Help to determine convection coeficent or temperatures

Click For Summary

Discussion Overview

The discussion revolves around the design of a flash steam boiler, specifically focusing on the evaporation of water at high pressure and temperature conditions. Participants explore the necessary calculations for heat transfer, convection coefficients, and the thermal dynamics involved in the phase change of water within a piping system.

Discussion Character

  • Homework-related
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • The original poster seeks guidance on determining the surface temperature of pipes during the phase change of water, questioning whether the pipe temperature remains constant.
  • Some participants suggest that the design resembles a flash steam boiler and recommend researching related systems such as water tube boilers and steam superheaters.
  • Concerns are raised about the practicality of using a single straight pipe for the boiler design, with suggestions that real systems typically use coiled tubing for efficiency.
  • The original poster clarifies that their design involves five tubes and aims for a specific heat flux to ensure effective evaporation, while also needing to calculate the mass flow of combustion gases.
  • There is mention of the limitations of available literature, which often pertains to more complex boiler systems, and the original poster expresses the need for information tailored to their simplified design.
  • A request is made for a drawing of the system to facilitate better understanding among participants.

Areas of Agreement / Disagreement

Participants express differing views on the feasibility and design of the proposed boiler system. While some agree on the classification of the system as a flash steam boiler, others question the practicality of the design and the assumptions made by the original poster. The discussion remains unresolved regarding the specifics of the design and calculations needed.

Contextual Notes

The discussion highlights limitations in the original poster's assumptions about the system's operation, the dependency on specific design parameters, and the challenges posed by the high-pressure conditions. There is also an acknowledgment of the need for further research into applicable literature.

Vincenzo

Homework Statement



This is more like a design problem, I'm to evaporate water at 20°C and 2000 psi(Tsat=335.472°C), I have the heat flux the water is going to absorb during heating, and If that flux remains constant during all the length then, how can I find the surface temperature for the part where the water change phases?

Right now I have the conditions during heating; for the heat flux constant, but when steam appears I don't know how to proceed, what eqs should I use? Does the pipe reamins with a T=constant? Reasoning tells me that the pipe must always be at the same temperature along his lenght.

See, The pipes abosrb heat through turbulent external forced convection, and the water absorbs the heat through internal forced convection. And ultimatly I need to find the temperatures of the hot gas that is going to serve as the heat source. Any Ideas? Was I clear? I am so confused right now...sorry.

Homework Equations



Q'=Cp*m'*ΔT
Q'=h*As*ΔT
q'=h*ΔT
q=hfg

The Attempt at a Solution



Since I know all the heat transfer I need through all the pipe, I was thinking, that h=q'*ΔT, where I will force the Temperatures and see how much mass flow I need. I did a quick calculation and I got 8 kg/s of hot gas (combustion products) that starts at Ti=1227 °C and ends at To=350°C but i don't think this approach is correct, also it seems a lot ok kg/s of hot gas.

Any Ideas?

thanks!
 
Physics news on Phys.org
What you are describing is a flash steam boiler . There is extensive literature available about design of these .

Search on :

Water tube boiler .
Monotube steam generator .
Flash steam boiler .
Steam superheater .

I've never heard of a flash steam boiler with a single straight pipe though - real ones always have the tubing coiled up in some way so as to make the boiler compact and efficient . A single straight pipe would have to be hundreds of metres long which is clearly impractical .

Please confirm that this design work is just for your personal interest or some academic exercise . If you actually intend to manufacture steam generating equipment of any kind then I'll have to ask the mentors to close the thread on the grounds that it would be a far too dangerous a project to discuss on PF .
 
Last edited:
  • Like
Likes   Reactions: Vincenzo
Nidum said:
What you are describing is a flash steam boiler . There is extensive literature available about design of these .
...
I've never heard of a flash steam boiler with a single straight pipe though - real ones always have the tubing coiled up in some way so as to make the boiler compact and efficient . A single straight pipe would have to be hundreds of metres long which is clearly impractical .
.

Well the amount of steam that will produce is low, but it uses 5 tubes, around 13 m long. The idea was to have water injected through them and have a heated and enclosed ambient in which flows combustion gases to heat the pipes and evaporate the water that flows inside it. The thing is I need to find up the temperatures and the mass flow of those combustion products to ensure that the process happens.

It's no commercial or something similar, is a proposal of design for my thesis to receive my degree as mechanical engineer. It should work this design, I just need to get a quality higher than 80% as a goal, but I need to ensure that is possible, to get that heat flux to the water and that the pipes arent going to melt during operation. Of course I'm simplifiying the design as it isn't adaptable to different sizes I give because it will mean a recalculation of all parameters I have already, it will not aboard external operating parameters, nor water threatment, nor instrumentation and control, just to ensure that it is possible to get this proposal to work.

The thing about lierature that I have avaliable is that most things are correlations to boilers which have drums, uses more tubes, have reheaters and many more accesories, and have lower pressure based operations while I just have 5 pipes that get water and one that throws Combustion gases around those pipes, all inclosed in another pipe, also I would have to propose a combustible in a fluid phase. Still, I'll do a search for information again.

Thanks!
 
Please post a drawing of the system so that we can properly understand what we are dealing with .
 

Similar threads

Transient Energy Balance for Heating a Tank of Water
  • · Replies 4 ·
Replies
4
Views
3K
Flow inside pipe, heat transfer
  • · Replies 15 ·
Replies
15
Views
3K
Heat Transfer - Conduction/Convection Question - Which area?
  • · Replies 3 ·
Replies
3
Views
2K
What is the preheat temperature for the water entering the gas boiler?
  • · Replies 10 ·
Replies
10
Views
3K
Steam boiler efficiency calculations
  • · Replies 10 ·
Replies
10
Views
3K
1-D Heat Distribution in Long Cylinder
  • · Replies 1 ·
Replies
1
Views
2K
Finding thermal conductivity with conduction and convection.
  • · Replies 5 ·
Replies
5
Views
2K
Heat transfer- determine heat of chip using convection
  • · Replies 13 ·
Replies
13
Views
5K
Improving heat transfer in domestic hot water tank?
  • · Replies 15 ·
Replies
15
Views
4K
Simplifying the Natural Convection Heat Transfer Correlation
  • · Replies 8 ·
Replies
8
Views
3K