Where is the warm air coming from? (thermodynamic kinda question)‏

Discussion in 'Other Builds' started by acs55812, Feb 8, 2010.

  1. acs55812

    acs55812 Fire Starter

    Ok so I discovered a "problem" with my converted commercial oven for a smoker.

    Last time I smoked cheese I blamed myself/or the heat controller/or just plain bad luck for the cheese overheating and melting (or partially melting) the cheese.

    I smoked some yesterday again and it heated to 97° when I had the heat controller set to 85° so this time I blamed the "cold" smoke generator for adding a small amount of heat that was too much.

    Today to experiment I disconnected the heat source entirely (unplugged the element right at the element), did not add any smoke, simply had the circulation fan running in the oven, started out at 61° and 3 hours later it was up to 97° again.

    It has 1/2 hp motor that is mounted outside the oven for the fan---with a 1/2 inch airgap between motor and oven, (makes sense that if the oven is at 350° or whatever it can't overheat the motor)---the label on the oven is marked 8 amps--about right for 1/2 hp.

    I have my thoughts where it coming from --- want to hear yours.
  2. bbally

    bbally Master of the Pit OTBS Member

    fan is hooked to the amature of the motor. Motor is running about 385 watts. Heat is sinking from the amature, through the shaft to the fan blades. Fan blades lose heat moving in the cooler inner air of the chamber.. allowing the shaft to sink more heat from the motor to the fan blades.
  3. acs55812

    acs55812 Fire Starter

    Interesting reply, and one I had not thought of.

    With your theory lets assume the motor is near 160° (it is quite warm to touch on the outside casing), with the shaft-fan, fan running at (85-90-97)°,,,,would the heat be drawn from the warmer surface towards the cooler surface (that being the fan blade),,,,therefore even more heat being transferred to inside of cabinet?

    Or would the cooler air in the cabinet be drawn out thru the fan/shaft towards the hotter surface (motor)?

    (I am envisioning this similar to warm air inside the house being drawn towards the colder outside air in the winter time)

    And the air in the garage where this sits is 60 so I am truely getting a 37 degree warmup inside the oven.
  4. bbally

    bbally Master of the Pit OTBS Member

    Thermodynamics states that heat always moves toward cold. Cold can not move as it is a lower energy level.

    The motor created heat... heat is a wavelength of light in the infrared spectrum. To heat up molecules start to move faster and faster. As they do they give off a wave of infrared light. That wave is accepted by the lower energy molecule next to it and it passes it off to a low energy molecule next to it. As the first molecule gives off a wave, its energy state goes lower allowing it to accept more energy from another molecule of higher energy.

    It is commonly called heat sinking. Heat moving to a lower temperature area by transfer of infrared energy.

    (I am a trained atomic spectroscopist with an electrical engineering background.)
  5. gnubee

    gnubee Master of the Pit OTBS Member SMF Premier Member

    Wow pretty good explanation. I will add my 2 cents worth.

    I used to be a qualified weather observer and an aviation briefer in the coast guard. The difference between calm air and air moving at 15 knots is about a 4 degree F higher temp. The friction of the air molecules bashing into each other is the reason for the temperature rise.
    To the layman and casual observer it seems that with the wind chill the temperature is cooler but the actual ambient temp rises 4 degrees.

    This is not nearly enough to account for all your increase in heat but may be a small contributing factor. It all adds up.

    I never could convince my Wife or her family that a fan does not cool the air but rather it raises the temperature a tiny bit. Plus the heat from the motor is added to the room.
  6. pignit

    pignit Master of the Pit OTBS Member SMF Premier Member

    I don't have the training or the words to explain it but if the temp is rising in your smoker and the fan is the only thing attached to the smoker that creates heat.... well, there ya go. Has to be coming from the fan.
  7. slareau

    slareau Fire Starter

    I like the fan (ha) acting as a dynamic heat sink theory. To summarize and modify some of the above points: Energy (heat) would be moving from the motor through the shaft to the blades of the fan and into the control volume (inside the oven). Zeroth law of themo tells us that this must be the direction of the flow of energy. Also, first law of thermo says that for the total internal energy of the control volume to be increasing we must be adding energy as the system is producing no work.

    Now to the heat sink part of the theory. As it was stated earlier, the work of the fan alone moving air is going to add energy albeit a small amount. The substantial amount of energy is coming from the forced convection over what can be thought of as heat sink.

    The other explanation is that the mere proximity of the hot motor is heating up the oven. Or, alternatively, it is a combination of both of those theories. If you wanted to really nerd out you could calculate the heat flux through the fan blades given the information you have access to: Tf, To, Q, size of the fins, material etc.

    To give a little background I'm a mechanical engineer by education.
  8. fourthwind

    fourthwind Master of the Pit OTBS Member SMF Premier Member

    While everyone agree's the fan motor is creating the heat, I am wondering why so much. Perhaps the motor has some high resitance issue's internally? Wouldnt expect an electric motor to be producing so much heat. Might get the factory specs and put an ohm meter to it to see if the thing is just getting worn out and working too hard. That being said my mind also wonders about too much internal friction. bearings wearing out? I used to see this in aquarium water pumps back when I serviced tanks as a young man.
  9. bbally

    bbally Master of the Pit OTBS Member

    Go with the mechanical guys....... they are smart.
  10. rambler

    rambler Smoke Blower SMF Premier Member

    I would be worried if the motor is operating higher than a 30 degree delta T over the ambient temperature. That would indicate problems with the motor (i.e. windings, bearings, dirt blocking air flow).
  11. acs55812

    acs55812 Fire Starter

    I am going with the theory that the fan is a heat sink to the 1/2 HP motor, the motor (and fan) spins freely by hand so I suspect no problem there.

    The unit is labeled 8.5 amps or close to that and that is pretty much the same consumption as the 1/2 HP motor (its a gas oven until I added the electric element which I unhooked for this experiment)

    I am using a cold smoke generator, wanted to use the circulation for large loads of food, which will still work for jerky and such, will have to see how well the smoke will circulate for cheese without the motor running.

    Thanks all
  12. fourthwind

    fourthwind Master of the Pit OTBS Member SMF Premier Member

    What about putting a variable potentiometer on the unit? I would expect it to reduce the airflow and heat being generated, but at least you would still have airflow.
  13. rambler

    rambler Smoke Blower SMF Premier Member

    Even if it turns free you should check the air flow through the motor.
  14. rambler

    rambler Smoke Blower SMF Premier Member

    Also check your fan to see that it is turning with the motor
  15. pit 4 brains

    pit 4 brains Master of the Pit Group Lead OTBS Member SMF Premier Member

    Build a belt-driven fan and isolate the motor...
  16. pit 4 brains

    pit 4 brains Master of the Pit Group Lead OTBS Member SMF Premier Member

    Or have a fan inside shafted to a fan outside. Now, put another fan so it blows on the fan outside of the smoker. The two will slowly couple with the moving air and spin the inner fan. Now any heat sinking will be from the inside- out. [​IMG]
  17. gnubee

    gnubee Master of the Pit OTBS Member SMF Premier Member

    Have a smaller fan blowing on the body of the bigger fan from a side angle. That Should cool the entire thing down. No mechanics or building need be involved. Even if you did that just for a test to see if it cures your problem.

    Everyone has a small fan around plus they are cheap to buy.
  18. duck killer 1

    duck killer 1 Smoking Fanatic SMF Premier Member

    is there a light on inside the oven? just curious.
  19. acs55812

    acs55812 Fire Starter

    nope no light
  20. sqwib

    sqwib Smoking Guru OTBS Member

    If the fan is blowing air into the oven, reverse it so it DRAWS the air from the oven.

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