Battery Room Ventilation Fan 1

[jliu1]

I understand hydrogen is generated during battery charge mode and the generation will stop after switching off the charging current. My question is if the ventilation fans can be turned off at this moment? You may say hydrogen gas is still trapped in the plates and inside the cell and may come out to the atmosphere slowly. But what if the prior charging is just a float one, hydrogen emission is not much and there was continuous ventilation going on until the moment.

And after this moment, if the battery is in discharge mode, will the residual hydrogen recombines with the H2O or new hydrogen will be generated? I am talking about a critical situation, when the battery is in discharge mode, AC may be completely lost. If I need to provide ventilation fan, the power has to be from the battery itself, which is not preferred.

Any comments are appreciated.

 

[wayne440]

I'm no expert on battery chemistry, but have seen many communications sites with charger and battery installations. The ones I am most familar with have the vent fan on a timer to run for a few minutes each hour, plus a thermostat to turn on in case of air conditoning failure. They are powered from the service panel at 120 VAC, and if that power fails, they stop. I do not recall ever seeing DC powered vent fans.

 

[Gamaliel57]

As an ex-submarine electrician, this scares me. As long as you are passing current through the electrolyte (H2SO4 and H2O, yes?) you are generating some amount of hydrogen and oxygen. You do generate more hydrogen and oxygen on charge than discharge, but electrolysis is electrolysis.

Submarines do have DC vent fans for the battry compartment.

For a small UPS battery in a large room, maybe not much risk. For a large battery bank in a small space, I would want air flow constantly and maybe some H2 sensors.

 

[alehman]

The International Fire Code require ventilation to maintain hydrogen concentration less than 1% (which can be controlled by a hydrogen sensor), or continuously operating ventilation sized at one cubic foot per minute for every square foot of room floor area.

Alan
“The engineer's first problem in any design situation is to discover what the problem really is.” Unk.

 

[wayne440]

That IFC information will be useful the next time one of our landlord's site inspectors gets on his high horse. A DC powered fan might be the most straighforward way to comply.

 

[alehman]

Interestingly, the IFC doesn't specify a charging condition for the 1% requirement.

Alan
“The engineer's first problem in any design situation is to discover what the problem really is.” Unk.

 

[jliu1]

IEEE 484 requires less than 2% hydrogen accumulation, while NRC RG 1.128 goes further by requiring 1% for any new plant projects.

 

[NCTHAI]

As a prudent O&M practice, Ventilation fans should never be switched off. Hydrogen concentration should less than 1%. You need to refer to country specific Fire Standard for real required values.

 

[dpc]

Ventilation fans should never be switched off

This will depend entirely on the installation. If adequate natural ventilation is provided, dedicated fans may not even be necessary. The key is to keep hydrogen concentration at a low level.

David Castor

http://www.cvoes.com

 

[mike4472]

Have you considered a fan with a flameproof motor? Have you checked the number of 'wet' cells against the volume of the room. Can you generate enough hydrogen to exceed the lower explosive limit.

 

[alehman]

The purpose of the fan is to avoid a flammable atmosphere, so hopefully an explosion proof motor would not be necessary. If so, any other electrical equipment in the room would need to be similarly rated.

Alan
“The engineer's first problem in any design situation is to discover what the problem really is.” Unk.

 

[Coward]

Consider when you have to restart the fan motor when hydrogen might have accumulated above LEL.
Is this scenario possible ?

 

[hollandhvac]

What I normally do is, I have a duty standby fan for ventilation and you need an explosioin proof fan Eex(d) IIC T3 with spark proof fan construction. But if you stay below 25% of LEL or 1% of hydrogen you have not got an explosove mixture, only if you rach 3,8% or 4% depending on what document you read. For this reason you need high level hydrogen detection.

 

[alehman]

Yes, the system should be designed such that the fan comes on well below the LEL level. U.S. codes do not require explosion-proof equipment for this application.

Alan
“The engineer's first problem in any design situation is to discover what the problem really is.” Unk.

 

[Compositepro]

It is important to keep in mind that that hydrogen has a very powerful tendency to rise and that the exhaust vent should be placed at a high point in the room.

 

[hollandhvac]

Yes, supply at low level and exhaust at high level. No falce ceiling in the building or beams in the ceiling to create area's where hydrogen can accumilate. If there are beams you need to put a ductwork to exhaust the hydrogen from each pocket.
Aleman, can you indicate where it say's you do not need explosion proof equipment?

 

[gepman]

There isn't a lot of hydrogen gas produced in battery charging unless the batteries are overcharged either by rate (current) or potential (voltage). This question of the fan and it's classification is answered in 2008 NEC Article 480.9(A). There will be relatively little volume of hydrogen gas left in the batteries once the charging circuit is off. If you think clearly about the situation, the concentration of hydrogen gas (or any other gas) will be lower at the exhaust fan than at the source (unless, potentially, the fan is started after battery charging has occurred for a long period). If you feel that the exhaust fan is in a classified location then it is likely that the remaining area (or at least some of the remaining areas) of the room will need to be classified also. This NEC article does not answer the question of what happens in the room if the exhaust fan should fail. If I feel that there is NOT sufficient natural ventilation in the room to prevent a buildup of explosive gasses then I design a circuit to set an alarm contact and disable the charging circuit in case of fan failure. Disabling the charging circuit stops the potential for forming hydrogen gas. Depending on the location sometimes I design a circuit to start the fan only when battery charging occurs (i.e. forklift battery charging) but for dedicated battery storage rooms (such as for emergency station power) then the ventilation should be continuous and I “lock” the circuit breaker in the panelboard in the “on” position.

I will not go into the details of calculating the formation rate of hydrogen or the rate of natural ventilation although both are reasonably well documented.

See the commentary of the 2008 NFPA NEC Handbook on this issue.