-17

    RSES CMS, HVAC Electrical Specialist

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