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FIRE SERVICE AND GENERAL FIRE SAFETY TOPICS => Fire Safety => Topic started by: GB on July 11, 2007, 01:48:47 PM

Title: Air changes per hour
Post by: GB on July 11, 2007, 01:48:47 PM

I have a common escape which I have to ventilate within Domestic Apartments using mechanical extraction (inlet air being mechanically put in). How many air changes per hour do I use?

Title: Air changes per hour
Post by: wee brian on July 12, 2007, 10:32:13 PM

This isnt a simple thing - if you have to ask the question, then you arent competent to do it.

dont take it personnaly - I'm not competent to design a system like this.

Title: Air changes per hour
Post by: GB on July 15, 2007, 08:27:30 PM

I have a skin like a rhino - don't take too much personally (unless you start slagging my football teams close season purchases!!)

However (as there is always a however) I intended to use 10 air changes per hour similar to ADB section 18.13 within the simulation I have modelled. The system will be designed by others (who have the competence - hopefully). I have modelled a medium growth t-squared fire within a sprinklered compartment adjoining the corridor. The corridor is tenable without the air changes but want to compare natural ventilation utilising AOV's ADB section 2.26 and 2.27.

I wanted to guage the view of others to see if anyone disagreed and why.

Title: Air changes per hour
Post by: wee brian on July 17, 2007, 10:04:40 AM

10 air changes would be a bad idea - far too simplistic.

The ventilation specified in ADB 2006 for flats is intended to prevent smoke entering the stairway.

It does this by creating a pressure difference between the corridor and the stairway.

If you bung an extract fan in the coridor you would probably achive this but it will draw smoke from the flat on fire into the corridor - this is a bad thing. Unless you are using state of the art CFD you cant model this properly.

Some systems use a variable rate fan to reduce the risk of this happening but its all too complicated and will probably never work in a real incident.

Why are you messing about with fans anyway?

Title: Air changes per hour
Post by: GB on July 17, 2007, 12:46:07 PM

The flats are fully sprinklered as is the common escape.

The common escape is single direction but over 7.5m to protected stair. The CE is divided in that we have a section where there is no means of natural ventilation.

The CFD is FDS which while being a pain to set up is pretty good.

Title: Air changes per hour
Post by: wee brian on July 17, 2007, 10:35:15 PM

So the vent is to cover an extended travel distance - by how much?

How tall is the building, are sprinks required anyway or is this a trade off?

Title: Air changes per hour
Post by: GB on July 19, 2007, 04:06:40 PM

TD is 21m single direction within 4 storey domestic apartment block. Corridor split into three sections.

Inner hallways within each apartment. Sprinklers are trade off.

Must say Wee Brian, fascinating post you put on technical advice section in similar vain to what we have here regarding AOV's in stairs. Has inspired me to do a bit of reading which is quite feat!!

Title: Air changes per hour
Post by: wee brian on July 19, 2007, 07:41:45 PM

I get out as much as I put in, that's what is good about this forum.

I'd be tempted just to let it go just with the sprinks (maybe with an uprated supply for at least 30 mins on two heads)

I'm never sure how well these fans will actually work 10 or 20 years down the line.

Title: Air changes per hour
Post by: Advanced Smoke Technology on July 25, 2007, 04:29:10 PM

Why do you need mechanical extract, have you dicounted natural ventilation?

Title: Air changes per hour
Post by: wee brian on July 25, 2007, 10:04:27 PM

Natural is usually better as it self regulates.