Transcript Lecture 1 – Putting Safety Into Perspective

Part 2 - Dilution Ventilation
(General Ventilation)
General Dilution Ventilation
For the supply and exhaust of air in a building, there are 2
types of general dilution ventilation:
 Type1: dilution ventilation (D.V.)
 D.V. Is the dilution of contaminated air with uncontaminated air
for controlling potential airborne health hazards, fire and
explosive conditions, odors and nuisance type contaminants
 D.V. Also includes the control of airborne contaminants such
as vapors, gases and particulates generated within tight
buildings
 D.V. Is not as satisfactory for health hazard control as is local
exhaust ventilation
 Type 2 : heat control ventilation
 It is the control of indoor atmospheric conditions found in hot
industrial environments. The purpose is to prevent discomfort
or injury to workers
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Dilution Ventilation for Health
Dilution ventilation is generally used to control the
vapors from organic liquids with a TLV of 100 ppm or
higher.
The limiting factors for D.V. For health are:
 The quantity of contaminant generated must not be too great or
the air flow rate necessary for dilution will be impractical.
 Workers must be at an appropriate distance from the
contaminant source or the exposed contaminant must be in
sufficiently low concentrations so that workers will not have an
exposure in excess of the established TLV.
 The toxicity of the contaminant must be low.
 The emission rate of contaminants must be reasonably uniform.
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Parameters Required for Determination
of Dilution Ventilation Rates
 Solvent vapor per minute (i.e. evaporation rate )
 Specific gravity of liquid
 Molecular weight
 Acceptable health standard (threshold limit value
i.e. TLV)
 K factor for incomplete mixing
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General Dilution Ventilation Equation
Rate of accumulation = Rate of generation – Rate of
removal
Vdc = Gdt – Q’Cdt
Where
V = Volume of room
G = Rate of generation
Q’ = Effective volumetric flow rate
C = Concentration of gas or vapor in ppm
t = time
For steady state condition, change in concentration, dC = 0
Gdt = Q’Cdt
For constant concentration C and uniform generation rate
G, the above equation may be integrated as
Q’ = G/C
Now, G = (403 * SG * ER)/MW
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General Dilution Ventilation
Equation
Q’ = (403 * 106 * SG * ER)/MW * C
Where
SG = Specific gravity
ER = Emission rate in pints/minute
MW = molecular weight
G
= Rate of generation in cfm
C
= Concentration of gas or vapor in ppm
Actual Ventilation Rate Q = Q’ * K
Where
K = factor for incomplete mixing and lies between 1 and 10 and
depends on:
 Efficiency of mixing
 Toxicity of chemicals
 Duration of the process
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Mixtures-dilution Ventilation for
Health
When two or more hazardous substances are present,
then their combined effect known as the additive
effect should be given primary consideration
If
(C1/TLV1) + (C2/TLV2) +……… (Cn/TLVn) > 1
then the threshold limit of the mixture is considered to
be exceeded
Where
C
= observed atmospheric concentration
TLV = corresponding threshold limit
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Dilution Ventilation for Fire and
Explosion
It is necessary that the concentration of vapor in the work area
should be below the lower explosive limit (LEL)
(preferably<25% LEL). This is for fire and explosion only
and not for health hazard
Q = 403 * SG *100 *ER * SF/MW * LEL *B
Where
LEL
SG
ER
SF
MW
B
= lower explosive limit, parts per 100
= specific gravity
= emission rate, pints/min
= safety factor
= molecular weight
= constant ( 1 for temperatures up to 2500 F
0.7 for temperatures > 2500 F)
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End of Part 2