Low-pressure zones

Cavitation Loosely regarded as related to water hammer and hydrauhc transients because it may cause similar vibration and equipment damage, cavitation is the phenomenon of collapse of vapor bubbles in flowing liquid. These bubbles may be formed anywhere the local liquid pressure drops below the vapor pressure, or they may be injected into the hquid, as when steam is sparged into water. Local low-pressure zones may be produced by local velocity increases (in accordance with the Bernouhi equation see the preceding Conservation Equations subsection) as in eddies or vortices, or near bound-aiy contours by rapid vibration of a boundaiy by separation of liquid during water hammer or by an overaU reduction in static pressure, as due to pressure drop in the suction line of a pump. 

Damage will be confined to the bubble-collapse region, usually immediately downstream of the low-pressure zone. Components exposed to high velocity or turbulent flow, such as pump impellers and valves, are subject. The suction side of pumps (Case History 12.3) and the discharge side of regulating valves (Fig. 12.6 and Case History 12.4) are frequently affected. Tube ends, tube sheets, and shell outlets in heat exchanger equipment have been affected, as have cylinder liners in diesel engines (Case History 12.1). 

Pumps arc used to transfer liquids from low-pressure zones to high-pressure zones ... 

This is a low flow condition where the discharge flow of the pump is restricted and the product cannot leave the pump. The liquid is forced to re-circulate from high-pressure zones in the pump into low-pressure zones across the impeller. 

Wear rings provide for a close running, renewable clearance, which reduces the amount of liquid leaking from the high pressure zones to the low pressure zones in the pump. They are commonly fitted in the pump easing and on the impeller (Figure 16-1, next page). 

Drift velocity The velocity of the air as it drifts from a high-pressure zone to a low-pressure zone in a building. 

Autumn (e.g. October) While the distribution of precipitation is similar to that exhibited in spring, the Central Massif as well as the south-eastern periphery of the Alps exhibit particularly high precipitation. Because of more intensive evaporation of warm maritime surfaces, coupled with greater frequency of low-pressure zones above the Mediterranean at this time of the year, more... 

This heat flux is responsible for the burning rate given by Eq. (3.71) in the low-pressure zone, as shown graphically in Fig. 7.7. 

As shown in Figure 11.8, a loop of nitrogen is realized between a low-pressure zone (flash evaporator EV01, heat exchanger EC01, and bioreactor R01), a medium-pressure zone (gas/liquid separator V04), and a high-pressure zone (final condenser V05). 

Chemical ionization is, as might be expected from its name, more chemically interesting and is closely allied to ion cyclotron resonance, which will be discussed in the next section. The principle of chemical ionization is simple. The molecule to be studied is injected into the ionizing region of the mass spectrometer in the presence of 0.5-1.5 mm Hg pressure of a gas, usually methane. Electron impact causes ionization of the methane, which is present in relatively large concentration. The ionization products of methane then react with the compound to be analyzed and convert it to ions. The gas mixture then exits into a low-pressure zone (10 4 mm) and the ions are analyzed according to mje in the usual way. 

A sharp decrease in wall friction in the flow of visco-elastic liquids (including chemically reactive liquids) leads to the formation of low pressure zones. The gas flows into these zones, which leads to disruption of the liquid stream in nozzle 5. Additional gas blown into the disrupted liquid stream through the ring gaps by pipelines 11 and 15 causes separation of the liquid into smaller droplets, thus improving spray dispersion. 

In reduced-pressure backflow preventers (Fig. 7), a zone is provided between the check valves, in which a relief valve continually and automatically maintains pressure at a level lower than supply pressure. Because fluids w ill not flow from a low-pressure zone to a higher-pressure zone, these devices are able to provide excel-... 

In the earliest commercially available freeze-dryers, the functions of the low-pressure zone and the vapour trap were both served by a chemical or ice trap. 

A major change in the design of the low-pressure zone was introduced in 1955. Dry ice used as coolant was replaced with mechanically cooled low-temperature condensers, which proved much easier to use and maintain. The earliest condensers were made of stainless steel. As the scope of freeze-drying expanded, a need for a chemically resistant type of trap arose. In response, titanium traps that proved much more resistant to corrosion were developed. 

During freeze-drying, the condenser temperature creates a low-pressure zone in the dryer for vapour to migrate. Such a zone should therefore be maintained at a preset, low level. If such a level is exceeded, then a pressure rise in the system can cause the product to melt down. Freezer-dryers equipped with condenser overload controls automatically cool the shelves until the emergency is solved. 

The flow round the stirrer blades interacts with the stationary baffles and produces a complex, circulating turbulent flow. When gas is sparged in a tank it collects in low pressure zones behind the stirrer blades forming gas cavities, which considerably influence the flow and the turbulence in the vessel. 

Zone 1—low-pressure zone prior to pump. This is a noncritical area served by Teflon tubing. A fritted filter is placed at the inlet to prevent particulates from entering the column. 

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