Number of passes

With an even number of tube-side passes the floating-head cover serves as return cover for the tube-side fluid. With an odd number of passes a nozzle pipe must extend from the floating-head cover through the shell cover. Provision for both differential expansion and tube-bundle removal must be made. 

The ultimate distribution after an infinite number of passes is also shown in Fig. 22-6 and can be calculated for x < L — 1) From the following equation (Pfann, op. cit., p. 42) ... 

Figure 4.6 is interesting because it shows that for the best refining performance we need both a long zone and an impurity that is relatively insoluble in the solid (low k). Unfortunately long liquid zones can be destabilised by convection, and impurities with a low k do not come to order Commercial zone refining processes may therefore involve a large number of passes done one after the other (Fig. 4.7). This obviously adds a lot to the cost of the pure material, but the process can be speeded up considerably by using the multi-heater arrangement shown in Fig. 4.8.

Callan and Coleman [1977] have obtained this formula in the limit -r cc by summing up all multi-instanton contributions to Im F, i.e., taking into account the trajectories that pass through the barrier more than twice. These trajectories enter into Im F with factors exp( — nSinsj/n (where n is the number of passes) and, therefore, they can be neglected when Sins > 1-... 

The arithmetic average of the tube pass ATm s is the ATm corrected for number of passes. F = ATm cor-rected/ATM unconnected. 

Multipass Tray Number of Passes Minimum Diameter (ft) Preferred Diameter (ft)... 

Roll down pressure and speed, and number of passes. 

For a given shell length the number of pas.ses for each coil can be determined. Since the number of passes both upstream and downstream of the choke must be an even integer, actual Tj and T2 may differ slightly from that assumed in the calculation. The actual values of Tj and T2 can be calculated from actual coil lengths L, and L2. 

Once the total number of passes is known, the coil can be laid out geometrically assuming that the center-line minimum radius of bends is 1 times nominal pipe size. The required shell diameter is then determined. 

Other selections of shell length, number of passes and required diameters can then be made to obtain an optimum solution. 

Fire uihe area required and heater size (shell diameter, shell length, fire luhe rating, coil length and number of passes). 

Single-pass Tube Side. For these conditions, no baffle is in either the head or the return end of the unit. The tube-side fluid enters one end of the exchanger and leaves from the opposite end. In general, these baffles are not as convenient from a connecting pipe arrangement viewpoint as units with an even number of passes in which the tube-side fluid enters and leaves at the same end of the exchanger. See Figures 10-IC and 10-lG and Table 10-1. 

Four-pass Tube Side Even Number of Passes Tube Side. These conditions are often necessary to provide fluid velocities high enough for good heat transfer or to prevent the deposition of suspended particles in the tubes and end chambers. The higher the number of passes, the more expensive the unit. 

Size (In.) Number of Passes Net Free Distance 2 Passes Rows Across Size (In.) Niunber of Passes Net Free Distance 2 Passes Rows Across... 

Size (In.) Number of Passes Net Free Distance 2 Passes Rows Across... 

If this value is not reasonable, reassume the tube length, and/or the size of tubes. Try to keep the number of passes fewer than 8 except in special cases, as construction is expensive. 

Tube Number of passes Number of passes Side... 

Shell side Number of cross-passes (number of baffles -t 1) Number of passes Side two... 

Operated in this manner, the shell-and-tube type is a flooded evaporator (see Figure 7.3) and has oil drainage pots if using ammonia, or a mixture bleed system if the refrigerant is one of the halocarbons. The speed of the liquid within the tubes should be about 1 m/ s or more, to promote internal turbulence for good heat transfer. End cover baffles will constrain the flow to a number of passes, as with the shell-and-tube condenser. (See Section 6.4.)... 

Evaporators of this general type with dry expansion circuits will have the refrigerant within the tubes, in order to maintain a suitable continuous velocity for oil transport, and the liquid in the shell. These can be made as shell-and-tube, with the refrigerant constrained to a number of passes, or maybe shell-and-coil (see Figure 7.4). In both these configurations, baffles are needed on the water side to improve the turbulence, and the tubes maybe finned on the outside. Internal swirl strips or wires will help to keep liquid refrigerant in contact with the tube wall. 

In these designs there is one pass for the fluid on the shell-side and a number of passes on the tube-side. It is often an advantage to have two or more shell-side passes, although this considerably increases the difficulty of construction and, very often therefore, several smaller exchangers are connected together to obtain the same effect. 

A dimensionless dispersion number, based on the probable number of passes through the shear zone for an ingredient to be dispersed, was developed for scale-up of mixing of short fiber... 

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