Like any other devices, the boiler energy efficiency is a factor of its load factor. Figure (i) and also Figure (iii) show typical of such efficiencies.

Figure (iii) – Impact of boiler load factor on efficiency

Accordingly, for this boiler the efficiency is highest at certain point and then drop off with changes in load. Thus boiler load management could be considered as one aspect of energy efficiency. Figure (iii) shows that at 70% load, the efficiency is 80% and at 30% load, the efficiency is 63%; a significant drop.

Operating the boiler at low load is thus inefficient. Avoiding low load boiler operation will depend on ship type, number of boilers and where the steam is used. Generally, if there are two auxiliary boilers on a ship, they must not run in parallel if one can supply the whole steam demand; unless safety issues dictates the need for such a parallel operation case.