Selection of Equipment: Tonnage Vs CFM

[PS]

Hello Professionals,

According to heat load calculations, I'm getting 80 TR and 32800 dehumidified CFM for a subjected space, a shopping complex. The situation is I need to use only 4 TR cassette split units for the space. The cassette units are rated for 4 TR at 1140 CFM.

Now should I go for 4 TR x 20 Nos at 22800 CFM or should I match the CFM, i.e. 4 TR x 29 Nos at 33060 CFM?

Would anyone please suggest a solution with explanations?

Thanks in advance!

 

[lukaiENG]

You are getting a mismatch between CFM and tonnage because the unit performance is based on a difference differential temperature than your heat load calculation.

Suggestions:
1. Review your heat load calculation to confirm you have correct/appropriate supply/return air conditions and then get performance data from the manufacturer which matches these conditions.
OR
2. Edit your heat load calculation to reflect the manufacturer supply/return conditions IF these make sense from a space comfort perspective. Note this is only relevant if it is the supply condition which changes. i.e. if the cooling unit supplies air at 50degF vs a design SAT of 50degF, this may be acceptable to you.

It rarely works out (in my experience) that the catalog data is exactly at your operating parameters.

 

[MintJulep]

Break the space down into smaller zones and calculate individually.

No chance at all the the load will be homogeneous and that all units will share it anywhere close to equally.

 

[PS]

Thanks a lot @lukaiENG. The concern is I need to use the 4 TR units accordingly to suffice my heat load. No other go for me. That is troubling my calculations. I also tried to match the off coil temperature of the units as my supply temperature but I'm ending up nowhere in that case.

Thanks @MintJulep!

Are there any other ways to use the same 4 TR units for my conditioned space?

 

[GT-EGR]

That calculation is useful for a big picture idea of your cooling needs. But to do this correctly you need to calculate the load on a smaller scale - for each zone/enclosed area where you want a thermostat. If you have 40 zones, you’ll need 40 units regardless of the load you calculate.

You also should be questioning why you have to use only 4 ton units and nothing else, no reason to just box yourself into an unnecessary corner and end up with undersized or oversized units.

Not sure what load calc software you use, but it may have a system selection option, and if you set the system conditions to match the performance (dishacrge temp and return air temp) of the units you seem to want, then your CFMs will end up matching. The CFM number means little on its own and really is only good if your supply air temp and space temperatures match what you want.

 

[KesTheHammer]

The cooling required is TR (or kW) not CFM. CFM is airflow and if you reduce the airflow for the same Q you increase the dT.

Just bear in mind that the rated power is not necessarily what you can achieve with it. Quite often the rated power is calculated at 26C indoor not 21 C.



Kestell Laurie
South Africa