Based on the steam flow:
the steam is admitted at the one end of the turbine in the first stage blades. Steam flows axially through the radially-mounted blades and exhausts at the other end after expansion (specific volume is increased).
In this type of arrangement the steam is admitted at the center of the cylinder and the flow is divided to flow in two opposite axial directions towards both the ends of the rotor.
The major advantage of this kind of arrangement is to avoid the excessively longer blades at the rotor ends in case of single-flow turbines. For that reason this arrangement is invariably used for the LP turbines and at some places for the IP turbines, for turbines with large capacity, it is normal to have several double flow cylinders to reduce the size of turbine.
The second advantage is that the double-flow cylinder self-balances the axial thrust caused by the steam flow on the moving blades.
In this kind of arrangement, the steam flows in one direction through a particular number of stages and then it admitted by the help of a duct (can be internal or external) to the rest number of stages, where the steam flows in the opposite direction axially. Now a day this kind of arrangement is not used and double flow is used in most of the places.
(Single Flow & Double Flow)
Depending upon the Number of cylinders:
Turbines driving the electrical generators have a certain limit on the capacity; generally from a single-cylinder turbine the maximum power that can be generated is around 100MW, above that multi cylinder arrangement is used. Now a day the Morden power generating companies are going for larger capacities so almost everywhere the Multi-cylinder arrangement is used due to the increase in steam generation.
Depending upon the Shaft Arrangement:
Tandem-compounded: In this kind of arrangement, all the cylinders of the turbine are present on a single shaft being rigidly coupled.
Cross-compounded: Here the cylinders are mounted on two different shafts, which are connected to two different generators. This kind of arrangement is broadly used where the grid frequency is around 60 Hz. The other advantage of this kind of arrangement is that we can two generators at two different speeds. For example if we want to run one generator at 1500 rpm and the other generator at 3000 rpm then we can use cross-compound arrangement.