Thermodynamics HW Solutions 681

Thermodynamics HW Solutions 681 - ° C which is more than...

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Chapter 8 Internal Forced Convection ( c ) The energy consumed by the pump (except the heat dissipated by the motor to the air) is eventually dissipated as heat due to the frictional effects. Therefore, this problem is equivalent to heating the water by a 5118 kW of resistance heater (again except the heat dissipated by the motor). To be conservative, we consider only the useful mechanical energy supplied to the water by the pump. The temperature rise of water due to this addition of energy is C 0.55 ° = ° × = = Δ Δ = ) C kJ/kg 229 . 4 ( /s) m 5 . 1 )( kg/m 6 . 950 ( kJ/s) (5118 0.65 3 3 in elect, motor - pump elect p p C V W T T C V W ρ η Therefore, the temperature of water will rise at least 0.55
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Unformatted text preview: ° C, which is more than the 0.5 ° C drop in temperature (in reality, the temperature rise will be more since the energy dissipation due to pump inefficiency will also appear as temperature rise of water). Thus we conclude that the frictional heating during flow can more than make up for the temperature drop caused by heat loss. Discussion The pumping power requirement and the associated cost can be reduced by using a larger diameter pipe. But the cost savings should be compared to the increased cost of larger diameter pipe. 8-43...
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