# Help to determine convection coeficent or temperatures

• Vincenzo
In summary, the conversation discusses designing a flash steam boiler to evaporate water at 20°C and 2000 psi, with a heat flux constant during heating. The pipes absorb heat through turbulent external forced convection and the water absorbs heat through internal forced convection. The ultimate goal is to find the temperatures and mass flow of the hot gas serving as the heat source. However, there is limited literature available on this specific design, as most literature focuses on boilers with drums and other accessories. The design is for personal interest and academic exercise, and not intended for commercial use.
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.

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!

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:
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 .

## 1. What is a convection coefficient?

A convection coefficient is a measure of the rate at which heat is transferred between a solid surface and a fluid, such as air or water, by convection. It is typically represented by the symbol "h" and has units of watts per square meter per degree Celsius (W/m2·°C).

## 2. How is the convection coefficient calculated?

The convection coefficient can be calculated using the following formula: h = (q / AΔT), where q is the heat transfer rate, A is the surface area, and ΔT is the temperature difference between the solid surface and the fluid. It can also be determined experimentally through thermal testing.

## 3. What factors can affect the convection coefficient?

The convection coefficient can be affected by several factors, including the properties of the fluid (such as density and viscosity), the properties of the solid surface (such as roughness and emissivity), and the flow conditions (such as velocity and turbulence).

## 4. How is the convection coefficient used in engineering and science?

The convection coefficient is an important parameter in many engineering and scientific applications, such as designing heat exchangers, predicting weather patterns, and analyzing thermal performance of buildings and equipment. It is also used in materials testing and research to understand and improve heat transfer processes.

## 5. How can temperatures be determined using the convection coefficient?

Temperatures can be determined using the convection coefficient by rearranging the formula h = (q / AΔT) to solve for ΔT. This allows for the calculation of the temperature difference between the solid surface and the fluid, which can then be used to determine the temperatures at different points in the system. It is important to note that the accuracy of this method relies on the accuracy of the convection coefficient calculation.

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