Precision HVAC for Laboratory

[bjmccall2]

Dear Eng-Tips Community,

I run a 1200 sq. ft. laser laboratory (12' ceilings). We would like to maintain the air in our lab at ~70 F, with a relative humidity less than about 35%. We have one fume hood in the room, which exhausts about 400 cfm. The remainder of the exhausting is performed by a single large duct leading to a rooftop fan.

Our supply air comes in through diffusers on four separate ducts running in our ceiling. These ducts originate from the hallway outside our lab, where there are four mixing boxes that mix the building hot & cold air. The building cold air is outside air that has been cooled to ~56 F, and the hot air is re-heated building cold air. This system provides good temperature stability (despite our variable heat loads from the lab equipment), but the humidity varies a lot and gets very high when it rains outside.

A colleague recommended that I should re-engineer the system to use 75% recirculated air and only 25% fresh air, use one (or 4?) DX unit(s) to cool the air to 33 F, and then use SCR reheat for temperature control. I imagine I could install such units in place of the mixing boxes, and still utilize the existing ductwork. But I would have to find a way to draw room air (75%) into them.

Does this seem like the right approach? I would greatly appreciate any advice or suggestions you might be able to offer.

Thanks very much!!

Ben

 

[quark]

The building cold air is outside air that has been cooled to ~56 F, and the hot air is re-heated building cold air.

My brain is as bad as excel in handling Circular Reference Errors.

Is your present setup a once through system (100% fresh air)?

Do you have the data of your sensible and latent heat loads?

Are there any regulations for minimum quantity of fresh air?

what your friend suggested is, conceptually, OK. However, going to 33F with DX systems is a bit costlier. Desiccant dehumidifiers can be a better option if there are no flammable substances in the lab.

You would get better responses if you explain it in a better way.

 

[bjmccall2]

Dear Quark,

Thanks for your reply, and I'm sorry for the confusion (as you can tell I'm not a professional HVAC engineer!). To clarify, the building system takes outside air and cools it all down to ~56 F. A portion of that cooled air is then reheated, and the "cold" and "hot" air are mixed in 4 mixing boxes just outside our room to deliver temperature control. This is completely a once through system, and I believe that is why we have such problems with the humidity (and why it seems difficult to remove the necessary amount of humidity from 3200 cfm of fresh air).

I'm afraid I don't have much good data on our heat loads, which are mostly sensible but are quite variable. We tallied up all of the equipment in our lab, and if everything is on at once (probably this would never happen), it comes to 50 kW, or about 14 tons. The latent heat load is entirely due to the occupants (roughly 5 people) and, of course, the outside humid air streaming in.

As for the minimum amount of fresh air, I think we just need enough for our fume hood (~400 cfm) and perhaps a little to keep the room positive to keep dust out.

I've heard that desiccant dehumidifiers tend to have higher maintenance requirements, and have a larger form factor than DX units (we have somewhat limited space for the air handling units). Although cost is always a consideration, I'm really looking for the best solution rather to trying to save a few bucks.

If there's anything else I can clarify, please let me know...at this point, I don't even know the factors that might be important. I will certainly involve a professional engineer for the final design, but am looking for pointers in the right direction at this point.

Thanks again!

Ben

 

[williamsGA]

You are correct that desicaant dehumidifiers have higher maintenance requirements than DX units, however they will give you the best humdidity control. To get 33F leaving air temperature you are really talking about a refrigeration unit not a typical DX unit. Is there a reason you want such a lower relative humidity? At that low of a RH you will start to have static electricity and nose bleed problems.

 

[bjmccall2]

All I really know for sure is that our present humidity level is way too high. Our optics are degrading, our lasers become unstable, and we get excessive frosting on our equipment that is cooled by liquid nitrogen. I've been told by other laser lab operators that 35% RH is a good benchmark, but I was not aware that this would cause problems with static and nosebleeds! At what RH does that become a major problem? Perhaps I am aiming a bit too low?

Can you help me understand why desiccant dehumidifiers afford better humidity control that DX/refrigeration units? Also, what is the lower temperature limit for a DX system (you indicated that 33F is too low)?

Thanks!! - Ben

 

[RossABQ]

A typical laser lab spec is 35-40% RH, enough for static control, not so much that rusting is a problem.

 

[quark]

If the fresh air requiremnt is only for Fume Hood and occupants, then recirculation type system is OK. However, it is better to check the local codes once again.

If your lab condition is 73+/- and 35% RH then 45F DP (the economic lower limit for a DX system) is fair enough and this can be done with a DX system.

Your present setup seems to be cumbersome. Ideally, you should design to treat the air for the minimum DP required and then reheat the entire quantity to get the desired RH. The technique is to contol the cooling by a humidistat and then reheating by room thermostat. I used otherway also (cooling by return temperature controller and heating by a humidistat) and no problems, so far.

Assuming 89.3DB and 78.7WB (1% exceeding values of maximum wet bulb and mean coincidental dry bulb temperatures), to treat entire 3200cfm to the required room conditions of 73F and 35%RH, the load can be as high as 22TR. Your coil capacity seems to be lower.

If you go for a recirculation system, your load reduces drastically (more precisely, you are wasting less energy in terms of exhaust)

I have been operating desiccant dehumidifiers for 12 years and you shouldn't worry about them. The earlier models with silica filling cassettes used to give trouble but the quoted paper type wheels are good and you may have to change the cup seals once in a year. The drawback is that you can't use them with flammable chemicals.

Good luck,

 

[bjmccall2]

Thanks everyone for your help so far. To provide some additional information on the layout, I've attached a PPT file with a drawing (sorry for my poor drawing skills!) of the current system, and a drawing of my proposed system. Some of the unique challenges of this retrofit project are:

1) I cannot modify the building air handling system.

2) I wish to minimize the disruption inside the laser lab, so I'd like to utilize existing ductwork as much as possible.

3) There is limited space in the ceiling above the hallway where all of the mixing boxes are located. Where each mixing box is currently located, I have about a 4 foot cube of volume where a DX unit, reheat, and/or fan system could go. I think this must limit me severely in my choices?

4) It seems a recirculation system is most preferable, but I currently have no ductwork (or air handling) for return air; this will all have to be added. I may be able to wall off part of the hallway for return air ducting and/or a fan unit?

I humbly seek your advice on my proposed scheme. I will certainly involve a professional engineer in the final design work, but I am hoping to converge on a rough idea of what the solution will look like so I can seek funding for the design study and renovations.

Thanks again!!

Ben

 

[quark]

What are the ambient design conditions or which whether station is nearer to you?

 

[bjmccall2]

We are located in Urbana, Illinois...but since we don't (and can't) take any outside air directly, the maximum dewpoint of our supply air is ~56 F (assuming the building systems are functioning correctly). Thanks!

 

[quark]

Involving a professional engineer is the best idea. You can use the following pointers for the discussion and understanding the process.

First, get some idea of your room loads. You can countercheck the design from existing data. Check the DBT and WBT (or RH) of room incoming air and exhaust air (just at the room entry and room exhaust). Get the enthalpies from a psychrometric chart. The total load can be calculated by 4.5 x cfm x dT. Add fresh air heat load to this and then you will get your total coil capacity. I couldn't get the climate data of your place. However, Chicago's conditions are relatively worse and the enthalpy corresponding to 1% exceeding values of MCDB and WB is 39.6 btu/lb.

Fresh air rate should equal the direct exhaust rate (+the recommendation of local codes). Design the total flowrate with a TD of not more than 20F across the room. If latent heat is negligible, the DP can be obtained by plotting a straight line to saturation line from the room condition. You can also get actual DP by plotting the two room conditions and extending it to saturation curve.

Treat the entire quantity of air to a common DP. The off coil temperature can be controlled by a humidistat. Provide reheat (again to entire quantity of air) for the comfort condition. Take return from multiple points. Separate out the fume hood exhaust.

Good luck,