Heat-transfer duty

FIG. 11-64 A pneumatic-transport adaptation for heat-transfer duty. (Courtesy ofWerne [Pg.1097]

The [nimary objective of the VOC-condensation system is to meet mass-recovery objectives. However, heat is a key element in realizing the mass objectives. Hence, the mass and heat interactions of the problem have to be identified and reconciled. This can be achieved by converting the VOC-recovery task from a mass-transfer problem to a heat-transfer duty. This can be accomplished by relating the... 

Sc = Schmidt number, dimensionless Pr = Prandtl number, dimensionless Cg = gas specific heat, Btu/lb-°F a = interfacial area, fti/fti Q, = sensible heat transfer duty, Btu/hr Qj. = total heat transfer duty, Btu/hr... 

Although an increase in flowrate can result in increased film transfer coefficients, an increase in flowrate is also usually accompanied by an increase in heat transfer duty. In turn, this might lead to a need for increased heat transfer area. 

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. 

It rather seems that 40 percent of the surface area of the radiator in Fig. 13.1 is submerged under water. If the water is drained out, does this mean that the rate of steam condensation will increase by the same 40 percent. Answer—yes Does this mean that the radiator heat transfer duty will increase by 40 percent Answer—not quite. 

Reduce the overall heat-transfer duty from the radiator. 

Let s assume that the cooling airflow to all five banks are the same. Banks A and B in Fig. 14.6 have low outlet temperatures. Banks C, D, and E have must hotter outlets. Question Which coolers are handling most of the heat-transfer duty Is it A and B or C, D, and E ... 

Actually, retrofitting a tube bundle with low fin tubes often reduces heat-transfer capacity. This happens when the controlling resistance to heat transfer is shell-side fouling. The fouling deposits get trapped between the tiny fins. This acts as an insulator between the shell-side fluid and the surface of the tubes. In severe shell-side fouling services, I have replaced fin tubes with bare tubes, and doubled the heat-transfer duty on the exchanger. 

The final subject discussed in this chapter is the issue of reactor scaleup. Moving from a laboratory test tube in a constant temperature bath to a 20-L pilot plant reactor to a 200,000-L commercial plant reactor involves critical design and control decisions. One major problem is the reduction of the heat transfer area relative to the reactor volume (and heat transfer duty) as we move to larger reactors. This has an important effect on temperature control and reactor stability. 

Figure 7 -64 shows an example. This arrangement is really a high capacity steam trap. In the layout, the top of the condensate pot should be at least in line with the bottom of the exchanger to avoid flooding the tubes with condensate and adversely affecting the exchanger heat transfer duty. 

Cooney et al. (1969), Junker (2004), and Benz (2011) suggest that the heat transfer duty can be estimated as 110 kcal/mol of oxygen uptake. Using this value, the heat evolved is 550 kcal/mVh, which is 25% lower than the value obtained from... 

The need for a heat exchanger is to satisfy process requirement in terms of heat duty Q) and temperatures (LMTD or ATijvi). Thus, a heat exchanger is designed to have a certain surface area (A) to fulfill the process requirement. Based on the process temperature requirement, a certain amount of heat duty must be transferred. Under the basis of heat transfer duty and process temperatures, the required U value ean be calculated from the Fourier equation (6.1) ... 

First, heat exchanger heat balance calculations are conducted in a flowsheet simulation software, which has adequate thermal data and can describe process streams according to their physical properties and operating conditions. By providing measured temperatures, the simulation can determine the heat transfer duty from Q = m Cp AT. At the same time, the simulation calculates transfer capability by lumping overall heat transfer coefficient and surface area together as U - A = 2/ATlm> where ATlm is defined in equation (6.7) in Chapter 6. 

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. 

For heat exchangers, the key operating parameter is the U value, which affects heat transfer duty. 

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