Thermal efficiency internal

Although the [CHTJr cycle is internally reversible, external irreversibility is involved in the heat supply from the external reservoir at temperature Tr and the heat rejection to a reservoir at temperature 7. So a consideration of the internal thermal efficiency alone does not provide a full discussion of the thermodynamic performance of the plant. If the reservoirs for heat supply and rejection are of infinite capacity, then it may be shown that the irreversibilities in the heat supply ( r) f d the heat rejection respectively, both positive, are... 

The internal thermal efficiency increases as 6 is increased, but unlike the CHT]r cycle efficiency, drops with increa.se in pre.ssure ratio r. This is because the heat transferred gj decreases as r is increased. 

Thus the cooled reversible cycle [CHT]rci with a first rotor inlet temperature, Tj, will have an internal thermal efficiency exactly the same as that of the uncooled cycle [CHTJru with a higher turbine entry temperature 3 = Tr, and the same pressure ratio. There is no penalty on efficiency in cooling the turbine gases at entry but note that the specific work output, w = (wj — wc)/CpT = [(0 /x) — 11(j — 1), is reduced, since 0 < 0. 

Because of the intermediate clinker breaker and hot air return, the multiple-stage cooler with duotherm air circuit is characterized by high exhaust airtemperatures in conjunction with good internal thermal efficiency. This type of cooler will therefore always constitute the most economical solution in cases where the raw material has a high moisture content and therefore needs considerable heat input for drying. 

Another hydrogenation process utilizes internally generated hydrogen for hydroconversion in a single-stage, noncatalytic, fluidized-bed reactor (41). Biomass is converted in the reactor, which is operated at about 2.1 kPa, 800°C, and residence times of a few minutes with steam-oxygen injection. About 95% carbon conversion is anticipated to produce a medium heat value (MHV) gas which is subjected to the shift reaction, scmbbing, and methanation to form SNG. The cold gas thermal efficiencies are estimated to be about 60%. 

Helping to propel capacities upward has been the advent of greatly improved preheaters, which partially calcine the stone and significantly improve thermal efficiency. Modem preheaters improve capacity by 15—20% and decrease fuel consumption a similar percentage. Other kiln appurtenances and accessories that enhance efficiency and lime quahty are the contact coolers, and such kiln internals as metal refractory trefoil systems that act as heat exchangers, dams, and lifters. 

An improved approach from the point of view of thermal efficiency is the electrothermal process in which the mixture of zinc oxide and carbon, in the form of briquettes, are heated in a vertical shaft furnace using the electrical resistance of the briquettes to allow for internal electrical heating. The zinc vapour and CO(g) which are evolved are passed tluough a separate condenser, the carbon monoxide being subsequently oxidized in air. 

The objective of the gas turbine designer is to make all the proces.ses in the plant as near to reversible as possible, i.e. to reduce the irreversibilities, both internal and external, and hence to obtain higher thermal efficiency (in a closed cycle gas turbine plant) or higher overall efficiency (in an open gas turbine plant). The concepts of availability and exergy may be used to determine the location and magnitudes of the irreversibilities. 

The thermal efficiency, the work output as a fraction of the fuel exergy (the maximum reversible work), is shown as no. 1 in the figure and is 0.368. The internal irreversibility terms, are shown as nos. 2, 3, and 4 in the diagram, for the combustion... 

However, it is important to note that this conclusion becomes invalid if the air for cooling the LP turbine is taken from compressor delivery (as in Fig. 4.3b) and then throttled at constant temperature (T2 = Ty) to the lower pressure before being mixed with the gas leaving the HP turbine. The thermal efficiency drops as another internal irreversibility is introduced it can be shown [5] that... 

Rudolph Diesel s original intent was to produce a low-heat-rejection internal-combustion engine without the need for a cooling system. He believed that this would provide less heat losses from the combustion process and provide him with a superior heat, or thermally efficient (TE), design concept. To his chagrin. 

A diesel-electric locomotive uses as its prime mover a large, self-igniting, internal combustion engine of the type invented by RudolfDiesel and first successfully demonstrated in 1897. Thermal efficiency of these engines exceeded 30 percent, compared... 

High heat-transfer rates and sufficient cooling via internal, two-phase flow and circulation are consequently necessary to ensure that modem boilers operate below their particular critical heat-flux (CHF) conditions. This minimizes poor thermal efficiency performance, risks of thermal metal fatigue, and probable tube failures from excessive temperatures. 

Flue gas recirculation Flue gas recirculation, alone or in combination with other modifications, can significantly reduce thermal NO,. Recirculated flue gas is a diluent that reduces flame temperatures. External and internal recirculation paths have been applied internal recirculation can be accomplished by jet entrainment using either combustion air or fuel jet energy external recirculation requires a fan or a jet pump (driven by the combustion air). When combined with staged-air or staged-fuel methods, NO emissions from gas-fired burners can be reduced by 50 to 90 percent. In some applications, external flue-gas recirculation can decrease thermal efficiency. Condensation in the recirculation loop can cause operating problems and increase maintenance requirements. 

As an example, if the hot temperature is 1273 K (1000 °C) and the cold temperature 373 K (100 °C) then the efficiency is approximately 70%. In practice the operation of a real engine does not follow the Carnot cycle and the efficiency is considerably lower. For a medium sized motor car with an internal combustion engine the fuel efficiency is about 12%, much of the wasted 88% demanding water cooling. There are continuous improvements made in petrol and diesel engine technologies and in the fuels and projections suggest that thermal efficiencies a little over 50% will eventually be achieved. 

Internal quantum efficiencies in the inorganic light emitters can approach 100%. Thermal conductivity of the inorganic semiconductors is high. Large currents can be injected in to the diodes. The inorganic semiconductors can be doped to high electron and hole... 

A power plant operating on heat recovered from the exhaust gases of internal-combustion < uses isobutane as the working medium in a modified Rankine cycle in which the upper pressure I is above the critical pressure of isobutane. Thus the isobutane does not undergo a change of p" as it absorbs heat prior to its entry into the turbine. Isobutane vapor is heated at 4,800 kPa to 2 and enters the turbine as a supercritical fluid at these conditions. Isentropic expansion in the turh produces superheated vapor at 450 kPa, which is cooled and condensed at constant pressure, resulting saturated liquid enters the pump for return to the heater. If the power output of the modi Rankine cycle is 1,000 kW, what is the isobutane flow rate, the heat-transfer rates in the heater condenser, and the thermal efficiency of the cycle ... 

Heaters can be located in the space between the trms and the dryer housing, where they are not in direct contact with the product, and thermal efficiencies up to 3500 kj/kg (1500 Btu/lb) of water evaporated can be obtained by reheating the air within the dryer. Steam is the usual heating medium. The hi cost of heating electrically generally restricts its use to relatively small equipment. For materials which have a tendency to foul internal heating surfaces, an external heating system is employed. 

Nomura, M., Kasahara, S., Okuda, H., and Nakao, S., Evaluation of the IS process featuring membrane techniques by total thermal efficiency, International Journal of Hydrogen Energy, 30, 1465-1473, 2004. 

---