Positive/overpressure with exhaust/intake fanss

[stickerpen]

Hello all,

just started as HVAC engineer, with 10 years as process equipment mechanical engineer, my company could not find any hvac specialists in a short urgent time.
I have a process room with clarifier unit, where I have to create overpressure with intake and exhaust fans working together.

How is that really possible? Calculations suggest I use intake fan use higher flowrate and exhaust fan with lower flowrate. Mathematically simple, but air is a compressible fluid, and no matter how bigger is intake fan, exhaust fan will keep sucking more and more of compressed air.

I need help from experienced HVAC engineers! Please!

 

[berkshire]

So you have an exhaust system with makeup air. The process you are looking for is called ramping. You are right , as the room pressure rises, the flow will increase on the exhaust.
This article may help:

https://w3.usa.siemens.com/building...ments/room-pressurization-control-methods.pdf

You are judged not by what you know, but by what you can do.

 

[LittleInch]

How is it possible?

Well you balance flows in and out to create the required over pressure.

The big unknown normally is room air leakage and often turns into a hunt the leak when you're commissioning it such that you can actually achieve your required overpressure.

Is there a reason you need exhaust fans? Often the overpressure created will vent via an exhaust duct which restricts flow with a damper controlled to maintain the overpressure.

If you have big losses in your exhaust ducting then you start with a positive pressure into the exhaust fan which needs to be taken into consideration when sizing the exhaust fan.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[SAK9]

Do not use an exhaust fan. Provide a supply air fan and a relief vent to the outside with a volume control damper. Manually adjust the volume damper to achieve the required positive pressure in the room. Your relief air flow balance results may look like :80% exhaust through the relief vent and 20% exhaust through door/ceiling/penetrations leaks.

If you have specific exhaust requirements from certain locations such as over a vessel or low& high level exhaust this approach will not work. You will require a dedicated exhaust system to meet those requirements

 

[ChasBean1]

You’re thinking PV = mRT (ideal gas law). No… you want cfm supply vs. cfm exhaust. Use the approximation: Q = 2610 A (dP)^.5.

Q = cfm (sorry, not LPS or m3/hr); I think in US units
A = leakage area in ft2
dP = the pressure you want in the space, inches w.c.

Let’s say you have a total leakage area of 1 ft2 and you want a pressure of 0.02” w.c. Crank the math and you get 370 cfm. So if I were to design a space with a +0.02” pressure requirement, and I anticipate a total of 1 ft2 of leakage, I might have 2,000 cfm supply and 1,600 cfm exhaust/return. 400 cfm difference. Close enough to 370 cfm.

 

[jspath2339]

What is your pressure requirement, relative to various spaces? Are there multiple entrances, is this on an exterior?

Now of the options that exist for maintaining positive pressure, the two most common are:
-DP (differential pressure sensors) which try to affect the pressure by modulating a fan. Typically the way this would work would be that one system would be constant (supply or exhaust) and the other would modulate to maintain the pressure. It's a simple system, but it's predicated on DP sensors. These are prone to issues.
-Tracking airflow. Now I did not see anyone recommend this approach. If precise airflow/pressurization is required, Venturi Air Valves (Phoenix valves are the Post-It/Kleenex version of venturi air valves). These are extremely accurate valves that are used specifically for ensuring a particular airflow. They are rampantly used through critical spaces such as bio-labs & health care spaces. They are expensive, but using tracking airflow between the exhaust and supply will be the most assured way of ensuring pressurization.
-Relief vent: This will work, but is not ideal. Would never be used if you needed negative pressurization but may work with positive. Things to be cautious of: backflow if the supply fan isn't running & what type of vent it is. If it's a downblast gravity ventilator, little issue. If it's a sidewall-pressure relief damper, these can be closed by the wind, and should not be relied upon for pressure relief.

One of the options, you may be presented with is just providing constant supply & constant exhaust that the supply is 10% greater than the exhaust. This may work during commissioning but will not guarantee anything after. I would feel better with variable airflow for both.

As far as the air being a compressible fluid, remember that the fan is small and the room is big. Air is compressible, but we aren't using a compressor. As such, air will not compress easily in a large room.