Steam Engine Efficiency and Losses
Notes taken from Steam Engineering: A Text Book by William R. King
Constructs such as the Carnot Perfect Heat Engine are imaginary since it is constructed of perfect conductors and non-conductors. The results of this theoretical device enable us to point out the imperfections of an existing engine and hot it might waste heat. The Carnot cycle shows that no existing engine can be made that has a greater efficiency than (T1 - T2) / T1, in which T1 is the absolute temperature of the source of heat and T2 is the absolute temperature of the source of cold. Efficiency would be 100% when T2 is zero or when the source of cold is at absolute zero, which cannot be practically attained.
The best that can be done is to make T1 very large and T2 very low. Unfortunately, the practical range of temeratures in the cylinder of a steam engine is limited and the efficiency is thus quite low. The maximum temperature that might be attained is the products of fuel combustion, but if there was some practical way to use the temperatures of furnace gases something other than steel would have to be used in the construction of the engine. As such, we are limited to the temperature of the steam in the boiler and for the minimum temperature to a condenser temperature of about 129 degrees F - an absolute pressure of about 2 pounds.
The conditions of the four operations used in the Carnot cycle are very difficult to attain in practice in a modern steam engine. The best that might be gained is about 50 to 60 percent of the ideal efficiency. With current limits of temperatures used in steam engines, the idea efficiency varies from 18 to 30 percent and actual efficiencies range from 9 to 18 percent under favorable conditions.
The cycle of operations of the steam engine only approximate those of the Carnot cycle. Heat is taken in at the upper limit (boiler temperature); expansion follows during which work is done and the falling temperature approaches, but does not reach the lower limit at which condensation takes place, during which heat is given out. At this point, adiabatic compression is not quite achieved, the working substance in the condition of condensed water is returned to the boiler by mechanical means. The heat of the feed water improves the efficiency of the cycle, but heat supplied at a temperature below the upper limits results in less than ideal performance during the cycle.
Losses that Affect Engine Efficiency
Loses that reduce the efficiency of an engine to less than the idea case include:
1. Steam (the working substance) is far from being a perfect gas. It is impossible to compress the exhaust steam to its initial state, thus requiring re-heating in the boiler
2. Steam is not rejected into the condenser (the source of cold) at the condenser temperature and pressure, but suffers a fall due to incomplete expansion.
3. Due to inevitable leakages, an equal amount of water to the steam supplied to the cylinder is not returned to the boiler, which means additional feed water at a lower temperature than the water in the boiler must be added.
4. The exchange of heat between the steam and metal of the cylinder results in initial condensation and then re-evaporation inside the cylinder.
5. The drop in pressure from cylinder to cylinder in stage-expansion engines gives no external work because of free and uresisted expansion of the steam.
Cylinder Condensation and Re-evaporation
When steam comes into contact with the metal of the cylinder (which has just been chilled by the exhaust connection with the condenser), initial condensation is formed. This has latent heat that raises the temperature of the cylinder walls. As the stroke proceeds, the reduction in pressure from expansion lowers the temperature of the steam. This results in heat being extracted from the cylinder walls causing a partial re-evaporation fo teh water and thus causing the expansion curve to rise. This re-evaporation never makes up for the loss due to condensation and there is always a quantity of water rejected at release. The low back pressure during exhaust causes partial re-evaporation and adds to the back pressure.
It is imporant to note that the temperature during condensation is higher than it is during re-evaporation and as such the metal fo the cylinder receives more heat per pound of of steam condensed than it gives up per pound re-evaporated. As such, if the abstraction of heat from the cylinder walls could be prevented, there would be no initial condensation. If the walls of the cylinder are heated (using a steam jacket, for example), all condensation during expansion would be re-evaporated.
The above assumes that the steam is dry. If it contains moisture, re-evaporation is not just limited to initial condensation, but extends to the moisture contained in the steam as well. As such, it is important to use dry steam achieved by super heating.
A slide valve engine promotes initial condensation by requiring the steam to enter the cylinder though a passage that was immediately chilled just before. This source of inefficiency is remedied by the use of four valves: two for steam and two for exhaust.
Reducing Cylinder Condensation
Four methods are recommended to reduce cylinder condensation.
A jacket of cylinder is formed by the insertion of a cylindrical line within the cylinder proper, leaving an annular space inside. the liner is flanged at one end and secured to the cylinder with screws. At the other end, a stuffing box allows for expansion adn to make a steam tight joint with the cylinder.
Steam from the boiler is circulated through the jacket to keep the liner at the same temperature as that is entering the cylinder, which reduces initial condensation. The steam jacket is best applied to slow speed simple engines. It is less effective for stage expansion engines and almost useless for high speed engines.
Normally, steam is produced in a saturated form, which is a saturate vapor of maximum density (and thus the smallest volume per pound) consistent with its pressure or temperature. It is steam at the point of condensation when any reduction in temperature will cause liquefication.
If saturated steam is heated so that its temperature rises above the saturation point (pressure remaining constant), it is said to be superheated. This is to say that the that at a given boiler pressure, the temperature is higher than that of water from which it was evaported. There is no moisture in superheated steam and its behavior is much closer to a perfect gas than a saturated steam. Since the specific heat at a constant pressure is 0.48, it requires less than a thermal unit to raise the temperature of a pound of it by one degree.
Adding heat to saturated steam will raise the temperature and leave the pressure unchanged only when the steam is allowed to expand as heat is added. This is usually done on its way from the boiler to cylinder by passing through coils which are surrounded by hot gases from the furnace. the steam absorbes the heat from the gases and the temperature rises. The pressure remains constant because the steam is used almost as fast as it is generated and the displacement of the piston in the cylinder causes a virtual extension to the volume of the boiler, which allows for expansion.
Early attempts at superheating were difficult due to the inability of hemp packing to withstand high temperatures. This caused existing designs to shift to high steam pressures, high ratios of expansion, high piston speeds and steam jacketing. Further economy in steam engines lie in the use of super heated steam. the use of matallic packing and improved lubrication have made the use of superheated steam possible. The use of superheated steam doesn't increase the power of an engine. Rather, it increases the economy by which the power is developed (by reducing cylinder condensation). for every 7 degrees of super heat, a reduction of 1% of steam consumption can be expected.
Using higher steam pressures, with increased rates of expansion, can widen the range of temperatures in a single cylinder didn't work in practice. A division of tempurature ranges was attempted by allowing steam to to expand successively through two, three, and sometimes four cylinders. These are know as double, triple, and quadruple expansion engines.
Engines with a high RPM have insufficient time to transfer heat to and from cylinder walls, thus reducing cylinder condensation and re-evaporation.