For drying materials, air heated in an air heater or flue gases mixed with air are most often used as heat carriers and moisture carriers. Considering that the mixture of flue gases with atmospheric air differs little in its thermophysical properties from heated humid air, we will consider the most important characteristics of humid air.
Humid air is a mixture of dry air and water vapor. Moist air is characterized by the following main parameters:
Absolute humidity determined by the amount of water vapor in kg,
Relative humidity , or the degree of air saturation () is the ratio of the mass of water vapor in 1m 3 of moist air () to the maximum possible mass of water vapor in 1m 3 of air (density of saturated vapor) under the same conditions (t, P).
As the temperature rises, the (saturated vapor density) increases faster than (vapor density), i.e. when heated, the relative humidity decreases.
Moisture content is the amount of water vapor (in kg) contained in humid air and per 1 kg of absolutely dry air.
where u is the mass of water vapor and the mass of absolutely dry air in a given volume of moist air, kg.
According to the Mendeleev-Cliperon equation,
Substituting these values \u200b\u200binto the formula for (x) moisture content, we obtain
Molecular weight of water vapor (18)
Molecular Weight of Dry Air (29)
Ratio 18/29 \u003d 0.622
According to Dalton's law, the total pressure of the gas mixture (P) will be equal to the sum of the partial pressures of the components, i.e. for our case 
considering that, then 
,
where is the saturation pressure
Barometric pressure
Heat content or the enthalpy of humid air is expressed as the sum of the enthalpies of 1 kg of dry air () and water vapor () contained in it.
since heat capacity of air 
, and the heat capacity of water vapor 
... Water vapor is in the process of drying in a superheated state mixed with air, then
Enthalpy of superheated steam at 0 0 С (\u003d 2493 kJ / kg)
Dry Bulb Temperature - denoted by the letter (or), this is the temperature that is around us.
Wet bulb temperature - temperature of adiabatic saturation (i.e. without heat exchange with the environment) or it is the temperature of evaporation of water from a free surface (indicated).
Drying potential - denoted (g) is the difference between the air temperature () and the temperature of the wet thermometer (), characterizes the ability of air to absorb moisture from the material.
Dew point temperature () is the saturation temperature of the air at constant moisture content.
Moisture partial pressure is the pressure that moisture vapor would create if these vapors occupied the volume occupied by the vapor-air mixture.
The main instruments used to measure air parameters: (barometers, thermometers, psychrometers, hygrometers, recorders, barographs, thermographs).
As known, dry air (CB) consists of 78% nitrogen, 21% oxygen, and about 1% carbon dioxide, inert and other gases. If there is in the air, then such air is called moist air (BB). Considering that during ventilation of premises the composition of the dry part of the air practically does not change, and only the amount of moisture can change, it is customary to consider explosives in ventilation as a binary mixture consisting of only two components: SV and water vapor (VP). Although all gas laws are applicable to this mixture, however, with ventilation, it can be assumed with sufficient accuracy that the air is under atmospheric pressure almost all the time, since the fan pressures are quite small compared to barometric pressure... Normal atmospheric pressure is 101.3 kPa, and the pressures developed by fans are usually no more than 2 kPa. Therefore, heating and air ventilation occurs at a constant pressure.
From the thermodynamic parameters of explosives, which are operated in the course of ventilation, one can single out following:
- density;
- heat capacity;
- temperature;
- moisture content;
- partial pressure of water vapor;
- relative humidity;
- dew point temperature;
- enthalpy (heat content);
- wet bulb temperature.
Under the influence of various factors, it can change its parameters. If the air trapped in a certain volume (for example, a room) is in contact with hot surfaces, it heats up, that is, its temperature rises. In this case, those layers that are adjacent to hot surfaces are directly exposed to heating. Changes due to heating, and this leads to the occurrence convective currents: a process of turbulent exchange takes place. Due to the presence of turbulent mixing of air in the process of vortex formation, the perceived boundary layers are gradually transferred to more distant layers, as a result of which the entire volume of air is somehow enhances your temperature.
From the considered example, it is clear that the layers close to hot surfaces will have a temperature higher than the remote ones. In other words, the temperature is not the same throughout the volume (and sometimes it differs quite significantly). Therefore, temperature, as a parameter of air, at each point will have its own individual, local value. However, the nature of the distribution of local temperatures over the volume of the room is extremely difficult to predict, therefore in most situations we have to talk about a certain average value of one or another air parameter. Average temperature value is derived from the assumption that the perceived heat will be evenly distributed over the volume of air, and the air temperature at each point in space will be the same.
The issue of the temperature distribution over the height of the room has been more or less studied, however, even in this issue, the distribution pattern can vary greatly under the influence of individual factors : jet streams in the room, the presence of shielding surfaces of building structures and equipment, temperature and dimensions of heat sources.
Surrounding us atmospheric air is a mixture of gases. It is almost always wet. Water vapor, unlike other components of the mixture, can be in the air, both in a superheated and in a saturated state. The content of water vapor in the air changes, both in the process of moisture treatment in supply ventilation systems and air conditioners, and in the assimilation of moisture in the room by air. The dry part of humid air usually contains (by volume): about 75% nitrogen, 21% oxygen, 0.03% carbon dioxide and a small amount of inert gases - argon, neon, helium, xenon, krypton), hydrogen, ozone and others. The specified components of the gas mixture of air make up its dry part, the other part of the air mass is water vapor.
Air is viewed as mixture of ideal gases, which makes it possible to use the laws of thermodynamics to obtain calculation formulas.
According to Dalton's law, each gas in the mixture, which makes up air, occupies its own volume, has its own partial pressure
P i ,
and has the same temperature as other gases in this mixture.
Attention! An important definition:
The sum of the partial pressures of each of the components of the mixture is equal to the total barometric pressure of the air.
B \u003d Σ P i, Pa.
Consider the concept of what is partial pressure ?
Partial pressure - This is the pressure that the gas would have in the composition of this mixture if it were in the same amount, in the same volume and at the same temperature as in the mixture.
In calculating ventilation, we consider humid air as a binary mixture, i.e. a mixture of two gases, which consists of water vapor and a dry part of air. We conventionally take the dry part of the air as a homogeneous gas.
Thus, barometric pressure equal to the sum of the partial pressures of dry air P r.v. and water vapor P n , i.e.,
B \u003d P r.v. + P n
Under normal indoor conditions when the water vapor pressure R p approximately equal to 15 mm. rt. Art., share of the second member P r.v. in the barometric pressure formula, taking into account the difference in the density of moist and dry air, with other conditions being equal, it is only 0.75% of the value of the density of dry air ρ r.v. ... Therefore, in our engineering calculations, it is assumed that
ρ air. \u003d ρ r.v.
ρ air. \u003d ρ r.v.
When the air humidity changes in ventilation processes, the mass of its dry part remains unchanged. Based on this, it is customary to refer the mass of water vapor contained in the air to 1 kg. dry part of the air.
Let's go directly to those physical quantities that determine the parameters of humid air. It is the combination of these parameters that determines the state of humid air:
is a quantity characterizing body temperature... It is a measure of the average kinetic energy of the translational motion of molecules. Currently, the Celsius temperature scale and the Kelvin thermodynamic temperature scale are used, which are based on the second law of thermodynamics. There is a relationship between temperatures expressed in Kelvin and Celsius, namely:
T, K \u003d 273.15 + t ° C
It is important to note that the parameter of the state is the absolute temperature, expressed in Kelvin, but the degree of the absolute scale is numerically equal to the degree Celsius, i.e.
dT \u003d dt.
Air humidity is characterized by the mass of water vapor contained in it. The mass of water vapor in grams per 1 kg of dry part of moist air is called moisture content of air d, g / kg.
The quantity d is equal to:
where: B
- barometric pressure, equal to the sum of the partial pressures of dry air.
P r.v.
and water vapor P n
;
P n
- partial pressure of water vapor in unsaturated humid air.
The quantity φ equal to the ratio of the partial pressure of water vapor in unsaturated humid air P p. to the partial pressure of water vapor in saturated humid air P n.p. at the same temperature and barometric pressure, i.e.
At a relative humidity of 100%, the air is completely saturated with water vapor, and it is called saturated humid air , and the water vapor contained in this air is in a saturated state.
If a φ < 100%, then the air contains water vapor in a superheated state and is called unsaturated humid air .
The pressure of saturated water vapor depends only on temperature. Its value is determined experimentally and is given in special tables. There are a number of formulas that approximate the dependence Pn.p. in Pa or in mm. rt. st... on temperature in t ° C.
For example, for the region of positive temperatures from 0 ° C and higher pressure of saturated water vapor in Pa, approximately expressed by the dependence:
P n.p. \u003d 479 + (11.52 + 1.62 t) 2, Pa
Using the concept of relative humidity φ , the moisture content of the air can be defined as
For ventilation processes, the temperature range is constant and equal to
From s.v. \u003d 1.005 kJ / (kg × ° C).
In the temperature range usual for ventilation processes, this value can be considered constant and equal to
C n \u003d 1.8 kJ / (kg × ° C).
J r.v. \u003d C s.v. × t,
where: t - air temperature, in ° C.
Enthalpy of dry air J r.v. at t \u003d 0 ° C take equal to 0.
for water at t \u003d 0 ° C equals 2500 kJ / kg.
in air at any temperature t, is
J p \u003d 2500 + 1.8 t.
consists of the enthalpy of its dry part and the enthalpy of water vapor.
Enthalpy J humid air, referred to 1 kg dry part of humid air, in kJ / kg, at an arbitrary temperature t and arbitrary moisture content d, is equal to:
where: 1,005
– C s.v. heat capacity of dry air, _kJ / (kg × ° С);
2500
– r specific heat vaporization, kJ / (kg × ° С);
1,8
– C p heat capacity of water vapor, kJ / (kg × ° С).
If the air transfers obvious warmth, it heats up, i.e. its temperature rises. When humid air is heated, enthalpy changes as a result of changes in the temperature of the dry part of the air and water vapor. When water vapor with the same temperature enters the air from external sources (isothermal steam humidification), it is transferred latent heat vaporization. In this case, the enthalpy of humid air also increases, because the enthalpy of water vapor is added to the enthalpy of the dry part of the air. At the same time, the air temperature almost does not change, which was the reason for the introduction of this term - latent heat.
In general, the enthalpy of humid air consists of sensible and latent heat; therefore, enthalpy is sometimes called total heat.
For further calculations of ventilation and air conditioning systems, we need the following basic parameters of humid air:
- temperature t in , ° C ;
- moisture content d in , g / kg ;
- relative humidity φ in , % ;
- heat content J in , kJ / kg ;
- concentration of harmful impurities FROM , mg / m 3 ;
- travel speed V in , m / sec.
Ministry of Education and Science of the Russian Federation
Federal Agency for Education
Saratov State Technical University
DETERMINATION OF WET AIR PARAMETERS
Methodical instructions
for students of specialties 280201
full-time and part-time forms of study
Saratov 2009
Objective: deepening of knowledge in the section of technical thermodynamics "Moist air", study of methods for calculating the parameters of moist air and obtaining skills in working with measuring instruments.
As a result of the work, the following should be learned:
1) basic concepts of humid air;
2) a method for determining the parameters of humid air by
calculated dependencies;
3) method for determining the parameters of humid air by
I-d diagram.
1) determine the value of the parameters of humid air by
calculated dependencies;
2) determine the parameters of humid air using
I-d charts;
3) draw up a report on the laboratory work performed.
BASIC CONCEPTS
Air that does not contain water vapor is called dry air. Dry air does not occur in nature, since atmospheric air always contains a certain amount of water vapor.
A mixture of dry air with water vapor is called wet air. Humid air is widely used in drying, ventilation, air conditioning, and more.
A characteristic feature of the processes taking place in humid air is that the amount of water vapor contained in the air changes. The steam can partially condense and, conversely, the water evaporates into the air.
A mixture of dry air and superheated water vapor is called unsaturated moist air. The partial vapor pressure рп in the mixture is less than the saturation pressure рн corresponding to the temperature of humid air (рп<рн). Температура пара выше температуры его насыщения при данном парциальном давлении.
A mixture of dry air and dry saturated water vapor is called saturated moist air. The partial pressure of water vapor in the mixture is equal to the saturation pressure corresponding to the temperature of humid air. The vapor temperature is equal to the dew point at a given vapor partial pressure.
A mixture consisting of dry air and moist saturated water vapor (that is, there are particles of condensed vapor in the air that are in suspension and fall out in the form of dew) is called oversaturated humid air. The partial pressure of water vapor is equal to the saturation pressure corresponding to the temperature of humid air, which in this case is equal to the condensation temperature of the vapor in it. In this case, the temperature of the humid air is called the dew point temperature. tr... If the partial pressure of water vapor is, for some reason, higher than the saturation pressure, then part of the steam will condense in the form of dew.
The main indicators characterizing the state of humid air are moisture content d, relative humidity j, enthalpy I and density r.
The calculation of the parameters of humid air is carried out using the Mendeleev-Clapeyron equation for an ideal gas, to which humid air obeys with sufficient approximation. We consider humid air as a gas mixture consisting of dry air and water vapor.
According to Dalton's law, the pressure of humid air r equally:
where pv - partial pressure of dry air, Pa;
rn - partial pressure of water vapor, Pa.
The maximum value of the partial pressure of water vapor is equal to the pressure of saturated water vapor ph, corresponding to the temperature of the humid air.
The amount of water vapor in the mixture in kg per 1 kg of dry air is called the moisture content d, kg / kg:
https://pandia.ru/text/78/602/images/image003_38.gif "width \u003d" 96 "height \u003d" 53 "\u003e, since, then; (3)
Since, then, (4)
where V - volume of the gas mixture, m3;
Rin, Rp - gas constants of air and water vapor, equal
Rin\u003d 287 J / (kg × K), Rp\u003d 461 J / (kg × K);
T - humid air temperature, K.
Considering that 
, and, substituting expressions (3) and (4) into formula (2), we finally obtain:
DIV_ADBLOCK64 "\u003e
Relative humidity j called the ratio of vapor density (that is, absolute humidity rp) to the maximum possible absolute humidity (density rpmax) at a given temperature and pressure of humid air:
As rp and rpmax are determined at the same temperature of humid air, then
https://pandia.ru/text/78/602/images/image013_6.gif "width \u003d" 107 "height \u003d" 31 "\u003e. (8)
The density of dry air and water vapor is determined from the Mendeleev-Clapeyron equation, written for these two components of the gas mixture according to (3) and (4).
R is found by the formula:
https://pandia.ru/text/78/602/images/image015_6.gif "width \u003d" 175 "height \u003d" 64 src \u003d "\u003e.
Enthalpy of humid air I is the sum of the enthalpies of 1 kg of dry air and d kg of steam:
I= iin+ d× ip . (11)
Enthalpy of dry air and steam:
https://pandia.ru/text/78/602/images/image017_4.gif "width \u003d" 181 "height \u003d" 39 "\u003e, (13)
where tm- readings of a wet thermometer, ° С;
(tc- tm) - psychrometric difference, ° С;
x - correction to wet bulb temperature,%, determined
according to the schedule located at the stand, depending on tm and speed
A barometer is used to determine the pressure of humid air.
PROCEDURE AND PROCESSING TECHNIQUE
EXPERIMENTAL RESULTS
Measure the temperature of dry and wet bulb thermometers. Determine the true value of the wet bulb temperature using the formula (13). Find the difference Dt = tc - tm ist and determine the relative air humidity using the psychrometric table.
Knowing the value of the relative humidity, from expression (7) find the partial pressure of water vapor.
by (12), (13).
The specific volume of humid air is found by the formula:
The mass of moist air M, kg, in the laboratory room is determined by the formula:
where V - room volume, m3;
r - wet air pressure, Pa.
Enter the calculation results and instrument readings in the table in the following form.
Meter readings recording protocol
and calculation results
The name of the determined quantity | Designation | Dimension | Numerical magnitude |
|
Wet air pressure | ||||
Dry Bulb Temperature | ||||
Wet bulb temperature | tm | |||
Relative humidity | ||||
Saturated steam pressure | ||||
Partial pressure of water vapor | ||||
Partial pressure of dry air | ||||
Density of humid air | ||||
Absolute humidity | rp | |||
Wet air gas constant | ||||
Enthalpy of humid air | ||||
Wet air mass |
Next, you should determine the main parameters of moist air according to the measured tc and tm using an I-d diagram. The intersection point on the I-d diagram of the isotherms corresponding to the temperatures of the wet and dry thermometers characterizes the state of the moist air.
Compare the data obtained from the I-d diagram with the values \u200b\u200bdetermined using mathematical relationships.
The maximum possible relative error in determining the partial pressure of water vapor and dry air is determined by the formulas:
https://pandia.ru/text/78/602/images/image022_2.gif "width \u003d" 137 "height \u003d" 51 "\u003e; 
,
where D denotes the limit of the absolute measurement error
The absolute error limit of the hygrometer in this laboratory work is ± 6%. The absolute permissible error of the psychrometer thermometers is ± 0.2%. A barometer with an accuracy class of 1.0 is installed in operation.
WORK REPORT
The report on the performed laboratory work should contain
following:
1) short description work;
2) a protocol for recording the readings of measuring instruments and
calculation results;
3) a drawing with an I-d diagram, where the state of the wet is determined
air in this experiment.
TEST QUESTIONS
1. What is called humid air?
2. What is saturated and unsaturated humid air?
3. Dalton's law applied to humid air.
4. What is called the dew point temperature?
5. What is called absolute humidity?
6. What is the moisture content of humid air?
7. To what extent can moisture content change?
8. What is the relative humidity of the air?
9. In the I-d diagram, show the lines j \u003d const, I \u003d const; d \u003d const, tc \u003d const, tm \u003d const.
10. What is the maximum possible vapor density at a given temperature of humid air?
11. What determines the maximum possible partial pressure of water vapor in humid air and what is it equal to?
12. What parameters of humid air does the temperature of a wet thermometer depend on and how does it change when they change?
13. How can the partial pressure of water vapor in a mixture be determined if the relative humidity and temperature of the mixture are known?
14. Write the Mendeleev-Clapeyron equation for dry air, water vapor, moist air and explain all the quantities included in the equation.
15. How to determine the density of dry air?
16. How to determine the gas constant and enthalpy of humid air?
LITERATURE
1. Lyashkov basics of heat engineering /. M .: Higher school, 20s.
2. Zubarev on technical thermodynamics /,. M .: Energy, 19p.
DETERMINATION OF WET AIR PARAMETERS
Methodical instructions for laboratory work
in the courses "Heat engineering", "Technical thermodynamics and heat engineering"
Compiled by: Valentin Mikhailovich SEDELKIN
KULESHOV Oleg Yurievich
KAZANTSEVA Irina Leonidovna
Reviewer
Editor
License ID No. 000 dated 14.11.01
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