Pressure head and

The basic design is that of the Ostwald viscometer a U-tube with two reservoir bulbs separated by a capillary, as shown in Figure 24a. The Hquid is added to the viscometer, pulled into the upper reservoir by suction, and then allowed to drain by gravity back into the lower reservoir. The time that it takes for the Hquid to pass between two etched marks, one above and one below the upper reservoir, is a measure of the viscosity. In U-tube viscometers, the effective pressure head and therefore the flow time depend on the volume of Hquid in the instmment. Hence, the conditions must be the same for each measurement. 

Examine pressure heads and maximise whenever reasonably practicable to minimise vapour cavity formation. 

Pandit and co-workers have shown that scale-up may be possible on a more rational basis if cavitation is employed, and some data have been reported by Pandit and Mohalkar (1996), Mohalkar et al. (1999), Senthil et al. (1999), and Cains et al. (1998). A variety of reactors can be used, viz. the liquid whistle reactor, the Branson sonochemical reactor, the Pote reactor, etc. The principal factors affecting the efficiency of a hydrodynamic cavitation reactor are irreversible loss in pressure head and turbulence and friction losses in the reaction rates. 

In this case, the flow rate is to be determined when a given fluid is transported in a given pipe with a known net driving force (e.g., pump head, pressure head, and/or hydrostatic head). The same total variables are involved, and hence the dimensionless variables are the same and are related in the same way as for the unknown driving force problems. The main difference is that now the unknown (Q) appears in two of the dimensionless variables (/ and 7VRe), which requires a different solution strategy. 

The flow of fluids is most commonly measured using head flowmeters. The operation of these flowmeters is based on the Bernoulli equation. A constriction in the flow path is used to increase the flow velocity. This is accompanied by a decrease in pressure head and since the resultant pressure drop is a function of the flow rate of fluid, the latter can be evaluated. The flowmeters for closed conduits can be used for both gases and liquids. The flowmeters for open conduits can only be used for liquids. Head flowmeters include orifice and venturi meters, flow nozzles, Pitot tubes and weirs. They consist of a primary element which causes the pressure or head loss and a secondary element which measures it. The primary element does not contain any moving parts. The most common secondary elements for closed conduit flowmeters are U-tube manometers and differential pressure transducers. 

FIGURE 3.24 Groundwater conditions near the ground surface. Saturated and saturated zones (a), profile of moisture content vs. depth (b), pressure head and hydraulic head relationships insets = water retention under pressure heads less than (top) and greater than (bottom) atmospheric (c), profile of pressure head vs. depth (d), and profile of hydraulic head vs. depth (e). (After Freeze and Cherry, 1979.)... 

The term (z - 2i) may be called a static pressure head, and it can be expressed in feet or inches of water, or some height of any liquid. For example, barometric pressure can be expressed in inches of mercury. 

Equation (5.52) is Bernoulli s theorem [16], an energy equation that is well known to be equal to the sum of the elevation head, the pressure head, and the velocity head. Here the elevation head has been left out because the fan suction air pressure and the exiting air pressure from the air cooler are both equal. 

Head-type flowmeters include orifice plates, venturi tubes, weirs, flumes, and many others. They change the velocity or direction of the flow, creating a measurable differential pressure, or "pressure head," in the fluid. Head metering is one of the most ancient of flow detection techniques. There is evidence that the Egyptians used weirs for measurement of irrigation water flows in the days of the Pharaohs and that the Romans used orifices to meter water to households in Caesar s time. In the 18th century, Bernoulli established the basic relationship between the pressure head and velocity head, and Venturi published on the flow tube bearing his name. 

Figure 10.4 Radius of influence of a Rhizon sampler as a function of the pressure head and the soil type.

Cavitation will not occur as long as the pressure at the suction side of the pump is sufficiently high. The suction pressure required to avoid cavitation depends on the pump design and is specified by the pump manufacturer. The term manufacturers use to describe the pressure required is NPSH (net-positive-suction-head). NPSH is defined as the difference between the pressure head and the head corresponding to the liquid vapor pressure at the pump inlet, i.e.,... 

Other models directly couple chemical reaction with mass transport and fluid flow. The UNSATCHEM model (Suarez and Simunek, 1996) describes the chemical evolution of solutes in soils and includes kinetic expressions for a limited number of silicate phases. The model mathematically combines one- and two-dimensional chemical transport with saturated and unsaturated pore-water flow based on optimization of water retention, pressure head, and saturated conductivity. Heat transport is also considered in the model. The IDREAT and GIMRT codes (Steefel and Lasaga, 1994) and Geochemist s Workbench (Bethke, 2001) also contain coupled chemical reaction and fluid transport with input parameters including diffusion, advection, and dispersivity. These models also consider the coupled effects of chemical reaction and changes in porosity and permeability due to mass transport. 

In the method of the Jailing meniscus a liquid-wetted tapering tube is placed vertically in a reservoir, as in fig. 1.26. Inside the tube liquid is held by the capillary pressure. The tube is now moved upwards - or the liquid in the vessel downwards - to increase the hydrostatic pressure head, and this is continued until the liquid in the capillary collapses. From the hydrostatic head the Laplace pressure is obtained and from that the surface tension. The method is very simple and may be considered as the counterpart of the maximum bubble pressure technique there are also similarities to the situation sketched in fig. 1.8a. The idea is rather old... 

Thus, these three laws may be used to determine the effect of fan speed, fan size, and gas density on flowrate, developed static pressure head, and horsepower. For two conditions, where the constants k remain unchanged, these equations become... 

This unit consists of a steel, aluminum, or stainless-steel cylinder (preferably furnished with a dip tube), a pressurized head, and a device allowing cooling for long enough to ensure equilibration. 

The principle of the orifice meter is identical with that of the venturi. The reduction of the cross section of the flowing stream in passing through the orifice increases the velocity head at the expense of the pressure head, and the reduction in pressure between the taps is measured by the manometer. Bernoulli s equation provides a basis for correlating the increase in velocity head with the decrease in pressure head. 

The process engineer has to size both the centrifugal pump and the control valve. The bigger the control valve, the less pressure drop it requires. This results in a pump with a lower pressure head and lower energy costs because less power is consumed by the pump motor. Without considering control, the process engineer wants to design a system that has a low pressure drop across the control valve. From a steady-state standpoint, this makes perfect sense. 

We assume (1) the validity of Antoine s equation and Raoult s law, (ii) that pressure head and surface tension effects on the boiling point are negligible. 

BERNOULLI S THEOREM - In stream of liquid, the sum of elevation head, pressure head and velocity remains constant along any line of flow provided no work is done by or upon liquid in course of its flow, and decreases in proportion to energy lost in flow. 

Pressure forming is the reverse of vacuum forming. The plastic sheet is clamped, heated until it becomes soft, and sealed between a pressure head and the RIM of a mold. By applying air pressure (Figure 2.40),... 

The main purpose of the pressure head, as a connection element between extmsion barrel and die, is to create uniform inflow conditions over the entire die area. To start with, a universal pressure head was employed for all types of clays and products, but it was quickly discovered that plastic materials required a longer pressure head than lean bodies. Also perforated bricks needed longer pressure heads than solid bricks to allow for ample space between core bow and end auger. Therefore a tripartite pressure head was designed, which made it possible to adjust the correct length of the pressure head, for instance in the event of alternating production of solid bricks and perforated bricks. Also there were lubricated pressure heads and those with a lateral cleaning door. 

Steam-heated pressure heads and extruder barrels had been known in the United States since the 1890s in context with stiff extrusion. After 1950, with the increasing importance of advanced ceramics and the use of nonplastic materials or bodies which were difficult to extrude and which had to be plastified with the aid of suitable bonding agents, there was also a... 

As already stated, the required compacting pressure P is determined mainly by the rheological features of the mass and execution of the pressure consumer, in detail, through the angle in the pressure head and mouthpiece, the friction conditions, the degree of transformation etc. 

The characteristic feature of flow laminations is the directional orientation of the anisometric particles in the shearing speed gradient of the auger channel, the pressure head and the die. Figure 3 pictures the fracture surfaces of extruded clay-body columns after drying showing various flow laminations caused by different clay mineral content. 

Row laminations do, of course, also occur when using piston extrusion presses as the ceramic body is subjected to shearing stress in the pressure head and die. It follows that flow laminations are primarily linked to the nature of the material particles. There would be no flow laminations with... 

The so-called S-crack or Y-crack is typical for a screw extruder with a double-blade or triple-blade point auger. This type of lamination is a combined consequence of overlapping of flow laminations, the rotational movements of the body, as well as the hollow space created by the auger hub in the centre of the emerging body clog , which will not entirely rejoin as a result of body characteristics or the geometry of the pressure head and die, all as described above. 

Twisting of the column transforms the initial round cavity, which has been tapered at the pressure head and influenced by the double-blade or triple-blade point auger, into an S-shaped or Y-shaped crack-line, and this may lead to substantial quality problems above all with solid columns. 

The following article deals mainly with pressure heads and dies in the shaping of ceramic body by means of extrusion. The state-of-the-art regarding the accomplishment of extrusion with maximum-achievable stress relief is described for the various fields heavy clay ceramics, fine ceramics, and technical ceramics. In addition to pressure head and die, the extruder itself also has a major influence on shaping. Extruder, pressure head and die must not be viewed individually they should be regarded as a full complete shaping assembly . However, for the purposes of the following article, the extruder itself is not dealt with in detail. 

Combination Pressure Head and Die Assemblies for Technical Ceramics... 

Fig. 2 Combination pressure head and die assemblies for ceramic monoliths...

Fig. 3 Laboratory production with a combination pressure head and die assembly for oval ceramic monoliths at 400 c.p.s.i.

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