Although the drying of plastic granules is a relatively simple process, in some cases the granules cannot be completely dried. Factors affecting the drying effect are:
●Drying temperature:
Heat is the key to opening the resultant force between water molecules and hygroscopic polymers. When it is above a certain temperature, the gravitational force between the water molecules and the polymer chain is greatly reduced, and the water vapor is carried away by the dry air.
●Dew point:
In the dryer, the moist air is first removed to contain a very low residual moisture (dew point). Then, by heating the air to reduce its relative humidity. At this time, the dry air has a low vapor pressure. By heating, the water molecules inside the particles are free from the binding force and diffuse to the air around the particles.
●Time:
In the air surrounding the particles, the absorption of heat and the diffusion of water molecules to the surface of the particles take a certain amount of time. Therefore, the resin supplier should specify the time it takes for a material to dry effectively at the proper temperature and dew point.
●Airflow:
The dry hot air transfers heat to the particles in the dry silo, removes moisture from the surface of the particles, and returns the moisture to the dryer. Therefore, there must be sufficient gas 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 discovered from the following three aspects.
1. dryer condition
Pay particular attention to air filters and hoses when inspecting the dryer. A blocked filter or a flattened hose can reduce airflow and affect the operation of the dryer; a damaged filter can contaminate the desiccant and inhibit its ability to absorb moisture; a broken hose can introduce moist ambient air into the dry airflow. In the case of causing premature moisture absorption and high dew point of desiccant; hoses and dry silos with poor insulation measures also affect the drying temperature.
2. Dry gas path
In the dry gas path, the drying temperature should be checked at the inlet of the silo to compensate for the heat loss of the dryer in the hose. The air temperature at the entrance to the silo is low, possibly due to improper regulation of the controller and lack of insulation, or failure of the heater element, heater contactor, thermocouple or controller. In addition, it is important to monitor the drying temperature throughout the drying process and to observe temperature fluctuations during desiccant replacement.
If the material does not dry properly after it has exited the dryer, check that the dry silo has sufficient space to provide adequate and effective drying time. Effective drying time means that the particles are actually exposed to the appropriate drying temperature and dew point. Time in. If the residence time of the granules in the silo is insufficient, proper drying is not obtained. Therefore, attention should be paid to the size and shape of the pellets or crushed materials, which affect the bulk density and residence time of the dried materials.
A writhing hose or a clogged filter can restrict airflow and affect dryer performance. Therefore, if the inspection dryer does not find such a problem, it is impossible to judge whether the airflow is sufficient. Here, there is a quick, simple, and accurate method to check whether the airflow in the dryer is sufficient, that is, to measure the vertical temperature curve of the material in the dry silo.
Assume that the material supplier recommends a drying time of 4 h and a processing capacity of 100 lb/h (1 lb = 0.4536 kg). To determine if the dryer airflow is sufficient, you can measure the temperature profile in the dry silo. Here, pay special attention to the temperature at 4h (400lb). If the temperature at the 400 lb level in the dry silo reaches the set point, then the air flow is considered sufficient. If only 1 h, 2 h or 3 h of material in the dry silo is heated sufficiently, the gas flow cannot complete the heating and drying of the material at the predetermined yield. Insufficient heating may indicate that the drying bin is too small for this productivity, or that the airflow is limited due to filter clogging or hose damage. Too much gas can cause problems, not only wasting energy, but also causing high return air temperature and deteriorating desiccant performance.
The return air filter prevents the filamentous material from contaminating the desiccant and affecting its hygroscopic properties. These filters must be kept clean to ensure adequate airflow.
When the dry air comes out of the top of the dryer, most of the heat has been released. Most dryers work efficiently when the desiccant temperature is in the range of 120oF to 150oF. If the return air overheats the desiccant, it will reduce its ability to adsorb moisture from the dry 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 this productivity, or the temperature of the material entering the dry silo is high. For example, PET has crystallized before the dry silo, or only some materials ( Drying temperatures such as PET) are above the normal temperature range. In order to prevent the temperature of the return air from becoming high, it is ensured that the desiccant can effectively remove moisture in the dry air by installing a heat exchanger on the return gas path.
3. Desiccant regeneration and cooling
The hygroscopic capacity of the desiccant is limited, so the moisture it adsorbs must be removed by regeneration. The process is: when ambient air is drawn in, it enters the blower through a filter and is then fed into a group of heaters. The heated air passes through a desiccant bed. When the temperature of the desiccant rises, the adsorbed moisture is released. When the hot air absorbs water vapor and saturates it, it is discharged into the atmosphere. The high temperature regenerative desiccant must be cooled before returning to the drying loop to restore the hygroscopic function of the desiccant.
Dew point readings can help identify problems, so the dry air dew point value 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 20 ゚F to 50 ゚F. Of course, small fluctuations caused by the replacement of the desiccant are normal. If the dryer is operating normally, the dew point at the dry air inlet should be at least 30 ゚F below 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 sufficiently cooled before it is placed, so that it does not adsorb moisture well. After cooling, the dew point of the desiccant will fall to the normal standard if the desiccant is cooled. Improper results in peak temperatures, which can reduce the drying ability of the desiccant to heat sensitive materials such as ionomers, amorphous polyesters, and certain nylon grades.
If the dew point reading is normal after the desiccant bed is replaced, but the dew point rises rapidly before the end of the desiccant drying cycle, indicating that ambient air may have entered the closed gas path, causing the desiccant to absorb prematurely. Another possibility is that the desiccant is not completely regenerated or contaminated. If the dew point reading is close to the return air dew point reading, it indicates that the regeneration gas path has completely failed or the desiccant has been seriously contaminated.