How much power is needed to heat water to 200C through a tube?

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Discussion Overview

The discussion revolves around determining the power output required for a tape heater to heat water to 200°C in a pressurized system. Participants explore the calculations involved in heating water, the implications of high temperatures and pressures, and the safety considerations associated with such a setup.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Homework-related

Main Points Raised

  • One participant calculates the power requirement using the equation Q=m*cp*ΔT and arrives at 1.25 kW, questioning if this is accurate for the given flow rate of water.
  • Another participant raises a concern about the feasibility of heating water to 200°C at atmospheric pressure, noting that water would turn to steam at 100°C.
  • A third participant suggests that the calculated power requirement may be an order of magnitude too high, indicating a potential error in the original calculation.
  • One participant clarifies that the system will be fully pressurized to prevent steam formation, which addresses the concern about heating water to 200°C.
  • Another participant warns against the dangers of creating high pressures, suggesting that the pressure being discussed is too high for safety.
  • A later reply emphasizes that the participant has experience with high-pressure systems and reassures others about the safety measures in place, while still expressing difficulty with the heating calculations.
  • One participant corrects the power requirement to 125 W, suggesting that the original figure of 1.25 kW was likely a typo.
  • The original poster acknowledges a mistake in using a calculator, indicating a misunderstanding rather than a fundamental error in the heating requirements.

Areas of Agreement / Disagreement

Participants express differing views on the power requirement, with some suggesting a much lower figure than initially calculated. Concerns about safety and pressure levels are also contested, with some participants emphasizing the risks while others assert their experience and safety measures. The discussion remains unresolved regarding the exact power requirement and the implications of the pressure involved.

Contextual Notes

There are unresolved assumptions regarding the calculations and the specific conditions under which the heating is to occur, including the effects of pressure on boiling points and the accuracy of the initial calculations.

jonjmorrison
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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.
 
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200 degrees? Is this pressurized or something, because it will want to turn to steam at 100C if it's atmospheric pressure.
 
My caculator gives a factor 10 smaller than yours. But "water at 200C" makes this an interesting question :smile:
 
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.
 
That is way too high a pressure for you to be attempting to create. Please stop before you get yourself killed.
 
russ_watters said:
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!
 
Based on the numbers you provided your result should be 125W, 1.25kW must be a typo.
 
Thanks - turns out I can't use MS calculator properly, how embarrassing.

Thanks for the help everyone!
 

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