Best Ductwork Material for Moisture Removal Process 2

[m2e]

In a process building, a machine involves with hot water generates a lot of moisture/steam, and needs an exhaust system to bring the air outdoor. Because of building restriction, the exhaust duct will exit building horizontally through the wall, and run the exhaust stack along the side of the building up to the roof.

The flow rate is 40,000 cfm, and the winter outside design temperature is -40F.

The client intent to use galvanized inside and change to HDPE outside. This setup seems have an advantage of having the rust-proof duct outside and maybe insulation is not required?

What would be the best material for such application? Galvanized, stainless, PVC, HDPE, or FRP?

Also, how to determine if insulation is required, and how much, on the outside duct, if it's plastic duct or material duct?

Thanks.

 

[berkshire]

m2e (Mechanical)
I cannot reccomend which to use, But I can tell you which not to use. Having had experience with Galvanized ducting on dish washer systems, I can tell you the galvanizing being sacrificial will be worn away in spots inside 2 years by the humid air allowing the base steel to corrode away, resulting in holes in your system. We replaced the Galvanized with Stainless steel.
B.E.

The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them. Old professor

 

[KiwiMace]

I think the size of your duct lends itself to SS. You don't need to insulate if you don't mind it condensing. A good drain on the bottom of the stack (which should be there anyway) will probably allow you to not insulate the outdoor section.
You should insulate the indoor duct based on the temperature of the moist airstream and likelyhood of condensing in the duct - I would probably want to insulate to some extent, just calculate how thick you need.

 

[willard3]

Aluminum ductwork will serve also and is readily available.

 

[MintJulep]

HDPE not so good for outside as it has relatively low UV resistance.

 

[m2e]

Thanks guys.

I don't mind the condensation, and yes, there will be drains. The only thing I worry about is the freezing in the duct. If it's -40F outside plus wind chill, would the condensation freezes inside, causing weight problem, or even blockage if the ice build up, fall down, and accumulate? How do you estimate/calculate if freezing would happen or not?

Aluminum seems a good option, but the exhaust fan is running at round 4" w.g., would that be too high for aluminum?

 

[willard3]

>>>>>>>>>>>>>>>>>>Aluminum seems a good option, but the exhaust fan is running at round 4" w.g., would that be too high for aluminum?

Get thicker aluminum.

 

[m2e]

I remember reading somewhere that rectangular aluminum duct has a certain pressure limit. I've found where I saw it. It's in the June 2000 ASHRAE journal, "Metal Duct Selection and Application".

"Some people still falsely believe that aluminum duct wall can simply be two Brown and Sharpe gages heavier than galvanized sheet gage for equivalent performance. That rule of thumb was published in the ASHRAE Handbook in the 1950s and still appears in some model and local codes. It is not valid."

"With a 10 million psi modulus of elasticity, 21 ksi (145 MPa) yield strength, and an ultimate strength of 22 ksi (152 MPa), aluminum has never been recommended for rectangular duct at 4 in. w.g. (995 Pa) or higher."

 

[Compositepro]

I hope you realize that exhausting 40,000 cfm when it is -40F outside means that you have to heat that amount of make-up air. It maybe more economical to control your moisture emissions at the source.

 

[willard3]

Get the SMACNA duct tables for the proper thickness. I find it hard to believe that your exhaust system is operating above pressure limits for aluminum ductwork.

 

[UtilityLouie]

If you are continuously exhausting hot humid air, freezing shouldn't be a huge concern.

Use SS duct.

Drain the heck out of your duct. The drains that are outside will need to be insulated and heat traced because they'll be P-trapped, right?

I think that insulation would be a cost to make people feel safer. If the owner wants it and it makes them feel safe, put it on. Otherwise, do a simple heat balance. Calculate the BTUs being brought into the duct versus how much is lost to the cold even on the windiest of days. You'll probably see that the you won't have an issue.

Also make sure you interlock the exhaust fan to operate for some period of time after the equipment shuts down so you can purge the ductwork with drier air before you shut the system down for routine maintenance.

 

[MechEngNCPE]

304 SS would be the way to go, specify -6" pressure test, leakage class A. Insulate outside and down through the roof a few feet so if the fan is off in the winter you reduce the chance that condensate will form on the outside of the duct.

 

[trashcanman]

Consider routing that hot humid air through an energy recovery device. You are throwing away a LOT of BTUs.

 

[resilientsoma]

Isn't the whole point of galvanized steel its ZINC layer protecting against corrosion?

So with moisture present, it will still corrode after X amount of years?

What is normally the price difference between GALV vs SS ?