This note is intended to assist users, decision makers and consultants, in addressing some of the more common questions and concerns related to use of dehumidification:
Dehumidification & Drying
Section – A ( About Dehumidification )
01. What is Relative Humidity ?
Atmospheric air is a mixture of various gases and water vapour. The invisible water vapour in the air is called humidity.
The amount of water that can be contained in a pound of air depends upon ......
1. Temperature of the air
2. Pressure of the air
At a constant pressure, the warmer the air the more water vapour it can retain. If air at a certain temperature has absorbed all the moisture it can hold at that temperature, it is said to be saturated.
The amount of water present, or the degree of saturation is evaluated in terms of Relative Humidity or Saturation ratio.
Hence, Relative Humidity is the actual water content in % of that of a saturated volume.
When air is 50% saturated, it contains only one half the amount of water than it can contain at the same temperature and pressure. As the relative humidity approaches 100%, the air can take on less and less moisture and at 100% relative humidity, that air cannot hold more water.
If air is saturated with water vapour at a given temperature, a drop in the temperature will lead to condensation of water in the form of droplets. The temperature at which moisture condenses out is the dewpoint temperature.
02. How do you measure Relative Humidity?
Relative Humidity is determined by means of wet bulb and dry bulb thermometers. The dry bulb temperature is the temperature of air as determined by a standard thermometer. The wet bulb temperature is determined by tying a wet wick over the bulb dipped in a reservoir containing distilled water. Airflow around the wick causes the evaporation of moisture thus lowering the temperature and producing a reading lower than that on the dry bulb thermometer.
The comparison between the two readings gives us the measurement of water vapour in the air.
1. The lesser the difference - the wetter the air.
2. The greater the difference - The dryer the air.
The readings can be plotted on a chart known as the psychrometric chart from where the properties of air vapour mixture like relative humidity, absolute humidity, dewpoint can be directly determined.
03. How Humidity Effects Industry ?
Humidity is a constant threat to production efficiency and product quality.
The damage which can be caused by excessive relative humidity are principally:-
- corrosion of steel and metals
- deteriorated characteristics of hygroscopic material
- increased harmful activity of micro organisms.
04. What are the typical methods of dehumidification?
Principally there are three methods of dehumidification:-
- Over compression
- Refrigeration Dehumidification
- Sorption Dehumidification
05. How can Compression be used to Dry Air?
Air compression suggests a method of reducing moisture content in air. As air is compressed, partial pressure of the water vapour in the water gas mixture rises to the point where moisture can be condensed from the air at a higher temperature. This approach is some times quite practical for very small volumes of air but the cost of compressed air equipment, BHP requirement and the amount of cooling water required for after-cooling makes it very impractical for large volumes of air.
06. How can Refrigeration be used to Dry Air?
Another method most commonly employed is the reduction of moisture in the air by means of reducing the temperature. By examination of the dew point alone or saturation curve on the psychrometric chart. It can readily be seen that as the temperature of the air is lowered, the amount of moisture it can hold is reduced considerably. Thus by cooling the air below the dew point, the moisture contained in that air can be condensed out and some of the moisture vapour removed in liquid form, but cooling to very low temperature makes the refrigeration process impractical, as it requires a great deal of subsequent re-heating. The reduction in air temperature is also limited by the freezing point of water condensing on the cooling coil, which in some designs is offset by complicated brine spray and liquid lithium chloride type systems available using a combination of refrigeration and adsorbent liquid. These are very bulky and involve complicated control systems for the proper maintenance of solution density.
07. How can Sorbents be used to Dry Air?
Sorbents are solid or liquid materials which have the property of extracting and holding other substances (usually water vapour) brought into contact with them.
Sorbents can be classified into two general categories.
Absorbent : A sorbent which changes either physically, chemically or both during the sorption process.
Lithium Chloride is an example of solid absorbent. When water is absorbed on this material it changes to hydrated state.
In liquid sorption dehumidification system, the air is passed through sprays of a liquid sorbent such as lithium chloride or glycol solution. The sorbent in an active state has a vapour pressure below that of the air to be dehumidified and absorbs moisture from the air stream. The sorbent solution during the process of absorption becomes diluted with moisture which during regeneration is given up to an air stream in which the solution is heated.
Typically absorbent used is lithium chloride in either liquid form or as solid crystals in a honey-comb shell.
Adsorbent : A sorbent which does not change physically or chemically during the sorption process. Adsorbents are normally granular beads or solids with porous structures making it capable of holding large amounts of water on their surface.
The principle behind desiccant dehumidification is that the desiccant is exposed to moisture laden air, from where it extract the moisture. The saturated desiccant is heated, which drives off the collected moisture into the exhaust air stream. The regenerated desiccant is ready for use again. Thus a continuous cycle of sorption and regeneration can be set up giving very low dew points. Typical absorbents used are Silica gel, Molecular Sieve and Activated Alumina.
Section – B ( Equipment Related Questions )
01. How does a Dehumidifier work?
The new range of Bry-Air Dehumidifiers are fluted media based and remove moisture through a process of continuous "physical adsorption".
The moisture is adsorbed in the dehumidification sector by the fluted, metal silicate desiccant synthesized rotor and is exhausted in the reactivation sector by a stream of hot air in the counter flow. Following the reactivation process, the adsorption sector is again ready to adsorb the moisture. Thus, the two processes of "moisture adsorption" and "reactivation" are taking place with separate airflows continuously and simultaneously. Positive sealing between chambers prevents mixing of the process and reactivation air streams.
02. What is the range and capability of Desiccant Dehumidifiers and to what level can relative humidity (RH) be controlled by them ?
A. Bry-Air dehumidifiers incorporating desiccant synthesized rotor are available in a range of 170 cmh (100 cfm) to 40,000 cmh (24,000 cfm) as standard and engineered/packaged systems
B. As the dehumidifiers use desiccants, they work independent of apparatus dew point (ADP), and can continuously maintain relative humidity as low as 1%, even at subzero temperatures.
03. Are the humidity levels, being specified, not easily achievable with regular air conditioning or refrigeration?
Two items are significant in deciding the choice of the final equipment :
i) The amount of latent (moisture) load in relation to the total load.
ii) The relative humidity specified to be maintained
a) Generally, if the relative humidity is to be maintained between 50-60%, it is more economical and preferred to employ standard air conditioning. However, if the latent load, even at this RH levels, is predominant or significant, e.g. Supermarkets, large fresh air equipments, it is recommended and preferred to divide the sensible load on the refrigeration equipment and latent load on desiccant dehumidification equipment.
b) For relative humidity to be maintained below 45 to 50%, desiccant dehumidifiers offer the most economical, efficient and simplest method of humidity control.
c) Often humidity levels are not correctly specified as the user is not aware as to what RH levels and moisture will have ill-effect on its products and processes.
04. What are the limitations in controlling RH with central air-conditioning plants/window air conditioners?
In tropical areas like India, with normal central air conditioning plants, lowest relative humidity of 50%-60%, at best, are attainable economically, at room temperatures between 20°C to 24°C. This again is only possible with special design of the cooling coil and appropriate selection of the apparatus dew point and provision of sufficient reheat.
With window air-conditioners or split air-conditioning units, in the absence of specially designed cooling coils and reheat provision, typical relative humidity of 60-70% are commonly experienced at times of high outside moisture ambient conditions like monsoon.
Removal of large water vapour/moisture through cooling requires after cooling to apparatus dew point close to freezing and substantial subsequent reheating, thus requiring both over sizing of the cooling equipment, as also increasing the operating cost with higher BHP per ton consumed, and reheat energy.
05. How is the dehumidification equipment sized?
To size the equipment, it is first necessary to correctly estimate the moisture load. At low humidity, water vapour will permeate at very significant rate and large quantity based on vapour pressure difference. As ASHRAE has not laid out moisture estimation procedures for low humidity requirements, most air conditioning engineers are not fully equipped to correctly estimate the moisture load. Estimation of the moisture load procedure is explained in a very easy to understand manner in the Bry-Air design and engineering data manual.
Once the moisture load has been correctly estimated the sizing of the dehumidifier is a relatively simpler task. Please see Bry-Air engineering data manual or call our nearest office for support.
06. Is there any quick thumb rule applicable for determining the equipment selection ?
No, the moisture load in a particular area is directly influenced by :
a) The volume of the room
b) Outside absolute humidity
c) Inside absolute humidity to be maintained
d) The vapour pressure resulting from the difference between b) and c) above
e) The rate of migration of vapour resulting from d) above i.e. vapour pressure difference
f) Conveyor openings, if any
g) Door size and door openings per hour
h) Number of people and their activity level
i) The fresh air quantity
All of these can significantly change the dehumidifier equipment selection for the same size room.
07. Will the relative humidity being maintained inside an environment, with the help of dehumidifier, vary from season to season, depending upon the ambient temperature?
What are the controls used to maintain the desired RH, when there is a wide variation in ambient temperatures and RH?
Like any refrigeration plant the load on the dehumidifier system varies due to ambient conditions. The temperature control is maintained by a thermostat. This RH control is achieved with the help of humidistat or a proportionate RH controller. There are several control option for optimizing dehumidifier equipment energy/performance by one or more of many methods like proportionate reactivation energy, switching on and off the reactivation energy, switching off the dehumidifier, switching off the rotary bed and the reactivation energy and keeping the air flows on, etc.
08. Are higher size ducts required because the latent heat has been removed from the supply air?
This is not necessary as duct size is calculated on the basis of volume of air (measured in cfm). Any marginal increase in cfm can be absorbed in the standard higher sized ducts normally adopted.
09. Are there any specific recommendations about building materials and paints to be used internally to reduce humidity ingress, particularly in areas where low humidity is to be maintained?
To prepare any space for humidity control, the choice of vapour barrier is based on the degree of dryness or RH.
Commercial vapour barriers - moisture resistant construction material, paints and other coating offer a variety of design alternatives. Manufactures of vapour barrier materials can supply specific information on their products.
Between 40 - 50% RH, special vapour barrier paints will provide an economical reduction in moisture ingress. Below 35% RH, it is recommended to prepare the conditioned space properly with good vapour barrier treatment including polythene etc. as an inside envelope for reducing the moisture load which has a significant effect on the sizing of the equipment.
10. Is the equipment fire safe, specially where inflammable or ignitable material is stored?
Dehumidifier equipment can be easily supplied with flameproof motors and controls to make it suitable for hazardous areas and flame proof/explosive proof applications. The Bry-Air Dehumidifier incorporates inflammable, ECODRY metal silicate desiccant synthesized rotor.
11. Are there any references/commendations from users regarding the satisfactory usage of dehumidifiers?
Yes, references from various users of Bry-Air dehumidifiers are available.
The fact that we have tens of thousands of Bry-Air Dehumidifiers operating around the world and we are producing this product in India and Malaysia in addition to USA and Brazil. The export of the same to Middle East, various countries in American continent and Europe, South East Asia, including Taiwan, Hong Kong, Thailand, Indonesia, Singapore etc., China, African continent including South Africa and West Asia, serves as an adequate testimony of both the quality and capability of Bry-Air Dehumidifiers.
12. What are the common maintenance problems with dehumidifier? Are the spares and consumables available easily?
Bry-Air dehumidifiers are easy to maintain and do not require special skills either to operate or to maintain them. Elaborate maintenance manuals provide easy to follow instructions for installation, start-up and also a comprehensive trouble shooting guide.
Consumables and spares are easily available through any of the Representative, Regional or Branch offices located in all major cities in India and abroad.
Bry-Air also offers training programmes for service technicians, maintenance staff of user organisations, from time to time, to familiarise them with the operation and maintenance of the dehumidifier.
Section – C ---Application Related Questions
01. What are the common problems caused by moisture ?
Rust, mold, mildew, rot, decay, warping, stretching, lumping, caking, agglomeration and decomposition are all common enough problems encountered in our daily life. It is interesting to note that the root cause of all these problems and many more, is the presence of moisture or humidity in the air.
The storage, manufacture and transportation of material often takes place in a humid environment which is not suited to the moisture sensitivity of the material, leading to deterioration of stored material, machinery, equipment and reduced product appeal.
Humidity makes for mushy transfers in the printing industry and irregular operation of packaging machines.
Seeds lose their germinating potential and dry fertilizers agglomerate. In pumping stations and high voltage rooms, expensive installations may rust as a result of unchecked condensation.
02. What are the problems caused by moisture in the Pharmaceutical Industry?
In the Pharmaceutical Industry, chemists have common problems of decomposition and difficulty in compression of tablets leading to breaking of tablets; lumping and caking of dry powders, improper adhesion under pressure of tablets, improper drying of gelatine capsules.
The presence of high humidity in the air is the cause of all these problems.
The rapid advance in the discovery, manufacture and use of modern drugs has focussed attention on an infinite number of moisture control problems. Too much moisture in the air may retard the growth of certain organic cultures. At the very least, shelf life is particularly shortened by the presence of humidity during the manufacturing process.
03. What are the problems caused by moisture in the Food Industry?
Potato chips, dry breakfast cereals and soda crackers exhibit an affinity for water when exposed to high humid conditions and will become soggy and unappetising.
In processing of powdery foods such as cocoas, gelatines, the sticking or lumping of powder due to high humidity conditions prevents its flow in the manufacturing process.
04. What are the problems caused by moisture in the Electronic Industry?
In the electronic industry, wirings on PCBs gets corroded or short circuited due to presence of high humidity. Transistors may break down or suffer a decrease in longevity and for wafer production facilities the uniform growth of crystals is unachievable.
05. When packing products, we have a problem with water condensing in the package. Is there a simple way to avoid this?
This is common when packing cold products in a warmer environment, where the relative humidity, i.e. moisture content, is too high. This causes the moisture in the air to condense on the product's cold surface.
1. Make sure the dew point of the air surrounding the product is below surface temperature of the product by enveloping the packaging equipment and dehumidified air into the envelope or shroud.
06. When transporting powders, we have persistent problems like sticking and caking. Why?
Powders are generally very hygroscopic, meaning they absorb moisture easily. A preferred solution is to lower the relative humidity in areas where the powder is processed and also if conveying powders with air, dry the air to avoid absorption of moisture by powder during conveying.
07. Will a Dehumidifier prevent mould growth?
Yes. Mould growth appears on continually damp areas. A dehumidifier will remove the moisture that is the root cause.
08. What is Corrosion?
Corrosion is defined as destruction of a metal or alloy by chemical or electrochemical reaction with its environment. In most instances, the reaction is electrochemical in nature: a flow of electricity between certain areas of a metal surface through a solution capable of conducting an electric current. This electrochemical action causes destructive alteration (eating away) of a metal at areas.
Though corrosion is a complex function of many factors, the three most important are-
- a voltage differential between pure and impure areas
- physical conditions of temperature and humidity
- and oxygen in the air.
Higher humidities may lead to higher condensation of water on the metal surfaces. The concentration of molecules of water vapour increases with increasing RH. This molecular thickness of the layers of water eventually permits ionic conduction which accelerates the rate of corrosion.
With iron or steel, the ferrous ion may react with hydroxyl ion in water to form ferrous hydroxide and with oxygen to produce ferric hydroxide (rust).
The rate of corrosion is faster where surfaces are exposed to polluted air in combination with high relative humidity. Industrial pollutants like sulphur dioxide enhance the corrosion rate.
Corrosion or rusting of stored material can be seen in every Industry-Defence, Marine, Precision parts etc.
Air & Gas Purification
Resin Drying & Conveying