When it comes to air movement, understanding the properties of air is vital. In this series of articles, we look at how different parameters affect air density, and how air density changes take place within an Air Handling Unit (AHU).
The Contents Of Air
The air in our atmosphere is made up predominantly of two gases: nitrogen (78%) and oxygen (21%). This leaves just 1% for all of the other gases in the air.
Actually, the properties of air differ considerably depending on where in the world you happen to be. Clearly, the air in the Sahara is very dry, whereas in the Amazonian rain forest it is much more moist. Also, air pressure varies considerably depending on altitude, with the vertical distance from sea level to the summit of Mount Everest (the highest point on Earth) being 8848 metres – about 5½ miles.
An important metric in air property is the level of carbon dioxide (CO2). The average CO2 level is approximately 360 parts per million (ppm), although this value is rising due to both pollution and the greenhouse effect.
Good ventilation systems will reduce the levels of indoor CO2 – the World Health Organisation recommends that these should be below 1000ppm. Once the level gets to 1500ppm, the air does not smell fresh, and people breathing it will start to feel lethargic.
There is a ‘Standard Condition’ of air which is used to compare ventilation products. This is defined as a density of 1.2kg/m3, which occurs at 20°C and an atmospheric pressure of 1013 milliBbar.
The amount of moisture in air is an important property of it, and it can be measured in a number of ways.
The first is the Wet-Bulb Thermometer. When you wrap a water-soaked cloth around a thermometer bulb, the temperature will drop, due to the energy needed to evaporate the water in the cloth. The drier the air is, the more water can evaporate, and the more the temperature decreases.
The temperature drop is known as the wet-bulb temperature; it can be used as a measure of the humidity in the air, and can be input into a Mollier chart (a graphical representation of thermodynamic properties of – in this case – air).
The second way of expressing air humidity is Absolute Humidity, which is exactly what the name suggests: the absolute content of water in the air, without temperature or pressure as a reference. It is usually expressed in kilograms of water per kilogram of air, or else in grams of water per kilogram of air.
Normally the absolute humidity indoors would be in the region of 5-10g per kg of air; in a bathroom after someone had taken a shower, it could be as high as 20g/kg.
The third way of describing air humidity is Relative Humidity. As the name suggests, this is the ratio between the maximum water that the air can hold (100%) and the actual water content at the current temperature.
Air is able to hold more water at a higher temperature than at a lower temperature, so if saturated air cools, it gives off condensate (this is what happens when the bathroom window becomes wet in winter, because the air around the cold glass is cooled).
The Dew Point is the temperature below which the water vapour in a volume of humid air (at a constant barometric pressure) will condense into liquid – this is why condensed water formed on a solid surface is called dew.
The dew point is a water-to-air saturation temperature. So air with a high relative humidity indicates that the dew point is closer to the current temperature; when the air’s relative humidity is 100%, the dew point is equal to the current temperature (which means even a minimal cooling will cause condensate). Likewise, when the dew point remains constant and the temperature increases, relative humidity decreases.
Enthalpy is a measure of the total energy in a thermodynamic system. It includes the initial energy (the energy required to create the system in the first place), and the amount of energy required to make room for it by displacing its environment and establishing its volume and pressure.
Air is such a thermodynamic system; so enthalpy can describe the thermal energy present in the air. It is normally described as kiloJoules (kJ) per kilogram of air, but can also be described as kilocalories (kcal) per kilogram of air.
The enthalpy measure of dry air at 0°C is 0 kJ/kg.
Calculating Cooling/Heating Power
Calculating the cooling and heating power can be done in two ways, one of which uses the temperature and the other the enthalpy. Using the temperature difference to calculate the power can only be done when there is no condensation in the process, as this method only calculates the sensible power, and not the latent power.
Sensible power is the amount of power required to be added to an object to increase its temperature, or the amount of power removed to reduce its temperature. Latent power, on the other hand, is the power required to change an object’s state (from solid to liquid, or from liquid to gas, and vice-versa). Total power is the sum of sensible power and latent power.
So if both sensible and latent power form part of the colling calculation, it is important to use the enthalpy formula to calculate total power; when calculating the heating power you can use either method, as it will give the same result.
Using the temperature difference to calculate the power involves the following formula:
P = Δt * qv * ρt = (tB – tA) * qv * ρt
Whereas the enthalpy formula is as follows:
P = Δh * qv * ρt = (hB – hA) * qv * ρt
In both cases:
P Heating/cooling power, kW Δh Change in enthalpy per kg dry air, kJ/kg qv Airflow m3 moist air/s ρt Density in kg of dry air/m3 moist air (ca 1,2 kg/m3)