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Proven in use

Advances in indirect cooling and heating of bulk solids
Proven in use

During many bulk materials processing operations it is necessary to cool or heat the bulk material. The Bulk-X-Change bulk materials heat exchanger, which indirectly cools or heats free-flowing bulk materials, is now employed successfully in conveying systems. The following article describes several concrete applications and answers the most frequent questions asked by cus-tomers in recent months.

Dr. Bernhard Stark

In contrast to a fluidising bed the Coperion Bulk-X-Change indirect bulk materials heat exchanger has the advantage that it is run with far less complex equipment. Investment and especially operating costs are also likely to be much lower, and space requirements and the area necessary for installation are considerably smaller. In the Bulk-X-Change the bulk material flows vertically downwards through an upright tube arrangement due to gravitational force while the heat transfer medium flows around the tubes in the shell. A bulk material mass above the tubes, which is monitored by a level indicator system, ensures that each tube is completely filled. A rotary valve regulates the bulk solid flow during discharge. A major additional feature of the heat exchanger is the special method of product infeed above the tubes. The inlet into each tube is machined out of the plate in the form of a funnel and arranged so that there is no intact horizontal surface between two funnels.
Pressure-resistant version
A pressure-resistant version of the heat exchanger is possible due to its round design. This shell-and-tube heat exchanger can therefore also be used in positive or negative-pressure process systems. This offers considerable advantages during heating or cooling of bulk materials with a tendency to fluidise, which can be thermally treated in batches. During continuous operation with fluidising bulk materials there is always a risk that the product will “shoot” too quickly through individual tubes, in which case correct thermal treatment is prevented. During batch operation the bulk materials heat exchanger is filled with the bulk material and the product subsequently remains stationary in the tubes for a specific period time for cooling or heating. The entire heat exchanger can be slowly pres- surised during the stationary phase. The overpressure is then utilised for emptying and the product flows rapidly out of the heat exchanger. It can also be conveyed pneumatically over short distances, i.e. the bulk materials heat exchanger additionally serves as a pneumatic pressure vessel.
Placed in the conveying system
Another very interesting variant is a heat exchanger integrated in a pneumatic conveying line. This is advantageous if the overhead height is insufficient to install the heat exchanger above the pick-up point of the pneumatic conveying system, especially if the system needs to be extended, or if the conveying route length is not adequate for product cooling in the pneumatic conveying line with a jacketed tube. The top section of the heat exchanger is a separator into which the pneumatic conveying line opens. An overflow opening leads to a bypass tube through which the conveying air and a residual quantity of conveyed product flow around the heat exchanger. The majority of the conveyed product passes through the tubes due to gravitational force and is heated/cooled. Below the discharge valve the conveying gas flow re-enters the main tube and picks up the flow of product. The speed of the discharge valve is set so that its capacity is somewhat lower than the overall conveyed volume. The resulting overflow of a small residual quantity ensures that the neces-sary bulk solid mass is always available above the tubes without any technical monitoring of the filling level. An integrated bypass line with two diverter valves allows the conveying process to continue while the heat exchanger is being serviced.
Typical applications
One typical application of a Bulk-X- Change heat exchanger involves heating 100 to 400 kg/h of PET from 50 to 110 °C for fibre manufacture with the aid of steam in a nitrogen atmosphere. The plastic pellets flowing in the tubes are blanketed with a small flow of nitrogen for inerting to prevent oxidative degeneration of the product and expel small quantities of released moisture with the nitrogen. This pre-heating effect can be utilised to enhance the capacity of the extruder and improve the quality of the produced fibres.
Heating of a free-flowing powder during a production process is a further example. It serves to improve the properties of the final product. In order to investigate production conditions in this case, a bulk materials heat exchanger was supplied to a customer for a test centre system.
Another customer ordered a bulk materials heat exchanger as part of a system extension for cooling terephthalic acid to prevent melting of the polyethylene inliners during filling into Big Bags. The throughput is in the region of 20 t/h and the product is cooled from approximately 120 °C to between 60 and 80 °C. It is conveyed from a silo to the Bulk-X-Change bulk materials heat exchanger by means of a conveying screw with a length of around 15 m. The heat exchanger was subsequently installed above the existing bagging station.
Hall 5.0, Booth F2
cpp 429

Frequently asked questions
How is the product supplied to the tubes?
Each tube on the top tubesheet features a steep, funnel-shaped supply opening to ensure that the product is supplied to the tubes without dead flow zones, thereby preventing bridge formation. Due to the closeness of the individual tubes and the arrangement of many tubes next to one another, the selected tube diameter can be very small. The tube diameter and the angle of the supply funnel are selected on the basis of empirical values.
How is bulk flow achieved in the tubes?
The cone angle is determine by measuring the wall friction angle at the corresponding product temperatures. As reliable bulk flow requires small cone opening angles and therefore a high structure, it is optionally possible to lower the design height of the bulk materials heat exchanger’s outlet cone with specially developed internal components.
What is the residence time of the bulk material in the heat exchanger?
The residence time of the bulk material in the tubes varies from a few minutes to half an hour. This time is mainly determined by the temperature difference compared to the cooling or heating medium.
At which velocities does the product
flow through the tubes?
The velocity of the bulk solid in the tubes is generally around a few millimetres per second. The product is therefore very gently conveyed through the tubes.
Which throughputs are possible?
The throughputs of the heat exchangers requested to date range from a few kilos up to 150 t/h.
Can the bulk material be blanketed with a
flow of gas?
If the bulk materials heat exchanger is used for pellets it can be blanketed with a flow of gas, for example for inerting with nitrogen. Blanketing with dry air can also be effective for removing small quantities of residual moisture and therefore prevent the condensation of water on the tube walls.
How are the tubes cleaned?
The tubes can be manually cleaned with compressed air or a concentrated water jet – simple purging of the tube interior with air or water is often sufficient to remove product residues and accumulated dust. More stubborn residues can be removed with a plastic brush and/or suitable chemical cleaning agents.

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