Within a boiler system, the operating steam pressure and hot water temperature must always be maintained at a constant level regardless of the load demand placed on the system. “In a multi-boiler plant, demand-load management optimizes the distribution of steam demand among the units and adjusts the overall output to meet working requirements.”1
By adopting a proper demand load scheduling technique, one can easily cut down the running cost of the boiler plant since via this technique, firing of boilers take place only in case of actual need. On the other hand, demand-load scheduling can be used to run each boiler for exactly equal periods of time.
Major features of demand-load management in boiler systems are listed below:
- Base load or modulating operation
- Parallel or serial demand sharing
- Boiler banking operation and programmable sequence selection
Base-load or Modulating Operation
In order to achieve perfect demand load sharing, boilers can be operated in two ways i.e. base-load operating mode or modulating operating mode. In the former mode, the entire load demand placed on the system gets distributed amongst all the base-loaded boilers based upon the user input whereas in the latter mode the load sharing gets automatically done without any user interference. Actually, out of total load demand, the demand which doesn’t get fulfilled by the boilers operating in base load mode, gets enforced upon the modulating boilers and distributed among them depending upon their capacities. “Effective load allocation is based on real-time calculations that account for operating safety margins, load fluctuations, shutdown characteristics and boiler capacities.”2
Parallel or Series Demand Sharing
In parallel mode of demand sharing, the entire load demand gets distributed among the boilers operating at same firing rates. With the increase in load demand, the firing rate of all boilers rises by same amount till the point where the demand load necessitates an extra boiler. Now, for making adjustment for the extra boiler, the firing rate of the previous boilers needs to be decreased. This method of demand sharing is frequently applied for steam boilers.
In series method of demand sharing, the firing rate of modulating boiler tends to rise till the point where an extra boiler comes into play. To meet load demand, this newly added boiler occupies the place of modulating boiler. This method of demand sharing is mainly put into use for heated water systems and meeting rapidly changing steam demands.
In this method, the boilers which are always active are known as lead boilers whereas the other boilers are referred to as lag boilers. The boilers showing effective performance tends to start first while less efficient boilers always stop first.
“Boiler banking keeps boilers in hot standby mode by intermittently firing unused boilers, thus maintaining a required pressure. Boiler banking acts as a warm-start facility, improving the plant’s response to sudden load changes.”3