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For larger steam loads it is advisable to opt for a battery of boilers instead of one large boiler to ensure optimum fuel consumption.

Often we are faced with the decision of selecting the right boiler capacity, either during a project or when choosing a replacement boiler for our existing system.

The main characteristics necessary to selecting the right boiler capacity are the peak steam demand and the average steam load and the minimum steam load. The peak and the minimum steam demand are the maximum and minimum steam that would be required in the plant at any point in time. The average steam load refers to the average working load of the plant at which the plant runs or would run for the maximum time during the day.

Especially in case of high steam loads where the peak steam demand is roughly double the average steam demand or when the peak steam loads only occur for a few hours during the day, it may be appropriate to go in for a battery of boilers instead of one large boiler. This is because a battery of boiler will ensure that the average steam loads are met with one of the boiler running at its full capacity and the second boiler can fire to meet the peak load.

In case of one large boiler, during the average and low loads the frequent switch on and switch off causes the heat in the combustion chamber to be carried away by the air purge. This results in greater stack loss thereby reducing the boiler efficiency. Thus splitting the steam load in a battery of boilers is recommended to ensure optimum fuel bill for the boiler house.

Illustration:

Consider a plant with the following steam load characteristics:

In such a case if we will consider two scenerios where we have a single 9 TPH boiler and where we have a combination of 6TPH and 3TPH boiler.

1.    A single boiler of 9TPH capacity, the daily FO requirement will be: 8408lts/day

2.    Combination of 6TPH and 3TPH boiler, the daily FO requirement: 7946lts/day