Best Ductwork Material for Moisture Removal Process
Best Ductwork Material for Moisture Removal Process
(OP)
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.
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.





RE: Best Ductwork Material for Moisture Removal Process
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
RE: Best Ductwork Material for Moisture Removal Process
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.
RE: Best Ductwork Material for Moisture Removal Process
RE: Best Ductwork Material for Moisture Removal Process
RE: Best Ductwork Material for Moisture Removal Process
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?
RE: Best Ductwork Material for Moisture Removal Process
Get thicker aluminum.
RE: Best Ductwork Material for Moisture Removal Process
"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."
RE: Best Ductwork Material for Moisture Removal Process
RE: Best Ductwork Material for Moisture Removal Process
RE: Best Ductwork Material for Moisture Removal Process
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.
RE: Best Ductwork Material for Moisture Removal Process
RE: Best Ductwork Material for Moisture Removal Process
RE: Best Ductwork Material for Moisture Removal Process
So with moisture present, it will still corrode after X amount of years?
What is normally the price difference between GALV vs SS ?