Auxiliary boiler
An auxiliary boiler is a smaller steam generator in a larger steam boiler plant that ensures or supports the steam supply while the main boiler is out of order or not supplying enough steam.
Auxiliary boilers in power plants and industrial plants
Most of the larger steam power plants have an auxiliary boiler, which is used to keep the system warm when the main boiler is idle, to shorten the start-up process by preheating and to supply some internal and external systems that always require heating steam. In particular, if external steam consumers ( district heating or industrial process heat consumers ) are connected, there is usually a contractual obligation to deliver. In such a case, an auxiliary boiler is also provided to secure unplanned system shutdowns.
Since auxiliary boilers are not optimized for continuous operation, when choosing the type, the focus is not on maximum efficiency and lowest operating costs, but on other aspects such as compact design, fast start times and high load gradients. Often, auxiliary boilers are gas or oil-fired shell boilers .
If there are several blocks with several independent steam boilers at the site, it may be possible to connect them via an auxiliary steam busbar. In such a case there is no need for an auxiliary boiler, since the boilers are available to each other as auxiliary boilers.
Auxiliary boiler on ships
On larger merchant ships , the steam is mainly required for processing heavy oil (around 95 ° C) and for final preheating (around 130 ° C). It is also used to heat living areas and provide hot water. The exhaust gas boiler and the auxiliary boiler are connected via the common steam system. The burner and the fan of the auxiliary boiler are switched on via a simple two-point control when the pressure in the steam system falls below a lower limit value (e.g. 6 bar). The shutdown then takes place z. B. at 7 bar. When operating at sea, the exhaust gas boiler generally supplies enough steam so that the auxiliary boiler only switches on when the load is low (area operation) or when the engine is at a standstill (in port).
Another heating method, which is rarely used on ships, is the use of thermal oil instead of the water-steam cycle. Here, too, a heat exchanger heated by exhaust gas is used, similar to the water-steam cycle. The advantage is a "pressureless" system, the disadvantage is the cost of the thermal oil and the higher risk: cracked pipes in the exhaust system lead to oil leaks and fires.
On smaller ships that run on marine diesel oil (MDO) as fuel, lower temperatures are sufficient. Therefore, a hot water boiler is often used here to heat the living space and to supply hot water. When operating at sea, however, the fresh cooling water from the main engine is mainly used for heating (waste heat utilization).