Steam boiler for a workshop: which fuel and which design

A steam boiler is chosen in this order: first the fuel, then the amount of steam, then the pressure. The fuel is the one you have all year round at a clear price. For gas and liquid fuel, take a fire-tube boiler with a burner. For coal and dry biomass, a boiler with a grate. For wet biomass, slag and low-grade coal, a fluidized bed boiler: it burns what a grate cannot take. The amount of steam is counted in tons per hour from the workshop's consumers, the pressure by the most demanding of them.

Fuel first

The design of the boiler follows almost entirely from the fuel. A gas boiler cannot be converted to coal: they have a different furnace and a different path for the flue gases.

• Natural gas, diesel, fuel oil. The fuel is fed by a burner. The furnace is compact, and the boiler arrives assembled. You need a gas supply or a liquid fuel store.
• Hard coal and dry biomass. The fuel burns as a bed on a moving grate. You need a fuel store, feeding to a hopper and ash removal.
• Wet biomass, slag, low-grade coal. The fuel burns suspended in a stream of air, in a fluidized bed. This is how burns what goes out or cakes on a grate.

Ask yourself: will you have this fuel in five years? A boiler lasts longer than one coal contract.

What t/h and MPa mean

Steam output is how much steam the boiler gives per hour, in tons. It is counted from the sum of consumers: autoclaves, drying, heating, steaming. Take it with a margin for simultaneous operation, but not double: a boiler that runs all day at low load burns fuel less efficiently.

Working pressure is the steam pressure at the outlet, in megapascals. 1 MPa is about 10 bar or 10 kgf/cm². Chinese catalogs often list the series 1.25; 1.6; 2.0; 2.5 MPa. The pressure is chosen by the consumer that needs the most, allowing for losses in the steam line.

If the pressure in the workshop's old documents is given in kgf/cm² or in bar, convert it before ordering. How to convert units from a Chinese catalog is covered in a separate article.

How to read a Chinese boiler designation

The designation of an industrial boiler usually has three parts: the type letters, the steam output and pressure, then the fuel. For example, at one manufacturer the designation DZL4-1.25-AII means a boiler of type DZL at 4 t/h and 1.25 MPa; the last code refers to the kind of coal. The rule for building designations in China is set by an industry standard on the designation of industrial boilers. Its text could not be found in open access, so the meaning of the letters below is taken from manufacturers' catalogs. Check the fuel code at the end of the designation with the plant: it depends on the edition of the standard.

A fire-tube boiler has the hot gases inside the tubes and the water outside. A water-tube boiler is the opposite: water in the tubes, gases outside. Water-tube boilers are usually taken where more steam is needed or a packaged boiler does not fit the space: they are delivered in parts and assembled on site.

What to ask about water treatment

A boiler lives as long as its water lives. Hardness salts settle as scale on the tube walls. Scale conducts heat poorly, and the metal under it overheats.

Ask the plant:

• what requirements it sets for feed water and boiler water and under which standard;
• whether a water softening plant and a deaerator are included in the supply or are selected separately;
• how and how often to blow down the boiler.

Take an analysis of the water you will feed the boiler with and attach it to the inquiry. Without the analysis, water treatment is selected blind.

What to ask about automation

• What protections are fitted: by water level, by pressure, by flame failure for a gas boiler.
• What the operator sees and what the event log records.
• Whether there is remote monitoring and in which language the interface is.
• Whose sensors, valves and controller are in the cabinet and whether they can be bought separately if one fails.

What the boiler passport should contain

In Uzbekistan a boiler is imported under the technical regulation on the safety of pressure equipment. The regulation lists what must be in the boiler passport. It is convenient to use this as a list of questions even before ordering: the design fuel and its calorific value, fuel consumption, burner type, design, working and test pressure, rated, minimum and maximum steam output, safety devices, water level indicators, automation and protection, pumps. Which approvals are needed before start-up is covered in the article on boiler approvals.

How we do it

• We select the boiler at the manufacturing plant for your fuel, steam output and pressure. What we find, with the plant's specifications, we send before the order.
• Where the answer must come from the plant (water treatment, automation, drum material), we ask it and pass on its word.
• Supervised installation and start-up are led by the manufacturing plant. We arrange the visit of its engineers, interpreting and site access.