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This post was last edited by wcq1010 on 2015-9-2 at 12:23. Solutions for dew point in dryers and dehumidifiers. Applications of dew point meters in freeze-dryers, adsorption dryers, low-dew-point dehumidifiers, deliquescence dryers, and plastic/dryer machines: Gas law: PT = PA + PB + PC + …… + PW (where PW represents the partial pressure of water vapor). kPT = k(PA + PB + PC + …… + PW). PW/PT = C. Meaning of dew point: The temperature at which condensation, in the form of dew or other substances, occurs when a gas is cooled at constant pressure. Generally, it also includes the “dew point”. Explanation of Ppmv: It is the ratio of the partial volume of water vapor in the humid air to the partial volume of the dry air, multiplied by 106; it is sometimes also expressed as the ratio of the partial volume of water vapor to the total volume of the humid air (i.e., the total gas volume). I. There are currently many different types of dryers and dehumidification methods available on the market. Let’s take a look first at: 1. The working principle of freeze-dryers: Freeze-drying is a drying technique that utilizes the principle of sublimation. It involves rapidly freezing the substance to be dried at low temperatures, and then, in an appropriate vacuum environment, causing the frozen water molecules to sublimate directly into water vapor and escape. The product obtained through freeze-drying is called a freeze-dried product, and this process is referred to as freeze-drying. A freeze dryer consists of a refrigeration system, a vacuum system, a heating system, and an electrical instrumentation control system. The main components include a drying oven, condenser, refrigeration unit, vacuum pump, heating/cooling device, etc. Its working principle is to first freeze the item to be dried below its triple-point temperature, and then, under vacuum conditions, cause the solid water (ice) within the item to sublimate directly into water vapor, which is then removed from the item to dry it. After pre-treatment, the material is sent to a quick-freezing chamber for freezing, then to a drying chamber for sublimation dehydration, and finally packaged in the post-treatment workshop. The vacuum system creates a low-pressure environment in the sublimation drying chamber, the heating system supplies the latent heat of sublimation to the material, and the refrigeration system provides the necessary cooling capacity for the cold trap and the drying chamber. The dew point of freeze-dryers is generally around 10°C to -5°C. Oubert recommends that the range of dew point meters be from -20°C to 40°C, with an accuracy of ±2°C. This allows for an accurate and rapid determination of the dew point value, as well as an assessment of whether the drying system is operating properly, thereby ensuring that the dew point of the gas to be dried is within the desired range. 2. Working principle of adsorption dryers: Adsorption drying involves using adsorbents that have the ability to selectively absorb water vapor from compressed air, thereby achieving dehydration. The new generation of adsorption dryers combines the advantages of pressure swing adsorption and temperature swing adsorption, allowing adsorption to take place at normal temperatures and high vapor pressures ； At higher temperatures and low vapor partial pressures, the adsorbed water – that is, the moisture absorbed by the adsorbent during the adsorption process – is completely removed during regeneration through the combined action of heat diffusion resulting from high-quality regeneration (heating with dry air) and the low vapor partial pressure. Adsorption: Wet air enters the A drying tank through valve A via the lower piping system, flows upward through the adsorbent bed, and the dried air is discharged through the upper piping system. Regeneration and cooling: A small amount of dry air enters the heater for heating after being depressurized through the regeneration gas control valve in the upper piping system; this heated air (referred to as regeneration gas) then enters drum B. The adsorbent in tank B is desorbed and regenerated to restore its drying capacity, and the regenerated gas is discharged into the atmosphere through valve B of the lower piping system and a silencer. Pressure equalization: After the adsorbent regeneration is complete, valve B is closed, and the B drying tank’s pressure is gradually increased to the operating pressure online, in preparation for the switchover. Switching: Valve B of the lower piping system is opened, while valve A is closed; then valve A is opened, completing the switching between the two drying tanks. Tank B enters adsorption mode, while tank A undergoes pressure release and regeneration. The working sequence, working time, and heating temperature are automatically controlled by the controller to achieve the drying effect. Based on the principle of adsorption drying, the dew point value is generally around -40°C. Oubert recommends that dew point meters have a range of -80°C to +20°C, with an accuracy of ±2°C; this ensures the reliability of the measurement values while also enhancing the resistance of the dew point meter to contamination and oil. 3. Working principle of low-dewpoint dehumidifiers: a. Cooling dehumidification method: The air is cooled to below its corresponding dew point temperature, causing the gaseous water in the air to condense into liquid water and be removed from the air, thereby achieving dehumidification. The cooling medium can be chilled water, low-temperature brine, refrigerants, etc. Since water in humid air tends to freeze on surfaces at temperatures below 0°C, and there is also a certain temperature difference between the treated air and the surface of the heat exchanger, the lowest dew point temperature that can be achieved with this method is 0°C. To meet the general dehumidification requirements needed in industry, it is necessary to use low-temperature brine and increase the volume of air being treated; this inevitably increases the energy consumption of refrigerators, fans, and water pumps, and requires larger-sized equipment. b. Compression dehumidification method: The air is compressed to a certain pressure to saturate the water content in it, and then the water is removed through cooling. This method can achieve a low dew point, but it is only suitable for instruments that require a small amount of dried air, and its operating costs are high. c. Chemical dehumidification method: Solution absorption dehumidification: Water is removed by bringing a solution with a high concentration of moisture-absorbing capacity into contact with air. This method allows for continuous dehumidification, but the solution is corrosive, tends to be carried away by air, crystallizes easily during regeneration, makes it difficult to control its concentration, and requires complicated maintenance. d. Rotary wheel adsorption dehumidification: Utilizes the moisture-absorbing materials (silica gel or molecular sieve) in the honeycomb-shaped rotary wheel to absorb water from the air. This method enables continuous dehumidification, resulting in stable dry air. It has an unparalleled advantage over other dehumidification methods when removing moisture at low humidity levels, and the humidity can be easily controlled. Machines of different specifications can be manufactured according to actual needs. Rotary wheel dehumidifiers offer relatively good dehumidification performance, with a dew point that can reach around -80°C. For dehumidifiers used in such low-humidity environments, Oubert Company recommends dew point meters with a range of -100°C to 20°C and a precision of ±2°C. Looking at these figures: at a dew point of -80°C (under standard atmospheric pressure and a temperature of 20°C), the relative humidity is 0.002%, while the moisture content is 0.55 PPM. This shows the relationship between the dew point, relative humidity, and PPM of the dried air. Therefore, our sensors need to have a wide range, high precision, and high stability; otherwise, it will be impossible to accurately measure the true dew point value. The dew point transmitters distributed by Oubert Company, which are from the British company MICHELL, have obtained approval from China’s administrative metrology authorities. Most of the measurement standards used in metrology institutes today are also products from MICHELL, offering reliable precision and good stability – making them the ideal choice for dew point meters in low-dew-point dehumidifiers. 4. Working principle of hygroscopic dryers: Air passes through filters and is then heated by a heater (which can be electric heaters, fuel-fired hot air furnaces, natural gas hot air furnaces, etc.; the inlet air temperature must be designed accordingly based on the type of material). The heated air enters the drying chamber through the spiral channels at the top of the chamber, where a hot air distributor creates a uniform, rotating airflow within the chamber. The liquid feed is pumped from the feed tank, through a filter, to a centrifugal atomizer or high-pressure nozzle located at the top of the dryer, where it is converted into a fine mist. This mist comes into contact with the hot air in a swirl pattern, causing the water content to evaporate rapidly, and thus the material is dried into a finished product in a very short time. The finished product is discharged from the bottom of the drying tower and the cyclone separator, while the waste gas is extracted and removed by a fan. 5. Working principle of plastic/dryers: The drying process requires a large amount of thermal energy. To save energy, certain materials with high moisture content, as well as suspensions or solutions containing solid substances, are first subjected to mechanical dehydration or heating evaporation before being dried in the dryer to produce dry solids. During the drying process, it is necessary to simultaneously transfer heat and mass (moisture). This ensures that the vapor pressure (concentration) of moisture on the surface of the material is higher than that in the surrounding environment, as well as that the temperature of the heat source is higher than the temperature of the material. Heat is transferred from the high-temperature heat source to the wet material in various ways, causing the moisture on the surface of the material to vaporize and escape into the surrounding space, thereby creating a difference in moisture content between the surface and the interior of the material. Internal moisture diffuses toward the surface and vaporizes, thereby continuously reducing the moisture content of the material and gradually completing the drying of the entire material. The drying rate of a material depends on the surface vaporization rate and the diffusion rate of moisture inside it. Typically, the drying rate in the early stage of drying is controlled by the surface vaporization rate ； Thereafter, as long as the external drying conditions remain unchanged, the drying rate of the material and its surface temperature stay stable; this stage is known as the constant-rate drying stage ； When the moisture content of the material decreases to a certain level, the rate of diffusion of internal moisture toward the surface slows down and becomes less than the rate of vaporization at the surface. At this point, the drying rate is primarily determined by the rate of internal diffusion, and it continues to decrease as the moisture content further drops. This stage is known as the reduction drying stage. Why do plastic products need to be dried? Because in most plastic manufacturing plants, the main reason for poor molding of plastic products is insufficient dehumidification and drying. The American Plastics Association has recommended that plastics prior to molding must be dried to a moisture content of less than 0.02%, as otherwise it is difficult to ensure consistent product quality. For example, it is prone to defects such as poor transparency, runner sticking, burrs, bubbles, cuts, cracks, poor dimensional stability, internal stress, and insufficient strength of the product’s physical properties. Plastics with particularly high water absorption, such as PET, PBT, PA, PC, PS, ABS, POM, PP, PE, PVC, EVA, etc., require drying equipment with an ultra-low dew point of -40°C. To address the aforementioned issues, Oubert has introduced a series of dew point meters with multiple selectable measurement ranges: -100°C to 20°C, -80°C to 20°C, -60°C to 20°C, and -50°C to 20°C, aiming to provide customers with the most suitable dew point measuring instrument for their needs. It offers an excellent cost-performance ratio while ensuring reliable measurement accuracy and stable performance. II: Comparison of dehumidification capacity: As can be seen from the above comparison, different dehumidification methods have varying capacities; in other words, the level of the dew point depends entirely on the method used for dehumidification. Whether using a dryer or a dehumidifier to remove moisture, it is particularly important to be able to accurately measure the humidity level in plastic raw materials. British company MICHELL is the world’s largest manufacturer of humidity and dew point instruments; it holds more than half of the market share in the field of humidity measurement in Europe, and boasts over 35 years of expertise in this technical area. Standard laboratories in developed countries such as the EU and the US almost all use MICHELL products as standard instruments. For the dehumidification and drying industry, Oubert Company has carried out further development using MICHELL’s dew point sensors, enabling effective real-time monitoring of the dew point of dehumidification and drying machines, which greatly simplifies things for users in terms of controlling product quality. The EA2-TX-100-HZ series of dew point meters, specially developed for dehumidifiers, feature a wide measurement range, high precision, good stability, and ease of use, making them the ideal products for measuring dew points in drying and dehumidifying equipment.