HVAC system for an eye surgery room 1

[johncooling]

Hi to all,
I am new to the forum, but reading it for quite a while.
I was lucky enough to get a project for HVAC of an operating room for eye surgery, so I am writing to get some advice if possible
It is rather on the small side with ceiling (no false ceiling) at 2.75m, length 5.7m and width 3.7m (so H=9ft L=18ft 8in and W=12ft 2in).
There are 6 people in the room so the latent load=450W and the sensible load=450W.
There are 3 pieces of equipment (microscope, FAKO equipment and anesthesia machine-which is used sporadically) with sensible gain 500W.
Lights 250W.
Calculated cooling load is about 2400W=8190 Btu/hr (21 degrees Celsius internal temp=70F).
I cannot mount ceiling diffusers because of the ceiling height and already mounted lights (and the doctors do not want them over their head), so I am planning on putting the diffusers with HEPA filters like on the picture attached. I would use an 100% OA AHU with dx-coil and reheater which will be positioned on the terrace which is 1m above ground so the device can be serviced from the backyard. The ducts I am planning to use are preinsulated PU.

Weather data:
Altitude: 87m
(august)
Outdoor dry bulb: 32 degrees Celsius
Outdoor wet bulb: 23 degrees Celsius

I am concerned that the surgical team is too close to the outlets, so maybe I will turn the diffusers (with HEPA filters) downwards or straight out? Also, to find the right flow and supply air temperature to meet the requirements (12ACH minimum if 100%OA used=850m3/hr), but not to blow too cold or high velocity air onto the surgical team. Thinking of it to be in range from 800-1600m3/h variable with four 0.6mx0.6 diffusers with H14 HEPA filters.
Am I planning this right? Any ideas or comments are welcome.
Regards,
Johnny

 

[immsk]

I am concerned that you wouldn't get adequate mixing in the room. what if you ran ducts along the sides and installed slot diffusers, something like thisLink

Also your return is at the bottom, we typically install return grilles higher in the room, as fresh cold air is more dense and hangs around at the bottom, while the warmer less dense air rises up high. So if you return is up high, you won't be exhausting fresh air you just brought in.

Lastly I don't have first hand experience with it but I've read about it here and there over the past few years, so take this with a grain of salt. Look into displacement ventilation: The concept relies on introducing cold air at a low level within the room, the cold air displaces the warmer room air. The warm air is exhausted from the room at a high level

 

[johncooling]

Immsk, thank you for the fast reply!
According to the standard I must use high positioned supply and, at least two low positioned returns because contaminated particles must travel downwards toward the low returns.
Regarding the air mixing, I did not write about that, but- it would be a plus if the anesthesia team got a bit warmer air (common complain that is too cold for them, because they move much less).
Thank you for the slot diffuser idea, maybe I could put it instead of those 4 diffusers. Then, I would have to place a HEPA filter in front of it, is there a max air velocity value that a HEPA can take?
I will read about displacement flow, thx again, but I must use the high positioned supply.
Do you know maybe, what would be a good value that I can blow the air from the supply, that the occupants wouldn’t be bothered with at about 1,5m (surgical team from the wall)?

 

[immsk]

Hi Johncooling, regarding the filter - you can install it in the ductwork upstream of the diffusers, so that takes care of that.

Regarding the air velocity - you are looking for the throw/drop from the diffusers.
I don't have the answer but I bet there is some useful info here especially in the first few pages that will be helpful here

Another guide on EH price's website that may be useful
Link

 

[johncooling]

Thx for the readings, so that is the problem, the height, so the laminar flow diffusers can not be mounted. There just isn't enough space, but to mount the supply onto that wall. I found HEPAs can take about 0.5m/s. I will try using Comsole.

 

[goutam_freelance]

Looking at the air distribution path there is a possibility of relatively stagnant air above the outlets which may have more contaminant concentration. It may be worthwhile to install smaller capacity outlets above present outlets. Comsol is good for CFD but FLUENT is more well known. It will be interesting if you simulate contaminant concentration also.

Engineers, think what we have done to the environment !

 

[BronYrAur]

I know this is unrelated to your question, but what are you doing about humidity in this room? It is my understanding that humidity is important for eye surgery procedures. I can't be more quantitative than that. Do you have provisions to humidify/dehumidify?

 

[johncooling]

Thank you colleges for your input!
@goutam_freelance I will try Fluent and will consider the high positioned outlets (at about 25% of return airflow). I will put the simulation results here.
@BronYrAur I understand your question and to answer it- the doctors want the RH to be in the range 30-60%RH I found laser eye surgery needs to be spot on 50%RH, but it is not laser eye surgery in this case. Planning to use dx-coil cooling and plate recuperator with re-heater (humidifier in the winter). The humidity and temperature gets high here in July/August, so the psychrometric chart looks something like this:

 

[johncooling]

Fluent:

 

[nuuvox000]

I believe air is supposed to come from the ceiling and blow down towards the floor. Also, spend some money and get HVAC Design Manual for Hospitals and Clinics from ASHRAE. This isn't something you want to mess up because it could be expensive to fix a mistake.

 

[johncooling]

I have the manual, I am aware of the laminar diffusers that have to be mounted over the operating area. But the problem is the concrete ceiling height of 2.75m (9 foot), which is too low for an op room. Also the doctors don't want them over their heads and the lights are already mounted on the ceiling.

 

[nuuvox000]

If they are straying from industry standards, make sure they are aware (in email) and then you can do what they want. Maybe see if they need to have the room certified by any group to make sure you're covered. Owners will ask for things to be custom but when inspectors come they will forget what they told you and point the finger at you.

 

[goutam_freelance]

I believe there are two options with outlets at upper or lower levels. It appears that 2nd option gives more uniform temperature. I suggest you can play with direction of inlet air flow and outlet location to arrive at optimum temperature distribution.

Engineers, think what we have done to the environment !

https://www.linkedin.com/in/goutam-das-59743b30/

 

[dbill74]

The laminar flow diffuser above the patient is a requirement for a reason, it is to protect the patient from germs and disease from the doctors. If space limits are preventing this install, then the surgery room needs to be relocated to where you can do laminar diffusers above the patient.

Ask the doctors if they want to face a lawsuit because a patient got an infected eye and dies because one of them had a cold.