Comparing Heat transfer through metals

[goalieman01]

Hey everyone,

This is my first time posting and am unsure of how things work here. I am new to the engineering world and would like some help in solving two problems. I am comparing to separate parts; one made with Stainless steel (304) and one made with Cast Iron (Grade:65-45-12). My problem isn't real complicated but for some reason I am struggling.

Problem:

1.) The main difference between these two parts is the geometry I can machine the cast part a hair smaller but it is still roughly twice the thickness of the Stainless steel. I am looking ad placing an RTD sensor on one of the two but would like to know how to find out which will give me a more accurate reading of the fluid flowing in the pipe.

2.) same situation except I would like to know how to specify a heater (such as a band heater) to heat the fluid inside the pipe to 400 Deg. F.

Again any help would be great some dimensions are bellow.

Thickness SS - 13.5cm
CI - 26 cm
Length SS - 100cm
CI - 84 cm

Thanks for any help.

I will be looking in my old textbooks this weekend but thought this couldn't hurt.

 

[goalieman01]

Sorry another thing I would like find out is if I want the inside of my pipe to be 400 deg. F how hot must the heater be on the outside..

Diameter SS - 76 mm
CI - 100 mm

 

[goalieman01]

Sorry another mistake ~ all measurements are in mm NOT cm

 

[metengr]

Is the fluid flowing in the pipe as heat is applied?

 

[goalieman01]

The fluid is initially a solid (not flowing) and needs to be heated to 400 deg F before it can be moved. Once moving it will most likely stay moving for a most of the day then rest and cool over night. Another thing is how long it takes to heat the fluid to that temp but that would depend on the substance and its properties (unknown at this point). If you can guess I am dealing with a pump and trying to find optimal placements for heaters and temp sensors.

 

[metengr]

This will be a combination of heat convection and conduction. So, you need to set up an energy balance equation and work from there. You will need to know the fluid properties as indicated for the metals below

The thermal conductivity (k) for
304 ss at 200 deg C is 18 W/m.K

for CI

40 W/m.K

You will need to know the fluid density, density of ss is
7900 kg/m³ and for CI is 7272 kg/m³

You will need the heat capacity of the fluid, ss and CI

for ss 477 J/kg.K and for CI 420 J/kg.K

Thermal diffusivity

304 ss is 4 x 10^-6 m²/s
CI is 1.7 x 10 ^-5

Have fun

 

[Compositepro]

What you are doing is called "heat tracing". Go to the websites of heat tracing manufacturers and you will find information there. Chomalox and Watlow are two.

400 degree heat tracing is very challenging. After rereading your posts it almost sounds like you are working on a plasics extruder.

 

[unclesyd]

You didn't specify what type heater power to be used in your OP and two electrics were mentioned. Here is another approach using different types of heat transfer media.

There are several big caveats in defrosting lines especially at your specified temperature of 400F. You have to control the direction of the applied heat in defrosting the line, or simply you cannot start in the middle. You have to be careful about overshoot if your product is heat sensitive.
If the line is any size the process fluid will have to have a good thermal conductivity.


Can you comeback with the material in the line and some sizes of your piping sizes?

What are you going to use for heating media, electric,thermal fluids?

http://www.csiheat.com/Products/Bolt-On/ControTrace/ControTrace.htm

 

[MintJulep]

The fluid is initially a solid (not flowing)

There is a really big difference between a solid and a stagnant liquid.

Which do you have?

 

[goalieman01]

I appologize for taking so long to reply. I have been out of town and really busy and have not had time to get back to this topic. The material inside varies from Butyl rubber, Ethylene Vinyl Acetate and any type of sealant or adheasive that must be heated to be applied.

Currently we are using 2 - 750 watt band heaters on the outside of some cast iron, which is roughly 100mm in diameter and on average 21 in thick.

And RTD is placed on another part away from the heaters to measure the temperature. (poor design and looking to improve) It sits on a stainless steel part that is 70mm in dia. and 10.5 mm thick.

At the moment my ideas to increase the time to heat the fluid is to:
A. seal the top of the enclosure as to not create a bouyancy effect with the air around the heaters and pipe
and
B. to add insulating tape around the heaters and RTD