CHECKING YOUR VENTILATION SYSTEM

By Michael McCann, Ph.D., C.I.H.

(Reprinted from Art Hazards News  Vol. 16 No. 4)



   In the last issue of Art Hazards News, I wrote an article,

"Industrial Ventilation for a New Art Building", which discussed

the types of ventilation, choosing a ventilation engineer,

planning your ventilation system, and important design factors. 

Much of this article is also applicable to individual artists

planning ventilation systems for their studios.

   Once a ventilation system is installed, or if you have an

already existing one, the crucial question is does it work?  If

the ventilation system was designed and installed by a

ventilation engineer, air flow measurements should be taken to

ensure that design specifications are met.  In addition, smoke

tube observations should be made to ensure that toxic

contaminants are being captured adequately.  This should also be

done periodically to ensure that the ventilation system is still

working properly.

   However, it does not take a trained engineer to do some simple

tests to see if your ventilation system is working.  There are

some common sense rules and techniques you can use to test the

effectiveness of your ventilation system.  



Local Exhaust Systems

   A local exhaust system, for example a spray booth or slot

hood, should capture the toxic contaminants before they get into

the air you breathe.

   If you can see dusts or mists in the air or settling on

surfaces, that is an indication that a local exhaust hood is not

working properly.  If you can smell gases or vapors, that is

another indication, although you cannot rely on your sense of

smell since many chemicals do not have detectible odors at

hazardous levels.

   You can use commercially available smoke tubes to generate a

visible smoke so you can actually see the air flow into an

exhaust hood.  A less expensive, but still valid way to check air

flow, is to use a child's soap bubble kit.  If the soap bubble

just slowly drifts to the floor, this indicates poor air flow. 

The soap bubbles (or smoke) should be steadily drawn into the

hood.  In particular, you should always make sure that the soap

bubbles or smoke are not drawn past your face when in working

position, since this could indicate you are being exposed to the

toxic contaminants being exhausted.

   If the soap bubble test indicates that the local exhaust

ventilation system is not working properly, then you have to try

and find the problem.  The following are some common types of

problems with local exhaust systems.

* Is there adequate makeup air?  One of the most common problems

with exhaust systems is insufficient makeup air to replace the

air being exhausted.  The resulting decrease in air pressure

affects the performance of the exhaust system.

* Is the makeup air source properly positioned?   If the makeup

air source is located too close to a local exhaust hood, it can

create turbulence and interfere with proper functioning of the

local exhaust system, and might even blow the contaminants into

your face.  Soap bubbles will show if there is turbulence.

* Are there cross-currents?  Local exhaust hoods are often very

sensitive to cross-currents caused by heavy traffic around the

hood, nearby air conditioners, other exhaust systems, etc.  Check

with soap bubbles to see if this is occurring.

* Is the process enclosed adequately?  The more enclosed a

process is, the more effective is the hood. 

* Are there obstructions or holes in the ducts?  With dust

collectors and woodworking machines, one common problem is that the

duct is not properly connected to the machine, allowing air to

enter the duct at the connection.  This can interfere with proper

functioning of an exhaust system.

* Do the ducts have a lot of bends and sharp elbows?  The more

elbows in a duct, the greater the loss of efficiency.  This is

similar to the effect on water flow of crimping a garden hose. *

Have the filters in spray booths been checked?  Clogged filters or

ripped filters will interfere with the operation of the spray

booth.  Installation of a manometer (obtainable from a spraybooth

manufacturer) across the filter can tell you when to change

filters.

* Are the fans connected up properly?  I have seen instances where

fans were connected in reverse, or even where the fan motor was not

connected to the power switch.  Connecting a centrifugal fan

incorrectly will cause a decrease in air capacity.

* Has the fan been checked?  Worn or slipping fan belts, burnt-out

fan motors, damaged electrical connections are all reasons why

ventilation systems stop working.

* Have there been additions to the ventilation system?  A common

problem with older ventilation systems is that additional hoods and

ductwork have been connected to an existing ventilation system

without determining whether a stronger fan is needed. * Is the

exhaust air being recirculated?  Recirculation of

exhaust air means that the toxic contaminants are also being

recirculated.  



Dilution Ventilation

   Dilution ventilation systems bring in clean, makeup air to mix

with and dilute the contaminants to a safe concentration, and then

exhaust the contaminants to the outside with a fan.     The

following are some problems that occur with dilution ventilation

systems.

* Is there adequate makeup air?  As with local exhaust systems,

there must be adequate makeup air to replace the air being

exhausted.

* Is the makeup air source positioned properly?  If the makeup air

source is located too close to the exhaust outlet of a

dilution ventilation system, the makeup air can short circuit

directly to the exhaust duct and not mix properly with the

contaminated air in the room. 

* Are the contaminants being drawn past your face?  It is

important to make sure that clean air enters the room, passes your

face, gets contaminated, and then is exhausted. Soap bubble

observations can tell you direction of air flow. 

* Are obstructions interfering with air flow?  A common problem

with ventilation for oil painting is that the vertical easels can

block the free flow of air.  Carefully position makeup air source

and exhaust fans to prevent interference.

* Is the fan connected properly?  Connecting propeller fans in the

wrong direct will cause air to blow into the room instead of being

exhausted.  This is easily detected. 

* Is the exhaust air being recirculated?  Recirculation of

exhaust air means that the toxic contaminants are also being

recirculated.  

   Using these simple rules, you should be able top see if your

ventilation systems are working properly.  For further

information on ventilation, see CSA's book Ventilation.

           

(c) copyright Center for Safety in the Arts 1993