As a rule of thumb, feedwater tank should be sized to be 1.5 times the peak steam demand.
It is general practice to have a feedtank as large as possible with an idea to ensure one has sufficient water at all points in time. However this is not a good practice.
Large feedtanks present two problems:
- The incremental heat addition by returning condensate is diminished
- Increased radiation loss due to holding large quantities of hot water.
Both of the above points result in lower feedwater temperatures and thus lower boiler efficiencies. As a rule of thumb, every 6 degC rise in feedwater temperature reduces the fuel bill by 1%.
As a rule of thumb, the feed water tank should be sized to 1.5 times of the peak steam demand. This ensures that sufficient water is available for the boiler at any point in time while keeping the above losses to a minimum.