Overall heal transfer coefficient

Figure 3. Effect of sphere diameter on overall heal transfer coefficient. Top curve, G = 2.73 kg/rn2 s bottom curve, G = 1.40 kg/m s.

C Why are the windows considered in three regions when anolyzing heat transfer tlirough them Name those regions and explain how the overall LZ-value of the window is determined when the heal transfer coefficients for all three regions are known. 

Steam at 40°C condenses on the outside of a 3-cm diameter thin horizontal copper rube by cooling water that enters the tube at 25°C at an average velocity of 2 m/s and leaves at 35°C. Determine the rale of condensation of steam, the average overall heal transfer coefficient between the steam and the cooling water, and the mbe length. 

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. 

Work will an overall heal transfer coefficient U oi a total thermal resistance R, expressed as... 

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%... 

I- shell-pass and 4-lube-passes condenser, with 30 lobes in each pass, at 30 C (fi — 2431 kJ/kg). Cooling water (Cp = 4.18 kJ/kg C) enters the lubes at 12 C at a rale of 2 kg/s. If the heal transfer area is 14 m . ind the overall heal transfer coefficient is 1800 W/m °C, the rate of heal transfer in this condenser is... 

Assume U, overall heal transfer coefficient (Benz, 2011) 250.00 wW c... 

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. 

An open cylindrical tank 500 mm diameter and I m deep is three quarters filled with a liquid ol density 980 kg/mJ and of specific heat capacity 3 kj/kg K. If the heat transfer coefficient from the cylindrical walls and the base of the tank is 10 W/m2 K and front the surface is 20 W/m3 K, what area of heating coil, fed with steam at 383 K. is required to heat the contents from 288 K to 368 K in a half hour The overall heat transfer coefficient for the coil may be taken as 100 W/m2 K, the surroundings we at 288 K and the heal capacity of the tank itself may be neglected. 

The heat transfer area between the reactor and jacket is 140 The overall heat transfer coefficient is 70 Btu/h °F ft. Mass of the metal walls can be negleaed. Heal losses are negligible. 

The overall heat-transfer coefficient U depends upon the properties of the dry product and the method of heal transfer. The heat-transfer rate A is influenced by the mechanical design of the heating elements and the conditioning of the frozen mass. The temperature gradient AT is limited by the maximum allowable temperatures al the sublimation interface and dry-layer surface. In the constant-rate period, the lirst one-half to two-thirds of the drying cycle, about 8fl + of the water is removed. 

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 Overall heat transfer coefficient (the /-value) of a wall under winter design conditions is U = 2,25 W/m °C. Now a layer of 100-mm face brick is added to the outside, leaving a 20-mm air space between the wall and die bricks. Determine the new /-value of the wall. Also, determine the rate of heal transfer through a 3-m-high, 7-m-long section of the wall after modification when the indoor and outdoor temperatures are 22°C and -25°C, respeciively. 

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. 

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