The main physical quantities used in air conditioning


Since air conditioning systems affect the air, it is very important to know its composition and other parameters. Air consists of water vapor and dry parts (oxygen, nitrogen and other gases). But for heating and air conditioning systems, air parameters such as temperature (T), atmospheric pressure (B) and moisture content (D) are also important, as well as dry air pressure (Pg), relative humidity (F), air density (P), enthalpy (H), impurity content.

Basic concepts and definitions


This is a term that reflects how much a substance or body is heated or cooled. Most often, temperature is measured in degrees Kelvin (thermodynamic or absolute temperature), as well as in degrees Celsius.
1. Celsius scale (°C): the boiling point of water is 100 degrees, and the freezing point is 0 degrees.
2. The Kelvin scale (K): the absolute zero of this scale is the temperature when the thermal movement of particles stops. On the Kelvin scale, the boiling point is 373.15 K and the freezing point is 273.15 K.
To convert the temperature to the Kelvin scale from the Celsius scale, you need to use the following formula: T (temperature on the Kelvin scale) u003d t + 273.15.
In practice, temperature is measured by two types of thermometers: a wet bulb thermometer and a dry bulb thermometer.

If the relative humidity of the air is less than 100% (that is, the air does not appear to be water vapor), then the wet and dry bulbs will show completely different temperatures. The dry bulb temperature will be higher than the wet bulb temperature. And the temperature difference will be greater at lower relative humidity.


1. Moisture content (d), which is expressed in g/kg and is the amount of vapor per kilogram of dry air.
2.Absolute humidity is measured in kg/cu. m and displays how much steam is in a cubic meter of air.
3. Relative humidity (F) is the ratio of the maximum possible moisture content at a given temperature and pressure to its actual content. If Ф=0, then there is no water vapor in the air and it is absolutely dry. Ф=100% corresponds to wet steam. For humans, comfortable humidity is approximately 50-70%.

Quantity of heat

This is the energy that is transferred to a substance or body as a result of its heat exchange with the external environment. The unit of measure is the joule (J).
Heating or cooling power is a quantitative characteristic of the ability to cool or heat bodies. It is denoted in watts (W), and 1000 watts is equal to a kilowatt (kW). Power also depends on the time during which heat is transferred. For example, if the power of your heater is 3000 W, then this means that it gives off 3000 J of heat in one second.

Units of measurement of physical quantities

SI — the international system of units of measurement is currently recognized as the main system of units in physics (including refrigeration and air conditioning). Its basic units are meter (length), second (time), kilogram (mass), kelvin (temperature), ampere (current), candela (light intensity) and mole (amount of substance).

Air conditioning also uses many derived quantities, such as specific heat capacity. It is equal to the amount of heat that, in order to increase the temperature of a body by 1 K, must be brought to a unit of its mass. Specific heat capacity is measured in J / (kg * K).

In foreign technical literature, off-system units of measurement are also often used.


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