Humidification

[edit] Introduction

Absolute humidity is the mass of water vapour in a volume of air divided by the mass of dry air. Relative humidity (RH) is a measure of the water vapour density of air compared to the water vapour density for saturated air at the same temperature and pressure (that is, the maximum amount of moisture that air can 'hold' at that temperature and pressure).

Relative humidity is expressed as a percentage. Typically, a relative humidity of 40% to 60% is appropriate in many buildings.

[edit] Adjusting levels of humidity

Humidity in buildings is regulated by Approved Document C (Site preparation and resistance to contaminates and moisture) and Approved Document F (Ventilation), and further guidance is available in BS 5250 Code of practice for the control of condensation in buildings.

Humidity can influence thermal comfort. Air that is too dry can cause problems such as dry eyes, nose, ears and throat. It can also make some natural materials (such as wood) shrink and crack, and it can cause wall coverings to peel at the edges.

Low humidity levels can be controlled by making dry air 'wetter'. When moisture, water vapour or humidity are added to the air without changing its dry bulb (DB) temperature (a measure of air temperature taken when the thermometer bulb is dry), this procedure is referred to as humidification.

While temperature modifications are not an essential part of humidification, the process frequently takes place in conjunction with temperature control in systems associated with air conditioning and heating. Other types of humidifiers use different methods to introduce moisture into the air - including ultrasonic, high frequency vibrations that create a type of cold mist.

[edit] Cooling and humidification

The process of humidification can be incorporated into air conditioning equipment. Moisture is added to the air by releasing a spray or stream of water that is kept at a temperature that is lower than the dry bulb temperature of the surrounding air. When that air comes into contact with the cooler stream of moisture, small droplets of water in the stream evaporate as they adjust to the warmer temperature of the surrounding air.

The water from the evaporated stream is absorbed along with its moisture content into the air to increase humidity levels. The cooler temperature from the evaporated stream can also be incorporated into the air, which decreases the temperature of the air.

Cooling and humidification techniques do not work in hot, humid conditions. They can only be used in places with hot, dry climates.

An evaporative cooler (also known as a desert cooler) is a type of cooling and humidification device that operates on the same general principle as a conventional air conditioning and humidification system, but it is simpler and less expensive. For more information, see Evaporative cooling.

[edit] Heating and humidification

The process of humidification can be incorporated into heating systems and then activated when this equipment is being used to make the air warmer. This form of humidification increases the dry bulb temperature of the air as well as the humidity. Moisture is added to the air by releasing steam kept at a temperature that is higher than the dry bulb temperature of the surrounding air. When the surrounding air mixes with the warm steam, the moisture droplets in the steam evaporate as they adjust to the temperature of the surrounding air.

This process causes the relative humidity level and the dry bulb temperature of the air to increase. The wet bulb temperature and dew point temperature increase as well.

[edit] Humidification in homes

In a typical house with modern standards of insulation and heating, the average level of humidity in the winter months can be as low as 15% (as opposed to the recommended level range from 40% to 60%). This is why it is not uncommon for residents to increase the level of humidification in the home by using a humidifier.

One common type of home appliance is a cool air humidifier. This device incorporates a fan that blows air through a wet permeable membrane. It incorporates a reservoir for water and an electric motor that drives a fan. In addition, there is usually a speed selector switch, a timer, an air filter and a broad belt of permeable material to soak up the water. A separate motor moves the moist permeable belt through the airflow from the fan.

When the appliance is working, moisture is absorbed by the air as it passes through the belt and out of the appliance. This increases the humidity of the air in the room and reduces its temperature.

[edit] Related articles on Designing Buildings

• Air conditioning.
• Condensation.
• Cooling systems for buildings.
• Dehumidification.
• Dry-bulb temperature.
• Evaporative cooling.
• Heating ventilation and air conditioning HVAC.
• Humidity.