Frost Protection on AHU Unit with Plate Heat Exchanger Heat Recovery

[trueman]

I am designing a 100%OA system where frost protection (35%glycol) will be running through heating coil (AHU unit has Plate heat exchanger). Winter outdoor temperature is -7 C. My question is what temperature I need to bring the air at the heating coil to avoid any damage on the coil? There are also reheat coils at each zone.

Any design suggestion??

 

[willard3]

There are a lot of variables in this, the primary one being stratification of the cold air stream, i.e. the coldest air will come in on the bottom of the heating coil.

 

[tys90]

I'm not sure I understand the question. The discharge air temperature has nothing to do with preventing damage to the coil itself during cold conditions. 35% glycol has freeze protection down to roughly -16C so you should have no issues with freezing damage to the coil. Is this a preheat coil? Do you mean what temperature do you need to prevent damage to the plate heat exchanger? That depends on your dew points of the exhaust and outdoor air streams, most manufacturer's have literature to help you out with that.

 

[trueman]

Yes, it is a pre-heat coil. The AHU runs only during the day, and it will be off night time thats why we added glycol system to protect the coil.
I'd like to know what temperature I need to bring the outside air to. I think I have to heat outside air up to 55F but I saw someone was saying it should be enough to bring outside air to +5 C (41 F) somewhere in the forum. Is there a common application/rule for this temperature? Regards

 

[cry22]

Use face and by-pass on the wheel, by-pass the wheel when LAT (or EAT of OA after the wheel) is below freezing point.
Look at ASHRAE it has LAT calculations formula - if you still can't find it, then we will. It is easy to give you the answer, but I think you should be able to dig it easily so you can remember it.
Use a condensate drain for the wheel, and don't forget to heat trace it.
Locate you pre-heat coil upstream of wheel.

 

[trueman]

Ok, will do.
Thanks for the tips!