Pressure Drop in the Tube Side

The pressure drop for flow of a liquid or gas without phase change through straight tubes can be calculated using the following equation  

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An alternative method is presented in the following sections which can be used to determine pressure drop in the tube side and the shell side of the shell and tube heat exchanger. 

Estimating the pressure drop in the tube side is much easier than calculated pressure drop in the shell side. The pressure drop in the tube side is calculated using the following equations [6] ... 

The pressure drop in the tube side is calculated using the following equations [6] ... 

Pressure drop on the tube-side of a shell and tube exchanger is made up of the friction loss in the tubes and losses due to sudden contractions and expansions and flow reversals experienced by the tube-side fluid. The friction loss may be estimated by the methods outlined in Section 3.4.3 from which the basic equation for isothermal flow is given by equation 3.18 which can be written as ... 

Equation 25 calculates the effective tube length, L, where L is the tube length, ft. Ngg is the number of exchangers in series. This equation takes into consideration pressure drops on the tube side other than that inside the tube. 

I then asked the day-shift foreman if he had any thoughts on the cause of the high pressure drop across the tube side of the condensers. He indicated he really didn t know, but he wondered if it could have something to do with coke fines that accumulated in the fin passages of the condenser tube bundle. 

A large process plant air cooler may have 10, 20, 30, or more banks of air coolers, arranged in parallel. Figure 19.6 shows such an arrangement. Let s assume that the inlet header is oversized and has zero pressure drop. Let s also assume that the outlet header is oversized and also has no AP. The pressure drop across the tube side of all such air coolers arranged in parallel is then identical. 

The shape of the cooling and warming curves in coiled-tube heat exchangers is affected by the pressure drop in both the tube and shell sides of the heat exchanger. This is particularly important for two-phase flows of multicomponent systems. For example, an increase in pressure drop on the shell side causes boiling to occur at a higher temperature, while an increase in pressure drop on the tube side will cause condensation to occur at a lower temperature. The net result being both a decrease in the effective temperature difference between the two streams and a requirement for additional heat transfer area to compensate for these losses. 

Coiled-tube heat exchangers frequently have flow distribution problems that include (1) tube distribution (2) two-phase tube distribution and (3) two-phase shell distribution. Good flow distribution within the tubes can be obtained by designing the headers in such a way that their pressure drop is considerably less than that for the frictional pressure drop in the tubes. To obtain good shell-side distribution one must use symmetric bundles and separately introduce the vapor and liquid phases to the bundles. It is also advisable to arrange for downflow of the shell-side fluid. For two-phase annular flow, the vapor will flow mostly in the space between the tube layers while the liquid needs to be carefully distributed in the radial direction for proportionate vapor-liquid flow normal to each tube layer. To avoid convection on the shell side due to density gradients, it is normal practice to use sufficiently large pressure drops on the shell side. 

Subcooling in a shell-and-tube condensers. Figure 13.3 is the same propane condenser shown in Fig. 13.2. Let s assume that the pressure drop through the shell side is zero. Again, we are dealing with a pure component propane. The inlet vapor is at its dew point. That means it is saturated vapor. Under these circumstances, the outlet liquid should be saturated liquid, or liquid at its bubble point. As the inlet dew-point temperature is 120°F, the outlet bubble-point temperature should be 120°F. But, as can be seen in Fig. 13.3, the outlet shell-side liquid temperature is 90°F, not 120°F. Why ... 

The baffle cut, shown in Fig. 19.3, is usually about 20 to 30 percent of the diameter of the baffle. The smaller the baffle cut, the more perpendicular the flow across the tubes. Perpendicular flow encourages desirable cross-flow velocity and vortex shedding. But a smaller baffle cut will also increase the pressure drop on the shell side. 

Ans. (a) The viscous fluid will be directed in the shell side. The higher the viscosity, the higher will be the pressure drop (i.e., resistance to flow), and if it is directed in the tube side, the pressure drop will be still larger, (b) The corrosive fluid will be directed in the tube side, and if any tube becomes corroded, it can be replaced. 

CALCULATE THE TOTAL PRESSURE DROP EXCLUDING THE NOZZLE IN THE TUBE SIDE, psi. 

Plain Flat Fins on a Staggered Tubebank. This geometry, as shown in Fig. 17.146, is used in the air-conditioning/refrigeration industry for cost considerations as well as where the pressure drop on the fin side prohibits the use of enhanced/interrupted flat fins. An inline tube-bank is generally not used unless very low finside pressure drop is the essential requirement. The heat transfer correlation for Fig. 17.146 for flat plain fins on staggered tubebanks is provided by Gray and Webb (see Webb [47]) as follows for four or more tube rows with the subscript 4. 

Shellside Pressure Drop. Surprisingly little attention has been devoted in engineering literature to estimate two-phase pressure drop on the shell side of shell-and-tube heat exchangers [77, 78]. In engineering practice, the estimation of the two-phase flow pressure drop can be performed in some situations using modified single-phase flow correlations. This approach is, however, highly unreliable. 

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