Types of steam engines
Steam engines are mechanical devices capable of transforming heat energy into mechanical energy in a rotating axis. This heat energy takes advantage of the energy contained in water vapor at high pressure and temperature.
We consider steam engines all those machines that transform the thermal energy of a fluid into mechanical energy. In general, the fluid must be pre-heated and at the outlet of the steam machine it must be cooled to repeat the process.
Steam machines can be classified into these two types:
Plunger steam engine
The piston steam machines are the first steam engines were developed using a piston or piston coupled to a piston-rod crankshaft type mechanism. This mechanism was applied to steam at high pressure and temperature synchronized with a set of valves to obtain a kinetic energy and, therefore, a mechanical movement.
Water vapor is generated in a steam generator such as a boiler. The steam is introduced into a chamber where there is a control valve. This control valve is driven in a synchronized manner by a mechanism coupled to the crankshaft of the machine. The displacement movement of the control valve causes the input chamber, where the supply vapor is, to communicate alternately to the upper or lower part of the piston. The steam pushes the plunger in both directions to rotate the crankshaft. At the same time, this control valve establishes the communication from the opposite side of the plunger, to the outlet duct to let out the cold vapor and at low pressure useless. The temperature and pressure of the output steam are not high enough to continue to use its energy in this type of machine.
This elemental steam engine is very inefficient. The steam discharged to the outside is still hot and under enough pressure to do more useful work. To improve the efficiency of this type of steam engine, multi-stage machines are used. In multi-stage machines, waste vapor from one stage is introduced into another with a larger plunger to further exploit the energy it contains.
Steam of a stage is introduced in the following to drive an increasingly larger piston. In this way, the energy of the final steam output has been maximized.
This increase in piston size is necessary so that each stage of the steam engine can deliver approximately the same driving force. We must consider that each time the steam has less pressure. Increasing the size of the plunger increases its surface. According to the laws of physics, the pushing force is the product of pressure, by the area of the piston.
Within the piston machines we highlight the following types:
Multiple expansion steam engine
With the high-pressure steam from the boiler, the first piston is driven, the piston with the smaller diameter downwards.
In the upward movement of the first piston, the partially expanded steam is driven into a second cylinder that is beginning its downward movement.
The lowering of the second piston generates an additional expansion of the relatively high pressure released in the first chamber.
Also, the intermediate chamber discharges to the final chamber, which in turn is released to a condenser. A modification of this type of engine incorporates two smaller pistons in the last chamber.
The characteristics of this type of steam engine made it an optimal engine to use on steam ships. The advantage was that the condenser, recovering a bit of the power, converted the steam back into water that could be reused in the boiler.
The terrestrial steam engines this advantage was not so important. Land-based machines could exhaust much of their steam and be filled with a freshwater tower, but at sea this was not an added difficulty.
Before and during World War II, the expansion engine was used in marine vehicles that did not need to go at high speed. However, when more speed was required, it was replaced by the steam turbine.
Uniflow or uniform flow motor
Another type of piston machine is the uniflow or uniform flow motor. This type of engine uses steam that only flows in one direction on each half of the cylinder.
The thermal efficiency of this steam engine is achieved by having a temperature gradient along the cylinder. The steam always enters through the hot ends of the cylinder and exits through openings in the center of the cooler.
In this way, the relative heating and cooling of the cylinder walls is reduced.
The inlet valves are opened to admit steam when the minimum expansion volume is reached at the beginning of the movement.
At a certain moment of the return of the crank, the steam enters and the inlet of the cap is closed, allowing the continuous expansion of the steam. The steam inlet allows the piston to be operated, transmitting a certain kinetic energy.
At the end of the movement, the piston will discover a ring of exhaust holes around the center of the cylinder. These holes are connected to the condenser. This action will lower the pressure in the chamber causing a quick release. The continuous rotation of the crank is what moves the piston.
Turbine steam engine
Turbine steam engines are the next evolutionary step for piston machines.
The old steam engines have been giving way to the turbines. Turbine steam engines improve their durability, safety, relative simplicity and are more efficient. In the turbine, a jet of water vapor at high pressure and temperature. This jet of steam is suitably influenced on a propeller with blades with a certain section. During the passage of the steam between the blades of the propeller, this one expands and cools delivering the energy and pushing the blades to turn the propeller placed on the axis of exit of the turbine.
High-powered steam turbines use a series of rotating discs that contain a kind of propeller type blades on their outer edge. These mobile discs or rotors alternate with stationary rings or stators, fixed to the structure of the turbine to redirect the steam flow.
With this mechanism a very high speed of rotation is obtained. Due to the high speed the turbines are normally connected to reducer to convert kinetic energy into power. The reducer is connected to another mechanism such as a ship's propeller.
Steam turbines require less maintenance and are more durable than piston machines. The rotational forces they produce are softer at their output shaft, which contributes to less wear and less maintenance.
The main use of steam turbines is in the electricity generation stations. In this type of application, its high speed of operation is an advantage and its relative volume is not a disadvantage. Both in the fields of thermal power plants and nuclear energy this type of steam engine is used. Virtually all nuclear power plants generate electricity by heating the water and feeding steam turbines.
Another application of turbine steam engines is the drive of large ships and submarines.
Last review: November 16, 2017