By: Matthew Fickett AIA, LEED, CPHC | Principal
In Part 1 of this series, we discussed why 2 CFM/SF is the wrong metric for lab ventilation, and costs you twice as much as you need to spend. In Part 2, we showed that an efficient laboratory ventilation system doesn’t compromise your ability to provide for fume hood usage. In this part, we’ll discuss another big driver of lab ventilation: vivariums.
Ventilation in Vivariums
Recall that the commonly-used standard for lab ventilation is to replace the air once every ten minutes, or 6 Air Changes per Hour (6 ACH). It is generally agreed that vivariums benefit from greater air change rates. In general, 10-15 ACH is commonly used for small animal vivariums. Giving a range that wide may seem like I’m avoiding the question, but let’s look at an example.
The NIH requires 10 ACH for holding rooms (where the animals live) provided that the animals are in ventilated cages: that is, the cages are mounted on racks, like books on a shelf, and each cage gets its own individual feed for fresh air and exhaust air, from the rack. The rack works much like a fume hood, in that it pulls air from the room, uses it, and then exhausts it through a duct to the lab exhaust system. It also has very much the same goals: preventing air from the animals from getting into the room at large.
A very large cage rack (a double-sided one which holds nearly 200 cages) might use 150 CFM. Can we use the same method we did for fume hoods, and depend on that “background” 10 ACH ventilation to provide for the cage rack?
Although cage racks are kept in small holding rooms, the room is hardly ever just cage racks. There is a sink, a safety shower, a transfer station, and other equipment, plus some open space to move around in with bulky wheeled carts and racks. A 130 SF holding room might contain one double-sided rack as described above, plus a single-sided one, for a total draw of 150 + 75 = 225 CFM. That same room, if it has a 9’-0” ceiling, is 1170 cubic feet, which must be provided with 195 CFM of air to achieve 10 ACH. So, unfortunately we can’t just depend on background ventilation for this room!
To meet that 225 CFM draw, we have to provide the room with 11.5 air changes per hour (ACH). That’s not far off the mid-point of the 10-15 ACH range.
Big Picture Vivarium Planning
You might be saying to yourself, that’s all very well for one specific holding room, but all I know is that I want to provide for one whole floor to be a vivarium. How big does my air handler (and my shaft ductwork) need to be?
You can work through examples for other room types, and find values in the same range. There are some rooms (e.g. necropsy) where 15 ACH is required, and some rooms (e.g. the corridor) where nothing will push you above 10 ACH in any case. In general you’ll find that for rodent facilities, splitting the difference, and providing 13 ACH for the whole footprint of the vivarium, will be enough.
Coming back to our original terminology, 13 ACH in a 9’-0” ceiling space is 1.95 CFM/SF. So even high-level space like vivariums can be accommodated in less than the original 2 CFM/SF!
For normal lab space, even labs with a lot of fume hoods, 2 CFM/SF is more than twice as much air as you need.
For high-ventilation spaces like vivariums, which need an average of 13 ACH, 2 CFM/SF is still more than needed – but just by a little bit.
If you are planning a whole building, and expect that perhaps 10% of the lab will be a vivarium, you can size your equipment based on the average:
0.9 x 6 ACH + 0.1 x 13 ACH = 6.7 ACH
Or, if it’s easier in CFM/SF, a 9-foot ceiling would make that just plain 1.0 CFM/SF. So save money on capital costs and energy bills, check your ceiling heights, and cut that 2 CFM/SF in half.