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 beginning colleagues!

At the very beginning of my professional journey, 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 can then, starting from the foundation obtained, independently delve into this web of air characteristics.

This is roughly what it looks like in textbooks. It's getting kind of creepy.

I will remove all the unnecessary things that I will not need for my explanation and present the i-d diagram in this form:

(to enlarge the picture, click and then click on it again)

It’s 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” on one concept right away. Let's get rid of one stereotype that is firmly ingrained in us (at least in me) about what steam is. Since childhood, they pointed me to a boiling pan or kettle and said, pointing a 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. Doesn't have colors, taste and smell.” These are just H2O molecules in a gaseous state that are not visible. And what we see coming out of the kettle is a mixture of water in a gaseous state (steam) and “water droplets in a borderline 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. On most i-d diagrams, this value is measured in [g/kg], i.e. how many grams of steam (H2O in gaseous state) are in one kilogram of air (1 cubic meter of air in your apartment weighs about 1.2 kilograms). In your apartment, for comfortable conditions, 1 kilogram of air should contain 7-8 grams of steam.

On the i-d diagram, moisture content is depicted by vertical lines, and gradation information is located at the bottom of the diagram:

(to enlarge the picture, 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. On most ID charts, this value is measured in degrees Celsius [°C]. On the i-d diagram, temperature is depicted by inclined lines, and information about the gradation is located on the left side of the diagram:

(to enlarge the picture, click and then click on it again)

The third element of the ID diagram is relative humidity (φ). Relative humidity is exactly the humidity that we hear about on TV and radio when we listen to the weather forecast. It is measured in percentage [%].

A reasonable question arises: “What is the difference between relative humidity and moisture content?” I will answer this question step by step:

First stage:

Air can hold a certain amount of steam. Air has a certain “steam carrying capacity”. For example, in your room, a kilogram of air can “take on board” no more than 15 grams of steam.

Let's assume that your room is comfortable, and each kilogram of air in your room contains 8 grams of steam, and each kilogram of air can contain 15 grams of steam. As a result, we get that there is 53.3% of the maximum possible vapor in the air, i.e. relative air humidity - 53.3%.

Second phase:

Air capacity varies at different temperatures. The higher the air temperature, the more steam it can contain; the lower the temperature, the lower the capacity.

Let's assume 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 remained 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 there is 29.6% of the maximum possible steam, i.e. relative air humidity - 29.6%.

The same goes for cooling. If we cool the air to +11 degrees, we get a “carrying capacity” of 8.2 grams of steam per kilogram of air and a relative humidity of 97.6%.

Note that there was the same amount of moisture in the air - 8 grams, and the relative humidity jumped from 29.6% to 97.6%. This happened due to temperature fluctuations.

When you hear about the weather on the radio in winter, where they say that it is minus 20 degrees outside and the humidity is 80%, this means that there is about 0.3 grams of steam in the air. When this air enters your apartment, it heats up to +20 and the relative humidity of such air becomes equal to 2%, and this is very dry air (in fact, in the apartment in winter the humidity is kept at 10-30% due to moisture released from the bathrooms, from kitchens and from people, but that is also below comfort parameters).

Third stage:

What happens if we lower the temperature to 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 that does not fit into the “body” (for us 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 windows fog up due to the fact that the temperature of the glass is lower than the average temperature in the room, so much so that there is little room for moisture in the air and steam, turning into liquid, settles on the glass.

On an i-d diagram, relative humidity is depicted by curved lines, and gradation information is located on the lines themselves:

(to enlarge the picture, click and then click on it again)

The fourth element of the ID chart is enthalpy (I or i). Enthalpy contains the energy component of the heat and humidity state of the air. Upon further study (outside this article, for example in my article on enthalpy ) It is worth paying special attention to it when it comes to dehumidifying and humidifying the air. But for now we will not focus special attention on this element. Enthalpy is measured in [kJ/kg]. On an i-d chart, enthalpy is represented by slanted lines, and gradation information is located on the graph itself (or on the left and at the top of the chart).

Considering what is the main object of the ventilation process, in the field of ventilation it is often necessary to determine certain air parameters. To avoid numerous calculations, they are usually determined using a special diagram called the Id diagram. It allows you to quickly determine all air parameters using two known ones. Using a diagram allows you to avoid calculations using formulas and visually display the ventilation process. An example of an Id chart is shown on the next page. The analogue of the Id chart in the west is Mollier diagram or psychrometric chart.

The design of the diagram can, in principle, be somewhat different. A typical general diagram of an Id diagram is shown below in Figure 3.1. The diagram is a working field in the oblique coordinate system Id, on which several coordinate grids are plotted and auxiliary scales along the perimeter of the diagram. The moisture content scale is usually located along the bottom edge of the diagram, with the constant moisture content lines representing vertical straight lines. The constant lines represent parallel straight lines, usually running at an angle of 135° to the vertical moisture content lines (in principle, the angles between the enthalpy and moisture content lines can be different). The oblique coordinate system was chosen in order to increase the working area of ​​the diagram. In such a coordinate system, lines of constant temperatures are straight lines running at a slight inclination to the horizontal and slightly fanning out.

The working field of the diagram is limited by curved lines of equal relative humidity 0% and 100%, between which lines of other values ​​of equal relative humidity are plotted with a step of 10%.

The temperature scale is usually located along the left edge of the working area of ​​the diagram. The values ​​of air enthalpies are usually plotted under the curve Ф = 100. The values ​​of partial pressures are sometimes plotted along the upper edge of the working field, sometimes along the lower edge under the moisture content scale, sometimes along the right edge. In the latter case, an auxiliary partial pressure curve is additionally plotted on the diagram.

Determination of humid air parameters on the Id diagram.

The point on the diagram reflects a certain state of the air, and the line represents the process of changing the state. Determination of the parameters of air having a certain state, displayed by point A, is shown in Figure 3.1.

I-d diagram for beginners (ID state diagram humid air for dummies) March 15th, 2013

Original taken from Mrcynognathus in I-d diagram for beginners (ID diagram of the state of humid air for dummies)

Good day, dear beginning colleagues!

At the very beginning of my professional journey, 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 can then, starting from the foundation obtained, independently delve into this web of air characteristics.

This is roughly what it looks like in textbooks. It's getting kind of creepy.


I will remove all the unnecessary things that I will not need for my explanation and present the i-d diagram in this form:

(to enlarge the picture, click and then click on it again)

It’s 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” on one concept right away. Let's get rid of one stereotype that is firmly ingrained in us (at least in me) about what steam is. Since childhood, they pointed me to a boiling pan or kettle and said, pointing a 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. Doesn't have colors, taste and smell.” These are just H2O molecules in a gaseous state that are not visible. And what we see coming out of the kettle is a mixture of water in a gaseous state (steam) and “water droplets in a borderline state between liquid and gas,” or rather, we see the latter. As a result, we get that at the 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. On most i-d diagrams, this value is measured in [g/kg], i.e. how many grams of steam (H2O in gaseous state) are in one kilogram of air (1 cubic meter of air in your apartment weighs about 1.2 kilograms). In your apartment, for comfortable conditions, 1 kilogram of air should contain 7-8 grams of steam.

On the i-d diagram, moisture content is depicted by vertical lines, and gradation information is located at the bottom of the diagram:

(to enlarge the picture, 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. On most ID charts, this value is measured in degrees Celsius [°C]. On the i-d diagram, temperature is depicted by inclined lines, and information about the gradation is located on the left side of the diagram:

(to enlarge the picture, click and then click on it again)

The third element of the ID diagram is relative humidity (φ). Relative humidity is exactly the humidity that we hear about on TV and radio when we listen to the weather forecast. It is measured in percentage [%].

A reasonable question arises: “What is the difference between relative humidity and moisture content?” I will answer this question step by step:

First stage:

Air can hold a certain amount of steam. Air has a certain “steam carrying capacity”. For example, in your room, a kilogram of air can “take on board” no more than 15 grams of steam.

Let's assume that your room is comfortable, and each kilogram of air in your room contains 8 grams of steam, and each kilogram of air can contain 15 grams of steam. As a result, we get that there is 53.3% of the maximum possible vapor in the air, i.e. relative air humidity - 53.3%.

Second phase:

Air capacity varies at different temperatures. The higher the air temperature, the more steam it can contain; the lower the temperature, the lower the capacity.

Let's assume 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 remained 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 there is 29.6% of the maximum possible steam, i.e. relative air humidity - 29.6%.

The same goes for cooling. If we cool the air to +11 degrees, we get a “carrying capacity” of 8.2 grams of steam per kilogram of air and a relative humidity of 97.6%.

Note that there was the same amount of moisture in the air - 8 grams, and the relative humidity jumped from 29.6% to 97.6%. This happened due to temperature fluctuations.

When you hear about the weather on the radio in winter, where they say that it is minus 20 degrees outside and the humidity is 80%, this means that there is about 0.3 grams of steam in the air. When this air enters your apartment, it heats up to +20 and the relative humidity of such air becomes equal to 2%, and this is very dry air (in fact, in an apartment in winter the humidity is kept at 20-30% due to moisture released from the bathrooms and from people, but that is also below the comfort parameters).

Third stage:

What happens if we lower the temperature to 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 that does not fit into the “body” (for us 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 windows fog up due to the fact that the temperature of the glass is lower than the average temperature in the room, so much so that there is little room for moisture in the air and steam, turning into liquid, settles on the glass.

On an i-d diagram, relative humidity is depicted by curved lines, and gradation information is located on the lines themselves:

(to enlarge the picture, click and then click on it again)
Fourth elementID diagrams - enthalpy (I ori). Enthalpy contains the energy component of the heat and humidity state of the air. With further study (beyond the scope of this article), it is worth paying special attention to it when it comes to dehumidifying and humidifying the air. But for now we will not focus special attention on this element. Enthalpy is measured in [kJ/kg]. On an i-d diagram, enthalpy is depicted as slanted lines, and gradation information is located on the graph itself (or on the left and at the top of the diagram):

(to enlarge the picture, click and then click on it again)

Then everything is simple! The chart is easy to use! Let's take, for example, your comfortable room, in which the temperature is +20°C and the relative humidity is 50%. We find the intersection of these two lines (temperature and humidity) and see how many grams of steam are in our air.

We heat the air to +30°C - the line goes up, because... There is still the same amount of moisture in the air, but only the temperature increases. We put an end to it and see what the relative humidity turns out to be - it turned out to be 27.5%.

We cool the air to 5 degrees - again we draw a vertical line down, and in the region of +9.5 ° C we come across a line of 100% relative humidity. This point is called the “dew point” and at this point (theoretically, since in practice deposition begins a little earlier) condensation begins to form. We cannot move lower along a vertical straight line (as before), because at this point the “carrying capacity” of air at a temperature of +9.5°C is maximum. But we need to cool the air to +5°C, so we continue to move along the relative humidity line (shown in the figure below) until we reach a slanted straight line of +5°C. As a result, our final point was at the intersection of the +5°C temperature line and the 100% relative humidity line. Let's see how much steam is left in our air - 5.4 grams in one kilogram of air. And the remaining 2.6 grams were released. Our air has become dry.

(to enlarge the picture, click and then click on it again)

Other processes that can be performed with air using various devices (dehumidification, cooling, humidification, heating...) can be found in textbooks.

Besides the dew point, another important point is the “wet bulb temperature”. This temperature is actively used in the calculation of cooling towers. Roughly speaking, this is the point to which the temperature of an object can drop if we wrap this object in a wet rag and begin to “blow” intensively on it, for example, using a fan. The human thermoregulation system operates on this principle.

How to find this point? For these purposes we will need enthalpy lines. Let's take our comfortable room again, find the point of intersection of the temperature line +20°C and relative humidity 50%. From this point it is necessary to draw a line parallel to the enthalpy lines up to the 100% humidity line (as in the figure below). The point of intersection of the enthalpy line and the relative humidity line will be the point of the wet thermometer. In our case, from this point we can find out what is in our room, so we can cool the object to a temperature of +14°C.

(to enlarge the picture, click and then click on it again)

The process ray (angular coefficient, heat-moisture ratio, ε) is constructed in order to determine the change in air from the simultaneous release of heat and moisture by a certain source(s). Usually this source is a person. An obvious thing, but understanding the processes i-d diagrams will help detect a possible arithmetic error, if one occurs. For example, if you plot a beam on a diagram and, under normal conditions and the presence of people, your moisture content or temperature decreases, then it’s worth thinking about and checking the calculations.

In this article, much is simplified for a better understanding of the diagram at the initial stage of studying it. More accurate, more detailed and more scientific information must be sought in educational literature.

P. S. In some sources

It is very convenient to determine the parameters of moist air, as well as solve a number of practical issues related to the drying of various materials, graphically using using i-d diagrams, first proposed by the Soviet scientist L.K. Ramzin in 1918.

Built for 98 kPa barometric pressure. In practice, the diagram can be used in all cases of calculating dryers, since with normal fluctuations atmospheric pressure values i And d change little.

Diagram in i-d coordinates is a graphical interpretation of the enthalpy equation for moist air. It reflects the relationship between the main parameters of moist air. Each point on the diagram highlights a certain state with very specific parameters. To find any of the characteristics of moist air, it is enough to know only two parameters of its state.

The I-d diagram of moist air is constructed in an oblique coordinate system. On the ordinate axis up and down from the zero point (i = 0, d = 0), the enthalpy values ​​are plotted and the lines i = const are drawn parallel to the abscissa axis, that is, at an angle of 135 0 to the vertical. In this case, the 0 o C isotherm in the unsaturated region is located almost horizontally. As for the scale for measuring moisture content d, for convenience it is transferred to a horizontal straight line passing through the origin of coordinates.

The water vapor partial pressure curve is also plotted on the i-d diagram. For this purpose, use the equation:

R p = B*d/(0.622 + d),

Solving which for variable values ​​of d we obtain that, for example, for d=0 P p =0, for d=d 1 P p =P p1, for d=d 2 P p =P p2, etc. Specifying a certain scale for partial pressures, a curve P p =f(d) is constructed at the bottom of the diagram in a rectangular coordinate system at the indicated points. After this, curved lines of constant relative humidity (φ = const) are plotted on the i-d diagram. The lower curve φ = 100% characterizes the state of air saturated with water vapor ( saturation curve).

Also, on the i-d diagram of moist air, straight lines of isotherms (t = const) are drawn, characterizing the processes of moisture evaporation, taking into account the additional amount of heat introduced by water having a temperature of 0 o C.

During the process of evaporation of moisture, the enthalpy of the air remains constant, since the heat taken from the air to dry the materials returns back to it along with the evaporated moisture, that is, in the equation:

i = i in + d*i p

A decrease in the first term will be compensated by an increase in the second term. On the i-d diagram, this process runs along the line (i = const) and is called the process adiabatic evaporation. The limit of air cooling is the adiabatic temperature of the wet thermometer, which is found on the diagram as the temperature of the point at the intersection of the lines (i = const) with the saturation curve (φ = 100%).

Or in other words, if from point A (with coordinates i = 72 kJ/kg, d = 12.5 g/kg dry air, t = 40 °C, V = 0.905 m 3 /kg dry air. φ = 27%), releasing a certain state of moist air, draw down a vertical beam d = const, then it will represent a process of cooling the air without changing its moisture content; the value of relative humidity φ gradually increases. When this ray is continued until it intersects with the curve φ = 100% (point “B” with coordinates i = 49 kJ/kg, d = 12.5 g/kg dry air, t = 17.5 °C, V = 0 .84 m 3 /kg dry air j = 100%), we get the lowest temperature t p (it is called dew point temperature), at which air with a given moisture content d is still able to retain vapors in non-condensed form; a further decrease in temperature leads to the precipitation of moisture either in a suspended state (fog), or in the form of dew on the surfaces of fences (car walls, products), or frost and snow (pipes of the evaporator of a refrigeration machine).

If air is able to be humidified in state A without supplying or removing heat (for example, from an open water surface), then the process characterized by the AC line will occur without a change in enthalpy (i = const). Temperature t m at the intersection of this line with the saturation curve (point “C” with coordinates i = 72 kJ/kg, d = 19 g/kg dry air, t = 24 °C, V = 0.87 m 3 /kg dry air φ = 100%) is wet bulb temperature.

Using i-d, it is convenient to analyze the processes that occur when mixing moist air flows.

Also i-d chart humid air are widely used to calculate air conditioning parameters, which is understood as a set of means and methods of influencing the temperature and humidity of the air.

I-d diagram of moist air is a diagram widely used in calculations of ventilation, air conditioning, drying systems and other processes associated with changes in the state of moist air. It was first compiled in 1918 by the Soviet heating engineer Leonid Konstantinovich Ramzin.

Various I-d diagrams

I-d diagram of humid air (Ramzin diagram):

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Description of the diagram

The I-d diagram of moist air graphically connects all the parameters that determine the thermal and moisture state of the air: enthalpy, moisture content, temperature, relative humidity, partial pressure of water vapor. The diagram is constructed in an oblique coordinate system, which allows you to expand the area of ​​​​unsaturated moist air and makes the diagram convenient for graphic construction. The ordinate axis of the diagram shows the values ​​of enthalpy I, kJ/kg of the dry part of the air; the abscissa axis, directed at an angle of 135° to the I axis, shows the values ​​of the moisture content d, g/kg of the dry part of the air.

The diagram field is divided by lines of constant values ​​of enthalpy I = const and moisture content d = const. It also shows lines of constant temperature values ​​t = const, which are not parallel to each other - the higher the temperature of the moist air, the more its isotherms deviate upward. In addition to the lines of constant values ​​of I, d, t, lines of constant values ​​of relative air humidity φ = const are plotted on the diagram field. At the bottom of the I-d diagram there is a curve that has an independent ordinate. It connects the moisture content d, g/kg, with the water vapor pressure pp, kPa. The ordinate axis of this graph is the scale of partial pressure of water vapor pp.