Steam Engine vs Steam Turbine
While, steam engine and steam turbine use the large latent heat of vaporization of steam for the power, the main difference is the maximum revolution per minute of the power cycles that both could provide. There is a limit for the number of cycles per minute that could provide with a steam driven reciprocating piston, inherent in its design.
Steam engines in locomotives, normally have double acting pistons run with steam accumulated at both faces alternatively. The piston is supported with piston rod connected with a cross head. Cross head is further attached to the valve control rod by a linkage. The valves are for supply of the steam, as well as, for exhausting the used steam. The engine power generated with the reciprocating piston is converted to a rotary motion and transferred to the drive rods and the coupling rods that drive the wheels.
In turbines, there are vanes designs with steels to give a rotary movement with the steam flow. It is possible to identify three major technological advancements, which make the steam turbines more efficient to steam engines. They are steam flow direction, the properties of the steel that is used to manufacture the turbine vanes, and the method of producing “supercritical steam”.
The modern technology used for steam flow direction and flow pattern is more sophisticated compared to the old technology of peripheral flow. The introduction of direct hit of steam with blades at an angle that produces a little or almost no back resistant gives the maximum energy of the steam to the rotary movement of the turbine blades.
The supercritical steam is produced by pressurizing the normal steam such that, the water molecules of the steam are forced to a point that it becomes more like a liquid again, while retaining the gas properties; this has excellent energy efficiency compared to the normal hot steam.
These two technological advancements were realized through the use of high quality steels to manufacture the vanes. So, it was possible to run the turbines at much high speeds withstanding the high pressure of the supercritical steam for the same amount of energy as traditional steam power without breaking or even damaging the blades.
The disadvantages of the turbines are: small turndown ratios, which are the degradation of performance with the reduction of steam pressure or flow rates, slow start up times, which is to avoid thermal shocks in thin steel blades, large capital cost, and the high quality of steam demanding feed water treatment.
The main disadvantage of steam engine is its limitation of the speed and the low efficiency. Normal steam engine efficiency is around 10 – 15 % and newest engines are capable of operating at much higher efficiency, around 35% with the introduction of compact steam generators and by keeping the engine in an oil free condition thus, increasing the fluid life.
For small systems, the steam engine is preferred to steam turbines since the efficiency of turbines depend on the steam quality and the high speed. The exhaust of the steam turbines is at very high temperature and thus, low thermal efficiency too.
With the high cost of the fuel used for internal combustion engines, the rebirth of steam engines is visible at present. Steam engines are very good in recapturing the waste energy from many sources including steam turbines exhaust. The waste heat from steam turbine is used in combined cycle power plants. It further allows discharging the waste steam as exhaust in much low temperatures.