Expander thermal efficiency

Mechanical Expanders Reciprocating expanders are very similar in concept and design to reciprocating compressors. Generally these units are used with inlet pressures of 4 to 20 MPa. These machines operate at speeds up to 500 rpm. The thermal efficiencies (actual enthalpy difference/maximum possible enthalpy difference) range from about 75 percent for small units to 85 percent for large machines. 

In a combined cycle plant, high steam pressures do not necessarily convert to a high thermal efficiency for a combined cycle power plant. Expanding the steam at higher steam pressure causes an increase in the moisture content at the exit of the steam turbine. The increase in moisture content creates major erosion and corrosion problems in the later stages of the turbine. A limit is set at about 10% (90% steam quality) moisture content. 

Steam is generated in the boiler of a steam power plant operating on an ideal Rankine cycle at 10 MPa and 500° C at a steady rate of 80 kg/sec. The steam expands in the turbine to a pressure of 7.5 kPa. Determine (1) the quality of the steam at the turbine exit, (2) rate of heat rejection in the condenser, (3) the power delivered by the turbine, and (4) the cycle thermal efficiency (%). 

The thermal efficiency of the Rankine cycle can be increased by the use of regenerative heat exchange as shown in Fig. 2.15. In the regenerative cycle, a portion of the partially expanded steam is drawn off between the high-and low-pressure turbines. The steam is used to preheat the condensed... 

Loffler et al. successfully deposited a-Si H solar cells on their in-house ZnO grown by expanding thermal plasma CVD [52]. They reached initial efficiencies of around 10%, and supported also the need to improve the ZnO/p-layer interface. 

Show that for the same compression ratio the thermal efficiency of the air-standard Otto engine is greater than the themial efficiency of the air-standard Diesel cycle. Hint Show that the fraction which multiplies (l/r) in the above equation for is greater than unity by expanding rc in a Taylor series with the remainder taken to the first derivative. 

Rankine cycle A cycle of operations in a heat engine. The Rankine cycle mote closely approximates to the cycle of a teal steam engine that does the Carnot cycle. It therefore predicts a lower ideal thermal efficiency than the Carnot cycle. In the Rankine cycle (see illustration), heat is added at constant pressure Pi, at which water is converted in a boiler to superheated steam the steam expands at constant entropy to a pressure pz in the cylinder heat is rejected at constant pressure pj in a condenser the water so formed is compressed at constant entropy to pressure p, by a feed pump. The cycle was devised by William Rankine (1820-70). 

High-temperature, high-pressure steam is generated in the boiler and then enters the steam turbine. At the other end of the steam turbine is the condenser, which is maintained at a low temperature and pressure. Steam rushing from the high-pressure boiler to the low-pressure condenser drives the turbine blades, which powers the electric generator. Steam expands as it works hence, the turbine is wider at the exit end of the steam. The theoretical thermal efficiency of the unit is dependent on the high pressure and tanperature in the boiler and the low temperature and pressure in condenser. 

Almost all steam and hot-process piping systems operate in the temperature range of 212 to 1000°F (100 to 538°C).The products most frequently used are calcium silicate, fiberglass, mineral wool, and expanded perhte. Choices are usually based on thermal conductivity and resistance to physical abuse where applicable. Hberglass products are the most thermally efficient at lower temperatures, with calcium sihcate being the best at higher temperatures. It is important to use the insulation mean temperature when comparing thermal conductivities in order to make the proper comparison. 

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