Orifice constant

Qt = stoichiometric composition of combustible vapor in air expressed as a volume percent Co = sonic flow discharge orifice constant, varying VvTth Reynolds number... 

For large Q p 0, and large values of the dimensionless orifice constant,... 

Pf = fluid density in hole at conditions in inlet to hole AP = pressure drop in consistent units, kPa or Ib/ft" c = orifice constant, dimensionless (typically 0.8 for gas distributors)... 

In many industrial processes one of the thermodynamic functions is constant or nearly constant. Examples include compression in a highspeed compressor (near-constant entropy) and a gas flowing through an orifice (constant enthalpy). These concepts are discussed below. This type of process typically is described in... 

Variable-Area Flow Meters. In variable-head flow meters, the pressure differential varies with flow rate across a constant restriction. In variable-area meters, the differential is maintained constant and the restriction area allowed to change in proportion to the flow rate. A variable-area meter is thus essentially a form of variable orifice. In its most common form, a variable-area meter consists of a tapered tube mounted vertically and containing a float that is free to move in the tube. When flow is introduced into the small diameter bottom end, the float rises to a point of dynamic equiHbrium at which the pressure differential across the float balances the weight of the float less its buoyancy. The shape and weight of the float, the relative diameters of tube and float, and the variation of the tube diameter with elevation all determine the performance characteristics of the meter for a specific set of fluid conditions. A ball float in a conical constant-taper glass tube is the most common design it is widely used in the measurement of low flow rates at essentially constant viscosity. The flow rate is normally deterrnined visually by float position relative to an etched scale on the side of the tube. Such a meter is simple and inexpensive but, with care in manufacture and caHbration, can provide rea dings accurate to within several percent of full-scale flow for either Hquid or gas. 

Viscometer Orifice length, mm Orifice diameter, cm Viscosity range, mm /s(=cSt) Approximate constants k K Main apphcations... 

The hydrostatic-pressure test is performed on fabric mounted under the orifice of a conical weU. The fabric is subjected to increasing water pressure at a constant rate until leakage occurs at three poiats on the fabric s undersurface. The rating is the height of the water head ia centimeters above the fabric (AATCC Test Method 127 INDA Standard Test 80.4-92). 

Osmotic Control. Several oral osmotic systems (OROS) have been developed by the Alza Corporation to allow controUed deHvery of highly water-soluble dmgs. The elementary osmotic pump (94) consists of an osmotic core containing dmg surrounded by a semi-permeable membrane having a laser-drilled deHvery orifice. The system looks like a conventional tablet, yet the outer layer allows only the diffusion of water into the core of the unit. The rate of water diffusion into the system is controUed by the membrane s permeabUity to water and by the osmotic activity of the core. Because the membrane does not expand as water is absorbed, the dmg solution must leave the interior of the tablet through the smaU orifice at the same rate that water enters by osmosis. The osmotic driving force is constant until aU of the dmg is dissolved thus, the osmotic system maintains a constant deHvery rate of dmg until the time of complete dissolution of the dmg. 

General Principles The underlying principle of an ideal area meter is the same as that of a head meter of the orifice type (see subsection Orifice Meters ). The stream to be measured is throttled by a constriction, but instead of observing the variation with flow of the differential head ac-ross an orifice of fixed size, the constriction of an area meter is so arranged that its size is varied to accommodate the flow while the differential head is held constant. 

For conditions approaching constant flow through the orifice, a relationship derivea by equating the buoyant force to the inertia force of the liquid [Davidson et al., Tran.s. In.stn. Chem. Engr.s., 38, 335 (I960)] (dimensionally consistent),... 

For conditions approaching constant pressure at the orifice entrance, which probably siiTuJates most industri appheations, there is no independently verified predictive method. For air at near atmospheric pressure sparged into relatively inviscidhqiiids (11 - 100 cP), the correlation of Kumar et al. [Can. J. Chem. Eng., 54, 503 (1976)] fits experimental data well. Their correlation is presented here as Fig. 14-92. 

Orifice plates. Either the concentric orifice, eccentric orifice, or segmented orifice-type. Choice depends on the quality of the fluid handled. Venturi tubes. These consist of a well-rounded convergent section at the entrance, a throat of constant diameter, and a divergent section. Their accuracy is high however, installation, unless planned for in advance, is very difficult in the field. 

The set pressure of a conventional valve is affected by back pressure. The spring setting can be adjusted to compensate for constant back pressure. For a variable back pressure of greater than 10% of the set pressure, it is customary to go to the balanced bellows type which can generally tolerate variable back pressure of up to 40% of set pressure. Table 2 gives standard orifice sizes. 

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