Designing and maintaining a steam system which can deliver precise temperature control efficiently and safely requires an integrated approach using products which are carefully specified for the application. Bürkert has considerable expertise in designing complete packages as well as offering practical advice about crucial calculations and design specifications.
Bürkert has developed a wide range of control valves, pressure, temperature and flow sensors which can all be used to develop an efficient and accurate steam process control system. It has also produced a detailed Steam Site Guide, which can be downloaded from the company website, to help designers and maintenance engineers ensure that both new and existing facilities are properly managed.
Within the pet food industry, the cooking process used in 95 percent of all production is extrusion, a process which includes a pre-conditioner, the extrusion cooker and a die/knife assembly which shapes the final product. Prior to this, the raw ingredients are carefully blended, mixed and then ground, in order to produce a substrate that can be cooked and often dried to produce the finished product.
The pre-conditioner is designed to increase the temperature and the moisture content of the substrate by the addition of steam and water. The steam is fed in to the bottom of the pre-conditioner and must be carefully controlled in order to ensure a uniform distribution and even temperature rise within the substrate. The steam should be free from condensate and therefore will only contribute approximately 5 percent moisture to the substrate; to achieve the desired level of 10 to 30 percent, water is added from the top of the pre-conditioner.
In most applications, the steam is carefully controlled to elevate the substrate temperature to between 70 and 90 degrees C before it is discharged into the extrusion cooker. A typical example would be the Bürkert type 2301 globe control valve, combined with a type 8694 TopControl positioner. Following the Bürkert philosophy for modular components, the stainless steel valve is available in flange, threaded or welded formats, while the type of valve controller can be configured to suit each installation.
Once the moisture level has been adjusted, the substrate passes to the extruder itself, which uses a rotating screw to work the substrate into dough and also increase the pressure and temperature of the dough using a combination of friction and steam energy. By using a combination of sensors connected to a controller, such as the Bürkert type 8619 MultiCELL controller, the process can be monitored and adjusted to ensure that the optimum parameters are maintained. Extrusion temperature is often set at a critical control point (CCP) which manufacturers require as part of their food safety programme, which is designed to reduce the specific risk of Salmonella. Using a properly controlled system with data-logging facilities will help to ensure that the programme is consistently implemented and the evidence recorded. However, heating costs need to be properly managed as well, so finding the right balance between heat produced mechanically and heat from steam is essential to maximise the efficiency of the process.
Pet food safety is also a critical aspect of the production process that demands reliable and accurate control of the cooking process. Failure to meet specific legislation can have serious consequences, while operating an inefficient production process can significantly affect the financial margins for the product. In both cases, proper steam management is crucial.
The process of generating, conveying and controlling steam is one which requires expert advice and engineering knowledge, otherwise, at best, the process is inefficient, but at worst, serious injury can result.
