Batch process- resistive heater overtemperature safety

[analogkid2digitalman]

Howdy:

We have a batch process that 'cooks' material in a chamber under high pressure and moderate temperature. The system was built in-house, it uses a few kW of resistive heater bands controlled by a PID unit. Well, the SSR that supplies power to the bands shorted closed, luckily someone was around and noticed the problem.

I was thinking of using a capillary thermostat, similar to the Thermtrol TS series. But I could not find adjustable units in the range of 125-350 degrees C. My thought was to have this set a two coil latching relay to drop AC mains via a contactor that supplies the bands. User would have to push a pushbotton to reset the latch.

Is this a reasonable approach to the issue?

Any ideas of methods typically employed for this scenario?

Is a capillry therostat a good match for the application? We would mount the capillary to the outside of the chamber.

Any leads of some units that operate in the 125-230C sensing range?

Thanks in advance for any feddback and information.

-AnalogKid2DigitalMan

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[analogkid2digitalman]

I missed it

Above should read 125-350C sensing range.

-AK2DM

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"It's the questions that drive us"
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[cranky108]

Have you though of using a thermocouple, and reading it with a transducer?

 

[danw2]

A mechanical temperature switch driving a latched relay to interlock the contactor will work.

The switch's sensor bulb should be IN the process in a thermowell, not strapped to the outside for the setpoint to be 'accurate'. If strapped to the outside of a vessel, the bulb will lose heat on its exposed surface making it trip at a higher vessel temperature than the switch setpoint. When the bulb is immersed in the process or a thermowell, the bulb heats evenly and only loses minimal heat through its capillary.

There are UL (US agency) approved high limit electronic temperature controllers that use thermocouple or RTD sensors to do exactly what you propose - latch off when tripped.

I've used those small digital temperature controllers with a latching relay to do limit control drop-out.

The last one was a Cal Controls unit, but I can't recall which model I used, one of the ones listed on this web page:

http://www.cal-controls.com/products/3300m.php

Maybe they all have a latching alarm relay?

 

[jraef]

There are specific Temperature Limit Alarm Relays that are designed for this exact application, it's not uncommon and in fact some critical equipment specifications require them. The Siemens 3RS10 comes to mind (because I used to work for them) but pretty much anyone in the temperature control business has something like that available.




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[roydm]

A thermocouple would be the usual way as Cranky108 suggests.
BTW an electric range (stove) oven thermostat covers that range nicely, cheap too.

Roy

 

[analogkid2digitalman]

Good Morning to all:

Thanks for the inputs, it looks like a TC with a separate temperature monitoring relay is the way to go. I was initially trying to keep it simple and not familiar with the devices typically used in the field today.

jraef, thanks for the link to the Siemens, it gives me a better lead on what to look for now.

danw2, the chamber is enclosed in a tie-up insulation blanket, but I do agree that the sensor should be in a thermowell for accurate readings.

Our current PID unit does not have latching alarm outputs, regardless I would like the limiting function to be an independant system.

Thanks again for your time and advice!

-AK2DM

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"It's the questions that drive us"
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[Mike2468]

A variation on using a latching, independent system is a "FM Approved High Limit". This is a similar to a PID controller, but the output is always a latching relay. Factory Mutual requires these for high temperature applications, such as industrial furnaces. You can google on "FM Approved High Limit" to see suppliers. Some research companies require that every PID loop have a FM High Limit next to it, in their laboratory installations.

 

[roydm]

It's also a good idea to shut down the heater if the indicated temperature is less than ambient. A reading below normal might indicate that the measuring circuit is not working. I have seen this happen when someone turned off the 24VDC power supply feeding a RTD/Transmitter. The control system indicated 0°C so the heater turned full on.
Roy

 

[analogkid2digitalman]

I went with the Siemens 3RS1140-1GW60 unit that jraef suggested. I've read the manual twice and am just beginning to figure out how to wire it up and program it. The manual is pretty cryptic, mainly all graphics, very little text.

-AK2DM

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"It's the questions that drive us"
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