Determination of boiler and system output
Several concepts and variants are usually possible for selecting the individual boiler output, the total system output and the distribution of the individual boiler outputs across the total system output.
In good planning, various aspects must be taken into account.
- Security of supply
- Business management aspects
- Country- and project-specific requirements
Single boiler systems
In single-boiler systems, heat is supplied by a single boiler. The boiler must be able to serve the entire annual load duration curve. The heat output of the generator is adjusted to the heat output of the consumer(s) by means of a modulating or staged burner. Modulating burners can variably (continuously) adjust their output to the heat energy demand within a modulation range. Multi-stage burners, on the other hand, can regulate their output in different power levels. These burner functions prevent the burner from frequently switching on and off (known as cycling). Cycling is problematic with single-stage burners, as they can only influence their output by switching on and off. Frequent switching on and off shortens the service life of the burner and reduces the efficiency of the boiler due to the pre-ventilation required. In this process, some of the heat from the boiler is dissipated unused via the flue system, which increases heat loss. Accordingly, the longest possible duration should be achieved for single boiler systems. To optimise this, the heat is temporarily stored in buffer cylinders and consumed by the system as required. The temporary storage can also be used to cover temporary peak loads. A sufficiently dimensioned buffer cylinder can also reduce the maximum heat output required from the heat generator.
Multi-boiler systems
In multi-boiler systems, several boilers or heat generators are connected in cascade. These are used when, for example, the required heat output cannot be covered by a single heat generator or to increase reliability. Furthermore, multi-boiler systems can be used to increase the modulation range, thereby achieving better performance adjustment of the heat generator to the consumer system. An increase in the modulation range is useful for heat consumers that require a permanent but highly fluctuating heat supply. Examples of this are daily fluctuations due to reduced loads, shift work in production facilities, or seasonal fluctuations in building heating. The greatest quantity of heat for heating buildings is required in winter, whereas the least amount of heat is required in summer. The annual load duration curve shows a significant difference between base load and peak load, particularly for large heat consumers. In contrast, when providing heat for process heating with continuous heat consumption, almost constant quantities of heat are required throughout the year. However, cyclical load requests may also necessitate a large modulation range.
Multi-boiler systems therefore offer the possibility of operating boilers with high efficiency, with one boiler covering the base load at its optimum operating point and additional boilers being used to cover peak loads.
Similarly, multiple boilers allow for greater security of supply through redundancy. The heat supply is therefore fully or at least partially guaranteed in the event of failure, maintenance or replacement of a heat generator. Furthermore, multi-boiler systems are ideal for integrating sustainable heat generators. For this, a single boiler can be replaced with a heat generator that uses renewable energy.
An important component in multi-boiler systems is the boiler sequence control. This is responsible for switching the boiler on and off. It also determines the order in which the boilers are switched on. For this purpose, one boiler is defined as the lead boiler, while additional boilers are switched on as required and feed their heat into the system. The decision as to which boiler is the lead boiler is determined by the operating hours of the burner in the sequence control system of Bosch Industriekessel. The burner operating hours should be distributed as evenly as possible among the individual boilers. This should prevent excessive down time for the follow-in boilers.