Heat transfer driving force

The design procedure starts by identifying the minimum utility cost for a given heat-transfer driving force. Next, the fixed and operating costs are traded off by iterating over the driving forces until the minimum total annualized cost TAC is attained. 

Heat exchange area can be estimated from the balanced composite curves. The simplest is adopting the hypothesis of counter-current, as well as vertical heat transfer driving force. The total area A. is obtained by summing the differential heat-exchange area in different temperature intervals, as expressed by the relation ... 

The reactor bed is not assumed to be adiabatic, but loses heat from its walls. Presently a constant heat flux from the walls is assumed. A flux profile can easily be imposed, based on heat transfer driving force, but its effect will be negligible. In fact, if the reactor were assumed adiabatic the results would be essentially unchanged. The heat loss from the reactor walls was retained, so that the effects of the heat losses can be easily demonstrated. 

Are there any safety considerations in the choice of heat transfer driving force (AT) in Problem 4 Explain. 

Logarithmic mean tcmperaruie diffnence is calculated by Eq. (5). Logarithirtic mean tiemperaturc difTerence multiplied by correction factor (F> is the effective mean temperature difference. It is a measure of effective heat transfer driving force in a heat exchanger. 

By employing a minimum heat-exchange driving force of A T , one can establish a one-to-one correspondence between the temperatures of the hot and the cold streams for which heat transfer is feasible, i.e.. 

Bcomb mass transfer driving force with combustion Rvap = mass transfer driving force without combustion c = effective normalized reactant concentration Cp = specific heat (per g of mixture)... 

Effective thermal conductivity and effective moisture diffusivity are related to internal heat and mass transfer, respectively, while air boundary heat and mass transfer coefficients are related to external heat and mass transfer, respectively. The above transport properties are usually coefficients in the corresponding flow rate and driving force relationship. The equilibrium material moisture content, on the other hand, is usually related to the mass transfer driving force. 

Note that the flow rate of aqueous ammonia is limited, but chilled methanol is considered to be available in unlimited amounts. Note, also that the total amount of H2S to be transferred to the absorbent(s) is 0.06255 -I- 0.00507 = 0.06762 kg/s. This is less than the capacity of the aqueous ammonia. However, as in heat exchange, where a driving force is necessary to transfer the heat, mass exchange also requires a driving force and, at this point in the synthesis, it is not known whether sufficient mass-transfer driving forces exist to utilize the capacity of the aqueous ammonia. If not, then the use of chilled methanol must be considered. 

Figure 6.6 illustrates what happens to the cost of the system as the relative position of the composite curves is changed over a range of values of AT ir,. When the curves just touch, there is no driving force for heat transfer at one point in the process, which would require an... 

Here p is the chemical potential just as the pressure is a mechanical potential and the temperature Jis a thennal potential. A difference in chemical potential Ap is a driving force that results in the transfer of molecules tlnough a penneable wall, just as a pressure difference Ap results in a change in position of a movable wall and a temperaPire difference AT produces a transfer of energy in the fonn of heat across a diathennic wall. Similarly equilibrium between two systems separated by a penneable wall must require equality of tire chemical potential on the two sides. For a multicomponent system, the obvious extension of equation (A2.1.22) can be written... 

In the macroscopic heat-transfer term of equation 9, the first group in brackets represents the usual Dittus-Boelter equation for heat-transfer coefficients. The second bracket is the ratio of frictional pressure drop per unit length for two-phase flow to that for Hquid phase alone. The Prandd-number function is an empirical correction term. The final bracket is the ratio of the binary macroscopic heat-transfer coefficient to the heat-transfer coefficient that would be calculated for a pure fluid with properties identical to those of the fluid mixture. This term is built on the postulate that mass transfer does not affect the boiling mechanism itself but does affect the driving force. 

Likewise, the microscopic heat-transfer term takes accepted empirical correlations for pure-component pool boiling and adds corrections for mass-transfer and convection effects on the driving forces present in pool boiling. In addition to dependence on the usual physical properties, the extent of superheat, the saturation pressure change related to the superheat, and a suppression factor relating mixture behavior to equivalent pure-component heat-transfer coefficients are correlating functions. 

AT driving force for the binary macroscopic heat transfer ... 

An analogy exists between mass transfer by diffusion and heat transfer by conduction. Each involves coHisions between molecules and a gradient as the driving force which causes flow. Eor diffusion, this is a concentration gradient for conduction, the driving force is an energy gradient. Eourier s... 

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