Chapter 7, Problem 7.27P

Steady-state operating data are shown in the figure below for an open feedwater heater. Heat transfer from the feedwater heater to its surroundings occurs at an average outer surface temperature of 50°C at a rate of 100 kW. Ignore the effects of motion and gravity and let To = 25°C, po = 1 bar. Determine (a) the ratio of the incoming mass flow rates, m/ṁ2. (b) the rate of exergy destruction, in kW. P2 = 1 bar Tz = 400°C 1 ṁy = 0.7 kg/s Pi = 1 bar T, = 40°C Feedwater heater X3 = 25% P3 = 1 bar Tp = 50°C %3D 2)

7.25 As shown in Fig. P7.25, a 1-lb metal sphere initially at 2000°R is removed from an oven and quenched by immersing it in a closed tank containing 25 lb of water initially at 500°R. Each substance can be modeled as incompressible. An appropriate constant specific heat for the water is c 1.0 Btu/lb °R, and an appropriate value for the metal is cm = 0.1 Btu/lb oR. Heat transfer from the tank contents can be neglected. Determine the exergy destruction, in Btu. Let To = 77°F. System boundary Metal sphere: Tmi=2000°R m=0.1 Btu/lb R mm= 1 lb Water: Twj=500°R =1.0 Btu/lb R m 25 lb FIGURE P7.25

1. The first law of thermodynamics discussesa. Thermal equilibriumb. Energy conservationc. Direction of heat flowd. Entropy is zero at absolute zero temperature 2. A tank contains 1 kg mass gas whose density is 700 kg/m3. The pressure is increased from 1 bar to 3 bar. The approximate specific boundary work of the system isa. Cannot be find since some data is missingb. 285 kJ/kgc. 0 kJ/kgd. 0.285 kJ/kg 3. The nozzle is a device in whicha. Area decreases b. Area increasesc. Velocity decreases d. Velocity increases 4. Choose the correct statement/s with respect to entropy change during a processa. Entropy increases with increase in pressure at constant temperatureb. Entropy increases with increase in temperature at constant pressurec. Entropy can be kept constant by systematically increase both pressure and temperatured. Entropy can not be changed 5. The isentropic process is also called asa. Adiabatic processb. Irreversible adiabatic processc. Reversible adiabatic processd. Reversible…