Temperature Control Help

[Charlie Miller]

Hi, as part of my university project ive been tasked with keeping an enclosure of around 200x300x400 (LxWxH) at a constant temperature of around 80 degrees Celsius. I have been looking at doing this with a PID controllers, thermocouple and heating element of some kind. I am however pretty clueless with electronics etc (studying mechanical engineering).

Any information on PID controllers, temperature probes, types of heating elements would be great. Firstly, please forgive my naivety to all things electrical but ive been looking at a couple of controllers and have a few questions, what is the difference between a relay output and a SSR? One of the controllers states an output of 'Output Voltage:24-380V AC' does this mean that the output level can be controlled via the unit or will it still have to be transformed down to use a 12V heater for example. Another controller states it has 'one output', i have been given a small 12v fan heater unit that has four wires, red/black for the fan and brown/blue for the heater, would this controller only be able to control one of the variables, ie power to the heater or power to the fan?

With regards to heaters does anyone have any information on what sort would be best for heating the air in an area this size?

Any information would be greatly, greatly appreciated.

Thanks,
Charlie

 

[chopnhack]

Hi Charlie, does your mechanical engineering education include HVAC? I ask because this would seem more like homework help, for which the forum has an excellent section to cover this.

175 degrees F is fairly hot for ~1400 cu. inches. Do you also have to build the enclosure? The material the box is made out of will affect your heating/cooling characteristics. How accurate do you have to be? Will there be net exchanges of air to be concerned with or is it simply a fixed space with no air exchange? Will you have to put small fans to ensure proper heat distribution?

First figure out what energy will be required to raise the temperature of the box from room temp to your desired temperature. This will give you an idea of the minimum required wattage of your heaters. To come up with this figure consider using this equation:

Q = m*Cp*(T1-T2) was m= mass of air in box
Cp= specific heat cap of air
T1 and T2 are the final and initial temps of air.

I don't know what the equation would be for maintaining the constant temperature - you would have to account for insulative properties of the box, which you have not made mention of the material or thickness, etc. - air infiltration, out flow that would cause temperature changes, etc, etc.

I hope this is a good starting point, you may have already known all this, but you questioned heaters so I thought we should start here.

To my limited knowledge, a SSR stands for a solid state relay.

 

[hevans1944]

The days when mechanical engineers could remain clueless about electrical engineering are over. Today's engineers, no matter their specialization, must be able to understand systems. This requires a broad understanding of physics, chemistry, optics, computers and software, hydraulics, pneumatics, fluid mechanics, machines, and ... yes, electricity. Maybe not an in-depth understanding is necessary, but you must know enough to ask the right questions and determine if the answers you receive are reasonable. So it appears you are making that effort with your university project. Don't be afraid to seek the help of students in other disciplines to help solve your problems.

And BTW, welcome to Electronics Point. I am sure you will find someone here who can help you.

The advantage of a solid state relay (SSR) is they can be switched on and off much more rapidly than a mechanical relay. Some PID temperature controllers take advantage of this by modulating the duty cycle of an SSR to obtain a proportional power output. An excellent resource for learning all about temperature control and the heaters, controllers, temperature sensors, etc. used in industry is Omega Engineering. Try to get them to send you their full line of hardback technical manuals, or you can review their entire product line at their web site.

A few years ago I used Omega products to build a small electric furnace for melting aluminum. It was part of a test rig I built that alternately and repeatedly dipped coated steel rods into molten aluminum and then into a cooling water bath to test the thermal fatigue resistance of the coatings. It used solenoid valves to control pneumatic linear actuators to move a fixture that held the sample rod., A small microcontroller, which I learned to program, controlled everything. My training is as an electrical engineer, but I learned enough to get the job done with mostly off-the-shelf components. You can too.

The thermodynamic engineering required for your air-heated box is not trivial. If you re-circulate the air, which is a virtual necessity to obtain a uniform temperature profile inside the box, pay attention to the heat transferred to the fan(s) and bearings you use. It would be wise to thermally insulate the shafts and bearings from the heated air in the box. It will also help with your energy budget if you adequately insulate the box from conductive, convective, and radiation losses. Even then you may need to provide an outer box with forced ambient air cooling in between it and the heated inner box to avoid a burn hazard from touching the heated box.

Good luck with your project! Please visit again and upload pictures of what you come up with.

 

[ADRT]

Can you give model numbers for these controllers? Do you have the manuals for them? Normally controllers are designed to work with certain types of heating elements and temp. sensors. There are several different types out there. RTD's, thermocouples, thermistors, ... Your controllers may only work with one or the other of these, or may have to be converted to whatever the specific input the controller calls for.

If your going to use the 12volt heater your first controller will not work if it indeed is outputting 24-380 VAC. you will just cause damage to the heating element and/or controller. The controller with the single output could indeed be setup to control both the fan and the heating element, but I do not think that would be very efficient for what you are trying to do. I would power the fan separately, on continuously, and use the controller to control the heating element on/off.

Some more details about the controller's in question would be most helpful.