Discussion of Careful Lab Exhaust Design

One of the most challenging HVAC designs involve labs.  They typically require 100 % outside air, special room pressurization, tight temperature and humidification control, special filtration and eventually proper exhaust.  All of these have to work in harmony for good lab performance. 


The last item, proper exhaust, can be tricky to design.  This applies not only to labs but also to any exhaust duct conveying and discharging air properly outside the building.  The key points to remember,


1.       Proper duct velocity.  If you are conveying certain particles, higher velocities are needed.  If so, don’t make the mistake of saying the pressure drop is 1.5 times the duct pressure drop.  At high velocities this may not be the case.  Use computer duct design software or careful spreadsheet calculations to be sure. 

2.       Carefully calculating the correct fan size (many fans motors are too small).  Again, calculating fitting pressure drops and velocity pressures are very important.

3.       Consider condensation in the design.  If the ductwork is running outside the building then putting drain tees in may be necessary along with tightly sealed ductwork systems.  Ducts can also be insulated but if the duct pressures are fairly low, drain tee or dip legs like used in steam systems can work and save a lot on insulation costs. When dealing corrosive exhausts special duct material and condensate disposal needs to be planned.

4.       Proper make up air.  Don’t assume proper makeup air will come from some where else.  Make sure it does.  This is especially true in industrial applications.

5.       For room pressure control, use fast acting air valves.  If you have critical labs, safe operation only occurs with good control valves.  Know every detail about how they work and how they interface with your control system.  These can make or break your system.  In critical application consider direct wire safeties verses software safety control.  Software glitches can occur where direct wire to safeties can be more reliable.

6.       Additionally, use CFM difference not room differential pressure sensors for proper pressure control.

7.       Careful air balancing is critical.  Keep fan system operating at the lowest possible static pressure to prevent fan surging.

8.       Fan surging can occur when fans operate in unstable regions of their fan curve.  This can cause early fan bearing failures.

9.       Fan surging can also cause pulsing CFM reading.  If you are controlling your labs on CFM difference (which you should) a pulsating exhaust fan will not be able to hold the CFM steady.  This will make it very difficult to hold correct room pressures.

10.   The quick fix for pulsing CFM’s is to dumbed-down your control loop so it operates very slowly.  Unfortunately, if you need that control system to react fast, a dumbed-down system will not do that so if you have to do an emergency fan switch that can be difficult to do. A better solution is to fix the fan surging problem by re-balancing and reevaluating air flow and pressure requirements.

11.   Make sure everyone on your design team knows exactly what equipment is going to be in your exhaust system.  This seems obvious but when you start using engineered stack nozzles it is really important that your environmental engineer understands what that is and how it performs.  Don’t assume they will understand it right away because they can be very good environmental engineers but they don’t always understand HVAC system design.

12.   Many times, the design engineer forgets to add the discharge velocity into the fan calculations.  Most exhaust designs require 3000 fpm exhaust air.  That equates to 0.5 inches of static.  Some may say well that’s not a big number.  Well that depends.  So let’s say you have 6000 CFM @ 1.0 inches of duct pressure drop and a 55 % efficient fan.  That’s 1.75 BHP.  So you install a 2 HP motor, starter, run the wiring from the basement to the roof.  Building your discharge cone to get 3000 fpm and turn the switch on.  Guess what happens.  You don’t get 6000 CFM out of the system because your actual static pressure 1.5 inches.  6000 CFM at 1.5 inches = 2.57 BHP.  You should have a 3 HP motor on the roof.  Well, I would not want to have to go back to the owner and say that you have to run new wire and conduit up to the roof because you need a three HP motor.  Now what happens in real life is the cone gets chopped off so that the velocity drops below 3000 fpm.  This can put your roof workers at risk or you can get lab exhaust air going  back into your air intakes.