Humid air is a mixture of dry air and water vapor. Properties humid air characterized by the following main parameters: dry bulb temperature t, barometric pressure P b, partial pressure of water vapor P p, relative humidityφ, moisture content d, specific enthalpy i, dew point temperature t p, wet bulb temperature t m, density ρ.
The i-d diagram is a graphical relationship between the main air parameters t, φ, d, i at a certain barometric air pressure P b and is used to visualize the results of calculating the processing of moist air.
The i-d diagram was first compiled in 1918 by the Soviet heating engineer L.K. Ramzin.
The diagram is built in an oblique coordinate system, which allows expanding the area of unsaturated humid air and makes the diagram convenient for graphical plotting. The ordinate of the diagram shows the values of the specific enthalpy i, the abscissa, directed at an angle of 135 ° to the i axis, shows the values of the moisture content d. The field of the diagram is divided by lines of constant values of specific enthalpy i = const and moisture content d = const. The diagram also shows lines of constant temperature values t = const, which are not parallel to each other, and the higher the temperature of the humid air, the more the isotherms deviate upward. The lines of constant values of relative humidity φ = const are also plotted in the field of the diagram.
Relative humidity is the ratio of the partial pressure of water vapor contained in humid air of a given state to the partial pressure of saturated water vapor at the same temperature.
Moisture content is the mass of water vapor in humid air per 1 kg of its dry mass.
Specific enthalpy is the amount of heat contained in humid air at a given temperature and pressure, referred to 1 kg of dry air.
i-d curve diagram φ = 100% is divided into two areas. The entire area of the diagram above this curve characterizes the parameters of unsaturated humid air, and below - the area of fog.
Fog is a two-phase system consisting of saturated humid air and suspended moisture in the form of tiny water droplets or ice particles.
To calculate the parameters of humid air and plotting i-d charts use four basic equations:
1) The pressure of saturated water vapor over flat surface water (t> 0) or ice (t ≤ 0), kPa:
where α in, β in are constants for water, α in = 17.504, β in = 241.2 ° С
α l, β l - constants for ice, α l = 22.489, β l = 272.88 ° С
2) Relative humidity φ,%:
where P b - barometric pressure, kPa
4) Specific enthalpy of humid air i, kJ / kg d.w .:
Dew point temperature is the temperature to which the unsaturated air must be cooled so that it becomes saturated while maintaining a constant moisture content.
To find the dew point temperature on the i-d diagram through the point characterizing the state of the air, you need to draw a line d = const until it intersects with the φ = 100% curve. The dew point temperature is the limiting temperature to which humid air can be cooled at a constant moisture content without condensation.
Wet bulb temperature is the temperature that unsaturated humid air takes with the initial parameters i 1 and d 1 as a result of adiabatic heat and mass exchange with water in a liquid or solid state, having a constant temperature t in = t m after it reaches a saturated state that satisfies the equality:
where c in - specific heat capacity of water, kJ / (kg ° C)
The difference i n - i 1 is usually small, therefore the process of adiabatic saturation is often called isenthalpic, although in reality i n = i 1 only at t m = 0.
To find the temperature of the wet thermometer on the i-d diagram through the point characterizing the state of the air, you need to draw a line of constant enthalpy i = const until it intersects with the curve φ = 100%.
The density of humid air is determined by the formula, kg / m 3:
where T is the temperature in degrees Kelvin
The amount of heat required to heat the air can be calculated using the formula, kW:
The amount of heat removed from the air during cooling, kW:
where i 1, i 2 is the specific enthalpy at the initial and end points respectively, kJ / kg d.m.
G s - dry air consumption, kg / s
where d 1, d 2 - moisture content at the start and end points, respectively, g / kg d.m.
When mixing two air streams, the moisture content and specific enthalpy of the mixture are determined by the formulas:
On the diagram, the point of the mixture lies on line 1-2 and divides it into segments inversely proportional to the mixed amounts of air:
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A case is possible when the point of the mixture 3 * will be below the line φ = 100%. In this case, the mixing process is accompanied by the condensation of a part of the water vapor contained in the mixture, and the point of the mixture 3 will lie at the intersection of the lines i 3 * = const and φ = 100%.
On the presented site, on the "Calculations" page, you can calculate up to 8 states of humid air with the construction of rays of processes on the i-d diagram.
To determine the initial state, you need to specify two of the four parameters (t, φ, d, i) and the dry air flow rate L c *. The flow rate is set assuming an air density of 1.2 kg / m 3. From here, the dry air mass flow rate is determined, which is used in further calculations. The output table displays the actual values of the volumetric air flow corresponding to the real air density.
The new state can be calculated by defining the process and setting the final parameters.
The diagram shows the following processes: heating, cooling, adiabatic cooling, steam humidification, mixing and general process defined by any two parameters.
| Process | Designation | Description |
| Heat | O | The target end temperature or the target heat output is entered. |
| Cooling | C | The target end temperature or the target cooling capacity is entered. This calculation is based on the assumption that the surface temperature of the cooler remains unchanged, and the initial air parameters tend to the point with the temperature of the cooler surface at φ = 100%. It is as if the air of the initial state is mixing with fully saturated air at the surface of the cooler. |
| Adiabatic cooling | A | The target final relative humidity, or moisture content, or temperature is entered. |
| Steam humidification | P | The target final relative humidity or moisture content is entered. |
| General process | X | The values of two parameters out of four (t, φ, d, i) are introduced, which are final for a given process. |
| Mixing | S | This process is defined without setting parameters. The previous two air flow rates are used. If the maximum permissible moisture content is reached during mixing, then adiabatic condensation of water vapor occurs. As a result, the amount of condensed moisture is calculated. |
LITERATURE:
1. Burtsev S.I., Tsvetkov Yu.N. Wet air. Composition and properties: Textbook. allowance. - SPb .: SPbGAKhPT, 1998 .-- 146 p.
2. Reference book ABOK 1-2004. Wet air. - M .: AVOK-PRESS, 2004 .-- 46 p.
3. ASHRAE Handbook. Fundamentals. - Atlanta, 2001.
I-d chart humid air - a diagram widely used in calculations of ventilation, air conditioning, dehumidification systems and other processes associated with a change in the state of humid air. It was first compiled in 1918 by the Soviet heating engineer Leonid Konstantinovich Ramzin.
Various I-d charts
I-d diagram of humid air (Ramzin diagram):
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Description of the diagram
I-d-diagram of humid air graphically connects all the parameters that determine the thermal and humidity state of the air: enthalpy, moisture content, temperature, relative humidity, partial pressure of water vapor. The diagram is built in an oblique coordinate system, which allows expanding the area of unsaturated humid air and makes the diagram convenient for graphical plotting. The ordinate of the diagram shows the values of enthalpy I, kJ / kg of dry air, and the abscissa, directed at an angle of 135 ° to the I axis, shows the values of moisture content d, g / kg of dry air.
The field of the diagram is divided by lines of constant values of enthalpy I = const and moisture content d = const. It also contains lines of constant temperature values t = const, which are not parallel to each other - the higher the temperature of humid air, the more its isotherms deviate upward. In addition to the lines of constant values of I, d, t, lines of constant values of the relative humidity of the air φ = const are plotted on the diagram field. In the lower part of the I-d-diagram there is a curve with an independent ordinate axis. It binds moisture content d, g / kg, with water vapor pressure pп, kPa. The ordinate axis of this graph is the scale of the partial pressure of water vapor pп.
With a more rigorous definition, it should be understood as the ratio of the partial pressures of water vapor pn in unsaturated humid air to their partial pressure in saturated air at the same temperature
For the temperature range typical for air conditioning
Density of humid air
ρ
equal to the sum of the densities of dry air and water vapor
where is the density of dry air at a given temperature and pressure, kg / m 3.
To calculate the density of humid air, you can use another formula:
It can be seen from the equation that with an increase in the partial pressure of steam at constant pressure p(barometric) and temperature T the density of humid air decreases. Since this decrease is insignificant, it is accepted in practice.
Saturation degree of humid airψ is the ratio of its moisture content d to the moisture content of saturated air at the same temperature:.
For saturated air.
Enthalpy of humid airI(kJ / kg) - the amount of heat contained in the air, referred to 1 Kg dry or (1 + d) kg humid air.
The zero point is the enthalpy of dry air ( d= 0) with temperature t= 0 ° C. Therefore, the enthalpy of humid air can have positive and negative values.
Enthalpy of dry air 
where is the mass heat capacity of dry air.
The enthalpy of water vapor includes the amount of heat required to convert water to steam when t= 0 o C and the amount of heat spent on heating the resulting steam to a temperature t o C. Enthalpy d kg of water vapor contained in 1 Kg dry air:,
2500 - latent heat of vaporization (evaporation) of water at t = 0 o C;
- mass heat capacity of water vapor.
The enthalpy of humid air is equal to the sum of enthalpy 1 Kg dry air and enthalpy d kg of water vapor:
where 
is the heat capacity of humid air, referred to 1 kg of dry air.
When the air is in a foggy state, there may be suspended droplets of moisture d waters and even ice crystals d l... The enthalpy of such air in general view
Enthalpy of water = 4.19t, enthalpy of ice.
At temperatures above zero degrees ( t> 0 ° C) there will be droplet moisture in the air, at t< 0°С - кристаллы льда.
Dew point temperature is the air temperature at which the partial pressure of water vapor in the isobaric cooling process p p becomes equal to the saturation pressure. At this temperature, moisture begins to drop out of the air.
Those. dew point is the temperature at which airborne water vapor at its constant density becomes due to air cooling with saturated steam(j =100%). For the above examples (see table 2.1), when at 25 ° C the absolute humidity j becomes 50%, the dew point will be a temperature of about 14 ° C. And when at 20 ° C the absolute humidity j becomes 50%, the dew point will be a temperature of about 9 ° C.
A person at high values of the dew point feels uncomfortable (see table 2.2).
Table 2.2 - Human sensations at high dew point values
In areas with a continental climate, conditions with a dew point between 15 and 20 ° C cause some discomfort, and air with a dew point above 21 ° C is perceived as stuffy. Lower dew point, less than 10 ° C, correlates with lower temperature environment and the body requires less cooling. The lower dew point can go along with high temperature only at very low relative humidity.
Wet air d-I diagram
Calculation and analysis of the processes of heat and humidity treatment of air according to the above dependencies is complicated. To calculate the processes occurring with air when its state changes, use the thermal diagram of humid air in coordinates d-I(moisture content - enthalpy), which was proposed by our compatriot professor L.K. Ramzin in 1918.
L.K. Ramzin (1887-1948) - Soviet heating engineer, inventor
direct-flow boiler. http://ru.wikipedia.org/wiki/Ramzin
It has become widespread in our country and abroad. Diagram d-I humid air graphically connects all the parameters that determine the thermal and humidity state of the air: enthalpy, moisture content, temperature, relative humidity, partial pressure of water vapor.
The plotting is based on dependency.
Most often the diagram d-I is built for air pressure equal to 0.1013 MPa(760 mm Hg). There are also diagrams for other barometric pressures.
Due to the fact that the barometric pressure at sea level varies from 0.096 to 0.106 MPa(720 - 800 mm Hg), the calculated data on the diagram should be considered as average.
The diagram is built in an oblique coordinate system (at 135 °). In this case, the diagram becomes convenient for graphical constructions and for calculating air conditioning processes, since the area of unsaturated humid air expands. However, in order to reduce the size of the chart and make it easier to use, the values d demolished to a conventional axis located at 90 ° to the axis I .
Diagram d-I shown in Figure 1. The field of the diagram is divided by lines of constant values of enthalpy I= const and moisture content d= const. It also contains lines of constant temperature values. t= const, which are not parallel to each other - the higher the temperature of the humid air, the more its isotherms deviate upward. Except lines of constant values I, d, t, lines of constant values of relative air humidity are plotted on the diagram field φ = const. Sometimes a line of partial pressures of water vapor is applied p p and lines of other parameters.
Figure 1 - Thermal diagram d-I humid air
The following property of the diagram is essential. If the air has changed its state from a point but to the point b, no matter which process, then in the diagram d-I this change can be represented as a straight line segment ab... In this case, the increment in air enthalpy will correspond to the segment bc = I b -I a... Isotherm drawn through a point but, will split the segment bw into two parts:
line segment bd, representing a change in the proportion of perceptible heat (a supply of thermal energy, a change in which leads to a change in body temperature): 
.
line segment dv, which determines on a scale the change in the heat of vaporization (a change in this heat does not cause a change in body temperature): 
.
Line segment ah corresponds to a change in the moisture content of the air. The dew point is found by lowering the perpendicular from the point of the state of air (for example, from the point b) on the conditional axis d before crossing the saturation line (φ = 100%). In fig. 2.6 K-dew point for air, the initial state of which was determined by the point b.
The direction of the process in air is characterized by changes in enthalpy I and moisture content d .
Wet air is widely used in various industries, including rail transport in heating, cooling, dehumidification or humidification systems. Recently, the introduction of the so-called indirect evaporative cooling method is considered a promising direction in the development of air conditioning technology. This is due to the fact that such devices do not contain artificially synthesized refrigerants, in addition, they are quiet and durable, since there are no moving and quickly wearing elements in them. For the design of such devices, it is necessary to have information on the patterns of heat engineering processes occurring in humid air when its parameters change.
Thermal calculations related to the use of moist air are performed using i-d diagram (see Figure 4), proposed in 1918 by Professor A.K. Ramzin.
This diagram expresses the graphical dependence of the main parameters of air - temperature, relative humidity, partial pressure, absolute humidity and heat content at a given barometric pressure. To build it on the auxiliary axis 0-d on a scale, with an interval corresponding to 1 gram, the moisture content d is deposited and vertical lines are drawn through the points obtained. The ordinate to scale is the enthalpy i with an interval of 1 kJ / kg dry air. At the same time, upward from point 0, corresponding to the temperature of humid air t = 0 0 С (273K) and moisture content d = 0, they postpone positive, and downward - negative values of enthalpy.
Through the obtained points on the ordinate, lines of constant enthalpies are drawn at an angle of 135 0 to the abscissa. On the grid obtained in this way, lines of isotherms and lines of constant relative humidity are applied. To construct isotherms, we use the equation for the heat content of humid air:
It can be written as follows:
, (1.27)
where t and С sv are the temperature (0 С) and heat capacity of dry air (kJ / kg 0 С), respectively;
r is the latent heat of vaporization of water (in calculations it is assumed
r = 2.5 kJ / g).
If we assume that t = const, then equation (1.27) will be a straight line, which means that the isotherms in coordinates i – d are straight lines and for their construction it is necessary to determine only two points characterizing the two extreme positions of humid air.
Figure 4.i - d diagram of humid air
To construct an isotherm corresponding to the temperature value t = 0 ° С (273K), first, using expression (1.27), we determine the position of the heat content coordinate (i 0) for absolutely dry air (d = 0). After substituting the corresponding values of the parameters t = 0 0 C (273K) and d = 0 g / kg expression (1.27), it is seen that the point (i 0) lies at the origin.
. (1.28)
For fully saturated air at a temperature of t = 0 ° C (273K) and = 100% from the reference literature, for example, we find the corresponding value of the moisture content d 2 = 3.77 g / kg dry. air and from the expression (1.27) we find the corresponding value of the enthalpy: (i 2 = 2.5 kJ / g). In system coordinates i-d we put points 0 and 1 and through them we draw a straight line, which will be the isotherm of humid air at a temperature of t = 0 0 С (273K).
Any other isotherm can be constructed in a similar way, for example, for temperature plus 10 0 С (283). At this temperature u = 100%, according to the reference data, we find the partial pressure of fully saturated air equal to P p = 9.21 mm. rt. Art. (1.23 kPa), further and from the expression (1.28) we find the value of the moisture content (d = 7.63 g / kg), and from the expression (1.27) we determine the value of the heat content of humid air (i = 29.35 kJ / g).
For absolutely dry air (= 0%), at a temperature of T = 10 ° C (283K), after substituting the values into expression (1.27), we get:
i = 1.005 * 10 = 10.05 kJ / g.
On the i-d diagram, we find the coordinates of the corresponding points, and drawing a straight line through them, we obtain an isotherm line for the temperature plus 10 0 С (283 K). A family of other isotherms is constructed in a similar way, and by connecting all isotherms for = 100% (on the saturation line), we obtain a line of constant relative humidity = 100%.
As a result of the performed constructions, the i-d diagram was obtained, which is shown in Figure 4. Here, the values of the temperatures of humid air are plotted on the ordinate axis, and the values of moisture content are plotted on the abscissa axis. The oblique lines show the values of the heat content (kJ / kg). Curves diverging from the center of coordinates in a beam express the values of relative humidity φ.
The curve φ = 100% is called the saturation curve; above it, water vapor in the air is in an overheated state, and below it, in a state of supersaturation. An oblique line going from the center of coordinates characterizes the partial pressure of water vapor. Partial pressures are plotted on the right-hand side of the ordinate.
Using the i - d diagram, at a given temperature and relative humidity of air, it is possible to determine its other parameters - heat content, moisture content and partial pressure. For example, for a given temperature plus 25 ° C (273K) and relative humidity and φ = 40%, on the i-d diagram, we find the point BUT. Moving down from it vertically, at the intersection with the inclined line, we find the partial pressure P p = 9 mm Hg. Art. (1.23 kPa) and further on the abscissa - moisture content d А = 8 g / kg of dry air. The diagram also shows that the point BUT lies on an inclined line expressing the heat content i A = 11 kJ / kg dry air.
Processes that occur during heating or cooling of air without changing the moisture content are depicted in the diagram by vertical, straight lines. The diagram shows that when d = const, during the heating of the air, its relative humidity decreases, and during cooling, it increases.
Using the i - d diagram, you can determine the parameters of the mixed parts of humid air for this, the so-called slope of the process beam is built . The construction of the process beam (see Figure 5) starts from a point with known parameters, at this case this is point 1.
Using a system of equations that includes dependencies 4.9, 4.11, 4.17, as well as a functional connection R n = f(t), OK. Ramzin built J-d diagram of humid air, which is widely used in calculations of ventilation and air conditioning systems. This diagram is a graphical relationship between the main air parameters t, , J, d and R n at a certain barometric air pressure R b.
Building J-d diagrams are described in detail in the works.
The state of humid air is characterized by a dot on the field J-d charts bounded by a line d= 0 and curve = 100%.
The point position is set by any two of the five parameters listed above, as well as the dew point temperatures t p and wet thermometer t m . The exceptions are combinations d - R n and d - t p, since every value d only one table value matches R n and t p, and the combination J - t m.
The scheme for determining the air parameters for a given point 1 is shown in Fig. one.
Taking advantage of J-d diagram in the appendix. 4 and the diagram in Fig. 1, we will solve specific examples for all 17 possible combinations of the given initial air parameters, the specific values of which are indicated in table. 7.
The solution schemes and the results obtained are shown in Fig. 2.1 ... 2.17. Known parameters air are highlighted in the figures with thickened lines.
5.2. Process Beam Angle on J-D Diagram
The ability to quickly graphically determine the parameters of humid air is important, but not the main factor in use J-d charts.
As a result of heating, cooling, dehumidification or humidification of humid air, its heat-humidity state changes. The change processes are depicted in J-d the diagram with straight lines that connect the points characterizing the initial and final states of air.
Rice. 1. Scheme for determining the parameters of humid air on J-d diagram
Table 7
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Figure number |
Known air parameters |
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t 1, ° C |
kJ / kg d.m. |
R n1, kPa |
t p1, ° C |
t m1, ° C |
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These lines are called beams of processes changes in the state of the air. Process beam direction to J-d diagram is defined slope . If the parameters of the initial state of air J 1 and d 1, and the final one - J 2 and d 2, then the slope is expressed by the ratio J/d, i.e .:
.
(5.1)
The slope is measured in kJ / kg moisture.
If in equation (29) the numerator and denominator are multiplied by the mass flow of the processed air G, kg / h, we get:
,
(5.2)
where Q n is the total amount of heat transferred when the state of the air changes, kJ / h;
W- the amount of moisture transferred during the change in the state of the air, kg / h.
Depending on the ratio J and d the slope can change its sign and magnitude from 0 to .
In fig. 3 shows the rays of characteristic changes in the state of humid air and the corresponding values of the slope.
1. Moist air with initial parameters J 1 and d 1 is heated at constant moisture content to the parameters of point 2, i.e. d 2 = d 1 , J 2 > J 1 . Slope the process beam is equal to:
Rice. 3. Slope on J-d diagram
Such a process is carried out, for example, in surface air heaters, when the temperature and enthalpy of the air increase, the relative humidity decreases, but the moisture content remains constant.
2. The humid air is simultaneously heated and humidified and acquires the parameters of point 3. The angle coefficient of the process beam 3> 0. This process takes place when the supply air assimilates the heat and moisture release in the room.
3. Humid air is humidified at a constant temperature to the parameters of point 4, 4> 0. In practice, such a process is carried out when the supply or internal air is humidified with saturated water vapor.
4. Humid air is humidified and heated with an increase in enthalpy to the parameters of point 5. Since the enthalpy and moisture content of the air increase, 5> 0. Usually, this process occurs when air comes into direct contact with warmed water in irrigation chambers and cooling towers.
5. The change in the state of humid air occurs at constant enthalpy J 6 = J 1 = const. The angular coefficient of such a ray of the process is 6 = 0, since J = 0.
The process of isenthalpic air humidification with circulating water is widely used in air conditioning systems. It is carried out in irrigation chambers or in devices with an irrigated nozzle.
When unsaturated humid air comes into contact with small drops or a thin film of water without removing or supplying heat from the outside, the water as a result of evaporation humidifies and cools the air, acquiring the temperature of a wet thermometer.
As follows from equation 4.21, in the general case, the slope of the process beam with isenthalpic humidification is not equal to zero, because
,
where with w = 4,186 - specific heat water, kJ / kg ° С.
A real isenthalpy process, in which = 0 is possible only for t m = 0.
6. Humid air is humidified and cooled to point 7. In this case, the slope 7< 0, т.к. J 7 – J 1 0, a d 7 – d 1> 0. This process takes place in spray irrigation chambers when air comes into contact with chilled water, which has a temperature above the dew point of the processed air.
7. Humid air is cooled at constant moisture content to the parameters of point 8. Since d = d 8 – d 1 = 0, a J 8 – J 1 < 0, то 8 = -. Air cooling process at d= const occurs in surface air coolers when the temperature of the heat exchange surface is higher than the dew point of the air, when there is no moisture condensation.
8. Moist air is cooled and dried to the parameters of point 9. The expression for the slope in this case is:
Cooling with dehumidification occurs in irrigation chambers or in surface air coolers when humid air comes into contact with a liquid or solid surface with a temperature below the dew point.
Note that the process of cooling with drying with direct contact of air and chilled water is limited by the tangent drawn from point 1 to the saturation curve = 100%.
9. Deep drying and cooling of air to the parameters of point 10 occurs during direct contact of air with a cooled absorbent, for example, a solution of lithium chloride in irrigation chambers or in devices with an irrigated nozzle. Slope 10> 0.
10. The humid air is dehumidified, i.e. gives off moisture, at constant enthalpy up to the parameters of point 11. The expression for the slope has the form
.
This process can be carried out using solutions of absorbents or solid adsorbents. Note that the real process will have a slope 11 = 4.186 t 11 where t 11 - final dry bulb temperature.
Fig. 3.It can be seen that all possible changes in the state of humid air are located on the field J-d charts in four sectors, the boundaries of which are lines d= const and J= const. In sector I, the processes occur with an increase in enthalpy and moisture content, therefore the values > 0. In sector II, air is dried with an increase in enthalpy and the value < 0. В секторе III процессы идут с уменьшением энтальпии и влагосодержания и >0. In sector IV, air humidification processes occur with a decrease in enthalpy, therefore < 0.