“We have built a variety of labs with vivaria in them in the last six years and we’ve tried three different models for ventilating them. We are currently developing a fourth on yet another model to see what works best for different applications,” says Clifford Roberts, director of the Laboratory Animal Resource Center for UCSF.

Approximately 90 percent of the University’s new vivarium space is designed to house rodent populations ranging in size from 5,000 cage units to 40,000 cage units.

“Racks for mice are ventilated in a variety of ways so it is important to understand which racking system you are going to use before the engineers design the HVAC system. This is not how most universities work, but you need to make that decision early in the process” says Roberts.

According to Roberts, hood or canopy type cages require more exhaust air than supply air, while directly ventilated cages require finer airflow control.

“These can be extremely complex systems. It is not good to start out with engineers who have never done this before,” he says.

Rack-Mounted Supply and Exhaust

A cage rack-mounted supply and exhaust model has been found to offer the greatest flexibility and is the simplest to incorporate in existing facilities. In this design there are pre-filter and HEPA supply and exhaust blowers on each rack unit. Filtered room air is blown into the cage from the room and exhausted through a custom-fabricated thimble connection.

The principle disadvantage with this design, in addition to carrying an initially higher cost, is that it may not remove animal odors from the room. The need to regularly change the numerous HEPA filters also makes it inapplicable for use in large facilities.

“We had one facility designed to house approximately 400 racks and it was pretty obvious from the outset that we didn’t want to use this configuration,” says Roberts.

Another disadvantage of the rack mount design is that the room and rack are the same temperature.

“Mice have a thermo-neutral zone and a preferred temperature of approximately 75 degrees Fahrenheit. However, the workers, because they are wearing PPE, would prefer it to be more like 67 degrees. We are looking for better solutions to keep the mice in the thermo-neutral zone,” says Roberts.

Ceiling height can be an issue with this design. Roberts recommends having at least nine feet of ceiling height to accommodate the top-mounted units.

“These units can also tell you what the airflow is from moment to moment, which is more difficult with a central system,” says Roberts.

Rack Supply Fan in Interstitial Space

A variation on the rack-mounted supply and exhaust model is placing the air supply fan in the interstitial space. The primary advantages of this design include lower initial costs, better temperature control, and less maintenance.

“A simpler solution is to use the same fan and the same HEPA filter to ventilate all the racks. If you take the air out of the room, then it has had a chance to mix and balance so it is a relatively even temperature,” says Roberts.

According to Roberts, the extensive amount of ductwork required for this model dictates having an interstitial space with a maintenance walkway.

“The manifold requirements make it almost impossible to do this with an ordinary amount of ceiling space,” he says.

Central Direct Supply and Exhaust

The design most utilized by UCSF involves a centralized direct air supply and exhaust system that is housed in the interstitial space.

“It’s a fairly complicated system, but it gives us the most control over both the animal environment and the worker environment. It can be a difficult system to balance properly.
We have found that it has to be balanced with the racks in place using metered cages and metered exhaust; balancing it with rack simulators is not really effective,” says Roberts.

In addition to being able to run the room and the racks at different temperatures, there is also less maintenance and cost involved because there are no additional fans and everything is centrally lodged in the interstitial space.

One of the disadvantages is that, because the supply air is coming directly from the central system into the cages, there is no temperature buffer.

“If a reheat coil or a valve fails, or if it permits wide variations in temperature, you are in trouble. You need a level of control that is plus or minus two degrees Fahrenheit and that is not necessarily a simple thing to do,” says Roberts.

Another challenge with the central design is that it only uses HEPA filters at the main air handlers, which can create problems if the filters fail or if the filter racks leak because they are not engineered properly. Centralized air systems also require a considerable amount of well-designed interstitial space.

Rack-Mounted Supply, Direct Exhaust

A variation on the centralized system is a rack-mounted supply combined with a direct connection to the building exhaust. This design requires one fewer air control valves per room and has the distinct advantage of eliminating odors—a significant issue with rodent facilities. The direct exhaust connection also removes the bulk of allergens from the room.

This model is applicable to existing facilities and is less expensive to implement than having both supply and exhaust mounted on the rack, though ceiling height is still an issue. Other disadvantages include pre-filters that require regular changing by animal room staff and numerous HEPA filters, which must also be changed.

“It is possible to adjust the racks to be positive or negative to the room, but it is important to have a visual gauge available to achieve the necessary degree of balance,” says Roberts.

And the Winner Is

When it comes down to quantifying which ventilation system is best and why, Roberts says it depends on the size and intended function of the facility.

“We have found that no one system fits all. The idea is to balance cost with function.
The central system has penned out to be the best solution for our largest facility of approximately 40,000 cages.

In a 10,000 cage facility, we are using a supply blower on the rack and a direct exhaust connection to the building, and it looks like the best solution for that size facility,” says Roberts.

UCSF did not implement the design of ventilating multiple racks with one blower, which would have cut costs but also reduced the flexibility of rack placement.

The University’s latest iteration, a 16,000-cage cancer research institute currently under construction, will utilize a central system. The design will forgo individual rack temperature control and blow centrally controlled air into each rack.

“This design will spare the associated reheats, valves, and controls for those throughout the facility, and our value engineering indicates that this will provide a highly efficient solution,” says Roberts.

Ceiling height is a significant planning factor when dealing with rack mounted solutions.
When it comes to designs that involve interstitial space, Roberts emphasizes that it is important to put all of the moving parts that need maintenance along the walkway so workers can access them easily.

“One of the buildings that we finished had some issues with the air valves and all 300 of the mechanisms had to be replaced after the building started operation. Because of the interstitial space design we were able to do that and continue to operate the building,” says Roberts.

By Johnathon Allen

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