Part 2 - Dilution Ventilation (General Ventilation)

1. 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 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

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 value i.e. TLV) • K factor for incomplete mixing

5. 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

6. 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

7. 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

8. 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 = 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)

9. End of Part 2