Temperature control of a semi-sealed vessel with a heat gun

[Howard]

Planning on making a sort of oven using a heat gun. I will have an RTD or thermistor to measure the temperature, and either a relay or a triac to modulate the output from a heat gun ducted into the vessel.

I would like to stay with the relay if at all possible, but I worry about the frequent cycling of the heating element and fan. The oven would need to stay on for up to 24 hours and it would of course be ideal if the heat gun could last indefinitely.

To reduce the effect of the cycling, I could instead use a triac, but I do not know how this would affect the operation of the heat gun's integrated fan. Also, the complexity of the (DIY) project would increase significantly.

Thoughts?

 

[MongGrel]

Oven for what and what vessel ?

Sounds like you're trying to reinvent the wheel a bit.

 

[Howard]

Building my own BBQ smoker.

Heat gun preferred over a hot plate because I don't have to worry about the power cord overheating.

 

[CycloWizard]

There are a few ways to do this and it really depends on how sophisticated you want to get. I would use on/off control rather than PWM. This will reduce wear and tear and won't maintain the temperature exactly but should be good enough for the purpose. You could just set it to kick on when the temp is 1-2° below the setpoint and shut off 1-2° above to prevent rapid cycling. An appropriate AC relay will do the job no problem.

 

[Howard]

Think I found a solution to the problem. I can separate the controls for the heating element and the fan. I can run the fan constantly and have the relay control just the heating element.

 

[Eli]

Interesting project.

My only concern is the longevity of the heat gun. They aren't designed to blow into something that is already hot; the heating elements will run hotter than normal, but that may or may not be a problem?

Use a good quality heat gun. We've gone through a lot around here - Milwaukee makes a good one. Anything under $50 is garbage.

I would definitely use the triac instead of a relay. I assume you'll be using a microcontroller to monitor temperature?

http://playground.arduino.cc/Main/ACPhaseControl

Should be pretty easy, and you'll have much better temperature control and better reliability than a mechanical relay.

 

[Rakehellion]

Building my own BBQ smoker.

Your heat gun won't last two pork shoulders once the moisture and oil get to it.

I wouldn't use anything that isn't approved for outdoor use. The hotplate might be a better idea since you can completely separate it from the oven using a piece of metal

 

[sm625]

You need some way of controlling hysteresis, usually done by a deadband control. It is going to cost some minimal amount of dollars to implement that. For not much more, you can buy an off the shelf solution, like a Action Instruments G118 which can be found for around $50. I dont know if the relay in that module is good enough, but if not you can easily use it to drive a larger relay. You also get a decent break detection circuit when you use an instrument like this, which is nice because you want to make sure the heat gets shut off if there is a wire break in the path of the temperature sensor.

 

[Eli]

But what's the fun in that?

Some PWM(or rather, phase control) and PID action with an Arduino should be just the ticket.

 

[bryanl]

I also would not trust a heat gun to last. Get a convection oven and modify it. A relay or mechanical thermostat will stress the heating element more than any solid state temperature regulation will, including a triac. Zero-crossing control of the triac will prevent AC line noise. Use wiring and connectors rated for heater use; solder and ordinary insulation will melt.

 

[Howard]

The heat gun would not be placed inside the oven. The heat gun would be connected to the chamber either through a flex hose or a pipe. The fan would be running 100% of the time so the temperatures would not be excessive.

 

[bryanl]

I still would not trust a heat gun for use over extended periods. You need a heat source rated for a duty cycle of 100%.

 

[Eli]

I am sure any good heatgun is rated for 100% dutycycle. We use them all day long here at the shop.

You'll need to put the outlet of the heatgun as close to the chamber as possible to minimize heat loss.

I would probably take all the plastic bits off the heat gun. What you end up with then is a steel tube with heating elements in it, and a fan on one end.

If you just want to play around, you can often find heat guns at Harbor Freight on sale for $8. They will last anywhere from 10 minutes to a couple of months. After proclaiming that I'll just exchange them when they die - after 2-3 died, I just ended up buying a Milwaukee. We also have a DeWalt adjustable unit that sees a fair bit of use.

 

[herm0016]

I question your heat source wattage... will that be enough to mantain the temp you want?
If you have the fan running all the time, you will be blowing cold air into the system all the time, increaseing the cycleing of the heating ellement and decreasing the stability of the tempature.
I really can't imagine the motor being more than a dc motor with diodes for the fan, and even if not, why do you think the fan will not last with constant switching on/off? heat guns will not last if they are blowing into a box that has no exhaust. You will decrease air Velocity and increase the tempatures in the heating ellement dramaticly.

why not buy a cheap toaster oven with heating ellements and a temp control, and take it apart and use that.

 

[bryanl]

I am sure any good heatgun is rated for 100% dutycycle. We use them all day long here at the shop.

You'll need to put the outlet of the heatgun as close to the chamber as possible to minimize heat loss.

I would probably take all the plastic bits off the heat gun. What you end up with then is a steel tube with heating elements in it, and a fan on one end.

If you just want to play around, you can often find heat guns at Harbor Freight on sale for $8. They will last anywhere from 10 minutes to a couple of months. After proclaiming that I'll just exchange them when they die - after 2-3 died, I just ended up buying a Milwaukee. We also have a DeWalt adjustable unit that sees a fair bit of use.

I would definitely avoid the Harbor Freight. It uses a 5 amp diode in a circuit that draws 12A.

 

[Howard]

I question your heat source wattage... will that be enough to mantain the temp you want?
If you have the fan running all the time, you will be blowing cold air into the system all the time, increaseing the cycleing of the heating ellement and decreasing the stability of the tempature.
I really can't imagine the motor being more than a dc motor with diodes for the fan, and even if not, why do you think the fan will not last with constant switching on/off? heat guns will not last if they are blowing into a box that has no exhaust. You will decrease air Velocity and increase the tempatures in the heating ellement dramaticly.

why not buy a cheap toaster oven with heating ellements and a temp control, and take it apart and use that.

My calculations show anywhere from 1 kW to 2.5 kW depending on the amount of insulation and airflow.

So basically I've given up on this idea, unfortunately.