Heat exchanger operation

During vaporization or condensation, thermal and hydraulic performances depend essentially on the two-phase flow structure. Furthermore, as very often in industrial processes, the heat exchanger operates in thermosyphon or under natural circulation knowledge of the pressure drop and liquid holdup is of major importance. But there is very little information on two-phase flow characteristics in compact geometries. 

Heat exchangers operate for long periods with no change in the operating conditions thus, they can be modelled as steady-flow devices, for which the following assumptions are valid ... 

A shell-and-tube heat exchanger operates with two -shell passes and four tube passes. The shell fluid is ethylene glycol, which enters at I40°C and leaves at 80°C with a flow rate of 4500 kg/h. Water flows in the tubes, entering at 35°C and leaving at 85°C. The overall heat-transfer coefficient for this arrangement is 850 W/m2 - °C. Calculate the flow rate of water required and the area of the heat exchanger. 

Calculate the area of heat transfer of a shell and tube heat exchanger operated in a countercurrent fashion under the following conditions. Overall heat transfer ([/) = 4000 kcal/h m2 °C, heat duty (Q) = 6,00,000 kcal/h,... 

Figure 4.17. Temperature profiles for heat exchangers operated in cocurrent and countercurrent modes.

Example 5.7 Equipartition principle Heat exchanger For a heat exchanger operating at steady state, the total entropy generation P is obtained by integrating over the surface area... 

Many devices such as turbines, compressors, boilers, condensers, and heat exchangers operate for long periods of time under the same conditions, and they are classified as steady flow devices. (Note that the flow field near the rotating blades of a turbomachine is of course unsteady, but we consider the overall flow field rather tlian the details at some localities when we classify device.s.) During steady flow, the fluid properties can change from point to point within a device, but at any fixed point they remain constant. Therefore, the volume, the mass, and the total energy content of a steady-flow device or flow section remain constant in steady operation. 

In general, the major problem in heat exchanger operation is the accumulation of deposits on surfaces. To avoid taking the exchanger out of service for cleaning, on-line fouling control is often employed. The choice of method will, of course, very much depend on the fouling problem encountered. There are two basic methods control may be achieved by the use of chemical additives and by physical methods, or in some instances, with a combination of the two. 

Recent developments in low pressure methanol synthesis processes result from improvements in energy efficiency obtained in various parts of the plant including converter, separator, and heat exchange operations. There is also the on-going development of improved co-precipitated catalysts. However, there have been no major advancements to rival that achieved by I.C.I. in the mid 1960 s. 

Verify that AHUs, fans, and heat exchangers operate per design ratings. 

AHU, fan, and heat exchanger operations must meet or exceed their respective design ratings. 

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