Loss in Pressure Drop

It is unusual to find a loss in pressure drop under nominal operating conditions for most industrial applications.4 If a loss in pressure drop is recorded, it is typically a result of faulty instrumentation. 

---

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. 

These numbers show that, first, the theoretical work can be closely approached by actual work after known inefficiencies are identified and, second, the dominant driving force losses are in pressure drop and temperature difference. This is a characteristic of towers having low relative volatiUties. 

The shape of the coohng 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 shellside causes boiling to occur at a higher temperature, while an increase in pressure drop on the tubeside will cause condensation to occur at a lower temperature. The net result is 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. 

Example B. Suppose in the previous example that the loss rate had been specified as not to exceed 40 lbs/hr. To minimize the increase in pressure drop accompanying any increase in inlet velocity necessary to reduce Dth to a value which would bring performance up to the desired level, it might be more expedient to instead increase the gas discharge velocity. 

Pipe entrance and exit pressure losses should also be calculated and added to obtain the overall pressure drop. The loss in pressure due to sudden expansion from a diameter dtl to a larger diameter dl2 is given by the equation... 

The precursors for the formation of heavy polymer and coke deposits are initially formed as a result of an ineffective or damaged feed injection system. Loss of pressure drop across the injector nozzle(s) is an indication the flow has been lost an increase in pressure drop indicates a plugged nozzle. In either case, the catalyst to... 

Pressure drop measures the loss in pressure from the feed to the concentrate. In effect, it measures the loss in driving force for water across the membrane (see Chapter 12.3.1.3 and Equation 12.6). Factors that result in an increase or decrease in pressure drop are discussed below. 

A number of factors can lead to high pressure drop, including membrane scaling, colloidal fouling, and microbial fouling. These three factors all involve deposition of material onto the surface of the membrane as well as onto components of the membrane module, such as the feed channel spacer. This causes a disruption in the flow pattern through the membrane module, which, in turn, leads to frictional pressure losses or an increase in pressure drop. 

Hie most commonly found shape of catalyst particle today is the hollow cylinder. One reason is the convenience of manufacture. In addition there are often a number of distinct process advantages in the use of ring-shaped particles, the most important being enhancement of the chemical reaction under conditions of diffusion control, the larger transverse mixing in packed bed reactors, and the possible significant reduction in pressure drop. It is remarkable (as discussed later) that the last advantage may even take the form of reduced pressure losses and an increased chemical reaction rate per unit reactor volume [11]. 

Loss of pressure drop due to restrictions and friction, the pressure drops in the pipelines. This causes the boiling temperature to decrease and thus a fraction of hydrogen to evaporate. This part can be calculated in dependency of the vapor fraction x ... 

Hydraulic Losses.—The hydraulic losses are the losses in pressure caused by the gas friction and by the sudden changes in the gas velocity or direction of flow. On the basis of D. W. Taylor s experiments on the flow of air in pipes, the pressure drop in the suction and in the discharge pipes (pounds per square inch) = L = lv s/4 00,-GOOD, where I is the length of pipe, feet, v the velocity of gas, feet per second, s the specific gravity of gas referred to free air (0.0764) as unity, and D the diameter of pipe, inches. For pipes of first-class workmanship and in very best condition, this loss may be reduced by about 20 per cent. The same care to have smooth pipe walls and to avoid too short bends should be taken with gases as with liquids. 

Pressure recovery. If the flow through the venturi meter were frictionless, the pressure of the fluid leaving the meter would be exactly equal to that of the fluid entering the meter and the presence of the meter in the line would not cause a permanent loss in pressure. The pressure drop in the upstream cone — pj would be completely recovered in the downstream cone. Friction cannot be completely eliminated, of course, and a permanent loss in pressure and a corresponding loss in power do occur. Because of the small angle of divergence in the recovery cone, the permanent pressure loss from a venturi meter is relatively small. In a properly designed meter, the permanent loss is about 10 percent of the venturi differential Pa Pb and approximately 90 percent of the differential is recovered. 

The influence of these losses in temperature drop on the capacity of a multiple-effect evaporator is shown in Fig. 16.11. The three diagrams in this figure represent the temperature drops in a single-effect, double-effect, and triple-effect evaporator. The terminal conditions are the same in all three i.e., the steam pressure in the first effect and the saturation temperature of the vapor evolved from the last effect are identical in all three evaporators. Each effect contains a liquid with a boiling-point elevation. The total height of each column represents the total temperature spread from the steam temperature to the saturation temperature of the vapor from the last effect. 

---