Heal transfer coefficient heat exchangers

SOLUTION Hot oil is cooled by water in a douhle-tube counter-flow heat exchanger. The overall heal transfer coefficient is to be determined. Assqmptions 1 The thermal resistance of the inner tube is negligible since the tube rhaterial is highly conductive and its thickness is negligible. 2 Both the oil and water flow are fully developed. 3 Properties of the oil and water are constant. 

C Under what conditions can the overall heal transfer coefficient ofa heat exchanger be determined from H = (l//i, -E... 

A tesi is conducted lo detennine the overaU heal transfer coefficient in a shell-and-Cube oil-to-watcr heat exchanger that has 24 tubes of internal diameter 1.2 cm and length 2 m in a single. shell. Cold water (c, = 4180 J/kg - "C) enters the tubes at 20°C at a rate of 3 kg/s and leaves at 55°C. Oil Cp = 2150 J/kg C) flows through the shell and is cooled from 120 C to 45 C. Detennine the overall heal transfer coefficient Ui of this heat exchanger based on the inner surface area of the tubes, /tnsiver 8.31 kW/m C... 

The mass flow rate, specific heat, and inlet temperature of the tube-side stream in a double-pipe, parallel-flow heat exchanger are 2700 kg/h, 2.0 kJ/kg K, and 120°C, respectively. The mass flow rate, specific heat, and inlet temperature of the other stream are 1800 kg/h, 4.2 kJ/kg K, and 20°C, respectively. The heat Iranster area and overall heal transfer coefficient are 0.50 and 2.0 kW/m K, respectively. Find the outlet temperatures of both streams in steady operation using (a) the LMTD method and (6) the effcctivcncss-NTU method. 

A polymer solution (c = 2.0 Id/kg K) at 20°C and 0.3 kg/s is heated by ethylene glycol (c = 2.5 kJ/kg K) at 60 C in a thin-walled double-pipe parallel-flow heat exchanger. The temperature difference between the two outlet fluids is 15°C. The overall heal transfer coefficient is 240 W/m K and the heal transfer aiea is 0.8 ra . Calculate (a) die rate of hem Uansfer, (b) the outlet temperature of polymer solution, and (c) the mass flow rare of ethylene glycol. 

In a parallel-flow, liquid-to liqiiid heat exchanger, Ute inlet and outlet temperatures of the hot fluid are 150°C and 90°C while that of the cold fluid ate 30°C and 70°C, respectively. For the same overall heal transfer coefficient, the percentage decre.ise in the. surface area of the heat exchanger if coimter-flow arrangement is used is (o) 3.9% (b) 9.7% (c) 14.5%... 

The heat flow to the reactor, AQ, is given in terms of the overall heal transfer coefficient. U, the heat exchange area, AA, and the difference between the ambient temperature T and the reactor temperature T. 

A shell-and-tube heat exchanger is used as an ammonia condenser with ammonia vapor entering the shell at 50°C as a saturated vapor. Water enters the single-pass tube arrangement at 20°C and the total heat transfer required is 200 kW. The overall heat-transfer coefficient is estimated from Table I0-I as 1000 W/m2 °C. Determine the area to achieve a heat exchanger effectiveness of 60 percent with an exit water temperature of 40°C. What percent reduction in heal transfer would result if the water flow is reduced in half while keeping the heat exchanger area and V the same ... 

The inner and outer surfaces of a 25-cm-thick wall in summer are at 27"C and 44 C, respectively. The outer surface of the wall exchanges heal by radiation with surrounding surfaces at 40°C, and convection svith ambient air also at 40 C with a convection heat transfer coefficient of 8 W/m - °C. Solar radiation is incident on the surface at a rate of 150 W/m If both the emissivity and the solar absorptivity of the outer surface are 0.8, determine the effective thermal conductivity of the wall. 

Perhaps you are wondering why we have two overall heat transfer coefficients (/, and U for a heal exchanger. The reason is that every heat exchanger has two heat transfer surface areas A-, and A, which, in general, are not equal to each other. 

EXAMPLE 11-1 Overall Heat Transfer Coefficient of a Heal Exchanger... 

A double-pipe parallel-flow heat exchanger is to heal water (c = 4180 I/fcg °C) from 25°C to 60°C at a rate of 0.2 kg/s. The healing is to be accomplished by geothermal water (c = 4310 J/kg °C) available at 140"C at a mass flow rale of 0.3 kg/s. The inner tube is thin-walled and has a diameter of 0.8 cm. If the overall heat transfer coefficient of the heat exchanger is 550 W/m °C, determine the length of the tube required to achieve the desired heating. 

Cold water (Cp = 4180 J/kg °C) leading to a shower enters a Ihin-wallcd double-pipe counter-flow heat exchanger at I5°C at a rate of 1.25 kg/s and is healed to 45"C by hot water (Cp = 4190 J/kg °C) that enters at 100°C at a tale of 3 kg/s. If the overall heat transfer coefficient is 880 W/ni °C, determine the rate of heat transfer and the heat transfer surface area of the heat exchanger. 

A shell-aod-tube heal exchanger vvilb 2-sheII passes and 12 lube passes is used lo heal waicr (c = 41801/kg "Q in the lubes from 20°C to 70°C at a rate of 4.5 kg/s. Heat is supplied by hot oil (c, = 2300 JAcg "C) that enters the shell side at I70°C at a rate of 10 kg/s. For a tube-side overall heat transfer coefficient of 350 W/m °C, determine the heat transfer surface area on the lube. side. Answer 25.7 rn ... 

C Consider a shell-and rube waler-lo-waler heat exchanger with identical mass flow rales for both the hot- and cold-water streams. Now the mass flow rale of the cold water is reduced by half. Will the effectiveness of this heal exchanger increase, decrease, or remain the same as a result of this inodl-ficaiion Explain. Assume the overall heat transfer coefficient and the inlet temperatures remain the same. 

A thin-walled double-pipe parallel-flow heal exchanger is used to heat a chemical whose specific heat is 1800 J/kg - C with hot water (c, = 4180 J/kg °C). The chemical enters at 20 C at a rate of 3 kg/s, while the water enters at 110°C at a rale of 2 kg/s. The heal transfer surface area of the heat exchanger is 7 and the overall heat transfer coefficient is 1200 W/m -"C. Delermine Ihe outlet temperatures of the chemical and the water. 

Cold water (Cp = 4180 J/kg C) enters the tubes of a heal exchanger with 2-shell passes and 23-Uibe passes at 14"C at a rate of 3 kg/, while hot oil (Cp 2200 J/kg Q enters the shell at 2(X) C at Ihe same mass flow rate. The overall heat transfer coefficient based on the outer surface of Ihe tube is 300 W/m "C and the heat transfer surface area on that side is 20 m Deieiinine the rate of heat transfer using (a) the LMTD method and (b) the e-NTU method. 

Hot oil is to be cooled in a multipass shell-and tube heal exchanger by water. The oil flows through the shell, with a heat transfer coefficient of/i = 35 W/m - X, and the water flows through the tube with an average velocity of 3 in/s. The lube is made of brass (k = 110 W/m X) with Internal and external diameters of 1.3 cm and 1,5 cm, respectively. Using water properties at 25X, determine the overall heat transfer coefficient of this heat exchanger based on ihe inner surface. 

Air(Cp 1005 J/kg °C) is to be preheated by hot exhaust gases in a cross-flow heal exchanger before it enters the furnace. Air enters the heal exchanger at 95 kPa and 20°C at a rale of 0.4 mV.s, The combustion gases (Cp = 1100 J/kg °Q enter at 180°C at a rate of 0.65 kg/s and leave at 95°C. The product of the overall heat transfer coefficient and the heal transfer surface area is UA - 1620 W/ C. Assuming both fluids to be unmixed. determine the rate of heat Iran.sfer. 

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