Spray Booth Airflow and Makeup Air Load

Booth ventilation is not sized for comfort or even for the paint -- it is sized to keep the vapor concentration far below the lower flammable limit and to keep overspray moving away from the operator. NFPA 33 and OSHA 1910.107 set that as a FACE VELOCITY across the booth opening, commonly 100 fpm for an open-face booth, and the airflow follows from the opening area whether the booth is spraying or not. That air leaves the building and it has to be replaced, and replacing it in January is the expensive part. The sensible load is 1.08 x cfm x delta-T, and at booth airflows the delta-T does not have to be large before the number is enormous: a 14 by 9 ft opening at 100 fpm is 12,600 cfm, and tempering it from 20 F to 70 F is 680,400 BTU/hr -- more than most residential furnaces put out in a day, running whenever the booth runs, and at $1.20 a therm and 80% burner efficiency about $10 an hour in gas alone. That is why booth discipline, spraying in batches and not leaving the fan running, is worth real money. Note what the face velocity does and does not allow: nothing about it is negotiable downward, because it is a life-safety requirement, but a smaller opening is a smaller airflow, and a 10 ft wide booth at the same 100 fpm needs 9,000 cfm and costs 29% less to temper. Airflow and the sensible makeup-air load only. It does not size the fan, the ductwork, or the makeup air unit, does not compute the negative pressure the booth must hold relative to the shop, and does not address filter selection, filter loading and its effect on airflow, the interlocks between the spray equipment and the fan, the electrical classification of the booth and the area around it, or the exhaust stack height and discharge location. Air-solvent concentration, recirculation, and heat recovery are all separate questions with their own code limits. NFPA 33, OSHA 1910.107, the adopted mechanical code, the equipment manufacturer, and the AHJ govern.

opening = width x height; exhaust cfm = opening x face velocity; makeup = exhaust; heating load = 1.08 x cfm x (indoor - outdoor); gas input = load / burner efficiency; therms/hr = input / 100,000.

Spray booth exhaust airflow from the design FACE VELOCITY across the booth opening, the requirement NFPA 33 and OSHA 1910.107 set (commonly 100 fpm open-face), cited by name and not reproduced; makeup-air sensible load from the standard-air relation 1.08 x cfm x delta-T. Airflow and the sensible load only. NFPA 33, OSHA 1910.107, the adopted mechanical code, and the AHJ govern.

Area times velocity, and the standard-air sensible heat relation; the face velocity is cited to its standards and entered by the user rather than reproduced from a table.

Estimate. AHJ and licensed professional govern.

Field names used by the API: opening_width_ft, opening_height_ft, face_velocity_fpm, indoor_temp_f, outdoor_temp_f, burner_efficiency, hours_per_year, price_per_therm, opening_sqft, exhaust_cfm, heating_btu_hr, gas_input_btu_hr, therms_per_hour, cost_per_hour

• Face velocity is a requirement it comes from NFPA 33 and OSHA 1910.107 and is not a design choiceNFPA 33 / OSHA 1910.107
• Airflow follows the opening the fan runs at the design flow whether or not the gun is sprayingspray booth practice
• Sensible only 1.08 x cfm x delta-T is the standard-air constant; no latent, filter loading, or negative-pressure allowanceASHRAE Fundamentals