Reference datum

H = Total enthalpy, above reference datum, of vapor mixture at tray or specified conditions, Btu/lb mol, or Btu/lb... 

If a reference datum plane (h = 0) is set at the bottom of each of the capillaries shown in the previous Fig. 5.5, the expression for the total driving force, that is the piezometric pressure difference along the penetration length h for a capillary with an upward (I pgh cos a) or a downward (—pgh cos a) orientation, can be represented by following equation (Bouchon and Pyle, 2005b),... 

Subatmospheric pressure usually is expressed in reference to perfect vacuum or absolute zero pressure, lake absolute zero temperature (the concept is analogous), absolute zero pressure cannot be achieved, but it does provide a convenient reference datum. Standard atmospheric pressure is 14.695 psi absolute, 30 inches of mercury absolute, or 760 mmHg of density 13.595 g/cm3 where acceleration due to gravity is g = 980.665 emir. ] mmHg. which equals 1 torr. is the most commonly used unit of absolute pressure. Derived units, the million or micrometer, representing 1/1000 of 1 mmHg or 1 torr, are also used for subtorr pressures. 

Wepfer, W.J. and Gaggioli, R.A., "Reference Datums for Available Energy", Thermodynamics Second Law Analysis, A.C.S. Symposium Series, 122, 77-92, 1980. 

Elevation of fluid from a reference datum Specific weight of water pump efficiency Density of water... 

Table 5.2. d spacings of Uie cubic phase and reference datum... 

Wepfer, W. Gaggioli, R. Reference Datums for Available Energy," this symposium. 

The purpose of this article is to provide appropriate criteria for the practical selection of reference datums for the calculation of available energy. The selection of a reference datum generally depends on the commodity whose available energy is being evaluated, upon the particular process (or device) being analyzed, upon the complex of processes (devices) with which the particular process interacts, and upon the ambient environment of the complex. 

Proper choice of a reference datum (a dead state) is important to efficiency analysis and costing. It needs to be recognized that the reference datum for available energy is an altogether different concept than the reference base for property tabulations. 

Some contend that the chemical reference datum for available energy can also be selected arbitrarily, just like a base for thermochemical tables (while admitting that the thermal reference datum—the "dead state temperature"—is not arbitrary). The contention is erroneous changing the various values of the available energy of a specific material by a constant amount (as a consequence of changing the reference datum) leads to misconceptions, to misevaluations, and to misallocations—in the determination of inefficiencies and costs. Absolute values of available energy can and should be evaluated. 

Before proceeding to the criteria for practical selection of the dead state reference datum for analyzing a particular process, some background fundamentals will be presented. 

The Selection of Reference Datums for Subsystem Available Energy. 

This article has provided the guidelines for practical selection of reference datums for available energy. The application of these guidelines is illustrated by the various Second Law analyses presented (or referred to) in this volume. 

Several authors have proposed chemical reference datums for several elements and compounds, in various environmental circumstances e.g., se (8,9,12,13,14). 

Although it is often intimated to the contrary, a crucial point 1s this In engineering practice the selection of an appropriate reference datum must take into account the plant... 

Difficulties with the selection of chemical reference datum should not be allowed to be an obstacle to the application of Second Law analysis. The practitioner is encouraged to rely on judgment, and then proceed. Substantial errors will result only if the judgment is very poor. (Furthermore, judgment is improved by experience.)... 

However, the evaluation of the efficiency of a process requires absolute values of the available energy— for example, the available energy content of the fuel. Similarly, Second Law costing depends upon evaluation of absolute values of available energies. Thus, in order to make efficiency analyses, or to do costing, at least an approximation to the reference datum must be made. 

The key to the generalization is to base the Second Law on the property availability (l,2.,j3) the availability at any state of a system reflects the extent to which the system can affect any other because the state is not in equilibrium with some reference datum. 

Table 1. Relationship between sampling level, height above datum and immersion time ( sine calculation ). The reference datum used to calculate immersion times was 2.9 m below ordnance datum (see text for details)...

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