Heat Recovery

Eugene Silberstein 3 · April 2007

As Brad mentioned, humidity has a great effect on system operation. Areas that are prone to high humidity tend to have problems when it comes to low ambient operation. At lower temperatures, the coil becomes very efficient, the temperature drops and the rate of dehumidification, at least at first, tends to rise. The problem with this is, as the coil temperature drops, the excessive amount of condensate freezes on the coil.

One way to increase the dehumification whuile preventing the coil from freezing is to operate the coil at a warmer temperature for longer periods of time.

Moist areas have always been a challenge and, if not addressed properly at the design stage, tend to provide long term service problems.

Good luck and keep us posted on your findings and progress.

larry_15 · April 2007

I think I have a hard one for you

Professor:

What I was wondering is: what information do I need to get and how would I verify the results? I have a pool room with a air to air heat exchanger. The room contains two small boilers and and step down transformer. Plus, an open filteration tank for caening the water and adding chemicals to the pool water. These are my only heat sources. We supply 750 CFM to the room and exhuast the same amount of CFM. This is an interior room, with only an exit door to the out side. Block constrution.

In the winter time we are designed to heat the incoming are to 43 degrees at minus 15.

The problem is that we are freezing up the heat exchanger in low tempatures. Likewise I am told that the room is exstreamly cold when checking out the chemicals and boilers. I know this design is based on 78 degree air in the room. With the evaporation form the pump tank supplying some of that heat tempature.

Based on the supply and exhaust CFM and haet gain can you figure out how if the 43 degree supply air tempature is bring that room down to a tempature cuasing it to fall out of the design parameters? And hwat real information would I need?

larry_15 · April 2007

I think I have a hard one for you

Professor:

What I was wondering is: what information do I need to get and how would I verify the results? I have a pool room with a air to air heat exchanger. The room contains two small boilers and and step down transformer. Plus, an open filteration tank for cleaning the water and adding chemicals to the pool water. These are my only heat sources. We supply 750 CFM to the room and exhuast the same amount of CFM. This is an interior room, with only a narrow hallway and an exit door laeding to the out side. Block constrution heated space above the room too.

In the winter time we are designed to heat the incoming are to 43 degrees at minus 15.

The problem is that we are freezing up the heat exchanger in low tempatures. Likewise I am told that the room is exstreamly cold when checking out the chemicals and boilers. I know this design is based on 78 degree air in the room. With the evaporation form the pump tank supplying some of that heat tempature.

Based on the supply and exhaust CFM and haet gain can you figure out how if the 43 degree supply air tempature is bring that room down to a tempature cuasing it to fall out of the design parameters? And heat real information would I need?

Thanks for your help

larry

Brad White_9 · April 2007

Without stepping on the professor's toes, may I

offer an observation?

Your problem is one of humidity. Any heat exchanger under those conditions will drop the air temperature below the dewpoint at some time. The fact that you mention "evaporation from the pump tank" is a key indicator.

The good news is that outside air at those temperatures has relatively little moisture and is an effective drying agent. It can absorb a lot of moisture before it is sent out. The bad news is that you are freezing your plate air exchanger in the process. This in turn lessens the effectiveness so you get colder air. Even with a defrost cycle (bypass type especially) you defeat the operation even temporarily. A duct heater may be in order and if chemicals are involved, coated coils or SS sheathed electrical elements. Sounds like a harsh environment.

I suggest that you not rely on an open tank for heat; take steps to minimize moisture by sealing the water side. Vicious circle I know, but I see few options. Lessen the moisture and ventilate the remainder.

larry_15 · April 2007

Thanks Brad

Brad:

That is what we are figuring the problem is, I am just trying to find a way to verify it. That's what I am really looking for. Just to clearify... the tank it is an open pit like they have in most indoor pools. I am trying to ask the engineer to consider a preheat coil for the entering air which will keep the tempature up in the room.

I think over time, because the unit is running 24/7 that it loses room temp and it starts acting like a cooler box. The duct is dripping with condensation and the room becomes very cold, this is what I am told anyway.

I have 48% rh in the room at 68 degrees dry bulb with the out side air temp at 65 degrees. Now, it looks like I have to wait until next winter to find out the real problem.

larry

Brad White_9 · April 2007

You are welcome, Larry-

You may want to invest in some data loggers (Onset Computer is what I use) and monitor the temperature and RH over time, even within the HEX. 68F and 48% does not sound bad, but drop that temperature to 48 degrees and as the AOL guy says, "You've Got Rain!"

A cover for the tank is where I would start. Adding heat to the airstream without mitigating humidity input is a vicious cycle.

Heat Recovery

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