In the Scotch marine boiler, one or more large flues (approximately 18 to 24 inches or 45 to 60 centimeters in diameter) are used for furnaces or combustion chambers within the shell, and hot gases are returned through groups of small-diameter tubes or flues. The flues that form the combustion chamber are corrugated to prevent collapse when subjected to the water and steam pressure. These boilers may be oil-fired, or solid fuel can be burned on grates set within the furnace chambers. Scotch marine boilers have, with few exceptions, been superseded by water-tube marine boilers.
Gas tube boilers are sometimes used for the absorption of waste heat from process gases or the exhaust from internal combustion engines, particularly when their installation provides a simple and economical means for the recovery of low-grade heat. The boiler shell may be augmented by an external steam-separating drum and downcomer and riser connections to provide for proper circulation.
In a one-pipe boiler heating system, a single main serves to separate supply and return connections. Because steam cannot flow through the piping or into the heat exchanger until all the air is expelled, automatic air-venting valves must be provided on all exchangers and at the ends of all mains. These valves may be of a type which closes whenever steam or water comes in contact with the operating element, but which also permits air to flow back into the system as the pressure drops. A vacuum valve closes against sub-atmospheric pressure to prevent return of air.
A two-pipe boiler system operates at a few ounces above atmospheric pressure, and in this system, a thermostatic trap is located at the discharge connection from the heat exchanger which prevents steam passage, but permits air and condensation to flow into the return piping.
Small boilers, such as the type used for home heating, operate at pressures of up to about 200 pounds per square inch (psi), while the largest industrial boilers produce as much as 3,000 psi. (In metric units, these pressures are 1.4 megapascals and 20.7 megapascals.) If the pressure inside the boiler becomes dangerously high, a device called a safety valve permits some of the steam to escape.