Fume Hood Face Velocity and Exhaust CFM

ScopeTwo things make that simple product worth stating. THE SASH POSITION IS THE LARGEST VARIABLE AND THE ONLY ONE A USER TOUCHES. A hood at full sash open needs far more air than the same hood at a working height, and on a variable-air-volume hood that difference is continuous: a sash left up overnight exhausts conditioned air all night for no benefit. The energy in that air is real and it belongs to the building, not the laboratory, which is why it is often nobody's problem until someone adds it up. TOO FAST IS NOT SAFER, WHICH IS THE PART PEOPLE GET BACKWARDS. Face velocity has an optimum rather than a floor. Above roughly 125 feet per minute, turbulence at the face and in the wake of a person standing at the hood can pull contaminants OUT of it, so a hood running well above its design velocity may contain worse than one at specification. Raising the setpoint is not a safety improvement, and a hood that fails a containment test is more often fighting room air currents than short of air. AND FACE VELOCITY IS A SURROGATE FOR THE THING THAT MATTERS. It became the field check because an anemometer is cheap and quick and the reading correlates loosely with containment. What actually matters is whether the hood keeps material inside it, and that is what tracer gas testing to ASHRAE 110 determines. A hood can pass a face velocity survey and fail containment outright because of a cross-draft from a door or a ceiling diffuser, someone walking past, clutter blocking the rear baffle, or a large piece of equipment set too near the face -- none of which a face velocity reading sees. THE USEFUL PRACTICE MESSAGE IS THAT THE TWO GOALS AGREE. Working at the lowest practical sash height improves containment and cuts the exhaust at the same time, which is unusual enough to be worth telling users directly, because the instinct is that a more open hood is a safer one. This computes airflow from an entered geometry and velocity. It does not evaluate containment or substitute for tracer gas testing, determine the required face velocity (which the applicable standard and the institution set, and which varies with the material handled), address hood type -- constant volume, variable volume, auxiliary air and ductless hoods behave differently -- size ductwork, an exhaust fan or a stack, evaluate the discharge height and re-entrainment, or address the room-level airflow and pressure the hood depends on. ASHRAE 110, ANSI/AIHA Z9.5, the institution's chemical hygiene plan, and the industrial hygienist govern.

Formula or tableexhaust = sash width x sash height x face velocity; the sash difference is the two exhaust figures subtracted, and heating it is 1.08 x cfm x temperature rise.

Source and editionThe face-velocity airflow relation, with the 1.08 sensible heat constant at standard air. The too-fast threshold is ENTERED because the institution and the standard set it.

Free accessOne product, one difference, and the standard sensible heat relation.

What governsEstimate. AHJ and licensed professional govern.

API field namessash_width_ft, sash_height_in, face_velocity_fpm, alt_sash_height_in, heating_rise_f, hours_per_year, energy_cost_per_mmbtu, too_fast_fpm, open_area_ft2, exhaust_cfm, alt_open_area_ft2, alt_exhaust_cfm, extra_cfm, heating_btuh, annual_cost

• Sash opening only exhaust here is sash area x face velocity; total open area also includes the airfoil slot and bypass opening, so a hood exhausted at this figure runs below the target face velocitySiemens Technology Report 149-989 (2004), p. 3
• Face velocity is a surrogate containment is what matters and tracer gas testing is what measures itASHRAE 110
• Standard air for the heat figure the 1.08 constant shifts with altitude and temperaturestandard psychrometric practice
• The required velocity is entered the standard and the institution set it, and it varies with the materialANSI/AIHA Z9.5