Humidifier before or after cooling coil?

[Takket3]

Curious if anyone has an opinions on placing a humidifier before or after a cooling coil?

We have a process where we need to deliver 55 grains (+/-5). The AHU is 100% outdoor air.

So in the cold/dry winter it is no mystery. We heat the outdoor air and humidify.

On hot/moist summer days, again no mystery. We drive the cooling coil to 55 deg F which is more or less 55 grains. No problem.

But what about hot and dry days where, after cooling my outdoor air to 55 deg F, i'm still short on humidity? Should I pre-humidify my outdoor air to 55 grains, then pass it across the cooling coil, or should I cool it to 55 deg F first, and then humidify afterwards up to 55 grains? Is one method better than the other for control/energy usage or life expectancy of my equipment?

 

[berkshire]

If you humidify before the cooling coil, that coil can strip out humidity you just put in. Re humidify after the cooling coil if needed.
B.E.

You are judged not by what you know, but by what you can do.

 

[IRstuff]

"But what about hot and dry days where, after cooling my outdoor air to 55 deg F, i'm still short on humidity?"

Part of the reason you are short on humidity is the cooling coils condense water out of the air. Cooling coils are typically below the dew point of outside air.

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[berkshire]

Ir stuff,
Isn't that what I just said?
B.E.

You are judged not by what you know, but by what you can do.

 

[itsmoked]

Yes! But he used more words..

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Takket3]

Let me clarify......... Suppose I'm out in the desert bringing in 100% outdoor air. 95 degrees, 5% RH

I want 55 grains in my supply to my space.

Should I humidify to 55 grains before I hit the cooling coil (assuming the coil is set to a temperature so as to NO dehumidify the air below 55 grains), or should I cool the air, and then humidify to 55 grains after the coil? (Again, assuming I'm smart enough to not set the cooling coil so low that when I humidify, i make it rain in my ductwork)

The cooling coil is set to 50 F discharge air which is 55 grains (more or less) at saturation. The problem i see is if I try to humidify AFTER the cooling coil, if i'm not RIGHT on set point or my humidity sensor is off even a few %, I can add too much water to the air and it will rain in the ductwork. (Possible solution, when humidifying is needed based on outside air conditions, reset cooling coil setpoint to 55 so the air can hold more water)

But on the other hand if I humidify before the coil i risk over humidifying and having the coil condense it out. i'll hit setpoint but all the water condescending out is just money down the drain. I know the obvious solution is to have the humidifier far enough upstream that i can have a humidistat before the cooling coil i can use to modulate the humidifer, but unfortunately my existing conditions don't allow me that luxury.

 

[LittleInch]

I really think you're going to have trouble whichever way because as you say, at your 50F air temp, 55 grains = 100% RH, so whether you add water before and risk the excess coming out in the coil or add it after, you're in danger of condensation / rain and or mist coming out of the vents.

So either mix some warmer air in before the humidifier or have a separate humidifier just using the space air or change your 55 grains requirement.

Getting water drop out / condensation / mist coming out the vents is not a good idea.

I don't know what the normal limits for RH are coming out of an AHU going into a duct, but I would have thought you don't want more than 90%?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[berkshire]

Takket3
You are trying to pick a point on the chart where you are right at the condensation point, you need to see if you can get a few degrees away from that, such as setting your coil to 55 degrees. From the numbers you are quoting it sounds like you are running a chiller.
B.E.

You are judged not by what you know, but by what you can do.

 

[Takket3]

I agree berkshire. Have a DDC system so should be able to do this with programming.

 

[MedicineEng]

It is always dangerous to run a coil in the condensation point of your air.
Remember that the modulating valve has hysteresis and there will be always a dead band, so if your requirement is so strict you need to go for precise air conditioning or something like a CRAC.

 

[simangunsong]

Interesting thread,,
I like to read more..

 

[shane21]

"Let me clarify......... Suppose I'm out in the desert bringing in 100% outdoor air. 95 degrees, 5% RH"

Hot dry incoming, and you want to cool & raise humidity of it, I'd be looking at evaporative cooling as a first stage
and see how close that gets you, odds are AC will have much easier job then of tweaking it the rest of the way.

 

[ChasBean1]

Trouble with humidifiers is that if you put them too close to end of the AHU they set off duct smoke detectors.

You find yourself in a quandary as to where to put them.

Don’t think about grains of moisture but rather desired conditions. During the humidification season (winter), you might want to maintain a space of 70°F and 30% RH. That desired winter dew point is 37°F.

Now, if your chilled water is warmer than 37°F (which it usually is; 42-45°F is the norm), there will be no condensation on the chilled water coil while you humidify during winter to the desired condition.

During summer, you DEhumidify to some condition that does not need to involve the humidifier (the humidifier only matters when the OA dew point is low). This is likely a dew point of about 56-59°F, which means your cooling coil temperature is about 55°F.

Because of the above and due to AHU geometry, the humidifier often best goes BEFORE the cooling coil. That’s my preferred configuration, although there are other ways to skin a cat and the more I’ve done this, the more I’ve become open to all ideas..