1. An open feedwater heater operates at steady state with liquid water entering inlet 1 at 10 bar, 50°C, and a mass flow rate of 10 kg/s. A separate stream of steam enters inlet 2 at 10 bar and 200°C. Saturated liquid at 10 bar exits the feedwater heater. Stray heat transfer and the effects of motion and gravity can be ignored. Let T0 5 20°C, p0 5 1 bar. Determine (a) the mass flow rate of the streams at inlet 2 and the exit, each in kg/s, (b) the rate of exergy destruction, in kW, and (c) the cost of the exergy destroyed, in $/year, for 8400 hours of operation annually. Evaluate exergy at 8.5 cents per kW ? h.
2. Figure and the accompanying table provide a schematic and steady-state operating data for a mixer that combines two streams of air. The stream entering at 1500 K has a mass flow rate of 2 kg/s. Stray heat transfer and the effects of motion and gravity are negligible. Assuming the ideal gas model for the air, determine the rate of exergy destruction, in kW. Let T0 5 300 K, p0 5 1 bar.
|
State
|
T(K)
|
p(bar)
|
h(kJ/kg)
|
s°(kJ/kg ?K)a
|
|
1
|
1500
|
2
|
1635.97
|
3.4452
|
|
2
|
300
|
2
|
300.19
|
1.7020
|
|
3
|
-
|
1.9
|
968.08
|
2.8869
|
as0 is the variable appearing in Eq. 6.20a and Table A-22. Mixes with a separate stream of steam entering at 20 lbf/in.2, 250°F with a mass flow rate of 0.38 lb/s. A single mixed stream exits at 20 lbf/in.2, 130°F. Heat transfer from the mixing chamber occurs to its surroundings. Neglect the effects of motion and gravity and let T0 5 70°F, p0 5 1 atm. Determine the rate of exergy destruction, in Btu/s, for a control volume including the mixing chamber and enough of its immediate surroundings that heat transfer occurs at 70°F.