Shell type steam boilers offer better fuel efficiencies as compared to coil type boilers.
Coil type boilers are 'once through' water tube boiler and as a result, water has to be converted to steam only in a single pass. This is against shell type boilers which are generally 3 pass, with the result water comes in contact with the hot gases 3 times. This allows all the water to convert to steam, ensuring a higher dryness fraction.
The dryness fraction represents the percentage of steam that is dry and the percentage that is moisture in a given volume. A low dryness fraction indicates steam contains more hot water and less steam. Thus, because the maximum heat content is released when steam condenses, with the same steam flow rates the energy output of dry steam will be more than wet steam. So, for a given fuel consumption the quality of steam produced is poor, eventually indicating poor generation efficiency.
When coil type boilers are being used as steam generators, because of the temperature limitation of the positive displacement pump, condensate return is not possible. Even if this were made possible the temperature limitation on the economizer prevents recovery of condensate. Thus with coil type boilers condensate is required to be drained. This loss is equivalent to about 9%-10% of total fuel consumption.
On the whole because of the poor generation efficiencies and the loss of condensate the effective fuel consumption of a coil type boiler tends to be atleast 20% higher that a comparative shell boiler.
Below is an excerpt from a joint study done by CII and Forbes Marshall comparing coil and shell boilers.