Lasik Procedure Room HVAC

[BronYrAur]

I have a 20'x20' room within an eye clinic that will be used for Lasik. The requirements given to me are to maintain the room at 68-72 deg F and 40-45% RH. These conditions are to be held "stable" with stability defined as temperature not deviating more than 1 deg F in a 10 min period and RH not deviating more than 2% in the same 10 min period.

I am going next week to look at the room. I will take a general survey of the materials of construction etc. This room is just another space inside a large clinic. Right now it is served from an AHU that serves the whole floor.

I'm thinking that I need to divorce this room from the central AHU and put in a dedicated unit. I have hot and chilled water available. Any recommendations on manufacturers or strategies to achieve the required stability? Anything specific that I need to investigate while I'm at the site? Location is near Baltimore, MD.

Thanks for your help.

 

[DrRTU]

A separate system is a must. I have not designed an "Lasik" room but 40-45% during summer operation is going to be the hardest number to maintain. Most DX equipment will be pushed to maintain 47- 48% with a normal amount of OA. The problem gets tricky If the room has a decent sensible gain from equipment, then the equipment cycles off and your RH increases. Accuracy of RH sensors will also be an issue - 2% of 42.5% is on .85% swing in 10 min. The better quality RH sensors have only 1% accuracy. Chilled water on a deep row coil will help, but you maybe looking at dedicated system to address RH via a desiccant system. I have completed other presion temperature & humidity rooms via DX but I used a terminal reheat design. Overcool the air all the time and reheat the air to the correct temperature via SCR re-heat coil. The room must be considered a process so you don't get trapped by energy codes.

 

[Zesti]

Maybe this can be of interest:

http://www.surgicube.com

 

[Zesti]

Maybe you can still use the air from the central AHU to supply the necessary OA-ventilation to the room.

Maybe there is already a humidification system on the central AHU.

I would ask if they really need the RH to be between 40% and 45%. It might just be that this is their minimum requirement and that levels of up to 65% are acceptable.

Then, depending on the specs of the central AHU, this might lower the requirements on any circulating system you are going to design for this room.
Chances are there is already humidification on the central AHU since this is a medical clinic of some sort.

Also ask about filtration requirements.

In general: be sure you know what requirements REALLY are necessary because often clients write whatever they want concerning RH and T without knowing what consequenses this has on system-design and energy consumption. Raising or lowering RH limits by 5% can make a huge difference.

 

[BronYrAur]

I will have access to the space today and tomorrow. Any special things that I should definitely investigate? I think I'm safe to say that I will not be able to hit 40%RH with chilled water, agree? Since the adjacent spaces don't have such tight requirements, is construction of the room (walls, vapor barrier, etc.) important? Remember, this is just another room in a hospital setting (long corridor with rooms on both sides). Should I consider conditioning the corridor so that there is no difference between the room and hallway? Easier said than done, but I'll check it out. Perhaps I could even build an ante room.

I'm heading out now, but will be in tonight to check out any suggestions you have. I still have tomorrow to go back to the room.

Thanks!!

 

[mauricestoker]

BronYaur,

I'm working on a similar project. The sensible and latent loads are very low in this case, about 4.4 W/SF sensible and 800 Btu latent. Very first thing is get the exact criteria. Project I'm working on uses VA criteria, 4 ACH total, 1 ACH OA, 30-50% RH, 78/72* F DB. First thing needed is replacement of the existing differential enthalpy sensor on the economizer.

If you're using ASRAE 170, criteria changes dramatically. If using another source, who knows?

The real difficulty I'm expecting is when weather is like it has been recently, below 55 * F and raining. It's on a dual duct system that covers the entire floor, so when recent weather occurs, the OA is preheated up to 55* F on the cold deck. I'm thinking either small dessicant system or small DX unit along with reheat. Last similar job I did I intended up using a roof mounted 3 TR unit along with SCR reheat. Central chilled water plant delivers at about 49* F, so not much dehumidifying is goting to go on with the house loop. I'd definitely check on CHW supply temp, regardless of what temp it leaves the chiller. If you have to maintain postive, then I'd recommend looking at the boxes in adjoining areas. If monitoring is required, I'd also recommend looking inot what controls are used/compatible.

 

[Zesti]

Google "lasik room requirements humidity"

and I found this:

http://www.news-medical.net/news/2004/04/20/691.aspx#

Seems RH is an important factor in LASIK.
It doesn't say which RH is best though...

The article talks about winter and summer conditions which suggests that a lot of these treatments are done in rooms with no RH-control at all...

 

[sridhar1312]

you can use fabric duct of EURO air for maintaining almost same temperature gradient through out the hall.
As it is for eye Lasik the fabric duct adds as an additional filtration.

 

[AbbyNormal]

I would assume you have a high air change requirement for filtration, meaning a lot more air than what you would normally need to maintain temperature.

Now the %RH is going to be sort of problematic to control off of as you will get the 2% change perhaps easily just if the air temperature changes by 1F

I would do something like the following and it would involve working off of dew points.

With a lot of outside air in the summer, once you start cooling the mixed air down to around the dewpoint you are trying to maintain, the air off the coil approaches fog.

I would make the chilled water coil give me 45F air in the summer then I would use reheat to keep the room temperature tight.

I did something similar with a custom chilled water air handler with a built in DX dehimidifier unit. 3 coils in series- chilled water coil selected to get the air down to 52F, then dx coil to get air down to 49, then DX condensing coil that heated air up to about 65F (with the help of 3 degrees of fan heat)

I did not have the tight restrictions like you do, generally same temperature range and 50 to 60% RH.

I actually found it easier to hold 37 to 40% because you hit the brick wall of a coil freezing (got to love hot gas bypass) than it was to keep it in the 50 to 60% range.

Now on this job I had to deal with some civil servants who were acting in the project manager role as well as the controls contractor role, and let's say neither of them could read or comprehend a sequence of operation and they were both pyschrometrically challenged.

So I had to make due with the controls they figured were needed and this forced me to work off of RH percentages rather than the dew points I preferred.

I found the percentage RH very fickle in this situation

I have been year round cooling these last 11 years but for winter I would say humidify the air to get a 45F supply dewpoint

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

 

[AbbyNormal]

I have also seen Lieberts hold computer rooms to your conditions.

I did a major telephone switch that Nortel originally claimed had to be a class 100000 clean room. The client decided that they could not afford it to be quite so clean, but none the less, I had some DX 100% outside air units still pressurizing the space with dry 65F air and I had some recirculating DX units controlling the space temperature.

System had a natural affinity for 70F and low 40% range RH. If it fell below 40% I just disengaged one compressor stage on the DOAS, and RH rose right back up.DOAS units had 4 stages on a main coil, then a small stage on a secondary downstream evap then inline condenser rehaet coil. As you can tell I love hot gas reheat.

Summer time I get ambient dewpoints in the 77 to 84F range, that averages 78 from June to end of November, year round average condition here is about 82F db and a 74.5 dew point.

Anyways it is easy for me to cool mixed air down and assume it will be a fog

Again, I find the brick wall of a DX coil freezing (forcing on hot gas bypass) makes it easier to drive RH down to 40% or slightly lower.

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

 

[AbbyNormal]

anyways the two projects I am talking about used Engineered Air equipment, they have a Chicago Office

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