Thermodynamics

236 | ThermodynamicsDividing by the mass flow rate ṁ and substituting, h 2 is determined to beh 2 h 1 q out V 2 2 V 12Then, 1354.4 Btu/lbm2 11371.4 1.22 Btu/lbm 1900 ft/s22 1134.4 ft/s2 2P 2 200 psiah 2 1354.4 Btu/lbm fT 2 662.0°F1Table A–6E2Discussion Note that the temperature of steam drops by 38.0°F as it flowsthrough the nozzle. This drop in temperature is mainly due to the conversionof internal energy to kinetic energy. (The heat loss is too small to cause anysignificant effect in this case.)2a 1 Btu/lbm25,037 ft 2 /s 2 b2 Turbines and CompressorsIn steam, gas, or hydroelectric power plants, the device that drives the electricgenerator is the turbine. As the fluid passes through the turbine, work isdone against the blades, which are attached to the shaft. As a result, theshaft rotates, and the turbine produces work.Compressors, as well as pumps and fans, are devices used to increase thepressure of a fluid. Work is supplied to these devices from an externalsource through a rotating shaft. Therefore, compressors involve work inputs.Even though these three devices function similarly, they do differ in thetasks they perform. A fan increases the pressure of a gas slightly and ismainly used to mobilize a gas. A compressor is capable of compressing thegas to very high pressures. Pumps work very much like compressors exceptthat they handle liquids instead of gases.Note that turbines produce power output whereas compressors, pumps,and fans require power input. Heat transfer from turbines is usually negligible(Q . 0) since they are typically well insulated. Heat transfer is also negligiblefor compressors unless there is intentional cooling. Potential energychanges are negligible for all of these devices (pe 0). The velocitiesinvolved in these devices, with the exception of turbines and fans, are usuallytoo low to cause any significant change in the kinetic energy (ke 0).The fluid velocities encountered in most turbines are very high, and thefluid experiences a significant change in its kinetic energy. However, thischange is usually very small relative to the change in enthalpy, and thus it isoften disregarded.EXAMPLE 5–6Compressing Air by a CompressorAir at 100 kPa and 280 K is compressed steadily to 600 kPa and 400 K.The mass flow rate of the air is 0.02 kg/s, and a heat loss of 16 kJ/kg occursduring the process. Assuming the changes in kinetic and potential energiesare negligible, determine the necessary power input to the compressor.