Under specific heat substances understand the amount of heat that needs to be reported or subtracted from a unit of a substance (1 kg, 1 m 3, 1 mol) in order to change its temperature by one degree.
Depending on the unit of a given substance, the following specific heat capacities are distinguished:
Mass heat capacity With, referred to 1 kg of gas, J/(kg∙K);
molar heat capacity µC, referred to 1 kmole of gas, J/(kmol∙K);
Volumetric heat capacity WITH', referred to 1 m 3 of gas, J / (m 3 ∙K).
The specific heat capacities are interconnected by the relation:
where υ n- specific volume of gas under normal conditions (n.o.), m 3 /kg; µ - molar mass of gas, kg/kmol.
The heat capacity of an ideal gas depends on the nature of the process of supply (or removal) of heat, on the atomicity of the gas and temperature (the heat capacity of real gases also depends on pressure).
Relationship between mass isobaric C P and isochoric C V heat capacities is established by the Mayer equation:
C P - C V = R, (1.2)
where R- gas constant, J/(kg∙K).
When an ideal gas is heated in a closed vessel of constant volume, heat is spent only on changing the energy of motion of its molecules, and when heated at constant pressure, due to the expansion of the gas, work is simultaneously performed against external forces.
For molar heat capacities, Mayer's equation has the form:
µС р - µС v = µR, (1.3)
where µR\u003d 8314J / (kmol∙K) - universal gas constant.
Ideal gas volume V n, reduced to normal conditions, is determined from the following relation:
(1.4)
where R n- pressure under normal conditions, R n= 101325 Pa = 760 mm Hg; T n- temperature under normal conditions, T n= 273.15K; P t, V t, T t– operating pressure, volume and temperature of the gas.
The ratio of isobaric heat capacity to isochoric is denoted k and call adiabatic exponent:
(1.5)
From (1.2) and taking into account (1.5) we obtain:
For accurate calculations, the average heat capacity is determined by the formula:
(1.7)
In thermal calculations of various equipment, the amount of heat that is required to heat or cool gases is often determined:
Q = Cm∙(t 2 - t 1), (1.8)
Q = C′∙V n∙(t 2 - t 1), (1.9)
where V n is the volume of gas at n.c., m 3 .
Q = µC∙ν∙(t 2 - t 1), (1.10)
where ν is the amount of gas, kmol.
Heat capacity. Using heat capacity to describe processes in closed systems
In accordance with equation (4.56), heat can be determined if the change in the entropy S of the system is known. However, the fact that entropy cannot be measured directly creates some complications, especially when describing isochoric and isobaric processes. There is a need to determine the amount of heat with the help of a quantity measured experimentally.
The heat capacity of the system can serve as such a quantity. The most general definition of heat capacity follows from the expression of the first law of thermodynamics (5.2), (5.3). Based on it, any capacity of the system C in relation to the work of the form m is determined by the equation
C m = dA m / dP m = P m d e g m / dP m , (5.42)
where C m is the capacity of the system;
P m and g m are, respectively, the generalized potential and the coordinate of the state of the form m.
The value C m shows how much work of the type m must be done under given conditions in order to change the m-th generalized potential of the system per unit of its measurement.
The concept of the capacity of a system with respect to a particular work in thermodynamics is widely used only when describing the thermal interaction between the system and the environment.
The capacity of the system with respect to heat is called the heat capacity and is given by the equality
C \u003d d e Q / dT \u003d Td e S warm / dT. (5.43)
Thus, heat capacity can be defined as the amount of heat that must be imparted to a system in order to change its temperature by one Kelvin.
Heat capacity, like internal energy and enthalpy, is an extensive quantity proportional to the amount of matter. In practice, the heat capacity per unit mass of a substance is used - specific heat, and the heat capacity per mole of the substance, molar heat capacity. The specific heat capacity in SI is expressed in J/(kg·K), and the molar heat capacity is expressed in J/(mol·K).
The specific and molar heat capacities are related by the relation:
C mol \u003d C beat M, (5.44)
where M is the molecular weight of the substance.
Distinguish true (differential) heat capacity, determined from equation (5.43) and representing an elementary increase in heat with an infinitesimal change in temperature, and average heat capacity which is the ratio of the total amount of heat to the total change in temperature in this process:
Q/DT . (5.45)
The relationship between true and average specific heat capacity is established by the relation
At constant pressure or volume, heat and, accordingly, heat capacity acquire the properties of a state function, i.e. become characteristics of the system. It is these heat capacities - isobaric C P (at constant pressure) and isochoric C V (at constant volume) that are most widely used in thermodynamics.
If the system is heated at constant volume, then, in accordance with expression (5.27), the isochoric heat capacity C V is written as
C V = 
. (5.48)
If the system is heated at constant pressure, then, in accordance with equation (5.32), the isobaric heat capacity C P appears as
C P = 
. (5.49)
To find the connection between С Р and С V , it is necessary to differentiate expression (5.31) with respect to temperature. For one mole of an ideal gas, this expression, taking into account equation (5.18), can be represented as
H=U+pV=U+RT. (5.50)
dH/dT = dU/dT + R, (5.51)
and the difference between the isobaric and isochoric heat capacities for one mole of an ideal gas is numerically equal to the universal gas constant R:
C P - C V \u003d R. (5.52)
The heat capacity at constant pressure is always greater than the heat capacity at constant volume, since heating a substance at constant pressure is accompanied by the work of expansion of the gas.
Using the expression for the internal energy of an ideal monatomic gas (5.21), we obtain the value of its heat capacity for one mole of an ideal monatomic gas:
C V \u003d dU / dT \u003d d (3/2 RT) dT \u003d 3/2 R "12.5 J / (mol K); (5.53)
C Р \u003d 3 / 2R + R \u003d 5/2 R \u003e 20.8 J / (mol K). (5.54)
Thus, for monatomic ideal gases, C V and C p do not depend on temperature, since all the supplied thermal energy is spent only on the acceleration of translational motion. For polyatomic molecules, along with a change in the translational motion, a change in the rotational and vibrational intramolecular motion can also occur. For diatomic molecules, rotational motion is usually taken into account, as a result of which the numerical values of their heat capacities are:
C V \u003d 5/2 R "20.8 J / (mol K); (5.55)
C p \u003d 5/2 R + R \u003d 7/2 R \u003e 29.1 J / (mol K). (5.56)
In passing, we touch on the heat capacities of substances in other (except gaseous) aggregate states. To estimate the heat capacities of solid chemical compounds, the approximate Neumann and Kopp additivity rule is often used, according to which the molar heat capacity of chemical compounds in the solid state is equal to the sum of the atomic heat capacities of the elements included in this compound. So, the heat capacity of a complex chemical compound, taking into account the Dulong and Petit rules, can be estimated as follows:
C V \u003d 25n J / (mol K), (5.57)
where n is the number of atoms in the molecules of the compounds.
The heat capacities of liquids and solids near the melting (crystallization) temperature are almost equal. Near the normal boiling point, most organic liquids have a specific heat capacity of 1700 - 2100 J/kg·K. In the intervals between these phase transition temperatures, the heat capacity of the liquid can differ significantly (depending on temperature). In general, the dependence of the heat capacity of solids on temperature in the range of 0 - 290K in most cases is well represented by the semi-empirical Debye equation (for a crystal lattice) in the low-temperature region
C P » C V = eT 3 , (5.58)
in which the coefficient of proportionality (e) depends on the nature of the substance (empirical constant).
The dependence of the heat capacity of gases, liquids and solids on temperature at ordinary and high temperatures is usually expressed using empirical equations that have the form of power series:
C P \u003d a + bT + cT 2 (5.59)
C P \u003d a + bT + c "T -2, (5.60)
where a, b, c and c" are empirical temperature coefficients.
Returning to the description of processes in closed systems using the method of heat capacities, let us write down some of the equations given in Section 5.1 in a slightly different form.
Isochoric process. Expressing the internal energy (5.27) in terms of heat capacity, we obtain
dU V \u003d dQ V \u003d U 2 - U 1 \u003d C V dT \u003d C V dT. (5.61)
Given that the heat capacity of an ideal gas does not depend on temperature, equation (5.61) can be written as follows:
DU V \u003d Q V \u003d U 2 - U 1 \u003d C V DT. (5.62)
To calculate the value of the integral (5.61) for real monatomic and polyatomic gases, it is necessary to know the specific form of the functional dependence C V = f(T) of the type (5.59) or (5.60).
isobaric process. For the gaseous state of matter, the first law of thermodynamics (5.29) for this process, taking into account the expansion work (5.35) and using the method of heat capacities, is written as follows:
Q P \u003d C V DT + RDT \u003d C P DT \u003d DH (5.63)
Q P \u003d DH P \u003d H 2 - H 1 \u003d C P dT. (5.64)
If the system is an ideal gas and the heat capacity C P does not depend on temperature, relation (5.64) becomes (5.63). To solve equation (5.64), which describes a real gas, it is necessary to know the specific form of the dependence C p = f(T).
isothermal process. Change in the internal energy of an ideal gas in a process proceeding at a constant temperature
dU T = C V dT = 0. (5.65)
adiabatic process. Since dU \u003d C V dT, then for one mole of an ideal gas, the change in internal energy and the work done are equal, respectively:
DU = C V dT = C V (T 2 - T 1); (5.66)
And fur \u003d -DU \u003d C V (T 1 - T 2). (5.67)
Analysis of equations characterizing various thermodynamic processes under the following conditions: 1) p = const; 2) V = const; 3) T = const and 4) dQ = 0 shows that they can all be represented by the general equation:
pV n = const. (5.68)
In this equation, the exponent "n" can take values from 0 to ¥ for different processes:
1. isobaric (n = 0);
2. isothermal (n = 1);
3. isochoric (n = ¥);
4. adiabatic (n = g; where g = C Р /C V is the adiabatic coefficient).
The relations obtained are valid for an ideal gas and are a consequence of its equation of state, and the considered processes are particular and limiting manifestations of real processes. Real processes, as a rule, are intermediate, proceed at arbitrary values of "n" and are called polytropic processes.
If we compare the work of expansion of an ideal gas produced in the considered thermodynamic processes with a change in volume from V 1 to V 2, then, as can be seen from Fig. 5.2, the greatest work of expansion is performed in the isobaric process, the smallest - in the isothermal and even smaller - in the adiabatic. For an isochoric process, work is zero.
Rice. 5.2. P = f (V) - dependence for various thermodynamic processes (shaded areas characterize the work of expansion in the corresponding process)
Lab #1
Definition of mass isobaric
air heat capacity
Heat capacity is the heat that must be supplied to a unit amount of a substance in order to heat it by 1 K. A unit amount of a substance can be measured in kilograms, cubic meters under normal physical conditions and kilomoles. A kilomole of a gas is the mass of a gas in kilograms, numerically equal to its molecular weight. Thus, there are three types of heat capacities: mass c, J/(kg⋅K); volume c', J/(m3⋅K) and molar, J/(kmol⋅K). Since a kilomole of gas has a mass μ times greater than one kilogram, a separate designation for the molar heat capacity is not introduced. Relations between heat capacities:
where = 22.4 m3/kmol is the volume of a kilomole of an ideal gas under normal physical conditions; is the density of the gas under normal physical conditions, kg/m3.
The true heat capacity of a gas is the derivative of heat with respect to temperature:
The heat supplied to the gas depends on the thermodynamic process. It can be determined from the first law of thermodynamics for isochoric and isobaric processes:
Here, is the heat supplied to 1 kg of gas in the isobaric process; is the change in the internal energy of the gas; is the work of gases against external forces.
In essence, formula (4) formulates the 1st law of thermodynamics, from which the Mayer equation follows:
If we put = 1 K, then, that is, the physical meaning of the gas constant is the work of 1 kg of gas in an isobaric process when its temperature changes by 1 K.
Mayer's equation for 1 kilomole of gas is
where = 8314 J/(kmol⋅K) is the universal gas constant.
In addition to the Mayer equation, the isobaric and isochoric mass heat capacities of gases are interconnected through the adiabatic index k (Table 1):
Table 1.1
Values of adiabatic exponents for ideal gases
Atomicity of gases | |
Monatomic gases | |
Diatomic gases | |
Tri- and polyatomic gases |
GOAL OF THE WORK
Consolidation of theoretical knowledge on the basic laws of thermodynamics. Practical development of the method for determining the heat capacity of air based on the energy balance.
Experimental determination of the specific mass heat capacity of air and comparison of the obtained result with a reference value.
1.1. Description of the laboratory setup
The installation (Fig. 1.1) consists of a brass pipe 1 with an inner diameter d =
= 0.022 m, at the end of which there is an electric heater with thermal insulation 10. An air flow moves inside the pipe, which is supplied 3. The air flow can be controlled by changing the fan speed. In pipe 1, a tube of full pressure 4 and excess static pressure 5 are installed, which are connected to pressure gauges 6 and 7. In addition, a thermocouple 8 is installed in pipe 1, which can move along the cross section simultaneously with the full pressure tube. The EMF value of the thermocouple is determined by potentiometer 9. The heating of the air moving through the pipe is regulated using a laboratory autotransformer 12 by changing the heater power, which is determined by the readings of the ammeter 14 and voltmeter 13. The air temperature at the outlet of the heater is determined by thermometer 15.
1.2. EXPERIMENTAL TECHNIQUE
Heat flow of the heater, W:
where I is current, A; U – voltage, V; = 0.96; =
= 0.94 - heat loss coefficient.
Fig.1.1. Scheme of the experimental setup:
1 - pipe; 2 - confuser; 3 – fan; 4 - tube for measuring dynamic head;
5 - branch pipe; 6, 7 – differential pressure gauges; 8 - thermocouple; 9 - potentiometer; 10 - insulation;
11 - electric heater; 12 – laboratory autotransformer; 13 - voltmeter;
14 - ammeter; 15 - thermometer
Heat flux perceived by air, W:
where m is the mass air flow, kg/s; – experimental, mass isobaric heat capacity of air, J/(kg K); – air temperature at the exit from the heating section and at the entrance to it, °C.
Mass air flow, kg/s:
. (1.10)
Here, is the average air velocity in the pipe, m/s; d is the inner diameter of the pipe, m; - air density at temperature , which is found by the formula, kg/m3:
, (1.11)
where = 1.293 kg/m3 is the air density under normal physical conditions; B – pressure, mm. rt. st; - excess static air pressure in the pipe, mm. water. Art.
Air velocities are determined by dynamic head in four equal sections, m/s:
where is the dynamic head, mm. water. Art. (kgf/m2); g = 9.81 m/s2 is the free fall acceleration.
Average air velocity in the pipe section, m/s:
The average isobaric mass heat capacity of air is determined from formula (1.9), into which the heat flux is substituted from equation (1.8). The exact value of the heat capacity of air at an average air temperature is found according to the table of average heat capacities or according to the empirical formula, J / (kg⋅K):
. (1.14)
Relative error of experiment, %:
. (1.15)
1.3. Conducting the experiment and processing
measurement results
The experiment is carried out in the following sequence.
1. The laboratory stand is turned on and after the stationary mode is established, the following readings are taken:
Dynamic air pressure at four points of equal sections of the pipe;
Excessive static air pressure in the pipe;
Current I, A and voltage U, V;
Inlet air temperature, °С (thermocouple 8);
Outlet temperature, °С (thermometer 15);
Barometric pressure B, mm. rt. Art.
The experiment is repeated for the next mode. The measurement results are entered in Table 1.2. Calculations are performed in table. 1.3.
Table 1.2
Measurement table
Value name | |||
Air inlet temperature, °C | |||
Outlet air temperature, °C |
|||
Dynamic air pressure, mm. water. Art. | |||
Excessive static air pressure, mm. water. Art. |
|||
Barometric pressure B, mm. rt. Art. |
|||
Voltage U, V |
Table 1.3
Calculation table
Name of quantities |
|
|||
Dynamic head, N/m2 | ||||
Average inlet flow temperature, °C |
the Russian Federation Protocol of the State Standard of the USSR
GSSSD 8-79 Liquid and gaseous air. Density, enthalpy, entropy and isobaric heat capacity at temperatures of 70-1500 K and pressures of 0.1-100 MPa
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STATE STANDARD REFERENCE DATA SERVICE
Standard Reference Data Tables
AIR LIQUID AND GAS. DENSITY, ENTHALPY, ENTROPY AND ISOBARIC HEAT CAPACITY AT TEMPERATURES 70-1500 K AND PRESSURES 0.1-100 MPa
Tables of Standard Reference Data
Liquid and gaseous air Density, enthalpy, entropy and isobaric heat capacity at temperatures from 70 to 1500 K and pressures from 0.1 to 100 MPa
DEVELOPED by the All-Union Scientific Research Institute of the Metrological Service, the Odessa Institute of Marine Engineers, the Moscow Order of Lenin Energy Institute
RECOMMENDED FOR APPROVAL by the Soviet National Committee for the Collection and Evaluation of Numerical Data in the Field of Science and Technology of the Presidium of the USSR Academy of Sciences; All-Union Research Center of the State Service for Standard Reference Data
APPROVED by the GSSSD expert commission consisting of:
cand. tech. Sciences N.E. Gnezdilova, Dr. tech. Sciences I.F. Golubeva, Dr. of Chem. Sciences L.V. Gurvich, Doctor of Engineering. Sciences V.A. Rabinovich, Doctor of Engineering. Sciences A.M.Siroty
PREPARED FOR APPROVAL by the All-Union Research Center of the State Service for Standard Reference Data
The use of standard reference data is mandatory in all sectors of the national economy
These tables contain the most important for practice values of density, enthalpy, entropy and isobaric heat capacity of liquid and gaseous air.
The tables are based on the following principles:
1. The equation of state, which displays reliable experimental data on the , , -dependence with high accuracy, can provide a reliable calculation of caloric and acoustic properties from known thermodynamic relationships.
2. Averaging the coefficients of a large number of equations of state, which are equivalent in terms of the accuracy of describing the initial information, makes it possible to obtain an equation that reflects the entire thermodynamic surface (for a selected set of experimental data among equations of the accepted type). Such averaging makes it possible to estimate the possible random error in the calculated values of thermal, caloric and acoustic quantities, without taking into account the influence of the systematic error of the experimental , , -data and the error due to the choice of the form of the equation of state.
The averaged equation of state for liquid and gaseous air has the form
Where ; ; .
The equation is based on the most reliable experimental density values obtained in the works and covering the temperature range 65-873 K and pressures 0.01-228 MPa. The experimental data are described by an equation with a mean square error of 0.11%. The coefficients of the averaged equation of state were obtained as a result of processing a system of 53 equations that are equivalent in accuracy to the description of experimental data. In the calculations, the following values of the gas constant and critical parameters were taken: 287.1 J/(kg K); 132.5 K; 0.00316 m/kg.
Coefficients of the averaged air state equation:
Enthalpy, entropy and isobaric heat capacity were determined by the formulas
Where , , are the enthalpy, entropy and isochoric heat capacity in the ideal gas state. The values and are determined from the relations
Where and - enthalpy and entropy at temperature; - heat of sublimation at 0 K; - constant (in this work 0).
The value of the heat of sublimation of air was calculated on the basis of data on the heats of sublimation of its components and is equal to 253.4 kJ/kg Ar by volume). The values of enthalpy and entropy at a temperature of 100 K, which is an auxiliary reference point when integrating the equation for , are 3.48115 kJ/kg and 20.0824 kJ/(kg K), respectively.
The isobaric heat capacity in the ideal gas state is borrowed from the work and approximated by the polynomial
The root-mean-square error of approximation of the initial data in the temperature range 50-2000 K is 0.009%, the maximum is about 0.02%.
Random errors of calculated values are calculated with a confidence probability of 0.997 by the formula
Where is the average value of the thermodynamic function; - the value of the same function, obtained by the th equation from the system containing equations.
Tables 1-4 show the values of the thermodynamic functions of air, and tables 5-8 show the corresponding random errors. The error values in Tables 5-8 are presented for a part of the isobars, and the values for the intermediate isobars can be obtained with acceptable accuracy by linear interpolation. Random errors in the calculated values reflect the scatter of the latter relative to the averaged equation of state; for density, they are significantly less than the root-mean-square error of the description of the initial array of experimental data, which serves as an integral estimate and includes large deviations for some data characterized by scatter.
Table 1
Air density
Continuation
kg/m, at , MPa, |
|||||||||||
table 2
Air enthalpy
Continuation
KJ/kg, at , MPa, |
|||||||||||
Table 3
Air entropy
Continuation
KJ/(kg, K), at , MPa, |
|||||||||||
Table 4
Isobaric heat capacity of air
________________
* The text of the document corresponds to the original. - Database manufacturer's note.
Continuation
KJ/(kg, K), at , MPa, |
|||||||||||
Table 5. Root-mean-square random errors of calculated density values
, %, at , MPa |
|||||||||||||
Table 6. Root-mean-square random errors of calculated enthalpy values
KJ/kg, at , MPa |
||||||||||||||||
In connection with the use of the virial form, the equations of state in the tables do not claim to be an accurate description of the thermodynamic properties in the vicinity of the critical point (126–139 K, 190–440 kg/m).
Information about experimental studies of the thermodynamic properties of air, the method of compiling the equation of state and calculating tables, the consistency of calculated values with experimental data, as well as more detailed tables containing additional information about isochoric heat capacity, sound speed, heat of evaporation, choke effect, some derivatives and about properties on the boiling and condensation curves are given in .
BIBLIOGRAPHY
1. Hlborn L., Schultre H. die Druckwage und die Isothermen von Luft, Argon und Helium Zwischen 0 und 200 °C. - Ann. Phys. 1915 m, Bd 47, N 16, S.1089-1111.
2. Michels A., Wassenaar T., Van Seventer W. Isotherms of air between 0 °C and 75 °C and at pressures up to 2200 atm. -Appl. sci. Res., 1953, vol. 4, No. 1, p.52-56.
3. Compressibility isotherms of air at temperatures between -25 °C and -155 °C and at densities up to 560 Amagats (Pressures up to 1000 atmospheres) / Michels A.. Wassenaar T., Levelt J.M., De Graaff W. - Appl . sci. Res., 1954, vol. A 4, N 5-6, p.381-392.
4. Experimental study of specific volumes of air / Vukalovich M.P., Zubarev V.N., Aleksandrov A.A., Kozlov A.D. - Thermal power engineering, 1968, N 1, pp. 70-73.
5. Romberg H. Neue Messungen der thermischen ler Luft bei tiefen Temperaturen and die Berechnung der kalorischen mit Hilfe des Kihara-Potentials. - VDl-Vorschungsheft, 1971, - N 543, S.1-35.
6. Blance W. Messung der thermischen von Luft im Zweiphasengebiet und Seiner Umgebung. Dissertation zur Erlangung des Grades eines Doctor-Ingenieurs/. Bohum., 1973.
7. Measurement of air density at temperatures of 78-190 K up to a pressure of 600 bar / Vasserman A.A., Golovsky E.A., Mitsevich E.P., Tsymarny V.A., M., 1975. (Dep. in VINITI 28.07 .76 N 2953-76).
8. H. Landolt, R. Zahlenwerte und Funktionen aus Physik, Chemie, Astronomic, Geophysik und Technik. Berlin., Springer Verlag, 1961, Bd.2.
9. Tables of thermal properties of gases. Wachington., Gov. print, off., 1955, XI. (U.S. Dep. of commerce. NBS. Girc. 564).
10. Thermodynamic properties of air / Sychev V.V., Vasserman A.A., Kozlov A.D. and others. M., Publishing house of standards, 1978.
Transport energy (cold transport) Air humidity. Heat capacity and enthalpy of airAir humidity. Heat capacity and enthalpy of air
Atmospheric air is a mixture of dry air and water vapor (from 0.2% to 2.6%). Thus, the air can almost always be considered as humid.
The mechanical mixture of dry air and water vapor is called moist air or air/steam mixture. The maximum possible content of vaporous moisture in the air m a.s. temperature dependent t and pressure P mixtures. When it changes t and P the air can go from initially unsaturated to a state of saturation with water vapor, and then excess moisture will begin to fall out in the gas volume and on the enclosing surfaces in the form of fog, hoarfrost or snow.
The main parameters characterizing the state of moist air are: temperature, pressure, specific volume, moisture content, absolute and relative humidity, molecular weight, gas constant, heat capacity and enthalpy.
According to Dalton's law for gas mixtures wet air total pressure (P) is the sum of the partial pressures of dry air P c and water vapor P p: P \u003d P c + P p.
Similarly, the volume V and the mass m of moist air will be determined by the relations:
V \u003d V c + V p, m \u003d m c + m p.
Density and specific volume of humid air (v) defined:
Molecular weight of moist air:
where B is the barometric pressure.
Since the air humidity continuously increases during the drying process, and the amount of dry air in the vapor-air mixture remains constant, the drying process is judged by how the amount of water vapor changes per 1 kg of dry air, and all indicators of the vapor-air mixture (heat capacity, moisture content, enthalpy and etc.) refers to 1 kg of dry air in moist air.
d \u003d m p / m c, g / kg, or, X \u003d m p / m c.
Absolute air humidity- mass of steam in 1 m 3 of moist air. This value is numerically equal to .
Relative humidity - is the ratio of the absolute humidity of unsaturated air to the absolute humidity of saturated air under given conditions:
here , but more often the relative humidity is given as a percentage.
For the density of moist air, the relation is true:
Specific heat humid air:
c \u003d c c + c p ×d / 1000 \u003d c c + c p ×X, kJ / (kg × ° С),
where c c is the specific heat capacity of dry air, c c = 1.0;
c p - specific heat capacity of steam; with n = 1.8.
The heat capacity of dry air at constant pressure and small temperature ranges (up to 100 ° C) for approximate calculations can be considered constant, equal to 1.0048 kJ / (kg × ° C). For superheated steam, the average isobaric heat capacity at atmospheric pressure and low degrees of superheat can also be assumed to be constant and equal to 1.96 kJ/(kg×K).
Enthalpy (i) of humid air- this is one of its main parameters, which is widely used in the calculations of drying installations, mainly to determine the heat spent on the evaporation of moisture from the dried materials. The enthalpy of moist air is related to one kilogram of dry air in a vapor-air mixture and is defined as the sum of the enthalpies of dry air and water vapor, that is
i \u003d i c + i p × X, kJ / kg.
When calculating the enthalpy of mixtures, the starting point of reference for the enthalpies of each of the components must be the same. For calculations of humid air, it can be assumed that the enthalpy of water is zero at 0 o C, then the enthalpy of dry air is also counted from 0 o C, that is, i in \u003d c in * t \u003d 1.0048t.