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  1. #1
    Join Date
    May 2007
    Posts
    739

    RTAA Low Differential Lockout

    We tried to tell them not to do it.

    Trane RTAA chiller has intermittent Low Differential lockout during cold weather (below 20F or so). Ambient finally got cold enough today (11F) that I never got the chiller to run long enough to create 40 PSID. All the chiller was trying to do was chill the building loop (apx 350 gallons) from 70F to 42F, there was no other load. In their wisdom, the engineers insisted on using the building chiller for some process chilling, even though the building load is about 60 Tons and the process load will never be more than 10 Tons. Result in cold weather when no process is going on is that the chiller only comes on once every hour or longer just long enough to cool the loop and the condenser gets stone cold in the interim. After today's experience I am going to suggest a fluid cooler for the process.

    Thoughts? Similar experiences?

  2. #2
    Join Date
    Apr 2009
    Location
    In the south
    Posts
    30
    Is it equipped with low ambient controls for the fans.

  3. #3
    Join Date
    May 2007
    Posts
    739
    It never built up enough heat to turn on the fans. Wind was blowing at about 30MPH on one side. My first thought was wind shields. I disabled the windward side compressor and let the other compressor run a few times but it never warmed up enough.

  4. #4
    Join Date
    Dec 2006
    Location
    Henryville IN
    Posts
    248
    This may help. Trane service bulletin RTAA-SB-21. Kit number is 06300 which is an upgrade for fan cycling.

  5. #5
    Join Date
    Mar 2004
    Location
    ottawa canada
    Posts
    2,095
    This may help even more .
    Its called a dry cooler !!!!!!!!!!!!!!!!!!!!!!!!!!!!
    No load is no load ,fan staging dont matter S@$% when theres not even enough load to heat the condenser .
    The toy chest is officially full ... I got a new toy..... 2007 Aston Martin V8 Vantage
    You bend em" I"ll mend em" !!!!!!!
    I"m not a service tech.. I"m a thermodynamic transfer analyst & strategic system sustainability specialist
    Best Austin Healey In Show twice in 2013 .....All those hrs paid off .

  6. #6
    Join Date
    May 2007
    Posts
    739
    Graham,

    I suggested a dry cooler after going back when it was warmer and verifying charge in both circuits. I was told by the engineer they would talk about it which for this site is code for not going to do anything until it happens repeatedly.

  7. #7
    Join Date
    Dec 2007
    Location
    chicago
    Posts
    249
    Graham, Can you elaberate a bit on the dry cooler. The reason I'm asking is I recently got an account with a RTAA and a dry cooler on it thats supposed to takeover when the oat drops below 20 degrees. I don't see how pulling 20 degree air over a coil is going to drop the temp enough to make a difference. Incedently:; they don't use it, they run the rtaa. thanks

  8. #8
    Join Date
    Mar 2004
    Location
    ottawa canada
    Posts
    2,095
    This aint rocket science . If you need comfort cooling in the winter you dont need 42 degree water 46/50 may do even if you do need 42 degrees if you change over to the dry cooler at lets say 35 degrees and your return water is say 50 degrees as long as the dry cooler coil is big enough you will get cooling .
    Im no engineer just a guy in the trenches so you would need to determine your winter load and size the dry cooler accordingly .
    Ps dont forget the glycol. Square water dont fit through round pipes !!!!!!!!!!
    The toy chest is officially full ... I got a new toy..... 2007 Aston Martin V8 Vantage
    You bend em" I"ll mend em" !!!!!!!
    I"m not a service tech.. I"m a thermodynamic transfer analyst & strategic system sustainability specialist
    Best Austin Healey In Show twice in 2013 .....All those hrs paid off .

  9. #9
    Join Date
    May 2008
    Location
    Central Pennsylvania
    Posts
    442
    It's all about the humidity. You need to remove the latent heat before you see a change in the sensible heat so....50* entering water in the winter may make you the same supply air temp as 42* entering water in the summer. Those are just off the wall examples of course for the whole water temp thing that Graham was talking about. I'm sure in your case a drycooler would pay for it's self pretty quick between electrical usage and wear and tear on they're chiller. If they insist on using the chiller for such a small load sell them a big ol' buffer tank.

  10. #10
    Join Date
    Dec 2007
    Location
    chicago
    Posts
    249
    Quote Originally Posted by graham View Post
    This aint rocket science . If you need comfort cooling in the winter you dont need 42 degree water 46/50 may do even if you do need 42 degrees if you change over to the dry cooler at lets say 35 degrees and your return water is say 50 degrees as long as the dry cooler coil is big enough you will get cooling .
    Im no engineer just a guy in the trenches so you would need to determine your winter load and size the dry cooler accordingly .
    Ps dont forget the glycol. Square water dont fit through round pipes !!!!!!!!!!
    Thanks for responding, I'm guessing my customers unit is undersized.

  11. #11
    Join Date
    Feb 2010
    Location
    Ontario, Canada
    Posts
    15
    Quote Originally Posted by fxb80 View Post
    We tried to tell them not to do it.

    Trane RTAA chiller has intermittent Low Differential lockout during cold weather (below 20F or so). Ambient finally got cold enough today (11F) that I never got the chiller to run long enough to create 40 PSID. All the chiller was trying to do was chill the building loop (apx 350 gallons) from 70F to 42F, there was no other load. In their wisdom, the engineers insisted on using the building chiller for some process chilling, even though the building load is about 60 Tons and the process load will never be more than 10 Tons. Result in cold weather when no process is going on is that the chiller only comes on once every hour or longer just long enough to cool the loop and the condenser gets stone cold in the interim. After today's experience I am going to suggest a fluid cooler for the process.

    Thoughts? Similar experiences?
    I am working on a RTAA125 now and have similar problems but the Variable speed fan is on as well as 2 others when it it is 36F outside and low load conditions at process. It runs great for 3 seasons just not winter .I will pass on the info as I get it if it helps. The dry type fluid cooler sounds good .

  12. #12
    Join Date
    May 2007
    Posts
    739
    Thanks for the offer, just chillin', but I'm keeping track of your thread on the fan problems, or lack thereof.

    What Madhat said in your thread about a "set-up" got me thinking so I read the IOM some today, then spoke with a couple of our engineers. Especially after seeing my troublesome RTAA lock out again last night (it was in the mid teens here).

    The lead engineer says our customer's process could tolerate 60F chilled water with 70F for short times. So tomorrow I'm going to set up the chiller for outside air temperature reset. Process is the only thing this chiller is serving below 55F OAT. Maybe it will load more when it runs (maybe not) and it will keep the chiller a little warmer during those loooooong off times.

    BTW, the IOM was really good reading on Condenser Fan Staging.

  13. #13
    Join Date
    Feb 2010
    Location
    Ontario, Canada
    Posts
    15
    Quote Originally Posted by fxb80 View Post
    Thanks for the offer, just chillin', but I'm keeping track of your thread on the fan problems, or lack thereof.

    What Madhat said in your thread about a "set-up" got me thinking so I read the IOM some today, then spoke with a couple of our engineers. Especially after seeing my troublesome RTAA lock out again last night (it was in the mid teens here).

    The lead engineer says our customer's process could tolerate 60F chilled water with 70F for short times. So tomorrow I'm going to set up the chiller for outside air temperature reset. Process is the only thing this chiller is serving below 55F OAT. Maybe it will load more when it runs (maybe not) and it will keep the chiller a little warmer during those loooooong off times.

    BTW, the IOM was really good reading on Condenser Fan Staging.
    FXB80 thanks for the input i will dig in to that manaul again

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