Page 188 - Chemical process engineering design and economics

170 Chapter 4

Table 4.3 continued

mixtures containing appreciable fractions of these components will generally have substantially higher heat transfer
coefficients.
1 Superheat of a pure vapor is removed at the same coefficient as for condensation of the saturated vapor if the exit
coolant temperature is less than the saturation temperature (at the pressure existing in the vapor phase) and if the
(constant) saturation temperature is used in calculating the mean temperature difference. But see note k for vapor
mixtures with or without noncondensable gas.
' Steam is not to be condensed on conventional low-finned rubes; its high surface tension causes bridging and retention
of the condensate and a severe reduction of the coefficient below that of the plain tube.
* The coefficients cited for condensation in the presence of noncondensable gases or for multicomponent mixtures are
only for very rough estimation purposes because of the presence of mass transfer resistances in the vapor (and to some
extent, in the liquid) phase. Also, for these cases, the vapor-phases temperature is not constant, and the coefficient
given is to be used with the mean temperature differences estimated using vapor-phase inlet and exit temperatures,
together with the coolant temperatures.
'As a rough approximation, the same relative reduction in low-pressure condensing coefficients due to noncondensable
gases can also be applied to higher pressures.
* Absolute pressure and noncondensables affect condensing coefficients for medium and heavy organics in
approximately the same proportion as for light organics. Because of thermal degradation, fouling may become quite
severe for the heavier condensates. For large fractions of noncondensable gas, interpolate between pure component
condensation and gas cooling coefficients.
" "Narrow condensing range" implies that the temperature difference between dew point and bubble point is less than
the smallest temperature difference between vapor and coolant at any place in the condenser.
" "Medium condensing range" implies that the temperature difference between dew point and bubble point is greater
than the smallest temperature difference between vapor and coolant, but less than the temperature difference between
inlet vapor and outlet coolant.
f Boiling and vaporizing heat transfer coefficients depend very strongly on the nature of the surface and the structure of
the two-phase
flow past the surface in addition to all of the other variables that are significant for convective heat transfer in other modes. The flow
velocity and structure are very much governed by the geometry of the equipment and its connecting piping. Also, there is a maximum
heat flux from the surface that can be achieved with reasonable temperature differences between surface and saturation temperature of
the boiling fluid; any attempt to exceed this maximum heat Qux by increasing the surface temperature leads to partial or total coverage
of the surface by a film of vapor and a sharp decrease in the heat flux.
Therefore, the heat transfer coefficients given in this table aie only for very rough estimating purposes and assume the use of plain or
low-finned tubes without special nucleation enhancement. Ars//_ max is the maximum allowable temperature difference between
surface and saturation temperature of the boiling surface. No attempt is made in this table to distinguish among the various types of
vapor-generation equipment, since the major heat transfer distinction to be made is the propensity of the process steam to foul Severely
fouling streams will usually call for a vertical thermosiphon or a forced-convection (tube-side) reboiler for ease of cleaning.
'Subcooling heat load is transferred at the same coefficient as latent heat load in kettle reboilers, using the saturation temperature in
the mean temperature difference. For horizontal and vertical thermosiphons, a separate calculation is required for the sensible heat
transfer area, using appropriate sensible heat transfer coefficients and the liquid temperature profile for the mean temperature difference.
r
Aqueous solutions vaporize with nearly the same coefficient as pure water if attention is given to boiling-point elevation and if the
solution does not become saturated and care is taken to avoid dry wall conditions.
J
For boiling of mixtures, the saturation temperature (bubble point) of the final liquid phase (after the desired vaporization has taken
place) is to be used to calculate the mean temperature difference. A narrow-boiling-range mixture is defined as one foi which the dif-
ference between the bubble point of the incoming liquid and the bubble point of the exit liquid is less than the temperature difference
between the exit hot stream and the bubble point of the exit boiling liquid. Wide-boiling-range mixtures require a case-by-case analysis
and cannot be reliably estimated by these simple procedures.
Example 4.1 Optimum Cooling-Water Exit Temperature___________
The exit water temperature could be calculated by minimizing the total cost of
operating a heat exchanger. This optimization problem is approached by listing all
the relationships and variables to determine if there are any degrees of freedom.
Table 4.4.1 lists the equations for the optimization. The mass flow rate of cooling
water into the heat exchanger equals the mass flow rate of water out, as given by
Equation 4.4.1, where the subscript, w, refers to water. Also, we must calculate the
amount of heat transferred from the process stream to the water stream, so that an
energy balance is written for the tube side instead of over the entire heat ex-
changer, which would eliminate Q. Because the kinetic energy and potential en-
ergy changes are usually insignificant, and the work term is zero, the energy equa-

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