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  1. #1
    Join Date
    May 2011
    Location
    CA
    Posts
    123

    Floating Suction

    So the market I work at recently (last fall) installed floating head pressure controls on all of our racks (4 racks, Hussmann protocol, 404a). It was one of those federal rebate programs, it was free for us. Anyway, management of my market is tickled that KwH are down and are attributing it to the floating head (yeah, me cleaning the condensers for the first time in 3 years and removing 1/2 inch of crap off evap coils kinda helped a bit. But like Rodney Dangerfield, I get no respect.). Anyway, I've been trying to wrap my head around how a floating head pressure control works, and getting great info from older threads here. That said, now I'm looking at floating suction controls for my store.

    Here's my understanding of the system now:
    1) Sensor in case/walk in sends temp info to cpc;
    2) If above temp setpoint, cpc opens LL solenoid.
    3) Refrigerant flows through txv and vaporizes, absorbing heat.
    4) There's now more refrigerant in the SL than before, raising SL pressure
    5) Suction header pressure transducer senses this, and when pressure gets above set point turns on another compressor.
    6) Suction pressure falls below setpoint.

    The MT racks all have a suction setpoint of 58 psig. I realize that raising the setpoint will save us energy by having fewer compressors cycle on...but...I don't get the "floating" aspect of it. What would you peg the floating suction setpoint *to*? And, I'm assuming the present setpoint was carefully calculated at install so that all the txvs would work properly. I'm assuming that if it was raised, one might mess with the txv's function...at least some of them.

    I've been trying to figure this out on my own and I can't. Now, I know I'm about to get yelled at here...but I was considering an experiment. I was thinking of slowly raising the suction setpoint for one rack up gradually and seeing what happens. Checking superheat before and after on the same cases, etc. But I thought I'd check it out first with those that know better n' me first.

  2. #2
    Join Date
    Aug 2007
    Location
    Oregon
    Posts
    226
    Im a little confused at what exactly you are trying to accomplish with the racks ? Are you mainly concerned about saving money and having less run time ?

  3. #3
    Join Date
    Feb 2008
    Location
    California
    Posts
    2,057
    You're in the ballpark...not spot on, but getting there.

    There is no need for you to raise the suction setpoint as the floating suction setup should be doing this for you. (assuming it was setup properly)

  4. #4
    Join Date
    May 2011
    Location
    CA
    Posts
    123
    Quote Originally Posted by Phase Loss View Post
    You're in the ballpark...not spot on, but getting there.

    There is no need for you to raise the suction setpoint as the floating suction setup should be doing this for you. (assuming it was setup properly)
    It hasn't been installed...yet. And the government won't pay for it. We'll be paying for it.

    The reason I wanted to raise the suction setpoint was for me to see what effect it would have. I mean, if the system can run o.k. with a higher setpoint (not floating, just a higher setpoint) then why not run it higher? As long as there is sufficent pressure drop across the txv, why *not* run it higher? And now I'm scratching my head wondering what would happen to the head pressure if I raised the suction pressure setpoint up too high. As I understand it, the compressors *only* cycle on as a result of suction pressure. Hmmm. I'm frying my brain thinking about this...I need a beer.

  5. #5
    Join Date
    Feb 2008
    Location
    California
    Posts
    2,057
    Frying your brain with thinking is a good thing keep it up.

    you're not even in the industry, yet. and thinking about things that many guys whom are in the industry have no interest in.

    so keep on thinking, you'll get it.

  6. #6
    Join Date
    May 2011
    Location
    CA
    Posts
    123
    Aha...got it now. Read some old posts. So the coldest circuit on the rack (ice cream freezer...) is what the suction float will be 'pegged' to.

    1) Ice cream freezer suction set point is at 14 psig.
    2) Ice cream freezer satisfies; LL solenoid for that circuit shuts.
    3) Since that's the coldest case on the rack, we don't need all the other cases at 14 psig...since they have higher discharge air setpoints, they could go up to...17 or 18?. This would allow compressors to cycle off.
    4) When ice cream case calls again, cpc resets suction setpiont to original 14 psig.

    Pretty cool...but we only have one LT rack. I'm guessing this isn't something that's normally done with MT racks. Also, the rack that we have LT cases on has split suction....I'm betting that we wouldn't save much energy because of that.

  7. #7
    Join Date
    Jul 2000
    Location
    Guayaquil EC
    Posts
    10,078
    You're getting much better now that you see the controller is watching that ice cream case to float the suction.

    It's not about when that ice cream case is cycled off though. Basically, that would mean the ice cream case has overshot it's setpoint...meaning it was easily satisfied at the rack's normal suction pressure.

    This is where the CPC steps in and says, Hey! it's starting to fall below the design temperature, but before I cycle that case off, I'll try kicking the suction pressure up a notch to see if it stays satisfied. If it continues to drop in temperature, I'll kick the SP up some more.

    So in your example, you may well be able to satisfy the ice cream case at 18# SP.

    That's how basic suction float works.

  8. #8
    Join Date
    Dec 2010
    Location
    new jersey
    Posts
    362
    Quote Originally Posted by Iceneck View Post
    Aha...got it now. Read some old posts. So the coldest circuit on the rack (ice cream freezer...) is what the suction float will be 'pegged' to.

    1) Ice cream freezer suction set point is at 14 psig.
    2) Ice cream freezer satisfies; LL solenoid for that circuit shuts.
    3) Since that's the coldest case on the rack, we don't need all the other cases at 14 psig...since they have higher discharge air setpoints, they could go up to...17 or 18?. This would allow compressors to cycle off.
    4) When ice cream case calls again, cpc resets suction setpiont to original 14 psig.

    Pretty cool...but we only have one LT rack. I'm guessing this isn't something that's normally done with MT racks. Also, the rack that we have LT cases on has split suction....I'm betting that we wouldn't save much energy because of that.
    don't mind if I chime in....

    MT works the same as you have outlined above, the low setting for fresh meat, and the warmer cases would be produce/floral.

    no ,you wouldn't save as much on a split header...but you could float for fzn food.

    If you haven't already gone I would recommend the Sporlan supermarket seminar...check it out on line for dates if interested.

    keep up the good work,so to speak, and good luck! most of us excel at things we enjoy.

  9. #9
    Join Date
    May 2011
    Location
    CA
    Posts
    123
    Icemeister... got it. The cpc doesn't necessarily raise suction setpoint when the LL solenoid closes for the ice cream circuit, in other words. But I'm missing something...if the ice cream case discharge air reaches setpoint (or is in the setpoint deadband) the LL solenoid would close too, wouldn't it? Are you saying the the cpc would begin to raise the suction setpoint before the ice cream case satisfies?

  10. #10
    Join Date
    Jul 2000
    Location
    Guayaquil EC
    Posts
    10,078
    Quote Originally Posted by Iceneck View Post
    Icemeister...Are you saying the the cpc would begin to raise the suction setpoint before the ice cream case satisfies?
    Yes, if the case can cycle off then it means the suction pressure is still a little too low, and can be adjusted a bit higher.

    At least that's the way this worked when I last played with it 20 years ago.

  11. #11
    Join Date
    May 2011
    Location
    CA
    Posts
    123
    Supermarkettech,

    Thanks for the info. We do, actually, have our service meat cases and meat walk ins at a much lower discharge air setpoint (we don't have product sensors, much to my dismay...the employees take temps. Waste of labor if you ask me, since einstein is capable of it AND automatically logging temps for health dept.).
    Will check out the sporlan supermarket seminar! And, I do enjoy it...i'm perfectly happy keeping my cases and coils clean all day then going home and reading about refrigeration.

  12. #12
    Join Date
    Feb 2008
    Location
    California
    Posts
    2,057
    get the thought of the system cycling "off" the liquid line solenoid when the case reaches setpoint out of your head If you setup the rack for floating suction, but your cases are still cycling off on EPR's, T-stat/solenoids, Sensor/solenoids than your not setup for floating suction properly. With a functioning suction float the racks suction pressure IS your temperature control for the cases and all other temp controls should be taken out of play. (except for the cases that have a higher DA requirement)

    If you could...imagine your rack as a large self adjusting EPR based on lowest case temp.

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