Draining Condensate from Return-side Duct

[handee]

Hello, we've got an air cooler that consists of a cold water coil and a blower. The only place we can locate the coil is upstream (return, or suction side) of the fan, so I'm a bit concerned about corrosion if condensation droplets reach the fan. But my first concern is draining the condensate from the ductwork at the coil - what kind a trap would allow the condensate to drain out against the negative pressure differential? Thanks for any ideas.

Fran McConville
author of the Pilot Plant Real Book

http://www.pprbook.com

 

[quark]

This is a common practice, if not standard, to put cooling coil in fan suction side. What you require is a U loop seal(or a P Trap type thing), with a maximum difference of height in both legs equal to that of the fan static.

If you maintain a coil face velocity below 500fpm, moisture carryover is minimum.

Regards,

 

[HVAC68]

If you are still worried, use a drift eliminator

HVAC68

 

[handee]

Thanks gentlemen, that's great to know. Now how can I estimate the maximum static right in the vicinity of a 6000 CFM fan fed by 24 inch ducts?

Fran McConville
author of the Pilot Plant Real Book

http://www.pprbook.com

 

[quark]

Check on the fan or with fan manufacturer. This can be an overkill but I follow this as I work in a pharmaceutical company and don't want any outside air coming into the AHU without prefiltering. Further this condition may occur when the suction damper is fully closed and fan still running.

You can use 2 to 3" difference for a seal.

Regards,

 

[handee]

This isn't pharma, but we've got the same requirement to keep fresh air out. The air is circulated through prefilters, through carbon odor-control filters, then the cooling coil, all upstream of the fan. The pressure drop may be high enough that assuming the suction damper is closed is not overkill. So you think the fan draws only 2-3 in. wc? That's good to hear.

Fran McConville
author of the Pilot Plant Real Book

http://www.pprbook.com

 

[walkes]

To verify your static pressure drop, get the information from the manufacturer and have an Air Balancing and Testing firm confirm the pressure drop.

 

[atilashorse]

For 6000 fpm in a 24" duct, you will be moving aprox 18,600 CFM, and your pressure drop will be 1.6 in per 100 feet of ductwork.

If you have about 24" between the condensate drain and the fan, that will be 0.067 in in pressure drop.

 

[handee]

Ramcode, it's actually a 6000 CFM system. To size the P-trap I need a ballpark for what the minimum static pressure is at the fan suction, assuming the suction side damper is closed (worst case). Unfortunately it looks like the manufacturer may have gone out of business.

Fran McConville
author of the Pilot Plant Real Book

http://www.pprbook.com

 

[quark]

The pressure drop at 6000cfm for a 2'x2' duct is about 0.123inches/100ft. As far as my experience is concerned, the blower static may not be morethan 6"(even with terminal HEPAs). Even if half of that static appears at the fan suction, a 3" seal will be sufficient.

Regards,

 

[atilashorse]

Sorry I misunterstood the data. I agree with quark, a 3" trap should suffice.
Now, when the damper is closed, the air handler shouldn't be working. Additionally, if the damper is closed because of a fire, what importance may have a few drops of water into the AC ductwork anyhow? I don't think you should desing the P-trap for that condition.

Regards,

PS: Regarding pressure drops, CFM and duct sizing, get yourself a ductulator from Trane or other manufacturer. Call your Trane dealer, they will probably give it to you for free, or charge like 5 or 6 bucks.