Results 1 to 12 of 12
  1. #1
    Join Date
    Jul 2000
    Location
    Guayaquil, EC
    Posts
    14,308
    Post Likes
    How to properly set a CPR valve has been a common question posed here in the Refrig & Icemaking section of this forum. It's one of the more misapplied and likely one of the more often misunderstood facets of commercial refrigeration that I know of.

    Just last week I caught myself thinking a valve was in fact an EPR and not a CPR installed on a low temp system. With my own realization that it was indeed a CPR, the source of my problem was actually a significantly undersized valve for the application. (Thanks, Dave )

    The point of this post relates to what I've found while digging for info on that same 3 HP LT system. It's an LALA-031A-TAC installed on a 0F freezer running on R402A (HP80). It had a CRO-6 CPR on it and the pressure drop was around 10#with the valve wide open and the box near 0F. I needed the CRO-10 to get the drop down to a more reasonable 1-2# allowing the pump to breathe a little easier. OK, so that part of the problem is covered. On to the next dilemma.

    I go to the Copeland Calculator program and print out the performance specs for this pump (for R502) to get a better idea of where my upper limits are for setting the CPR. I usually use the nameplate RLA ratings for setting CPRs, but this one set me to do some serious headscratching.

    The RLA on the nameplate is 10.9 amps. On the performance tables I pick 120F SDT and find the 10.9 amps RLA puts me way down at -30 SST or about 10 psig. This box should run at about -20F SST minimum. What to do?

    I go back to the calculator program and check the operating envelope and it tells me that it'll go from -5 to -40F SST and 90 to 130F SDT. Hell, at the -5F/130F condition the amp draw is like 18.1. I know the RLA's are set by the maximum continuous current allowable, which is 140% of RLA, but that only gets me to 15.3 amps absolute max. WTF.

    How far beyond the RLA can one safely go to cover yourself for post-defrost pulldown loads? I'm thinking I'll boot the head up to an expected maximum and set the post defrost amps at just under the 15.3 amps MCC figure....like 14-14.5 amps.

  2. #2
    Join Date
    May 2005
    Posts
    1,624
    Post Likes
    i did a search on this valve in question and came up empty..is there a plain english instructions on this little beauty?

  3. #3
    Join Date
    Jul 2000
    Location
    Guayaquil, EC
    Posts
    14,308
    Post Likes
    Thread Starter

    primmers......

    If you want to learn about CPR valves, go here. It's a good start.

    You can also go to Sporlan Bulletin #90-10 which covers CPRs from soup to nuts.

  4. #4
    Join Date
    May 2005
    Posts
    1,624
    Post Likes
    thxs, i should have all that commited to memory in a couple hours...

    thanks again..wally

  5. #5
    Join Date
    Sep 2004
    Posts
    159
    Post Likes
    icemeister,

    If Copeland's info on this compressor states it can pump -5 f (maximum allowable evaporating temp, then set the cro valve for -5f = 26 psig/404a. Set it , and forget it! What Copeland is telling you (inderectly) is the motor can handle this suction temp and not any higher. That's one advantage of setting cpr valves by pressure and not amperage. Once set for the max e.t., you don't have to recheck it when condensing temps are higher.

    Forget about the RLA info. It's nothing more than a mathematical derivation of something we don't need to be concerned about (in this case).

  6. #6
    Join Date
    Jun 2004
    Posts
    182
    Post Likes
    icemeister,

    Excellent catch here. I checked with the engineering department and this compressor envelope should have restrictions at the upper right corner of the evelope.

    "How far beyond the RLA can one safely go to cover yourself for post-defrost pulldown loads?" you asked. We say you can go 25% above RLA as long as the protector doesn't trip (typically 40% above RLA). This still leaves a 15% safety factor. If you go higher then 25% you just have less of a safety, nad you're operating closer to the trip point (MCC) of the protector.

    RLA is a mathematical formula, that's correct. As far as setting a CPR valve do use the RLA. Remember by using a CPR we are trying to limit the suction pressure so the compressor doesn't over amp and trip the overload. If you don't look at the amp draw you could actually be overrestricting and increasing the pull down time.

    Again, excellent catch. You're on the ball.

  7. #7
    Join Date
    Jul 2001
    Location
    Fairfield and NewHaven Counties in Commecticut
    Posts
    4,920
    Post Likes
    I have been using the 40% over rule or you find the damn fan klixon takes waaaaaay to long to stay closed and the ice cream melts. I would set this one for a small c-store freezer to 16 amps if you can fake out the head to 300+ by blocking the condenser coil. What i am saying is make sure 16 amps is the highest it will EVER get!
    Quality and Value Service and Repair

  8. #8
    Join Date
    Jul 2000
    Location
    Guayaquil, EC
    Posts
    14,308
    Post Likes
    Thread Starter

    Thanks for the replies.........

    As I mentioned earlier, this system is charged with HP80 so I expected to only get into the ball park with Copeland Calculator's numbers. I picked 14.0 amps as a goal giving me about 10% under the calculated MCC (140% of RLA). It was only about 85F today so I blocked the condenser to get me up to a 325# head (approx 120F SDT) while setting the CPR to get my target amps of 14.0.

    Lo and behold the resulting suction pressure was 30# (about -6F SST) so it was right up there where I normally like to see it. It must be the HP80, don't you think?

    That puppy was cranking out some heat at that condition, yet not technically overloaded in my book. If you go only by RLA, it was way over-amped. If the CPR were set strictly by RLA, I'd still be there at 8:30 PM waiting for the box to get to 0F.

    Thanks again for your comment and help.

  9. #9
    Join Date
    Feb 2003
    Posts
    1,173
    Post Likes
    Rule of thumb I've always used on CPR was 110% of FLA, as it should pull down to FLA and below within a few minutes of run time after defrost. Helps to get a better pull down time.
    Watts New, Ohm My, I been Electrically Commutated. Are U2.

  10. #10
    Join Date
    Apr 2001
    Location
    Camel City, NC
    Posts
    6,234
    Post Likes

    2

    I use 20% as a rule when setting a CPR. When a ZP head is used there is no adjustment so to speak. You can use one or the other but not both together, Sporlan does not recommend it.
    Be safe not fast. body parts don't grow back

  11. #11
    Join Date
    Jul 2001
    Location
    Fairfield and NewHaven Counties in Commecticut
    Posts
    4,920
    Post Likes
    You are correct about still being there at FLA, the fans would still be cycling.....!

    I like to revisit this topic every now and then.

    I have seen 5hp and larger units run 50% over fla for 30 mins trying to pull down a Large load. Sometimes they will cycle on overload a time or two on the way down, especially on HP-80.
    Quality and Value Service and Repair

  12. #12
    Join Date
    May 2004
    Posts
    2,358
    Post Likes
    sporlan says to set them to the Maximum suction pressure for the compressor. And alot of you say set them to the RLA on the data plate. Wich is it? i have to set one tomorrow

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •