![]() We are at sea level, with an atmospheric pressure of 1013 mbar – a typical summer’s day. Suppose the outdoor condition (position #1) is a temperature of 27☌ with 65% relative humidity. In Fläkt Woods’ product selection software ACON, a Mollier Chart with the specific air handling unit process can be plotted automatically.įigure Two shows a typical Mollier Chart.Ĭalculation Of Air In An Air Handling Unitįigure Three shows a typical Air Handling Unit, showing eight positions air must pass through, each of which will change the air on its journey from ambient air (position #1) to the desired air condition (position #8).įigure 3: The eight positions of a typical Air Handling Unit The Mollier Chart is used in order to design air conditioning processes and to calculate, among other things, the change in temperature and humidity, and the energy required to heat or cool the air. It is an enthalpy-entropy chart, describing the enthalpy of a thermodynamic system. ![]() The Mollier Chart was devised in 1904 by Richard Mollier in Dresden. The effect of changes in humidity are almost negligible: a large change of 25% in humidity will only change the air density by 0.37%. Pressure has a smaller but still tangible impact: a 500Pa change in pressure will change the density by 0.49%. The biggest environmental factor which will affect air density is temperature: a change of just 6☌ (from 20☌ to 26☌ will change the density by 1.7%. It can also be expressed as the ratio (in per cent) of the vapour partial pressure of the air to the saturation vapour partial pressure at the actual dry bulb temperature, or by the actual mass of the vapour and the air. The equation for calculating air density is shown in Figure One.Īt sea level, at a temperature of 20☌, at International Standard Atmosphere (ISA), air has a density of approximately 1.2 kg/m 3.Īs we saw in Properties Of Air Part One, Relative Humidity is the ratio between the maximum water that the air can hold (100%) and the actual water content at the current temperature. It is defined as the mass per unit volume of air, and its value is affected by a number of factors, including altitude (air density decreases with increasing altitude, like air pressure), and variations in temperature and humidity. The density of air is a useful value in measuring the air volume.
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