What is Relative Humidity :-
Relative humidity (RH) is the percentage of water vapour present in the air relative to the amount that would be present in the equilibrium state.
It is the above process that causes dry air in buildings. As cold incoming air is heated, its relative humidity value drops. Therefore moisture must be added to attain an acceptable level of humidity within the building.
The equilibrium point is temperature dependent. At higher temperatures the equilibrium occurs with more water vapor. If the container above was heated to 86ºF (30ºC) the water and water vapor would no longer be in equilibrium. The relative humidity of the vapor right after increasing the temperature would be 57%. This means that immediately after heating there are 57% as many water vapor molecules as would be present in the equilibrium state. Figure 2: Relative Humidity after Heating It is the above process that causes dry air in buildings. As cold incoming air is heated, its relative humidity value drops. Therefore moisture must be added to attain an acceptable level of humidity within the building.
The reasons for humidifying dry air vary from one building to another and from one geographic area to another, however there are three fundamental reasons.
These are:
- Static Electricity
- Poor Moisture Stability
- Health and Comfort Static
Electricity Static electricity is a condition caused by stationary charges of electricity and is a major problem in most unhumidified areas. Since static electricity is caused by friction, particularly when the elements in friction are dry, the problem increases proportionately with the speed of production machinery. Without sufficient humidification, high-speed machinery might well defeat its own purpose. Reduced efficiency is frequently the result of static electricity.
- In the Printing industry presses must self-feed paper evenly, one sheet at a time at very high speeds. When the static electricity causes sheets of paper to stick together, the paper bunches, the feeding becomes uneven, and eventually the paper jams the presses.
- In the Textile industry static electricity causes the yarns to adhere to each other, the shuttles miss threads and improper weaving patterns result.
- In offices, static electricity can disrupt operations and increase operating costs. In many photocopiers, sheets of paper stick together and jam the machine, wasting time and paper. Severely jammed equipment may even require service calls.
- In Computer rooms and data processing areas, the lack of humidity results in static electricity that causes problems such as circuit board failure, dust buildup on heads, and storage tape breakage.
- Static electricity can also be dangerous. Sparks caused by static are extremely hazardous in locations such as hospital operating rooms where flammable gases are present. Many flash fires – even explosions - are caused by static electricity.
- Controlling Static Electricity - Maintaining relative humidity above 35% is one important measure that can be taken to reduce static electricity.
One of the easiest and most common methods of minimizing static electricity is to increase the relative humidity level. Electrostatic charges do not dissipate through moist air, but through a moisture film that is absorbed on the charged surfaces. This moisture film decreases the surface resistance and causes static charges to be drained. This effect is most pronounced at RH above 30-35% and it also corresponds with a decrease in ozone production (a by-product of electrostatic discharge). Static electricity is a problem that should be of primary concern to any manufacturing plant interested in running efficiently and accurately.
2.Moisture Stability :-
When air is heated the relative humidity will decrease. When this occurs the rate at which water molecules leave objects containing water or the rate at which water evaporates is increased. All hygroscopic or fibrous materials either lose of gain moisture in direct relation to the relative humidity of the surrounding air.
Moisture stability is the ability of a material to maintain a level of moisture content despite fluctuations in the humidity of the environment. Many materials give off, or take on moisture rapidly which can result in serious damage to the material or the process in which it may be involved. The drying out of a material can result in product deterioration, while conversely, a dry material can also suffer damaging side effects of moisture regain. In many cases, product deterioration is directly related to the lack of moisture stability
Below Table gives the hygroscopic regain of some common hygroscopic materials. Hygroscopic regain is defined as the amount of water a completely dry material will absorb from the air. It is expressed as a percent of the dry weight. (For example the weight of completely dry timber will increase by 9.3% if it is stored at an RH of 50%)
- Products such as vegetable, cut flowers, fruit and many grocery items cannot be brought back to original quality once they have lost their moisture. By installing an efficient humidification system this costly loss of products can be avoided. Many food processors humidify their plant and storage areas and are able to store fruits and vegetables for months without any loss of product quality or weigh.
- For any product that requires a certain percentage of moisture to maintain its quality, loss of that moisture reduces its valve. Some products can be brought back to their original condition by returning the moisture to them. However, among those that cannot reabsorb moisture to regain their lost quality are fruit and vegetable products, paintings and art objects.
- Deterioration caused by loss of moisture is also a problem for treasures such as antiques, rare books, and works of art, all of which are susceptible to damage caused by moisture loss. It causes antiques, paintings, paper and book bindings to crack, warp and deteriorate. Fortunately, most libraries and museums are well aware of the need for controlled humidity to protect their collections. They know that proper humidity control is a very inexpensive preventive measure that will avoid costly and often impossible restorations.
- A specific moisture content in materials is essential to the quality of products produced by a wide range of manufacturers of hygroscopic or fibrous materials. Wood, paper and textiles are examples of materials particularly affected by changes in content. If these materials have a correct moisture content when they arrive at a plant, and if they are used immediately, they will respond properly to the manufacturing process. But problems can be anticipated if the materials are stored in a dry atmosphere.
- Paper provides a good example of the effects of dry air and the lack of moisture stability. When it is stored under dry atmospheric conditions, moisture from the outer layers and edges of the stacks escapes into the air. The moisture loss is much more rapid from the outer edges than from the center of the stacks. The result is not only curled stock, but also uneven moisture content, which creates printing and processing problems.
- If moisture stability in the surrounding atmosphere is the answer to a manufacturing operation, then complete humidification of the plant and storage areas is an absolute necessity. Humidification is the best and least expensive way of maintaining moisture stability. If the air surrounding the material is maintained at a proper and constant relative humidity level, so that no moisture is emitted or absorbed by the materials, then the products will remain stable in both moisture content and dimension.
- Ideally, humidification equipment should be installed in raw material storage areas, manufacturing facilities, and finished goods’ storage rooms, for full control of the product moisture content.
During the heating season, inside air dries to the point where the humidity is substantially lower or comparable to that of the Sahara Desert. The effect on people is to dry out nasal and throat membranes. For employees this means more susceptibility to colds and virus infections. The subsequent increased absenteeism proves costly for any employers. Another aspect of comfort is the fact that humidity in the air makes a room feel warmer, so there will be fewer requests to have the thermostat turned up.
Most employers provide air conditioning for employee comfort and productivity during the hot days of summer. Adding humidification for full winter comfort and productivity is just as important as air conditioning in the summer months. In fact, it is one of the most important functions of the complete air conditioning or “total comfort” system.
- The advantage of conditioning the interior space of a building to increase productivity and reduce the downtime of machinery has been documented many times. Unfortunately it is usually equipment, such as computers and communications systems, that is placed in separate climate controlled rooms, while the majority of employees have temperature control only.
- Temperature control must be combined with humidity control to maintain proper comfort parameters in an office environment. More than 75% of all I.A.Q. problems start with a comfort complaint. If this is not rectified, the employees will continue to complain and become less productive.
- Temperature control alone does not take into account the physiological aspects of the employees. As demonstrated in Figure 3, indoor RH variations above and below the 40-60% range have a dramatic effect on the comfort and well being of employees. Humidity conditions above this range are usually controlled easily by the normal dehumidification process of the air conditioning system. However, as the cold, dry weather of winter approaches or in arid climates, the indoor RH can easily drop well below the recommended 40% parameter. It is not uncommon to find relative humidities in the 10-15% range in most offices during this period. This low RH creates comfort, productivity, and absenteeism problems costing immeasurable dollars to employers worldwide. Studies conducted by Dr. George Green of the University of Saskatchewan indicates that increasing the indoor RH from 20 to 30% will reduce absenteeism by 15%. This, along with the productivity increase that can be gained from additional comfort result in a real economic benefit from general office humidification.
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