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-   -   Heating water through a tube (http://www.physicsforums.com/showthread.php?t=569801)

jonjmorrison Jan22-12 11:39 AM

Heating water through a tube
 
Hi all,

Sorry if this seems trivial but I think I may have managed to confuse myself and could do with some help.

I need to specify a tape heater power output for a small rig through which water will be heated to 200C. Having gone through the Q=m*cp*ΔT equation where:
m = 0.1658x10-3 kg/s
cp = 4186 J/kg K
ΔT = 180K

I come out with a power requirement of 1.25KW. This seems like an awful lot of power for such a tiny flow of water. Am I getting something wrong here or do I really need a 1.25KW heating element for this?

Thanks in advance for any help.

NascentOxygen Jan22-12 09:42 PM

Re: Heating water through a tube
 
200 degrees? Is this pressurized or something, because it will want to turn to steam at 100C if it's atmospheric pressure.

AlephZero Jan22-12 10:11 PM

Re: Heating water through a tube
 
My caculator gives a factor 10 smaller than yours. But "water at 200C" makes this an interesting question :smile:

jonjmorrison Jan23-12 08:37 AM

Re: Heating water through a tube
 
Yes the system will be fully pressurised to prevent any steam forming.

I don't think I phrased the question particularly well, it should probably have read:

"If I have a 0.5m long tube, how do I work out the power required to heat the water by 180C?"

Thanks for the help so far.

russ_watters Jan23-12 11:54 AM

Re: Heating water through a tube
 
That is way too high a pressure for you to be attempting to create. Please stop before you get yourself killed.

jonjmorrison Jan23-12 01:15 PM

Re: Heating water through a tube
 
Quote:

Quote by russ_watters (Post 3723731)
That is way too high a pressure for you to be attempting to create. Please stop before you get yourself killed.

This pressure is minimal by comparison to what I've working on for the past few years. We already have a piece of kit from the same company who will be building this rig that runs up to 110bar before the burst disc goes, the vessel itself is rated to 250 bar max safe limit and 350bar before failure. A vapour pressure of 15.5 bar is extremely small and, given the lab set up and my own experience of working with high pressure apparatus, poses a minimal danger.

The volume of liquid in use would also be very small, on the order of a few hundred cc's and with an appropriately set PRV and higher pressure burst disc assembly leading down an armoured tube to a dump tank, the most damage an explosive decompression could cause is to make me hit my head if I jump in surprise.

The pressure side of things is taken care of and I have a good bit of experience working safely with this kind of thing, the heating calculations are all that I have trouble with as I am not an engineer by training.

Cheers for the concern though!

Mech_Engineer Jan23-12 02:03 PM

Re: Heating water through a tube
 
Based on the numbers you provided your result should be 125W, 1.25kW must be a typo.

jonjmorrison Jan23-12 02:17 PM

Re: Heating water through a tube
 
Thanks - turns out I can't use MS calculator properly, how embarrassing.

Thanks for the help everyone!


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