Water Tube Boiler

[MechEng]

Guys,

I understand the basics of the water tube boiler. Pump water in, evaporates, superheated, goes to engine. However, what type of control is used to make water isn't being pumped in faster than it can evaporate? Anyone know a place I can get specifics on designing a water tube boiler?

FYI, this boiler will be a stationary boiler heated by wood.

 

[JorgensenSteam]

This is not really design information, but is some information on watertube boilers.

Pat J

http://www.classicsteamengineering.com/index.php?topic=131.0

http://www.classicsteamengineering.com/index.php?topic=131.0

Pat J

 

[Maryak]

Guys,

I understand the basics of the water tube boiler. Pump water in, evaporates, superheated, goes to engine. However, what type of control is used to make water isn't being pumped in faster than it can evaporate? Anyone know a place I can get specifics on designing a water tube boiler?

FYI, this boiler will be a stationary boiler heated by wood.

The simplest way is to throttle the water inflow using a SDNRV. One alternative is to use a float controller but this would be an awesome project in its own right at model sizes. If the supply is from a positive displacement pump then a relief must also be provided. Another is to use a steam injector and again manually operate it as required.

Water level fluctuations are far more rapid in Water Tube Boilers than tank boilers due to the much smaller volumes of water and much smaller amounts of reserve steam combined with much higher forcing rates.

Superheating is not a "normal" function of any type boiler. A superheater can be added to a boiler of any type.

Hope this helps.

Best Regards
Bob

 

[JorgensenSteam]

If you use a three-drum type water tube boiler arrangement, the drums should be able to hold sufficient water to keep the level from being too critical.

I believe the Doble style boiler was a flash type, and I have heard those can be tricky to control.

I have seen micro switch actuated pumps, but I would not rely on an electromagnetic device for boiler water level, since sooner or later that setup will fail.

Pat J

 

[MechEng]

Thanks for the information guys.

I am still new to boiler mechanical design. What is a SDNRV? Do you know any sites that explain the functioning of a float switch (I am already picturing something in my head but what to make sure there isn't an easier way)? Also, when you say the boiler doesn't super heat, you mean that function is done in a different set of pipes separate from the boiling pipes like the first picture on http://en.wikipedia.org/wiki/Water_tube_boiler ? Or that when running steam engines you use wet steam and superheating is only used at powerplants?

 

[Maryak]

Thanks for the information guys.

I am still new to boiler mechanical design. What is a SDNRV? Do you know any sites that explain the functioning of a float switch (I am already picturing something in my head but what to make sure there isn't an easier way)? Also, when you say the boiler doesn't super heat, you mean that function is done in a different set of pipes separate from the boiling pipes like the first picture on http://en.wikipedia.org/wiki/Water_tube_boiler ? Or that when running steam engines you use wet steam and superheating is only used at powerplants?

I'll try and answer as best I can:

SDNRV - Screw Down Non Return Valve.
On a boiler it's normally called a feed check valve, (because it controls or checks, as in reduces, the rate of feed water supply to the boiler.

Regulators come in several flavours from a simple float mechanism to 3 element controllers either air or electronically operated which measure drum water level, steam flow and feed water flow. e.g. when more steam is demanded the water level in the drum initially rises because the steam pressure initially drops, however that's not what we really want because a float control would say admit less water when in fact we need more water to make up for the excess steam demand and to allow for the fact that the firing rate is increased to keep up with the demand for more steam.

This is a bit esoteric for a model boiler and probably the most sophisticated would be a simple float regulator. Hand regulation of the feed check works satisfactorily for all but the most high powered boilers with fluctuating steam demands.

Yes, a superheater is another set of tubes placed in the flue passage after the steam has left the steam drum and after the heat from the fuel has done it's primary job of boiling the water.

Both reciprocating engines and steam turbines may use either saturated or superheated steam.

In condensing systems it is usual to use saturated steam in reciprocating engines because the water in the steam is used to lubricate the rings and cylinders and obviates the need to try and remove lube oil from the feed water before it goes back to the boiler.

For non condensing engines and most, (very nearly all), turbines, superheating is used because it's extracting more heat from the fuel giving greater power/efficiency and there is no water present to belt the hell out of the turbine blades.

Another very potted explanation but I hope it helps.

Best Regards
Bob

 

[JorgensenSteam]

Mech-

A word of caution about building your own boiler.
If you are not really familiar with boiler construction techniques, materials, operation, saftey, etc, then building and operating your own boiler can be a quick way to exit the earth.

There are many "no-no's" with boilers and boiler operation.

If you are not sure about boiler design, you probably should consider a tried and tested design. If you are not good with metal working and materials, then you should not attempt to build your own boiler.

A boiler is not something I consider turning on and walking away from.

Proceed with caution.
Even a very small boiler can be dangerous.

Pat J

Edit: Generally, superheat is not used with D-valves, since it causes too much valve and seat wear. Superheat can be used with piston valves. There are exceptions. I think the Stanley steam auto had D-valves with superheat.

 

[doubletop]

This thread may be of interest, especially now that flashtwo has contributed. He very successfully has a Microcontoller managing a flash steam plant in a radio controlled boat using Stuart D10 engine.


http://www.homemodelenginemachinist.com/index.php?topic=9721.0

Pete