Designing a Heating Circuit

Discussion in 'The Projects Forum' started by kevin.khan, Jun 2, 2015.

  1. kevin.khan

    Thread Starter New Member

    Jul 9, 2012
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    0
    Hi there,

    I am currently working on a project that requires me to design a heating circuit that would be used to heat a stainless steel chamber area to temperature of +60C. The size of the chamber area is 52" x 36" x 82".

    I have never had to design a heating circuit before and I am looking for some suggestions/advice as to how to approach this project.

    Some things that would be helpful to know would be:
    1. The type of wire or other material used as a heating element.
    2. How to provide even heat distribution within the chamber
    3. How to incorporate a heat sink or prevent the heating element from burning the chamber
    4. How to achieve this desire temperature of +60C and prevent over heating

    It would also be nice if someone could provide an example of a heating circuit that could be applied to my particular project.

    Please let me know if further clarification is needed.
     
  2. KMoffett

    AAC Fanatic!

    Dec 19, 2007
    2,576
    231
    What is the project...what is the chamber used for? It often helps to know it's purpose as it can eliminate a lot of questions.

    Ken
     
  3. wmodavis

    Well-Known Member

    Oct 23, 2010
    737
    150
    You just need to heat up a stainless steel chamber or is there something in the chamber that you desire to heat?
    Does it matter how long it takes to heat the chamber along what ever is in it?
    Do you know what the ambient temperature is?
    To what precision do you need the temperature maintained?
    What power source is available for the heater?
     
  4. BR-549

    Well-Known Member

    Sep 22, 2013
    2,007
    395
    Is the chamber insulated?

    What is the environment out side the chamber?

    What is in the chamber?

    Does the chamber have flow?

    What is the chamber pressure?

    What power supply is available?

    As Ken said, how can we help you when we don't know what you are doing and what your doing it to?
     
  5. kevin.khan

    Thread Starter New Member

    Jul 9, 2012
    23
    0
    Hi there,

    Allow me to try and answer as many questions as I possible could here.

    The main goal of this project is to create a climate chamber. This would be used to test electronic devices (in my case medical devices) and stress test them under different environment conditions (hot and cold environment conditions in this case).

    The chamber is insulated (in fact I am using a old freezer as the chamber for my project).

    The chamber is located indoors, and the ambient temperature is around +23C.

    The heat within the chamber needs to be maintained and kept constant for a minimum of one hour duration.

    The precision of the temperature needs to be precise (-/+ 3C).

    I have two options for a power supply. I have a 220V, 15A power supply. Or I could use power from the main line (110V).

    I don't know whether there is flow inside the chamber or what is the current pressure.

    Let me know if there is any other information that I would need to provide.
     
  6. wmodavis

    Well-Known Member

    Oct 23, 2010
    737
    150
    So you are basically heating the air and device under test inside a freezer. Your power source is AC mains power - 120 or 240 VAC.
    You would likely need a fan to move the air inside to help keep the temperature uniform. The wattage of the heating element would determine how quickly the chamber could be heated. I might suggest as a place to start consider using a replacement electric range oven or stove top element as the heater. A higher wattage one will raise the temperature faster. Use a PID temperature controller and a thermocouple probe with a SSR to control power to the heater. If you are not adverse to using EBay search for "PID Temperature controller" and you will find some packages containing the controller, a K type thermocouple and a SSR that won't even break the bank.
     
  7. wayneh

    Expert

    Sep 9, 2010
    12,156
    3,063
    What you are doing is basically an incubator, just warmer. I'd use a light bulb or two for heat, controlled by a thermostat, and a fan or two that run continuously to circulate the air. Achieving ±3°C should be trivial. Better than ±1°C throughout the chamber would start to be tricky.

    Just get an off-the-shelf thermostat rated to handle the wattage of the light bulbs (or heating element, if you go that way),
     
  8. kevin.khan

    Thread Starter New Member

    Jul 9, 2012
    23
    0
    Hi wmodavis,

    I like your idea of using a PID temperature controller. Do you happen to know if there are temperature controllers that can control a heating circuit and cooling circuit with a timer function.

    What I would like to have is a temperature controller would run the heating circuit at +60C of one hour and then after that the heating function would stop, the cooling circuit would start up and run for another hour and then the system would shut off.
     
  9. wayneh

    Expert

    Sep 9, 2010
    12,156
    3,063
    There are lab timers built for this exact purpose. They're basically like the cheap lamp timers you use at Christmas, but more precise.

    PID control is huge overkill for your application. After all, the freezer as designed will hold to better than ±3°C and it did that without PID. Probably used a $5 thermostat.

    Airflow to avoid stratification is your bigger challenge.
     
  10. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Electric range-top elements are designed to warm solid objects; ie: pots and pans - using them to heat air could result in a short life, or even a fire hazard.
    Light bulbs do generate a lot of heat, particularly the heat-lamp type, but they are fragile and easily shattered, which then exposes the user to mains voltage.

    I'm thinking that a couple of hair dryers or heat guns might be a more appropriate option; they are designed to heat air and move it in the first place. There are also small ceramic room heaters that may be appropriate as well - and they also have built-in fans. You will need to ensure that the power cord temp ratings are not exceeded, or you may melt them. You may have to remove the original supply cord(s) and replace them with wire rated for at least 150°C.

    It may be difficult to avoid overshooting your desired temperature when starting up, using just a standard thermostat.
     
  11. wayneh

    Expert

    Sep 9, 2010
    12,156
    3,063
    Good suggestions. The more purpose-built stuff you can use, the better. You might even be able to take advantage of the less-than-full-on settings of these devices, although this would require a fancier control scheme.
     
  12. kevin.khan

    Thread Starter New Member

    Jul 9, 2012
    23
    0
    One thing I forgot to mention is that I need to heat the space of my chamber (The size of the chamber area is 52" x 36" x 82") to +60C in 30 minutes or less. Do you guys think a space heater or heat gun would still be able to achieve this.

    Also could someone please briefly explain to me the concept of stratification.
     
  13. wayneh

    Expert

    Sep 9, 2010
    12,156
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    If the inside of the chamber is mostly air (low thermal mass) and the walls are insulated, a 1500W hair dryer or space heater may well do the trick. Easy enough to test.

    Stratification is simply heat rising - the small density difference due to temperature exhibited by fluids like air and water. Without a fan, the top of the chamber will be significantly warmer than the bottom.
     
  14. #12

    Expert

    Nov 30, 2010
    16,355
    6,852
    I estimate 60 watts for 30 minutes. Stick a 60 watt light bulb and a thermometer in there. Close the door on the power cord and check the temperature every ten minutes.

    When you add your load, the time required will increase, therefore you will need to increase the watts.
     
  15. wayneh

    Expert

    Sep 9, 2010
    12,156
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    Just passing by, #12?

    I have nothing but gut to go on, but 60W seems light. (A pun?) The TS mentioned stainless steel walls, so I don't think they will warm to 60°C that easily.

    But really, the answer is to run the test. One profile of temp versus time with a known wattage will tell all.
     
  16. #12

    Expert

    Nov 30, 2010
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    Meh. It's thermodynamics and a dead freezer, two things I am good at, so I caught this while exploring the orangeness.

    Obviously, I made an educated guess, based on the idea that most refrigerators have the stainless steel on the outside. It's better to be wrong on the low side because a stuck light switch has melted many plastic interiors. :eek:

    You got the bottom line correct: One experimental result is worth a hundred guesses. I merely pointed to a safe way to do the experiment. The slope of that line will tell everything except the heat cost of the machinery that needs to be baked.
     
  17. kevin.khan

    Thread Starter New Member

    Jul 9, 2012
    23
    0
    So I have another question or I should say I would like some guidance.

    How would I determine the length of time it would take for a heating circuit to reach the appropriate of +60C. I just want to know the factors that I would need to consider to figure this out.
     
  18. #12

    Expert

    Nov 30, 2010
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    6,852
    Do you mean the unknown size of heater heating the unknown substance lining the cavity and the unknown things you want to bake? What are the specific heat capacities of each substance and the conductivity of each one to the outside of the box?
    Is it too much trouble to plug in a light bulb and read a thermometer?
    The science is called, "thermodynamics". I spent 6 months in that class.
    You can get the answer you need for a practical heater with one experiment.

    Make an effort to help yourself!

    Either that or go enroll in school. You've been asked for all the relevant parameters, and you can't provide them. You haven't said whether the stainless steel is inside or outside the box. You can't say what the insulation factor is for the box and door gasket combined. We can't either. One simple measurement is all we ask. From that we can work backwards through layer after layer of education..or would you rather have us type the pages of the books we studied into this thread?
     
    Last edited: Jun 18, 2015
    Reloadron likes this.
  19. Reloadron

    Active Member

    Jan 15, 2015
    969
    235
    What you appear to be building is commonly known as an environmental chamber or environmental test chamber and names like Tenny and Thermotron come to mind as manufacturers of commercial units. If you look at the insides as a box with heat having the heat uniform is known as temperature uniformity. It's like dividing the air in your box into zones and each zone is the same temperature (and humidity) as the zone beside it. I didn't notice if you had humidity concerns as most environmental chambers allow for temperature and humidity control. If any vibration concerns figure in you can call it Shake & Bake. :)
    As mentioned there is PID types of control depending on exactly what you want to do. Also as mentioned you can be concerned with the temperature of the atmosphere in the box or the temperature of the product(s) under test. Timing can involve for example Ramp and Soak another popular term in controllers. For example I put my widgets in my chamber and ramp up to 500 F. I can control my ramp speed. Now when my parts temp is 500 F I can start my timing or when the atmosphere temp reaches 500 F I can begin my soak timing, which do you want?

    Here is what I suggest you do starting with choosing the heating elements. Hundreds of companies in the US sell heating elements including globally. Watlow comes to mind. They offer great engineering support. If you want temperature uniformity in your box then moving forced air is a good way to go. Their engineers will suggest heating element styles and designs. They will also suggest controller methods and designs tailored for your needs. Another popular distributor is Omega Engineering for heaters and controllers.

    On a home brew device as you speak of if testing medical equipment is there any requirements to maintain records? What are the temperature accuracy requirements? Temperature uniformity requirements? Additionally if using forced air you need to make sure the air flow is present removing heat from the heating elements because a heating element can self destruct if producing heat that is not going anywhere. You also need to consider over temp protection so if for any reason heaters are not turned off and controlled they have a means to shut down at an over temperature set point.

    You really need to sit down with a blank sheet of paper and well define all of your needs. Normally used ovens and chambers can be had reasonable from surplus industrial supply houses. Sometimes it's cheaper to buy than attempt to build. While I doubt you need an environmental chamber you may find for example a used Blue M oven in some industrial surplus place that meets or can be modified to meet your needs.

    Ron
     
  20. #12

    Expert

    Nov 30, 2010
    16,355
    6,852
    That was a very nice way to say, "Go buy one". :)
    Still, I fear that would require some effort.
    I'm going to mark this one as a pie in the sky fantasy that will never get built. :(
     
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