Dilution Ventilation for a Solvent Vapour

The vapour a solvent generates is its mass rate over its molecular weight times the molar volume, 387 cubic feet per pound-mole at 70 degF and one atmosphere, and the airflow is that volume divided by the target concentration and multiplied by a mixing factor. WATCH THE INPUT UNIT ON THE PUBLISHED CONSTANT. The familiar 403 belongs to a form taking PINTS PER MINUTE of liquid at a given specific gravity -- it is 387 times the 1.043 pounds a pint of water weighs -- and applying it to a rate in pounds per hour understates the airflow by a factor of sixteen. This uses the mass-rate form directly, so the input unit and the constant cannot drift apart. TWO REQUIREMENTS COME OUT OF THE SAME EMISSION AND THEY ARE NOT THE SAME NUMBER. The health requirement holds the room at an exposure limit measured in parts per million. The flammability requirement holds it at a fraction of the lower explosive limit, which is measured in percent -- four orders of magnitude higher. For a solvent with a low limit the health figure governs by a wide margin, and for one with a high limit the fire figure can govern instead, so both are computed and the larger is reported. A ventilation rate set on one without checking the other is set on half the question. THE MIXING FACTOR SWINGS THE ANSWER BY A FACTOR OF TEN and it is the least defensible input. It runs from about 1 for a well-mixed room with the emission remote from anyone, up toward 10 for a room that mixes poorly, a worker standing at the source, a limit with a serious consequence, or an emission rate that is itself uncertain. It is a judgement about how badly the assumption of uniform mixing fails, and it is entered rather than derived because nothing in the arithmetic can supply it. AND THE ASSUMPTION OF UNIFORM MIXING IS THE LIMITATION THAT MATTERS MOST. This computes a room average, and the person leaning over the tank is breathing a concentration higher than the average -- before mixing, in the plume, at the point where the emission actually occurs. Dilution ventilation is appropriate for a low-toxicity vapour released steadily at a distance from workers. For anything more hazardous, released faster, or released where people are, LOCAL EXHAUST VENTILATION that captures at the source is the control, and it does the job on a fraction of the airflow. A dilution rate that comes out implausibly large is itself the argument for local exhaust. This is a steady-state single-contaminant estimate. It does not size a ventilation system, place supply and exhaust (the air has to sweep the room rather than short-circuit between the openings, which no airflow figure captures), account for mixtures with additive or synergistic effects, model a non-steady or peak release, address make-up air, heating cost, or the effect on other processes, evaluate the exposure a person actually receives, or substitute for air monitoring. The ACGIH Industrial Ventilation manual, the applicable exposure limit, and a certified industrial hygienist govern.

dilution airflow = molar volume 387 ft^3/lbmol (at 70 degF, 1 atm) x 1e6 / 60 x (lb/h) x K / (molecular weight x ppm), with the flammability requirement computed the same way against a fraction of the LEL and the larger governing.

Dilution ventilation as the ACGIH Industrial Ventilation manual states it. NOTE THE INPUT UNIT ON THE PUBLISHED CONSTANT: the familiar 403 is 387 ft^3/lbmol x 1.043 lb/pint and belongs to a form taking PINTS PER MINUTE of liquid; applying it to a mass rate in lb/h understates the airflow by a factor of sixteen. The mass-rate form is used here so the unit and the constant cannot drift apart.

The ideal gas law and one entered judgement factor.

Estimate. AHJ and licensed professional govern.

Field names used by the API: evaporation_lb_hr, molecular_weight, tlv_ppm, mixing_factor, lel_pct, lel_safety_fraction, room_volume_ft3, alt_mixing_factor, health_cfm, lel_target_ppm, lel_cfm, governing_cfm, room_ach, alt_health_cfm

• Mixing factor is entered 1 to 10; nothing in the arithmetic supplies ita certified industrial hygienist
• Uniform mixing assumed the person at the source breathes more than the averagelocal exhaust ventilation design
• Single contaminant, steady state mixtures and peak releases are not modelledthe ACGIH Industrial Ventilation manual