Flow measurement specific weights

Being simply the quotient of the thrust and the total weight flow, the specific impulse is a performance parameter readily measured experimentally with good accuracy. This fact accounts for its popular acceptance. With regard to convenience there is no greater merit in the use of I P instead of c. As for the effective exhaust velocity, the specific impulse is evaluated for optimum conditions when theoretical comparisons are made between various propellant combinations. For pe = pa then ... 

II. A. 3, Being simply the quotient of the thrust and the total weight flow, the specific impulse is a performance parameter readily measured experimentally with good accuracy. This fact accounts for its popular acceptance. 

For quality control purposes, rather than measuring molecular weight averages, properties that correlate with molecular weight can be measured. Examples are the dilute solution intrinsic viscosity, and the melt viscosity under specific conditions (see the melt flow indexer in Section 7.1.1). 

On-line calculations of mass flow, weight liquid level/ and composition from measurements of differential pressure, density, pressure, and temperature are being made in many industries today. In addition, calculations are being made of the mass flow of specific substances in mixtures of gases, solutions, and slurries. Analog computation is also being used to determine the flow of heat in boilers, cooling systems, and reactors. The possibilities are virtually unlimited. 

A pitot static tube (Fig. P6.10) is used to measure the volume rate of flow in a pipe Q). For the fully turbulent condition viscous forces may be ignored. Perform a dimensional analysis for the volume rate of flow (0 as a function of A/> (between A and B which is equal to yji where = specific weight of manometer fluid), D, and r for the fluid in the pipe. IfAp increases by a factor of 2, what is the corresponding increase in the rate of flow ... 

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. 

The ratio of rocket thrust to propellant mass flow, commonly called the specific impulse (/9p) of the propellant, represents a measure of the force developed per unit mass flow of propellant. From Eq. (2), it is apparent that high propellant-flame temperatures and low molecular-weight combustion products are required to produce high 7sp. 

However, it should be pointed out that any given column, operated at a specific flow rate, can exhibit a range of efficiencies depending on the nature of the solute that is chosen for efficiency measurement. Consequently, under exceptional circumstances, the predicted conditions for the separation of the critical pair may not be suitable for another pair and the complete resolution of all solutes may not be obtained. This could occur if the separation ratio of another solute pair, although larger, was very close to that of the critical pair but contains solutes, for example, of widely different molecular weight. However, the possibility of this situation arising, in practice, is extremely remote and will not be considered in this discussion. It follows, that the efficiency required to separate the critical pair, numerically defined, is the first performance criterion... 

Molecular weights are not often measured directly for control of production of polymers because other product properties are more convenient experimentally or are thought to be more directly related to various end uses. Solution and melt viscosities are examples of the latter properties. Poly(vinyl chloride) (PVC) production is controlled aceording to the viscosity of a solution of arbitrary concentration relative to that of the pure solvent. Polyolefin polymers are made to specific values of a melt flow parameter called melt index, whereas rubber is characterized by its Mooney viscosity, which is a different measure related more or less to melt viscosity. These parameters are obviously of some practical utility, or they would not be used so extensively. They are unfortunately specific to particular polymers and are of little or no use in bringing experience with one polymer to bear on problems associated with another. 

Here M is the molecular weight and V the partial specific volume of the solute, N the Avogadro number, k the Boltzmann constant, and T the absolute temperature s and D are the sedimentation and translational diffusion coefficients (after extrapolation to infinite dilution). The translational frictional coefficients from both measurements are regarded as identical, i.e., f, = fd. The rotary frictional coefficient, designated as f, can be determined from either flow birefringence or non-Newtonian viscosity measurements. 

---