This article offers a comprehensive overview of air dryers.
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An air dryer is a specialized system or device engineered to remove moisture from the air, particularly in compressed air systems. Typically, the relative humidity of ambient air falls between 30% and 50%. However, once air is compressed, its moisture content rises within a confined space, elevating the relative humidity to 100%. This increase in moisture results in condensation of excess water.
When too much moisture is present, it can adversely affect equipment performance and lifespan. Potential issues include contamination of process streams, the risk of premature equipment failure, excessive part wear, and the corrosion of machinery, among other problems.
Moisture removal isn't limited to compressed air processes alone. Various industrial and manufacturing activities also require moisture extraction, such as dewatering, baking, industrial drying, food dehydration, steam heating, and cleaning. These operations utilize different types of dryers, primarily aimed at drying, solidifying, and purifying products or materials through moisture removal. These dryers rely on fundamental principles of heat transfer, namely conduction, convection, and radiation. Air drying, however, distinguishes itself by using methods like refrigeration, adsorption, absorption, diffusion, and filtration to extract moisture.
The dew point temperature refers to the temperature at which air reaches its saturation point with water vapor. At this specific temperature, the water vapor starts to condense and form droplets on surfaces like pipe walls, tank interiors, or even on dry products.
Relative humidity is the measure of the current water vapor concentration in the air compared to the maximum amount the air can hold at a given temperature. It is expressed as a percentage and changes with temperature. When relative humidity reaches 100%, the air is fully saturated, and any extra moisture will condense. The temperature at this point is known as the dew point temperature.
While relative humidity can be adjusted by altering the temperature, the total amount of water vapor and the dew point temperature remain unchanged.
The dew point temperature is a key indicator for air drying processes because it directly reflects the moisture level in the air. A higher dew point indicates that the air holds more moisture and is more prone to condensation. For instance, if a drying system has a dew point temperature of 3°C (37°F), it means that the system effectively removes moisture from the air to prevent condensation when the ambient temperature exceeds 3°C.
Additionally, it's crucial to recognize that pressure influences the dew point temperature. An increase in pressure raises the dew point. When discussing dew point temperatures under varying pressures, other than atmospheric pressure, the term pressure dew point is used.
An air dryer plays a crucial role in compressed air systems. As air is compressed, moisture and condensation naturally occur, which can lead to water buildup both within the compressor and in downstream equipment and processes. Here are some key benefits of incorporating air dryers into your system:
Water contamination poses a significant challenge in industries that rely on high-purity compressed air, such as laser cutting and welding, plasma generation, microelectronics manufacturing, food and pharmaceutical production, shot blasting, painting, and coating. Contaminants like water can impact the performance of compressed air systems in various ways. For instance, in laser cutting, compressed air is used to cool the resonator, which produces intense light beams. If the air is contaminated with water, the cooling process becomes less effective, leading to overheating and reduced energy efficiency.
Compressed air often contains a significant amount of water vapor, which can condense when the temperature is lowered or when the air is further compressed and pressurized. This condensation can lead to water accumulation in small crevices or cavities within downstream equipment. Such water buildup can interfere with the functioning of sensitive instruments, including measuring and monitoring devices.
Water entering equipment internals can lead to corrosion of steel surfaces. Components such as pipes, tanks, vessels, drums, and the interiors of mixing equipment can collect water resulting from the condensation of saturated air. This water accumulation can accelerate corrosion on internal surfaces, potentially causing contamination of products or process streams.
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 machinery rely on compressed air to operate air motors or turbines. Examples include pneumatic grinders, drills, and jackhammers. Water contamination within these tools can lead to fouling of their internal components, which in turn reduces the efficiency and power output of the air motor.
In the food and beverage industry, compressed air is utilized for tasks like product mixing and conveying. However, water in the compressed air system can harbor microbes that may contaminate and spoil food products. Similarly, pharmaceutical manufacturing facilities demand highly pure air, as even minor impurities can compromise an entire batch of products.
As previously discussed, air dryers eliminate moisture using various methods such as refrigeration, adsorption, absorption, diffusion, and filtration. This section will explore each type of air dryer and explain how they operate.
Refrigerant dryers function by cooling the compressed air stream to a temperature low enough to cause water vapor to condense. This cooling is typically achieved to a temperature that meets or falls below the dew point. Compressed air is usually saturated, meaning it holds the maximum amount of moisture possible and is at or above ambient temperature. Since warmer air can retain more moisture than cooler air, reducing the temperature decreases the air’s capacity to hold water. As a result, the excess water vapor condenses and is removed from the air.
Refrigerant air dryers are ideal for high-capacity applications but are less effective than other types of air dryers at moisture removal. These dryers typically achieve dew point temperatures ranging from 2 to 3°C (35 to 37°F).
Refrigerant dryers are composed of two main systems: the air circuit and the refrigeration circuit.
The air circuit is responsible for extracting water vapor from the air. This system operates through the following steps:
The air side of the process consists of three key components: the air-to-air heat exchanger, the air-to-refrigerant heat exchanger, and the condensed water drain. The air-to-air heat exchanger recovers heat by transferring it from the incoming warm air to the outgoing cool air. This heat recovery enhances the efficiency of the dryer by reducing the cooling load on the refrigeration cycle and eliminating the need for additional heating elements to adjust the temperature and lower the relative humidity of the outgoing air.
The air-to-refrigerant heat exchanger, also known as the evaporator, serves as the primary cooling unit that reduces the air temperature to or below its dew point. Lower temperatures increase the amount of moisture that can be removed, with a typical target around 3°C (37°F). This temperature effectively removes most moisture while avoiding the freezing of condensates, which can obstruct air flow and hinder heat transfer through the cooling coils. To address potential freezing issues, auxiliary components like an electrical heat tracing unit can be added.
At the bottom of the air side unit, there is a boot, drain, or water separator where the condensed water collects and is expelled from the system via 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 categorized based on their evaporating units and operational modes. The two primary types of refrigerant dryers are outlined below.
Direct expansion (DX) refrigerant dryers are among the most commonly used types. They operate on a straightforward refrigeration cycle where heat is directly transferred from the air to the refrigerant. These systems lack intermediate components such as water lines or reservoirs. Typically using halocarbon-based refrigerants (like Freon) in a closed-loop system, DX dryers are more cost-effective and compact compared to other types. However, they generally run continuously at a fixed speed regardless of load variations, which can be less economical over time. Recent advancements have introduced cycling and variable-speed DX dryers that can adjust or shut down the compressor during periods of low demand, improving overall efficiency.
This type of dryer utilizes an intermediate medium to absorb heat from the hot air stream, rather than relying directly on a refrigerant. The intermediate medium, often a blend of water and glycol or materials like sand or clay, serves as a thermal reservoir. As this medium absorbs heat, it transfers this energy to another heat exchanger. The secondary side of this heat exchanger is connected to a DX refrigeration circuit or a cooling water supply from chillers or cooling towers. Thermal mass refrigerant dryers operate as cycling dryers, allowing them to shut down once the thermal mass has absorbed enough heat. This capability can lead to lower operating costs, potentially offsetting the higher initial investment.
Desiccant dryers utilize hygroscopic substances to remove moisture from the air. These substances, called desiccants, are dry, solid materials that function based on the principle of adsorption. The desiccants have porous surfaces that attract and hold moisture molecules through intermolecular forces. This process is known as physisorption. Common desiccants used for physisorption include silica gel and activated alumina (molecular sieves). These desiccants are available in various forms, such as powders, pellets, or beads, to maximize their surface area in contact with the air.
Another category of desiccants operates through chemical reactions, known as chemisorption desiccants. These materials have surfaces that strongly attract water molecules, forming new chemical bonds with them, unlike physisorption desiccants that rely on intermolecular forces.
Calcium sulfate is one of the most effective and commonly used chemisorption desiccants. It is safer compared to calcium oxide and can achieve dew point temperatures as low as -40°C (-40°F) and dry air down to -73°C (-100°F). Desiccant dryers are among the most efficient types of air dryers available.
A desiccant dryer typically consists of one or more vessels filled with desiccant material. Inside each vessel, components like screens, trays, or beds are used to hold the desiccant in place while allowing air to pass through. Air is usually introduced at the bottom of the vessel and exits at the top. As the air passes through and is dried, the desiccant gradually becomes saturated and less effective at capturing moisture. To restore its drying capability, a regeneration phase is performed before the desiccant reaches full saturation.
Regeneration is the process of expelling the absorbed water molecules from the desiccants by heating and purging. In desiccant dryers designed for continuous operation, there are typically at least two vessels: one vessel is active in the drying phase, while the other is undergoing the regeneration phase.
There are three main methods for regeneration: pressure swing, heat of compression, and blower regeneration. The choice of regeneration method determines the type of desiccant dryer being used.
This type of dryer removes adsorbed moisture from the desiccant by redirecting a portion of the discharged dry air to the regeneration vessel. Since adsorption generates heat, the dry air exiting the system is warm enough to effectively purge moisture from the desiccant. This design, known as a heatless dryer, does not require an external heat source like electric heaters or steam. However, if the heat generated by adsorption is insufficient, external heating may be used to enhance the regeneration process.
In this type of dryer, instead of using a portion of the dried air for regeneration, hot, moist air directly from the compressor discharge is employed. The air from the compressor is heated due to the compression process. In some designs, this heat alone is sufficient for regeneration, eliminating the need for external heating. This method is highly energy-efficient because it avoids the loss of dry air and operates without requiring additional external heat, making it a heatless design.
This dryer relies on externally heated atmospheric air for the regeneration process. It features an integrated blower that channels air through either an electric or steam heating coil. This method consumes a significant amount of additional energy, as it does not take advantage of or recover the heat produced during the compression and adsorption stages.
Single-tower desiccant dryers effectively manage moisture in pipelines by lowering the pressure dew point by 20°F or more. They work by passing the air through a bed of hygroscopic desiccant within the dryer, which removes water vapor. This type of air dryer is known for its versatility and cost-effectiveness.
With no moving parts and minimal maintenance requirements, single-tower desiccant dryers offer a durable and economical solution for air drying. Their robust construction makes them resistant to corrosion, chipping, and cracking, even in challenging environments.
These dryers require no power to operate. The only maintenance needed involves replenishing the absorbent desiccant, typically performed two to three times a year.
Single-tower desiccant air dryers feature a single tower filled with desiccant material. Moist air enters at the bottom of the tower, travels upward through the desiccant bed, and exits at the top as dry air with a significantly reduced dew point.
They are commonly used for point-of-use applications, offering benefits such as low initial and maintenance costs and minimal pressure drop. These dryers can be installed outdoors and in corrosive or hazardous environments, and are capable of removing oil and solid particulates from the air.
Unlike desiccant dryers that rely on adsorption, deliquescent dryers operate based on the principle of absorption. These dryers use hygroscopic materials that dissolve as they absorb moisture, which is why they are referred to as deliquescent. Deliquescent dryers can achieve dew points as low as -7°C (20°F).
Typical drying agents for deliquescent dryers include salts like sodium hydroxide, potassium hydroxide, and calcium chloride. These drying agents are often formulated in proprietary blends by manufacturers.
The operation of deliquescent dryer vessels is similar to that of desiccant dryers. The vessel contains a bed of hygroscopic materials supported by a screen or tray. Hot, humid air is introduced at the bottom of the vessel and exits at the top. The key difference lies in the behavior of the drying media. Instead of becoming saturated, deliquescent materials dissolve and form a liquid as they absorb moisture. This liquefied solution collects at the bottom of the vessel and is periodically drained off.
Since the drying media is consumed and not regenerated, it must be replenished regularly to maintain effective performance. The absence of a regeneration phase and moving parts allows deliquescent dryers to operate passively, making them suitable for remote and hazardous locations where electrical power may be unavailable or unsafe.
Membrane dryers remove moisture by directing humid air through tiny tubes or hollow fibers made from a semi-permeable material. These fibers are grouped within a canister that features multiple openings for compressed air input, dry air output, and moisture exhaust. Membrane dryers can achieve dew points as low as -40°C (-40°F), comparable to desiccant dryers.
Moist air enters one end of the membrane canister and exits as dry air from the other end, driven by a pressure gradient between the two ends. As the moist air moves through the canister, water molecules diffuse through the semi-permeable membrane of the fibers. This diffusion is influenced by a concentration gradient, where water molecules migrate more rapidly towards areas of lower moisture concentration, which is outside the hollow fibers.
While concentration gradient diffusion is one mechanism, other diffusion methods include pore diffusion and molecular sieving.
To sustain the concentration gradient across the fiber walls, a portion of the dry air output is redirected to the opposite side of the membrane. This stream helps to carry away the water molecules that have passed through the membranes and directs them towards the canister's exhaust.
Membrane dryers are ideal for applications requiring reliable, continuous operation without the need for automated control or external power sources. They produce high-quality dry air due to their efficiency in removing moisture and other contaminants. However, they have a limited capacity compared to other types of dryers and can experience high-pressure loss because the air must be forced through the bundled fibers.
Coalescing dryers operate primarily as filtering devices designed to capture tiny water droplets rather than water vapor dispersed in compressed air. These units also trap other microscopic contaminants such as oil and particulate matter, which is why they are often referred to as filters rather than traditional dryers.
The effectiveness of coalescing dryers and filters relies on three primary mechanisms: diffusion, interception, and impaction. Diffusion occurs when sub-microscopic particles or aerosols move randomly and independently of the main air stream. These particles eventually collide with and adhere to the filter surfaces, allowing water droplets to accumulate and coalesce, eventually trickling down and being removed from the system.
Microscopic particles and aerosols, which move less randomly and follow the air flow, are removed through the interception mechanism. Impaction filtration captures these contaminants by trapping them in the gaps between the filter fibers.
The design of coalescing dryers varies based on the size of the contaminants they are targeting and their required removal efficiency. For bulk water removal, these dryers typically use thin, multi-layered, corrugated plates to create microscopic gaps. Water is separated from the air stream primarily through impaction.
Coalescing dryers are often used in combination with other air dryers. They usually function as a preliminary drying stage immediately after the compressor or aftercooler, helping to reduce the overall drying load and remove contaminants that could impair the effectiveness of subsequent drying media. They are positioned upstream of desiccant and membrane dryers.
In the case of refrigerant and deliquescent dryers, coalescing dryers are more effective when placed downstream. This is because water droplets resulting from condensation or liquefaction can travel with the discharge air stream and pose a significant problem to downstream equipment. Coalescing dryers address this issue by removing these water droplets, which can cause more damage than water vapor alone.
For larger contaminants like water droplets, the impaction mechanism is key. Due to their higher mass and inertia, these droplets collide with filter fibers and become trapped, accumulating with other 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 powder drying, a liquid is atomized into fine droplets and introduced into a chamber filled with hot air. The combination of the droplets' small size and the high air temperature causes the liquid to evaporate quickly, transforming the droplets into powder particles. Once the powder exits the chamber, it undergoes a gas-solid separation process, which involves both dry and wet separation techniques.
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