After reading this article, I recommend reading the article about enthalpy, latent cooling capacity and determination of the amount of condensate formed in air conditioning and dehumidification systems:
Good day, dear novice colleagues!
At the very beginning of its professional path I came across this diagram. At first glance, it may seem scary, but if you understand the main principles by which it works, you can fall in love with it: D. In everyday life, it is called an i-d diagram.
In this article, I will try to simply (on fingers) explain the main points, so that you then, starting from the resulting foundation, independently delve into this web of air characteristics.
It looks something like this in textbooks. It becomes somehow creepy.
I will remove all that is superfluous that will not be necessary for me for my explanation and present the i-d diagram as follows:
(to enlarge the picture, you must click and then click on it again)
It is still not entirely clear what it is. Let's break it down into 4 elements:
The first element is moisture content (D or d). But before I start talking about air humidity in general, I would like to agree on something with you.
Let's agree “on the shore” about one concept at once. Let's get rid of one stereotype that is firmly entrenched in us (at least in me) about what steam is. From my very childhood they pointed to a boiling pot or kettle and said, pointing their finger at the “smoke” pouring out of the vessel: “Look! This is steam. " But like many people who are friends with physics, we must understand that “Water vapor is a gaseous state water... Does not have colors, taste and smell ”. These are just H2O molecules in a gaseous state that are not visible. And what we see pouring out of the kettle is a mixture of water in a gaseous state (steam) and “water droplets in a boundary state between liquid and gas”, or rather we see the latter (also, with reservations, we can call what we see - fog). As a result, we get that in this moment, around each of us there is dry air (a mixture of oxygen, nitrogen ...) and steam (H2O).
So, moisture content tells us how much of this vapor is present in the air. In most i-d diagrams, this value is measured in [g / kg], i.e. how many grams of steam (H2O in the gaseous state) is in one kilogram of air (1 cubic meter of air in your apartment weighs about 1.2 kilograms). For comfortable conditions in your apartment, there should be 7-8 grams of steam in 1 kilogram of air.
On i-d diagram moisture content is plotted with vertical lines, and gradation information is located at the bottom of the diagram:
(to enlarge the picture, you must click and then click on it again)
The second important element to understand is air temperature (T or t). I think there is no need to explain anything here. Most i-d charts measure this value in degrees Celsius [° C]. In the i-d diagram, the temperature is depicted with oblique lines, and information about the gradation is located on the left side of the diagram:
(to enlarge the picture, you must click and then click on it again)The third element of the ID chart is relative humidity (φ). Relative humidity is exactly the kind of humidity that we hear about from televisions and radios when we listen to the weather forecast. It is measured in percent [%].
A reasonable question arises: "What is the difference between relative humidity and moisture content?" I will answer this question in stages:
First step:
Air can hold a certain amount of steam. Air has a certain “steam capacity”. For example, in your room a kilogram of air can “take on board” no more than 15 grams of steam.
Suppose that your room is comfortable, and there is 8 grams of steam in every kilogram of air in your room, and 15 grams of steam can hold each kilogram of air. As a result, we get that 53.3% of the maximum possible vapor is in the air, i.e. relative air humidity - 53.3%.
Second phase:
Air capacity is different at different temperatures. The higher the air temperature, the more steam it can hold, the lower the temperature, the less capacity.
Suppose that we heated the air in your room with a conventional heater from +20 degrees to +30 degrees, but the amount of steam in each kilogram of air remains the same - 8 grams. At +30 degrees, the air can "take on board" up to 27 grams of steam, as a result, in our heated air - 29.6% of the maximum possible steam, i.e. relative air humidity - 29.6%.
It's the same with cooling. If we cool the air to +11 degrees, then we get a "carrying capacity" equal to 8.2 grams of steam per kilogram of air and a relative humidity equal to 97.6%.
Note that the moisture in the air was the same amount - 8 grams, and the relative humidity jumped from 29.6% to 97.6%. This was due to temperature fluctuations.
When you hear about the weather on the radio in winter, where they say that outside is minus 20 degrees and humidity is 80%, this means that there is about 0.3 grams of steam in the air. Getting into your apartment, this air heats up to +20 and the relative humidity of such air becomes 2%, and this is very dry air (in fact, in the apartment in winter, the humidity is kept at the level of 10-30% due to the release of moisture from the bathrooms, from kitchen and from people, but which is also below the comfort parameters).
Stage three:
What happens if we drop the temperature to such a level where the “carrying capacity” of the air is lower than the amount of vapor in the air? For example, up to +5 degrees, where the air capacity is 5.5 grams / kilogram. That part of gaseous H2O, which does not fit into the “body” (in our case, it is 2.5 grams), will begin to turn into liquid, i.e. in water. In everyday life, this process is especially clearly visible when the windows fog up due to the fact that the temperature of the glasses is lower than average temperature in the room, so much so that there is little room for moisture in the air and the vapor, turning into a liquid, settles on the glass.
In the i-d diagram, the relative humidity is depicted in curved lines, and the gradation information is located on the lines themselves:
(to enlarge the picture, you must click and then click on it again)
The fourth element of the ID diagram is enthalpy (I or i). The enthalpy contains the energy component of the heat and humidity state of the air. Upon further study (outside of this article, for example, in my article on enthalpy ) it is worth paying special attention to it when it comes to dehumidification and humidification of the air. But for now, we will not focus on this element. The enthalpy is measured in [kJ / kg]. In the i-d diagram, the enthalpy is depicted by oblique lines, and the information about the gradation is located on the graph itself (or on the left and at the top of the diagram).
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 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, in practice it is accepted.
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 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 to 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 humid air, the more its isotherms deviate upward. In addition to the 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 of essential importance. If the air has changed its state from a point a 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 a, will split the segment bw into two parts:
section 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): 
.
section 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): 
.
Section 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 air condition (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 .
The basic properties of humid air can be determined with sufficient accuracy for technical calculations at help i-x- diagram developed by L.K. Ramzin (1918). I-x diagram(Fig. 1, 2) built for constant pressure p = 745 mm Hg. Art. (about 99 kN / m 2), which, according to long-term statistical data, is taken as the average annual for central regions the former USSR.
On the ordinate axis the enthalpies i are plotted on a certain scale, and the moisture content x is plotted on the inclined abscissa axis. The angle between the coordinate axes is 135 °, but for ease of use, the moisture content x values are projected on an auxiliary axis perpendicular to the ordinate axis.
The diagram has lines:
- · Constant moisture content (x = const) - vertical straight lines parallel to the ordinate axis;
- Constant enthalpy (i = const) - straight lines parallel to the abscissa axis, i.e. directed at an angle of 135 ° to the ordinate;
- · Constant temperatures, or isotherms (t = const);
- · Constant relative humidity (c = const);
- · Partial pressures of water vapor (p) in humid air, the values of which are plotted to scale on the right ordinate axis of the diagram.
Rice. 1. Wet air diagram i - x (a)
Lines of constant temperatures, or isotherms, are set at a given temperature t = const by two arbitrary values x 1 and x 2. The i value corresponding to each x value is then calculated. The resulting points (x 1, i 1) and (x 2, i 2) are plotted on the diagram and a straight line is drawn through them, which is the isotherm t = const.
Lines of constant relative humidity express the relationship between x and p at q = const. Taking at a given q = const several arbitrary temperatures t 1, t 2, t 3 for each of them, the corresponding values of p are found from the tables of water vapor and the corresponding value of x is calculated. Points with known coordinates (t 1, x 1), (t 2, x 2), (t 3, x 3), etc. connect the curve, which is the line q = const.
Rice. 2.
At temperatures t> 99.4 ° C, the value of q does not depend on temperature (since in this case p = 745 mm Hg, for which the diagram is plotted) and is practically constant. Therefore, the lines μ = const at 99.4 ° C have a sharp break and go almost vertically upward.
The line u = 100% corresponds to the saturation of air with water vapor at a given temperature. Above this line is the working area of the diagram corresponding to unsaturated humid air used as a drying agent.
The partial pressure lines at the bottom of the diagram allow you to determine the partial pressure if you know the position of the point on the diagram corresponding to the state of air.
By diagram i-x for any two known parameters humid air, you can find a point characterizing the state of the air, and determine all its other parameters.
L.K.Ramzin built " i, d»- a diagram that is widely used in calculations of drying, air conditioning in a number of other calculations related to changes in the state of humid air. This diagram expresses the graphical dependence of the main air parameters ( t, φ, p NS, d, i) at a given barometric pressure.
Elements " i, d»- diagrams are shown in fig. 7.4. The diagram is built in an oblique coordinate system with an angle between the axes i and d 135 °. The ordinate is the enthalpies and air temperatures ( i, kJ / kg dry air and t, ° С), along the abscissa - the values of moisture content of humid air d, g / kg.
Rice. 7.4. Approximate " i, d"- diagram
It was mentioned earlier that the parameters ( t° C, i kJ / kg, φ%, d g / kg, p P Pa), which determine the state of humid air, by " i, d»- the diagram can be graphically depicted as a point. For example, in Fig. below point A correspond to the parameters of humid air: temperature t= 27 ° С, relative humidity φ = 35%, enthalpy i= 48 kJ / kg, moisture content d= 8 g / kg, partial vapor pressure p P = 1.24 kPa.
It is necessary to take into account the fact that the parameters of humid air obtained graphically correspond to a barometric (atmospheric) pressure of 760 mm Hg. Art., for which was built shown in Fig. " i, d"- diagram.
The practice of using graphical analytical calculations to determine the partial pressure of steam using " i, d»- the diagrams show that the discrepancy between the results obtained (within 1 - 2%) is explained by the degree of accuracy of the diagrams.
If the parameters of point A are on " i, d"- the diagram (Fig. 7.5) i A , d A, and final B - i B, d B, then the ratio ( i B - i A) / ( d B - d A) · 1000 = ε-is the slope of the line (ray), characterizing the given change in the state of the air in the coordinates " i, d"- diagrams.
Rice. 7.5. Definition slopeε using “ i, d"- diagrams.
The value of ε has the dimension of kJ / kg of moisture. On the other hand, in the practice of using “ i, d»- the value of ε obtained by calculation is known in advance in the diagrams.
In this case, on " i, d»- the diagram can construct a ray corresponding to the obtained value of ε. To do this, use a set of rays corresponding to different values of the slope and plotted along the contour " i, d"- diagrams. The construction of these rays was carried out as follows (see Fig. 7.6).
To construct the angular scale, various changes in the state of humid air are considered, while taking the same initial air parameters for all cases considered in Figure 4 - this is the origin ( i 1 = 0, d 1 = 0). If the final parameters are denoted by i 2 and d 2, then the expression for the slope can be written in this case
ε = 
.
For example, taking d 2 = 10 g / kg and i 2 = 1 kJ / kg (corresponds to point 1 in Fig. 1.4), ε = (1/10) 1000 = 100 kJ / kg. For point 2 ε = 200 kJ / kg and so on for all considered points in Figure 1.4. For i= 0 ε = 0, i.e. rays on i, d"- the diagram is the same. In a similar way, beams with negative slope values can be applied.
On the fields " i, d»- the diagrams show the directions of the scale rays for the values of the angular coefficients in the range from - 30,000 to + 30,000 kJ / kg of moisture. All of these rays come from the origin.
The practical use of the angular scale is reduced to the parallel transfer (for example, using a ruler) of the scale beam with a known value of the slope to a given point at " i, d"- the diagram. In fig. shows the transfer of a ray with ε = 100 to point B.
Building on " i, d"- a diagram of an angular scale.
Determination of dew point temperaturet P and wet bulb temperaturet M using "i, d "- diagrams.
The dew point temperature is the temperature of the saturated air at a given moisture content. On " i, d"- a diagram for determining t P it is necessary from the point of this air state (point A in the figure below) to descend along the line d= const to the intersection with the saturation line φ = 100% (point B). In this case, the isotherm passing through point B corresponds to t R.
Definition of values t P and t M to " i, d"- the diagram
Wet bulb temperature t M is equal to the temperature of the air in a saturated state at a given enthalpy. V " i, d"- the diagram t M passes through the point of intersection of the isotherm with the line φ = 100% (point B) and practically coincides (with the parameters taking place in air conditioning systems) with the line I= const passing through point B.
An image of the processes of heating and cooling air on "i, d "- diagram. The process of heating air in a surface heat exchanger - air heater in " i, d"- the diagram is depicted by the vertical line AB (see the figure below) at d= const, since the moisture content of the air does not change upon contact with a dry heated surface. Temperature and enthalpy increase when heated, and relative humidity decreases.
The air cooling process in a surface heat exchanger-air cooler can be implemented in two ways. The first way is to cool the air at a constant moisture content (process a in Fig. 1.6). This process at d= const flows if the surface temperature of the air cooler is higher than the dew point temperature t P. The process will take place along the VG line or in last resort- along the line VG '.
The second way is to cool the air with a decrease in its moisture content, which is possible only when moisture falls out of the air (case b in Fig. 7.8). The condition for the implementation of such a process is that the temperature of the surface of the air cooler or any other surface in contact with air must be lower than the dew point of the air at point D. In this case, condensation of water vapor in the air will occur and the cooling process will be accompanied by a decrease in moisture content in the air ... In fig. this the process will go along the SJ line, and the point W corresponds to the temperature t P.V. air cooler surface. In practice, the cooling process ends earlier and reaches, for example, point E at a temperature t E.
Rice. 7.8. An image of the processes of heating and cooling air on " i, d"- the diagram
The processes of mixing two air streams in "i, d "- the diagram.
Air conditioning systems use the processes of mixing two air streams with different states. For example, using recirculated air or mixing prepared air with indoor air supplied from an air conditioner. Other cases of confusion are also possible.
It is of interest for calculating mixing processes to find a connection between analytical calculations of processes and their graphic images on " i, d"- the diagram. In fig. 7.9 presents two cases of the implementation of mixing processes: a) - point of the state of air on " i, d»- the diagram lies above the line φ = 100% and case b) - the point of the mixture lies below the line φ = 100%.
Consider case a). Air of the state of point A in the amount G And with parameters d And i A mixes with the air of the state of point B in an amount G B with parameters d B and i B. In this case, the condition is accepted that calculations are made for 1 kg of air of state A. Then the value of n = G V / G And it is estimated how much air of the state of point B falls on 1 kg of air of the state of point A. For 1 kg of air of the state of point A, one can write down the balances of heat and moisture when mixing
i A + i B = (1 + n)i CM;
d A + nd B = (1 + n)d CM,
where i Mass media d CM are the parameters of the mixture.
From the equations you get:
.
The equation is the equation of a straight line, any point of which indicates the mixing parameters i Mass media d CM. The position of the mixing point C on the line AB can be found by the ratio of the sides of similar triangles ASD and CBE
Rice. 7.9. Air mixing processes in " i, d"- the diagram. a) - the point of the mixture lies above the line φ = 100%; b) - the point of the mixture lies below φ = 100%.
,
those. point C divides straight line AB into parts inversely proportional to the masses of the mixed air.
If the position of point C on line AB is known, then we can find the masses G A and G B. From the equation it follows
,
Likewise
In practice, the case is possible when in cold period year, the point of the mixture С 1 'lies below the line φ = 100%. In this case, moisture condensation will take place during the mixing process. The condensed moisture falls out of the air and after mixing will be in a saturation state at φ = 100%. The parameters of the mixture are quite accurately determined by the point of intersection of the line φ = 100% (point C 2) and i CM = const. In this case, the amount of precipitated moisture is equal to Δ d.
The I-d diagram of humid air was created in 1918 by L.K. Ramzin. The fruits of the labor of this Russian scientist are still used today. His diagram is currently still a valid and reliable tool for calculating the basic properties of humid air.
Since the calculation of the change in the state of atmospheric air is associated with complex calculations, a simpler and more convenient method is usually used. Those. use Ramzin, which is also called a psychrometric diagram.
In the coordinates of the i-d diagram, the dependences of the main parameters of humid air are plotted. These are temperature, moisture content, relative humidity, enthalpy. At a given barometric pressure on the ordinate, the enthalpy is plotted per kg of dry air (kJ / kg). Along the abscissa, the moisture content of the air is plotted in g per 1 kg of dry air.
System coordinates i-d the diagram is oblique. The angle between the axes is 135º. This arrangement of the axes makes it possible to expand the area of unsaturated humid air. Thus, the diagram becomes more convenient for graphical constructions.
The lines of constant enthalpy I = const pass at an angle of 135º to the ordinate axis. Lines of constant moisture content d = const run parallel to the ordinate axis.
The mesh formed by the lines I = const and d = const consists of parallelograms. On them lines of isotherms t = const and lines of constant relative humidity φ = const are plotted.
It is worth noting that although the isotherms are straight lines, they are not at all parallel to each other. The angle of their inclination to the horizontal axis is different. The lower the temperature, the more parallel the isotherms are. The temperature lines shown in the diagram correspond to the dry bulb values.
A curve with a relative humidity φ = 100% is built based on the data from the tables of saturated air. Above this curve, the diagram shows an area of unsaturated humid air. Accordingly, below this curve, there is an area of oversaturated humid air. The moisture in saturated air, characterized by this area, is in a liquid or solid state. Those. represents fog. This area of the diagram is not used in calculating the characteristics of humid air, so its construction is omitted.
All points on the diagram represent a specific state of humid air. To determine the position of any point, you need to know two parameters of the state of moist air from four - I, d, t or φ.
Humid air in any point i-d the diagram is characterized by a certain moisture and heat content. All points located above the curve φ = 100% characterize the state of humid air in which water vapor in the air is in an overheated state. The points located on the φ = 100% curve, the so-called saturation curve, characterize the saturated state of water vapor in the air. All points located below the saturation curve characterize the state in which the temperature of humid air is below the saturation temperature. Consequently, there will be moist vapor in the air. This means that the moisture in the air will be a mixture of dry steam and water droplets.
When addressing practical tasks i-d the diagram is used not only for calculating the parameters of the air condition. With its help, changes in its state are also built during the processes of heating, cooling, humidification, dehumidification, as well as their arbitrary combination. In calculations, such air parameters are often used as the dew point temperature t p and the wet bulb temperature t m. Both parameters can be plotted on the i-d diagram.
The dew point temperature t p is the temperature corresponding to the value to which humid air must be cooled in order to become saturated at a constant moisture content (d = const). On the i-d diagram, the dew point temperature t p is determined as follows. A point is taken that characterizes the given state of humid air. From it, draw a straight line parallel to the ordinate until it intersects the saturation curve φ = 100%. The isotherm that will intersect this curve at the obtained point, and will show the dew point temperature t p at a given moisture content of the air.
Wet bulb temperature t m is the temperature at which humid air, when cooled, becomes saturated with constant moisture content. To determine the temperature of a wet bulb on the i-d diagram, do the following. A line of constant enthalpy I = const is drawn through the point characterizing the given state of humid air until it intersects with the saturation curve φ = 100%. The wet bulb temperature will correspond to the isotherm through the intersection point.
On the i-d diagram, all the processes of air transition from one state to another are depicted by curves passing through the points characterizing the initial and final state of humid air.
How to apply a wet air i-d chart? As mentioned above, to determine the state of the air, you need to know any two parameters of the diagram. For example, let's take any dry bulb temperature and any wet bulb temperature. Having found the point of intersection of the lines of these temperatures, we obtain the state of the air at the given temperatures. Thus, this point clearly characterizes the state of the air. Similarly to the example, these temperatures can be used to find the state of the air at any point in the i-d diagram.
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