Of the indoor environmental conditions that affect human comfort and equipment performance (temperature, cleanliness, air movement, and relative humidity) relative humidity is often paid the least amount of attention. All of us will recognize and react more quickly to temperature changes, odors or heavy dust in the air, drafts, or radiant heat from sunlight or a radiator, than we will to a change in relative humidity. However, as relative humidity interrelates with temperature and others of these variables, it becomes a vital ingredient in total environmental control. To understand why, let's first define our terms and examine the interrelationship between temperature and relative humidity.   What is "humidity"? Humidity is the water vapour or moisture that is constantly present in the air. The level of humidity can be taken as an absolute measure: the amount of water vapour in a unit of air, but a more meaningful measurement is relative humidity. Relative humidity (RH) is the amount of water vapour in the air compared to the total amount the air can hold at a given temperature.   Air temperature and humidity The amount of water vapour air can hold depends on the temperature of the air. Warm air has the capacity to hold more moisture than cold air. For example, air at a temperature of 70ºF can hold more than ten times the water vapour of the same volume of air at 10ºF.   Humidity and human comfort Conditions and circumstances vary, but generally speaking, relative humidity maintained between 35% and 55% seems most conducive to human comfort. When air is dry, moisture evaporates more readily from the skin and produces a feeling of chilliness even with the temperature at 75ºF or more. As we mentioned earlier, human perception of relative humidity is the least evident of the variables that make up our environment. However, changes in RH are often sensed as changes in temperature. Drier air feels cooler than more humid air at the same temperature.   This universal human tendency to wrongly sense a condition of relative humidity as a condition of temperature reinforces the benefits of controlled relative humidity. For example, you can achieve a comfortable condition during the heating season at lower temperatures in a humidified building than in a building where RH is not controlled. The savings in heating costs can be significant over the course of just a single heating season.   Evaporative cooling and hygroscopic materials Changing RH can cause temperatures to change. As moisture in the air evaporates, it removes heat from the air, thereby cooling it. This is the principle of evaporative cooling. The moisture can come from people or from wood, paper, textiles and other hygroscopic materials (materials that can absorb and release moisture) in the building. Conversely, if hygroscopic materials absorb moisture from humid air, the vapour condenses as it is absorbed and gives off heat to the air, thereby warming it.   The amount of moisture that hygroscopic materials absorb or give off depends on the relative humidity of the surrounding air. No single level of relative humidity provides adequate moisture content in all hygroscopic materials. Moisture content requirements vary greatly from one material to the next.   Humidity and static electricity Static electricity is being generated around us at all times, caused by the movement of materials of high electrical resistance against one another. Typical examples are people walking across carpeted floors, paper going through a printing press, or sheets coming off a flatwork ironer. Only when atmospheric conditions allow static electricity to accumulate does it become a problem. People may receive unpleasant shocks; paper, films and fabrics are repelled or attracted to each other or to processing machinery; the components inside computers can become damaged. Sparks from static electricity are extremely dangerous in the presence of gases, volatile liquids, or explosive dusts such as is found in munitions plants, paint spray booths, rubber cement plants, flour mills and other places where volatile chemicals are used or stored.   How relative humidity deters the accumulation of static electricity charges For most environments, maintaining sufficiently high RH levels provides the most effective and economical means of preventing the build-up of static charges. When RH is sufficiently high, an invisible film of moisture forms on surfaces in the room. The presence of normal impurities makes this film a conductor that carries static electricity harmlessly to the ground before it can build up to a spark-producing potential. A relative humidity of at least 45% is needed to reduce or prevent the accumulation of static charges, although some materials such as wool and certain synthetic fabrics may require higher RH levels. Similarly, heat-generating machines may require higher relative humidities to provide sufficient moisture in proximity to the machine to dissipate static charges. How to humidify If you are humidifying a hospital operating room, obviously your design criteria are different than those for humidifying a textile mill, an office building, or even a laboratory. Different types of operations have substantially different requirements for the achievement of proper relative humidity. These requirements determine what means of humidification you should use.   Three main humidifying methods are available: steam, evaporative pan and water spray. Each has particular advantages and limitations which determine its suitability for a particular application.   Steam is ready-made water vapour that needs only to be mixed with the air. With evaporative pan humidification, air flows across the surface of heated water in the pan and absorbs the water vapour. Both steam and evaporative pan humidification do not affect the temperature of the humidified air. Water spray humidification disperses water as a fine mist into the air stream where it evaporates. As it evaporates it draws heat from the air and cools it. Recommended applications: Steam - Recommended for virtually all commercial, institutional and industrial applications. Where steam is not readily available, self-contained steam generating units or central system steam humidifiers are most effective and economical. Evaporative Pan - Recommended only as an alternative to self-contained steam generating unit humidifiers for small load commercial or institutional applications. Water Spray - Recommended for industrial applications where evaporative cooling is required. It is typically employed in textile mills in the southern U.S. during summertime. When properly specified and installed, humidification systems offer great benefits for the safe operation of machinery and equipment, the optimization of employee comfort and large potential savings in heating and cooling costs.   Source: The Armstrong Humidification Handbook     back to PilotLight online