Pressure/heat transfer duties

In retrofit situations, existing heat exchangers might be subjected to changes in flowrate, heat transfer duty, temperature differences or fouling characteristics. Heat transfer coefficients and pressure drops can be approximated from... 

For large heat transfer duties, it is good practice to recover the steam that is flashed as the condensate reduces in pressure. Such an arrangement is shown in Figure 23.19. Steam enters the steam heater and condensate (in practice, with some steam) passes through the trap. Flashing occurs before the mixture enters a settling drum that allows the flash steam to be separated from the condensate. The flash steam would then be fed to a steam main at the appropriate pressure. 

Number of transfer units Pressure Vapor pressure Heat-transfer rate Condenser duty Reboiler duty External-reflux ratio... 

The design change was implemented to replace existing tube bundles with single hehcal baffle bundles (Figure 7.11). The helical flow eliminated dead flow zones as well as enhanced heat transfer on the shell side due to the countercurrent flow. Not only was heat transfer duty augmented greatly, but pressure drop was reduced at the same time. 

Process flow diagrams (PFD) are drawings, wtudr show how to process the raw material to products. On the drawings, it shows all the equipment, major process lines, stream number, all the control loops, and normal operating conditions (temperature and pressure). It shows how the raw material will be processed from one equipment to another, and at what conditions. A brief description of each equipment will also be shown on PFD, see Figure 1. For the vessel, its dimension will be shown. For heat exchanger or fire heater, its normal heat transfer duty will be shown. For pump or compressor, it normal flow rate and differential pressure will be shown. 

FIG. 11-62 Vibratory-conveyor adaptations as indirect heat-transfer equipment, a) Heavy-duty jacketed for bqiiid coolant or high-pressure steam, (h) Jacketed for coolant spraying, (c) Light-duty jacketed construction, (d) Jacketed for air or steam in tiered arrangement, (e) Jacketed for air or steam with Mix-R-Step surface, Coutiesy of Jeffrey Mfg. Co.)... 

For liquid/liquid duties, the plate heat exchanger will usually give a higher overall heat transfer coefficient and in many cases the required pressure loss will be no higher. 

For those condensing duties where permissible pressure loss is less than 0.07kpf/cm there is no doubt but that the tubular unit is most efficient. Under such pressure-drop conditions only a portion of the length of a plate heat exchanger plate would be used and a substantial surface area would be wasted. However, when less restrictive pressure drops are available the plate heat exchanger becomes an excellent condenser, since very high heat transfer coefficients are obtained and the condensation can be carried out in a single pass across the plate. 

Another advantage in retrofit is that an existing frame can often accommodate additional plates if a higher capacity is required. For the same flowrate, an increase in the number of plates decreases the flowrate through the channels and therefore the heat transfer coefficients. However, if the flowrate increases, the number of plates can be increased to accommodate a higher duty (at the expense of increased pressure drop). 

Garimella and Bandhauer [32] conducted heat transfer experiments using the same test sections that were used for the pressure drop experiments of Garimella et al. [24, 25, 27, 28] described above. The high heat transfer coefficients and low mass flow rates in microchannels necessitate modifications to the test facility and test procedures described above. For the small zlx required in the test section, the heat duties at the mass fluxes of interest are relatively small. Calculating this heat duty from the test section inlet and outlet quality measurements would result in considerable... 

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