Suitability of Materials for heater in septic vault vent 1

[FishScreener]

I'm working on developing an electric heater to be inserted into a 10-inch id polyethylene pipe, that is the vent for the vault on a vault toilet.

The vault is normally vented by the convection generated in the black pipe as it heats in the sun. Which warms the air and causes air to flow in through the toilet assembly, and out the vent, carrying the noxious gasses with it.

I know that the gases include ammonia, methane, and hydrogen sulfide, in varying but low concentrations.

My thought for the heater is: a phenolic plastic board with glass insulators bolted to it; and, 80/20 Nichrome wire as the heating element wrapped around, and between the insulators. By sizing the wire length, based on the resistance of the wire, and the maximum output of the solar cell I use to power it, I should be able to limit the maximum temperature to roughly 170-degrees F.

The electrical engineer involved wants to use an encapsulated resistor, bolted to an aluminum heat sink.

Which will have the least problems with corrosion?

Thanks, Randy

 

[ScottyUK]

If you have methane then you will need a certified heater for use within an explosive atmosphere, so any home-brewed contraption with bare heating elements is definitely out. There are plenty of air heaters available which are certified for hazardous areas and are built in corrosion-resistant materials such as stainless steel or the duplex alloys. I'll let those more knowledgable on the materials front comment on the suitability of stainless steel for your conditions.


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If we learn from our mistakes I'm getting a great education!

 

[cloa]

The original poster either needs to have analysis for whether the gases form a explosive mixture or just assume that.

 

[ScottyUK]

With a source of methane and no permanent ventilation to dilute it I expect that it would be entirely plausible for a combustible / explosive atmosphere to develop and would design accordingly, but you're right: it may be possible to prove by calculation that it isn't possible for such a condition to arise. On the other hand I'd like to think that the oxygen level in the toilet would remain capable of supporting life.


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If we learn from our mistakes I'm getting a great education!

 

[gr2vessels]

Geez, that toilet is a real hazard, I assume that every time you switch the light on, kaboom, up she goes...It must be a solid vault, if you still have the room to fit in there a heater. I wonder also, what kind of people could generate that explosive atmosphere and what they might eat...
I would think that the moist air ladden with some of that explosive gas mixture (I will never ever again visit a public toilet, brrrr...) would be corrosive to the aluminium, in long term. However, you have not advised the MOC of your heater inserted (like a fuse!) in the toilet vent.
What's next, self-igniting toilet paper?!
Cheers,
gr2vessels

 

[ScottyUK]

gr2vessels,

Agreed!

Of course, we don't know what the source of the methane actually is - I'd like to think it is not from 'direct emissions'!


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If we learn from our mistakes I'm getting a great education!

 

[FishScreener]

I guess I should have given some more information on what Vault Toilets are:

Vault toilets are used in areas with out water/sewer and usually without power.

The vault is a waterproof concrete vault with an epoxy lining, which holds, the waste. The toilet is installed on the concrete top slab, and drops directly into the vault. When the vault fills up, the waste is pumped out, and disposed of.

While the vault is sitting between pumping, the decomposition results in the off gassing.

I don't expect any explosion hazards, for a few reasons: The properly sized heating element, won't be able to generate a high enough temperature to ignite the mixture, even if the air-fuel ratio was perfect fo combustion; If the vent system is functioning correctly there is very little build up of gasses. And, I feel far more confident the heating element is less likely to cause an explosion, than the sparks from the universal motors on the proposed fans.

 

[stevenal]

It's the thermostat, not the heating element that has arcing contacts that could ignite an explosive mixture.

Heat sounds like a very inefficient way to move air. There are blowers available that are meant to vent explosive gases, marine blowers for example.

 

[FishScreener]

There will be no thermostat. The Solar panel will be direct connected to the resistive element, and the element sized to limit the total heat gain, based on the maximum output of the solar panel.

We are trying to mimic the thermal cycle, which would occur with an exposed semi-gloss black pipe, that has roughly twelve square-foot of surface exposed to the sun.

Since the architects and recreation staff decided the exposed black pipes were ugly, and either shrouded them with walls, or painted them a light tan, the pipe doesn't heat up, and the restrooms aren't venting properly.

The solution we are investigating is the installation of the solar powered heaters, to generate the heat the black pipe would generate if the installations had been properly orientated, with the pipe left exposed.

 

[ScottyUK]

Fishscreener,

No offence intended but your opinion (or mine!) won't count if there's a flammable atmosphere present. The rules for flammable atmospheres are well-defined and aren't exactly compromising. You can't apply judgments such as saying that the heater won't reach ignition temperature of the gas / air mixture, even if that conclusion is where common sense might lead you: you will need a certified heater and, if you use one, a certified 'stat too.


stevenal,

Does US legislation not require flameproof (explosion-proof) heaters for hazardous areas? I agree that there are possibly better ways of doing this using a fan rather than a heater , but I also get the impression this is an off-grid installation where power for a standard flameproof fan is probably not available.


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If we learn from our mistakes I'm getting a great education!

 

[FishScreener]

The combustible gas is methane, which is lighter than air. And the vault is vented. Thus, the methane concentration can't get to the levels they are explosive or combustible.

However, I'm not sure how corrosive methane is. I do know that ammonia, and the hydrogen sulfide, are corrosive.

I need advice on which materials will be the most resistant to corrosion in this atmosphere.That was why I posted in the corrosion engineering forum.

 

[ScottyUK]

Methane is pretty innocuous in terms of corrosion of the metals you'd find in electrical equipment. Your H₂S is probably the biggest problem - even low concentrations will attack copper, and anything with silver plating will end up with silver whiskers which eventually short out with varying degrees of destruction depending on how much energy the source can deliver. H₂S is a problem which costs the waste water industry a lot of money fighting a losing battle to control the damage to their electrical equipment. The best solution is to keep electrical equiment away from it completely.

Ideally the electrical connections should be in a sealed terminal compartment with a gas-tight gland for the cable entry. At least, that's how we would do it in the power and O&G industries.


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If we learn from our mistakes I'm getting a great education!

 

[stevenal]

ScottyUK,

Yes, but I guess we've established this is not an explosive atmosphere. The marine blower I suggested is DC powered, so it should work well powered by solar cells. Being ignition protected, it's metallic parts are well isolated from the atmosphere. Also resistant to salt induced corrosion. Just unsure how it would stand up to continuous duty.

I believe it will take a massive amount of solar cells to achieve any appreciable amount of heat.

 

[FishScreener]

Here's my reasoning:

If I assume that the solar panels are 75% efficient, then it should take approximately 16-sf of solar panels to equal the solar gain of the 12-sf of black body absorber that the original pipe is.

Since, the solar panels are direct connected to the resistive heat generator, located a short distance away, there should be very little loss of energy, between the panel and the heating element.

Thus, if I install 16-sf of panels, I should have an equal heat gain in the resistor, and thus air column, that I would with the solar heating of the pipe.

Every time the solar arrays for the radio system get upgraded, they give my shop the old panels, and related mounting hardware. So, I have several hundred square feet of panels in storage, that are paid for.

I figure that if I can put them to use, the total installation will be quite inexpensive.

 

[btrueblood]

"If I assume that the solar panels are 75% efficient"


Try more like 10 or 15% efficiency.

A better idea may be to source a light-colored paint with high solar absorbtivity. Dark, flat green perhaps.

 

[stevenal]

Two entirely different processes at work. Solar heating of the black pipe comes from the infrared spectrum, while electricity production from your cells will be more from the visible spectrum. I'm not sure how to properly compare the two.

Do you just let the gasses accumulate all night?

 

[gr2vessels]

FishScreener,
Assuming (again) that the vault vent pipe end is somewhere up above the roof, generally speaking 'always in the wind', why not consider the use of the wind as source of energy;- I suggest using a turbine ventilator, similar to the domestic roof ventilators. They are very effective, extermely simple in construction and very cheap to buy, install, maintain or replace.
Try google 'roof turbine ventilators' for details and suppliers.
Cheers,
gr2vessels