Increase cfm

[AUTOKAD]

I'm working on a retail space and heres my prob.

the heat loss calc comes up to 14 ton. The existing roof top unit only has 12 ton. The client doesn't want to add anymore ac units to cover the required 14 ton. Is there a way to increase the cfm to 14 by just using the 12 ton. sorry i'm new to hvac. I was thinking maybe there's a unit you can add on to the duct to increase it's cfm or is there such thing? Thank you all for your help

 

[MintJulep]

Are your design calculation and the unit's rating done at the same conditions?

 

[AUTOKAD]

The unit is a shell space with roof top unit. The base building engineers assume that whoever occupy the space, 12 ton would be enough. So to answer your question, I'm going to say no.

 

[sprinkler1000]

I assume you are talking of cooling (so it should be heat "gains")
Review your calculation to see if you are being too conservative.
If your calcs are right, you have too options

1- try to adjust the building (lights installed,insulation), to meet the 12 tons and use the existing installation

2- propose small split a/c unit if you have small rooms to help the rooftop

 

[IRstuff]

What's the square footage?

TTFN

FAQ731-376

 

[MintJulep]

Is the fan direct drive or belt drive. If it's belt drive you can play with pulleys to get some more cfm. If it's direct drive, is it across the line, or driven by a VFD?

So you could probably get some more CFM and thus some more capacity. But probably not 2 tons.

But, where are you getting "it's a 12 ton unit" from? Is it the rated capacity on the nameplate? That capacity happens only at a specific set of conditions that should be (but probably isn't) published in the technical data for the unit.

If your actual site conditions are not as severe as the rating conditions then you will get more than the rated 12 tons.

If your load calculations are overly conservative you probably don't need a full 14 tons.

So you might be fine.

Or the space might be a bit warm on a "design day".

Time to sharpen a pencil and take a closer look at what you really need and what you have.

 

[cry22]

Please post some engineering checks of your output, climate, space use, etc. so one can check at least the rule of thumb on the tonnage.

One thing I would do is throw in a CO2 sensor and reduce ventilation rates, it should pull 2-ton reduction along with the proposed VFD above.