A water-cooled screw conveyor combines mechanical conveying with indirect water cooling. It transports hot powders, ash, slag, and granular materials while removing heat through cooling passages inside a hollow screw shaft and, where required, a water jacket around the conveyor trough. Its main components include the screw flight, hollow water-cooled shaft, cooling water pipes, rotary joint, drive assembly, and conveyor housing.
This type of conveyor is used to transport and cool hot bulk materials continuously. Specially designed units may handle materials at temperatures of 800 to 1000°C. However, the achievable discharge temperature depends on the material flow rate, inlet temperature, heat transfer area, cooling water conditions, and operating parameters. The presence of a water-cooled shaft alone does not determine the final material temperature.
Main Components of a Water-Cooled Screw Conveyor
Screw Flight and Screw Assembly
The screw flight is mounted on the hollow water-cooled shaft. As the shaft rotates, the flight moves material from the feed inlet toward the discharge end. Because the conveyed material may be at a high temperature, the flight material must be selected according to the operating temperature, wear conditions, and service environment.
Some designs use segmented screw flights that can be removed from the shaft. Certain units also use trapezoidal flight sections and trapezoidal slot connections to simplify installation and local replacement. These features depend on the machine design and should be specified according to the engineering drawings and actual load conditions.
A tapered flight section may also be used near the feed inlet or in other areas where material tends to accumulate. Its purpose is to improve material flow into the conveying section.
Hollow Water-Cooled Shaft
The water-cooled shaft usually has a hollow construction. It supports the screw flight and transmits torque from the drive assembly. Cooling passages inside the shaft allow water to circulate while the screw rotates.
Heat from the hot material passes through the screw flight into the shaft and is then transferred to the circulating cooling water. The shaft material must be selected according to operating temperature, torque requirements, thermal expansion, and corrosion conditions. Room-temperature mechanical strength alone is not sufficient for material selection.
Cooling Water Pipes and Internal Flow Passages
Cooling water pipes are usually installed inside the hollow shaft to form the supply and return passages.
One common arrangement uses a central pipe to supply cooling water and the space between the pipe and shaft wall as the return passage. Water enters through the rotary joint, flows along the internal pipe toward the far end of the shaft, and then returns through the surrounding passage to the outlet.
The actual supply direction, return path, and internal passage arrangement depend on the shaft design and the number of channels in the rotary joint. Not all water-cooled screw conveyors use the same single-pipe circulation arrangement.
Rotary Joint
The rotary joint connects the fixed external cooling water lines to the internal passages of the rotating screw shaft. It allows cooling water to enter and leave the shaft while the conveyor remains in operation.
A typical rotary joint consists of a rotor, housing, seals, and bearings. The rotor turns with the equipment shaft, while the housing remains stationary and connects to the fixed piping. Internal seals prevent cooling water from leaking between the rotating and stationary parts. Systems with separate water supply and return passages generally require a rotary joint with the corresponding flow channels.
When selecting a rotary joint, consider the cooling medium, operating pressure, flow rate, rotational speed, and temperature. Seal wear or incorrect installation may cause water leakage, so installation and maintenance should follow the manufacturer's instructions.
Conveyor Trough and Cooling Jacket
The screw flight and shaft are installed inside a trough or cylindrical housing. The housing contains the material and supports and protects the screw assembly.
Some water-cooled screw conveyors also have a cooling jacket around the outside of the trough. Cooling water circulates through the jacket and removes heat transferred through the trough wall. This arrangement allows the housing and the material inside it to be cooled at the same time.
Not every water-cooled screw conveyor has an external jacket. Depending on the heat load and machine design, cooling may be applied to the hollow shaft alone or to both the shaft and trough.
Drive Assembly and Support Components
The drive assembly usually consists of an electric motor, gearbox, and coupling or another transmission component. The motor supplies power, while the gearbox reduces the speed to the required operating range. The drive connection then transfers torque to the screw shaft.
Bearing housings support the screw shaft and carry the loads generated during operation. Because hot material can raise the temperature near the shaft ends and support points, the bearings, seals, and lubrication system must be selected with heat insulation and heat dissipation in mind. Otherwise, heat transfer may cause bearing overheating or lubricant failure.
Working Principle of a Water-Cooled Screw Conveyor
During operation, the drive assembly rotates the screw shaft and flight, moving material from the inlet toward the discharge end. At the same time, cooling water enters the hollow shaft through the rotary joint and continuously absorbs heat from the conveying components.
Continuous Conveying of Hot Material
Material enters the trough through the feed inlet. As the screw flight rotates, it applies force to the material and moves it along the shaft. Screw speed, flight pitch, filling ratio, and conveying length all affect the conveying rate and material residence time.
Cooling Water Circulation and Heat Transfer
When the cooling system starts, water enters the hollow screw shaft through the external piping and rotary joint. In a two-pass circulation arrangement, water first travels through the internal supply pipe to the far end of the shaft. It then returns through the surrounding passage and flows back into the external cooling system.
During this process, heat passes from the hot material to the screw flight, shaft, and conveyor housing. It is then transferred through the metal walls to the cooling water. The heated water leaves the equipment and passes through a cooling tower or heat exchanger before returning to the circulation system.
When the conveyor is also equipped with a water-cooled trough jacket, the water in the jacket removes heat transferred through the trough wall, increasing the total heat transfer area.
Material Cooling and Discharge
The material continues to exchange heat with the cooled components as it moves through the conveyor. Its temperature gradually decreases before it reaches the discharge outlet.
The discharge temperature is not a fixed value. It depends on several factors:
The material's initial temperature, specific heat capacity, particle size, and bulk condition.
The material throughput per unit of time.
The effective heat transfer area of the shaft, flights, and conveyor housing.
Cooling water inlet temperature, flow rate, and outlet temperature.
Material residence time and actual heat transfer conditions.
Two conveyors with similar water-cooled shaft designs may therefore achieve different cooling results when their material throughput, particle size, or feed temperature differs.
Cooling System Parameters and Temperature Control
Cooling system design must account for water pressure, flow rate, velocity, and the temperature difference between the inlet and outlet. These parameters are related and should not be used individually to evaluate cooling capacity.
The original equipment information provides the following reference values:
| Parameter | Value in the original data |
|---|---|
| Cooling water operating pressure | 0.6 MPa |
| Cooling water velocity | 0.37 m/s |
| Cooling water flow rate | 4.285 t/h |
| Cooling water inlet temperature | 20°C |
| Cooling water outlet temperature | 100°C |
| Material inlet temperature | 950°C |
| Target material outlet temperature | Below 150°C |
These values should be treated as design parameters for a specific machine or operating condition, not as standard specifications for all water-cooled screw conveyors.
Based on a cooling water flow rate of 4.285 t/h, with the water temperature rising from 20°C to 100°C, the theoretical heat absorption rate is approximately 399 kW. This calculation assumes that the water remains liquid and uses an approximate specific heat capacity of 4.18 kJ/(kg·K). The result can be used for preliminary heat balance calculations, but it does not by itself prove that the conveyor can cool material from 950°C to below 150°C.
The 0.6 MPa pressure value also needs clarification as to whether it is gauge pressure or absolute pressure. The stated velocity of 0.37 m/s refers to flow velocity inside a pipe; it is not a substitute for volumetric or mass flow rate. Actual design calculations must also account for pipe cross-sectional area, circulation resistance, pump head, and pressure and temperature changes within the cooling circuit.
For systems in which the outlet water temperature approaches 100°C, local pressure drops, vaporization, and vapor locking must also be considered. The cooling circuit must operate within the allowable pressure and temperature ranges of the rotary joint, piping, pump, and heat exchanger. Appropriate monitoring and protection devices should be provided.
Suitable Materials and Industrial Applications
Water-cooled screw conveyors are mainly used for hot bulk materials that need to be cooled during conveying. Common applications include the following.
Hot Ash, Slag, and Furnace Discharge Materials
Ash and slag from coal-fired boilers, bottom ash from incinerators, and other hot industrial residues may require continuous cooling after leaving the furnace. A water-cooled screw conveyor combines conveying and indirect heat exchange, reducing the material temperature before it enters downstream conveying or processing equipment.
Metallurgical and Mineral Processing Materials
Some hot granular materials produced during metallurgical, calcination, and mineral processing operations need to be cooled before storage, screening, or further conveying. Equipment selection should account for material temperature, hardness, abrasiveness, and throughput.
Materials from Chemical and Building Materials Production
Certain powders and granules discharged from calcination or heat treatment processes in the chemical and building materials industries also require cooling. Whether a water-cooled screw conveyor is suitable depends on the material's thermal stability, sensitivity to moisture, and process requirements.
Plastic powders and grain should not be listed as typical materials handled at temperatures of 800 to 1000°C. Plastics may melt, thermally decompose, or undergo other chemical changes at these temperatures. Grain requires a completely different temperature range and process arrangement. If either material needs conveying and cooling, the operating temperature and equipment requirements must be specified separately.
Design and Operating Considerations
Determine Cooling Capacity from the Heat Load
During equipment selection, provide the material type, inlet temperature, throughput, target discharge temperature, particle size, and specific heat capacity. A heat balance for both the material and cooling water is needed to determine the required heat transfer area, cooling water flow rate, and conveyor length.
Select Materials for High-Temperature Conditions
The screw flight, shaft, and conveyor housing must withstand high temperatures, thermal cycling, and material wear. Material selection should account for the actual operating temperature, mechanical load, corrosive conditions, and thermal expansion. Differences in thermal expansion between components may also need to be considered.
Maintain Stable Cooling Water Circulation
During operation, monitor the cooling water inlet pressure, outlet pressure, inlet and outlet temperatures, and flow rate. For a closed-loop system, also check the pump, heat exchanger, and pipe connections to ensure that water circulation is not interrupted by insufficient flow or blocked passages.
Inspect the Rotary Joint and Shaft-End Seals
The rotary joint operates under continuous rotation and pressure. Its seals are subject to wear over time. Regular inspections should cover joint temperature, leakage, connection condition, and shaft-end seals. This helps prevent water leakage and damage to components around the shaft end.
Provide Appropriate Protection Devices
If cooling water supply is interrupted, the flow rate becomes too low, or the shaft temperature rises abnormally, the equipment may overheat locally because of insufficient cooling. Depending on the machine design, flow, pressure, and temperature monitoring can be connected to shutdown or feed interlocks to reduce the risk of damage.
When conveying hot furnace slag or similar materials, cooling water leakage may also cause rapid local vaporization, pressure buildup, or thermal shock. The design should therefore address water circuit sealing, leakage detection, and abnormal shutdown protection.
Conclusion
A water-cooled screw conveyor moves material through a screw mechanism while removing heat through a hollow water-cooled shaft, screw flight, and, where fitted, a cooling jacket around the trough. The rotary joint connects the stationary cooling water lines to the rotating shaft, allowing water to circulate while the conveyor is running.
Cooling performance depends on material throughput, effective heat transfer area, cooling water flow rate and temperature difference, and material residence time. For hot furnace materials at 800 to 1000°C, the equipment must be designed based on the actual process conditions and heat balance calculations. A water-cooled shaft or heat-resistant steel construction alone does not establish a specific achievable discharge temperature.
Hansheng Automation provides CNC precision machining of non-standard metal components according to customer-supplied engineering drawings. Our machining work also covers components for yacht equipment, RC racing cars, tobacco and packaging machinery, and filling machinery.
