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  1. #1
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    Chiller rating question LWT

    Are all chillers rated at 50F LWT? If I walk up to a chiller at it is rated at 200 tons and running 44F LWT is it safe to say it is losing 2% per degree below 50F. Therefore the actual tonnage would be 176 tons?
    I STARTED WITH NOTHING, AND I STILL HAVE MOST OF IT!

  2. #2
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    Are we talking air or water cooled?
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    It takes three people to do anything around here. Two do the work, one explains to the crowd of people who showed up when they seen smoke and flames.

  3. #3
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    Air cooled. does it matter?
    I STARTED WITH NOTHING, AND I STILL HAVE MOST OF IT!

  4. #4
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    Jan 2011
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    Not all chillers are rated at 50℉ LWT. The formula for determining tonnage in an evaporator is delta T times GPM divided by 24. If your 200 T. machine was a 10℉ evaporator then it would have to flow 480 GPM through the evaporator. If the machine was cooling water to 44℉ then the return water could be no warmer than 54℉.

    If you are talking about a centrifugal chiller the question would be if the compressor had enough gear to provide the lift at the lower evaporator pressure. A ton is a ton is a ton. I don't know where you came up with the 2% loss per degree. That doesn't figure into the tonnage formula.

  5. #5
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    I know how many tons the chiller is actually doing, but what do you do if it is less then the rated value? The chiller model number says it should do 200, but it is only doing 180. I know that certain process chillers are rated at 50F LWT, but what about say a Carrier or a Trane?
    I STARTED WITH NOTHING, AND I STILL HAVE MOST OF IT!

  6. #6
    Join Date
    Oct 2011
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    La.
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    All equipment is rated off of AHRI standards unless it is custom ordered to not meet standards.
    That way was the manufactures cannot play numbers games with customers. But like politicians they still find loopholes to do so.

    google AHRI-550/590 and have fun reading.

  7. #7
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    Prattville, Alabama
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    Quote Originally Posted by allstar08 View Post
    I know how many tons the chiller is actually doing, but what do you do if it is less then the rated value? The chiller model number says it should do 200, but it is only doing 180. I know that certain process chillers are rated at 50F LWT, but what about say a Carrier or a Trane?
    The model number listed on the nameplate may not reflect the design tonnage of your chiller. Contact the manufacturer for the design tonnage.

  8. #8
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    Quote Originally Posted by allstar08 View Post
    Air cooled. does it matter?
    Story I was always told is that the evaporator can only absorb what the condenser can reject.
    UA 100

    It takes three people to do anything around here. Two do the work, one explains to the crowd of people who showed up when they seen smoke and flames.

  9. #9
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    Jan 2011
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    Quote Originally Posted by Allstar08
    I know how many tons the chiller is actually doing, but what do you do if it is less then the rated value? The chiller model number says it should do 200, but it is only doing 180. I know that certain process chillers are rated at 50F LWT, but what about say a Carrier or a Trane?
    It is extremely difficult to determine the performance of a chiller in the field. If you have ever been to a factory performance test you will understand what I mean. How do you know it's doing 180 T.? I can tell you that you will not be able to accurately determine the tonnage of your machine with the instrumentation available to you in the field. How do you know the exact flow? What are you using to measure the flow rate? How about the temperatures? Even if you are using a state of the art electronic flow meter and some of the best sensors available you will not be able to pinpoint the tonnage. Most temperature sensors used on chillers are accurate within +/- .5℉. Even if you knew the flow to be constant at 480 GPM and one sensor was .5℉ high and the other was .5℉ low then you would be calculating 180 T.

    As stated earlier all chillers are rated by certain ARI standards. In the real world chillers almost never run at those standards. Process chillers are like any other they can have design temperatures all over the map. I worked on process chillers earlier in my career that had a leaving product temperatures of well below 0℉. Not sure what difference it makes if the chiller is a Carrier, Trane or any other manufacturer. A ton is a ton.

  10. #10
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    Quote Originally Posted by allstar08 View Post
    I know how many tons the chiller is actually doing, but what do you do if it is less then the rated value? The chiller model number says it should do 200, but it is only doing 180. I know that certain process chillers are rated at 50F LWT, but what about say a Carrier or a Trane?
    I looked at a product data book for a Carrier chiller. And with a leaving water temperature of 50 you get 215 tons at 85 degree entering air and 189.5 tons with 115 degrees entering air. I'm sure like Spinning Wheel said its all about playing with numbers.
    UA 100

    It takes three people to do anything around here. Two do the work, one explains to the crowd of people who showed up when they seen smoke and flames.

  11. #11
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    Nov 2005
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    1,350
    I just used the circuit setter to check for flow using a wheel and pressure.

    The reason behind my questions are that the chillers on this site are maxed out, and I wanted to know what kind of tonnage numbers I was doing vs. what is stamped on the machine. If the target water temp we are currently using is 2-3 degrees below design, then I am doing better then I thought I was. I will have to check with the manufacturer to see what the design was.

    The other reason would be for the site owner asking why things aren't getting cool fast enough. Then I can say we are doing all we can, in fact we are doing more then the machine is rated for. CYA
    I STARTED WITH NOTHING, AND I STILL HAVE MOST OF IT!

  12. #12
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    Oct 2011
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    Did you not read the ARI rating standard! 54 in 44 out 2.4GPM per ton 95degrees for ambient temp for condenser. If anything changes, your tonnage changes. 200 ton unit can do more if ambient is lower and less if higher. As stated earlier, you need very accurate instruments for testing capacity. If unit is fairly new and you can duplicate these conditions then you are going to see very close to 200 tons. The manufacturer cannot lie about rated capacity on an ARI tested unit. If you check the Product Data book on your unit, they will have capacity ratings for off design conditions.

  13. #13
    Join Date
    Feb 2010
    Location
    NE Alabama
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    The reason behind my questions are that the chillers on this site are maxed out, and I wanted to know what kind of tonnage numbers I was doing vs. what is stamped on the machine. If the target water temp we are currently using is 2-3 degrees below design, then I am doing better then I thought I was. I will have to check with the manufacturer to see what the design was.



    The chillers are maxed out? Have changes been made to the system? They most likely didn't start out maxed. With any system it's not what you think but can you make it work. With any system it starts with are the components clean and are the refrig levels where they belong along with airflow which goes back to clean. when in doubt clean it first then analyze.

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