Immersion Heater Question

  1. I have a closed 4" diameter cylindrical vessel with Volume=400in^3 filled with air at room temperature. A 3500W immersion heater with Watt Density=23.9W/in^2 will be used to heat the air. Can the air get up to 600 degrees Fahrenheit? How can I find out what temperature the air will get to?
     
  2. jcsd
  3. NascentOxygen

    Staff: Mentor

    Hi, and welcome to the Physics Forums. :smile:

    You have a closed volume to which you are constantly adding heat energy? is there any way for heat to be lost from this system?
     
  4. Well, heat can escape through the vessel walls. The vessel material is 316 stainless steel.
     
  5. Integral

    Integral 7,345
    Staff Emeritus
    Science Advisor
    Gold Member

    Emerson heaters are generally designed to work in water. You may burn out your heater if you try to use it in air.

    Computing final temperature of anything is non trivial given complete information. You have not given near enough info to get even a good guess.
     
  6. Chestermiller

    Staff: Mentor

    To estimate the rate of heat loss from the cylinder, you need to know the heat transfer coefficient on the outside of the cylinder, from the cylinder surface to the room air. This depends on the orientation of the cylinder and also on whether there are air currents in the room. You can do better if you insulate the cylinder.

    Chet
     
  7. From this video:



    I was able to get a convection heat transfer coefficient of 15 W/m^2*K.

    What can I do? I've attached a photo of some of my work.
     

    Attached Files:

    Last edited by a moderator: Sep 25, 2014
  8. I was told a watt density under 30W/in^2 shouldn't cause any problems with burnout
     
  9. Chestermiller

    Staff: Mentor

    The equation to work with is:

    hA(T-Tair)=Q

    where h is the convective heat transfer coefficient, T is the final temperature, A is the surface area of the cylinder, Tair is the room air temperature, and Q is the rate of heating supplied by the immersion heater (W/sec). This doesn't say anything about how long it will take for the system to reach that temperature. Make sure everything is in consistent units.

    Chet
     
  10. This is Newton's Law of cooling...based on my work from my photo attachment is this the Qout?
     
  11. Do you mean just Watts instead of W/sec? Units of Q are in Joules and then Qdot is J/sec
     
  12. NascentOxygen

    Staff: Mentor

    If the heater is valuable, you might consider first testing it with reduced voltages to the heater element, afterwards inspecting for signs of heat intolerance. A VARIAC would be handy for this, but make sure you are not supplying reduced voltage to any control circuitry associated with the heater.
     
  13. Chestermiller

    Staff: Mentor

    Yes.:redface:
     
  14. Chestermiller

    Staff: Mentor

    At steady state, the rate that the heater supplies heat is equal to the rate that heat is lost to the surrounding air.

    Chet
     
  15. I want to heat the air to 600F so there has to be some sort of energy accumulation. Wouldn't this mean the heat rate lost to air would be less than heat rate supplied by the heater? Therefore, not a steady state problem?
     
  16. NascentOxygen

    Staff: Mentor

    Sure! But it does mean it will be a steady state once the contents reach 600F! :wink:
     
  17. Chestermiller

    Staff: Mentor

    It depends. As the temperature rises, the rate of heat loss to the room increases until the system eventually reaches steady state. Do you want the 600F to be the final steady state temperature, or do you want to reach 600 F, and then have the possibility to get even higher if you continue supplying heat. The first thing to do is to see if the system can even reach 600F, or whether the steady state temperature would be below 600F, and you could never reach 600 F. So, the first thing to do is to determine what the steady state temperature would be.

    Chet
     
  18. Aha! thanks for all your help
     
  19. Remember that the immersion heater in a water tank will,probably be sitting at a max of 180 F (ish), if the element is in air at greater temperatures the wiring will reach the same temperature as the element so make sure your insulation and fittings are good for high temperatures, and or provide a way of cooling your wiring. I suspect that the element will burn out well before you reach your required temperature, but it's probably a good idea to check the data sheets for max operating temp. Please have your electrical safety devices checked prior to running this, it might be a good idea to check the max working temp of any fixtures and fittings you are using. BTW what are you trying to make/demonstrate?
     
  20. Thanks Jobrag for your input on keeping things working properly. We are seeing what temperatures our instruments can withstand if they were attached to a heated chamber.

    My goal is to achieve a uniform temperature of 600F on the outside of the metal chamber. Instead of air as the medium being heated, we decided on using sand to achieve a process temperature of 600F. We used three thermocouples one near the top portion on the outside of the chamber, one in the middle, and one on the bottom. The top thermocouple was hooked up to a temperature control that was set to 600F.

    The results were:
    Top Thermocouple- 600F
    Middle Thermocouple- 856F
    Bottom Thermocouple- 497F

    How can I make these temperatures more evenly distributed to 600F? What would you do?
     
  21. NascentOxygen

    Staff: Mentor

    Two smaller heaters in the chamber might come closer to giving a more even heat distribution. One nearer to the bottom.
     
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