what is maximum supply air in room without return?

[mikey0615]

Hello Members,

I'm designing office building and i have some offices with space limitation and i need to avoid provide return on every office.

My question is what is maximum supply CFM on a room without providing return? in another way what is maximum positive pressure allowed on a room and how i can calculate it?

Thank you

 

[irfansiddiqui]

1. provide supply cfms as per the load calculation and provide door louvers so that they can come out from them into other space where you can provide return for all of them,
2. Discuse with the architect and propose perforated ceiling then you will not require any return diffusers, or if it is gypsum ceiling then look at the borders to provide return

 

[mikey0615]

Thanks for your help

 

[dbill74]

How small are the offices and what is in the ceilings that you are not having enough space for RA grille?
I ask because I've never had an issue of not having enough space for SA and RA in an office.

Regular slot diffusers are an option for saving ceiling space. You don't see them often these days, but I've seen older buildings with light fixtures with integral slot diffusers on the sides.

Keep in mind also that grilles and diffusers do come in sizes other than 24"x24".

 

[willard3]

If you want to be able to balance this air system, you will supply both SA and RA through grilles to eqach office.

 

[RandomUserName]

For irfansiddiqui's suggestion no. 1: I would be careful of returning air through louvers into common corridors. I do not believe fire codes like that (Check IBC Chapter 10; 1018.5). In the perfect world egress corridors are positively pressurized to help remove smoke in a fire.

 

[Drazen]

maximum supply equals flow in room cracks for pressure exerted by fan. in theory you could make "crack curve" of the room, intersect it with fan/system curve and get "working point" of the room.

in practice that means nothing, such flows are so small that they do not belong to range of ventilation.

your fan is not compressor, you cannot "compress" the room, you can only very slighly increase room pressure until balance with cracks resistence is achieved.

in short, there is no ventilation without return. if you have no space for return ducting, you can always put transfer grill on some door toward some other room wich return than needs to pull out supply of both rooms. without that, it cannot work.

 

[mikey0615]

Thanks Drazen...

 

[cry22]

The allowable clearance for a one-hour fire rated door is 3/4" (Google it - or see NFPA-80)
Typical applications are for rooms adjacent to hallways, some rooms require smoke detectors and smoke dampers (storage rooms over 100 SF), by pressurizing the room, with air ex-filtrating through the door undercut, you save one smoke detector and smoke damper for each rooms. Hospitals have lots of soiled and clean utility rooms that fall in this category.

Normally, you can easily ex-filtrate as much as 75 CFM through such door without overpressurizing in the room. Small storage rooms do not need more than 75 CFM as they have no load.