OpenSpecEngineering

Finds the fresh-air flow that keeps a solvent vapor below a target concentration in a room, when the solvent evaporates at a steady rate. It follows the ACGIH ventilation manual [1]. Industrial hygienists and HVAC engineers use it for paint, cleaning and printing rooms where local exhaust is not practical.

Equations

V_m = R·T/P     G = (E/M)·V_m

Q = K·G/C

E is the evaporation rate, in g/h, and M the molar mass, in g/mol. R is the gas constant, 8.314 J/(mol·K). T is the absolute air temperature and P the pressure. V_m is the volume of one mole of vapor, and G the volume of vapor given off per hour. C is the target concentration as a volume fraction (ppm × 10⁻⁶). K is the mixing factor, and Q the air flow, in m³/h. This is the same as the ACGIH form Q = 403·10⁶·SG·ER·K/(M·C) in US units, with Q in cfm, ER in pints per minute and SG the liquid’s specific gravity [1].

Inputs

• Choosing a solvent sets M from its formula [5] and offers its NIOSH REL [3] or OSHA PEL [4] as the target. Choose Custom to enter M and the target.
• The evaporation rate is the mass of solvent that goes into the air: for example, the amount used in a shift divided by the hours of use.
• K allows for poor mixing. Use about 1 to 2 for good mixing near the worker, and up to 10 for poor mixing, a toxic solvent or a worker near the source [1].

Limits and common mistakes

• Dilution is for solvents of low toxicity released at a low, steady rate, away from the worker’s breathing zone. For a strong point source, use local exhaust [2].
• The target is a time-weighted average. Short peaks can still go above it.
• Q is the flow of clean outdoor air. Air that is recirculated without removing the vapor doesn’t count.
• Keeping a space below the lower flammable limit is a different check, with its own target and factors.

Worked example

Workers wipe down parts with toluene in a small room. About 1.2 kg is used in an 8-hour shift, so 150 g/h. Mixing in the room is poor, so take K = 5. How much air keeps the room at the NIOSH limit at 22 °C?

1. Choose Solvent Toluene and Target Concentration NIOSH REL (10-h TWA).
2. Enter Solvent Evaporation Rate 150 g/h and Mixing / Safety Factor 5.
3. Enter Air Temperature 22 °C, keep Pressure at 101.325 kPa and press Solve. The Solution shows: Molar mass M (from formula): 92.141 g/mol
   Target C = NIOSH REL (10-h TWA): 100 ppm
   Molar volume at T, P: 24.2192 L/mol
   Vapour generation rate G: 0.0394275 m³/h
   Required ventilation rate Q: 1,971.37 m³/h
   Required ventilation rate Q: 1,160.31 CFM

Check by hand: 150/92.14 = 1.628 mol/h, and 1.628 × 0.02422 = 0.03943 m³/h. Then Q = 5 × 0.03943/0.0001 = 1,971 m³/h. The ACGIH US form, with toluene’s specific gravity 0.867 and 150 g/h = 0.00609 pint/min, gives 1,155 cfm, a little lower because it takes the air at 70 °F.