Target temperature of nichrome heating element

[utimmer43]

newbie here, I've been searching many forums and cannot find the questions/answers I am looking for (close, but not quite). so, I am going to ask my question in a very specific, detailed manner.

i want to use nichrome wire to make 8 heating elements. they will each be a single strand running alongside the 8 injector lines of my diesel engine. their purpose is to heat the fuel (vegetable oil, in my case) in the lines.

i understand ohms law, but that only deals specifically with amps, volts, ohms, and watts.

my question is, is it possible to predict and achieve a target temperature range? in my case that range would be around 250* to 300*F. ultimately, my dilemma is particularly what gauge/resistance wire i should use, or if it makes no difference.

in other forums i have read about people doing this, and their target wattage is in the neighborhood of 80 to 100W. i guess a better way to ask the question would be... will a certain wattage (for sake of arguement, 100W) give the same temperature, regardless of the makeup of the wire itself?

i realize that the temperature would vary depending on the conditions surrounding the element, i.e. ambient temp, air flow, the heatsink effect from the fuel lines, etc. so, let's assume that the bare wire would be in open, still air at room temp. that should be a good starting point for me, i can probably adjust from there.

 

[Astronuc]

One would want to determine the resistance of each wire -
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/resis.html#c2

See also - http://hyperphysics.phy-astr.gsu.edu/hbase/electric/restmp.html#c3

and set the current so that I²R = the necessary wattage.

With 8 wires, one needs 10-12.25 W/wire for 80-100 W total, but I think one might need something like 1000 W total (like a hair dryer). That really depends on the rate of flow of diesel fuel to be heated, and the losses one assumes.

 

[Gokul43201]

In still air, the temperature will not be constant - it will increase almost logarithmically with time. You need to have a duty-cycle control to keep the temperature constant.

 

[utimmer43]

thanks for your replies guys.

i do understand how to achieve a particular wattage and amperage, and the wire comes labled with its resistance in ohms/foot.

ultimately, i need to decide what wire i want to get, as there is a huge variety. what i need to know is, will 1 foot of 2 ohm/foot wire act in the same manner (or at least close) as 2 feet of 1 ohm/foot wire? will one heat faster than the other?

even after serching and reading forums, and even posting my own questions, i do realize that in the end i will have to experiment with it to get it just the way i want it. but i would like to have a pretty good idea before i get to that point.

 

[Gokul43201]

ultimately, i need to decide what wire i want to get, as there is a huge variety. what i need to know is, will 1 foot of 2 ohm/foot wire act in the same manner (or at least close) as 2 feet of 1 ohm/foot wire? will one heat faster than the other?

It depends on the circuit you are using. Can you provide more details - include the circuit diagram and the heater geometry?

 

[Q_Goest]

Hi utimmer. In order for the heating element to come to some equilibrium temperature in the range you specify (and I'm assuming you want to allow the element to stay energized the entire time) then the heat created by running current through the wire must equal the heat dissipated. If you're putting this on a fuel line, you have a lot of variables, such as how much fuel is running through the line and what temperature it is. The fuel is removing heat. Heat is also being lost to the environment through conduction and convection.

In order to calculate the equilibrium temperature, one would need to calculate both heat losses so all of the parameters for those heat losses would be required such as type of fuel, flow rate, geometry of tubing being heated and what insulation is on it, etc... It's not an easy problem.

To make it an easy problem, you can put a temperature sensor on it that shuts the heat off when you reach a certain temperature. You still should do some calculations to determine how quickly it might heat up, but at least then you can be reasonably sure your final temperature is what you want it to be.

Note also that if you're creating some kind of fuel heating device, you really should comply with industry standards for hazardous electrical locations. In effect, you're putting a potential source of ignition into a flammable system. I'm not sure what standards the automotive industry uses, but any stationary installation would need to comply with NFPA and NEC codes for such installations.

 

[Panda]

I assume that you are taking a copper fuel line wrapped with thin electrical insulator then wrapped with heating coil which is over wrapped with thermal insulator.

Ignoring the effect of heat outside the insulated heating coils, You need to calculate the heat flow from the coil to the fuel. This will change with fuel flow rate.
1. Work out how much energy is required to raise a volume of your fuel from ambient to operating temp (Specific Heat Capacity*).
2. Work out the flow rate under normal conditions. This gives you power input.
3. Work out heat flow efficiency through electrical insulation and copper pipe. This will give you your input power.

You need to then calculate the nominal heat rise at low and high flow rates. If your input heat is too high you could vapour lock the fuel lines at idle speeds (common in petrol engines with poor shielding or fancy after market fuel lines), if too low then your not effectively heating the fuel at high rpm.

*Are you trying to run pure oil are are you adding a thinner. I know in the UK that if you use pure oil then you don't pay duty as soon as you add meths or similar to boost the fuel you have to pay the duty. More importantly though it may make calculating the SHC harder.

Heavy oil boilers run the fuel oil through the fire box to preheat. Although you can't do this with a diesel, could you loop the fuel lines around the manifold to use the waste heat to preheat your fuel. You would still need a preheat until the manifold reached operating temp but then you could turn it off. This makes your system not only simpler but also your vehicle more efficient which I assume is the point of running it on chip fat.

I'm a petrol head so I'm usually trying to keep heat out of the fuel.

 

[Q_Goest]

Some good info Panda.

Just a few more thoughts. Why do you want to make your own? They're already available and inexpensive, some come fitted with thermostats. If you're in the US or Canada, you can try McMaster Carr or any of the other industrial supply outfits like Grainger or MSC
http://www.mcmaster.com/
http://www.grainger.com/Grainger/wwg/start.shtml
http://www1.mscdirect.com/cgi/nnsrhm

Search on "heater", "heat", "heat tape", "heat strip", etc...

I suspect there are also packaged kits you can buy that are specifically for diesel engines. A little internet search might turn up quite a few pre-made systems that are already designed for automotive use.

 

[Koonism]

Why is it necessary to heat the fuel,( vegetable oil), to such high temperatures? Does it have to be at that temperature coming out of the injectors for proper combustion. I am a retired automation/Industrial controls engineer & I have had to solve many problems through the use of temperature controls. Viscosity changes takes place with variations in temperature and this has an effect on flow rates in oil based fluids. Perhaps heating the fuel in it's reservior with a non ignition source heating element might work better. Perhaps a 12 volt D.C. closed control loop. example ..a temperature sensor in the fuel tank to a controller with an adjustible temperature set point, and an output to a heating element heating a high temperature oil in a separate reservior that cirulates through a tubed coil in the fuel tank. Of course the fuel lines from the Fuel tank through the fuel pump to the injector ports would have to be insulated and both the fuel Tank and the high temp oil tank also would have to be insulated. You could achieve the disired results with a high level of safety. Eurotherm makes excellent small D.C. powered temperature controllers. Seems like a lot of expense to be able to burn Vegetable oil. But for a proto type system for testing..it can be done.

 

[Arun Jagga]

Could anyone point me in the right direction to keep a metal mold heated at a constant temperature using a heating element. I need to know what calculation would be required. According to this forum the temperature of a heating element will keep rising until the rate at which it is losing heat is the same as the rate at which it is gaining heat... from my understanding this would mean its power/heat capacity (j/s)/(j/K) should equal its heat loss (K/s) (i don't know wht physical measured property the heat loss is referring to but I am assuming it has something to do with conduction between the metal mold and the room temperature air surrounding it) also would i have to do a calculation between the heat element and the aluminum mold?...