Elementary Principles 254 - 7.20 (a) H = 1.04 T C - 25 b g...

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7- 9 7.20 (a) ± . ± HT H 104 25 C in kJ kg bg ± .. H out 9.36 kJ kg =− = 104 340 25 ± . H in 5 kJ kg = 104 300 25 20 Δ ± ... H =−= 9 36 520 16 4k J k g ΔΔ Δ ²² ² ² ² HE E Q W kp s ++= assumed no moving parts ² E k E p W s == = 0 0 ch ² ± QH n H 1.25 kW kg 1 kJ / s 10 g 1 mol 4.16 kJ kW 1 kg 28.02 g mol s 3 ⇒= = = ² ² ² . n Q H Δ 10 7 = 10.7 mol 22.4 L STP 303 K kPa s mol 273 K 110 kPa L/s L s ² . . V 1013 2455 246 (b) Some heat is lost to the surroundings, some heat is needed to heat the coil, enthalpy is assumed to depend linearly on temperature and to be independent of pressure, errors in measured temperature and in wattmeter reading. 7.21 (a) ± ±± . ± . ± Ha Tba HH TT bH a T =+ = = = = = U V | W | ° 21 11 129 8 258 50 30 52 52 30 130 2 1302 b g kJ kg C ± . . =⇒ = = ° 0 25 ref C Table B.1 = = × SG V . ± . CH l 3 3 61 4 m 659 kg mk
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