I-D diagram wet air It was composed of Professor Leonid Konstantinovich Ramsin in 1918. It graphically connects 5 wet air parameters:
· Specific heat generation (enthalpy) I B.,
· Temperature t.,
· Relative humidity φ ,
· Partial pressure of water vapor p P..
Knowing any two of these parameters, you can define all the others.
The diagram is compiled for a certain barometric pressure.
At the axis of the ordinate (vertical), the values \u200b\u200bof heat-containing (enthalpy) are postponed I S. dry air, on the abscissa axis (horizontal) - moisture content d.. Lines of permanent heat generation (enthalpy) I \u003d const (adiaba) are held at an angle of 135º to the ordinate axis. Lines of permanent moisture content d.\u003d const pass parallel to the axes of the ordinate.
Constant relative humidity curves are also applied φ \u003d Const and at an angle to the axis of the ordinate line isotherm T \u003d const.
Lines φ \u003d 0 I. d.\u003d 0 coincide, because the complete absence of moisture in the air is equally characterized.
Through the intersection point of lines with parameters d.\u003d 0 I. t.\u003d 0 passes line i \u003d 0. The values \u200b\u200bof the heat generation (enthalpy) above this line are positive, below are negative.
The line φ \u003d 100% divides the diagram into two parts. Above the line is the area of \u200b\u200bwet unsaturated air. Line itself φ \u003d 100% corresponds to saturated air - " saturation curve " Below the line is a surrounding air region, " zone Tuman "Where water is in the air of a suspended state in a liquid or solid phase.
I-D charts and schemes for determining wet air parameters for point A.
Basic air treatment processes
And their image on the i-D diagram
When considering the process of changing the state of wet air, the following is accepted assumption : air properties change throughout its volume at the same time.
In fact, this is not the case, since the layers closest to hot surfaces will have a temperature higher than deleted. Based on this, it follows that the average values \u200b\u200bof air parameters for the entire volume are accepted as active.
Processing of wet air - i.e., changing its parameters is made by special devices. The following is a description of only the appointment and principle of operation of such devices, without consideration of their design, varieties and installation.
To elementary devices that are tools for exposure to air parameters include:
· Calorifer
· Irrigation (nozzle) chamber (water humidifier)
· Steam humidifier (steam generator)
HEATER
Heater- This isople-banner, changing the temperature of the air without affecting moisture content.
Dry heating
The process is observed only in the heat exchanger (caloriefer).
Air heating occurs at constant moisture content (D \u003d const), since the moisture does not go anywhere, and it is not added to anywhere, since the processed air contacts only with the dry surface of the heat exchanger (Calrifer). Only the number of explicit heat change changes.
At the same time, the process does not change moisture content, the temperature and enthalpy increase, and falls relative humidity (t 2.>t 1.,I 2.>I 1.,φ 2.<Φ 1., d 2.=d 1.\u003d const).
Heat and heat for air heating in the caloriefer:
Q K. = ΔI ∙ G., kj / h \u003d, wt, where
ΔI. - the difference in the heat generations of KJ / kg of air after and to the carrier, respectively;
G. - air flow passing through the calorifer, kg / h
Dry cooling
Air cooling occurs with constant moisture content (D \u003d const), since the moisture does not go anywhere, and it is not added to anywhere, since the air contacts only with the dry surface of the heat exchanger (aircraft). Only the number of explicit heat change changes.
It does not change moisture content, the temperature and heat-containing (enthalpy) decreases, and relative humidity increases ( t 2.<t 1.,I 2.<I 1.,φ 2.>Φ 1., d 2.=d 1.\u003d const).
Cost costs in the caloriefer are determined in order similar to the calculations of heat. At the same time, the negative value of the heat of the ground will mean no heat costs, but the cold.
Dew point
If during dry cooling the process d.\u003d const reaches the lines of relative humidity φ \u003d 100%, then with a further decrease in temperature from the air, moisture begins to stand out, since the water steam condensation occurs.
Dew point - saturated air condition ( φ \u003d 100%) with this moisture content d.. It is at the point of intersection of lines d.\u003d Const I. φ \u003d 100%. Isothermary passing through this point corresponds dew point temperature T Tr..
The essence of the process is that when cooled air containing water vapors in a constant quantity, this temperature occurs, in which steam cannot be held with air and goes into a liquid state.
Cooling with drying
If the temperature of the heat exchanger surface (calorfor) t pov below the temperature point of the dew, then with a further decrease in the air temperature, the process after reaching the dew point further passes along the line φ \u003d 100%. At the same time, steam is condensed and, accordingly, the air moisture content decreases. Also, the enthalpy decreases during the process, and relative humidity reaches a maximum possible value of 100% ( t 2.<t 1.,I 2.<I 1.,Φ 1.<φ 2.≈100%, d 2.<d 1.).
Amount of moisture remote from everyone The air kilogram is defined as the difference of moisture content values \u200b\u200bat the dew point and at the end point of the process Δd.=d 2.– d Tr, D Tr \u003d D 1. Water consumption condensed in the caloriefer is determined by the formula: W \u003d G. .
It should be noted that in practice, the process can not go strictly along the line φ \u003d 100%, and along it, with values φ about 95%. At the same time, the final air temperature will be slightly higher than the temperature of the heat exchanger surface (calorfor).
I-D Wet Air Chart is a diagram, widely used in the calculations of ventilation, air conditioning, drying systems and other processes associated with a change in the state of wet air. For the first time was compiled in 1918 by the Soviet engineer-heat engineer Leonid Konstantinovich Ramzin.
Various i-D charts
I-D Wet Air Chart (Ramsin Diagram):
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Description of chart
The I-D diagram of wet air graphically binds all parameters that determine the heat-woofer state of the air: enthalpy, moisture content, temperature, relative humidity, partial pressure of water vapor. The chart is built in the coordinate rowing system, which allows you to expand the area of \u200b\u200bunsaturated wet air and makes a chart comfortable for graphic buildings. In the ordinate axis, the values \u200b\u200bof the enthalpy I, KJ / kg of the dry part of the air are postponed, along the abscissa axis directed at an angle of 135 ° to an axis I, the values \u200b\u200bof the moisture content D, g / kg of dry part of the air are postponed.
The field of the diagram is broken by the lines of permanent values \u200b\u200bof enthalpy I \u003d const and moisture content d \u003d const. The lines of permanent values \u200b\u200bof the temperature t \u003d const are also applied to it, which are not parallel between themselves - the higher the temperature of the wet air, the more its isotherms are rejected. In addition to the lines of constant values \u200b\u200bI, D, T, on the field of the diagram, the lines of permanent values \u200b\u200bof the relative humidity of the air φ \u003d const. In the lower part of the i-d-diagram there is a curve having an independent axis of ordinates. It binds moisture content D, g / kg, with elasticity of water vapor PP, kPa. The axis of the ordinate of this graph is the scale of partial pressure of water vapor PP.
Wet air is widely used in various fields of industry, including in railway transport in heating, cooling systems, drying or humidification. Recently, the promising direction of the development of air conditioning technology is considered to introduce the so-called indirectly evaporative method of cooling. This is explained by the fact that such devices do not contain artificially synthesized refrigerants, in addition, they are silent and durable, since they lack moving and fast wear elements. To design such devices, it is necessary to have information on the patterns of thermal processes flowing in wet air when it changes its parameters.
Heat engineering calculations associated with the use of wet air are performed using i-D. Charts (see Figure 4), proposed in 1918 by Professor A.K. Ramsin.
This diagram expresses the graphical dependence of the main parameters of air-temperature, relative humidity, partial pressure, absolute moisture content and heat generation at a given barometric pressure. To construct 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 carried out through the obtained points. On the axis, the ordents on the scale are postponed with enthalpy i. With an interval of 1 kJ / kg of dry air. At the same time, up from point 0 corresponding to the temperature of the humid air T \u003d 0 0 C (273K) and the moisture content d \u003d 0, postpone positive, and down the negative values \u200b\u200bof the enthalpy.
Through the obtained points on the axis, the ordents carry out the lines of constant enthalpy at an angle of 135 0 to the abscissa axis. On the thus obtained, the lines of the isotherm and the line of permanent relative humidity are applied. To build isotherms, we use the equation for the heat-containing of wet air:
It can be written in the following form:
, (1.27)
where t and with sv - respectively, the temperature (0 c) and the heat capacity of dry air (KJ / kg 0 s);
r - hidden heat of vaporization of water (in the calculations is accepted
r \u003d 2,5kj / g).
If we take that t \u003d const, then equation (1.27) will be a straight line, which means that isotherms in coordinates i-D. They are straight lines and for their construction it is necessary to determine only two points characterizing the two extreme positions of wet air.
Figure 4. I - D Wet air diagram
To construct an isotherm with the corresponding temperature value t \u003d 0 ° C (273K) first using an expression (1.27) we define the position of the coordinate of the heat content (I 0) for absolutely dry air (d \u003d 0). After substituting the corresponding values \u200b\u200bof the parameters T \u003d 0 0 C (273K) and D \u003d 0 g / kg, the expression (1.27) shows that the point (I 0) lies at the beginning of the coordinates.
. (1.28)
For fully saturated air at a temperature T \u003d 0 ° C (273K) and \u003d 100% of reference literature, for example, we find the corresponding moisture content D 2 \u003d 3.77 g / kg dry. Ware. and from the expression (1.27) we find the corresponding value of the enthalpy: (I 2 \u003d 2.5 kJ / g). In the coordinate system I-D, we apply a point 0 and 1 and through them we carry out a straight line, which will be the isotherm of wet air at a temperature T \u003d 0 ° C (273K).
Similarly, you can construct any other isotherm, for example, for temperature plus 10 0 C (283). At this temperature, and \u003d 100% on reference data we find the partial pressure of the fully saturated air equal to p \u003d 9.21 mm. RT. Art. (1.23kpa), further and from the expression (1.28) we find the value of moisture content (D \u003d 7.63 g / kg), and from the expression (1.27) we define the value of the heat-containing of wet air (I \u003d 29.35 kJ / g).
For absolutely dry air (\u003d 0%), at a temperature of T \u003d 10 ° C (283K) after substitution of values \u200b\u200bin the expression (1.27) we obtain:
i \u003d 1.005 * 10 \u003d 10.05 kJ / g.
In the I-D diagram, we find the coordinates of the corresponding points, and spending the line of isotherm directly through them for the temperature plus 10 0 s (283k). Similarly, a family of other isotherms is built, and by connecting all isotherms for \u003d 100% (on the saturation line) we obtain a line of constant relative humidity \u003d 100%.
As a result of the constructions made, the I-D diagram was obtained, which is shown in Figure 4. Here, the values \u200b\u200bof the temperatures of wet air are applied on the axis, on the abscissa axis - the values \u200b\u200bof moisture content. Inclined lines show the values \u200b\u200bof heat content (KJ / kg). The curves that diverge the beam from the center of the coordinates express the values \u200b\u200bof the relative humidity φ.
The curve φ \u003d 100% is called saturation curve; Above its water vapors in the air are overheated, and below - in a state of synassium. The inclined line coming from the center of coordinates characterizes the partial pressure of water vapor. The values \u200b\u200bof the partial pressure are applied to the right on the ordinate axes.
Using the diagram I - D, it is possible at a given temperature and relative humidity of the air to determine the remaining parameters - heat-containing, moisture content and partial pressure. For example, for the specified temperatures plus 25 ° C (273K) and relative humidity and φ \u003d 40% on the I - D diagram found a point BUT. Moving it from it vertically down, at the intersection with the inclined line we find partial pressure P n \u003d 9 mm RT. Art. (1.23kpa) and on the abscissa axis - moisture content D a \u003d 8 g / kg of dry air. The diagram also shows that the point BUTlies on an inclined line expressing heat-containing I a = 11 kJ / kg of dry air.
The processes occurring during heating or cooling the air without changing moisture content are depicted on a diagram by vertical, straight lines. The diagram shows that at d \u003d const, in the process of heating air, the relative humidity decreases, and during cooling - increases.
Using the I - D diagram, you can define the parameters of the mixed parts of wet air for this build the so-called corner coefficient ray of the process . Construction of the beam of the process (see Figure 5) begins on the point with known parameters, in this case it is point 1.
The main properties of wet air can be with accuracy sufficient for technical calculations using the I-X diagram developed by L.K. Ramsin (1918). The I-X diagram (Fig. 1, 2) is constructed for a constant pressure P \u003d 745 mm RT. Art. (about 99 kN / m 2), which, according to many years of statistical data, is accepted as an average annual for the central regions of the former USSR.
At the axis, the ordinates are postponed on a certain scale of enthalpy I, and on the inclined axis of the abscissa - moisture content x. The angle between the coordinate axes is 135 °, but for ease of use, the values \u200b\u200bof moisture content X are designed to the auxiliary axis, perpendicular to the ordinate axis.
The diagram has lines:
- · Permanent moisture content (x \u003d const) - vertical straight, parallel axes of ordinate;
- · Permanent enthalpy (i \u003d const) - direct, parallel axis of the abscissa, i.e. directed at an angle of 135 ° to the axis of the ordinate;
- · Permanent temperatures, or isotherms (t \u003d const);
- · Constant relative humidity (C \u003d const);
- · Partial pressure of water vapor (P) in wet air, the values \u200b\u200bof which are postponed on the scale on the right axis of the ordinate chart.
Fig. one. Diagram of wet air I - x (a)
The lines of constant temperatures, or isotherm, are set at a given temperature T \u003d const two arbitrary values \u200b\u200bX 1 and x 2. Then calculate the value I corresponding to each value x. The obtained points (x 1, i 1) and (x 2, i 2) are applied to the diagram and spend directly through them, which is isotherm with T \u003d const.
The lines of constant relative humidity express the relationship between X and P at C \u003d const. Taking with a given C \u003d const several arbitrary temperatures T 1, T 2, T 3 for each of them are found along the tables of water vapor the corresponding values \u200b\u200bof P and calculate the value of x. Points with known coordinates (T 1, x 1), (T 2, x 2), (t 3, x 3), etc. Connect the curve, which is the line C \u003d const.
Fig. 2.
At temperatures T\u003e 99.4 ° C, the value of C does not depend on the temperature (since it is p \u003d 745 mm Hg. Art. For which the diagram is built) and is practically the value of constant. Therefore, the C \u003d const lines at 99.4 ° C have a sharp fracture and go almost vertically upwards.
Line C \u003d 100% corresponds to air saturation with water vapor at a given temperature. Above this line is the working area of \u200b\u200bthe diagram that corresponds to unsaturated wet air used as a drying agent.
Parts of the partial pressure conducted at the bottom of the chart allow you to determine the partial pressure if the position of the point is known on the diagram corresponding to the condition of the air.
According to the i-x diagram for any two known parameters of wet air, you can find a point that characterizes the condition of the air, and determine all of its other parameters.
Using a system of equations, comprising 4.9, 4.11, 4.17, as well as a functional connection R N \u003d f.(t.), L.K. Ramsin built J.-d. Diagram of wet air, which is widely used in the calculations of ventilation and air conditioning systems. This diagram is a graphical relationship between the main air parameters t., , J., d. and R n with a certain barometric air pressure R b.
Building J.-d. Charts are described in detail in the works.
The state of wet air is characterized by a point applied on the field J.-d. Frames limited d. \u003d 0 and curve \u003d 100%.
The position of the point is given by any two parameters of the five, indicated above, as well as dew point temperatures. t. P and wet thermometer t. M. . The exception is combined d. - R P I. d. - t. p, because Each value d. only one table value corresponds R P I. t. p, and combination J. - t. m.
The scheme for determining air parameters for a given point 1 is shown in Fig. one.
Using J.-d. diagram in adj. 4 and scheme in fig. 1, solve specific examples for all 17 possible combinations of the specified initial air parameters, the specific values \u200b\u200bof which are indicated in Table. 7.
Schemes of solutions and the results obtained are shown in Fig. 2.1 ... 2.17. Known air parameters are highlighted in drawings of thickened lines.
5.2. The angular coefficient of the ray of the process on the J-D diagram
The ability to quickly graphically determine the parameters of wet air is an important, but not the main factor when using J.-d. Charts.
As a result of heating, cooling, drainage or moisture of wet air, its heat-humid state changes. Change processes are depicted on J.-d. A diagram with straight lines that connect points characterizing the initial and final air states.
Fig. 1. The scheme for determining the parameters of wet air on J.-d. diagram
Table 7.
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t. 1, ° C |
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R p1, kpa |
t. P1, ° C |
t. M1, ° C |
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These lines are called rays of processes changes in air condition. The direction of the beam of the process on J.-d. The diagram is determined angular coefficient . If the initial air condition parameters J. 1 I. d. 1, and the final - J. 2 and d. 2, then the angular coefficient is expressed by the attitude J./d.Ie:
.
(5.1)
The magnitude of the angular coefficient is measured in the KJ / kg of moisture.
If in equation (29) numerator and denominator multiplied by the mass flow rate of the air G., kg / h, then get:
,
(5.2)
where Q. P is the total amount of heat transmitted when a change in air condition, KJ / h;
W. - The amount of moisture transmitted in the process of changing the condition of air, kg / h.
Depending on the ratio J. and d. The angular coefficient can change its sign and value from 0 to .
In fig. 3 shows the rays of characteristic changes in the state of wet air and the corresponding values \u200b\u200bof the angular coefficient.
1. Wet air with initial parameters J. 1 I. d. 1 heats up with constant moisture content to point 2 parameters, i.e. d. 2 = d. 1 , J. 2 > J. one . The angular coefficient of the ray of the process is:
Fig. 3. Corner coefficient on J.-d. diagram
Such a process is carried out, for example, in surface air heaters, when the temperature and enthalpy of air increase, relative humidity decreases, but the moisture content remains constant.
2. The wet air is simultaneously heated and moistened and acquires parameters of the point 3. The angular coefficient of the beam of the process 3\u003e 0. Such a process proceeds when the dying air assimilates heat and mediating indoors.
3. The wet air is moisturized at a constant temperature to the parameters of the point 4, 4\u003e 0. Almost this process is carried out at moistening of the supply or internal air in a saturated water vapor.
4. The wet air is moistened and heated with an increase in enthalpy to the parameters of the point 5. Since the enthalpy and moisture content of air increase, then 5\u003e 0. Typically, such a process occurs with the direct contact of air with seppe water in irrigation chambers and in cooling towers.
5. Changing the state of wet air occurs at constant enthalpy J. 6 = J. 1 \u003d const. The angular coefficient of such a beam of the process 6 \u003d 0, because J. = 0.
The process of isentalpic humidification of air with circulation water is widely used in air conditioning systems. It is carried out in irrigation chambers or in devices with an irrigated nozzle.
Upon contact with unsaturated wet air with small drops or thin film of water without removal or heat supply from the outside, water as a result of evaporation moisturizes and cools the air, purchasing the temperature of the wet thermometer.
As follows from equation 4.21, in the general case, the angular coefficient of the ray of the process during isentalpine moisture is not equal to zero, because
,
where from w. \u003d 4,186 - the specific heat capacity of water, KJ / kg ° C.
A valid isenthalthalpy process, at which \u003d 0 is possible only when t. M. = 0.
6. Wet air is moistened and cooled to point 7. In this case, the angular coefficient 7< 0, т.к. J. 7 – J. 1 0, a d. 7 – d. 1\u003e 0. Such a process proceeds in nozzle irrigation chambers when air contact with cooled water having a temperature above the point of dewing air of the processed air.
7. Wet air is cooled at constant moisture content to point 8 parameters. Since d. = d. 8 – d. 1 \u003d 0, a J. 8 – J. 1 < 0, то 8 \u003d - Air cooling process with d. \u003d Const occurs in surface air coolers at the surface temperature of heat exchange above the temperature of the air dew point when there is no moisture condensation.
8. Wet air is cooled and dried to point 9 parameters. The expression of the angular coefficient in this case has the form:
Cooling with drying occurs in irrigation chambers or in surface air coolers, with a moist air contact with a liquid or solid surface having a temperature below the dew point.
It should be noted that the cooling process with drying during direct contact of air and chilled water is limited by tangent, carried out from point 1 to saturation curve \u003d 100%.
9. Deep drying and air cooling to the parameters of point 10 occurs with direct contact of air with a chilled absorbent, for example, a solution of lithium chloride in irrigation chambers or in devices with an irrigated nozzle. Corner coefficient 10\u003e 0.
10. Wet air is dried, i.e. Gives moisture, with permanent enthalpy to point 11 parameters. The expression of the angular coefficient has the form
.
Such a process can be carried out using solutions of absorbent or solid adsorbents. Note that the real process will have an angular coefficient 11 \u003d 4,186 t. 11, where t. 11 - Final air temperature over a dry thermometer.
From fig. 3. It can be seen that all possible changes in the state of wet air are located on the field J.-d. charts in four sectors whose boundaries are lines d. \u003d Const I. J. \u003d const. In sector I, processes occur with an increase in enthalpy and moisture content, so the values \u200b\u200b\u003e 0. In the II sector, air is drained with an increase in enthalpy and value < 0. В секторе III процессы идут с уменьшением энтальпии и влагосодержания и > 0. In the IV sector, air humidification processes occur with a decrease in enthalpy, so < 0.