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 (V
r
) of 900 ft
3
, a 60-cell
battery with a charge rate of 50 amps per hour, and a
maximum H
2
concentration of 0.8 percent by volume.
The total H
2
generation is given by:
G
t =
G
c
NA
Where:
G
t
=
total hydrogen generated, ft
3
/hr
G
c
= hydrogen generation per cell, ft
3
/hr/cell
N
= number of cells
A
= charging rate, amps
G
h
= (0.016)(60)(50) = 48 ft
3
/hr
The maximum acceptable volume of H
2
is given by:
V
h
= V
r
C
Where:
V
h
= volume of hydrogen, ft
3
V
r
= total room volume, ft
3
C
= acceptable hydrogen concentration, percent
V
h
= (900)(0.008) = 7.2 ft
3

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 = V
h
/G
h
= 7.2/48 = 0.15 hrs or 9 minutes
The minimum number of air changes per hour, N, is
determined by:
N = 60/9 = 6.7 changes/hr
The minimum airflow, Q, required, is given by:
Q = V
r
N/60 = (900)(6.7)/60 = 100 cfm
Ventilation rates for battery rooms are usually small (less than
100 cfm).
For preliminary sizing of ventilating requirements,
assume 1 cfm/ft
2
, or 6 air changes/hr.
Maximum hydrogen gas
generation occurs when batteries are approaching full charge.