Airflow balance in hospital 1

[seanhkim]

In a hospital floor I'm evaluating on, there are 25 patient rooms. Two of them are negative pressure rooms, and other are general patient rooms. According to the guide, negative pressure rooms have -0.0037"W. To make the negative pressure, return is set to 400cfm and supply is set to 200cfm. For other patient rooms, supply is set to 300 cfm and return is set to 200cfm to make a positive pressure. Meanwhile the corridor which connects all the patients rooms has a balance pressure having 1000 cfm for supply and 1000 cfm for return.
Here is my question. Then
Sum of supply in all patient rooms : 23*300 + 2*200 = 7300 cfm.
Sum of return in all patient rooms : 23*200 + 2*400 = 5400 cfm.
Since air volume in the corridor is balanced, the difference cfm (7300-5400 = 1900cfm) will go to the corridor.
Where does this air go toward? To the adjacent spaces through infiltration?

 

[quark]

The first exercise you have to do is to plot leakage from all the rooms on the layout drawing. This indicates you the flow pattern and total quantity of air leakage. I am not knowldegeable as to how you arrived at the respective supply and return figures. If they are based on intuition (seems most likely, IMHO)you may face problems.

For example, a DP of 0.0037" wg indicates a leakage area of not less than 1.13 sq.ft. This leakage is most unlikely to happen through closed doors. Further, 0.0037" wg is about 0.1mm or 1 Pa and I don't think you can acheive good containment, not to mention how you read this pressure and control it.

The method I follow is as below.

First calculate the leakage across each room (you can use this approximate equation Q = 2910xAxdp^1/2, where Q is in cfm, A is leak area in sq.ft and dp is in inches wg). Plot the leakage on a layout indicating inward leakage if it is a negative pressure room and outward leakage if it is a positive pressure room. Calculate the supply air flowrate based upon no. of ACPH required or any suitable method. The return from the room is supply rate+leakage for a negative pressure room and supply rate-leakage for a positive pressure room.

Once you complete with the individual rooms then add up supply and return flowrates of all the rooms under one AHU. For a recirculating system, if the supply rate is less than return rate then you have to bleed air from the return duct (this is the case with predominantly negative pressure control spaces). If the supply rate is more than the return rate then you have to take fresh air into the system.

Suppose, I consider a door size of 1.5mx2.1m from every room into the corridor and 1.5mx2.1m from corridor to the external area, then I get about 45cfm leakage from each room to the corridor. If corridor is kept at ambient pressure then, theoretically, there is no leakage from corridor to outside area. Your two negative pressure rooms have 200cfm supply and 250 cfm return. The positive pressure rooms have 300cfm supply and 250cfm return (the 50cfm leaks into the corridor). So, total leakage into the corridor is 23*50-2*50 = 1050 cfm. If the supply air flowrate into corridor is 1000 cfm then return will be 2050 cfm. Now the AHU supply is 23*300+2*200+1*1000 = 8300 cfm (assuming all areas are covered under one AHU) and the return will be 23*250+2*250+1*2050 = 8300 cfm. The flow is totally balanced as all rooms are being covered with one AHU.

If your corridor is on a separate AHU then, the room AHU supply will be 7300 cfm and return will be 6250 cfm. You have to provide fresh air equivalent to (7300-6250) = 1050 cfm. Correspondingly, you have to exhaust 1050 cfm from the corridor AHU return duct.

You should refer your local codes for AHU zoning, fresh air requirement and no. of ACPH.

Good luck,

 

[mechanicaldude]

Quark,

I was reading your reply and found it very interesting and informative, but I have one question about the equation you used. In the equation you listed as Q = 2910xAxdp1/2 shouldn't the constant be 2610 and not 2910. I'm not sure if its a typo, or if for some reason you are using a different constant.

thanks.

 

[mechanicaldude]

I have one more question regarding your calculation. IO'm not sure where how you arrived at 45 cfm leakage? Are you assuming the leak area is via the door uncut ?

 

[quark]

That depends upon the redundancy we consider interms of coefficient of flow. For intentional openings, the flowrate is equal to the pressure drop happening across the opening (i.e the difference between the room pressures) and can be calculated as 4005xAxdp^1/2. 2910/4005 gives you 0.73 and can be a good flow coefficient, where as 2610/4005 = 0.65 which is a bit too tight. In any case, that depends upon the door tightness and type of door.

For 45cfm, the underlying assumptions are 5mm gap in the bottom and 1 mm on top and sides. The door size is 1.2mx2.1m.

Hope, I tried to clarify the things.

For a strict leakage calculations, you can refer to ASHRAE handbooks or Contam (a free package) and both these deal with real life coefficients derived upon fan pressure tests.