Temperature build-up inside the milling chamber comes from particle collision, friction between material and mill components, and viscous air compression. Uncontrolled heat triggers protein denaturation, so multi-layer thermal management is required for dry fractionation of legume protein.
Cooled process air supply
Feed chilled, dry air into the grinding and classification loop. Cold airflow continuously removes frictional heat and transports fine particles out of the hot zone rapidly. For sensitive native protein, maintain inlet air temperature at 5–15 °C. Avoid humid cold air, which causes powder agglomeration. Nitrogen inert cooling can be adopted if dust explosion risk exists.
Jacket cooling for mill body and classifier housing
Fit water/glycol cooling jackets on the grinding chamber, rotor housing and air classifier casing. Circulate low-temperature coolant to dissipate conductive heat from metal surfaces and eliminate local hot spots. Monitor jacket coolant inlet/outlet temperature to maintain stable heat removal.
Optimize operational parameters to reduce heat generation
Avoid unnecessarily high rotor peripheral speed; reduce impact energy where selective liberation can still be achieved.
Maintain consistent, non-overloaded feed rate. Overfeeding creates material accumulation, longer particle residence time and heat stacking; underfeeding leads to excessive metal-to-metal collision and hot spots.
Shorten particle residence time inside the grinding chamber. Increase transport airflow to extract finished fines quickly so particles are not repeatedly impacted.
Raw material pre-conditioning
Control raw seed moisture at the optimal range (8–11% for pea and chickpea). Proper moisture reduces brittleness and lowers the impact force required for cell dissociation, cutting frictional heat generation. Over-dried beans absorb more impact energy and produce higher temperature spikes.
PLC temperature interlock and real-time monitoring
Install high-response temperature sensors at the mill outlet, grinding chamber wall and classifier zone. Link sensors to the control system: automatically slow feed rate or trigger temporary shutdown once powder temperature exceeds the set threshold (typically 40 °C for native protein). Track outlet powder temperature continuously rather than only casing temperature.
System design and heat loss reduction
Use polished, smooth rotors and liners to minimise friction. Keep the mill pipeline properly insulated to prevent hot air recirculation back into the grinding zone. Separate hot coarse return streams with heat exchange before recirculation, avoiding feeding warm middlings back into the milling chamber.
Excessively high airflow or aggressive cooling may increase energy consumption and alter the air classification cut point, affecting protein-starch separation efficiency. Thermal control must be balanced with target PSD and separation performance.
