Although the drying of plastic granules is a relatively simple process, in some cases the granules simply cannot be completely dried. Factors affecting the drying effect are:

● Drying temperature:

Heat is the key to opening up the forces between the water molecules and the hygroscopic polymer. Above a certain temperature, the attraction between the water molecules and the polymer chains is greatly reduced, and the water vapor is carried away by the dry air.

● Dew point:

In the dryer, the humid air is first removed so that it contains very low residual moisture (dew point). The air is then heated to reduce its relative humidity. 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 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 require a certain amount of time. Therefore, the resin supplier should specify the time it must take for a material to dry effectively at the appropriate temperature and dew point.

● Airflow:

The dry hot air transfers heat to the particles in the drying silo, removes moisture from the surface of the particles, and then sends the moisture back to the dryer. Therefore, there must be sufficient air flow to heat the resin to the drying temperature and maintain this temperature for a certain period of time.

When the problem of poor drying occurs, the problem should be found from the following three aspects.

1. Condition of dryer

When checking the dryer, pay special attention to the air filter and hose. A blocked filter or a flattened hose will reduce the airflow, thereby affecting the operation of the dryer; a damaged filter will pollute the desiccant and inhibit its moisture absorption capacity; a broken hose may introduce humid ambient air into the dry airflow, Causing 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. Dry gas path

In the drying gas circuit, the drying temperature should be detected at the silo inlet in order to compensate for the heat loss of the dryer in the hose. The low air temperature at the silo inlet may be due to improper adjustment of the controller and lack of insulation, or a failure of the 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 there is enough space in the drying silo to provide a sufficient and effective drying time. The effective drying time refers to the time when the particles are actually exposed to the appropriate drying temperature and dew point. If the residence time of the particles in the silo is insufficient, proper drying is not achieved. 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 dried material.

A kinked hose or a clogged filter can restrict air flow and affect the performance of the dryer. Therefore, if the inspection of the dryer does not find such problems, it is impossible to judge whether the airflow is sufficient. Here, there is a quick, simple and accurate method to detect whether the dryer airflow is sufficient, that is, to measure the vertical temperature curve of the material in the drying silo.

It is assumed that the material supplier recommend a drying time of 4h and a processing capacity of 100lb/h(1lb = 0.4536kg). To judge whether the dryer airflow is sufficient, the temperature curve in the drying silo can be measured. Here, special attention should be paid to the temperature at 4h(400lb). If the temperature at the 400lb level in the drying silo reaches the set value, the gas flow is considered sufficient. If only the materials at 1h, 2h or 3h in the drying silo are fully heated, it means that the gas flow rate cannot complete the heating and drying of the materials at the predetermined yield. Insufficient heating may indicate that the drying bin is too small for this production rate, or that the air flow is restricted due to a blocked filter or a damaged hose, etc. Too much gas will also cause problems, not only wasting energy, but also causing high return air temperature and destroying the performance of the desiccant.

The return air filter can prevent the filamentous material from contaminating the desiccant and affecting its hygroscopic properties. These filters must be kept clean in order to ensure sufficient air flow.

When the drying air comes out of the top of the dryer, most of the heat has been released. When the temperature of the desiccant is in the range of 120oF ~ 150oF, most dryers can work efficiently. If the return air overheats the desiccant, it reduces its ability to adsorb moisture from the drying air.

Check the return air temperature of the dryer from time to time. When the return air temperature is high, it may indicate that the dryer size is too large for this production rate, or the temperature of the material entering the drying silo is high, for example, PET has already crystallized before the drying silo, or only certain materials (such as PET) are drying at a temperature higher than the normal temperature range. In order to prevent the return air temperature becomes high, as long as a heat exchanger is installed in the return air circuit, it can ensure that the desiccant can effectively remove the moisture in the dry air.

3. Regeneration and cooling of desiccant

The moisture absorption capacity of the desiccant is limited, so the moisture adsorbed by it must be removed by regeneration. The process is: when the ambient air is sucked in, through a filter into the blower, and then is sent to a set of heaters. The heated air passes through the desiccant bed. As the temperature of the desiccant rises, the adsorbed moisture is released. When the hot air absorbs water vapor and reaches saturation, it is discharged into the atmosphere. The high temperature regeneration desiccant must be cooled before returning to the drying loop to restore the moisture absorption function of the desiccant.

Dew point readings can help find some 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 F ~ 50 F. Of course, small fluctuations caused by the replacement of the desiccant are normal. If the dryer is operating properly, the dew point at the dry air inlet should be at least 30 ° F lower than the dew point at the return air outlet.

On the other hand, the dew point peaks immediately after the desiccant is replaced, indicating that the desiccant is not cooled sufficiently before it is put in, so that it cannot absorb moisture well. After cooling, the dew point of the desiccant will drop to the normal standard. If the desiccant is not cooled properly, it will lead to a temperature peak, and sudden temperature changes will reduce the desiccant's drying ability to heat-sensitive materials such as ionomer, amorphous polyester and some nylon brands.

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, the ambient air may enter the closed air path, causing the desiccant to absorb moisture prematurely. Another possibility is that the desiccant regeneration is incomplete or contaminated. If the dew point reading is close to the return air dew point reading, it indicates that the regeneration gas circuit is completely failed or the desiccant is seriously polluted.