Heat-transfer coefficient doubling

Quality of fluidization increases for large particles heat transfer coefficients double from 1 to 10 bar for larger particles but not smaller ones. 

Minimum Area. The limit of minimum network area is presented in References 2 and 3. If idealized double-pipe exchangers are used, a heat-exchange network having minimum area can quickly be developed for any In the limiting case, where all heat-transfer coefficients are assumed to be equal, the area for this network can easily be obtained from the composite streams by... 

Sindlady, heating surface area needs are not direcdy proportional to the number of effects used. For some types of evaporator, heat-transfer coefficients decline with temperature difference as effects are added the surface needed in each effect increases. On the other hand, heat-transfer coefficients increase with temperature level. In a single effect, all evaporation takes place at a temperature near that of the heat sink, whereas in a double effect half the evaporation takes place at this temperature and the other half at a higher temperature, thereby improving the mean evaporating temperature. Other factors to be considered are the BPR, which is additive in a multiple-effect evaporator and therefore reduces the net AT available for heat transfer as the number of effects is increased, and the reduced demand for steam and cooling water and hence the capital costs of these auxiUaries as the number of effects is increased. 

Of these special surfaces, only the double-fluted tube has seen extended services. Most of the gain in heat-transfer coefficient is due to the condensing side the flutes tend to collect the condensate and leave the lauds bare [Caruavos, Proc. First Int. Symp. Water Desalination, 2, 205 (1965)]. The coudeusiug-film coefficient (based on the actual outside area, which is 28 percent greater than the nominal area) may be approximated from the equation... 

These devices are replacing the older tank and spiral-conveyor devices. Better provisions for speed and ease of fill and discharge (without powered rotation) minimize downtime to make this batch-operated device attractive. Heat-transfer coefficients ranging from 28 to 200 W/(m °C) [5 to 35 Btu/(h fF °F)] are obtained. However, if caking on the heat-transfer walls is serious, then values may drop to 5.5 or 11 W/(m °C) [1 or 2 Btu/(h fH °F)], constituting a misapplication. The double cone is available in a fairly wide range of sizes and construction materials. The users are the fine-chemical, pharmaceutical, and biological-preparation industries. 

To reduce the pressure drop, a batch reactor with a half-pipe jacket of length L and flowrate W can be partitioned into a two-zone jacket, each with a length L/2 and each supplied with W jacket flowrate. This doubles the jacket flow at a lower pressure drop in each zone. The flow in each zone can then be increased to increase the outside and overall heat transfer coefficients, which is similar to those of the single-zone jacket. 

The outer and inner tubes extend from separate stationary tube sheets. The process fluid is heated or cooled by heat transfer to/from the outer tube s outside surface. The overall heat transfer coefficient for the O.D. of the inner tube is found in the same manner as for the double-pipe exchanger. The equivalent diameter of the annulus uses the perimeter of the O.D. of the inner tube and the I.D. of the inner tube. Kem presents calculation details. 

A forward-feed double-effect standard vertical evaporator with equal heating areas in each effect is fed with 5 kg/s of a liquor of specific heat capacity of 4.18 kJ/kgK, and with no boiling-point rise, so that 50 per cent of the feed liquor is evaporated. The overall heat transfer coefficient in the second effect is 75 per cent of that in the first effect. Steam is fed at 395 K and the boiling-point in the second effect is 373 K. The feed is heated to its boiling point by an external heater in the first effect. 

A double-effect forward-feed evaporator is required to give a product which contains 50 per cent by mass of solids. Each effect has 10 m2 of heating surface and the heat transfer coefficients are 2.8 and 1.7 kW/m2 K in the first and second effects respectively. Dry and saturated steam is available at 375 kN/m2 and the condenser operates at 13.5 kN/m2. The concentrated solution exhibits a boiling-point rise of 3 deg K. What is the maximum permissible feed rate if the feed contains 10 per cent solids and is at 310 K The latent heat is 2330 kJ/kg and the specific heat capacity is 4.18 kJ/kg under all the above conditions. 

A liquor containing 15 per cent solids is concentrated to 55 per cent solids in a double-effect evaporator, operating at a pressure in the second effect of 18 kN/m2. No crystals are formed. The flowrate of feed is 2.5 kg/s at 375 K with a specific heat capacity of 3.75 kJ/kg K. The boiling-point rise of the concentrated liquor is 6 deg K and the steam fed to the first effect is at 240 kN/m2. The overall heat transfer coefficients in the first and second effects are 1.8 and 0.63 kW/m2 K, respectively. If the heat transfer area is to be the same in each effect, what areas should be specified ... 

A double-effect forward-feed evaporator is required to give a product which contains 50.0 per cent by mass of solids. Each effect has 10 m2 of heating surface and the heat transfer coefficients are 2.8 and... 

The process design of double-pipe exchangers is practically the simplest heat exchanger problem. Pressure drop calculation is straightforward. Heat transfer coefficients in annular spaces have been investigated and equations are rated in Table 8.10. A chapter is devoted to this equipment by Kern (1950). 

The contents of a reaction vessel are heated by means of steam at 393 K supplied to a heating coil which is totally immersed in the liquid. When the vessel has a layer of lagging 50 mm thick on its outer surfaces, it takes one hour to heat the liquid from 293 to 373 K. How long will it take if the thickness of lagging is doubled Outside temperature = 293 K. Thermal conductivity of lagging = 0.05 W/mK. Coefficient for heat loss by radiation and convection from outside surface of vessel = 10 W/m2 K. Outside area of vessel = 8 m2. Coil area = 0.2 m2. Overall heat transfer coefficient for steam coil = 300 W/m2 K. 

Double-pipe heat exchangers are widely used for smaller flow rates. When the heat-transfer coefficient outside is too low, a solution consists of using longitudinal finned tubes as an extended surface. 

Water at the rate of 68 kg/min is heated from 35 to 75°C by an oil having a specific heat of 1.9 kJ/kg °C. The fluids are used in a counterflow double-pipe heat exchanger, and the oil enters the exchanger at 110°C and leaves at 75°C. The overall heat-transfer coefficient is 320 W/m2 °C. Calculate the heat-exchanger area. 

Instead of the double-pipe heat exchanger of Example 10-4, it is desired to use a shell-and-tube exchanger with the water making one shell pass and the oil making two tube passes. Calculate the area required for this exchanger, assuming that the overall heat-transfer coefficient remains at 320 W/m2 °C. 

Hot water at 90°C flows on the inside of a 2.5-cm-ID steel tube with 0.8-mm wall thickness at a velocity of 4 m/s. This tube forms the inside of a double-pipe heat exchanger. The outer pipe has a 3.75-cm ID, and engine oil at 20°C flows in the annular space at a velocity of 7 m/s. Calculate the overall heat-transfer coefficient for this arrangement. The tube length is 6.0 m. 

A small steam condenser is designed to condense 0.76 kg/min of steam at 83 kPa with cooling water at 10°C. The exit water temperature is not to exceed 57°C. The overall heat-transfer coefficient is 3400 W/m2 °C. Calculate the area required for a double-pipe heat exchanger. 

A counterflow double-pipe heat exchanger is used to heat water from 20 to 40°C by cooling an oil from 90 to 55°C. The exchanger is designed for a total heat transfer of 29 kW with an overall heat-transfer coefficient of 340 W/m2 °C. Calculate the surface area of the exchanger. 

A counterflow double-pipe heat exchanger is used to heat liquid ammonia from 10 to 30°C with hot water that enters the exchanger at 60°C. The flow rate of the water is 5.0 kg/s and the overall heat-transfer coefficient is 800 W/m2 °C. The area of the heat exchanger is 30 m2. Calculate the flow rate of ammonia. 

For shells with triple or double segmental baffles, the heat-transfer coefficient calculated for turbulent flow (DG/fi greater than 8000) should be multiplied by a value of 1.3. 

Determine the overall heat-transfer coefficient The heat-transfer coefficient on the tube side is proportional to G0 8. The mass velocity G will be doubled because the exchanger is to be converted to two passes on the tube side. Therefore, = 178.6(2)0 8 = 310.9 Btu/(h)(ft2)(°F) [1764 W/(m2)(K)]. The new overall heat-transfer coefficient can now be calculated ... 

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