Reference no: EM13950358
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.
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State
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T(K)
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p(bar)
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h(kJ/kg)
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s°(kJ/kg ?K)a
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1
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1500
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2
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1635.97
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3.4452
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2
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300
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2
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300.19
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1.7020
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3
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-
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1.9
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968.08
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2.8869
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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.
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: 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 cen..
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