Encyclopedia of Environmental Science and Engineering, Volume I and II

516 INDUSTRIAL HYGIENE ENGINEERING

the previous section can be employed but its effectiveness is

limited because of the wide area now involved. Therefore it

may be necessary to resort to diluting the airborne material

to a safe level by providing quantities of uncontaminated air

sufficient for the purpose.

General Ventilation Although control efficiency is con-

siderably less than with a well designed extract system the

technique is usually easy to operate and is less susceptible to

mechanical or software failure. But because of its limitation it

should only be applied to places where the pollutant is of low

toxicity and such that localized clouds of high concentrations

cannot occur.

Dilution ventilation is based on the fact that a cloud of

contaminant mixes with the surrounding air thus reducing

the concentration in the resulting mix.

Assuming complete mixing the concentration of a pol-

lutant emitted at a rate of R m^3 /s into an airstream flowing at

a rate of Q m^3 /s is given by:

concentration (in m^3 of pollutant per m^3 of air)

R

Q

but is more commonly expressed as a percentage.

Concentration (per cent) 
 100

R

Q

or a parts per million

concentration (ppm)
 106

R

Q

.

These concentrations are expressed as volume for volume

but if a mass per volume concentration is required then the

density of the substance should be used as shown below:

mass concentration (in kg m
^3 )

R

Q

D

where D is the density of the airborne substance in kg/m^3

or mass concentration (in mg m 
^36 ). 10

R

Q

D

It must be noted that in the case of airborne solvent vapors

the density of the vapor is not the same as the density of the

liquid but can be calculated from:

Vapor density at STP
 Kg/m^3

M

22 4.

where M is the molecular mass of the vapor.

With dilution ventilation it is necessary to calculate the

required volume flow rate of air to dilute the pollutant to a

desired concentration i.e. to the TLV or below. The calcula-

tion can be summarized in the following expressions:—

Required flow rate

106

Q

RK

C



where C is the chosen concentration of the pollutant in parts

per million and K is a safety factor. The units of R and Q

must be consistent, that is, if R is in m^3 /s then Q will be the

same.

Where the solvent vapors are concerned Q can be found

by:—

Q

rK

M





22.4 10

C

ms

6

(^3) /
where r is the rate of evaporation of the solvent in kg/s.
The determination of the safety factor K is a matter of
judgment and experience and depends upon the relationship
between the source of pollution and the airflow distribution
of the diluting air, the uniformity of emission and the toxic-
ity of the pollutant. Where there is good mixing between
the diluting air and the pollutant and rates of emission are
uniform and the pollutant is of a low toxicity then K could
be as low as 2 or 3.
The figures calculated from the above formulae should
be used with great caution as dilution on its own has little
effect on the source of contaminant, serving only to reduce
the workers’ exposure by minimizing the dose accumulated
from the general working environment. If the pattern of air-
flow in the vicinity of the operator can be modified then a
greater effect on the breathing zone concentration can be
achieved (see section, Displacement ).
In practice, airflow patterns in buildings are complex,
the energy of motion of the contaminant often giving rise
to a plume effect which can remain in a concentrated cloud
for some time. Considerable energy in the form of a high
air velocity would be required to disperse it. In the case of
hot fume the plume can rise and form a layer in the upper
parts of the building and some gases in their concentrated
form can also do this if they are less dense than air. Likewise
concentrated clouds of denser gas released into still air can
form layers in the floor or pour into sumps and trenches.
When dispersing such accumulations air velocities in excess
of 2 m/s (400 ft/min) are required but the whole operation
must be undertaken with care to prevent spread into other
occupied areas.
However if there is no risk of a contaminant with acute
hazard potential accumulating in areas which may become
occupied, then the motion of a concentrated plume may
be used to good effect. Careful location of screening and
exhaust points can be used to remove the concentrated mate-
rial without the need to distribute and dilute. As a result
much greater effectiveness can be achieved at much lower
air volumes. Here empirical studies of contaminant flow
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