Too Much of a Good Thing: Exhaust Versus Supply in Laboratories

In my 47 years of research support, one of the most common complaints I hear is the need for more exhaust. It usually is not phrased that way. Instead, organizations say they need more hoods, more snorkels, more local exhausts, more ventilated enclosures – in effect, more exhaust for numerous needs. Research facility managers and support groups end up trying to do everything they can to eke out any additional capacity from an HVAC system that is usually already straining to meet the current demand. Variable air systems not only save money and energy but allow more exhaust to be available as it used only when it is needed. Efforts are made to ensure personnel keep hood doors closed when not in use to free up more capacity on average. Fans and ductwork are modified to try and squeeze out a bit more capacity. And, finally, additional exhaust fans are often added where they can be economically (or not so economically) provided.

Yet in all this effort, few look at the other side of the equation: the supply system capacity.

In a well-designed laboratory HVAC system, the supply and the exhaust are usually essentially the same capacity. The supply typically runs at a slightly lower routine capacity then the exhaust to keep the laboratory negative as required by code. ?This offset also, providentially, allows a facility to add some more exhaust over time (by whatever means) since there is some extra supply available. All too often, however, research facilities keep adding and adding exhaust until sooner or later the exhaust greatly exceeds the supply system’s capacity.

That isn’t a problem is often the position taken by those who really do not understand laboratory HVAC systems. If a laboratory is supposed to be negative by code (negative meaning the exhaust is more than the supply to keep anything from escaping the laboratory into a non-laboratory area) then more negative should be even better – or at least not a problem. ?The truth is that when the exhaust capacity greatly exceeds the supply numerous problems can arise.

·????????Hood no longer perform as well. Face velocities drop as the hood becomes starved for supply air to pull into the hood. Hence capture effectiveness drops as the laboratories are supply air starved. The hoods struggle to pull in enough air from outside the laboratory and become less and less efficient and effective. Often less knowledgeable sites try to address this issue by adding even more exhaust only making the overall problem worse.

·????????Temperature uniformity becomes much more problematic. Some areas are too hot, others too cold. Standard temperature control systems struggle to try and address this issue. Temperature and humidity variations become common and often significantly adversely impact tests which require uniform ambient conditions.

·????????Doors become harder to open or close (depending on their direction of swing). In extreme cases smaller personnel struggle to operate them without straining. Some doors will not close as the automatic door closers are too weak to overcome the air flow.

·????????Laboratories can become positive when one with a larger exhaust need draws supply air from a lower exhaust need location. This problem is difficult to address easily and can create hazards in non-laboratory spaces as the laboratory that is drawn upon becomes, effectively, positive.

In many cases, facilities are unpleasantly surprised when a new exhaust addition fails to produce as much actual exhaust inside the laboratory as planned. The laboratory is so supply air starved that the additional exhaust is so inefficient as to produce only a marginal increase in capacity while aggravating the other problems noted above.

How does one avoid this issue? Easily by simply recognizing that every increase in exhaust requires a corresponding increase in supply. Hence, projects to add more exhaust require adding more supply. As this effectively more than doubles the cost (since supply is more expensive than exhaust due to the need to heat and cool the incoming air) many organizations routinely chose to “roll the dice” installing only the exhaust and hoping it will work satisfactorily without additional supply. Miss leadingly, it often does the first time or two since, as discussed above, most well designed systems have some initial excess exhaust. So, the first small exhaust increases may be able to be accommodated, leading the organization to believe this is a continued viable path. This leads to costly and unpleasant surprises later.

In several cases I have been called in to troubleshoot, simply adding more supply has made the existing (larger) exhaust be so much more effective that more exhaust is not required.

So ,recognize that, playing with an old engineering truism, if you want more to go out you need to let more come in!

For further information and training on laboratory design, issues, and solutions you may want to consider Engineering Career Solutions course Successful Laboratory Design: Grass Roots, Renovation, or Relocation to be given virtually ?November 2-4, 2022. For more information and registration, please see https://ecstechtraining.com/lab-design-1 .