Question about HVAC Design for Laboratory Environments (Confused Intern, please help me)

[wsatchell]

Hi all,

This is my first time posting on this forum; however, through research for my project I've noticed that there is a lot of knowledge on HVAC coming from this website so I hope someone can help me with my problem. Basically, my boss wants me to design models that can predict flowrates between rooms (assuming doors with some cracks or leakage) given the input supply and return rates to each room and then using the minimum pressure drops between adjacent rooms figure out what the leakage areas should be. We can then use these models to check lab environments that are not performing exceptionally well to figure out what should be fixed.

I've been using the equation Q = 2610*A*dP^.5 to calculate the minimum flowrates across each room but I am having trouble figuring out the actual flowrates given the input supply and return values. I've been trying to apply control volumes on each room but the system of equations that results is never unique and depending on the grid size has more unknowns then equations.

My question is, how should I go about solving this and is it even possible to analyitically calculate the flowrates? If there is a better way to attack this problem I would love to hear it. I've been working on a 2x3 grid of rooms that have doors to each adjacent room and walls surroudnign the outside and I've tried to figure out the flow rates for (using Engineering Equation Solver) but am having no luck. Any help would be greatly appreciated!! I'm trying not screw up my first project as an intern.

Thanks

 

[MintJulep]

For any room

Supply + Leakage in = Exhaust + Leakage out

For any two rooms that have a leakage path between them

Leakage out (1-->2) = Leakage in (2-->1)

If you know (or assume) a dP for your leakage flow equation then you should have a unique solution for A between each pair of rooms.

 

[RossABQ]

Personal opinion, I've done rooms like that, and no amount of calculating turns out to be accurate. The flows depend completely on construction quality -- sealants at the base of walls, door seals, etc. A good T&B guy can set pressures right, provided the designer has given him enough supply and exhaust to work with.

I had to estimate leakage rates for one lab in particular that was in the middle of a row of negative pressure labs. This one needed to be less negative for various reasons (just barely negative). I used ASHRAE values for "reasonably tight" construction, which was supported by construction methods and by personal observations of installation quality. Come T&B time, the leakage rates were found to be double that. Fortunately, I had enough fat in my systems to accommodate the increased leakage.

 

[Zesti]

My personal rule of thumb is maximum one air change per hour for leakage when a room is at the correct pressure. If there is more leakage, then construction of the room needs to be checked - cracks filled, door seals adjusted etc.

If rooms with over-pressure leak too much to their surroundings then this will also lead to problems in other areas because you'll have to extract even more air from those areas.

 

[urgross]

The 1 ACH has worked well for me, or 100 CFM/door. As a lab, it will be set at, say 10 ACH on the exhaust. Once the room ACH are made with exhaust, the supply damper is moved back until you have 0.05 IWG. If 1 ACH or 100 CFM/door doesn't make it, I look for obvious leaks.