Pressure 1 Head

Note The totals are differentials and neither gauge nor absolute values. 

The above examples purposely disregarded pressure head, friction, entrance, and exit head losses. 

Note that both the discharge and suction pressures must be on the same base/units. These illustrations are for static head only, while overall the pump has to work against the static and the pressure heads. (To be discussed.) 

231 + 50 = 281 ft ( Note that no flow friction losses or entrance/exit losses are included in this example) + 50 psig = +115 ft water (system fluid) 

Friction, entrance and exit heads, valve losses 

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Once fluidized, the bed behaves as if it were a fluid. A level is maintained and a static pressure head is generated. No flow of soHds through a side outlet occurs in a packed bed however, flow through the opening does occur after a fluidized state has been achieved (Fig. Ic). 

Subsequent to stock preparation and proper dilution, the paper furnish usually is fed to the paper machine through one or more screens or other devices to remove dirt and fiber bundles. It then enters a flow spreader which provides a uniform flowing stream and which is the width of the paper machine. The flow spreader, or manifold, discharges the slurry into a headbox, where fiber flocculation is minimised by microturbulence and where the proper pressure head is provided to cause the slurry to flow at the proper velocity through the slice and onto the moving Fourdrinier wire. 

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. 

The thickness of the tape is controUed by the sHp characteristics, the height of the doctor blade, the casting rate, and the pressure head of the sHp reservoir behind the doctor blade. SHp viscosities in the range of 1 5 Pa-s (10—50 P) are used to cast tapes at 5—100 cm/s. To achieve the desired strength and flexibUity in the green tape, tape casting slurries contain more binder than those used in sHp casting, as weU as a plasticizer to ensure flexibUity. 

Another component of the TDH is the Hp, pre.ssure head. VVe can see in Figure 8-7 that both tanks have vent valves. These two ve,ssels are exposed to atmospheric pressure, which is the same in both tanks. So by simple observation, pressure head doesn t exist. AHp = 0. 

Let s consider now a system with dynamic pressures and a constant elevation. A classic example of this would be where a pump feeds a sealed reactor vessel, or boiler. The fluid level in the reactor would be more or less static in relation to the pump. The resistances in the piping, the Hf and Hv, would be mostly static although they would go up with flow. The Hp, pressure head would change with temperature. Consider Figure 8-14. 

The flow of the spent catalyst to the regenerator is typically controlled by the use of a valve that slides back and forth. This slide valve (Figure 4-50) is used to control the catalyst level in the stripper. The catalyst level in tlie stripper provides the pressure head that allows tlie catalyst to flow into tlie regenerator. The exposed surface of tlie slide valve is usually lined with a suitable refractory to withstand erosion. 

From the regenerator, the regenerated eatalyst flows down a transfer line eommonly referred to as a standpipe. A standpipe provides the neeessary pressure head needed to eireulate the eatalyst around the... 

Turbomachines can be compared with each other by dimensional analysis. This analysis produces various types of geometrically similar parameters. Dimensional analysis is a procedure where variables representing a physical situation are reduced into groups, which are dimensionless. These dimensionless groups can then be used to compare performance of various types of machines with each other. Dimensional analysis as used in turbomachines can be employed to (1) compare data from various types of machines—it is a useful technique in the development of blade passages and blade profiles, (2) select various types of units based on maximum efficiency and pressure head required, and (3) predict a prototype s performance from tests conducted on a smaller scale model or at lower speeds. 

With positive prewhirl, the first term of the Euler equation remains H = U Vg — UiVgi Therefore, Euler work is redueed by the use of positive prewhirl. On the other hand, negative prewhirl inereases the energy transfer by the amount U Vg. This results in a larger pressure head being produeed in the ease of the negative prewhirl for the same impeller diameter and speed. 

Process flow diagrams are more complex and show all main flow streams including valves to enhance the understanding of the process as well as pressures and temperatures on all feed and product lines within all major vessels and in and out of headers and heat exchangers, and points of pressure and temperature control. Also, information on construction materials, pump capacities and pressure heads, compressor horsepower, and vessel design pressures and temperatures are shown when necessary for clarity. In addition, process flow diagrams usually show major components of control loops along with key utilities. 

A good method for a simple calibration facility is a system where a constant airflow is produced by using two water containers and an arrangement of a virtually constant pressure head, The constant water flow into the second container displaces an equal airflow out of the container (Fig. 12.21). With this arrangement the difficult measurement of a small airflow is changed into a much easier and accurate measurement of a small water flow. 

Volume 1 explains that pumps ean be classified as either positive-displacement or kinetie. The same is true for compressors. In a positive displacement compressor the gas is transported from low pressure to high pressure in a device that reduces its volume and thus inereases its pressure. The most common type of positive displacement eompressors are reeiprocating and rotary (serew or vane) just as was the ease for pumps. Kinetic compressors impart a veloeity head to the gas, which is then converted to a pressure head in accordance with Bernoulli s Law as the gas is slowed down to the velocity in the discharge line. Just as was the case with pumps, centrifugal compressors are the only form of kinetic compressor commonly used. 

The terms fans and blowers are often used interchangeably, and no distinction is made between them in the discussion that follows. Whatever is stated about fans equally applies to blowers. Strictly speaking, however, fans are used for low pressure (drop) operation, generally below 2 psi. Blowers are generally employed when pressure heads in the range of 2.0 to 14.7 psi are generated Operations at higher pressures require compressors. 

Forderer, m. promoter accelerator conveyer. Ffirder-gut, n. goods to be conveyed or forwarded output, -hohe, /. delivery head, pressure head, -kohle, /. rough coal, coal directly from the mine, run-of-mine. forderlich, a. serviceable speedy. Forder-menge, /. output, -mittel, n. transportation means conveyer, fordem, v.t. demand, ask, require summon challenge. 

Never use cast iron fittings or pipe in process situations unless there is only grav ity pressure head (or not over 10 psig) or the fluid is nonhazardous. One exception is in some concentrated sulfuric acid applications however, extreme caution must be used in the design of the safety of the system area. Never use in pulsing or shock service. 

Note when used for pump system balance, this Zhf must be used as a negative number ( — 0.1863) because it is a pressure loss associated with the fluid flowing. For pipe line sizing, the pressure head on the tank of 5 psig and any elevation difference between tank outlet nozzle and pump suction centerline do not enter into the calculations. 

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