Positive Air Pressure Calculations in a Room

I can't see any point in trying to calculate such a thing. (But I am a cat)
Measure it*, if you are building a clean room facility then a few 'order of magnitude calculations' may be possible, but I expect you'd want to design in redundancy and overcapacity anyway.
I'll bet the final set up is adjusted empirically probably by throttling the outgoing air.
If you insist on trying to calculate it you would need the performance data for the blower and the leakage and volume of the room.
Del

* A U tube manometer with one limb either side of a window in the room would be simplest.

Positive pressure in operation theaters, ICU and burnt patient ward are very critical for avoiding infections. Ashrae - application-chapter 7 page 8 health care facilities on table 3 specify the pressure requirements.

As a thump rule every 100 CFM of air difference between supply air and exhaust gives 10 pascals pressure inside the room. For example, in an operation theater, if supply air is 2000 CFM and exhaust is 1800 CFM, inside will have 20 pascal positive pressure. Care should be taken to monitor inside pressure as the exhaust used to be a direct drive fan whereas the supply fan used to be belt driven. In case of belt slackness and dirty air filter supply air to room will be reduced whereas exhaust will remain constant thereby chances of reduced inside pressure or negative pressure. The exhaust fan should be electrically interlocked with supply fan in such a way that exhaust starts after some time lag of supply fan started to avoid initial negative pressure inside.

you just saved my life....hats off boss!

Sounds to me like energygod deserves a GA vote!

I will do the customary thing and hit the 'Rate' button in his post #3, and then vote "Good Answer".

Why are you dividing 0.25"W.C./360"W.C.?

That is to determine the RATIO of the desired over-pressurization compared to "standard" ambient (sea level) air pressure- which using "American" terms is about 30 feet water pressure (or 30 x 12 inches per foot = 360 inches).

The 0.25" water column (WC) pressurization requires (0.25 / 360) of excess air, or 0.006944 times the equilibrium value. This is based on gas laws relating pressure to air volumes (which is a direct ratio). 1" of excess pressure would require (1 / 360) times base volume as excess air supply.

Another way to calculate it is to compare the required NET pressure vs. base pressure, or (360.25 / 360) times the equilibrium supply air volume for total required supply air BUT the original way gives you a SPECIFIC VALUE for excess air which you can easily "bump" to provide a safety factor potential (which is why you use a VFD on the supply fan sized for the safety factor amount added to the equilibrium amount- allowing the fan to supply as much excess air as is needed to meet the required over-pressure setting.

Hope this helps. If not, send in a another reply to this note.

30 feet being an approximation of 14.7psi x2.31=33.957 feet?

That is correct- I used a short cut with my discussion.

Obviously, you should use the pressure for your elevation as the basis.

Shouldn't we be taking leakage into account using Q (CFM) = 2610 x A (ft2) x (DP)^0.5 ? Also, i would know how to calculate this area for say a door crack but what would you assume for leakage through the construction?

Start by calculating the CFM of outside air required by your local code, based on occupancy and usage of the space: clean room, laboratory, operating room, etc. Design the exhaust system to handle that same quantity of air. Theoretically this will give you zero space pressure difference referenced to the adjacent spaces. Equip the exhaust fan with a variable frequency drive (VFD) and slow the fan speed to arrive at the pressurization desired...usually 0.05 to 0.10 will do the trick. Employing the services of a professional balancing firm is recommended followed by a thorough commissioning process to assure that the design parameters have been met. That good start should be followed by a competently designed and executed, on-going maintenance program. The occupants of this space can now concentrate solely on carrying out the mission for which the space was designed and built.

determine the air leakages through the door and crevices etc and then decide the air changes required and always exhaust slightly less than the supply air to ensure pressurization is maintained.

Methodolgy is available in the book "design of clean rooms".

Doing anything is being silly. You have to learn the losses in order to proceed.

Presuming that you know how much of air you want to circulate in the room and how much positive pressure you need, you have only two known data.

Your variables are the air " leakages through the door and crevices" which can be roughly calculated however doors opening and closing which cannot be controlled.

Using a variable speed fan as indicated by " Lou Bindner" can be a solution if you do not require a precise positive pressure because frequent door openings will unbalance the system.(There is also the efficiency graphic to be considered)

An automatic damper controlled by a pressure gauge as I have earlier indicated in # 4 will solve your problem. It has been working in a hospital in my home town since 25 years.

Forget about leakage etc. Have a Pressure difference control set with 20 pascals set pressure for conditioned room and interface it with your BMS with audio and video annunciation so that you will be alerted the moment the inside pressure drops below the minimum set pressure to take remedial action

The actual measurement of pressure difference can take place after the design installation.If the design does not take care of the leakages and other requirements you may have to start from the begining.