Kinetic head

The calculation of the overall stage efficiency must also include losses encountered in the diffuser. Thus, the overall actual adiabatic head attained will be the actual adiabatic head of the impeller minus the head losses encountered in the diffuser from wake caused by the impeller blade the loss of part of the kinetic head at the exit of the diffuser (A(/ed), and the loss of head from frictional forces (A(/osf) encountered in the vaned or vaneless diffuser space... 

In equation 8.26, it is implicitly assumed that the kinetic head of the inlet liquid is available for conversion into pressure head. If this is not so, u /2g must be deducted from the NPSH. 

PRESSURE DROP THROUGH THE CONTACTOR ASSEMBLY. Causes for pressure drop through the contactor assembly are shown in Fig. 16-10 for bubble caps as (1) contraction, (2) friction in riser, (3) reversal of flow direction, and (4) friction in annular space. Similarly, Fig. 16-11 shows for sieve trays that this cause for pressure drop is (1) contraction and (2) friction in the sieve hole. The total pressure drop due to the preceding causes is primarily a function of the kinetic head. The pressure drop as feet of liquid equivalent to one kinetic head is... 

Chart for estimation of number of kinetic heads for calculation of... 

For sieve trays, the number of kinetic heads equivalent to the total pressure drop through the plate itself is a function of the ratio of the sieve-hole diameter to the tray thickness and the ratio of the hole area per tray to the active area per tray as shown in Fig. 16-5. This pressure drop for a reasonable sieve-tray design is generally in the range of 1 to 3 kinetic heads, and Fig. 16-12 can be used to choose the most reasonable number to use in preliminary designs Designating the number of kinetic heads obtained from Fig. 16-12 as K.H., the pressure drop due to gas flow through the holes for a sieve tray expressed as liquid head is... 

Head of liquid equivalent to the frictional flow resistance in the downcomer and in the passage of the fluid from the downcomer onto the plate. This head is usually relatively small, but it can be estimated as three, kinetic heads for the liquid, based on the linear liquid velocity at the minimum cross-sectional area for downooming liquid flow, or... 

A sieve-tray tower has an ID of 5 ft, and the combined cross-sectional area of the holes on one tray is 10 percent of the total cross-sectional area of the tower. The height of the weir is 1.5 in. The head of liquid over the top of the weir is 1 in. Liquid gradient is negligible. The diameter of the perforations is in., and the superficial vapor velocity (based on the cross-sectional area of the empty tower) is 3.4 ft/s. The pressure drop due to passage of gas through the holes may be assumed to be equivalent to 1.4 kinetic heads (based on gas velocity through holes). (Tray thickness = hole diameter and active area = 90 percent of total area-see Fig. 16-12). If the liquid density is 50 lb/ft3 and the gas density is 0.10 lb/ft3, estimate the pressure drop per tray as pounds force per square inch. 

In Equation 9.1, is the total pressure that is the sum of hydrostatic and pressure head that is acting on the orifice in the impeller from which gas bubbles come out. The tip velocity at the orifice, is generally used in estimating the kinetic head. 

The pressure at the outlet of the stator, P, that now includes the kinetic head generated by the rotor is given by... 

In Equation 9.40, the only operating variable is Fr, which is the ratio of the hydrostatic to kinetic head. According to this equation, the induction rate is solely decided by this ratio (Fr ) that is independent of the liquid-phase density. Continuing with this expression for f/, Zundelevich introduced the impeller performance parameter in terms of the pumping capacity (=K D ) and power input through the impeller (P=Nj,p N rP). The dispersion density, p, for use in estimation of power input was... 

Pressure Drop through Risers and Cap, This loss is chiefly a kinetic velocity effect due to the changing cross-sectional areas. The pressure drop in inches of the liquid equivalent to the kinetic head is... 

For the reversal loss at the bottom of the down pipe, it is recommended that a loss equal to one kinetic head be used with a coefficient of 0.6. 

In small-diameter columns, D/Ha is usually so small that the effect of cross flow is negligible, but in large columns it may become so great that the kinetic head equivalent to Vcf may be significant in terms of liquid head. Under these conditions, the pressure in the vapor space is not constant across the cross section, and its variation is such that it forces an increase in the hydraulic gradient. In order to make Hs as large as possible, any beams or projection on the bottom of a plate should be positioned to aid the vapor cross flow. Good vapor distribution on the plate will eliminate the effect of vapor cross flow. 

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