Losses during short cycles occur every time a boiler is switched off for a short period of time. The boiler cycle consists of a purge period, a post-purge, an idle period, a pre-purge and a return to firing. Part of the losses during the purge periods and idle period can be low in modern, well isolated boilers, but can increase rapidly in older boilers with inferior insulation.
Losses due to short term cycles for steam boilers can be magnified if the boilers can generate the required capacity in a very short period of time. This is the case if the installed capacity of the boiler is considerably larger than that generally needed. The steam demand for the process can change over time and should be reassessed periodically (see Section 2.2.2). Total steam demand may have been reduced through energy savings measures. Alternatively, boilers may have been installed with a view to a later expansion, which was never realised.
A first point for attention is the type of boiler in the design phase of the installation. Fire tube boilers have considerably large thermal inertia, and considerable water content. They are equipped to deal with continuous steam demand and to meet large peak loads. Steam generators or water tube boilers in contrast can also deliver steam in larger capacities. Their relatively lower water content makes water pipe boilers more suitable for installations with strongly varying loads.
Short cycling can be avoided by installing multiple boilers with a smaller capacity instead of one boiler with a large capacity. As a result, both flexibility and reliability are increased. An automated control of the generation efficiency and of the marginal costs for steam generation in each boiler can direct a boiler management system. Thus, additional steam demand is provided by the boiler with the lowest marginal cost.
Another option is possible where there is a standby boiler. In this case, the boiler can be kept to temperature by circulating water from the other boiler directly through the standby boiler. This minimises the flue-gas losses for standby. The standby boiler should be well insulated and with a correct air valve for the burner.
Energy savings can be obtained by boiler isolation or boiler replacement.
The negative impact of short cycling becomes clear when there is low usage of available boiler capacity for instance, less than 25 %. In such cases, it is good practice to review whether to replace the boiler system.
See examples in Annex 7.10.1.
Energy Efficiency (2009) 3.2.9