Cooling a resistor load at 6 to 10kW

Discussion in 'General Electronics Chat' started by stube40, Feb 24, 2010.

  1. stube40

    Thread Starter Member

    Feb 3, 2010
    I have some resistors to cool that I presume are going to get rather hot!!

    I am using 6x wirewound resistors in parallel to create a 3.75 Ohm load for a 150V source, hence generating 40A and 6kW. Later I may up the voltage to 200 so I need to spec the cooling for a max of 10kW. Each wirewound resistor is 28mm dia and 350mm long.

    I was thinking of a open perspex box filled with some oil (maybe olive oil). Then creating a custom lid for the box out of a thermally conductive material that is an electrical insulator. Then mounting the resistors on to the bottom side of the lid on long legs so that they are completely submerged. On the top side of the lid I would put a large heatsink and fan.

    The maximum length of time the power will flow through the load is 1 minute, so this will help reduce the overall specs.

    If anyone likes this idea then my next challenge is to spec the size of the box, amount of oil and the size of the heatsink and fan. This is the tricky bit obviously and I'm not really sure where to start.
  2. ifixit

    AAC Fanatic!

    Nov 20, 2008
    1. What is the rating for the wirewound resistors you are using?
    2. 150V AC or DC?
    3. What frequency if AC?
    Design Considerations...
    • If the surface area of the resistors is too small they will burn up, even in oil bath. Yours are likely big enough.
    • The wire of the resistor is the only part generating heat, the rest of the resistor is designed to take the heat away and pass it to the ambient surrounding, usually air, or a liquid. That needs to happen fast enough to prevent the resistors max operating temp from being exceeded. Likely 200°C. This ensures the resistance won't change too much. Check the spec of the resistor.
    • Warning! If the heat can't transfer fast enough to the oil a fire could ensue. Monitor your first tests with a thermalcouple on the resistor body. Start with less power at first.
    • Olive oil boils at 300°C. This is good... it's less than 200°C.
    • The specific heat capacity of olive oil is 1.97 kJ/kg °C
    • We can afford to raise the temp from room temp to 200C = 180°C.
    • That would req 355 kJ to heat a kg of oil to 200C from 20C.
    • 1 kJ is 1000W per second, therefore, is would take 355 seconds (5.9 minutes) to heat 1kg to 200C, or 59 seconds with 6kW of heat.
    • If you double the volume of oil the 59 seconds becomes 118 seconds, almost 2 minutes.
    • Agitate/circulate the oil well, in and through the resistors, to ensure effective heat transfer.
    • External heat sink and fan is unneccessary for a 1 minute test.
    I hope you get the idea of how to proceed.

    Good Luck
  3. SgtWookie


    Jul 17, 2007
    You need an engineer on site.

    This site is primarily intended for hobbyists. Your needs far exceed the hobbyist realm.

    I recommend that you hire an engineer, and that this thread be closed.
  4. eblc1388

    AAC Fanatic!

    Nov 28, 2008
    Seems like ifixit has covered nearly all angles. I just have two small comments to make.

    1. perspex can't stands 200°C, it has to be a metal tank, or a big wok.

    2. the olive oil used should be "extra virgin" for additional favor.
  5. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    1KW 'wirewound' resistors are readily available, I've used some recently while rebuilding the controller for a 3.5KW magnetic chuck.

    Six of those would give you your 6KW rating without any problems.

    Put a box or chimney around them and add a decent volume blower and they should be fine at 10KW for a while.

    There are no mountings supplied with them, I used lengths of 6mm threaded rod with stacks of oversize washers that fitted just in & over the ceramic core at either end, with double nuts at either end to keep the stacks of washers held tight.

    I fixed the complete assemblies in a metal enclosure using 'L' brackets (small shelf brackets), about 3" or 75mm per leg.

    You could use small 'C' rail or small angle iron to make a box frame and put two rows of three resistors across it, then add sides to contain the airflow if you need to add a blower.

    Example - 1000W resistor:
  6. stube40

    Thread Starter Member

    Feb 3, 2010
    Great advice guys, thanks for all the help.

    Sarge, point taken - I wont make any more posts.