Boiler Pressure

[At just flattened at!]

Hello all,

 

I humbly ask for some assistance from someone who's more capable when it comes to science, I'm a work with my hands type of chap and the science behind things takes a bit of grasping for me to understand sometimes. So here is what I'm trying to do and where I would very much appreciate some help:

 

I am building a steam boiler:

 

It will be made of stainless steel and contain two heated tubes to boil water.

 

These are the dimensions: W 25cm H 25cm D 10cm So a cuboid with two pipes running the 25cm length.

 

The heated pipes are 800F or 427C, without the water/steam touching them.

 

 

It will have a display to show the internal pressure, a blowoff valve to relieve pressure should it build too high and an outlet for the pressurised steam to be released.

 

 

 

I would like to know what pressure the outlet could output constantly, or would the boiler have to build pressure and then release it? . And how to calculate the output pressure?

What would happen if the output hole was reduced in size?

 

 

If anyone could work out something to show pressures that would be great, and even better if you could show me how you came to the end result. I have difficulty with equations but could understand it if all aspects were explained.

 

Thankyou in advance.

 

Alex

[InigoMontoya]

Not enough information to answer your questions.

 

At 800 F you should be above the critical temperature for water so you'll have dry steam regardless of pressure. You can have pretty much whatever pressure you want. However, that's somewhat meaningless without knowing other details.

 

How much energy are you putting into the system? Is it to be throttleable? What is your desired mass flow of steam?

 

Not enough information to answer your questions.

 

At 800 F you should be above the critical temperature for water so you'll have dry steam regardless of pressure. You can have pretty much whatever pressure you want. However, that's somewhat meaningless without knowing other details.

 

How much energy are you putting into the system? Is it to be throttleable? What is your desired mass flow of steam?

 

Not enough information to answer your questions.

 

At 800 F you should be above the critical temperature for water so you'll have dry steam regardless of pressure. You can have pretty much whatever pressure you want. However, that's somewhat meaningless without knowing other details.

 

How much energy are you putting into the system? Is it to be throttleable? What is your desired mass flow of steam?

[At just flattened at!]

It needs to be a constant flow. So not throttle-able if thats what you mean.

 

Desired mass flow is: 60-85 CFM

 

 

Im a little unsure of what you mean by how much energy am I putting into the system. Do you mean the energy created by the heat?

 

 

Thankyou for the reply.

[At just flattened at!]

This video shows a small boiler putting out a decent jet of steam, what I would like to know is how to calculate the volume of steam I could output and the constant pressure at which it was ejected. Ignoring everything above and: Taking the boiler as 1L in volume and a constant heat of 400C surely it is possible to calculate the maximum amount of steam produced per minute isen't it? In CFM.

 

 

www.youtube.com/watch?v=sz_Kgkzfaxk

Edited March 31, 2012 by At just flattened at!

[InigoMontoya]

Not without knowing your power source, thermal transfer coefficients, surface area, and other details. With a small oriface you could (in theory) generate an arbitarily small amount of steam or you could boil the whole damned lot in a second. Tweak one variable and everything changes.

[At just flattened at!]

Pm sent chap.

[CaptainPanic]

Make it a cylinder, not a cuboid. There is a reason that all pressure vessels are always cylinders or spheres.

[doG]

So a cuboid with...

 

 

Make it a cylinder, not a cuboid. There is a reason that all pressure vessels are always cylinders or spheres.

and not just a cylinder but a cylinder with spherical or parabolic end caps. See pressure vessel design...

 

 

editted out doubled post

Edited April 2, 2012 by doG

[At just flattened at!]

I see, ok so say the vessel is cylindrical with parabolic ends, I'll draw up a quick sketch so maybe we could calculate some outputs.

[paulhorth]

NO NO - STOP!!!!

 

I hope you realise that what you are proposing to do is highly dangerous and also illegal. If you build a steam boiler without knowing how to design and fabricate, you could easily kill yourself and anyone in the vicinity.

Any vessel built to withstand internal pressure MUST be properly designed and constructed, and inspected and tested by a competent person. That's the LAW (at least in the EU)as well as obvious engineering sense. Above a certain (small) size, in the EU you must comply with the Pressure Equipment Regulations (google it). The tests include a hydraulic pressure test and a steam test.

It's your responsibility to check the law elsewhere - but the engineering comments above, and following, are universally valid.

 

Welding stainless steel is difficult and requires profssional training. The boiler must have at the minimum a safety valve which can be demonstrated to release as much steam as can be raised, without the pressure rising more than 10% above the set pressure.

 

I write having constructed a model steam traction engine, including the boiler, which was made in copper, which is fired by coal and runs at 90 psig. This boiler is tested every year at my club and a certificate issued. Model steam engines of this kind are exempted from the Pressure Equipment Regulations if they are below the size threshold and because of the inspection and testing procedures defined and carried out by the clubs.

 

I am also a professional engineer and spend a lot of my time designing protection systems for pressure vessels. Making a pressure vessel is not a game.

 

Paul

Edited April 9, 2012 by paulhorth

[CaptainPanic]

paulhorth, thanks for an excellent post. I feel pretty bad that I hadn't written these warnings myself. I should have.

 

At least our do-it-yourself engineer included a relief valve.

[At just flattened at!]

Thank you for your words of wisdom, they are appreciated

 

I had not planned to weld nor build any part myself for the very reasons you outlined. The description of the boiler and details was for me at this stage not a concern as all the engineering aspects would be handled by a qualified person. The object of the posts was to ascertain if a boiler of such a size (even a very inefficient one) could produce what I needed. Unfortunately even if the boiler had produced enough steam I was stuck at another hurdle and that one I couldn't overcome.

 

Thankyou all for your help and if you arn't aware of it:

 

Google, 3D printing, its a clever thing.

 

All the best.

[paulhorth]

I'm relieved to hear that.

 

The first thing to decide when considering making a steam boiler is the PRESSURE you want to design it for. This will determine a lot of important features, not least, the cost. Then determine how much steam you want (in kg/h), look up the latent heat at your chosen pressure, that gives you the heat input rate required into the steam. Then decide on your fuel or means of heating.

With an electric boiler (like a kettle in your kitchen) the heat to steam translates directly into kW of power input. With a coal-fired boiler like on my model, it's not so simple because of efficiencies. Quite a lot of heat from the coal goes out the chimney with the flue gas, and not into the water. Similarly with a gas-fired boiler.

 

Then you need to consider how to keep the boiler filled. This is an essential safety feature in a fired boiler. You will need a pump and a water supply. You will need a level gauge to see the water level, and a relief valve (safety valve) to protect against overpressure. You will need to consider how the pressure is to be controlled. With electricity it's on-off. With gas it's turning the flame up and down like on your cooker. With coal, well there is no proper control at all, but you won't be doing it that way. With meths firing like on a Mamod steam toy, there is no real control either.

 

What's it for?

 

Paul