A typical modern medium to medium-large electricity plant may have a steam flow rate in excess of 3 million pounds per hour (lb/h). For comparison, the rate of steam that has to be generated would be equivalent to burning 20 gallons of gasoline (one car) 5-6 times per second. The factors that affect how steam is boiled are 1) heat transfer rate and 2) heat release rate. Think about a kettle of water heating to boiling on a stove. The more of the kettle that rests on the burner, the faster it will boil (heat transfer rate). The higher the heat is turned up, the faster the water boils (heat release rate).
Heat transfer can be affected in three ways: 1) conduction - direct contact of an object with the source of heat; 2) convection - heat carried by currents of fluid; and 3) radiation - heat that is transmitted by electromagnetic radiation from glowing objects. In our case, the heat to produce steam is made available by burning fuel. That heat must somehow be transferred to the water or steam. The rate at which heat can be transferred depends on:
- the nature of the material through which heat is transferred;
- its thickness;
- the difference in temperature across the material (losses);
- the total area across which the heat is being transferred.
Increasing the surface area is the most effective way to do this. A way to increase surface area is to transfer the heat through smaller tubes. Doing this will reduce the need to make the boiler bigger and bigger - and if you think about a pot of water boiling, the more water you put in a pan, the longer it takes to boil it keeping the surface area constant.
The first evolutionary step in boilers was the fire-tube boiler. Heated gases are in the tubes, and water and steam are in a big tank; the entire tank is under pressure. The problem with using this design was that if the tank burst, it created a major explosion. This design provides significantly more heat-transfer surface area than the corresponding flat plate boiler. Fire tube boilers are useful in industrial heating and in very small (by today's standards) electric plants. "Rolling fire tube boilers" were successful for 150 years as steam locomotives. However, the steady growth in electricity demand and the consequent increase in plant size and necessary steam rate meant that eventually not even the fire tube boiler could keep up.
This led to the next evolutionary design step, which was the water tube (or steam tube) boiler. This is the present state-of-the-art design. Depending on the fuel used and the necessary steam rate, a modern water tube boiler is 10-20 stories tall. The design changed so that the water/steam is in tubes within the boiler with hot gases surrounding the tubes.
For More Information
Visit howstuffworks.com for some additional diagrams of steam engines.
The following video shows an example of a research boiler in the EMS Energy Institute (0:34).
PRESENTER: The place that I'm located right now is at the EMS Energy Institute, and this is a research boiler. It's a much smaller size than what you would see in industry, but basically it's constructed in a similar way, where we have a unit for burning the material and it's injected in. And then the material heats up some kind of steam, or water to make steam, and the steam will go through and turn a turbine and essentially make electricity generation.