Fire/smoke alarms - advice please

[Adrian Brentnall]

Sorry - my bad terminology. 'House alarm' = set of existing linked fire alarms fitted inside the house.... No point in 'outside' alarms here - nobody but the chickens to hear 'em

[NT]

The point is it only costs =A33 to try an ionisation type, and it may prove ok. If it does, you can then get one that wires to the house.

NT

[Peter Parry]

The safety system may well be the simplest and most reliable of all, the kiln will withstand the elements at full power indefinitely or until the heater element itself or some other component overheats in a predicted way and fails.

[Owain]

But if the 'house alarm' goes off you grab the cat and run outside.

If the kiln alarm goes off, you stop to put your trousers on.

Owain

[Andy Dingley]

Depends on the kiln.

These kilns don't catch fire, because they're made of steel, firebrick and FIRE already. So their "fault conditions" are probably just that the temperature is outside prediction (a temperature of zero is especially bad, because it means the thermocouple broke) and the appropriate reaction is to just to cut the power and sound a "job trashed, building OK" alarm.

If the kiln actually has a "fire", more than it does normally, then you may not have time for the trousers.

I've got a similar problem with laser cutters. A machine that works by burning material away will tend to make a smoky exhaust. So how do I tell the difference between "It's working" and "It has set fire to the whole sheet" ? It's certainly ill-advised at present to leave lasers cutting on their own without anyone in attendance - several workers who've done this have lost =A310k+ machines from the resultant fires, and not dealing with the trivial fire quickly enough.

[Andy Dingley]

For a Skutt Firebox it's an evens bet between element failure, or the element overheating the bricks and cracking them. They're an unusually powerful element for the size of kiln.

[The Natural Philosopher]

They should have proper smoke extraction as smoke is bad for the laser beam and bad for the optics.

[Owain]

AIUI the kiln is in a timber shed and my assumption was that it was detached from the house by sufficient distance to give time to put on trousers before investigating.

depending on the material, some form of optical flame detection probably?

Owain

[Adrian Brentnall]

Ah - but it didn't - wouldn't have minded if it had... The controller's _now_ decided that it can't see the thermocouple (even when the thermocouple's replaced by a wire 'short' at the controller terminals - so it looks like it's a 'late controller'...

Still need to swap the controllers, but I suspect that the old controller in the new kiln will work fine....

Thanks for all the perspectives A

[Adrian Brentnall]

It's about 8 feet away from the house back door... would really rather it didn't catch fire!

A

[Adrian Brentnall]

Yes - good point. Of course, I'll never know if it really works unless the the kiln controller fails again and the shed burns down ... ..but it would prove that the cheapie detector isn't tripped by solder smoke / flux fumes etc!

Thanks Adrian

[Chris Hogg]

While I appreciate the desire for a fire alarm system, do you actually need one? The operating temperature of 700C, and by implication the maximum temperature attainable, isn't that high by pottery kiln standards (is yours an enamel kiln?). I have three pottery kilns, two electric ones in a spare room now used as a pottery workshop, and a gas-fired kiln in a wooden shed in the garden. All are capable of

1300C although seldom operated above 1260C. They are made of refractory brick and ceramic fibre, in a steel case. There is nothing there to burn.

The gas kiln is manually controlled, and is checked/regulated every thirty minutes over about an eight hour period. The shed is fully lined with fireproof fibre-board (similar to the old asbestos cement sheeting, but with another type of fibre reinforcement): walls, floor and ceiling. A doubly insulated flue passes up through the roof. The distance between the shed walls and the kiln is typically two feet all round. Despite the outside of the kiln being much too hot to touch when going full blast, the fibre-board shed lining adjacent to it only gets warm.

The two electric kilns in the bedroom-that-was are rated at 7kW and

3kW respectively. They are located in one corner of the room. The walls in that corner are lined with plasterboard on battens, up to a height of about four feet, such that there is a 2 inch gap between the board and the wall. The board is also covered with aluminium foil (the widest Baco cooking foil that I could get, stuck on with evostick. Be wary of modern 'aluminised' plasterboard, the stuff with a moisture barrier; it's covered with aluminised plastic film which is probably not very heat-resistant). To protect the wooden floor, the kilns stand on a sheet of the fibre-board resting on a wooden pallet to provide an air gap. The kilns are probably no more that 12 inches away from the aluminium-coated plasterboard at the nearest point. Firing the kilns is done overnight (Economy-7), using an automatic programmable controller. As with the gas kiln, even though the outsides of the electric kilns get far too hot to touch, the aluminised plasterboard only gets warm.

The only precaution I take when firing any of the kilns is to make sure there are no combustible materials close to the kilns before starting. In your case, I would make sure that the shed walls in the immediate vicinity of your kiln, and the floor it stands on, are protected from radiant heat in much the same way as I have done mine, and I see no problems. After all, the outside of the kiln doesn't get _that_ hot, and the amount of heat it radiates isn't going to do any harm if the shed walls and floor are reasonably well protected.

If you're thinking of a thermal fuse, then one idea might be a thick piece of ordinary 'silver' wire (92.5%Ag, 7.5% Cu), m.p. 920C, in a thermocouple-type ceramic holder, wired in series with the elements. Kiln overheats; silver wire melts; kiln cools. Not too difficult to make up. A friendly jeweller should have that sort of wire.

[NT]

Cheap ionisation detectors are very sensitive. All you need establish is that it doesnt false trigger. If it does, try an optical one for less sensitivity.

NT

[Adrian Brentnall]

OK - thanks! Adrian

[Dave Liquorice]

But thermocouples aren't just a simple bit of wire, they produce a voltage in proportion to the temperature. They produce enough voltage/current to hold open gas valves. So no wonder your controller is saying "no thermocouple" when there is a wire short in place, it's not bust it's telling the truth.

[Adrian Brentnall]

Hi Dave Sorry to disagree - but lots of controllers (including the one that I have) include this feature as a built-in test mode - simply because it is difficult for the average user to test a thermocouple

I've also got another similar kiln here, I've been able to do various combinations of controllers, relays and thermocouples - and the only thing that's consistently bust is the controller...

Good news is that the 'new' big kiln is now running using the older kiln's controller - so I just need to get the duff controller replaced under warranty and we're back where we should have been!

Have also asked the controller manufacturer how their 'current on, kiln not heating up' logic works, in the event of a partial failure of the temperature-sensing circuitry...

Adrian

[Chris Hogg]

Most controllers, certainly those for pottery kilns, have an 'open circuit detect' ability, i.e. they detect broken t/c's, a not uncommon occurrence. If this were not the case, they would not be able to distinguish between a broken t/c and one at ambient temperature producing zero emf. What AB did, replacing the t/c by a wire short, was an appropriate test IMO.