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  1. #1

    carrier rooftop unit

    On a rooftop pkg unit that has two circuits, and only first stage running should you only expect half the temp drop accross the evap. 72 RA temp and 60 SA and the other 10 degrees when the 2nd stage starts?

  2. #2
    Join Date
    May 2006
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    No. If it is a constant volume unit, I would expect the delta T across the evap to be higher than if all stages were operating.
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  3. #3
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    Jan 2008
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    On most carrier units running only first stage you are bypassing the top of the evaporator coil . First stage on bottom second stage on top. So it is very possible to have a lower delta T with just one stage running.
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  4. #4
    Join Date
    Mar 2010
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    Kent, WA.
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    I work on a lot of package units, I got a couple strip malls i take care of. In most of my cases the first stage will get about 15* delta with a constant volume of air and with the 2nd stage it will be 20*. I also live in an area where the design temp is 85* and the humidity in the 40% range. your numbers will be different if your climate is more/less humid. also another factor is 1st and 2nd stage sizing. I see lots of units that will have 1st stage be 10 tons and the 2nd 5 tons on a 15 ton unit for example.
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  5. #5
    Join Date
    Nov 2009
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    Quote Originally Posted by comfortdoc View Post
    No. If it is a constant volume unit, I would expect the delta T across the evap to be higher than if all stages were operating.
    I really don't understand your thinking here... Maybe I am missing/forgetting something? Please explain why you would get a higher delta T with only 1 stage vs 2 stages?

  6. #6
    Join Date
    Dec 2005
    Location
    Vancouver BC
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    126
    The overall temperature drop over the evaporator is lower with one stage running than with two. With the second stage running you have another circuit pulling heat from the airstream so the OVERALL delta T will always be lower. With systems that interweave both refrigerant circuits over the full area of the coil and two circuits of identical capacity the overall Delta T of one stage operating is about half of what it is with both. As was explained, and is very common, Carrier puts one evaporator on top of the other. When one stage is running, condensate on the coil restricts the airflow and more air is forced through the dry inactive coil. The overall Delta T is lower measured at the return and supply duct however the air temperature measured immediately before and after the one operating coil will have normal Delta T or possibly higher because the airflow is restricted by condensate.
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  7. #7
    Join Date
    Jan 2008
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    Quote Originally Posted by wisconsinapp View Post
    I really don't understand your thinking here... Maybe I am missing/forgetting something? Please explain why you would get a higher delta T with only 1 stage vs 2 stages?
    Read it again slowly.
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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.

  8. #8
    Join Date
    Nov 2009
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    132
    Quote Originally Posted by thegoodlistener View Post
    Read it again slowly.
    I did read it again slowly, and the way I read it is, if you have one stage of cooling running your delta T is going to be higher than if you have 2 stages of cooling running. And to me, if I have a 20 degree delta T and a 10 degree delta T a 20 degree would be higher, therefore I would have a 20 degree delta T with 1 stage and a 10 degree with 2 stages, which makes no sense to me because if that were the case then why wouldn't the unit start out with 2 stages of cooling then kick down to 1 when more demand was needed? Not trying to start an argument, like I said, just seeing if there is something I am missing here or maybe I am just reading it wrong.

  9. #9
    Join Date
    Nov 2010
    Location
    N. Idaho
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    Quote Originally Posted by thegoodlistener View Post
    Read it again slowly.
    Just reminded me of my local Bryant Tech Support guy. He'll ask me to read something, like say an error code. Then while I'm reading it, he'll say "STOP, Now what did you just tell me?" I'll repeat it and he says, "Thats where you start" or "what does that tell you" or some rhetorical question in a way to make me think about the problem and find a solution.

    Always better for me to fix it, rather than him telling me the answer.

  10. #10
    the single stage td was 12 and the 2 stage was 24

  11. #11
    Join Date
    Jun 2010
    Location
    Philadelphia PA
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    2,190

    Come on fellas

    The use of HIGHER in this case meant larger number/ value/ temp diff It is a simple case of saying the right thing in a different way that made it confusing
    You have got to learn from other people's mistakes! Because God knows you don't live long enough to make them all yourself !!!!!!!!

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