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KS19: Humidification
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KS19: Humidification

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Humidity may need controlling in almost every type of building, both for comfort and health and for the building’s contents and processes. In many cases this is undertaken using well-established natural mechanisms, but there are often circumstances where the total-life cost of the building would benefit from active humidification control.

Cold outdoor air will characteristically have a low moisture content, as will outdoor air in hot arid climates, and will frequently require humidification to provide comfortable and productive internal conditions.  Conversely, warm outdoor air (other than in arid climates) will characteristically have a high moisture content and may need to be dehumidified to meet comfort conditions. It is important that comfort  air-conditioning systems should not operate both cooling and humidifying equipment at the same time as this can be hugely wasteful; it is normally avoided through proper design and control.

This guide will specifically consider the means of increasing the humidity of air to ensure that it does not fall below appropriate levels – a process known as humidification.

This guide will explain:

  • what ‘humidity’ means

  • the reasons for controlling the humidity levels in the built environment

  • the processes that make humidifiers work

  • the main types of humidifiers used in buildings

  • how to determine the humidification load

  • the energy used when humidifying

  • the selection of humidifiers

  • the operational needs of humidifiers


1 The need for humidification

2 What is humidity?

2.1 Relative humidity or percentage saturation?

3 Why is humidity control important?

3.1 Reasons for controlling humidity
3.1.1 Health and comfort
3.1.2 Control of static electricity
3.1.3 Fabric and materials
3.1.4 Food storage
3.1.5 Industrial production
3.1.6 Equipment and processes

3.2 Design conditions for controlled environments
3.2.1 Humidity variations within a room

4 Humidification processes

4.1 Adiabatic humidification
4.2 Isothermal humidification
4.3 Heating to humidify

5 Humidification load calculations

5.1 An example to compare the application of adiabatic and isothermal humidification systems

6 Humidifier systems

6.1 Adiabatic humidifiers
6.1.1 Atomising humidifiers
6.1.2 Wetted surface or ‘evaporative’ humidifiers
6.1.3 Ultrasonic humidifiers
6.1.4 Adiabatic humidifier performance
6.2 Isothermal humidifiers
6.2.1 Centrally supplied steam
6.2.2 Electric steam generators
6.2.3 Local gas- or oil-fired steam generators
6.2.4 Isothermal humidifier performance

7 Selection considerations

7.1 System running costs
7.2 Absorption distances for duct-mounted humidifiers
7.3 Duct relative static pressures

8 Health concerns

8.1 Legionnaires’ disease

8.1.1 Reducing health risk

9 Installation, commissioning and maintenance

9.1 Humidifier installation
9.2 Commissioning
9.3 Maintenance

Further reading


Technical author: Tim Dwyer
Contributors: HEVAC Humidity Group

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