Drying Equipment: Operation and Daily Maintenance of Spray Granulation Dryers

The spray granulation dryer is a critical piece of equipment in the production of specialty ceramics. This article details the working principle, composition, daily operating procedures, and precautions for the spray granulation dryer, taking into account the specific characteristics of our company’s PTC thermosensitive ceramic production. Finally, it emphasizes the importance of equipment maintenance and outlines specific measures.

1. Introduction

The spray granulation dryer is a critical piece of equipment in the PTC thermosensitive ceramic production process. This equipment is relatively expensive, has a complex structure, and is difficult to operate and maintain. Ensuring the equipment operates smoothly not only guarantees the normal progress of PTC thermosensitive ceramic production but also helps extend the equipment’s service life and reduce production costs. Therefore, proper operation and enhanced maintenance of this equipment are essential.

2. Principles and Components of the Spray Granulation Dryer

Granulation is a critical process in the production of PTC thermosensitive ceramic discs; the quality of the granules directly affects the appearance, mechanical properties, and temperature-resistance characteristics of the PTC ceramic discs. Granulation involves adding a certain amount of binder to finely ground powder, mixing it uniformly to form granular powder. This powder possesses good flowability and calenderability, ensuring that the resulting wafers in the tableting process have good strength and are resistant to delamination and cracking. In industrial production, spray drying is used for granulation. The basic principle involves spraying the binder-containing powder into a granulation tower via a sprayer to atomize it; the droplets within the tower are then dried by a hot air stream into granular powder, which is subsequently discharged from the bottom of the drying tower.

The pressure-type spray granulation dryer used by our company primarily consists of a feeding system, a drying system, a dust collection system, a heating system, and an electrical system, with each system comprising several related pieces of equipment.

The feeding system consists of a mixing tank, a filter, a diaphragm pump, and a spray gun. The secondary slurry, which has been ball-milled, is transferred from the ball mill to the mixing tank, drawn through the filter by the diaphragm pump, and then fed into the drying tower via the spray gun.

The slurry enters the drying tower through the spray gun nozzle, initiating the spray granulation drying process, which consists of three stages:

(1) Slurry atomization. The slurry is forced into the drying tower from the nozzle at a certain pressure by a diaphragm pump in the feeding system. The energy of the pressure is converted into kinetic energy, and the slurry is ejected from the nozzle in an upward direction, forming a high-speed liquid film that immediately breaks up into droplets. The size of the droplets produced by atomization is inversely proportional to the pressure, while the nozzle’s production capacity is proportional to the square of the pressure.

(2) Drying of the droplets into spheres. The droplets interact with hot air in a mixed flow. Hot air enters the tower through a hot air distributor located on the top cover; this distributor generates a downward laminar airflow, and the droplets are sprayed upward into the hot air stream. Due to surface tension, the droplets form spherical shapes. At the same time, because the droplets have a large surface area, the moisture within them evaporates rapidly, causing them to shrink and eventually form dry spherical granular powder.

(3) Discharge of granular powder. The formed spherical granular powder gradually settles within the drying tower, separating from the hot air. A funnel-shaped chamber at the bottom of the tower collects the granular material, which is then discharged through the outlet. Finer particles are drawn along with the dry air by an exhaust fan connected to the upper part of the funnel-shaped chamber and enter the dust collection system.

The blower that supplies hot air to the drying tower, the drying tower itself, and the exhaust fan together constitute the drying system.

The dust collection system consists of a high-efficiency cyclone separator, a baghouse filter, a centrifugal fan, and other components. The exhaust fan conveys the finer granules, along with the dry air, into the high-efficiency cyclone separator. After effective separation, the finer granules enter the collection drum at the bottom of the separator for recovery, while the remaining exhaust gas—containing only a very small amount of fine particles—is drawn by the centrifugal fan into the baghouse filter for further dust removal and collection. This ensures the harmless treatment of the exhaust gas, which is ultimately discharged through the stack.

In addition, a heating system—comprising an electric heater and a gas-fired hot-air furnace—supplies hot air to the drying tower. The electrical system, consisting of an electrical control cabinet and on-site sensors installed at the air inlet and discharge port to monitor temperature, monitors and controls all major components of the spray granulation dryer, ensuring the normal operation of the entire equipment.

3. Daily Operations and Precautions

During daily production, necessary preparations must be made before starting the equipment. First, check whether the bearings and seals of each unit are loose, inspect the lubrication status of all mechanical components, and verify that the valves for water, air, and slurry lines are in the correct positions. Next, connect the power supply and verify that the voltage and gauges are functioning normally. Finally, check the volume and concentration of the slurry in the mixing tank; any issues should be resolved promptly.

Then, start the supply fan and exhaust fan in sequence, followed by turning on the heating switch to begin heating. When the outlet temperature reaches the set temperature (generally around 130°C), start the feed pump and the dust collection system. Once the pump pressure reaches 2 MPa, open the spray gun to begin granulation. After the equipment is running, monitor the atomization and the operation of the feed pump closely. If the spray gun becomes clogged, clean or replace the nozzle immediately.

Once the equipment is operating normally, collect the product at regular intervals, check the operation of each system periodically, record all process parameters, and be sure to clean the vibrating filter screen.

At the end of granulation, first shut down the heating system and stop the gas engine. Then close the spray gun valve, remove the spray gun, unscrew the nozzle, and rinse it thoroughly. Once the inlet temperature drops below 100°C, stop the operation of the blower and exhaust fan. Next, remove any residual material from the drying tower and dust collector, shut down the dust collector and air hammer, and finally turn off the main power supply to complete the production operation.

In the event of an emergency, the equipment must be shut down immediately. First, shut down the blower and the feed pump. If a power outage occurs suddenly, disconnect the gas engine to allow the tower to cool naturally. Then, open the drain valve to completely drain the slurry from the slurry pipes and clean the equipment.

4. Maintenance and Servicing

Granulation is a critical stage in the PTC production process, and maintaining the normal operation of the equipment is essential for ensuring stable PTC production. Regular cleaning, maintenance, and servicing help identify problems promptly, resolve potential malfunctions, reduce downtime, and ensure normal and stable production.

After prolonged operation or due to improper operation, material buildup may occur inside some equipment, affecting normal operation. In such cases, operations must be stopped for cleaning. To remove material buildup inside the drying tower, open the cleaning door and use a long-handled broom to sweep out the material accumulated at the funnel-shaped bottom. Then, open the discharge valve and rinse the interior of the tower with tap water. To remove dust accumulated inside the cyclone separator, open the cyclone separator, sweep out the material with a broom, and rinse with water if necessary. To clean the bag filter, turn on the control switch and tap the unit continuously. Then open the cleaning door, tap the bag filter, and finally replace the filter bags. To clean the slurry piping system, open the drain valve on the two-way filter, clean the filter screen and piping, and then turn on the feed pump to flush the pump, pipes, pressure stabilizer, and piping with water instead of material.

After a period of operation (generally around 600 hours), the spray granulation dryer must undergo necessary inspection and maintenance. For the feeding system, inspect the filter, piping, valves, and nozzles for blockages; clean them regularly; and check the nozzles for wear to ensure timely replacement. Check the feed pump for oil leaks, verify that the pressure is normal, and ensure the oil level is within the proper range. For the blower, check whether the shaft and bearings are overheating due to lack of lubrication, and monitor for vibration or noise. If necessary, clean the fan blades and perform balancing adjustments. For the heater, check whether the heat pipes are functioning normally; if necessary, clean the filters at the oil pipes, oil pump, and oil nozzles. Additionally, monitor all electric motors for overheating, vibration, or unusual noises, and verify that the instruments and electrical components in the control cabinet are operating normally.

Furthermore, after the equipment has been in operation for approximately 2,400 hours (typically during a production shutdown), maintenance personnel should perform a partial disassembly and maintenance on the equipment.