oversized?

[seniorbeeg]

this is a comercial paint booth oven with two seprate rooftop units feeding one tunnel in the building. everything in the units is rated for 800000 btuh and the units modulating valves are always at low end.i found a detail in the oem manual that shows the total btuh to be 800000, that is 400000 for each unit. what airflow charicteristics or other can i use to determine if the drawing is flawed or the install is flawed?

 

[walkes]

Determine what your required air temperature for the process is. Do both units operate simultaneously or is one supposed to be a standby unit?

 

[imok2]

Try turning one unit off and see if one unit can handle the load it does, then it's more then possible that one is for a stand by, if that's the case you should run them on a timing cycle so run time is evenly distributed.

 

[tombmech]

It should be a simple airflow process:

BTUH = 1.08 x CFM x DeltaT

Delta T should be the difference between ambient (or intake air) and the desired temperature in the oven. CFM is the air velocity times the cross-sectional area (perpendicular to the airflow).

Most cross-flow (horizontal) paint booths use 100% outside air intake and 100% exhaust to the outside, at 100 fpm (in the booth). It could be that your units are sized to provide continuous production capability during all seasons. In that case, they may be sized to handle the extreme of your particular winter design value, which may or may not be a common temperature.

Pull-down ratio is important in a production process, too - you don't want to stand around waiting all day for the oven to get up to temperature. So, the units may be over-sized for that as well.

Or, they could simply be redundant as the others suggest.

 

[ChrisConley]

Just a small addition to tombmech's formula.

If the units that are making up the exhaust are direct-fired then the Btu/hr formula is modified to:

Q = 1.18 x CFM x DeltaT

The additional capacity is due to the latent energy added from the products of combustion burned in the air stream.

 

[tombmech]

No offense, but a direct-fired unit in a paint booth is a recipe for disaster.

 

[ChrisConley]

I find myself defending direct-fired equipment again, despite the fact I seldom (if ever) specify it.

Direct-fired MUA are used regularily on paint-boots. Many manufacturer's offer 'paint-booth' units that allow the units to increase temperature for baking/curing.

Where else does the receipe for disaster contain? What disasters have you seen because of a direct-fired MUA?

 

[tombmech]

I'm not saying it's not done, just that it's not the best choice. Cure cycles usually need massive capacities due to the combination of single-pass airflow and the elevated temperature. So, a cheap and powerful BTU-rated source is economical, ala NG.

However, a typical paint booth is an explosive environment. A direct-fired make-up unit is NOT intrinsically safe. There are some failure scenarios where vapors/paint spray can migrate back up into the unit with catastrophic results.

 

[tombmech]

Chris,
There may also be a distinction between a "curing booth" or "oven" vs. a real paint booth. The application of direct-fired NG in a "real" paint booth is the concern - you have a potentially explosive, fine particle mist that can ignite.

Curing booths or ovens generally are dealing with outgassing, only, so the concentration potential is much less.

Some of this may just be my old-fart experience, too. Newer VOC-compliant paints are intrinsically safe anyway. It's the old volatiles that caused the issue. They are still used in some places.