switch gear room pressurization & infiltration

[123131415]

I am new to HVAC industry and assigned task to design air conditioning system for 33 KV substation wherein switch gear room has to be maintained positive pressure of 30Pa. Equipment heat gain is of 250kw and room to be maintained less than equal to 25C deg and RH of 75% max.my question is how much of air is required to maintained 30pa. is this pressure required to avoid the containments entering into the switchgear room if so than how it will be calculated. thank you

 

[LittleInch]

All depends on how well you can seal the room.

This will be the fresh air intake fan, not the HUGE volume of air you're going to need to circulate to cool 250kW. Are you sure for a switchroom?

what's in there? A bunch of VFD units?

It's been a long time since I was part of something like this, but there are standards on how much air leaks out of doors, cable glands, windows etc. Then add at least 50%.

You usually need to have an airlock type door arrangement to stand any chance of actually achieving this.

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

 

[Snickster]

Attached is a calculation of a previous project I worked on based on ASHRAE that is based on door cracks and permeablility of building walls, etc.

 

[nuuvox000]

Sizing a fan for pressurization is one of the harder calculations to perform so I'm sorry they assigned this to you. It's hard because you have to rely on the tightness of the room but you don't have a ton of control over it with your scope.

I would ask whoever is designing and installing the room to make it as tight as they possibly can. Then you can provide a supply fan and relief damper; even a manual relief damper should work fine.
I am also a little blown away by the cooling load of that equipment. Double check that but maybe your company does way bigger projects than I've ever worked on. With that much cooling and flowrate, you'll want to think about the location of your supply outlets and return inlets. Place return inlets away from doors/windows/walls if possible. You don't want a negative pressure next to a door.

You can get some rule of thumb numbers for room tightness (air leakage area) from 2015 ASHRAE HVAC Applications Manual, pg 53.7, table 2.
Then it looks like you can use Equation 11 from the same page of the manual for the CFM equation.

V = 776*C*A*(sqrt((2*deltaP)/rho))
V = volumetric flow in CFM
C = flow coefficient (see pg 53.5)
A = flow area (leakage area), square feet
deltaP = pressure difference, inches of water
rho = gas density in path, lb/cubic ft

Hope that helps.

 

[123131415]

thank you all of you for support help and time.

 

[MedicineEng]

nuuvox made a real good point that is often forgotten/ignored.
The location of the supply and return diffusers is really important to achieve a positive pressure in the entire room and keep contaminants away.
As a rule of thumb, ideally you would want the supply diffusers close to the door while the return/relief damper towards the end of the room.

 

[LittleInch]

When it comes to the FAT of the room, you'll find the HVAC engineer wondering around with a smoke wand looking for leaks with a roll of duct tape in the other hand....

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

 

[BronYrAur]

@snickster. Is your sheet available as a workable spreadsheet?