Vapor flow increase

Lowering the compressor discharge pressure can be achieved by changing to cooler water to the refrigerant condenser. But how does this affect the horsepower load or electrical power demand on the compressor s driver Well, it depends. It depends on how much the refrigerant vapor flow increases, as the compressor discharge pressure is reduced. 

Valve plates are proprietary designs (see Figure 11.22). They are essentially sieve plates with large-diameter holes covered by movable flaps, which lift as the vapor flow increases. 

Packed columns are not likely to be affected by inverse response, but may in principle experience a related problem. In the loading regime, a rise in vapor flow increases liquid holdup in the column. With scheme 16.4e, raising boilup will therefore induce a drop in bottom level, which in turn will reduce boilup. If sufficiently vigorous, this phenomenon can unsettle column boilup. The expectation of this behavior is based on first principles only the author is not aware of any troublesome reports of this behavior. 

Note that in Example 11.6-2 in the stripping section, the vapor flow increases slightly from 125.0 to 126.5 in going from the reboiler to near the feed tray. These values are lower than the value of 133.0 obtained assuming equimolal overflow. Similar conclusions hold for the enriching section. The enthalpy-concentration method is useful in calculating the internal vapor and liquid flows at any point in the column. These data are then used in sizing the trays. Also, calculations of <7r used... 

The self-regulation is, however, also rffected by the composition responses (the so-called composition self-regulation) a step change in H results, via a higher pressure and higher top temperature, in an increased top vapor flow. This will result in an increase of less volatile component in the top, as a result of which the top temperature increases even more and also the top vapor flow increases even more. As a result of this positive feedback, the pressure might eventually end up at a lower value (see Fig. 34.4). 

The reflxix to the top of this column is aromatic naphtha, which has been condensed and decanted in the water quench tower. The valve trays or sieve trays in the upper section of this column will cool the cracked gas from 320° to 350°F to between 210° and 250°F. This cooling is accomplished by evaporation of reflux so that the vapor flow increases from bottom to top of this upper section. The reflux flow is controlled to limit the outlet gas temperature to not less than 20°F above the water dew point, so that all the water in the cracked gas passes through this column. 

About 5 percent of the trayed towers have old style bubble-cap trays. Dirt and coke tends to accumulate under the cap. This cannot be seen unless the cap is removed. Coke underneath the bubble cap will restrict the vapor flow, increase vapor AP and promote downcomer... 

As may be seen from Figure 18.2-18.4 and 18.6, an increase of liquid flow onto a tray increases low boiler concentration. Therefore, although in the Icmg run a vapor flow increase will decrease low boiler concentration, in the short nm it increases it. This momentary change of ccHicentration in the wrong direction gives rise to the term inverse response. ... 

Inverse response or overshoot can be expected whenever two physical effects act on the process output variable in different ways and with different time scales. For the case of reboiler level mentioned above, the fast effect of a steam pressure increase is to spill liquid off the trays above the reboiler immediately as the vapor flow increases. The slow effect is to remove significant amounts of the liquid mixture from the reboiler through increased boiling. Hence, the relationship between reboiler level and reboiler steam pressure can be represented approximately as an overdamped second-order transfer function with a right-half plane zero. 

About 5 percent of the trayed towers have old style bubble-cap trays. Dirt and coke tends to accumulate under the cap. This cannot be seen unless the cap is removed. Coke underneath the bubble will restrict the vapor flow, increase vapor AP and promote downcomer backup and flooding, it only takes a moment to unscrew a bubble-cap using a crescent wrench. The maintenance folks will usually clean the deck, but not remove the caps. Liz injured herself crawling through a manway helping me discover this problem at the Motiva Refinery visbreaker fractionator in Convent, Louisiana. This tower flooded on... 

The upward flow of gas and Hquid in a pipe is subject to an interesting and potentially important instabiHty. As gas flow increases, Hquid holdup decreases and frictional losses rise. At low gas velocity the decrease in Hquid holdup and gravity head more than compensates for the increase in frictional losses. Thus an increase in gas velocity is accompanied by a decrease in pressure drop along the pipe, a potentially unstable situation if the flows of gas and Hquid are sensitive to the pressure drop in the pipe. Such a situation can arise in a thermosyphon reboiler, which depends on the difference in density between the Hquid and a Hquid—vapor mixture to produce circulation. The instabiHty is manifested as cycHc surging of the Hquid flow entering the boiler and of the vapor flow leaving it. 

Feed Slurry Temperature Temperature can be both an aid and a limitation. As temperature of the feed slurry is increased, the viscosity of the hquid phase is decreased, causing an increase in filtration rate and a decrease in cake moisture content. The limit to the benefits of increased temperature occurs when the vapor pressure of the hquid phase starts to materially reduce the allowable vacuum. If the hquid phase is permitted to flash within the filter internals, various undesired resiilts may ensue disruption in cake formation adjacent to the medium, scale deposit on the filter internals, a sharp rise in pressure drop within the filter drainage passages due to increased vapor flow, or decreased vacuum pump capacity. In most cases, the vacuum system should be designed so that the liquid phase does not boil. 

Absorbent Flow Failure - For lean oil absorption generally, no relief requirement results from lean oil failure. However, in a unit where large quantities of inlet vapor may be removed in the absorber, loss of absorbent could cause a pressure rise to relief pressure, since the downstream system may not be adequate to handle the increased flow. In such cases, the effect of this additional vapor flow into downstream equipment must be analyzed. 

This represents no change in overflow from the feed plate, and the increase in vapor flow is equal to the mols of feed. 

Slug/semi-annular flow. Here both slug and semi-annular flows were present. The vapor velocity increased with the heat flux and the rear of elongated bubbles began to break up (Fig. 2.30d). Coalescence was no longer clean and created a churn-like zone where the liquid slug had been. 

Since the pressure drop in two-phase flow is closely related to the flow pattern, most investigations have been concerned with local pressure drop in well-characterized two-phase flow patterns. In reality, the desired pressure drop prediction is usually over the entire flow channel length and covers various flow patterns when diabatic condition exist. Thus, a summation of local Ap values is necessary, assuming the phases are in thermodynamic equilibrium. The addition of heat in the case of single-component flow causes a phase change along the channel consequently, the vapor void increases and the phase (also velocity) distribution as well as the momentum of the flow vary accordingly. 

As the mixture accelerates, the globs of liquid break up downstream and are exposed to an accelerating vapor flow, with increasing relative velocity between the two phases. This relative velocity further breaks the drop into smaller ones depending on the Weber number,... 

The output of such high-speed dryers is limited by the increasing density of the water vapor flow. The grains of the product are floating in the vapor stream as in a fluidized bed, and the smallest particles are carried along with the vapor to the condenser. Even if only 1 % of the dried product is carried away, it sumps up to 10 kg per day if the throughput is 1000 kg per day. In 4 weeks, this totals to 280 kg or 1 m3 of coffee powder. To remove this out from the vapor stream very large filters have to be used in order to minimize the pressure drop in the filters. 

Type F. As the droplet diameter and/or impinging velocity further increases from the level in type V, the small droplets resulted from the breakup disperse in the radial direction, leading to transition into type F. In this mode, the radial film leaves the substrate surface in whole because the vapor beneath the film blows through in the radial direction. Thereafter, as the diameter of the film further increases, it is tom and broken up by the vapor into small droplets while the vapor flows beneath it. The small droplets are dragged by the vapor and disperse in the radial direction at velocities faster than... 

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