Recirculating discharge air to inhibit inlet filters from freezing 1

[crb1976]

I am new to the forum working on a project where we are going to recirculate a portion of our discharge air back across our inlet filters to stop them from freezing. We are drawing in atmospheric air @ ambient temperatures our discharge air is in excess of 100F. I would like to know if anyone can tell me at what conditions (%RH and ambient Temp) you would need to open the valve to start the defrosting process and for what amount time. The volume flow would also be helpful. Our filter is roughly 2500 square inches

Thanks in advance for you help

 

[MechEngNCPE]

So you have a 100% OA system without a pre-heat coil? You could install a preheat coil, and a discharge air coil and put them on a closed circuit loop with a temp sensor modulating the control valve and/or pump to maintain a constant leaving air temperature above freezing, so the filter would never see freezing temps. This would also prevent cross-contamination of your airstreams, as well as use that free heat for pre-heating in the wintertime.

 

[MechEngNCPE]

Down here in NC we don't need to worry about filters freezing, so we put the pre-heat coil after the inlet filters, I don't see anything wrong with putting the coil first, just make sure you have a good 1/4" steel mesh and louver to protect it.

 

[crb1976]

Thanks for the info, I think we are going to stay away fro melctric heaters / Coils as this may prove to be quite costly in energy consumption.

 

[ChrisConley]

Didn't see this one the first time around. We typically set preheat coils to 5 deg F to prevent frost build up. Most of our installations have a preheat coil with two filter positions (winter and summer) maintenance staff is then expected to move the filters each season.

In practice we frequently see filters in both positions, or left in winter position year round... but we try.

 

[tys90]

There isn't enough information to answer your original question.

I like MechEngNCPE's suggestion. The only additional energy you are using is pump energy. And since you are working with a large temperature delta, you shouldn't need to pump very much (relatively). I suppose you could recirculate some air but it sounds like this is a process application of some sort?

 

[MechEngNCPE]

crb - the closed circuit coil will only use heat from your airstream that will be exhausted anyways. It is free heat. The only cost is the 2 glycol coils and a pump/valve setup. This method is used widely in pharma/lab industry as a way to SAVE energy.

 

[crb1976]

Would you be kind enouigh to breifly explain how the Coil / Pump works?

 

[chicopee]

It seems that MechEngNCPE has one glycol coil in front of the filters and one downstream from the filters. Up stream and downstream are connected in to a closed circuit with a pump in the line to get the glycol moving.

 

[MechEngNCPE]

Actually, there is one coil in front of the filters, and that is connected to a coil outside the unit, where you can run your exhaust duct through. Think of two coils side by side, one with air flowing from left to right (ahu) and the other flowing right to left (exhaust). They are not in the same unit. There is hot water/glycol pumped on a closed circuit through both coils, as the exhaust air heats the glycol to say 70 degrees, it then flows to the preheat coil in the ahu, and provides free heat to raise the temp of the OA coming in to above freezing. The temp you will get will be deterimined by size of coils, LAT,EAT, CFM's and gpm's. The gpm can be controlled with a TCV (temperature control valve) to provide a minimum Leaving Air Temp (LAT) before the filters.

 

[ChrisConley]

For 'googling' purposes the system MechEngNCPE is describing is a 'run-around' heat recovery system. They are typically able to recover ~60% of the waste heat from an exhaust stream and would be a excellent way to deal with your frost issue as you've also got an energy recovery system installed.

 

[MechEngNCPE]

Exactly Chris, run around heat recovery. This would be a great application.

 

[crb1976]

Thanks for the info

 

[rmw]

What is your minimum ambient temperature? And what are your typical humidities at that point?

I can look at some info I have at work and see if you are into icing conditions. We have to deal with that all the time and it is surprising how little heat you have to add to get the temp/humidity ratio changed enough to get out of icing.

rmw

 

[crb1976]

-8F and 10%RH

 

[rmw]

I looked at the information I have which ranges from +40F to -40F and what I noticed is that below ~60% RH, there was no icing potential. There just isn't enough moisture in the air. Our controls would not activate the anti-icing equipment.

Now above 60+% at any temperature in that range...... well bring on the anit-icing.

rmw