Psychrometric Chart

The world of Psychrometrics

The human body can tolerate a wide range of environmental humidity levels, from the crisp dry air we experience on a sunny but frosty winter morning to the muggy closeness of a summer day in the rain. There are those, however, who do not tolerate the extremes well, suffering from symptoms of dry eyes in winter and feeling clammy in summer. Many manufacturing  processes,  R&D laboratories and healthcare premises are similarly intolerant of extremes and require a narrower range of humidity to function correctly. For this reason, having precise control over environmental humidity can be critical, both from a human health and building process perspective. The successful control of humidity requires a detailed knowledge of the subject known as Psychrometrics.


Pronunciation: /ˌsʌɪkrəˈmɛtrɪks/

Psychrometrics or psychrometry or hygrometry are terms used to describe the field of engineering concerned with the determination of physical and thermodynamic properties of gas-vapor mixtures. The term derives from the Greek psuchron (ψυχρόν) meaning "cold" and metron (μέτρον) meaning "means of measurement". (source)

Wherever humidity control is required, it is most frequently the job of the ventilation system to perform that task. The variance in humidity can be caused either by external environmental changes (seasonal or diurnal) or by internal processes which absorb or emit moisture. In either case, the ventilation system must add or remove moisture to maintain the humidity at the desired level. To add moisture requires the installation of a humidifier in the Air Handling Unit or distribution ductwork. In some specific cases, the humidifier may inject moisture directly into the space. To remove moisture is a little more difficult. The two most common methods are;

1) to pass the ventilation air over a cooling coil whose surface temperature is low enough to cause condensation and thus the removal of moisture from the airstream. The disadvantage of this method is that the air is also cooled in the process and is usually too cold to be supplied directly to the space. In which case, it is typical for a heating coil to be placed downstream of the cooling coil in order to reheat the air to the desired temperature.

2) to pass a portion of the ventilation airstream through a standalone device known as a dehumidifier. The dehumidifier commonly contains a rotating wheel filled with desiccant beads. The beads adsorb moisture from the passing air, thus reducing the quantity of moisture in the air delivered to the space. After a time the desiccant beads become saturated and require drying before they can perform any further dehumidification function. This is achieved by rotating them out of the airstream and drying them with a small quantity of heated air. The heated air removes the moisture back from the beads and regenerates them ready for reuse. The heated, moisture laden air is typically rejected to the external atmosphere via a ventilation duct.











In order to know how much moisture requires to be added to or removed from a room environment a series of calculations must be performed, utilising local winter and summer weather data. The results of the calculation are then utilised to select the correct size of plant to perform the humidification or dehumidification function. It is extremely important to arrange the ventilation components in a very strict sequence to achieve the desired result. Even if the plant components are adequately sized in themselves, they will not achieve the desired humidity control unless they are arranged in the correct manner.

Please contact us if we can be of assistance with your humidification, dehumidification or general psychrometric project.