How are steam boilers made?
A steam boiler heats water much like a teakettle, only on a much larger and more complex scale. While not every steam boiler is the same, all steam
boilers have some basic elements in common: burner, combustion chamber, heat exchanger, expansion tank, steam temperature controller, safety
relief valve, and low water shutoff.
Steam boiler designs and types vary greatly depending on their construction and purpose.
1. The purpose of the burner is to inject fuel and air into the combustion area. Fuels such as oil, natural gas or pulverized coal must mix easily with
air. Dampers are used to regulate the amount of air entering the burner. The impeller circulates air evenly around the burner. Pipes called spuds feed
fuel into the boiler and are ignited by an igniter.
2. The combustion chamber contains the reaction of fuel with air and uses it to generate heat to create steam. To increase the efficiency of the
combustion chamber, it needs to be adequately insulated to avoid heat loss through radiation. The combustion chamber has pipes containing
water and steam that pass through an open box with a burner and controls. Efficient combustion is an important part of boiler operation. The
heat generated in the combustion chamber is absorbed by the boiler surface at the top of the combustion chamber.
3. Heat exchangers allow heat exchange between fluids without allowing substances to mix. A heat exchanger consists of a long coil of tubing
immersed in the fluid to be heated. The gas or heated liquid passes through the pipe and heats the water around it. Heat exchangers are made f
rom a variety of materials, with stainless steel being the best as it does not corrode or rust.
4. Expansion tanks help maintain pressure in the boiler by providing a place for water to expand, absorb pressure, and regulate pressure. The water
tank has a diaphragm that is divided into two parts, one receiving water from the boiler and the other side controlled by an air valve to relieve
pressure. The air valve pushes another portion of the water until normal pressure is reached.
5. Steam temperature control prevents thermal stress; precise control of steam temperature is important. Control of pressure and temperature is
necessary to reduce fuel costs. Steam temperature is usually controlled by water injection into the first and second stage superheaters. This is done
using a thermostat or desuperheater.
Other methods of controlling steam temperature include the use of flue gas circulation, flue gas bypass, or the tilt angle of the burner firing furnace.
The simplest way to control steam temperature is to monitor the temperature of the steam as it leaves the boiler and change the position of the
water spray valve.
6. Safety relief valve
The pressure relief valve is the most important safety measure for steam boilers. It ensures that pressure build-up in the steam boiler is relieved;
this helps avoid catastrophic disasters. When the pressure reaches a critical level, the pressure relief valve opens and when the pressure returns to
normal, the pressure relief valve closes.
7. Low water cut-off
A low water cut-off device shuts down the burner or cuts off fuel to the steam boiler when the water level falls below the set point. Dry fired boilers
can rupture or suffer serious failure. Low water cut-off is another safety feature built into steam boilers that helps avoid boiler damage and injury.
They are a normal part of the boiler construction and are installed in steam boilers and hydronic heating units to shut down the boiler in the event
of water loss.