12b Dilution Ventilation

7/30/2019 12b Dilution Ventilation

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Part 2 - Dilution Ventilation

(General Ventilation)



2

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 andexplosive 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



3

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.



4

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 valuei.e. TLV)

K factor for incomplete mixing



5

General Dilution Ventilation Equation

Rate of accumulation = Rate of generation – Rate of

removalVdc = 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 rateG, the above equation may be integrated as

Q’ = G/C

Now, G = (403 * SG * ER)/MW



6

General Dilution Ventilation

Equation

Q’ = (403 * 106 * SG * ER)/MW * CWhere

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 anddepends on:

Efficiency of mixing

Toxicity of chemicals

Duration of the process



7

Mixtures-dilution Ventilation for

Health

When two or more hazardous substances are present,then their combined effect known as the additiveeffect should be given primary consideration

If

(C1 /TLV1) + (C2 /TLV2) +……… (Cn /TLVn) > 1then the threshold limit of the mixture is considered to

be exceeded

Where

C = observed atmospheric concentrationTLV = corresponding threshold limit



8

Dilution Ventilation for Fire and

Explosion

It is necessary that the concentration of vapor in the work areashould be below the lower explosive limit (LEL)(preferably<25% LEL). This is for fire and explosion onlyand not for health hazard

Q = 403 * SG *100 *ER * SF /MW * LEL *B

Where

LEL = lower explosive limit, parts per 100

SG = specific gravity

ER = emission rate, pints/min

SF = safety factor

MW = molecular weight

B = constant ( 1 for temperatures up to 2500 F

0.7 for temperatures > 2500 F)



End of Part 2

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12b Dilution Ventilation