Enthalpy adiabatic

In Figure 2 the lines, volume, m /kg dry air, indicate humid volume, which includes the volume of 1.0 kg of dry gas plus the volume of vapor it carries. Enthalpy at saturation data are accurate only at the saturation temperature and humidity however, for air—water vapor mixtures, the diagonal wet bulb temperature lines are approximately the same as constant-enthalpy adiabatic cooling lines. The latter are based on the relationship ... 

Cg = humid heat for humidity H in units of kj / (kg-K) and = latent heat of vaporization at / in kj /kg. The slope of the constant-enthalpy adiabatic cooling line is —C j which is the relationship between temperature and humidity of gas passing through a totally adiabatic direct-heat dryer. The humid heat of a gas—vapor mixture per unit weight of dry gas includes the specific heat of the vapor... 

Enthalpy data are given on the basis of kilojoules per kilogram of diy air. Entbalpy-at-saturation data are accurate only at the saturation temperature and humidity. Enthalpy deviation curves permit enthalpy corrections for humidities less than saturation and show how the wet-bulb-temperature hues do not precisely coincide with constant-enthalpy, adiabatic cooling hnes. 

Much of this book deals with the finding ofthe expression that relates the with different contacting patterns and various reaction parameters (intrinsic kinetic, reaction enthalpy, adiabatic temperature, etc.). 

In the case of the synthesis of alumina, chrome, nickel, iron and nanocrystalline cobalt oxides using the solution combustion technique, for example, we lack, so far, a deep understanding of the influence of the fuel-oxidant ratios well as a model of the thermodynamic variables associated with enthalpy, adiabatic flame temperature and the total number of moles of gas generated related to the powder characteristics, such as crystallite size and surface area. 

In modern separation design, a significant part of many phase-equilibrium calculations is the mathematical representation of pure-component and mixture enthalpies. Enthalpy estimates are important not only for determination of heat loads, but also for adiabatic flash and distillation computations. Further, mixture enthalpy data, when available, are useful for extending vapor-liquid equilibria to higher (or lower) temperatures, through the Gibbs-Helmholtz equation. ... 

A step-limited Newton-Raphson iteration, applied to the Rachford-Rice objective function, is used to solve for A, the vapor to feed mole ratio, for an isothermal flash. For an adiabatic flash, an enthalpy balance is included in a two-dimensional Newton-Raphson iteration to yield both A and T. Details are given in Chapter 7. 

Accurate enthalpies of solid-solid transitions and solid-liquid transitions (fiision) are usually detennined in an adiabatic heat capacity calorimeter. Measurements of lower precision can be made with a differential scaiming calorimeter (see later). Enthalpies of vaporization are usually detennined by the measurement of the amount of energy required to vaporize a known mass of sample. The various measurement methods have been critically reviewed by Majer and Svoboda [9]. The actual teclmique used depends on the vapour pressure of the material. Methods based on... 

The heat capacity of thiazole was determined by adiabatic calorimetry from 5 to 340 K by Goursot and Westrum (295,296). A glass-type transition occurs between 145 and 175°K. Melting occurs at 239.53°K (-33-62°C) with an enthalpy increment of 2292 cal mole and an entropy increment of 9-57 cal mole °K . Table 1-44 summarizes the variations as a function of temperature of the most important thermodynamic properties of thiazole molar heat capacity Cp, standard entropy S°, and Gibbs function - G°-H" )IT. 

The enthalpy hberated on the VDP of parylene is real and in an adiabatic situation causes a rise in temperature of the coated substrate. For Parylene C, 229.1 kj/mol (54.7 cal/mol) corresponds to 1654 J/g (395 cal/g) whereas its specific heat at 25°C is only 1.00 J/(g-K) [0.239 cal/(g-K)] (33). In most practical situations, however, the mass of parylene deposited is dwarfed by the substrate mass, and the heat of polymeriza tion is dissipated within the coated substrate over the time required to deposit the coating with minimal actual temperature rise. 

Fig. 27. Rankine cycle in terms of (a) pressure and volume (b) temperature and entropy and (c) MoUier (enthalpy vs entropy) chart, where adiabatic...

Adiabatic flame temperatures agree with values measured by optical techniques, when the combustion is essentially complete and when losses are known to be relatively small. Calculated temperatures and gas compositions are thus extremely useful and essential for assessing the combustion process and predicting the effects of variations in process parameters (4). Advances in computational techniques have made flame temperature and equifibrium gas composition calculations, and the prediction of thermodynamic properties, routine for any fuel-oxidizer system for which the enthalpies and heats of formation are available or can be estimated. 

The specific enthalpies ia equation 9 can be determined as described earUer, provided the temperatures of the product streams are known. Evaporative cooling crystallizers operate at reduced pressure and may be considered adiabatic (Q = 0). As with of many problems involving equiUbrium relationships and mass and energy balances, trial-and-error computations are often iavolved ia solving equations 7 through 9. 

An equation representing an energy balance on a combustion chamber of two surface zones, a heat sink Ai at temperature T, and a refractory surface A assumed radiatively adiabatic at Tr, inmost simply solved if the total enthalpy input H is expressed as rhCJYTv rh is the mass rate of fuel plus air and Tp is a pseudoadiabatic flame temperature based on a mean specific heat from base temperature up to the gas exit temperature Te rather than up to Tp/The heat transfer rate out of the gas is then H— — T ) or rhCp(T f — Te). The... 

Adiabatic-Saturation Temperature, or Constant-Enthalpy Lines If a stream of air is intimately mixed with a quantity of water at a temperature t, in an adiabatic system, the temperature of the air will drop and its humidity will increase. If t, is such that the air leaving the system is in equihbrium with the water, t, will be the adiabatic-saturation temperature, and the line relating the temperature and humidity of the air is the adiabatic-saturation line. The equation for the adiabatic-saturation line is... 

Solution. Figure 12-8 shows the path on a psychrometric chart. The leaving dry-bulb temperature is obtained directly from Fig. 12-2 as 72.2 F. Since the spray water enters at the wet-bulb temperature of 70 F and there is no heat added to or removed from it, this is by definition an adiabatic process and there will be no change in wet-bulb temperature. The only change in enthalpy is that from the heat content of the makeup water. This can be demonstrated as follows ... 

The simplest continuous-distillation process is the adiabatic single-stage equihbrium-flash process pictured in Fig. 13-25. Feed temperature and the pressure drop across the valve are adjusted to vaporize the feed to the desired extent, while the drum provides disengaging space to allow the vapor to separate from the liquid. The expansion across the valve is at constant enthalpy, and this facd can be used to calculate To (or T to give a desired To). 

Most of the analytical treatments of center-fed columns describe the purification mechanism in an adiabatic oscillating spiral column (Fig. 22-9). However, the analyses by Moyers (op. cit.) and Griffin (op. cit.) are for a nonadiabatic dense-bed column. Differential treatment of the horizontal-purifier (Fig. 22-8) performance has not been reported however, overall material and enthalpy balances have been described by Brodie (op. cit.) and apply equally well to other designs. 

Steam Rate Enthalpy data can be obtained from Mollier diagrams or from steam tables (see Sec. 2), from which the theoretical steam rate can be calculated. For example, a throttle inlet condition of 4137 kPa (600 psig) and 399° C (750° F) gives an enthalpy of 3.2 MJ/kg (1380 Btu/lb), and if the end point is at 348 kPa (50 psig), then adiabatic expansion is to 2.69 MJ/kg (1157 Btu/lb). This gives 0.52 MJ/kg (223 Btu/lb) available, and the theoretical steam rate is calculated from the Btu equivalent per Idlowatthour or horsepower-hour ... 

As a check on the assumptions made, a comparison can be made to a different method of checking the derivation of the head. Enthalpy difference, as a function of temperature change, for an adiabatic pro is... 

Comparing this value with the a) and b) point results ot Example 7, we discover that the line of constant enthalpy lies between the determination line of wet bulb temperature and the adiabatic humidification line. The nearer the Lewis number is to 1, the nearer the wet bulb temperature is to the adiabatic humidification temperature. 

The total enthalpy change across the whole (stationary) cooled blade row is straightforward and is obtained for the overall process (i.e. the complete adiabatic flow through control surfaces (A + B) plus (C)). Even though there is a heat transfer Q internally between the unit mainstream flow and the cooling air flow i//, from A to B, the overall process is adiabatic. 

But if combustion is adiabatic, then the steady flow energy equation for the open-circuit gas turbine (with exhaust of enthalpy (//p)s leaving the HRSG and entering the exhaust stack with a temperature Ts greater than that of the atmosphere. To) is... 

To calculate the heat duty it must be remembered that the pressure drop through the choke is instantaneous. That is, no heat is absorbed or lost, but there is a temperature change. This is an adiabatic expansion of the gas w ith no change in enthalpy. Flow through the coils is a constant pressure process, except for the small amount of pressure drop due to friction. Thus, the change in enthalpy of the gas is equal to the heat absorbed. 

Figure 12-12A. Illustration of isentropic path on log pressure-enthalpy diagram, showing Mollier chart method of finding final temperature and calculation of H for reversible and adiabatic compression. (Used by permission Edmister, W. C. Applied Hydrocarbon Thermodynamics, 1961. Gulf Publishing Company, Houston, Texas. All rights reserved.)...

After identifying the initial temperature (T) and pressure (P) values, the final temperature and both enthalpy values (H) can be read on the same entropy line of the appropriate gas Mollier chart. For the adiabatic process, the work done on the gas is equal to AH, see Figures 12-13A-D. The following is reproduced by permission of Edmister, W. C., Applied Hydrocarbon Thermodynamics, Gulf Publishing Company. ... 

Follow the line of constant entropy (s) to discharge pressure, pg, locating adiabatic discharge state point (2ad)- Read adiabatic enthalpy (hgad) direcdy below point (2,d). 

The specific enthalpy will increase with dry hulh (sensible heat of the air) and moisture content (sensible and latent heat of the water). The adiabatic (isoenthalpic) lines for an air-water vapour mixture are almost parallel with the wet bulb lines so, to avoid any confusion, the enthalpy scale is placed outside the body of the chart, and readings must be taken using a straight-edge. (See Figure 23.7.)... 

The adiabatic (constant enthalpy) line AC is almost parallel to the line of constant wet bulb. Had the latter been used, the final error would have been about 0.2 K, and it is sometimes convenient and quicker to calculate on the basis of constant wet bulb. (This... 

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