Increase in Pressure Drop

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. 

High pressure drop causes disruptions to the system hydraulics. Because of the high pressure drop, the lead membranes tend to operate at very high fluxes while the lag membranes operate at low flux. This increases the rate of membrane fouling for both the lead and lag membranes. Lead membranes foul faster because more water is forced to the membrane module faster and the rate of contaminant accumulation in the boundary layer on the membrane surface increases. The lag membranes, on the other hand, experience low flows since most of the water is removed through 

Kucera-Gienger, Studies on Halogen Interaction with Polyamide Type Reverse Osmosis Membranes, Master s Thesis, UCLA, March, 1984. 

Reverse Osmosis A Practical Guide for Industrial Users, 2nd Ed., Tall Oaks Publishing, Inc., Littleton, CO, 2002. 

Craig Granlund, Dow Water Solutions, personal communication, August 18,2008. 

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The most important feature of the pressure filters which use hydrauHc pressure to drive the process is that they can generate a pressure drop across the medium of more than 1 x 10 Pa which is the theoretical limit of vacuum filters. While the use of a high pressure drop is often advantageous, lea ding to higher outputs, drier cakes, or greater clarity of the overflow, this is not necessarily the case. Eor compressible cakes, an increase in pressure drop leads to a decrease in permeabiUty of the cake and hence to a lower filtration rate relative to a given pressure drop. 

Flow Maldistribution. One of the principal reasons for heat exchangers failing to achieve the expected thermal performance is that the fluid flow does not foUow the idealized anticipated paths from elementary considerations. This is referred as a flow maldistribution problem. As much as 50% of the fluid can behave differently from what is expected based on a simplistic model (18), resulting in a significant reduction in heat-transfer performance, especially at high or a significant increase in pressure drop. Flow maldistribution is the main culprit for reduced performance of many heat exchangers. 

OTSGs also experience deposition of material on the flow areas in the tube support plates which causes an increase in pressure drop and eventual reductions in plant power production. 

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. 

Flooding across a column section reflects itself in an increase in pressure drop and a decrease in temperature difference across the affected section. Product quality is also impaired, but it is hoped that the other indicators will allow correction of the situation before major change in product quality. When a column floods, the levels in the accumulator and bottom often change. It can occur that the accumulator fills with liquid carried over while the reboiler runs dry. 

In addition to the large increases in pressure drop, high liquid entrainment into the exiting gas stream also occurs. 

It is recommended that suspended solids be removed from the feed stream to a GAC column. If this is not possible in the scale-up design, then the effect of suspended solids should be included in the pilot run. In the upflow operation, most of the suspended solids work their way up through the GAC bed without a significant increase in pressure drop. 

This equation shows that the operating pressure drop is proportional to the square of the filtering velocity. For a fixed set of operating conditions, increasing filtering velocity to reduce the size of the collector will result in increases in pressure drop, fan power costs, and penetration and probably reduction in bag life. 

Hooding A condition that gives rise to a sharp decline in tray efficiency and a sharp increase in pressure drop. Flooding is commonly due to either an excessive carryover of liquid to the next tray, or to an inability of the system to convey the liquid flow to the tray below. 

A convenient chart for all fluids including a 20% increase in pressure drop over theoretical smooth tubes is given in the copyrighted figure of Reference 36 ... 

Check the recirculating CW pressure. An increase in pressure drop indicates fouled or plugged tubes, or a fouled tube sheet. 

Furthermore, it can be seen that for turbulent flow when n — 2, air injection would result in substantial increases in pressure drop. 

The comparison given above shows that a reduction in channel dimensions offers some substantial benefits in cases where surface reactions are involved or efficient heat and mass transfer are needed. One important conclusion to be drawn is that a decrease in the channel diameter at fixed efficiency does not necessarily mean an increase in pressure drop. Rather, the pressrue drop can be kept constant... 

The upper limit to vapour flow is set by the condition of flooding. At flooding there is a sharp drop in plate efficiency and increase in pressure drop. Flooding is caused by either the excessive carry over of liquid to the next plate by entrainment, or by liquid backing-up in the downcomers. 

The major disadvantage in using heat transfer enhancement is that it increases the pressure drop. In retrofit, this can be important, as the pumps driving the flow might be limited in their capacity to meet the required increase in pressure drop. 

Thus, all of the tube inserts will potentially provide the required enhancement without the need for extra heat transfer area. However, it would be preferred to have the enhancement with minimum increase in pressure drop. Table 15.9 shows the increase in friction factor for the various inserts. 

Also, as discussed in Chapter 15, rather than install additional heat transfer area to cater for the new operational requirements, heat transfer enhancement can be considered. Changes to the number of tube passes or the baffle arrangement might allow the heat transfer coefficient to be enhanced. Alternatively, tube inserts could be used. This was discussed in Chapter 15. The major disadvantage in using heat transfer enhancement is that it increases the pressure drop. In retrofit this can be important, as the pumps driving the flow might be limited in their capacity to meet the required increase in pressure drop. 

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. 

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... 

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