Dehumidification with cooling coils only

[drfloyd]

Hi all,

I am currently employed as an intern at a lyophilization facility. The facility is broken into zones including an office space, mechanical space and clean rooms for lyophilization. The relative humidity requirement for the clean rooms is 40% or lower to discourage the growth of mold. However, in summer months the HVAC system has been unable to maintain below about 60%. Our chillers are set to put 42 degree chilled water through the cooling coils in our makeup air unit and 2 air handler units.

The majority of the moisture is removed by the cooling coil inside the make up air unit. Once this air has entered the air handlers, the cooling action inside of those air handlers removes relatively (compared to the make up coil) little moisture from the air because it is 90% recycled from the building. I believe our humidity problems are due (at least partially) to the cooling coil in the make up unit being completely saturated with condensed moisture, making it unable to remove as much moisture as it could potentially do at that temperature, as well as possibly dispersing water droplets into the air.

My question is, is there any way that we can achieve 40% RH in the rooms using only cooling coils? Would it be practical to simply add another cooling coil within the makeup unit to extract more moisture? A dehumidification skid would be extremely difficult to fit into our mechanical space.

If that won't work, we thought about possibly adding a refrigerant coil (with compressors and all that fun stuff) as an alternative to an entire dehumidifier unit.

I apologize if my question is unclear. I would appreciate any help I can get on this!

 

[imok2]

You need to determine if in fact your pulling in only 10% OSA also what is the normal summer OSA temp and R.H. Can you tell us what the mixed air temp and w.b. on is for the cooling coils and the same for the air off the cooling coil
Maybe all you need is OSA adjustment. Did this system ever perform before?

 

[AbbyNormal]

Investigate reheating the air

The way we build has a far greater impact on our comfort, energy consumption and IAQ, than any HVAC system we install

 

[walz]

Use treated fresh air units ,,,, which is a chilled water cooling coil with heat pipe combination... i think thats the best option, you can easily acheive 40% RH

 

[EnergyProfessional]

you should look into some desiccant systems. Like an ERV with desiccant wheel (i.e. SEMCO Pinnacle systems). with that you can dehumidify air lower, also can dehumidify recycled air.

your approach to mainly treat fresh air is good, but you also have infiltration and internal water gains (humans, spilled water etc.).

you could look on a psychromatic chart and see how cold your air needed to be to give you 40% at space temperature. You also should investigate raising space temperature (since air is dry it wil be comfortable enough).

If you barely get below 60% you may have infiltration problems (or live in humid climate, or both). Achieving 40% will be difficult before you fix that.

 

[willard3]

What is the dry bulb temp that goes with the 40% rh. If you have low dry bulb, getting the right rh will be more of a problem.

 

[drfloyd]

@imok2:
Unfortunately we have limited data with which to determine the percent makeup air. The systems were designed for 10% fresh air vs. 90% recirculated air, but the exact percentage varies depending on the flow rates we set in the air handlers and mua. Based on the flow rate of the mua inlet into the air handlers, I think 10% is reasonable.

@willard3
The outside air is (currently) 76% RH and 82.3 degrees fahrenheit. Wet bulb and dry bulb temperatures are not given. Our system is able to maintain the correct RH in the winter when the outside air is cooler and dryer.

@abbeynormal
Could you be more specific? We have reheat coils before all the rooms. We have tried reducing the temperature setpoint within the air handlers so that the air temperature fluctuates more in the hope that this would wring out more moisture (this did not work).

@walz and HerrKaLeun
After a preliminary google it looks like these machines are still fairly large. Again, I am looking for a solution that preserves our ductwork and uses little to no floorspace. It might be impossible but I am just trying to confirm that.

 

[drfloyd]

thanks for the responses guys! I really appreciate the help.

 

[AbbyNormal]

if it is a clean room you have a high volume of air probably 20 air changes an hour for filtration purposes. This volume of air is probably a lot more than you would normally need for space cooling.

You have to have the supply air with a lower dew point than what you need to maintain in the room for your 40% RH.

So it could be possible that your system will have to cool the air down low enough to get the proper dew point but then it becomes too cold that it wants to drive the space temperature down low. You would use reheat to warm up the supply air a bit so that your clean room does not get over cooled.

A desicant system would be one way around using reheat.



The way we build has a far greater impact on our comfort, energy consumption and IAQ, than any HVAC system we install

 

[EnergyProfessional]

the fact that the ERV is large shouldn't effect your ductwork at all. You still supply the same amount of air. the ERV can be in the mechanical room or on the roof... wherever you air enters.

since you have a clean room, there may be a problem with cross-contamination (some % of return air will mix in the supply), but that isn't a problem as long as you keep one ERV for each ventialtion zone (i.e. don't use the same system for your bio lab and your chemical lab at the same time)

 

[mauricestoker]

I'd recommend only considering the air going to the dry freezing area, as office space and mechanical space would probably not have the stringent RH requirements. A reheat in the supply to the dry freezing area, followed by a cooling coil, much the same as done for OR's, would not take up any floor space.

 

[drfloyd]

@mauricestoker

That sounds like an easy solution. In fact many of our clean room supply units already have an additional cooling coil and reheat coil installed (though they are normally unused).

My understanding of this subject is very limited, so I would appreciate anyone here educating me should they spot ignorance.

We have tried further cooling the air in the air handlers and then reheating it at the rooms, which had almost no impact. Does the amount of cooling correspond to greater removal of moisture? Considering that all cooling coils are running at the same chilled water setpoint (approximately 42 degrees fahrenheit) is it feasible for us to remove additional moisture in this way?
As I understand it, reheating the air before cooling it should not make much difference as the dewpoint will be the same. Unless the moisture isn't fully condensing at our initial cooling coils I don't understand how cooling the air again at the same chilled water temp could help.
So my challenge right now (I think) is figure out the level of humidity that our cooling should achieve, and then compare it to what we are actually getting.
I've been trying to look up the percent humidity that corresponds to our level of cooling, with little success.

 

[AbbyNormal]

work out the dewpoint needed in that room where you want the 40% RH.

An example at sea level then could be maintaining 75F @ 40% RH. This is a dew point of 49F. The air off of the cooling coil will need a dew point lower than 49 by the amount that the internal latent load calls for.

Perhaps you would need a 48F supply dew point. So you would determine, "Can my cooling coil give me air with a dew point of 48F, using chilled water supply of 42F?" If the answer is yes, then most likely the dry bulb and wet bulb are going to be down at 49F as well, so this is where you need the reheat.

If the answer is no, then repeat the process and see if 40F water can get your supply dew point down below 49F (to whatever your calcualtions say you need)

If 40F Chilled water will not do it, maybe start thinking about supplemental dehumidification - mechanical or dessicant

If you are reheating and cannot control the humidity, the dew point of the supply air is too high

The way we build has a far greater impact on our comfort, energy consumption and IAQ, than any HVAC system we install

 

[AbbyNormal]

To control temperature in a space, you have to supply air to the space cooler than what you want to maintain the room at.

Likewsie to control humidity in a space, you have to supply air to the space with a dew point lower than the dew point that you wish to maintain in the space.



The way we build has a far greater impact on our comfort, energy consumption and IAQ, than any HVAC system we install

 

[AbbyNormal]

I use chilled water air handlers with a built in DX dehumidifacation and condenser coil for reheat from time to time.

Night clubs, surgical suites

The way we build has a far greater impact on our comfort, energy consumption and IAQ, than any HVAC system we install

 

[mauricestoker]

I would guess the reheat and cooling coils were probably included for that specific purpose, and their not being used may be the source of your problem. For a brief but detailed explanation and examples, I'd recommend looking at ASHRAE Handbook of Fundamentals, chapter 6.

 

[SAK9]

It appears that the high RH is due to insufficient dehumidification in summer.With a DX coil after the CHW coil in the make up air unit the lowest you could possibly go to is around 38F.Anything below that will be at the risk of coil frosting.You would need to check on the charts if this is enough to achieve 40%.Having done a few pharma jobs in the past I have not seen any installation achieving 40% RH with DX coils.A chemical dehumidifier seems to be the way to go.

 

[Zesti]

Let's go back to this sentence from the original question:


"The relative humidity requirement for the clean rooms is 40% or lower to discourage the growth of mold."


If this is indeed a clean room with HEPA-filtered air entering the room, where would the mold spores come from?
The HEPA-filters should be pretty effective at removing the spores (although mold spores can vary quite a bit in size over the different types of mold).

Having a RH lower than 50% or so will inhibit growth of mold. But if there are no spores to begin with, what is the use of actually lowering the RH to 40%?

What I mean is: Is there actually mold growth in the clean room when it is at 60% RH ???

 

[mauricestoker]

Maybe the issue is that for freeze drying, it's not desirable to have humidity as it defeats the process.

 

[BronYrAur]

drfloyd,

There has been a lot of info thrown at you here. A couple of key points: You cooling coils will only remove moisture down to the dew point and the air leaving the coil will be nearly saturated (i.e. 100% RH). Some of the air bypasses the coil surface, so even with a 42 deg chilled water temp, your discharge air might be only 55 degrees, again saturated. I don't see that you have posted the discharge air temp from the air handler or the desired room temp. Do you know them?

But if the air from the unit is near 55 deg, you will never get 40% RH unless you let the room temperature get very high (85 deg or so). This is obviously unacceptable. Reheating the air will only help if you have excess cooling. In other words, if you can drive the air down low enough and have enough cooling to drop the dew point, then you could reheat the air to make the space comfortable. But, how low do you need to go??? Probably down near 45 deg, which you probably can't get out of the coil. Room temp as I mentioned earlier plays a big part in the %RH. You want to keep the room temp as HIGH as possible without causing comfort or process issues in the room. The higher the temp, the lower the %RH.

I hope I'm not adding to the confusion, but one other thing. You mentioned that you only have 10% outside air. You might want to increase that amount to help pressurize the room and keep out the infiltration. Of course, when you do this, you are taxing your coil even more and probably increasing the %RH.

Bottom line, I wouldn't try to achieve 40% with a chilled water system if I were starting from scratch. I would definitely use a desiccant system, but they are large and expensive. Since you are trying to make an existing system work, I would drop the temperature as low as possible in the air handler and then re-heat it up (after the cooling coil) to as high as possible without causing issues in the room. You still won't hit 40%, but you should be able to beat 60%.