Callidus Design Consulting Engineers

Designers, engineers, consultants and technical specialists covering a wide range of building and process services.

Humidity Control

 Psychrometric Design Service

Outwith ventilation systems for comfort only, some processes rely on environments in which the close control of both temperature and humidity levels are very important. It would be unusual for a building not to require fresh air ventilation and therefore the admission of external air into the building will alter the internal environmental conditions if left uncontrolled. The external ambient humidity level rises and falls hourly, diurnally and seasonally. Typically the external ambient humidity level is low in winter and high in summer. Therefore, in winter, moisture must be added to the air in the building and in summer it must be removed to compensate for the external changes. The addition of moisture is achieved through a process of humidification and the removal of moisture through a process of dehumidification. The science of moisture addition and removal is called Psychrometrics.


Moisture levels can be increased using one of the following forms of humidifier:

  • Cold water spray under high pressure through a nozzle, with or without the use of compressed air
  • Cold water spray formed by dripping water onto a spinning disc
  • Cold water absorption by passing ventilation air through a wetted substrate
  • Steam injection either from a live steam system or a local unit incorporating an electrical or gas-fired kettle



Moisture levels can be decreased using one of the following forms of dehumidifier:

  • Absorption by passing air through a rotating wheel filled with a desiccant material
  • Condensation by passing ventilation air over a coil containing chilled water


Common pitfalls

The design and selection of all of the components of the ventilation system are critical if proper control of humidity is to be achieved. Improper selection of one or more components can result in any of the following symptoms:

  • Insufficient humidity levels achieved in the winter months
  • High humidity levels experienced in the summer months
  • Wetting out of the internal surfaces of Air Handling Units (AHUs) due to inadequate absorption of added moisture
  • Plumes of water vapour emanating from supply diffusers

Callidus Design understand the subject of psychrometrics exceptionally well and can design and specify all components of a ventilation system incorporating humidification and dehumidification functions which will operate in the desired manner.

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