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allenb
04/11/2024 6:33 PM
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Hot Coil on Shutdown
sahstim001
I just ran all of my electronics for my DIY FB roaster, and I'm thinking about potential failure modes...

Has anyone ever had an issue with shutting down the roaster while the heater coil is still hot? In normal operation everything upstream from the coil is cool since hot air is pushed downstream.

But what if you shut off the fan while the heater is hot? Can the risidual heat flow backwards and cause harm, such as to the wires powering the coil itself, or even the fan?
 
renatoa
For FB machines especially, is mandatory to keep a minimal airflow through the heater the whole roast cycle, for the reason you described above.
The most concerned people designed their own security systems, operating independent from the software solution used for roasting.
For example, the popular TC4 roasting system has a heater cutoff setting, when the fan is below a certain level... but this measure don't cover the scenario when the fan is stopped by a failure, and somewhat externally, not by a command from TC4 or Artisan.
So the ideal setup should have either a heater over temperature cutoff solution, as are those based on bi-metal, that cut at 270 C, either a low airflow detection, that led to same result, the heater cutoff.
 
allenb
Wiring and components will most likely not be harmed by an occasional complete shutdown while at full heat output but for optimal lifespan of heater element and surrounding components, best practice is to flow air across the element long enough after heater is de-energized to keep things from getting overly hot. Probably only need 30 seconds post-heat air flow.

One good rule to always follow is never allow the blower to be turned off except by manual deactivation and ensure that power to the heater is automatically cut whenever the blower is deactivated.
1/2 lb and 1 lb drum, Siemens Sirocco fluidbed, presspot, chemex, cajun biggin brewer from the backwoods of Louisiana
 
sahstim001
Thanks for the replies!

In addition to software cut-off for minimum airflow, I will also have a sail switch between the TC4+ and the heater SSR to protect against blockage / failure.

However that doesn't protect from heat after shutdown... Is there any safeguard, software or otherwise, against this scenario? Or is this generally left to user?

I know this is probably a rare scenario, as normally a roast has a cool down period anyway. But still, safety first!
 
renatoa

Quote

...as normally a roast has a cool down period anyway.


Indeed, one of the features that make FB design preferable to the others, is that the cooling is done in the same machine, and is part of the roast cycle.
So, in which case would you consider a shutdown without cooling ? Mains brownout?
 
sahstim001

Quote

renatoa wrote:

So, in which case would you consider a shutdown without cooling ? Mains brownout?


Either a failure like that, or user error, or some other "emergency shut down" situation
 
renatoa
Ok, back to your initial dilemma...

Quote

Can the risidual heat flow backwards and cause harm... ?


Why would that happen?... heat always is rising, never going down... is the basics of natural convection.
Else hot air balloons would no fly Grin
 
sahstim001

Quote

renatoa wrote:

Why would that happen?... heat always is rising, never going down... is the basics of natural convection.
Else hot air balloons would no fly Grin


My design is oriented with the fan above the heater. The air blows down then through a U into the RC
 
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