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When designing a battery room, ventilation requirements need to be
taken into consideration. Lead acid motive power batteries give off
hydrogen gas and other fumes when recharging and for a period after the
charge is complete.
Proper ventilation in the battery charging area is extremely
important.
A hydrogen-in-air mixture of 4% or greater substantially increases
the risk of an explosion. The concentration of hydrogen should be kept
below 1% to provide a safety factor.
Hydrogen gas is colorless and odorless. It is also lighter than
air and will disperse to the top of a building.
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The information below is provided for reference only. State and
local codes may apply that supersede these guidelines. The following is for
general understanding only, and GB Industrial Battery takes no
responsibility for these guidelines.
Step 1: Calculating Hydrogen
Concentration
A typical lead acid motive power battery will develop approximately
.01474 cubic feet of hydrogen per cell at standard temperature and
pressure.
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H = (C x O x G x A) ÷ R
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(H) = Volume of hydrogen produced during recharge.
(C) = Number of cells in battery.
(O) = Percentage of overcharge assumed during a recharge, use 20%.
(G) = Volume of hydrogen produced by one ampere hour of charge. Use .01474
to get cubic feet.
(A) = 6-hour rated capacity of the battery in ampere hours.
(R) = Assume gas is released during the last (4) hours of an 8-hour charge.
Example: Number cells per battery = 24
Ampere size of battery = 450 A.H.
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(H) = (24 x 20 x .01474 x 450) ÷ 4
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H = 7.9596 cubic feet per battery per hour
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Step 2: Calculating Room Volume
For a room with a flat roof volume is calculated W x L x
H less the volume of chargers and other fixed objects in the battery room.
W= Width
L = Length
H = Height
Example: Room size 80 feet long, 60 feet wide and 30 feet tall.
V = 60 x 80 x 30
V = 144,000 cu.ft.
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Step 3: Determining Ventilation
Requirement
Assume 75 batteries stored.
7.9596 x 75 = 596.97 cubic feet per hour (7.9596 calculated in Step 1)
Battery room 144,000 cu. ft. from example in Step 2
V = R x P ÷ H x 60 minutes
(V) = Ventilation required
(R) = Room cu. ft.
(P) = Maximum percentage of hydrogen gas allowed
(H) = Total hydrogen produced per hour
V = 144,000 x .01% ÷ 596.97 x 60
V = 144.73 or the air should be exchanged every 144.73 minutes (2 hours 24
minutes)
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Step 4: Determining Fan
Requirement
Fan Size = R x 60 minutes ÷ V
(R) = Room cu. ft.
(V) = Ventilation required
144,000 x 60 ÷ 144.73 = 59’ 697.36 cu. ft. per hour or 995 CFM.
The ventilation system should be capable of extracting 59,697.36 cu.ft. per
hour or 995 CFM.
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Step 5: Do You Need Forced
Ventilation
In theory the 596.97 cu. ft./hr. only represents .004%
which is < 1%. Therefore forced ventilation would not be required for
this example.
However, the following should be considered before ruling out forced
ventilation:
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Is the battery room closed in or open? If closed in no
natural ventilation may be possible.
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Since hydrogen gas rises are there areas in the
ceiling where gas may collect in greater concentrations.
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The above calculation represents worse case scenario
assuming all batteries are gassing at the same time. This is highly
improbable.
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If natural ventilation is sufficient in an open area forced ventilation
should not be required.
If your calculations determine a percentage <1% hydrogen concentration,
we recommend a Hydrogen Gas Detector for safe measure, part number HGD-1.
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Hydrogen Gas Detector (HGD-1)
Operation
Should the concentration of hydrogen gas in the air surrounding the sensor
reach 1% by volume, the "1% caution" yellow LED will light and
the 1% internal relay will close. Should the hydrogen gas concentration
reach 2% by volume, the "2% warning" red LED will flash and an 80
db alarm will sound; the 1% relay will remain closed and, if a Dual-Relay
model, the 2% internal relay will close. Either relay can activate a remote
exhaust fan and/or alarm.
Location
Hydrogen, colorless and odorless, is the lightest of all gases and thus
rises. The detector, therefore, should be installed at the highest,
draft-free location in the battery compartment or room where hydrogen gas
would accumulate.
The size of the area one detector will protect depends upon battery
compartment room. The detector measures the hydrogen gas in the air
immediately surrounding the sensor. If hydrogen gas might accumulate in
several, unconnected areas in the compartment or room, individual detectors
should be placed at each location.
Optional Accessories: steel junction box mounting on wall or ceiling;
modular jack (with duplicate LEDs; test button; and buzzer if needed) for
remote placement; telephone-type cable for connecting the modular jack to
the detector.
Added Benefits
In addition to protecting your employees and your property, the
detector also may reduce the following costs:Electricity – Heating – Air
Conditioning. Instead of continuously running an exhaust fan to prevent
hydrogen gas accumulation, use the detector to activate the fan only if the
concentration reaches 1%. Insurance. Installation of a detector in areas
where batteries are charged may result in a premium reduction.
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