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  1. #1
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    Chilled Water Coil Flow

    I am aware that counter flow is the preferred way to pipe a chilled water coil for an AHU. What I am trying to find is a numeric value making a comparison of the efficiency. For example What is the BTU difference assuming the same CFM and GPM values for both installation methods. Is this difference in efficiency a some what linear across coil sizes or is a large loss on small coils but not as much of an impact on very large coils.

    I am not the installing contractor trying to justify a mistake made. I am just curious how drastic of a difference it can make on cooling capacity more so as a controls tech who is curious

  2. #2
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    Dec 2006
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    Chilled Water Coil Flow

    Ive been out of school for a long time. I dont recall the formula off hand. I would have to look in my book which Im not be at right now and wont be until Im back from vacation. But you can calculate it out by determining the temperature drop of each piping configuration and plugging it into your BTUH formula. You would need to re pipe one to determine this but if it saves you from having to re pipe additional hundreds then it may be worth the exploration.



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  3. #3
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    May 2014
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    Thread Starter
    It will likely not be repiped. It's one large AHU with a 2 zone coil 108"" tall and 72" wide. It has been under performing due to VE'd booster pumps. I was really curious if there was any research or study's done comparing the actual measured result difference and trends associated based on coil size. I see people throw numbers around like a 3% loss per the trane rep or 17% when it's the guy who wants to sell the repipe work to the customer. But these are from forums and not citing manufacturers or industry specific articles and seem to be mostly opinions formed from logical thinking but citing no origin.

  4. #4
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    Thread Starter
    Correction VE'd OUT booster pumps that have now been decided to be installed to resolve the missing 100GPM of water flow.

  5. #5
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    Nov 2001
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    I was responsible for the engineering of large coils for a large air handler company for many years. There is no simple formula. It depends on your entering water temperature, entering air temperature, the number of rows, fins-per-inch, circuiting, etc. Here are some general guidelines:

    - If there is a small difference between the entering water temperature and entering air the effect is large. If a big difference (e.g. heating coil with 160 degF water) it is much smaller.

    - More rows, more effect. The rule-of-thumb that I use for cooling coils with typical conditions: 2 or 3 row coils it is typically 3 to 5%. 6 rows 10%, 8 rows 12%. Heating coil effectiveness losses would generally be half or less of these.

    The big performance cost is that in order to get the same leaving air temperature that you would get with a counterflow coil is that you have to supply colder water.

  6. #6
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    Feb 2017
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    Do you have access to the submittals for the AHU? Once you get your chilled water flow straightened out, then you can compare the engineered ∆T across the coil to what you have. Now you can calculate your performance loss for this specific unit.

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  8. #7
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    Dec 2006
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    Quote Originally Posted by klo View Post
    It will likely not be repiped. It's one large AHU with a 2 zone coil 108"" tall and 72" wide. It has been under performing due to VE'd booster pumps. I was really curious if there was any research or study's done comparing the actual measured result difference and trends associated based on coil size. I see people throw numbers around like a 3% loss per the trane rep or 17% when it's the guy who wants to sell the repipe work to the customer. But these are from forums and not citing manufacturers or industry specific articles and seem to be mostly opinions formed from logical thinking but citing no origin.
    Nobody is going to be able to tell you an accurate blanket percentage. Site conditions, unit specifications, and ambient conditions will have their own unique effect on the outcome. Youll have to do the calculations and run the numbers.

    Unit specs should be able to tell you whats its supposed to give at their designed test conditions. If you cant flip the pipes then youll have to use those numbers and compare to what you have.


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  9. #8
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    This is the way i understood it. The big loss is it doesn't maximize temperature difference throughout the coil thus inhibiting it's ability to drive down supply temp. The other way to overcome it would be additional water flow to obtain the same cooling capacity. The design is about 44 degree entering water temp and 74 degree mixed air temp. Thanks for the insight this definitely provides a better understanding off the overall trends. Thanks for the info

  10. #9
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    May 2014
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    Thread Starter
    Quote Originally Posted by coolcoil View Post
    I was responsible for the engineering of large coils for a large air handler company for many years. There is no simple formula. It depends on your entering water temperature, entering air temperature, the number of rows, fins-per-inch, circuiting, etc. Here are some general guidelines:

    - If there is a small difference between the entering water temperature and entering air the effect is large. If a big difference (e.g. heating coil with 160 degF water) it is much smaller.

    - More rows, more effect. The rule-of-thumb that I use for cooling coils with typical conditions: 2 or 3 row coils it is typically 3 to 5%. 6 rows 10%, 8 rows 12%. Heating coil effectiveness losses would generally be half or less of these.

    The big performance cost is that in order to get the same leaving air temperature that you would get with a counterflow coil is that you have to supply colder water.
    This is the way i understood it. The big loss is it doesn't maximize temperature difference throughout the coil thus inhibiting it's ability to drive down supply temp. The other way to overcome it would be additional water flow to obtain the same cooling capacity. The design is about 44 degree entering water temp and 74 degree mixed air temp. Thanks for the insight this definitely provides a better understanding off the overall trends. Thanks for the info

  11. #10
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    Aug 2009
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    Dehumidification will be degraded with piping backwards.
    In honor of RichardL: "Ain't 'None' of us as smart as 'All' of us".

  12. #11
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    So they want to pay for and have the additional cost of running a larger pump, plus run a higher water flow through the coil accelerating wear on the pipes, with the hopes that they will achieve a similar heat exchange?

    It sure sounds easier, and cheeper in the long run, to fix the known issue instead of continuing to bandaid it.


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  13. #12
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    Aug 2009
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    Prattville, Alabama
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    It may be easier to correct the piping than your customer realizes.
    When I came to work in my current job, I found 5 AHU's were piped backwards! Out of 17 total. Two were on a riser at one end of the building, and three were on a riser at the other end. I realized there was a problem when I saw one AHU was not maintaining setpoint, despite the CHW valve being wide open. Supply air was about 3-4 degrees above setpoint. While checking water flow at that AHU, I saw that it was backwards. In fact, the chilled water lines were mislabeled. That caused me to check other risers, and I found one other also mislabeled and piped backwards. The building had been renovated about 3-4 years earlier. When I reported this, the contractor was contacted. Their response: if we piped it wrong, we'll correct it. They verified it wrong, and took measurements of the piping to be corrected. We scheduled a Saturday on which to do it. They came with crossover sections they fabricated in their shop, and a welder, and other tools, plus an insulator they subbed out. They did the whole thing in about four hours.
    Did not cost us a penny, except for my overtime to be there. All I did was isolate and partially drain the risers, and refill them and test run the system. I was there maybe one hour before them and one hour after them.

    Just the peace of mind, knowing that it's right, is well worth that.
    In honor of RichardL: "Ain't 'None' of us as smart as 'All' of us".

  14. #13
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    Aug 2019
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    I use the general loss if the coil is piped backwards is 3% - 4 % per row . BTW , I agree with pretty much everyone above . I also agree that with planning and pre-fabricated pipe its a fairly easy job to correct the piping . It's amazing that someone would want to leave it as is . Do they know about the reduction in capacity and loss of dehumidification ? P.S. Have you tested the actual cooling btu/hr and compared it to design ?

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