Pasteurizer
Pasteurizer
The main purpose of pasteurizing beverages is to extend the product's shelf life. This process kills all microorganisms that can grow in the beverage and all enzymes that may cause chemical changes. The duration and temperature of the treatment are key to killing harmful bacteria in the beer.
This critical effect is often defined using the "Pasteurization Unit," abbreviated as PU. One pasteurization unit corresponds to the pasteurization effect produced by sterilization at 60°C for one minute. Most beers have a pasteurization unit of 8–15, low-alcohol beers 20–40, and malt beers 100–400. Pasteurization of juices and canned products often uses the "time priority over temperature" principle rather than the pasteurization unit principle. For example, orange juice containing pulp requires sterilization at 70°C for 5–10 minutes.
Tunnel pasteurization is a method of treating already filled containers. Untreated product is filled into containers, the containers are sealed, and then conveyed by a conveyor belt into the interior of the tunnel pasteurizer. In each temperature zone of the pasteurizer, the product temperature is precisely controlled. As they pass through the tunnel, the containers are sprayed with water at different temperatures, namely the heating stage, the sterilization stage, and the cooling stage.
Gradual heating at the front of the pasteurizer and gradual cooling at the rear are very important. This avoids both flavor loss caused by overheating and bottle breakage caused by sudden thermal stress on the containers.
In tunnel pasteurizers, three heat transfer methods are commonly used: one is a spray plate with many spray holes (called a spray plate system), where hot water is sprayed onto the containers through the spray plate; another is a spray pipe with several spray nozzles (called a spray nozzle system), where hot water is sprayed onto the containers through the spray nozzles; the third is a rectangular spray pipe with slots (called a spray pipe system), where hot water is sprayed onto the containers through the spray pipe.
At present, the most commonly used is the spray pipe system, which has a simple structure, is practical, and has relatively uniform water distribution. The spray plate system has the advantage of even more uniform water distribution but has higher manufacturing costs. Since the spray nozzle system has a relatively complex structure and takes longer to replace nozzles, its maintenance and servicing are relatively less convenient.
Design and structural requirements for tunnel pasteurizers
Several (6–8) spray zones are configured in the tunnel pasteurizer, each operating at a different temperature. The incoming containers are gradually heated until the required temperature and heat treatment effect are achieved. Similarly, the cooling of the containers is also gradual from the holding temperature down to the temperature required at the exit.
The structural design of the tunnel pasteurizer should meet actual production requirements, such as the passage time of containers and their size, weight, etc.; the characteristics of the product, the required treatment time and temperature, and the size of the workshop are factors that determine the optimal structural design of the machine.
Modern manufacturing technology can use computer-aided design to address these factors, and computer-aided design can meet various technical specifications of the machine. Tunnel pasteurizers are manufactured in a modular manner and can also be specially manufactured according to user-specific conditions, for example, extending individual treatment zones when processing special products.
Older pasteurizers mostly use hydraulic step-by-step conveying systems, while newer pasteurizers use bed-type conveying systems, i.e., chain mesh conveyor belts. This type of conveyor belt can transport products smoothly, containers are not easily tipped over, and it has good water permeability. At present, two types of chain mesh conveyor belts are commonly used: one is a chain plate conveyor belt made of stainless steel, and the other is a chain mesh conveyor belt made of plastic. Plastic conveyor belts are mainly used for conveying aluminum easy-open cans and lightweight disposable glass bottles, as they do not easily scratch the can bottoms, but they are prone to aging with long-term use. Stainless steel belts are mainly used for conveying heavier glass bottles and have a long service life. Some pasteurizers equipped with heavy-duty plastic conveyor belts can also convey heavier glass bottles. With the improvement of chain mesh manufacturing technology, stainless steel chain mesh is now also commonly used in can pasteurizers. New pasteurizers can also be equipped with automatic filter mesh belts, which can remove broken glass and other debris from the machine, minimizing maintenance workload.
PU value control requirements for tunnel pasteurizers
Good sterilization requires precise control of the pasteurization unit to ensure that the product is not under-sterilized and receives minimal over-sterilization. The pasteurizer converts the PU value set on the temperature controller into an actual control temperature curve for each temperature zone's water tank, and precisely controls temperature changes in each zone through temperature detection. The main drive motor drives the upper and lower layers of the bottle-conveying chain mesh to carry the material through the three temperature zones for gradual preheating, two sterilization zones, and three cooling zones for gradual cooling.
The PU value online control system is an electro-pneumatic hybrid automatic control system. Signals are sent directly from thermal resistors (Pt100) and pressure transmitters to the programmable logic controller (PLC), and then through communication cables to the industrial computer. Control of solenoid valves and pneumatic diaphragm valves enables full-process regulation of the PU value. The temperature of each zone is sent directly to the PLC via Pt100 platinum resistance thermometers; the water pump pressure of each zone passes through distributors and solenoid valves to pressure sensors, is converted into 4–20mA signals, and then sent to the PLC for sequential acquisition. The PLC performs input data processing and PID operations on one hand, and transmits various signals to the industrial computer on the other. As the human-machine interface, the industrial computer can monitor the operation status and various parameters of the sterilization process in real time, provide alarms, historical trend charts, and data archive reports, and can automatically calculate the set temperature for spray water in each zone based on the PU value set by the operator.
After the input temperature signal undergoes PID operations, the PLC controls the solenoid valves, which convert electrical control signals into pressure signals to control the pneumatic diaphragm valves, thereby controlling the amount of cold water or steam and achieving temperature control in each zone.
Within the PLC, the output signal of each zone is interlocked with the water pump, meaning that only when the pump is working can the pneumatic diaphragm valve perform automatic adjustment. Otherwise, the regulating valve will automatically close. If the bottle conveyor stops working for any reason, the sterilization zone and superheat zone will automatically adjust the temperature set values according to the duration of the stoppage to ensure sterilization quality.