Handbook of air conditioning and refrigeration / Shan K

4.24 CHAPTER FOUR
levels are as follows:
Process Air Conditioning Systems
Relative humidity, %
Summer 30–65
Winter
Commercial and public buildings 20–60
Health care buildings 30–60
Humidity affects the physical properties of many materials and, therefore, their manufacturing
processes.
Moisture Content. Relative humidity has a marked influence on the moisture content of hygroscopic
materials such as natural textile fibers, paper, wood, leather, and foodstuffs. Moisture content
affects the weight of the products and sometimes their strength, appearance, and quality.
Dimensional Variation. Hygroscopic materials often extend at higher relative humidity and contract
at lower humidity. A 2 percent increase in moisture content may result in a 0.2 percent increase in
dimension of paper. That is why lithographic printing requires a relative humidity of 45 � 2 percent.
Corrosion and Rust. Corrosion is an electrochemical process. Moisture encourages the formation
of electrolytes and therefore the corrosion process. A relative humidity greater than 50 percent may
affect the smooth operation of bearings in precision instruments. When indoor relative humidity
exceeds 70 percent, rust may be visible on the surface of the machinery and on parts made of steel
and iron.
Static Electricity. Static electricity may cause minute particles to repel or attract one another,
which is detrimental to many manufacturing processes. Static electricity charges minute dust particles,
in the air, causing them to cling to equipment and work surfaces. Static electricity exists in an
indoor environment at normal air temperatures when relative humidity is less than about 40 percent.
Loss of Water. Vegetables and fruits lose water vapor through evaporation from their surfaces
during storage. Low temperatures and high relative humidities, such as � � 90 to 98 percent, may
reduce water loss and delay desiccation.
It is important to specify the exact relative humidity required for product quality and cost
control. For process air conditioning systems, the specified relative humidity is either a year-round
single value or a range. A strict relative-humidity requirement always includes a basic value and a
tolerance, such as the relative humidity for lithographic printing mentioned before. When temperature
and relative humidity controls are both required, they should be specified as a combination.
Consider this example:
Temperature, °F (°C) Relative humidity, %
Clean room 72 � 2 (22.2 � 1.1) 45 � 5
Case Study 4.1. A factory workshop has the following environmental parameters during summer:
Indoor air temperature 79°F (26.1°C)
Indoor air relative humidity 50 percent