What factors affect the drying effect of the Drying Oven and how to solve it?

During the drying process, if the particles cannot be completely dried, we need to look for factors that affect the drying effect of the Drying Oven.
　　There are generally four types:
　　①Drying temperature: Heat is the key to opening the synergy between water molecules and hygroscopic polymers. When it is higher than a certain temperature, the attractive force between water molecules and polymer chains will be greatly reduced, and the water vapor will be carried away by the dry air.
　　②Dew point: In the dryer, the humid air is first removed to make it contain very low residual moisture (dew point). Then, lower the relative humidity of the air by heating it. At this time, the vapor pressure of the dry air is low. By heating, the water molecules inside the particles get rid of the binding force and diffuse into the air around the particles.
　　Dew point readings can help spot some problems, so the dry air dew point should be monitored throughout the drying process. The dew point reading of the dryer during normal operation should be a straight line in the range of 20oF to 50oF. Of course, small fluctuations caused by changing the desiccant are normal. If the dryer is functioning properly, the dew point at the dry air inlet should be at least 30oF lower than the dew point at the return air outlet.
　　On the other hand, after the desiccant is replaced, the dew point immediately peaks, indicating that the desiccant is not cooled enough before it is put in, so that it cannot absorb moisture well, and the dew point of the desiccant will drop to the normal standard after cooling. Improper cooling of the desiccant can result in temperature spikes that can reduce the desiccant's ability to dry heat-sensitive materials such as ionomers, amorphous polyesters, and certain nylon grades.
　　If the dew point readings are normal after a desiccant bed change, but the dew point rises rapidly before the desiccant drying cycle is complete, ambient air may have entered the closed air path, causing the desiccant to absorb moisture prematurely. Another possibility is incomplete regeneration or contamination of the desiccant. If the dew point reading is close to the return air dew point reading, it indicates that the regeneration air circuit has completely failed or the desiccant has been seriously contaminated.
　　③ Time: In the air around the particles, it takes a certain amount of time for heat absorption and water molecules to diffuse to the surface of the particles. Therefore, the resin supplier should specify how long it will take for a material to dry effectively at the proper temperature and dew point.
　　④ Airflow: The dry hot air transfers heat to the granules in the drying silo, removes the moisture on the surface of the granules, and then sends the moisture back into the dryer. Therefore, there needs to be enough air flow to heat the resin to the drying temperature and maintain this temperature for a certain period of time.
　　When there is a problem of poor drying, the problem should be found from the following three aspects:
　　1. The condition of the dryer
　　When checking the dryer, pay special attention to the air filter and hose. Clogged filters or pinched hoses can reduce airflow, which can interfere with dryer operation; damaged filters can contaminate the desiccant, inhibiting its ability to absorb moisture; cracked hoses can introduce humid ambient air into the drying airflow In the process, it will cause premature moisture absorption and high dew point of the desiccant; hoses and drying silos with poor insulation measures will also affect the drying temperature.
　　2. Drying gas path
　　In the drying gas path, the drying temperature should be detected at the entrance of the silo to compensate for the heat loss of the dryer in the hose. Low air temperature at the silo inlet may be due to improper adjustment of the controller and lack of insulation, or a faulty heater element, heater contactor, thermocouple, or controller. In addition, it is also important to monitor the drying temperature throughout the drying process and observe the temperature fluctuations when the desiccant is replaced.
　　If the material is not properly dried after coming out of the dryer, it should be checked whether the drying silo is large enough to provide sufficient and effective drying time. Effective drying time is the time the particles are actually exposed to the proper drying temperature and dew point. If the pellets do not have enough residence time in the silo, they will not be properly dried. Therefore, attention should be paid to the size and shape of the granular or crushed material, which will affect the bulk density and residence time of the dry material.
　　A kinked hose or a clogged filter can restrict airflow and affect your dryer's performance. Therefore, if no such problems are found when checking the dryer, it is impossible to judge whether the airflow is sufficient. Here, there is a quick, simple and accurate method to detect whether the airflow of the dryer is sufficient, that is, to measure the vertical temperature curve of the material in the drying silo.
　　Assume that the drying time suggested by the material supplier is 4 hours, and the processing capacity is 100lb/h (1lb=0.4536Kg). To determine whether the airflow of the dryer is sufficient, you can measure the temperature curve in the drying silo. Here, pay special attention to the temperature at 4h (400lb). If the temperature at the 400 lb level in the drying silo reaches the set point, then the air flow is considered adequate. If only the material at 1h, 2h or 3h in the drying silo is fully heated, it means that the air flow cannot complete the heating and drying of the material at the predetermined production rate. Insufficient heating may indicate that the drying silo is too small for this production rate, or that airflow is restricted due to conditions such as clogged filters or damaged hoses. Too much air volume will also cause problems, not only wasting energy, but also causing high return air temperature, which will damage the performance of the desiccant.
　　The return air filter can prevent filamentous material from contaminating the desiccant and affecting its hygroscopic performance. These filters need to be kept clean for adequate airflow.
　　When the dry air exits the top of the dryer, most of the heat is released. Most dryers work effectively when the desiccant temperature is in the range of 120oF to 150oF. If the return air overheats the desiccant, it reduces its ability to absorb moisture from the drying air.
　　Always check the return air temperature of the dryer. When the return air temperature is high, it may indicate that the dryer is oversized for the production rate, or that the temperature of the material entering the drying silo is high, for example, PET crystallization before drying, or just some materials (such as PET) The drying temperature is higher than the normal temperature range. In order to prevent the return air temperature from becoming high, as long as a heat exchanger is installed on the return air path, the desiccant can effectively remove the moisture in the dry air.
　　3. Regeneration and cooling of the desiccant
　　The moisture absorption capacity of the desiccant is limited, so the moisture it absorbs needs to be removed through regeneration. Here's how it works: When ambient air is drawn in, it passes through a filter and into a blower, where it is sent to a set of heaters. The heated air is passed through the desiccant bed. When the temperature of the desiccant rises, the adsorbed moisture is released. When the hot air is saturated with water vapor, it is released into the atmosphere. The high temperature regenerated desiccant needs to be cooled before being returned to the drying loop to restore the moisture absorption function of the desiccant.