Dilution And Local Exhaust
In the majority of ventilation systems comprising of local exhaust, the push aspect of the push - pull ventilation concept is extremely important. When you use the general ventilation in a room to provide the push aspect of the push - pull system, the general ventilation also acts to dilute contaminates which may escape into the room’s environment.
This may happen due to
contaminates escaping from the reach of the capture effect of the local exhaust or may
occur due to contaminates escaping into the room’s environment, a distance away from
the local exhaust hood. Dilution may be required in systems where a great deal of
evaporation is expected from liquids with which workers are working.
(See NRC Publication's #29,
#31 and #84)
To determine the correct volume flow rate for dilution to occur (Qd), it is first necessary to have a good estimate of the evaporation rate (qd) at the temperature which the liquid is being utilized. To arrive at these estimates, the following procedure may be utilized:
EQ (1). q(d) = 387 x lbs./(MW x Em x d)
where:
q(d) = Evaporation rate in cfm
387 = The volume in cubic feet formed by the evaporation of one lb-mole of a substance (e.g. a solvent)
MW = Molecular weight of emitted material
lbs. = Pounds of material evaporated
Em = Time of evaporation
d = Density correction factor
The appropriate dilution volume flow rate for toxic vapors is found as follows:
EQ (2) Q(d) = {q(d) x K(m)} x 106/C(a)
Where:
Q(d) = Volume flow rate of air, in cfm
q(d) = Evaporation rate, in cfm
K(m) = Mixing factor to account for poor mixing
C(a) =Acceptable airborne concentration of the material (OSHA indicates that this is typically half of the PEL)
With dilution as one of the contamination control concepts you are trying to accomplish you should also evaluate the following:
1. Position all local exhaust intakes very close to the emission source.
2. To improve dilution and mixing, use supplemental fans within the space to mix the air.
3. Make sure that the workers are up wind of the dilution zone and that contaminates will not penetrate the workers breathing zone undiluted.
In many installations it is a requirement that despite having local exhaust and general ventilation for dilution, the type of contamination which is being controlled necessitates the use of an airlock concept. The airlock is designed to assure that there is a separation between those areas that may contain contamination and those which must be protected against any possible contamination migration when there is movement from one area to another.
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