Heat transfer coefficient external coil

A reaction vessel is heated by steam at 393 K supplied to a coil immersed in the liquid in the tank. It takes. 1800 s to heat the contents from 293 K to 373 K when the outside temperature is 293 K. When the outside and initial temperatures are only 278 K, it takes 2700 s to heat the contents to 373 K. The area of the steam coil is 2.5 rn2 and of the external surface is 40 m2. If the overall heat transfer coefficient from the coil to the liquid in the vessel is 400 W/m2 K, show that the overall coefficient for transfer from the vessel to the surroundings is about 5 W/m2 K. 

The heat transfer term envisions convection to an external surface, and U is an overall heat transfer coefficient. The heat transfer area could be the reactor jacket, coils inside the reactor, cooled baffles, or an external heat exchanger. Other forms of heat transfer or heat generation can be added to this term e.g, mechanical power input from an agitator or radiative heat transfer. The reactor is adiabatic when 7 = 0. 

For the calculation of the heat transfer coefficient of the external film, some models are also available [2], describing the hydraulics of the flow in the jacket or in the half-welded coils. The results depend strongly on the technical design of the equipment. Thus, the direct experimental determination is mostly preferred. 

The inside convective heat transfer coefficient h, is the only element of the overall heat transfer coefficient U that varies with the agitation speed N. If heat is removed from an agitated reactor using an internal coil or external jacket, the overall heat transfer rate depends on the rotation speed of the agitator N and if the process side offers the major resistance. This is expressed by... 

For a heat-transfer coefficient of 500 W/m2/K, the necessary area is A = 190m2. This requires use of cooling coils, or performing the cooling operation in an external heat exchanger. The second alternative is presented in Figure 9.13. 

Table IX presents heat transfer data for an external catalyst cooler of the FFB type. Heat flux of 70-150 kW/m2 and heat transfer coefficient of 350-490 W/(m2-k) were obtained, both somewhat less than those of a turbulent bed internal cooler (horizontal coil type), but approximately equal to those of the external cooler of the bubbling bed type.

The overall heat transfer coefficient, U, does not remain the same for a long period of operation. Corrosion, dirt, or various other solids deposited on the internal or external surfaces of the heating coil result in a gradual decrease of the heat transfer coefficient. This, in turn, will cause the time constant of the system to vary. This example is characteristic of what can happen to even simple first-order systems. 

For reactions with a sizable heat effect, heat is abstracted in several ways reflux condenser, internal coil, external heat exchanger, cooling jacket, and half-round pipes wound on the reactor body. The overall heat transfer coefficient with water in the tank for a water-cooled or steam-heated jacketed vessel is 0.15-1.7, for a tank with water-cooled half-round pipe is 0.3-0.9, and for a tank with water-cooled internal coil is 0.5-1.2kW/m K (Rose, 1981). 

Heat Transfer Enhancements Heat transfer enhancements increase the film heat transfer coefficient, thus improving U and consequently heat tfansfer in the exchanger. In the case of the ubiquitous ST heat exchanger, heat transfer enhancement can be achieved on the shell and/or tube sides as required. Tube-side enhancements help in improving the film heat transfer coefficient on the tube side, and are useful if the exchanger involved has lower film heat transfer coefficient on the tube side. Tube-side enhancements include, but are not limited to, twisted-tape inserts, coiled-wire inserts and internal fins. Similarly, shell-side enhancements are used to improve the heat transfer coefficient on the shell side. They include helical baffles, external fins and Expanded Metal (EM) baffles. More details on heat transfer enhancements are available in Pan et al. (2013). 

Neglecting the thermal resistance of the coil wall, the overall heat transfer coefficient based on the external area of the coil, U is given by ... 

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