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welding exhaust system with air cleaners capable of removing particulates and odors, and recirculating air. f. Battery Rooms.— Provide continuous ventilation to maintain hydrogen gas concentration below 0.8 percent by volume during maximum gas generation conditions. The following should be noted when determining the required exhaust fan size:
Design Guide for Heating, Ventilating, and Air Conditioning Systems February 29, 2000 8:09 – Last Rev: September 21, 2006 100 Figure 33.—Weld ventilation hood.
Design Guide for Heating, Ventilating, and Air Conditioning Systems February 29, 2000 8:09 – Last Rev: September 21, 2006 101 Figure 34.—Welding ventilation movable exhaust hoods.
Design Guide for Heating, Ventilating, and Air Conditioning Systems February 29, 2000 8:09 – Last Rev: September 21, 2006 102 (1) When the battery is fully charged, each charging ampere supplied to the cell produces about 0.016 cubic feet of hydrogen per hour from each cell. This rate of production applies at sea level, when the ambient temperature is about 77 ºF, and when the electrolyte is “gassing or bubbling.” (2) Number of battery cells and maximum charging rate (not float rate) can be obtained from specifications or field inspection. (3) Hydrogen gas lower explosive limit is 4 percent by volume. Good practice dictates a safety factor of 5, which reduces the critical concentration to 0.8 percent by volume. This large safety factor is to allow for hydrogen production variations with changes in temperature, battery room elevation, and barometric pressure and also allows for deterioration in ventilation systems. The following example illustrates the procedure for determining battery room ventilation requirements. Assume a battery room volume (Vr) of 900 ft3, a 60-cell battery with a charge rate of 50 amps per hour, and a maximum H2concentration of 0.8 percent by volume. The total H2generation is given by: Gt = GcNA Where: Gt= total hydrogen generated, ft3/hr Gc= hydrogen generation per cell, ft3/hr/cell N= number of cells A= charging rate, amps Gh= (0.016)(60)(50) = 48 ft3/hrThe maximum acceptable volume of H2is given by: Vh= VrC Where: Vh= volume of hydrogen, ft3Vr= total room volume, ft3C= acceptable hydrogen concentration, percent Vh= (900)(0.008) = 7.2 ft3
Design Guide for Heating, Ventilating, and Air Conditioning Systems February 29, 2000 8:09 – Last Rev: September 21, 2006 103 The time, T, to reach critical concentration is given by: T = Vh/Gh= 7.2/48 = 0.15 hrs or 9 minutesThe minimum number of air changes per hour, N, is determined by: N = 60/9 = 6.7 changes/hrThe minimum airflow, Q, required, is given by: Q = VrN/60 = (900)(6.7)/60 = 100 cfmVentilation rates for battery rooms are usually small (less than 100 cfm). For preliminary sizing of ventilating requirements, assume 1 cfm/ft2, or 6 air changes/hr. Maximum hydrogen gas generation occurs when batteries are approaching full charge.