watt density? 1

[dkm0038]

I am using an electric heating element to heat up some SAE 10 hydraulic oil. Someone warned me to be carefull about the watt density of heating element, that is if the liquid is too dense or viscous you can actually overheat and ruin the electric heating element.

does anyone have any experience with this problem? that can offer me some advice

Thank you

 

[itsmoked]

That is an issue with any heating element. The elements themselves will melt if they cannot get rid of their heat.

In some cases whatever is being heated has to be taken into account too. Like your suggestion.

If the media you are heating can only move past the heating element via convection then you will have a greatly reduced heating rate over the media being pumped past the element or mechanically agitated.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[dkm0038]

thanks itsmoked,

if i have a heating element and i know the watts, is the watt density the watts divided by the surface area of the heating element?

I have seen the watt density for stagnant oil be 15watts/in, Am I correct to assume that this is the maximum alowable watt density for the fluid befor you burn up your electric heating element?

 

[CharlieGill]

That was a very good question and Keith is correct in his comments. It will depend greatly (almost totally) on the flow characteristics past the heating element and the thermal characteristics of the medium a bit less less. In truth, when I have to spec something like this, I call a couple manufacturers or reps for the product and just give them the application and ask for their recommendation. They generally know their products and capabilities pretty well.
When I was young and knew I was smarter than everybody else, I did all the calculations. Now that I have learned that I will not live long enough to learn everything, I try to concentrate on learning how to ask good questions of those who know.
Charlie

 

[7anoter4]

The watt density is indeed the rated wattage divided by overall surface. For General Purpose hydraulic oil heater will be about 10 watts/square inch.
But I agree with itsmoked and CharlieGill that better you'll contact the manufacturer for your application. See for instance:

http://www.warrenelectriccorp.com/applications/hydraulic-oil-heaters/default.asp

 

[itsmoked]

Yes, yes, yes, on the watt density comments all around.

Remember you will have a watt density that a heater can handle and then you will have 'other' watt densities relating to the medium being heated.

That nice link of 7anoter4's notes 10 watts per sq inch as being about right for lube oil heating. Higher temps and the elements will start to cook the oil which plates onto the element. Then the element starts running hotter yet. Small less coated areas are then pushing much higher WSI and they cook on more oil faster. It all sea saws around until the element fails due to the heating element melting or oxidizing into an open.

So you always want as low a watt density as you can afford or have room for (sqin).

Keith Cress
kcress -

http://www.flaminsystems.com

 

[OperaHouse]

A little tip. To reduce the watt density, buy a heater that is twice the voltage you will use it on. That will reduce the wattage to one quarter of rating. The same thing will happen with two heaters in series. The cheapest heaters always seem to be the ones with higher wattage and the smallest package. This could open up your options.

 

[roydm]

Oil doesn't circulate as well as water does so placement of the thermostat is important. Don't put it too far from the element if the oil is not pump circulated.
If you are using an analog signal e.g. RTD and PLC dont forget to include loss of signal protection in the heater circuit.

 

[rawelk]

All the previously mentioned points sound good to me. If you have enough control system to play with, and preventing heater failure due to overheating is a major concern you may even consider two temperature feedback devices - one mounted on the 'high point' of the heater element itself (a location that would first be exposed if oil level dropped), and the other monitoring oil temperature.

Control would be done from the oil-immersed feedback device, and overtemp limiting with the heater-mounted device.

 

[roydm]

I once saw the lube system on a crusher boiling. Someone turned the instrument power supply off before going home. The signal from temperature transmitter dropped to zero mA so the PLC turned on the elements. Since that day I always program in a low temperature cutout i.e. < 4mA cuts off the heaters.