What is Psychrometrics? Psychometric Charts and Tables

Psychometric, moist air using these properties and thermodynamic properties of moist air is a branch of thermodynamics dealing with procedures and conditions. Depending on the temperature and the studies conducted in recent years have found new correlations for thermodynamic properties. It belongs to the humid air and water thermodynamic properties according to the new formula is presented in the tables in Annex 1 and Annex 2.

simplicity, speed and economy in terms of providing solutions with the ideal gas law used in air-conditioning problems if the comparison of the results of these formulas can be seen that the difference is negligible in order. When using the ideal gas equation of 101 325 kPa and -50 to +50 ° C temperature heard from the air enthalpy and specific humidity in the error rate, at 0.7% which is smaller, it has been proven by Threkeld. This error rate is decreasing with increasing pressure more. general observations in this section will be used as the ideal gas approximation.

Psychometric Dry and Moist Air Composition

various contaminated gas with a mixture of a plurality of atmospheric air gas and water vapor, pollen (pollen) and consists of smoke. In general, away from atmospheric air pollution sources included dirty smoke and gas.

The dry air and water vapor in the atmospheric air is completely enclosed. Of dry air with relative precision measurement has been shown as fixed. Components within the geographic regions, according to the altitude and very little time and can be changed. Volumetric dry life of approximately 78 084% nitrogen, 20.9476% oxygen, 0.934% argon, 0.0314% carbon dioxide, 0.001818% Neon, 0.000524% helium 0.0007% methane, from 0% to 0.0001% sulfur dioxide, 0.0005% hydrogen and a total of 0.0002% Krypton, xenon is considered to consist of ozone gas. dry air molar mass by the carbon-12 scale 28 965 kg / mol and a gas constant of the following values.

RH = 8314.41 J / mol K / 28 965 kg / mol = 287 055 J / UPS = 0.287 kJ / UPS

Moist air as a mixture of dry air and water vapor is assumed to be composed of two components.The amount of air in the water vapor than zero (dry air) depending on the temperature and pressure ranges from a maximum value. max is the state of the water vapor in the air is called a saturated humid air and condensate (liquid or solid) is a natural equilibrium. Unless otherwise indicated, that the condensation surface of the contact portion in contact with the air is considered to be saturated.The molar mass of water relative to the carbon-12 scale 18.01534 kg / mol and a gas constant of water vapor of the following values.

RH2O = 8314.41 J / mol K / 18.01534 kg / mol = 461.52 J / UPS = 0.461 kJ / UPS

Standard Atmosphere

Atmospheric air barometric pressure and temperature, mainly as changes in sea level height, which varies with the geographical situation and weather conditions. According to a reference in air conditioning engineering, it is necessary to find a definition of the physical characteristics of standard atmospheric air at different elevations.

Standard sea level air temperature 15 ° C, the standard barometric pressure is 101,325 kPa. The troposphere (the lower layer of the atmosphere) until the air temperature, it is assumed that the height of the linear change and stratospheric reaches a constant value. the lower part is fixed in the atmosphere of dry air components, and is considered to represent the ideal gas behavior. Standard gravity of 9.807 m / s worth. standard temperature and pressure of the atmospheric air changes according to the height above sea level are given in Table 1.

Table 1. Standard features of the atmospheric air, sea level height changes

Height, mTemperature, ° CPressure, kPa
one thousand8.589874

Ideal Gas Equation of State

Ideally, the gas first law was enacted by R. Boyle and E. Mariotte. They are kept at a constant temperature for a certain amount of gas, it has found that the product of pressure and volume is constant (Boyle – Mariotte law).

PV = constant

This is dependent on the type of constant temperature and gas. T = constant volume of energy that is equal to that in 2 cases, including 1 case.

The representation of the isotherm P-V diagram
Figure 1. The representation of the isotherms in the PV diagram

Charles derivation of the law
Figure 2. derivation of Charles law

The ideal gas equation of state
Figure 3. The ideal gas equation of state

Psychometric Charts and Tables

Psychrometry psychometric chart we can account values, the physical properties of moist air is a diagram that can be plotted. The selection of the axes on a psychrometric chart is completely arbitrary. enthalpy axis of psychometric diagram of humid air graphics solutions to many problems in thermodynamics and be as specific humidity provides great convenience. axes in the first diagram prepared by Mollie is taken as enthalpy and specific humidity.

Using the same axes, seven types of Mollier psychrometric chart prepared by ASHRAE. Of these, 1, 2, 3 and 4 in the diagram, atmospheric pressure 101,325 kPa is taken as the standard sea level pressure. 5 numbered diagram above sea level 750 m (92.66 kPa) height, number 6 diagram above sea level 1500 m (84.54 kPa) in height, the diagram number 7 above sea level at 2250 m (77.04 kPa) is prepared for height. On the other hand;

• 1, 5, 6, 7 in the diagram: 0 to 50 ° C (normal temperature)
• No. 2 diagram: -40 to 10 ° C (low temperature)
• 3 from the diagram: 10 to 120 ° C (high temperature)
• Diagram No. 4: 100 to 1200 ° C (high temperature) are arranged according to the dry bulb temperature.

other than the pressure given in this diagram is a need for interpolation at atmospheric pressure.With all of the following comparison of these diagrams can be detected.

• enthalpy of a given age and specific humidity and relative humidity increased with height above sea level for the pair of dry bulb temperature varies very little.
• atmospheric pressure, specific volume for a given pair of wet and dry bulb temperatures vary too much. Practically, it is assumed that the specific volume is inversely proportional to atmospheric pressure changes.

In Table 2 below, at sea level (diagram No. 1) 1500 m above sea level with properties (No. 6 diagrams) shows a comparison of the features in height.

Comparison No. 6 which is the diagram of Table 2 with the Psychometric diagram

diagram No.T (dry)T (years)h (kJ / kg)X (g / kg)φ (%)v (m3 / kg)

Cartesian coordinate system in place for all these enthalpy diagrams and specific humidity, tilted axes is used. No. 1 which was prepared by the ASHRAE psychometric diagram is shown in Annex 3.’t. enthalpy of moist air on this diagram, specific humidity, dry bulb temperature, thermodynamic wet bulb temperature, relative humidity and specific volume are physical variables.

dry-bulb temperature of 0 to 50 ° C in this diagram, a relative humidity of 0 (dry air) 30 Hold the stylus I / kgkur ranges from air. The constant enthalpy changes in the form of oblique parallel confirms 1 kJ / kgkur is seen as indicated by the air gap. Although in the dry bulb temperature of straight lines are not exactly parallel to each other and are slightly deviates from the perpendicular. a little different from the true enthalpy thermodynamic wet bulb temperature is observed in case of oblique line.

narrow zones above the saturation curve, damp, foggy air is defined as the region. In this region, representing two phase, liquid water particles with a mixture of saturated moist air is their thermal stability is observed. Foggy accurate constant temperature in the region is in line with the extension of the thermodynamic wet-bulb temperature in humid weather. If the relative humidity in this area if necessary foggy, wet bulb temperature, enthalpy and thermodynamic extended.

seen in the upper left corner of the diagram has two scale semi-circle. Heat is emitted first scale shows the ratio of total heat illustrates the scale ratio of the specific enthalpy difference of the second humidity difference. This half-circle on the scale used to determine the direction of change in the psychrometric diagram.

Annex 1

Thermodynamic properties of moist air (standard atmospheric pressure of 101 325 kPa)
Thermodynamic properties of moist air

Thermodynamic properties of moist air (standard atmospheric pressure of 101 325 kPa) (Continued)
Continued thermodynamic properties of moist air

Annex 2

Thermodynamic properties of saturated steam
Thermodynamic properties of saturated steam

Thermodynamic properties of saturated water vapor (Continued)
Continued thermodynamic properties of saturated water vapor

Annex 3

Psychometric Diagram (ASHRAE Diagram No. 1)
psychometric Diagram

Psychometric Diagram


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1 Comment

Space_2022  -  17 March 2022 at 09:07

A psychrometric chart packs a lot of information into an odd-shaped graph. If we consider the components piece by piece, the usefulne 9608 ss of the chart will be clearer. Boundaries of the psychrometric chart are a dry-bulb temperature scale on the horizontal axis, a humidity ratio (moisture content) scale on the vertical axis, and an upper curved boundary which represents saturated air or 100-percent moisture holding capacity. The chart shows other important moist air properties as diagrammed in Figure 2: wet-bulb temperature; enthalpy; dewpoint or saturation temperature; relative humidity; and specific volume. See the for an explanation of these terms. Moist air can be described by finding the intersection of any two of these properties. This is called a “state point.” From the state point all the other properties can be read. The key is to determine which interest. Some practice with examples will help. Use Figures 2 and 3 with the psychrometric chart in Figure 1 to verify whether you can find each air property.

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