# PS_80 - drops so low that ice forms on the surface of ponds...

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ChBE421 Problem Set #8 Fall 2009 1. A countercurrent heat exchanger (area = 2 m 2 ) is being used to heat a liquid with C p =0.47 kcal/kg o C using hot combustion gases with C p = 0.47 Kcal/kg o C. The liquid enters at 25°C at a mass flow rate of 130 kg/hr and the gases enter at 550 °C at a flow rate of 40 kg/hr. The individual heat transfer coefficients in kcal/hr ∙m 2 o C are: convection for gas 20, convection for liquid 10. Conductive resistances are negligible. Find the outlet temperatures. 2. A countercurrent heat exchanger is needed to cool a liquid ( C p = 0.38 kcal/kg °C) from 90 °C to 25 °C at a flow rate of 1200 kg/hr, using cooling water at an inlet temperature of 10 °C. What flow rate of water is required if the outlet temperature of the water is not to exceed 30 °C? What is the effectiveness of a heat exchanger required for this application? 3. During a cold autumn in a frigid land (a.k.a. central Illinois), the air temperature occasionally
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Unformatted text preview: drops so low that ice forms on the surface of ponds and lakes. If the bulk temperature in a large lake is held constant at 36 oF, and the air temperature is constant at 25 F, find the thickness of the ice layer on a lake assuming steady state heat transfer. Thermal conductivity of ice: k = 1.3 Btu/hr ft F. Heat transfer coefficients: Ice to air (forced convection,wind) h = 5 Btu/hr ft 2 F Ice to water (natural convection) h = 2.5 Btu/hr ft 2 F 4. Hot combustion gases ( C p = 0.38 kcal/kg C) at 825 C are to be used to heat water from 25C to saturated steam at 145C. The mass flow rate of water is 45 kg/s. Heat transfer coefficients (kcal/m 2 hr C): gas, forced convection 55; water, forced convection 300; water, boiling 2800. H v = 530 kcal/kg Find the minimum flow rate of combustion gases. Find the required area of the heat exchanger for a flow rate 35% above this value....
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