Steam before cooling coil?

[Birchwood]

We have a large Trane air handler that was built with the steam humidifiers discharging upstream from the cooling (chilled water) coil. This results in the coil removing much of the humidity that the steam adds. Is there any possible justification? We're in Texas. About 2% of the year, we *can* use this configuration to pull the heat out of the steam and heat the chill water for use in other air handlers.

 

[MintJulep]

Bizarre.

 

[trashcanman]

Agree with Mintj. Put the humidifier after the cooling coil.

 

[Birchwood]

I'll call that a consensus, then. What puzzles me is that Trane built it that way, they should know better. I wonder if the design engineer forced them... or if the pieces were just assembled wrong by the installer.

Thanks!

 

[flyrfan]

Sometimes design engineers use the cooling coil as a moisture eliminator for the upstream humidifier (I personally don't). Theoretically, if the water vapoer has been fully absorbed in the airstream and the coil temperature is above the air/water vapor's dewpoint, there shouldn't be any problem.

Does the system have an economizer cycle? If so the cooling coil control valve should be fully closed. You should lock out the CC CV during humidification.

If there is no economizer, then the humidifier should be installed downstream of the CC.

 

[lowedogg]

What you have is most likely a controls issue. In a hot, humid climate, you should never be simulataneously cooling and humidifying. Another possibility is that your humidifier is junk. When you witness it in operation, can you see the mist coming out of it? The steam should be almost invisible to the naked eye at the point of injection.

The humidifier upstream of the the cooling coil is a point of debate amongst engineers. This arrangement provides some operational safety factor to prevent your HVAC system from supplying fog into the space. The cooling coil acts as a moisture eliminator and you can also put a high limit humidity sensor downstream that will activate the cooling coil. This can be particularly usefule if you are dealing with a cold space (like an operating room).

 

[Birchwood]

Ok, since this issue can be situational, here's our situation:

This air handler serves only one zone, the sanctuary of a large church with a very expensive organ. The humidifers (and most of the controls configuration) exist to keep that organ close to 50% RH. People come second, but we try to make them comfortable, also. Some of those people are on a stage under 30+ spotlights. This means we have to have the moist air as cool as possible to deliver to the stage area and keep them from dissolving into puddles of sweat. Much of the seating area ends up being tempered back with electric reheats. The humidifiers typically only run from October to March when the outside humidity drops. The rest of the time, we regulate (primarily) the chill water temperature setpoint to control humidity, as well as chill water flow rate. We are able to use outside air very little, as it rarely has the right combination of temperature and humidity to be of much use. Cold air needs to almost completely saturated to end up at 50%RH when heated to 70 degrees F.

Most of the time, this zone is unoccupied or lightly occupied and we can let the temperature vary as long as the sub-zone that surrounds the organ (partially enclosed) remains within a fairly tight temp range (very tight during organ usage). This sub-zone has electric heat (reheats).

But during occupancy, even if it is quite cold outside, we'll be running the cooling coil 100% and the humidifier 100% and they will be canceling each other out in terms of both temp and humidity, to some degree. Fortunately, lots of people add a little humidity, instead of just needing to be cooled down (which reduces humidity).

The humidifier unit is installed directly upstream from the coil (26" of separation), so there is little chance for the steam to mix in with the airstream before the coil condensates most of the moisture back out.

The unit has two electric heat sections, but they are virtually never used. The steam provides more than enough heat at those rare times when they would normally have some benefit.

We do have a fair amount of mold on the supply fan cages in the exit end of the air handler, but not much more than the other air handlers that have no humidifiers.

There are times when we have to abandon humidity control for a brief period of time (when the sanctuary is full and everyone has a lit candle, eg.) and do a 100% outside air exchange. 42 degree F water through the coil just isn't enough. Getting too dry won't damage the organ's components if it doesn't last very long. Temperature is critical during performance, it affects tuning.

===
So, if removing excess liquid moisture is the purpose of putting the coil downstream from the humidifier, they should have put the humidifier farther upstream, to give the air more time to integrate the steam. And if we ran the coil just cold enough to remove excess water, we wouldn't get enough cooling during occupancy.

But, if they *had* put the coil upstream of the humidifier, then we *might* have a bigger moisture problem in the exit end of the handler and those ducts... Running the humidifiers at 100% with the coil manually shut off, might tell us if there would be a moisture problem....

Hmmm....

 

[lowedogg]

do you have a preheat coil? when it is cold out, you go to economizer mode and preheat to 55 deg F (or whatever your cooling suppy temp is) and then humidify. cooling coil should be off. a close look at the psychrometrics can determine what your setpoints should be.

check out Dri-steem ultrasorb steam injection tubes. they have the shortest absorption distances (8" in some cases)that i have found.

 

[Birchwood]

We have a preheat coil, but sure can't understand when I would ever want to want waste $ by heating outside air except by mixing it with the zone air. If I'm using outside air for cooling, I'm going to use just enough to reach temp setpoint in the zone.

Also, I've yet to encounter a time when our cold outside air wasn't so dry that we could maintain our min setting (49%RH) even with both humidifiers set to 100%.

So we run 42 degree chill water through the coil until that isn't enough, then we shut down the humidifiers so they don't heat the air and then as a last resort, we give up entirely on humidity control and add outside air.

This scenario only happens a handful of times each year, and then only for a few hours.

On the other several days a year, when we need some cooling and some extra moisture (more temperate days than above situation), the chill water cooling is sufficient and we're facing using steam to replace the condensation off the coil (instead of using steam to bring 40 degree / 35% RH outside air up to 50 degree / 50%RH air.

Maybe it doesn't matter where the coil is, maybe it will condensate the same amount of moisture, whether it's pre or post steam. I would think that it would be better to give the steam a longer mix time though.

Our humidifiers are Dri-Steem, not sure if we have those exact injection tubes or not.

I would like to replace the steam with ionizing humidity devices, preferably in the duct that feeds the subzone that feeds organ chamber. Then I could run the overall zone a little drier and the humification process wouldn't add heat to the process.

 

[ChasBean1]

It's very common to have the humidifier upstream of the cooling coil because you never use both at the same time. If it's cold outside (lower than 42°F) and you're using 42°F chilled water to try to dehumidify it's not going to produce a result. There's more that can be said about this but I'm limiting my response. Lowedog came closest in his first reply. Best regards, CB