Psychrometry

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As psychrometry (from the Greek "psychros" cold and "metron" level), which is moisture determination by comparative temperature measurement with two thermometers , respectively. Of the two thermometers, one is in the environment of the humidity to be determined (room humidity), the other is in the same place when steam is saturated. The humidity results from the temperature difference depending on the room temperature, taking into account a device constant. Such a measuring device is called a psychrometer . Sometimes the entire investigation of a system consisting of a steam and a gas is called psychrometry and the properties of the (mostly air-water) system are called psychrometric properties.

All states of humid air can be represented as a gas-vapor mixture of dry air (referred to as "gas") and water vapor (referred to as "steam"). The gas-vapor mixture therefore consists of condensing and non-condensing components and, like its two components, behaves similarly to ideal gases under the boundary conditions typical in meteorology and air conditioning (room ventilation technology) with regard to temperature and pressure .

The states and changes in state of the humid air can be shown in the hx diagram ( enthalpy vs. vapor load of the air). In Germany, the Mollier diagram is mainly used, while in the USA the carrier diagram is used. In the Mollier diagram, the area of ​​the air unsaturated with water vapor was enlarged by rotating the abscissa of the coordinate system. The rotation was made so that the isotherm for zero degrees Celsius is straight on the horizontal. Accordingly, the enthalpy lines do not run horizontally, but run with a gradient that corresponds to the heat of vaporization of water at zero degrees Celsius. In addition to the isotherms, the diagram also shows the relative humidity and the density of the air mixture as parameters.

In the Mollier diagram, mixing processes of two air streams, heating of an air stream, cooling of air with and without dehumidification as well as humidification through water evaporation or steam can be represented.

Here one of the thermometers is moistened with water (wet bulb thermometer). The thermometer is cooled down by evaporation. The lower the relative humidity (provided there is sufficient air movement), the stronger the cooling.

example

The wet bulb thermometer is cooled to 22 ° C at a temperature of 30 ° C and 50% relative humidity.

See also

Individual evidence

  1. ^ Tibor Müller: Dictionary and Lexicon of Hydrogeology German English . Springer-Verlag, 2013, ISBN 978-3-642-58514-2 , pp. 254 ( limited preview in Google Book search).
  2. ↑ Onkar Singh: Applied Thermodynamics . New Age International, 2003, ISBN 978-81-224-1496-7 , pp. 913 ( limited preview in Google Book search).
  3. ^ Dennis R. Heldman: Encyclopedia of Agricultural, Food, and Biological Engineering (Print) . CRC Press, 2003, ISBN 978-0-8247-0938-9 , pp. 663 ( limited preview in Google Book search).
  4. ↑ a b David Shallcross: Handbook of Psychrometric Charts Humidity diagrams for engineers . Springer Science & Business Media, 2012, ISBN 978-94-009-0027-1 ( limited preview in Google book search).
  5. ^ Anton Pech, Klaus Jens: Ventilation and sanitation . Birkhäuser, 2015, ISBN 978-3-99043-094-1 , pp. 8 ( limited preview in Google Book search).