Jun,16

2-2.2 Specific Energy and Pressure absolute pressure, p a : the value of a pressure when the datum is absolute zero. It is always positive. barometric pressure, p b : the absolute pressure exerted by the atmosphere. differential pressure, Δ p : the difference between any two pressures. gage pressure, p : the value of a pressure when the datum is the barometric pressure at the point of measurement. It is the difference between the absolute pressure at a point and the pressure of the ambient atmosphere in which the measuring gage is located. It may be positive or negative. pressure, p : normal force per unit area. Since pressure divided by density may appear in energy balance equations, it is sometimes convenient to consider pressure as a type of energy per unit volume. specific energy, y : energy per unit mass. Specific kinetic energy is kinetic energy per unit mass and is equal to one-half the square of the fluid velocity. Specific potential energy is potential energy per unit mass and is equal to the gravitational acceleration multiplied by the elevation above a specified datum. Fluid pressure divided by density is sometimes called “specific pressure energy” and is considered a type of specific energy; however, this term is more properly called specific flow work. static pressure, p s , p sa : the pressure measured in such a manner that no effect is produced by the velocity of the flowing fluid. Similar to the static temperature, it would be sensed by a measuring instrument moving at the same velocity as the fluid. Static pressure may be expressed as either an absolute or gage pressure. Absolute static pressure is used as a property in defining the thermodynamic state of the fluid. total pressure, p t , p ta : sometimes called the stagnation pressure , would be measured when a moving fluid is brought to rest and its kinetic and potential energies are converted to an enthalpy rise by an isentropic compression from the flow condition to the stagnation condition.
2-2.5 Fan Performance 2-2.5.1 General. Fan performance can be expressed in terms of different sets of parameters. This Code provides the user with two choices. One set uses mass flow rate and specific energy. The other uses volume flow rate and pressure. The product of mass flow rate and specific energy and the product of volume flow rate, pressure, and a compressibility coefficient are each designated fan output power. However, values of output power calculated by the two methods are slightly different [1]. 2-2.5.2 Mass Flow Rate–Specific Energy Approach. The fan performance parameters that are associated with this approach are defined as follows: compressibility coefficient, K ρ : the ratio of the fan inlet density to the fan mean density; is useful in this approach. fan efficiency, η : the ratio of the fan output power to the fan input power. In this approach, there is only one definition of fan output power, so there is only one definition of fan efficiency. fan mass flow rate, F m ? : the mass of fluid passing through the fan per unit time.
2-2.5.3 Volume Flow Rate–Pressure Approach. The fan performance parameters associated with this approach are defined as follows. compressibility coefficient, K p : a dimensionless coefficient used to account for compressibility effects [2] and is calculated according to the procedure given in para. 5-11.4 [3]. fan efficiency, η : In this approach, fan efficiency is expressed as either fan total efficiency or fan static efficiency. fan static efficiency, η s : the ratio of fan output power to fan input power, in which the fan output power is modified by deleting the fan velocity pressure. This may also be called total-to-static efficiency. fan total efficiency, η t : the ratio of fan output power to fan input power. This may also be called total-to-total efficiency. fan gas density, ρ F : the total density of the gas at fan inlet conditions. fan output power, P o : the product of fan volume flow rate, fan total pressure, and compressibility coefficient K p . fan pressure : in this approach, three fan pressures are defined as follows: fan static pressure, p Fs : the difference between the fan total pressure and the fan velocity pressure. Therefore, fan static pressure is the difference between the average static pressure at the fan outlet and the average total pressure at the fan inlet. Refer to subsection 5-7 for appropriate averages.

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