Ventilation - Intermittent Occupancy

[Engineer6512]

I have a client installing a ventilation system in a movie theatre. The capacity is 400 people and from ASHRAE standard 62-1999 requires 15cfm/person equalling 6000cfm total.

My question is regarding the intermittent occupancy considerations (section 6.1.3.4 of the standard). This specifies that for peak occupancies occur for 3 hours or less, one half of the maximum ventilation (3000 cfm in this case) is permitted. What it doesn't specify however, is the period that the space must be unoccupied for this to be valid. In a movie theatre setting, yes there is intermittent occupancy, but sometimes probably for only 15-20 minutes at a time before the next group comes in.

Anyone have any experience dealing with a similar situation?

Thanks.

 

[darkvibe]

I believe ASHRAE says to use the average occupancy of the space but not less than 1/2 of the ventilation required for the space.

Also, what state are you in? ASHRAE 62 has a bunch of addednda that you might be required to use. One change is the way you figure out required ventilation air.

 

[quark]

Some past threads in this forum recommend demand ventilation by CO₂ sensors for similar applications. Have you thought of it?

 

[Engineer6512]

Actually I am in Canada (NS). Yes, I re-read the standard and it must have been the wording that was confusing me. You are correct.... an average occupancy is determined and then the cfm required is calculated based on that average occupancy. I assumed an average film is 90 minutes with 20 mins between shows. This works out to be 82% occupancy:

82% * 400 = 328 people on average
328 ppl * 15cfm/person = 4920 cfm required

I will also be using CO2 on demand sensors, as it will be rare the theatre will be at full capacity.

 

[darkvibe]

Look at ASHRAE 62 addendum n for new ventilation requirements.