please help...please 3

[678wer]

Machine ; extruder(heated pipe): lenth of 60cm. Dia of 8cm.Pipe is Heated to 150C

Pipe material: iron

Product flowing thru: Dough (dough temp. at entry:16C)

flow rate: 280kg/hr

How do I calculate heat transfer from heated pipe to the dough, residence time of dough in the pipe, exit temp of dough?
Anyone can help?please

 

[FiltrationEngineer]

You need more information. You need the specific heat of the dough, and the dough viscosity to measure heat transfer. It would probably be easier to go out and actually measure the exit temp of the dough.

To measure residence time, you need the density of the dough.

 

[678wer]

Thanks for ur reply.

I can measure the density and exit temp of the dough.Can i measure temp. at different points in the pipe, using any formula. After that, how do i measure the
1)heat transfer rate,
2)residence time(this can be done by running it offcourse)
but can i measure it using any formula.

the main issue is the heat tranfer rate.Waiting for ur reply.

thank u

 

[packdad]

Well, if you know the mass flow rate, specific heat, and temperature difference of the dough, then the overall heat transfer rate is simply:

mass flow rate * specific heat * (T_out - T_in)

Right? But if the dough is very thick and has a low thermal diffusivity, then perhaps the measured values for T_out and, possibly, T_in make this an oversimplification since the heating might not be uniform. I don't know.

Thinking back to my days in school (it's been a while), it seems that calculating the temperature profile along the length of the dough in the heated pipe can be done using the transient heat-conduction equations. I don't recall a 1-D solution being too awful. But, again, if the diffusivity is low, this might become a more complex 2-D problem (complex for me, anyway..!).

 

[MikeHalloran]

The dough is being kneaded in the extruder, right? So the heat transfer is going to be pretty effective, since the extruder screw is continuously tearing the bulk dough apart and wiping it against the extruder barrel bore.

I'm not sure how to model that, but diffusivity shouldn't be a big factor in it.



Mike Halloran
Pembroke Pines, FL, USA

 

[678wer]

Thanks for the reply,....very helpful......
and yes...diffusivity is effective in the extruder because of the screws...how can i calculate the specific heat of the dough?

 

[678wer]

also is there a way to calculate the residence time from one point of the extruder to the other end using a formula?...I cannot do a test on it using a colored die and see how much time it takes because its not possile to dismantle the extra arrangement thats attached after the extruder.
What i know is the rpm of the screws, the feed rate in kg/hr, dia of extruder etc.
Waiting for ur reply.

 

[MikeHalloran]

Ah. A twin screw extruder.

I worked for Werner & Pfleiderer in Waldwick, NJ, briefly, many, many years ago.

Call them and ask.



Mike Halloran
Pembroke Pines, FL, USA

 

[RossABQ]

If you know the RPM of the screws and their pitch, doesn't that give your the velocity down the tube, and therefore the residence time?

 

[MikeHalloran]

It may not be that simple. W&P extruder screws are produced in unbelievably precisely made sections and stacked. The joints are barely visible. The cross sections for a given extruder size are the same, but they come in several pitches, including some with no pitch, and the application engineers juggle the stack configuration to adjust the bulk transport velocity, and residence time, as the mix travels through different zones of the barrel, which may be at different temperatures.



Mike Halloran
Pembroke Pines, FL, USA

 

[678wer]

if it was a uniform pitch screw and we know the rpm, we know the feed rate in kg/hr,dia of pipe, length of pipe, pitch ....how do i calculate the speed of material flowing thru it. I also know the density of the dough.I think there is a formula for it. Anyone knows?
thanks

 

[HEC]

678wer
why not try an actual measurement of transit time in the barrel. An injection of brine or a handfull of salt into the inlet would be the start condition. Use a conductivity probe on the outlet to detect a rise in salt concentration will give a fairly accurate indication of fastest transit time, leave the probe in place and look for the corresponding drop in conductivity, will be the slowest transit time. We have used this method for holding tube testing.
Regards

Mark Hutton

 

[Ashereng]

Hi Mike,

Yes, you are correct. In a twin screw extruder, the engineer can indeed change residence times in certain sections of the barrel by using different configurations.

Also, to consider, is that in certain sections, the purpose may be to generate heat, in which case it is not just heat transfer into the dough, but also heat generation caused by mechanical work.



678wer,

To determin temperature at various points in the extruder barrel, the best way is to measure it I would think. However, with a 60 cm extruder, 8 cm in diameter (that is fairly small), would there be that much difference in temperature? I don't have any experience with an extruder that small.

You mentioned that you are interested in heat transfer rate. I am just curious as to why. I would think that for dough, the concern would be absolute temperature rather than heat transfer rate.

Anyway, just curious.

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
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[vpl]

Asherang,

I think he would be interested in the heat transfer rate as it affects the "proof" time of the dough. The yeast used in dough is heat sensitive and will start the chemical reaction that increases the volume of the bread. Depending where in the process the extruder is, this might have considerable impact on the consistency of the dough coming out.

678wer

The "standard" way to determine velocity from flow is: (flow/(density * area). This will give you an answer in m/hr. Then you divide the length of the extruder by the velocity to find the stay time.

Unfortunately, your density is not constant; however you should be able to set some bounding conditions.
In a quick Google search, I found bread dough density ranges from 2.2 kg/m[sup]2[/sup] to 8 kg/m[sup]2[/sup]. For these values, I got a stay time between 8.5 seconds and 31 seconds.

Of course these stay times could be way off, as it depends on your actual dough density. Is this something you could measure -- even as before and after to set bounds?

Patricia Lougheed

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[Ashereng]

Hi vpl,

Thank you for the explanation. I just always thought temperature was more important rather than heat transfer rate (I guess related to burning my cookings and croissants). I did not realise the "proof" time is due to heat, and not temperature.

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?