Answer the questions:

1. Why was interest to effects of isostatic pressure on microorganism inactivation and on polysaccharide denaturation triggered?

2. When is a novel technology interesting for manufacturers?

3. What are consumers concerned with?

4. What are advantages of high pressure processing?

5. What do suppliers primarily focus on?

6. What makes Flow International one of the leading manufacturers of high pressure equipment?

7. What are equipment developers working on?

Equipment for High Pressure Processing.

There are two major types of high pressure processing of food products: the (conventional) batch systems, derived from cold isostatic processing, and the semicontinuous systems. Batch systems can process both liquid and solid products, but these have to be prepacked. In-line systems can be applied only to pumpable products (e.g. orange juice). The product is pumped into the pressure vessel and pressurized using a floating piston, which separates the product from the pressure medium.

Batch Systems.

Vessels and Yoke.

Equipment for high pressure batch processing consists of three main parts: the vessel, its surrounding yoke, and the hydraulics. The actual pressure treatment takes place in the pressure vessel, which is considered to be the heart of the equipment. The volume of the vessel may vary from less than a liter (for laboratory-scale applications operating at 1000 MPa) up to 500 liters (for processing units operating at 600 MPa).

In many cases, the pressure vessel is a forged cylinder constructed in low-alloy steel of high-tensile strength. The wall thickness of these (monobloc) vessels is determined by the maximum working pressure, the vessel diameter, and the number of cycles for which the vessel is designed. The use of industrial-scale monobloc vessels is limited to working pressures up to 600 MPa. Above these pressures, stresses at the inner core approach the yielding stress of the material, resulting in a diminishing fatigue resistance. To overcome this problem, pressure vessels with residual compressive stress have been developed that result in a higher yield strength. However, the costs of manufacturing these vessels are significantly higher than for monobloc vessels that are forged and processed in one single piece. Therefore, prestressed vessels are used only for high pressure processes where monoblocs cannot be used. Three techniques can be used to apply a tangential compression on the inner core of pressure vessels.

Autofrettage. The vessel is subjected to an internal pressure exceeding the operating pressure. The maximum stress in the autofrettage zone exceeds the yield stress of the material, and plastic deformation occurs. Because tangential stresses decrease rapidly in a radial direction, plastic deformation takes place at the inner core only. After depressurization, the plastically deformed region is a compressive tangential state, thus opposing the tensile stresses resulting from high pressure processing. Consequently, a higher working load can be achieved.

Multilayer vessels. By building vessels in multiple layers, the inner layers can be prestressed by giving the outer layer a negative tolerance to the inner layer. Mounting can be performed after cooling the inner cylinder and heating the outer cylinder. After cooling, the inner cylinder is under a state of compression.

Wire-wound and strip-wound vessels. This is the process Flow Pressure Systems and Kobe Steel apply to their vessels. When a rectangular spring steel wire is wound around a steel cylinder, the cylinder becomes prestressed. Spring steel wire has a yielding strength of 1600 N/mm² and a rupture strength of 1900 N/mm², which is three times stronger than normal forged high-strength steels. The repetitive mechanical deformation of the wire (cold rolling) strengthens the spring steel wire. The winding process is computer controlled. To manufacture a production-scale vessel, up to a few hundred kilometers of wire is needed. Because a wire-wound vessel has a higher yield strength and the ability to withstand the optimum stress at every diameter, it can have halve the weight of other designs, thus reducing size of the enclosing frame as well. Kobe Steel indicates that a unit containing a multilayer vessel with an inner capacity of 100 liters weighs about 29 tons; a unit containing a wire-wound vessel weighs about 10 tons. UHDE (a German company) uses a similar pre-stressing technique. However, instead of wires, strips compress the inner core of the vessel. In this case, the vessel is constructed of pre-stressed elements that are fit onto a conical liner.

To cope with the high axial forces acting on the vessel and its end closures, the vessel is surrounded by a yoke. This yoke consists of either a number of steel plates (for instance, the ACB Pressure System-Alstom Hyperbar, Figure 1) or a wire-wound steel frame (Flow Quintus System). The position of the vessel in the yoke - either horizontal or vertical - depends on the type of processing. The vessel in the Flow Quintus System is positioned vertically, whereas the ACB Pressure System-Alstom Hyperbar has a horizontally placed pressure vessel. The main advantage of the horizontal placement is that charging and discharging can take place quickly (the untreated packages may push out the treated packages). ACB Pressure System-Alstom developed a quick opening-closing system for its Hyperbar system to speed up processing. Furthermore, it is technically preferable to scale up in axial direction, which is easier for horizontal constructions. An advantage of vertical construction is that the pressure medium is reused in the vessel after discharging the product. This lowers the total cycle time, since refilling of the ves­sel is less time consuming.