Resistance For Electric Heaters Question

[leeave96]

I have an electric heating strip. Let's say the heater's watts is 1000. The voltage is 208. From W=(V^2)*R, I determine that the resistance for the heater is 43.26 ohms. Is this correct?

If I change the voltage to a higher or lower number, does the resistance on the heater stay constant?

So if the voltage went to 220 volts, and the resistance stayed constant at 43.26 ohms, then my watts would go up. W=(V^2)*r => (220^2)*43.26= 1118.7 watts.

Is the above correct? Is the resistance of a resistor (in this case a heating strip) always constant?

I know that some resistors change resistance with temperature, but I should think that is not the case here.

Thanks in advance!
Bill

 

[magoo2]

The resistance should be constant for a strip heater. Your results look OK but your equation for power (in watts) should be

P = (V^2) / R

 

[IRstuff]

Almost all metals increase resistance with temperature:

http://www.allaboutcircuits.com/vol_1/chpt_12/6.html

The effect is small, but nonzero.

TTFN

FAQ731-376

 

[Compositepro]

Electric heaters are not limited to a certain wattage but to a certain core or sheath temperature. If heat transfer is very good, as in immersion heaters, they can safely put out greater wattage. If they are heating air the safe wattage will be greatly reduced. At a certain elevated air temperature the safe wattage will be reduced to zero, meaning the air is already at the maximum allowed temperature of the heater.

Many heaters fail because dirt or other deposits build-up and cause the heater temperature to rise, although the watt output is the same.

 

[davidbeach]

That's one of the reasons why switchgear often uses 240V heaters connected to a 120V source. Output wattage is 25% of what it would be at 240V, but the surface temperature is much lower and the life of the heater is greatly extended. They can also be run continuously without worry of overheating.

 

[waross]

The last switchgear I installed, in a sub-station, had it both ways. There were conventional heaters on a thermostat for freeze protection. Then for condensation protection we had the series connected heaters energized 24/7. Even in the tropics I see a lot of heaters on half voltage for condensation protection. The problem with condensation is that it can form at temperatures well above the temperature that a thermostat would have turned off the regular heaters. Another problem is detecting and anticipating condensation conditions. The mass of iron in the rotor and stator of a generator will respond slowly to changing ambient conditions and a warm moist breeze coming in from the sea in the morning may have a dew point above the temperature of the slowly warming mass of iron. The solution is to keep a small amount of heat on whenever the machine is stopped. I have had to repair the half voltage heaters, they don't usually burn out but the terminations occasionally fail and splices sometimes fail. Another failure point is the relay that turns the heater off when the generator starts, if so equipped.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[telecomguy]

The heater will have a maximum operating voltage. I guess if you're running at 208V, it's probably rated for 240V so 220V would be ok. But don't try to operate outside the operating voltage.

We use heater strips to simulate thermal load in electronic equipment testing. We've had heaters burn up so we usually dial down the voltage to a maximum of 75% allowable and use multiple heater strips to create the appropriate heat load.

</tg>

 

[leeave96]

Thanks everyone! Sorry about the typo on the power equation too!

Bill