Electrical math check please

[Badfish740]

So I'm in the process of designing a new filtration station for the waste vegetable oil that I burn in my truck. I'm going to be using an open top steel drum for heating the oil prior to filtering, because as you can imagine, trying to push vegetable oil through a filter at 6 degrees F doesn't work so well. The drum will have two heating elements (the kind that are normally used in hot water heaters) that will be immersed in the oil (I'm having a friend TIG weld bungs into the drum for them) in order to heat it up quickly on cold days for filtering. I'm going to have to run an extra circuit or two in my garage because all of the outlets are currently run off of one circuit (15A) which won't be enough. I just want to check my math on how much amperage the heaters will pull:

The elements are 5500W @ 240V

At 1/2 the voltage (120V) the wattage should quarter correct? That would make it 1375W.

1375W / 120V = 11.4A

If that's correct I'll need to run two separate 20A circuits into the garage in order to run both heaters at the same time. Can someone check my math and electrical theory so I don't burn the house down?

 

[oldspark]

Half is half, half the voltage will be half the watts.

 

[Badfish740]

Half is half, half the voltage will be half the watts.

Ok, now I'm confused-I've gotten this formula on the 'net from a few different places:

Power = Voltage applied divided by the resistance of the element. The resistance of the element can be determined by dividing the square of the rated voltage by the power rating. The net result is that at 120 volts (1/2 the voltage) a 240 volt element will only put out 1/4 of the power.

 

[oldspark]

Volts X amps = watts is what I am using.

 

[steam man]

The heaters should put out 1/4 the power at half the voltage. The resistance of the elements stays constant. I figured a 5500watt element at 240 volts to be 10.5ohms.

W=Vsqd/Resistance

@240 volts-(SQUARED), with 10.5 ohms you get 5500 watts.

@120 volts -(SQUARED), with 10.5 ohms resistance you now have 1371 watts of power.

Try http://www.csgnetwork.com/ohmslaw2.html for some quick calculations.

 

[oldspark]

I think we are talking about the same thing in a different manner, 11.4 amps is correct. I should have done the math before I posted.

 

[Badfish740]

I think we are talking about the same thing in a different manner, 11.4 amps is correct. I should have done the math before I posted.

Now I get it This place is the best!

 

[LittlePony]

A purely resistive 240 volt heater [baseboard unit] with no fan will operate when connected to a 120 volt source.

Either "hot" in the heater can be connected to the 120 volt grounded circuit conductor [neutral].

The wattage output will be only one quarter of the rated watts, so it won't get mad hot, but it will work.

A 2000 watt 240 volt baseboard heater unit [typically 8 foot] will only allow enough current to generate 500 watts at 120 volts.
If the heater has a fan, then it won't work, because the fan motor won't operate correctly at the lower voltage.


Read more: http://wiki.answers.com/Q/Can_you_s...e_neutral_branch_of_the_circuit#ixzz1CvcUin57

 

[btuser]

Why knock it down to 240v? Are you looking to slow it down?

 

[woodgeek]

1/4 power at half the voltage is correct--I assume you don't want to overheat the oil??

 

[Badfish740]

1/4 power at half the voltage is correct--I assume you don't want to overheat the oil??

Yeah-I've read about folks who have put 240V through these to heat oil and what ends up happening is the oil cokes around the element, causing it to fail prematurely. Running it on lower voltage (and lower wattage) causes the "watt density" to be less, which results in less intense heating. I'm basically copying someone else's design and they are able to get a cold (like cold as in unheated garage at 10 degrees F) 55 gallon barrel of oil to 100 degrees in about 30 minutes. That's quick enough for me.