carrier rooftop unit

[leavemcold]

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?

[comfortdoc]

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.

[Dallas Duster]

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.

[hotrodrob]

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.

[wisconsinapp]

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?

[seascott]

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.

[Dallas Duster]

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.

[wisconsinapp]

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.

[Damien]

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.

[leavemcold]

the single stage td was 12 and the 2 stage was 24

[genduct]

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