The importance of Psychrometrics
An understanding of psychrometrics is vital to HVAC and it is at the heart of Computair’s coil selection software. Psychrometrics describes the properties of moist air, which we consider as a mixture of dry air and water vapour.
The amount of water vapour in the air can vary quite a lot and has a significant effect on our perception of temperature and comfort. At a given temperature and pressure there is only so much water vapour that air can hold. The point at which water will no longer evaporate is called “saturation”. At low temperatures this happens at low humidity ratios. As temperature increases the saturation humidity ratio increases rapidly until the boiling point of water is reached and it can no longer exist as liquid.
Dry air is a mixture of several gases. It’s mostly Nitrogen and Oxygen, but also contains Argon and Carbon Dioxide. What we call ‘moist’ air also contains water vapour. Water vapour is a colourless and odourless gas, but it’s important for air conditioning. Consider some of the factors below.
The ability of the human body to regulate temperature is highly dependent upon both air temperature and humidity. Very humid air causes people to sweat. Very dry air also causes discomfort. ASHRAE Standard 55 defines comfortable conditions (or acceptable thermal environments).
Data centres are a rapidly-growing industry. Electrical power flowing into a data centre generates heat, which must be removed to keep computers running at an optimum temperature. Very dry air can also cause static build up and dust accumulation. Data centre HVAC needs to control both temperature and humidity.
When humid air flows over cooling coils, water can condense out as liquid and provision needs to be made for the water to drain away. Psychrometrics can determine at what rate water will condense and the amount of latent heat released by the water as it condenses from liquid to gas.
cientists and engineers have been using psychrometric calculations for a very long time, with mathematical models slowly improving over time. The ASHRAE Fundamentals manual contains some simple calculations for psychrometric properties which are easy to implement in software and which many engineers would consider as the standard reference. These are accurate enough for the normal range of conditions used for air conditioning, but lose accuracy outside of those conditions.
We consider ASHRAE RP 1485 as the definitive psychrometric model. It is valid between temperatures of -143C and 350C, air pressures of 100Pa and 10MPa and humidity up to 10kg/kg. This research paper was published in 2009 and provides a virial equation of state for moist air. The equation of state shows how the temperature, pressure and density of air are related. From that equation, all other psychrometric properties are derived.
Our coil selection software and air handler design software contain implementations of both ASHRAE Fundamentals and ASHRAE RP 1485.
A psychrometric chart is a clever chart that can be used to estimate psychrometric properties for a given air condition. Laminated charts can be used to plot out processes and given a visual representation of the effects of cooling, heating or humidification. Such charts are normally produced for standard air pressure (101325Pa) and so are limited. Of course, software can calculate a psychrometric chart for any air pressure within the range of validity of its psychrometric calculations.
A standard psychrometric chart shows dry bulb temperature along the X-axis and humidity ratio on the Y-axis, although the Y-axis is drawn to the right of the chart. In Europe a different convention is often used, known as a Mollier chart. This shows dry bulb temperature on the Y-axis and humidity ration on the X-axis. The X-axis is drawn above the chart.
Computair’s coil selection software and air handler selection software draws both standard psychrometric charts and Mollier charts as requested by the end user.
A few definitions
What most people refer to as “temperature” is normally called “dry bulb temperature” or just “dry bulb” in order to disambiguate it from wet bulb temperature. It is simply the temperature measured by a thermometer. Its SI unit of measure is Kelvin (K), but we normally measure temperature as Celsius (C) or Fahrenheit (F).
This is also known as absolute humidity or moisture content. It is the ratio, by mass (“specific humidity ratio”) or by mole (“molar humidity ratio”), of water vapour to dry air. Specific humidity ratio is normally used and its SI units of measure are kg/kg, although lb/lb or gr/lb are normally used in North America.
WET BULB TEMPERATURE
This is a tricky concept to explain, but a very important one. It can be thought of as how temperature is experienced by something that is wet. The name “wet bulb” comes from the fact that it can be measured with a thermometer with wet fabric wrapped around the bulb. If RH is low and temperature is high then water will evaporate readily from a wet surface. As the liquid water evaporates it absorbs latent heat from its surroundings causing a cooling effect. Thus wet bulb temperature is normally lower than dry bulb temperature and never higher. As humidity increases wet bulb temperature increases to become closer to dry bulb. At 100% RH wet bulb temperature is equal to dry bulb temperature. A wet bulb temperature of 35C or higher is normally reckoned to be fatal to human beings.
RELATIVE HUMIDITY (RH)
This is the ratio of the partial pressure of water vapour in the air to the partial pressure of water at saturation at a given temperature. It can be thought of as the ratio between the amount of water vapour in the air against the maximum amount of water vapour possible at the given temperature and pressure. At 100% RH liquid water cannot evaporate into the air, so our bodies are not cooled by sweating and can feel very uncomfortable at temperatures above 25C.
Psychrometrics tells us how the density of air changes with temperature, pressure and humidity. This is important because it defines the relationship between volume and mass of air. Density is measured in kg/m3, or in North America lb/ft3 is normally used. Specific volume is the inverse of density and tells us how much volume a given mass occupies. Its units of measure are m3/kg and ft3/lb.
This is a temperature below which, at a given humidity ratio air cannot be cooled without water condensing out as liquid and thus reducing the humidity ratio. At low temperatures small changes in humidity ratio can cause large changes in dew point.
Put simply, enthalpy is heat. As air is heated or cooled by a heat exchanger, enthalpy is added or removed from the air. Psychrometric calculations will tell us how this affects the temperature, density and humidity of the air. The SI units are kJ/kg, although in North America BTU/lb is normally used. Something to be aware of is that psychrometric enthalpy is defined slightly differently to thermodynamic enthalpy because in psychrometrics we tend to be less interested in the enthalpy at a specific condition than the difference in enthalpy between two conditions. So for convenience psychrometric enthalpy in SI units is set to 0 kJ/kg at 0 C and 0 absolute humidity. For IP units it is set to 0 BTU/lb at 0 F and 0 absolute humidity. Thus the two scales for psychrometric enthalpy are offset by a fixed value in the same way as the Celsius and Fahrenheit scales for temperature.