Pressure drop Friction loss

Tube-side Pressure Drop. Friction loss inside tubes can be computed from the general formulations given in Chap. 13, but the losses caused by enlargements, contractions, and turns must be included in the calculations. In order to account for nonisothermal flow, Sieder and Tate recommend multiplying the friction loss calculated by isothermal formulations such as F.q. (13-1), p. 395, b the dimensionless ratio for turbulent... 

Pressure drop and loss due to friction. When the fluid is in steady-state laminar flow in a pipe, then for a Newtonian fluid the shear stress is given by Eq. (2.4-2), which is rewritten for change in radius dr rather than distance dy, as follows. 

The first set of constraints represents the mass conservation law at each node of the water network. The second set describes the energy (head) losses for each pipe in the network to relate the pressure drop (head loss), due to friction, to the pipe flow rate and the diameter, roughness, material of construction, and length of the pipe. In this work, the commonly used Hazen-Williams empirical formula (Alperovits Shamir, 1977 Cunha Sousa, 1999 Coulter Morgan, 1985) is used. The third set of constraints includes bounds on variables such as minimum head or flowrate requirements. This set also includes constraints to ensure that only one diameter can be selected for each pipe (stream), a more realistic representation rather than having a split-pipe design. 

Flow Summary shows the values of fC-pipe and fC-fittings. These are the indications of pressure drop contribution by the pipe and fittings. In the Pressure Drop frame, there are a few pressure drop calculations, which include the contribution of static pressure drop (elevation loss or gain), velocity head (change in velocity due to reducer or expander), and control valve. The result also shows the frictional pressure drop per 100 m. This is sometimes very useful, because most operating companies establish some basic value of pressure drop per 100 m. 

Pressure Drop. The prediction of pressure drop in fixed beds of adsorbent particles is important. When the pressure loss is too high, cosdy compression may be increased, adsorbent may be fluidized and subject to attrition, or the excessive force may cmsh the particles. As discussed previously, RPSA rehes on pressure drop for separation. Because of the cychc nature of adsorption processes, pressure drop must be calculated for each of the steps of the cycle. The most commonly used pressure drop equations for fixed beds of adsorbent are those of Ergun (143), Leva (144), and Brownell and co-workers (145). Each of these correlations uses a particle Reynolds number (Re = G///) and friction factor (f) to calculate the pressure drop (AP) per... 

Pressure Drop. The pressure drop across a two-phase suspension is composed of various terms, such as static head, acceleration, and friction losses for both gas and soflds. For most dense fluid-bed appHcations, outside of entrance or exit regimes where the acceleration pressure drop is appreciable, the pressure drop simply results from the static head of soflds. Therefore, the weight of soflds ia the bed divided by the height of soflds gives the apparent density of the fluidized bed, ie... 

The upward flow of gas and Hquid in a pipe is subject to an interesting and potentially important instabiHty. As gas flow increases, Hquid holdup decreases and frictional losses rise. At low gas velocity the decrease in Hquid holdup and gravity head more than compensates for the increase in frictional losses. Thus an increase in gas velocity is accompanied by a decrease in pressure drop along the pipe, a potentially unstable situation if the flows of gas and Hquid are sensitive to the pressure drop in the pipe. Such a situation can arise in a thermosyphon reboiler, which depends on the difference in density between the Hquid and a Hquid—vapor mixture to produce circulation. The instabiHty is manifested as cycHc surging of the Hquid flow entering the boiler and of the vapor flow leaving it. 

The term pressure drop usually refers to the pressure loss that is not recoverable in the circuit, and it is lost energy that is dissipated into the fluid stream in the form of heat energy. The pressure drop in a flow circuit is associated with various forms of energy dissipation owing to friction, change in flow area, flow turning, and others ... 

Heat exchangers use energy two ways as frictional pressure drop, and as the loss in ability to do work when heat is degraded. 

Viscous Transport. Low velocity viscous laminar dow ia gas pipes is commonplace. Practical gas dow can be based on pressure drops of <50% for low velocity laminar dow ia pipes whose length-to-diameter ratio may be as high as several thousand. Under laminar dow, bends and fittings add to the frictional loss, as do abmpt transitions. 

The viscous or frictional loss term in the mechanical energy balance for most cases is obtained experimentally. For many common fittings found in piping systems, such as expansions, contrac tions, elbows and valves, data are available to estimate the losses. Substitution into the energy balance then allows calculation of pressure drop. A common error is to assume that pressure drop and frictional losses are equivalent. Equation (6-16) shows that in addition to fric tional losses, other factors such as shaft work and velocity or elevation change influence pressure drop. 

Note that the total pressure drop consists of 0.5 velocity heads of frictional loss contrihiition, and 1 velocity head of velocity change contrihiition. The frictional contrihiition is a permanent loss of mechanical energy hy viscous dissipation. The acceleration contrihiition is reversible if the fluid were subsequently decelerated in a frictionless diffuser, a 4,000 Pa pressure rise would occur. 

This is pressure drop (including friction loss) between run and branch, based on velocity in the mainstream before branching. Actual value depends on the flow split, ranging from 0.5 to 1.3 if mainstream enters run and from 0.7 to 1.5 if mainstream enters branch. 

K Empirical proportionality constant for cyclone pressure drop or friction loss Dimensionless Dimensionless ... 

When a PR valve is relieving at rated capacity, the total frictional pressure drop between a vessel and the inlet of the valve should be less than 3% of the set pressure (kPa). In this calculation, the effect on static pressure of fluid acceleration is ignored rather, only friction loss is considered. 

For a special fan situation, a straight air duct of uniform cross-sectional area is used on the leaving side. The outgoing velocity Cj is the same as the fan leaving velocity C2. The only minor loss is the outgoing loss = pcT Another part of the pressure drop is the frictional pressure drop Ap, . Equation (9.127) gives... 

Barth assumed that the pressure loss of a cyclone consists mainly of the pressure loss required to overcome the wall friction of the cyclone and the pressure drop to drive the fluid out of the cyclone outlet pipe. This leads to the following expression for the total loss factor C/ ... 

There are economic and operational reasons for considering an additional stage of compression. The addition of a stage of compression requires an additional scrubber, additional cylinder or case, and more complex piping and controls. In addition, there are some horsepower losses due to additional mechanical friction of the cylinder or rotating element and the increased pressure drop in the piping. This horsepower loss and additional equipment cost may be more than offset by the increased efficiency of compression. 

The basis for single-phase and some two-phase friction loss (pressure drop) for fluid flow follows the Darcy and Fanning concepts. The exact transition from laminar or dscous flow to the turbulent condition is variously identified as between a Reynolds number of 2000 and 4000. 

The single-phase friction loss (pressure drop) for these situations in chemical and petrochemical plants is still represented by the Darcy equation with specific limitations [3] ... 

The total piping system pressure drop for a particular pipe installation is the sum of the friction drop in pipe valves and fittings, plus other pressure losses (drops) through control valves, plus drop through equipment in the system, plus static drop due to elevation or pressure level. For example, see Figure 2-2. 

Rather than assuming a pressure drop across the control as 25%, 33%, or 40% of the other friction losses in the system, a logical approach [9] is summarized here. The control valve pressure drop has nothing to do with the valve size, but is determined by the pressure balance (See Equation 2-59 [9]). 

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