EQUATION: Hydrogen Rate (HR) = 0.016 x 0.24 x 2 x 1360 x 182 = 0.317 CFM (1)
0.016 = hydrogen emission rate per cell per ampere charge (ft /h)
60 = number of minutes per hour
0.24 = charging current per 100 AH cell capacity at 77 degrees
= doubling factor for 15 degrees F (8 degrees C) rise in
electrolyte temperature (above the normal 77 degrees F
(25 degrees C))
1360 = capacity of one cell at the eight-hour rate (ampere-hour)
= increment of cell capacity that will pass 0.24 ampere
current charge (ampere-hour)
= number of cells
Concentrations of hydrogen below four percent are not flammable.
To meet the Occupational Safety and Health Act (OSHA) requirements, limit
hydrogen concentration to one percent. The required ventilation rate is:
Ventilation Rate (VR) = 0.317 CFM = 31.7 CFM
Notice that this is a small volume. Other factors, such as, lighting, other
equipment and solar-thermal gains, will require additional ventilation.
Therefore, a ventilating rate of no less than 400 CFM should be considered.
To provide redundancy, use two fans, each sized for half the flow. Fans may
be wall-mounted, roof-mounted, or ducted in-line. Makeup air shall be
filtered as previously specified. A schematic of the heating and ventilating
scheme is shown in Figure 5. Fans shall be on the utility power circuit.
b) Heating. The preferred method of heating UPS battery rooms is
a simple ducted arrangement using waste heat from the UPS equipment. Backup
heat would then be required only when the UPS equipment is down for
maintenance or repair. Electric resistance unit heaters are preferred,
because of low first cost maintenance and utilization. The heater should be
on the utility power circuit.
c) Controls. In the scheme depicted in Figure 5, the modulating
damper should be controlled in winter by a wall-mounted thermostat, set at 65
degrees F (18 degrees C). A switch should make it possible to bypass the
thermostat in the summer to fully close the damper in the branch duct from the
UPS equipment exhaust irrespective of temperature to take full advantage of
daily range and to obtain a lower electrolyte temperature within the specified
range. Dampers should be of the low-leakage type. Fans should run
continuously, except when de-energized by the firestat. Fan failure alarms
and hydrogen gas buildup alarms are not required. Fans should, however, be