Vacuum Belt Dryer

Overview

A vacuum belt dryer dries a pumpable food paste continuously and gently. A thin layer of paste is spread onto a slow-moving stainless steel belt that carries it through a long sealed Vacuum Chamber. Because the chamber is held at low pressure, water boils off the paste at a low temperature, often well under 70 C, so heat-sensitive products keep their colour, aroma, and nutrients. The product dries to a porous solid that is scraped off and broken up at the cool end. It suits fruit and vegetable concentrates, coffee and tea extracts, flavours, and other high-value pastes that would scorch in a hot-air dryer and that benefit from running continuously rather than in batches.

The machine combines a continuous conveyor with a vacuum process. The Belt System carries the paste over a series of Heating Zones and a final Cooling Zone, the Feed System lays down the layer at the inlet, the Vacuum System holds the low pressure and condenses the vapor, and the Discharge System takes the dry product off. A Control System coordinates the vacuum, heat, and belt speed, which together set the dryness.

Why vacuum

The reason for the vacuum is simple physics: water boils at a lower temperature when the pressure over it is lower. At a chamber pressure of a few tens of millibar, water boils around 30-45 C, so the Heating Zones only need to warm the product to a gentle temperature to drive the water off fast. This protects the flavour and nutrition that high heat would destroy. The trade-off is that the dryer must be a sealed pressure vessel and must handle a very large volume of low-pressure vapor, which is why the Vacuum System includes a condenser as well as pumps.

The belt and how heat reaches the paste

The Belt System is a continuous thin Steel Belt running between a Drive Drum and a Tension Drum. The belt does two jobs: it carries the paste, and it conducts heat into it from the platens beneath. The Belt Drive Unit turns it slowly, because belt speed sets how long the paste spends in the heated zones, which is its residence time and a main control on final moisture. Tracking Roller idlers keep the belt centred so it does not wander off the platens, and a Belt Wash Station station cleans the return run before it picks up fresh paste.

Staged heating and final cooling

Drying is fastest when the product is wettest and slows as it dries, so the Heating Zones are split into several independently controlled platens down the length of the tunnel. Hot water or low-pressure steam circulates through each Heating Platen under the belt, driven by the Circulation Pump, and each Zone Valve sets that zone's temperature. Typically the early zones run warmest to boil off the bulk of the water and the later zones cooler to finish gently without overheating the now thin, fragile layer. At the end the Cooling Zone runs chilled water through its platens to bring the dry, brittle product down to a stable temperature so it does not soften or stick when it is scraped off.

Feeding the layer

The Feed System sets the other main capacity control: layer thickness. A Feed Pump meters paste from the Feed Hopper to an Applicator Roll that lays it onto the belt, and a Spreader Bar trims it to an even thickness across the width. A thinner wet layer dries faster and to a lower moisture but cuts throughput; a thicker layer raises output but needs more residence time. Because the chamber is under vacuum, a Feed Seal at the belt entry lets the paste in while holding the pressure.

Vacuum and vapor handling

The Vacuum System must do more than pull a vacuum once; it has to remove the steady stream of vapor the drying paste gives off. Most of that vapor is captured by the Vapor Condenser, a cold surface that condenses it back to water, which the Condensate Pump removes. The Vacuum Pump and Vacuum Booster then only have to handle the small remaining load of air that leaks in and gases that come out of the product. A Vacuum Gauge reading is a key process variable, since the pressure fixes the boiling temperature and so how hard the product can dry.

Discharge, control, and structure

At the cool end the Discharge System lifts the dry sheet off with a Discharge Scraper, a Product Crusher breaks it into flakes or powder, and a Discharge Airlock passes the product out of the chamber without breaking the vacuum, onto a Discharge Conveyor to packing. The Control System PLC holds the chamber vacuum and each zone temperature on a recipe and sets belt speed and feed rate through its Variable-Frequency Drive drives, with the Temperature Sensor product probes confirming the layer is dry by the cooling zone. The whole tunnel rides on the Frame & Structure, a long rigid structure that keeps it straight and level so the belt tracks true and that carries the heating, cooling, and vacuum service piping along its length.