Tube Side Rating

One of the two heat exchanging fluids has a smaller film coefficient than the other. It could be called the weaker of the two fluids. Its coefficient is, therefore, the governing factor of the overall heat transfer coefficient. To reduce the exchanger size to an absolute minimum, the weaker of the two fluids should consequently be passed through that side of the exchanger, resulting in a smaller heat resistance value at the given pressure drop. 

In the turbulent flow region and at the same pressure drop the tube side often offers better heat transfer conditions than the shell side. This depends, however, on various factors tube length, number of tube passes, tube diameter, physical properties of the fluid, etc. To arrive at the optimum solution in a minimum of time, use of the following calculation method is suggested. The method is best explained by examples. The equations used have been derived from well-known heat transfer principles. 

Example 1. The weaker medium, liquid propane, is to be cooled from 135 to 65°F. 

Average temperature difference between the two heat-exchanging fluids At = 39.2°F. 

Heat transfer coefficient of shell-side fluid /I2 = 950 Btu/hr. sq. ft.°F 

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Tube-side Rates. The film coefficient for fluids flowing in forced convection inside tubes has been studied thoroughly by Sieder and Tate, and they found that three regions exist, each of which exhibits different film characteristics. Between Reynolds numbers of 100 to 2,100, viscous... 

VVi, VK Total mass rate of flow on tube side and shell side respectively of a heat exchanger kg/s IVh... 

This tube is useful when the shell-side fluid is not compatible with the material needed for the tube-side fluid, or vice versa. The thicknesses of the two different wall materials do not have to be the same. As a general rule, 18 ga is about as thin as either tube should be, although thinner gages are available. In establishing the gage thickness for each component of the tube, the corrosion rate of the material should be about equal for the inside and outside, and the wall thickness should still withstand the pressure and temperature conditions after a reasonable service life. 

To determine the true overall temperature difference, the correction factors, F, shown in Figure 10-34 are used to correct for the deviations involved in the construction of multipasses on the shell and tube sides of the exchanger. Note that R of the charts represents the heat capacity rate ratio , and P is the temperature efficiency of the exchanger. 

This same concept incorporating the TEMA charts can be used to (1) determine the fti heat transfer area required for an exchanger and (2) determine flow rate and outlet temperature of the fluids (shell or tube side) . 

Figure 10-57. Effect of velocity on heat transfer rates and pressure drop shell-side and tube-side. (Used by permission Shroff, P. D. Chemical Processing, No.4, 1960. Putnam Publishing Co., Itasca, III. All rights reserved.)...

Calculate the tube-side flow rate based upon the assumed number of tubes per pass and the heat balance. 

If the tube-side pressure drop exceeds a critical allowable value for the process system, then recheck by either lowering the flow rate and changing the temperature levels or reassume a unit with fewer passes on tube side or more tubes per pass. The unit must then be rechecked for the effect of changes on heat transfer performance. 

Determine the gpm tube-side flow rate and temperature rise for the overall unit to be certain that they are reasonable and consistent with heat load. 

L = temperature of inlet water to interval, °F Lo = temperature of outside tube wall, °F W( = tube side flow rate, Ib/hr (Cp)( = tube side specific heat, Btu/lb (°F) q = heat load of previous interval, Btu/hr... 

Determine the inside film coefficient using Equation 10-41 and Figure 10-46 for tube-side heat transfer. If two or more coils are in parallel, be certain that the flow rate per pipe is used in determining hj. Correct hj to outside of tube, giving hjo. Note that Figure 10-46 also applies to cast iron cooling sections. 

R is equal to the shell-side fluid flow-rate times the fluid mean specific heat divided by the tube-side fluid flow-rate times the tube-side fluid specific heat. 

G, Tube-side mass flow-rate per unit area ML-2T- ... 

Suppose the overall heat transfer coefficient of a shell-and-tube heat exchanger is calculated daily as a function of the flow rates in both the shell and tube sides (ws and wt, respectively). U has the units of Btu/(h)(°F)(ft2), and ws and wt are in lb/h. Figures E2.3a and E2.3b illustrate the measured data. Determine the form of a semiempirical model of U versus ws and wt based on physical analysis. 

Variation of overall heat transfer coefficient with tube-side flow rate wt for ws = 4000. 

Case 2. Coolant flow rate is fixed. Here At2 is known, so the tube side and shell side coefficients and area are optimized. Use Equation (/) and (J) to find h0 and hv A0 is then found from Equation (b). 

The basic reason for using different control-valve trims is to keep the stability of the control loop fairly constant over a wide range of flows. Linear-trim valves are used, for example, when the pressure drop over the control valve is fairly constant and a linear relationship exists between the controlled variable and the flow rate of the manipulated variable. Consider the flow of steam from a constant-pressure supply header. The steam flows into the shell side of a heat exchanger. A process liquid stream flows through the tube side and is heated by the steam. There is a linear relationship between the process outlet temperature and steam flow (with constant process flow rate and inlet temperature) since every pound of steam provides a certain amount of heat. 

Heat exchange provided by sensible-heat transfer is improved when velocities are higher. Especially when the heating fluid is on the tube side of an exchanger, sensible-heat-transfer rates are always increased by high velocity. 

The tube-side fluid now flows into the floating head, which acts as a return header for the tubes. The tube-side flow makes a 180° turn and flows back through the top hah of the floating-head tubesheet. The floating head is firmiy attached to the floating-head tubesheet. But why is it that one end of the tubes must be left free to float The reason is thermal expansion—or, more precisely, the differential rate of thermal expansion between the tubes and the shell. 

But suppose we are operating a heat exchanger subject to rapid rates of initial fouling. The start-of-run heat-transfer coefficient U is 120 Btu/[(h)(ft2(°F)]. Four months later, the U value has lined out at 38. The calculated clean tube-side velocity is lV2 ft/s. This is too low, but what can be done ... 

The resulting four-pass tube bundle will have a tube-side velocity twice as high as it did when it was a two-pass exchanger 3 ft/s. Experience has shown that in many services, doubling this velocity may reduce fouling rates by an order of magnitude. That is fine. But what about pressure drop ... 

Correlations for friction factors and heat transfer coefficients are rated in HEDH. Some overall coefficients based on external bare tube surfaces are in Tables 8.11 and 8.12. For single passes in cross flow, temperature correction factors are represented by Figure 8.5(c) for example charts for multipass flow on the tube side are given in HEDH and by Kays and London (1984), for example. Preliminary estimates of air cooler surface requirements ram be made with the aid of Figures 8.9 and 8.10, which are applied in Example 8.9. 

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