Heat Exchanger Bulk Materials Heat Exchanger Makes Significant Inroads in the Plastics Industry

Heating or cooling of granulate or powder bulk materials plays a crucial role in a wide range of production and processing applications. The following article describes current enhancements to and the advantages of a bulk material heat exchanger which is based on indirect heat exchange and a lean design.

Related Vendors

Glatt Ingenieurtechnik GmbH

Italvacuum, Srl

Angst+Pfister AG

Coperion GmbH

The Bulk-X-Change heat exchanger is designed to indirectly cool or heat bulk materials. It has an excellent three-year track record and is being used in an increasing number of applications in a range of industries including the plastics industry. The following list contains some examples of plastics heating or cooling applications in production environments:

• heating of pellets before extrusion to increase throughput or reduce energy consumption of extruder
• heating of plastic pellets in order to accelerate diffusion rates of following degassing processes
• cooling plastics following granulation and drying
• cooling prior to loading into plastic bags
• cooling to avoid oxidation or postpolymerization, (e.g. polyamide granulate)
• cooling following a degassing or deodorizing process
• cooling PET granulate following crystallization

Coperion Waeschle’s Bulk-X-Change is based on the following basic principle. Bulk materials are flowing slowly and gently by gravity down through vertical tubes. The heat transfer medium circulates around the tubes on the shell. A rotary valve discharges the bulk materials in a controlled manner. The product distribution plate for feeding the bulk materials into the tubes is a major feature of the solution. The inlet at every tube is funnel-shaped, and there is no horizontal surface between the tubes which could cause deposits and product contamination.

New: supplemental gas counterflow

The effect of gas counterflow on granulate bulk materials such as plastic granulate was evaluated during the further development phase. The investigations showed that gas counterflow through the tubes enhanced heat transfer during heating and cooling of granulate bulk materials. The following example shows the extent of this effect. Granulate temperature at the heat exchanger inlet on all trials: 95 °C; cooling water temperature: 27 °C; the geometry of the heat exchanger was not altered during the trials.

• At a mass flow rate of 770 kg/h and no gas counterflow, the exit temperature of the granulate was 60 °C.
• At the same mass flow rate (770 kg/h) with gas counterflow, the exit temperature of the granulate was 50 °C.
• Only a mass flow rate of 530 kg/h through the heat exchanger could be maintained to achieve a granulate exit temperature of 50 °C. Using gas counterflow in this example, it was possible to increase the throughput by about 50 percent.

The trials even showed that heat transfer could be doubled or even tripled in some cases. As a result, the size of the heat exchanger could be reduced significantly. In addition to tighter construction, this solution has other advantages:

• Reduction in product changeover time because the product volume in the bulk material heat exchanger is significantly smaller
• Reduction in cleaning time, especially if tube length is retained and fewer tubes are used
• Supplemental gas counterflow can be used to remove surface moisture (and internal moisture under certain conditions)
• Cleaning effect: gas counterflow removes some of the dust and fine particulate matter from the bulk materials
• Enhances performance with sticky bulk materials, because gas flow during ongoing operation separates the material somewhat and reduces the risk of agglutination

The trials also revealed that there is an optimal heat exchange tube diameter as well as an optimal gas volume flow which results in an optimal heat exchange.

Integration in a pneumatic conveying system

Integration of the bulk material heat exchanger directly into a pneumatic conveying line is an interesting option. Inline design is attractive when the available vertical space is insufficient to mount the heat exchanger above the product infeed. This is often the case when existing systems are modernized or upgraded. The pneumatic conveying line feeds into the top section of the heat exchanger which is configured as a separator. An overflow opens into a bypass line which carries the conveying gas and a bulk material residue around the heat exchanger. The main product volume flows by gravity through the tubes where it is heated or cooled. Further down at the discharge gate, the conveying gas is fed back into the main line where it picks up the product flow. The speed of the discharge rotary valve is set so that the discharge rate is somewhat lower than the overall conveying rate, eliminating the need for level control.

Sample bulk materials heat exchanger applications

A number of orders were received in recent months from customers with polyolefin and technical plastics applications. In many cases, the intention was to cool plastic granulate following degassing or to install the heat exchanger downstream from a silo dryer. Two examples are described.

Alternative to fluidized bed

Because of its simple design with no moving parts, the Bulk-X-Change can be an attractive alternative to a fluidized bed cooler in which the bulk material is fluidized and cooled using a gas flow. The Bulk-X-Change eliminates costly gas processing and cleaning which would otherwise be needed. This is a big advantage when the process must be performed under a nitrogen atmosphere or when exhaust gas has to be subjected to thermal oxidation to remove hydrocarbons. The heat exchanger itself also features a much leaner design. It only requires a relatively simple steel structure and is easy to install. It needs relatively little maintenance, and the risk of failure is minimal.

Alternative to a vibrating spiral cooler

The flow diagram on page 50 shows a conceptual solution for cooling plastic granulate following the granulation process. This solution, which uses two vacuum lines, is very flexible and is an attractive alternative to a vibrating spiral cooler. Following granulation, the bulk material is sucked into the bulk materials heat exchanger which gradually fills until the level sensor in the product buffer above the heat exchanger is activated, triggering the start signal for the second vacuum line. The granulate is suctioned out of the discharge cone using a special delivery shoe, eliminating the need for a rotary valve.

Summary: The bulk material heat exchanger is a very versatile piece of equipment which features a relatively simple design. It is often superior to conventional technology in a range of applications, and it is currently making inroads in the plastics industry.

The authors work at Coperion Waeschle GmbH & Co. KG, Weingarten, Germany.

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