Two-pass flow

Refinery Presumably a fixtctionator A restrictive design of a transition tray converting sin e-pass flow to two-pass flow caused premature flooding. Avoid restrictive transition tray designs. 

Fig. 1.54 Two-pass flow scheme with downward flow in blanket assemblies and part of the seed assemblies...

During blowdown, the coolant flow in the reactor vessel of the Super FR is more complicated than that of the Super LWR due to the two-pass flow scheme. The coolant flow schemes are illustrated in Fig. 7.105 [37]. In order to analyze the blowdown of the two-pass core, the SPRAT-F is modified to the SPRAT-F-DP [37]. Flow redistribution among the downward flow seed channels, blanket channels, and downcomer are calculated in this code. The initial conditions for the LOCA analyses are the same as those for the safety analyses at supercritical pressure. 

The power raising phase in the plant startup was designed in consideration of the thermal and thermal-hydraulic stability criteria. Due to the two-pass flow scheme, the power to flow rate ratio became locally large in the downward flow channels during certain power and flow conditions. Thus, the flow rate required to satisfy those criteria was determined by the MCST and the decay ratio of thermal-hydraulic stability in the downward flow channels. 

Holding GPMAVFP below about 8 is preferred, although liquid as high as 20 GPMAVFP can and have been used. WFP is the width of the flow path in inches. Some companies like to use trays having no more than two passes. 

The following are several examples TEMA K-type shells, whieh allow for proper liquid disengagement for reboilers TEMA J-type shells, whieh aecommodate high vapor flows by allowing for divided flow in the shellside Two-pass TEMA F-type shells, whieh ean be used for applieations when a temperature eross exists (below) TEMA D-type front head designs, whieh are often the answer for high-pressure tubeside applieations. 

Eor one shell and multipass on the tube side, it is obvious that the fluids are not in true counter-current flow (nor co-current). Most exchangers have the shell side flowing through the unit as in Eigure 10-29C (although some designs have no more than two shell-side passes as in Eig-ures 10-IJ and 10-22, and the tube side fluid may make two or more passes as in Eigure 10-IJ) however, more than two passes complicates the mechanical construction. 

Figure 10-31. Fluid flows through two passes in tubes part of flow is parallel to shell-side fluid, and part is counterflow.

An oil cooler is to operate with an inlet of 138°F and an oudet of 103°F, and the cooling water enters at 88°F and is to be allowed to rise to 98°F. What is the corrected MTD for this unit, if it is considered as (a) a concentric pipe counterflow unit, (b) a single-pass shell-two-pass tube unit, and (c) a parallel flow unit ... 

Correction fectors should seldom be used when they fall below a value that lies on a curved portion of the P-R curves. Figure 10-34. That is, values on the straight portions of the curves have litde or no accuracy in most cases. For the single-shell pass—two or more than two passes unit chart, an F of less than about 0.8 would indicate consideration of a two-shell pass unit. As a general guide, F factors less than 0.75 are not used. To raise the F factor, the unit flow system, temperature levels, or both must be changed. 

Design faults in two-pass condensers and heat exchangers that can cause corrosion include poor division plate seals allowing the escape of water at high velocity between the passes, and flow patterns that produce stagnant zones. 

Figure 12.12. Shell types (pass arrangements), (a) One-pass shell (E shell) (b) Split flow (G shell) (c) Divided flow (J shell) (d) Two-pass shell with longitudinal baffle (F shell) (e) Double split flow (H shell)...

The loss in terms of velocity heads can be estimated by counting the number of flow contractions, expansions and reversals, and using the factors for pipe fittings to estimate the number of velocity heads lost. For two tube passes, there will be two contractions, two expansions and one flow reversal. The head loss for each of these effects (see Volume 1, Chapter 3) is contraction 0.5, expansion 1.0, 180° bend 1.5 so for two passes the maximum loss will be... 

A liquid reactant stream (1 mol/liter) passes through two mixed flow reactors in a series. The concentration of A in the exit of the first reactor is 0.5 mol/liter. Find the concentration in the exit stream of the second reactor. The reaction is second-order with respect to A and V2/V1 = 2. 

A stream of fully suspended fine solids (v = 1 mVmin) passes through two mixed flow reactors in series, each containing 1 m of slurry. As soon as a particle enters the reactors, conversion to product begins and is complete after two minutes in the reactors. When a particle leaves the reactors, reaction stops. What fraction of particles is completely converted to product in this system ... 

We note in passing that two-phase flow leads to circumstances where for a given phase there are source or sink terms. For example, consider situation wherein water droplets are evaporating in a moist-air flow. It is possible to write mass-conservation equations for the liquid and the vapor phases. The conversion of liquid to vapor (and vice-versa) causes... 

When we convert a tube bundle from two to four passes, the pressure drop increases by a factor of 8. For example, assume that the two-pass AP was 5 psig. With the same flow, the four-pass AP would be 40 psig. Let me explain ... 

For reasons of compactness of equipment, the paths of both fluids may require several reversals of direction. Two of the simpler cases of Figure 8.3 are (b) one pass on the shell side and two passes on the tube side and (c) two passes on the shell side and four on the tube side. On a baffled shell side, as on Figure 8.4(c), the dominant flow is in the axial direction, so this pattern still is regarded as single pass on the shell side. In the cross flow pattern of Figure 8.5(c),... 

T on the tubeside, T on the shellside. i = input, o = output, (a) One pass on shellside, any multiple of two passes on tubeside, (b) Two passes on shell side, any multiple of four on tubeside, (c) Cross flow, both streams unmixed laterally, (d) Cross flow, one stream mixed laterally, (e) Cross flow, both streams mixed laterally, (f) Effectiveness and number of transfer units in parallel and countercurrent flows, (g) Three shell passes, multiples of six on tubeside, (h) Four shell passes, multiples of eight on tubeside, (i) Five shell passes, multiples of ten on tubeside, (j) Six shell passes, multiples of 12 on tubeside. 

The correlations for sieve and bubblecap trays have no provision for multipass flow of liquid. Their basic data may have been obtained on smaller towers with liquid flow equivalent to two-pass arrangement in towers 8 ft dia. The sieve tray correlation should be adapted to multipass flow by comparison with results obtained by the valve tray correlation in specific cases. 

DIVIDE LIQUID FLOW RATE BY 2 -- 13) OBTAIN DIAMETER FROM TWO-PASS TRAY LINE... 

---