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This article offers a comprehensive overview of air dryers.
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An air dryer is a system or piece of equipment that is used to remove moisture present in the air, particularly compressed air. Ambient air typically has a relative humidity of around 30 to 50%. Compressing air packs higher quantities of moisture in a small volume. This increases the relative humidity to 100% while the excess moisture that cannot be held condenses.
Air with high moisture content is detrimental to plant operation and reliability downstream equipment as it can cause process stream contamination, premature failure and wearing of instrument parts, equipment corrosion, and so on.
Compressing air is not the only process that requires moisture removal. Some examples of other industrial and manufacturing processes that facilitate water removal are dewatering, baking, industrial drying, dehydration of foodstuffs, steam heating, and cleaning. These processes need different types of dryers which are mainly used for removing moisture from a product or mass to make it dry, solid, and pure. The dryers used for these applications operate through the basic principles of heat transfer which are conduction, convection, and radiation. Air drying differs in that it removes moisture through refrigeration, adsorption, absorption, diffusion, and filtration.
The dew point temperature is the temperature at which the air becomes saturated with water vapor. At this temperature, water vapor from air begins to condense and precipitate on surfaces of objects such as walls of a pipe, the internals of a tank, or surfaces of dry products.
Relative humidity, on the other hand, is the ratio or percentage of the current concentration of water vapor in the air in contrast with the maximum concentration. This value is dependent on temperature. At 100% relative humidity, additional moisture cannot be held since the air is already saturated. The temperature at 100% relative humidity is also the dew point temperature.
Relative humidity can be manipulated by increasing or decreasing the temperature. However, the amount of water vapor and the dew point temperature is still the same.
The dew point temperature is the more commonly used term to characterize air drying since it relates directly to moisture content. A high dew point temperature means the air is wetter and it is easier for water to condense. Take for example a certain drying unit that has a dew point temperature of say, 3°C (or 37°F). This means that the drying unit removes moisture from the air such that no condensation happens when the temperature of the ambient air is higher than 3°C.
It is important to note that pressure affects the dew point temperature. Higher pressure means a higher dew point. The term used for describing the dew point temperature at states with changing pressure (not atmospheric) is the pressure dew point.
An air dryer is an essential component of compressed air systems. Moisture and condensation are an inevitable byproduct of air compression. Water can condense and accumulate within the compressor unit and also to the equipment and processes downstream. Below are some of the advantages of using air dryers.
Water contamination is a serious problem in industries that use high purity compressed air such as laser cutting and welding, plasma generation, microelectronics production, food and pharmaceutical products manufacturing, shot blasting, painting, and coating. Water, along with other contaminants, can create various effects depending on how the compressed air is used. An example is laser cutting which uses air to cool the resonator—a component that generates high-intensity light beams. When water-contaminated air is used, cooling efficiency is decreased which results in overheating and loss of energy.
Usually, compressed air is fully saturated with water. Decreasing its temperature or further compression and pressurization can cause the diffused water vapor to precipitate. When compressed air at this state is supplied to downstream equipment, there is a risk of water buildup in small cavities or depressions within the equipment. Water buildup can affect the operation of sensitive equipment such as measuring and monitoring devices.
The presence of water into equipment internals can cause corrosion to steel surfaces. Pipe, tanks, vessels, drums, and mixing equipment internals can accumulate water from the condensation of saturated air. This can promote corrosion to the internal surfaces of equipment which may lead to product or process stream contamination.
Precipitated water in compressed air systems can freeze. They can jam moving components of pneumatic actuators of valves and measuring devices. Freezing of accumulated water on process lines can disrupt product or process fluid flow.
Air-powered tools and equipment use energy from compressed air for driving air motors or turbines. Examples of these are pneumatic grinders, drills, jackhammers, etc. Water can develop fouling on the internals of these devices which decreases the power delivered by the air motor.
Compressed air is used in the food and beverage industry for product mixing and conveying. Water from a compressed air system can carry microbes that can result in food contamination and spoilage. Pharmaceutical manufacturing plants require air with more stringent qualities as small amounts of impurities can potentially ruin a whole batch of products.
As mentioned earlier, air dryers remove moisture through refrigeration, adsorption, absorption, diffusion, and filtration. This chapter tackles each type of air dryer and its principle of operation.
Refrigerant dryers are types of dryers that work by cooling the stream of air to a temperature low enough to condense water vapor. This temperature is usually at or below the dew point temperature. Compressed air is normally at a saturated state which means it is at its maximum capacity for holding moisture. At this state, it also has a temperature the same as or above ambient. Since air with a higher temperature can hold more water than cold air, cooling removes water by decreasing the water holding capacity of the air. This forces the excess water vapor to precipitate or condense.
Refrigerant air dryers are suitable for high-capacity applications. However, they are not as effective as other air dryers in removing moisture. Refrigerant dryers can achieve dew point temperatures of 2 to 3°C (35 to 37°F).
Refrigerant dryers can be divided into two systems: the air circuit and the refrigeration circuit.
The air circuit is the system that removes water vapor from the air. Its process involves the following operations:
The three main components of the air side process are the air-to-air heat exchanger, air-to-refrigerant heat exchanger, and condensed water drain. The air-to-air heat exchanger is a heat recovery unit that is used to transfer heat from the incoming hot air to the outgoing cold air. This heat recovery process increases the efficiency of the dryer by lowering the cooling load of the refrigeration cycle and eliminating the need for heating elements for elevating the temperature and decreasing the relative humidity of the outgoing air.
The air-to-refrigerant heat exchanger, or evaporator, is the main cooling unit that lowers the temperature of the air at or below its dew point. The lower the temperature, the more moisture is removed. The typical target temperature is about 3°C or 37°F. This temperature is low enough to remove most of the moisture present in the air but high enough to prevent any precipitates from freezing. Freezing poses a problem by disrupting the flow of air through the unit and by decreasing the amount of heat transferred through the cooling coils. This can be solved by adding auxiliary components such as an electrical heat tracing unit.
At the bottom side of the air side unit is a boot, drain, or water separator where the condensed water is accumulated and removed from the system by an automatically actuated drain valve.
The refrigeration circuit is the system that provides cooling to the dryer to create condensation. This system uses a working fluid called a refrigerant which is subjected to a continuous cycle of heat absorption (evaporation), compression, heat removal (condensation), and expansion. Air-to-refrigerant exchange happens during the heat absorption phase wherein the refrigerant passes on one side of the heat exchanger while air passes on the other. In this phase, the refrigerant is initially in its sub-cooled liquid form which is then evaporated by the heat transferred by the hot air.
Refrigerant dryers can be classified according to their type of evaporating unit and mode of operation. Below are the two main types of refrigerant dryers.
Direct expansion or DX refrigerant dryers are the most common types, which are composed of a basic refrigeration cycle wherein heat is directly transferred from air to the refrigerant. There are no intermediate components such as water lines or reservoirs. The refrigerants used are usually halocarbon-based compounds (Freon) and are running in a closed-circuit system. This makes them cheaper and more compact than the other types. DX refrigerant dryers usually operate continuously at a fixed speed despite varying load conditions. Thus, it becomes a less economical investment in the long run. However, cycling and variable speed DX refrigerant dryers are now becoming available which can shut down or reduce its compressor speed during low demand.
In this type, an intermediate media is used to absorb heat from the stream of hot air instead of a refrigerant. This intermediate media acts as a cold reservoir which is typically composed of a mixture of water and glycol, or other materials such as sand or clay. As the thermal mass gains heat from the hot air, another heat exchanger is used to absorb this energy. The other side of the heat exchanger is a DX-type refrigeration circuit or a cooling water supply from air chillers or cooling towers. Thermal mass refrigerant dryers are operated as cycling dryers with the advantage of being shut down when sufficient cooling from the thermal mass is attained. This results in cheaper operating costs that can offset the high investment required.
Desiccant dryers use hygroscopic materials to capture moisture from the air. These materials, known as desiccants, are dry, solid particles that work using the principle of adsorption. The surface of these materials has pores that act as sites for adsorbing certain molecules through intermolecular attractions. This ability of such materials is specifically known as physisorption. Common physisorption desiccants in the market are silica and activated alumina (molecular sieves). These desiccants are supplied in powdered, pelletized, or bead-like form taking advantage of a larger surface area in contact with the air.
Another type of desiccant works by means of chemical reactions. These types of materials, known as chemisorption desiccants, have a surface with a strong affinity towards water molecules. In contrast with physisorption desiccants which operate through intermolecular forces, chemisorption compounds bind with water molecules by creating new chemical bonds.
One of the most effective types of desiccants and widely used is calcium sulfate. It is safer than calcium oxide and reaches dew point temperatures of -40°C (-40°F) and dries air to -73° C or -100° F. Desiccant dryers are one of the most effective types of air dryers.
A desiccant dryer is made up of one or more vessels that contain the desiccant material. Inside the vessel are internal components such as a screen, tray, or bed for containing or holding the desiccant in place while allowing passage of air. Air is usually introduced at the bottom of the vessel and discharged at the top. As air is supplied and dried, the desiccant becomes saturated which diminishes its ability to capture moisture. Before this state is reached, a regeneration phase is done to regain its drying effectiveness.
Regeneration is the process of removing the captured water molecules at the surface of the desiccants through heating and purging. Desiccant dryers featuring continuous operation usually have at least two vessels. One is in the drying phase while the others are in the regeneration phase.
Regeneration is done through three different methods: pressure swing, heat of compression, and blower regeneration. The regeneration method used represents the type of the desiccant dryer.
This type of dryer removes adsorbed moisture from the desiccant by directing some of the discharged dry air to the vessel under regeneration. Since adsorption is an exothermic process, the dried air is at a temperature enough to purge moisture from the desiccant. This regeneration process does not need an external source of heat such as electric heaters or steam. This is known as the heatless dryer design. However, when the heat of adsorption is insufficient, external heating is employed.
In this type of dryer, instead of redirecting some of the dried air, air from the compressor discharge is used. Hot, wet air directly from the compressor is at an elevated temperature due to the heat of compression. In some designs, this heat is sufficient for regeneration without the need for external heating. This is the most energy-efficient regeneration method since there is no dry air loss and has a heatless (no external heating) design.
This type of dryer uses externally heated atmospheric air for regeneration. The dryer skid has an integrated blower that directs air through an electric or steam heating coil. This setup uses the maximum additional energy since it does not utilize or recover the heat generated from compression and adsorption processes.
Single tower desiccant dryers prevent moisture in pipelines by suppressing the pressure dew point by 20o F or more. The removal of water vapor is completed by the air in the pipeline passing through a bed of deliquescent desiccant in the dryer. This form of air dryer is the most versatile and economical type of air dryer on the market.
The lack of maintenance and the absence of moving parts makes a single tower desiccant air dryer a cost effective and long lasting choice for air drying. The construction of single tower desiccant air dryers makes them corrosion, chip, and crack resistant regardless of how demanding the surrounding environment is.
No power is needed to operate a single tower desiccant air dryer. A single tower desiccant air dryer only needs the absorbent desiccant loaded, which only needs to be done two or three times a year.
Single-tower desiccant air dryers consist of a single tower packed with desiccant material. Moist air from the environment is introduced at the bottom of the tower. It flows upward and dries as it passes through the bed of desiccant material with dry air leaving at the top of the tower with a significantly reduced dew point.
This type is commonly designed for point-of-use applications. The advantages of single tower desiccant air dryers include low initial and maintenance costs and low pressure drop. They can be installed outdoors and in corrosive and hazardous environments as well as have the ability to remove oil and solid particulates from the air.
In contrast with desiccant dryers that work through adsorption, deliquescent dryers utilize the principle of absorption. This type of dryer also uses a hygroscopic material that gradually dissolves as it absorbs moisture, hence the term deliquescent. Deliquescent dryers can reach dew points of -7°C (20°F).
Common drying media used for deliquescent dryers are salts such as sodium hydroxide, potassium hydroxide, and calcium chloride. The drying media are formulated under proprietary compositions by manufacturers.
The vessels used for deliquescent dryers operate similarly to that of desiccant dryers. The vessel has a bed of hygroscopic materials contained by a screen or tray. Hot, wet air is also supplied at the bottom of the vessel and discharged at the top. The main difference is the behavior of the drying media. Deliquescent drying media does not become saturated. Rather, it becomes consumed as the water dissolves and liquefies the material. The liquefied solution falls at the bottom of the vessel where it is collected and drained.
Because the drying media is consumed, the vessel cannot be regenerated. The drying media is replenished regularly to ensure reliable performance. Since there is no drying and regeneration phase shifting, no moving parts are needed for it to operate. It can operate passively in remote and hazardous locations where electrical supply is unavailable, unsafe, or impractical.
Membrane dryers remove moisture by passing moist air through tiny tubes or hollow fibers made from a semi-permeable material. These hollow fibers are bundled together in a canister with several openings for the compressed air supply, dry air discharge, and moisture exhaust. Membrane dryers can be as effective as desiccant dryers which can reach a dew point of -40°C (-40°F).
Moist air is introduced on one side of the membrane canister and discharged as dry air on the other end. The flow is driven by a pressure gradient between these two ends. As the moist air travels through the canister, water molecules permeate through the membrane wall of the fibers. A concentration gradient drives this semi-permeable action of the membrane. Water tends to migrate at a faster rate to the side with lower moisture concentration which is outside the hollow fibers.
Concentration gradient is only one type of membrane diffusion mechanism. Other mechanisms include diffusion through pores and molecular sieving.
To maintain the concentration gradient across the fiber walls, part of the dry air discharge is directed to the other side of the membrane wall. This stream of dry air discharge carries the water molecules that traveled across the membranes towards the exhaust of the canister.
Membrane dryers are useful in applications that require reliable and continuous operation without the need for automatic control or external intervention. These dryers do not need external power sources or control units to operate. They can also produce high-quality dry air because of their inherent efficiency in removing moisture along with other contaminants. However, membrane dryers have a limited capacity compared to other dryers. High-pressure loss is also a problem since the air must be forced across the bundled fibers.
Coalescing dryers function as filtering devices that trap tiny water droplets instead of water vapor dispersed in the stream of compressed air. These devices do not only capture water, but also other sub-microscopic contaminants such as oil and particulate matter. Because of this, they are more widely referred to as filters instead of dryers.
The working principle of coalescing dryers and filters lies on three mechanisms: diffusion, interception, and impaction. Diffusion is the ability of sub-microscopic particles or aerosols to move randomly and independently of the bulk air stream. As these sub-microscopic particles move randomly, they eventually collide and adhere to the filter surfaces. Water droplets accumulate on the surface of the filters and form a larger mass which then trickles down from the filtering system.
For particles and aerosols on the microscopic scale, their movement is less random, and they flow together with the air stream. These contaminants are removed through the interception mechanism. Filtration by impaction works by trapping them on the gaps between the fibers.
Coalescing dryers are designed depending on the size of the target contaminant and removal efficiency. For bulk water removal, the usual design is thin, multi-layered, corrugated plates stacked together to form microscopic gaps in between. Water is removed from the air stream through impaction.
Coalescing dryers are typically used in tandem with other air dryers. They act as a preliminary drying unit directly after the compressor or aftercooler. This is to lessen the drying load and remove contaminants that can deactivate the drying media of the main drying unit. They are used upstream of desiccant and membrane dryers.
For refrigerant and deliquescent dryers, coalescing dryers are more useful downstream. This is because the water droplets from the condensation of the water vapor and the brine solution from the liquefaction of the deliquescent substance can travel together with the discharge air stream. Water droplets entrained in the air stream pose a larger problem to downstream equipment than the presence of water vapor; thus, the need for coalescing dryers.
For larger contaminants such as water droplets, the impaction mechanism governs. Water droplets moving with the air stream have high inertia because of their larger mass. As they collide with the filter fibers, they become trapped and accumulate with other water droplets.
Powder dryers, also known as spray dryers, are used to transform liquids and suspensions into a light porous powder. It is a production method used to make milk powders, coffee creamers, powdered cheese, instant coffee and tea, and powdered eggs to name a few. Powder drying is one of the many methods used to perform micro encapsulation.
In the powder drying process, a liquid is sprayed or atomized as a spray of fine droplets into an enclosed chamber filled with heated air. The small size of the droplets and the extremely high temperature of the air rapidly dries the droplets and changes them into particles of powder. When the droplets in powder form leave the chamber, they go through a gas solid separation system that includes dry and wet separation.
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