What is economical speed of the main engine ? How do we determine what is economical speed on any ship ?

Economical speed is the rpm at which an engine can be run where consumption of fuel is minimum. 

Now many of us confuse this with specific fuel consumption and other technical terms like this. Economical speed is for saving the fuel and at what rpm we can save the fuel most. At Dead slow ahead.
If we need to go from point A to point B, and if we run the engine at dead slow ahead, the total consumption to reach point B will be lesser than other higher rpm like slow ahead and half ahead and so on.
But we cannot run the engine at dead slow ahead for longer period of time as we don't want out blower to cut in/Cut off. The minimum rpm at which we can run the engine for longer period is the rpm where blower cuts off. We take another 2 rpm for fluctuations.
So the economical speed is the rpm where blower cuts off + 2 rpm for fluctuation.