Contractions pressure drop

Contraction Pressure Drop. The contraction pressure drop applies to the pressure drop produced by gas contracting from a large area to a small area. For internal cyclones, the most common application of the contraction pressure drop is for the contraction from the large area of the freeboard of a fluidized bed to the smaller area of the cyclone inlet. For external cyclones, the contraction pressure drop does not usually apply from the freeboard of the bed to the cyclone inlet. For external cyclones, the contraction pressure drop will usually have to be applied twice, from the area of the freeboard of the bed to the area of the exit gas line, and from the area of the exit gas line to the area of the cyclone inlet. The contraction pressure drop is given by... 

For external cyclones, people often measure the pressure drop from a point immediately before the cyclone inlet to a point immediately above the cyclone in the gas outlet tube. This measurement does not include the contraction pressure drop as gas flows from the freeboard of the bed into the exit gas line from the fluidized bed. For dipleg seal height calculations, this pressure drop should be taken into account. It is better to measure the total cyclone pressure drop as the difference between the pressure in the freeboard and the pressure in the gas outlet tube. 

Outlet Exit Contraction Pressure Drop. The outlet contraction pressure drop occurs because of the gas contracting from the area of the cyclone barrel to the area of the gas outlet tube. It is estimated by... 

Gas contraction pressure drop from freeboard to cyclone inlet, cm H2O = Solids acceleration pressure drop, cm H2O = Cyclone barrel friction pressure drop, cm H2O = Cyclone exit pressure drop, cm H2O = Gas viscosity, kg/(m-s)... 

Pressure Drop from Are Cha.nge, Pressure drop from area change occurs as a result of energy dissipation associated with eddies formed when a flow area is suddenly expanded or contracted. It is expressed in the following form ... 

Pinch The industrial equivalent of controlling flow bv pinching a soda straw is the pinch valve, Mves of this tvpe use fabric-reinforced elastomer sleeves that completely isolate the process fluid from the metal parts in the valve. The valve is actuated bv applying air pressure directly to the outside of the sleeve, causing it to contract or pinch. Another method is to pinch the sleeve with a linear actuator with a specially attached foot. Pinch valves are used extensively for corrosive material service and erosive sliirrv senice. This type of valve is used in applications with pressure drops up to 10 bar (145 psi),... 

For pass corrections use fluid flow expansion and contraction as an illustration of one approach to these pressure drop calculations. 

Fig. 27. Abrupt contraction cell for flow visualization, birefringence and degradation measurements A inlet (from a peristaltic pump of a pressurized reservoir B outlet (atmospheric pressure or partial vacuum) C interchangeable metallic nozzle with a sapphire tip D capillary flow meter E glass window for flow visualization AP pressure drop (from pressure transducers)...

For flow at right angles to the axes of the tubes, the cross-sectional area is continually changing, and the problem may be treated as one involving a series of sudden enlargements and sudden contractions. Thus the friction loss would be expected to be directly proportional to the number of banks of pipes j in the direction of flow and to the kinetic energy of the fluid. The pressure drop - APf may be written as ... 

Pressure drop on the tube-side of a shell and tube exchanger is made up of the friction loss in the tubes and losses due to sudden contractions and expansions and flow reversals experienced by the tube-side fluid. The friction loss may be estimated by the methods outlined in Section 3.4.3 from which the basic equation for isothermal flow is given by equation 3.18 which can be written as ... 

Calculate the pressure drop in, and the power required to operate, a condenser consisting of 400 tubes 4.5 m long and 10 mm internal diameter. The coefficient of contraction at the entrance of the nibes is 0.6. and 0.04 mJ/s of water is to be pumped through the condenser. 

The consideration of the pressure drop over the monoliths containing a variety of CPSI (cells per in ) for the modeling of honeycomb reactor may be required, since Ap of the reactor strongly depends on CPSI of monolith. Eqn. (7) for the pressure drop of the honeycomb was employed to develop the reactor model describing the performance of the honeycomb fabricated in the present work [8]. and Ke indicate contraction and expansion loss coefficient at the honeycomb inlet and outlet, respectively and o is the ratio of free flow area to frontal area. 

Another source of pressure drop will be the flow expansion and contraction at the exchanger inlet and outlet nozzles. This can be estimated by adding one velocity head for the inlet and 0.5 for the outlet, based on the nozzle velocities. 

The pressure drop over the inlet and outlet pipes could also be estimated, taking into account the bends, and expansions and contractions. 

The pressure drop due the contraction and expansion losses through the ports in the plates must be added to the friction loss. Kumar (1984) suggests adding 1.3 velocity heads per pass, based on the velocity through the ports. 

Table 13.4 gives some typical values of the loss coefficient for various fittings9. It should be noted that values for loss coefficient will vary for the same fitting, but from different manufacturers, as a result of differences in geometry. Table 13.5 gives head losses for sudden contractions, sudden expansions and orifice plates. Note that the relationship for orifice plates in Table 13.5 relates to the overall pressure drop and not the pressure drop between the pressure tappings used to determine the flowrate. 

Later, Weisman et al. (1978) also found that assuming homogeneous flow everywhere provided nearly as good a correlation of the data as the slip flow model. The total pressure drop across a contraction can be approximated by... 

The simplest and most common device for measuring flow rate in a pipe is the orifice meter, illustrated in Fig. 10-7. This is an obstruction meter that consists of a plate with a hole in it that is inserted into the pipe, and the pressure drop across the plate is measured. The major difference between this device and the venturi and nozzle meters is the fact that the fluid stream leaving the orifice hole contracts to an area considerably smaller than that of the orifice hole itself. This is called the vena contracta, and it occurs because the fluid has considerable inward radial momentum as it converges into the orifice hole, which causes it to continue to flow inward for a distance downstream of the orifice before it starts to expand to fill the pipe. If the pipe diameter is D, the orifice diameter is d, and the diameter of the vena contracta is d2, the contraction ratio for the vena contracta is defined as Cc = A2/A0 = (d2/d)2. For highly turbulent flow, Cc 0.6. 

In these expressions u2 is the mean velocity in the smaller exit pipe. When the contraction is very great, the pressure drop tends to h(pu2l2), or a head loss of half a velocity head, based on the smaller pipe. 

The same authors presented data on pressure drop at abrupt contractions for which their method of correlation has a rational basis. In this work both the fluid used and the pipe diameter were changed simultaneously hence the reasons for the absence of any correlation in this case cannot be specified. Their general method of approach appears to be a powerful and useful basis for further work but has not yet led to a recommended design procedure. 

Only the irreversible frictional pressure drop should be included in the calculation of upstream pressure drop, not the momentum pressure drop required to accelerate the fluid to the velocity at inlet to the valve. The irreversible frictional pressure drop includes both friction in the inlet contraction from the reactor.(K = 0.5 for a sudden contraction1151) and friction in the piping, bends and any fittings. ... 

Correlations are available for pressure drops in flowthrough pipe fittings, such as elbows, bends, and valves, and for sudden contractions and enlargements of the pipe diameter as the ratio of equivalent length of straight pipe to its diameter. 

The pressure drop across the cyclone is an important parameter in the evaluation of cyclone performance. It is a measure of the amount of work that is required to operate the cyclone at given conditions, which is important for operational and economical reasons. The total pressure drop over a cyclone consists of losses at the inlet, outlet and within the cyclone body. The main part of the pressure drop, i.e. about 80%, is considered to be pressure losses inside the cyclone due to the energy dissipation by the viscous stress of the turbulent rotational flow [9], The remaining 20% of the pressure drop are caused by the contraction of the fluid flow at the outlet, expansion at the inlet and by fluid friction on the cyclone wall surface. 

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