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high power tubular heating elements

high power tubular heating elements

(OP)
Hi,

I am asked to manufacture a tubular heating element with very special features, while I don't have experience enough in the heating elements field.
The requirement for the element is as below:

1- Type: tubular heating element with a coiled heating filament (buried inside insulation).
2- Length: 1600 mm (straight), OD: 9.5 mm, shealth material: Stainless steel
3- Power: 100 KW,
4- Resistance: 1.7-1.9 Ohm
5- Medium: flow of a liquid with temperature of 600 degree of Celsius, (surface of the element will be kept at 600 C).

100 KW power for an element with these dimensions is too high and I have a lot of worries about the element overheating and melting. How can I solve this problem?
The question is that, is such an element possible to manufacture?
if yes, can I use usual materials such as Nichrome for the heating filament and Mgo for the insulation??
what is the suitable materials for the heating filament and insulation of such element?

Thank you very much
Vahid

RE: high power tubular heating elements

Hmm: a flowing liquid at 600 C? Is the flowrate infinite, such that the inlet and outlet temperatures are BOTH 600C? A liquid that flows past an incredibly hot, ridiculously high watt-density element and apparently does not decompose? And does not dissolve or otherwise react with the element?

Call me skeptical- it sounds like the person who generated this spec is fantasizing rather than designing a real product.

RE: high power tubular heating elements

In all heating elements, the heating tube surface skin temp is at a much higher temp than the bulk fluid temp at any location. The film temp at any location is notionally set at

Tf = 0.5* (Ts + Tb)

In most cases, there is a max permissible film temp that the fluid can be exposed to without risk of thermal decomposition, and we usually operate at a Tf which is say 10degC or so below this decomposition temp. Once you have obtained / estimated values for these, you can begin the design of the heating element and operate at a flowing velocity that will keep Tf at this working upper limit.

Also note that the max transient Tf and Ts the fluid will experience is when there is a sudden reduction in flowing velocity coincident with max electrical heating load - this is due to the thermal response lag time inherent in the thermowell which is part of the temp control loop which sets the electrical heating rate. This is relevant to the the setting of the tubeskin temp trip on these elements.

RE: high power tubular heating elements

Since you are not in water you will have a lower power density than those heaters.
Typically in hot oil the limit is about 20W/in2.
This would put you at about 15kW total.
So unless you can tolerate very high surface temps and have much higher heat transfer rates you are off by about an order of magnitude.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

RE: high power tubular heating elements

Quote (vahid.z)

flow of a liquid with temperature of 600 degree of Celsius, (surface of the element will be kept at 600 C).

Errrr, how exactly is there any heat transfer here if surface temp = fluid temp?

this idea, presumably given to you appears to be unfeasible and doesn't obey the laws of thermodynamics.

62 kW/ m from a 10mm diameter tube? Someone has an order of magnitude error going on here.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: high power tubular heating elements

(OP)
Thank you for your very beneficial answers,

The customer asks me to use high temperature materials such as Sic for the heating filament and inconel for the shealth (instead of stainless steel) and BN for the insulation.
But I found from your answers that this can result in a very high film temperature and can decompose the liquid.

what is the solution? usage of for example #10, 10KW elements instead of one 100 KW element? the customer says there isn't enough space to use more elements :(

I result the right solution is change of design (power reduction) or rejection of the order smile


RE: high power tubular heating elements

(OP)

Dear LittleInch

Thank you for your hint for the surface temperature. it was a mistake.
The medium temperature will be 600 C and the surface temperature is not controlled.

RE: high power tubular heating elements

Quote (vahid.z)

or rejection of the order

Sometimes that is a really good option.

If you still have that opportunity here, take it.

RE: high power tubular heating elements

Looks a bit like the client hasn't really thought this through. Can you increase surface area? Add fins, larger OD,? Where are your design limits?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: high power tubular heating elements

The application is fundamentally infeasible- the watt density is too high, by probably an order of magnitude. I can say that even if you won't tell us what liquid they are putting through this. The only liquid I'm aware of which may be possible in this application is a liquid metal, and even then you'd need a huge flowrate.

They need to make the unit bigger- longer or more tubes. Fins are not an option at these conditions. And even then, since the fluid in question is likely not a liquid metal, the whole thing is unlikely to work anyway.

The "spec" or requirement in this case is based on wishful thinking.



RE: high power tubular heating elements

Right now, key information missing is the minimum thermal decomposition temp of this liquid. Also ask if this fluid is clean and not fouling ( any suspended solids) that can potentially blanket heating tubes / fins.

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