Gases specific heats

Gases, specific heats 794 -- thermal conductivities 796 -- viscosities of 798 Gastf.rstArt, J. 220,229 Gear pumps 321,322... 

A = Surface area ft based on tube ID C = Gas specific heat. Btu/lb°F d = Tube inner diameter, in. k = Gas thermal conductivity, Btu/ft-h°F L = Tube length, ft N = Total number of tubes in boiler Pr = Gas Prandtl number Q = Duty of the boiler. Btu/h... 

Gas Specific Heat Ratio K = Cp/C, Critical Flow Pressure Ratio, P,/P,... 

Pa = absolute downstream pressure (N/m ) y = gas specific heat ratio (Cp/Cv, dimensionless)... 

Sc = Schmidt number, dimensionless Pr = Prandtl number, dimensionless Cg = gas specific heat, Btu/lb-°F a = interfacial area, fti/fti Q, = sensible heat transfer duty, Btu/hr Qj. = total heat transfer duty, Btu/hr... 

The dependence of gas specific heats on temperature was discussed in Chapter 3, Section 3.5. For a gas in the ideal state the specific heat capacity at constant pressure is given by ... 

The expansion factor Y depends on the pressure drop X, the dimensions (clearance) in the valve, the gas specific heat ratio k, and the Reynolds number (the effect of which is often negligible). It has been found from measurements (Hutchison, 1971) that the expansion factor for a given valve can be represented, to within about 2%, by the expression... 

The burglar trips and breaks his flashlight and also a large bottle of methanol that spills into a holding tank of 1.8 m diameter. He then uses a cigarette lighter to see. Use property data from the tables in Chapter 6 the gas specific heat is constant at 1.2 J/g K, the density for the gas in the room can be considered constant at 1.18 kg/m3 and the heat transfer coefficient at the methanol surface is 15 W/m2 K. 

The recuperated Brayton cycle approaches Carnot efficiency in the ideal limit. As compressor and turbine work are reduced, the average temperatures for heat addition and rejection approach the cycle limit temperature. The limit is reached as compressor and turbine work (and cycle pressure ratio) approach zero and fluid mass flow per unit power output approaches infinity. It can be expected from this that practical recuperated Brayton cycles would operate at relatively low pressure ratios, but be very sensitive to pressure drop. With tire assumption of constant gas specific heat over the cycle temperature range, a good assumption for helium, the cycle efficiency of a recuperated Brayton cycle may be expressed ... 

Table A-1 Molar mass, gas constant, and ideal-gas specific heats of some substances 842...

Edmister (1948) published a generalized plot showing the isothermal pressure correction for real gases as a function of the reduced pressure and temperature. His chart, converted to SI units, is shown as Eigure 3.2. Edmister s chart was based on hydrocarbons but can be used for other materials to give an indication of the likely error if the ideal gas specific heat values are used without corrections. 

The major alternatives are then as follows (assuming gas specific heat capacity Cpc and latent heat of evaporation are fixed) ... 

The approach is predictive, and the only input data required are the critical parameters, molecular weight, Pitzer s acentric factor and, for thermal conductivity, the ideal gas specific heat. 

This ideal reaction produces the net enthalpy of combustion AH , (T,net) for the burning of one mole of CiHjSk. Measurements of this property are almost always made at 25°C (298.15 K). To convert from 25°C to another temperature requires the ideal gas specific heats (heat capacities) per mole,... 

With few exceptions, thermodynamic property tabulations are calculated from P-V-T meaz.urements and from zero-pressure specific heat values derived from spectroscopic measurements. It should be noted that the zero-pressure (ideal gas) specific heat values, Cp, for the cryogenic fluids are generally known with an uncertainty of less than 3 parts in 10,000 whereas the random deviations of the P-V-T data are of the order of 2 to 5 parts in 1000. The phase boundaries involve a further complication and, consequently, must be defined by additional experimental data. As a minimum requirement, measurements of the vapor pressure are sufficient for the calculation of thermodynamic property differences due to a phase change. This is indicated by the Clapeyron equation, which may be expressed... 

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