Heat exchangers
Heat exchangers
Different types of heat exchangers
The most widely used type of equipment at the end of the 19th century was the heater, one type of which is shown in figure 6.1.13. Despite its many shortcomings, this heat exchanger model was still in use in some dairies even in the 1950s.
In 1878 a German, Albert Dracke, was granted a patent on an apparatus in which one liquid could cool another by each flowing in a layer on opposite sides of series of plates. It is not known whether any such patents, one of which covers the heat exchanger shown in figure 6.1.14, ever left the drawing board. However, at the beginning of the 1920s the old German ideas were reappraised, and a regenerative heat exchanger based on these concepts. Since then plate heat exchangers have assumed a predominant role for heating and cooling purposes in the dairy industry.
The following three types of heat exchangers are the most widely used nowadays:
- Plate heat exchanger
- Tubular heat exchanger
- Scraped-surface heat exchanger
Plate heat exchangers
Most heat treatment of dairy products is carried out in plate heat exchangers. The plate heat exchanger (often abbreviated PHE) consists of a pack of stainless steel plates clamped in a frame.
The frame may contain several separate plate packs sections in which different stages of treatment such as preheating, final heating and
cooling take place. The heating medium is hot water, and the cooling medium cold water, ice water or propyl glycol, depending on the required product outlet temperature.
The plates are corrugated in a pattern designed for optimum heat transfer. The plate pack is compressed in the frame. Supporting points
on the corrugations hold the plates apart so that thin channels are formed between them.
The liquids enter and leave the channels through holes in the corners of the plates. Varying patterns of open and blind holes
route the liquids from one channel to the next.
Gaskets round the edges of the plates and round the holes form the boundaries of the channels and prevent external leakage and internal mixing.
Flow patterns
The product is introduced through a corner hole into the first channel of the section and flows vertically through the channel. It leaves at the other end through a separately gasketed corner passage. The arrangement of the corner passages is such that the product flows through alternate channels in the plate pack.
The service (heating or cooling) medium is introduced at the other end of the section and passes, in the same way, through alternate plate channels. Each product channel consequently has service medium channels on both sides.
For efficient heat transfer the channels between the plates should be as narrow as possible; but both flow velocity and pressure drop will be high if a large volume of product must pass through these narrow channels. Neither of these effects is desirable and, to eliminate them, the passage of the product through the heat exchanger may be divided into a number of parallel flows.
In figure 6.1.16 the blue product flow is divided into two parallel flows which change direction four times in the section. The channels for the red heating medium are divided into four parallel flows which change direction twice.
This combination is written as 4 x 2 / 2 x 4, i.e. the number of passes times the number of parallel flows for the blue product over the number of passes times the number of parallel flows for the red service medium. This is called the grouping of the plates. 
Tubular heat exchangers
Tubular heat exchangers Tubular heat exchangers (THE) are in some cases used for pasteurisation/ UHT treatment of dairy products. The tubular heat exchanger, figure 6.1.17, unlike plate heat exchangers, has no contact points in the product channel and can thus handle products with particles up to a certain size. The maximum particle size depends on the diameter of the tube. The tubular heat exchanger
can also run longer between cleanings than the plate heat exchanger in UHT treatment.
From the standpoint of heat transfer the tubular heat exchanger is less efficient than a plate
heat exchanger.
Tubular heat exchangers are available in two fundamentally different types; multi/mono channel and multi/mono tube
Multi/mono channel
The heat transfer surface of a multichannel tubular heat exchanger, shown in figure 6.1.18, consists of straight tubes of different diameters concentrically located on a common axis by headers (1) at both ends. The tubes are sealed against the header by double O-rings (2), and the whole assembly is held together by an axial compression bolt (3).
The two heat exchange media flow in countercurrent in alternate annular channels between concentric tubes. The service medium is always supplied to the outermost channel. A header at each end acts as both distributor and collector, supplying one medium to one set of channels and discharging the medium from the other set. The corrugated configuration of the tubes keeps both media in a state of turbulence for maximum heat transfer efficiency.
It is also possible to use this type of tubular heat exchanger for direct product/product regeneration. The monochannel is a version with only one annular product channel enclosed between two concentric channels for service medium.
Multi/mono tube
The multitube tubular heat exchanger operates on the classic shell and tube principle, with the product flowing through a group of parallel tubes and the service medium between and around the tubes. Turbulence for efficient heat transfer is created by helical corrugations on the tubes and shell.
The heat transfer surface consists of a bundle of straight corrugated or smooth tubes (1) welded into
tube plates at both ends, figure 6.1.19. The tube plates are in turn sealed against the outer shell by a double O-ring construction (2) (floating design). This design allows the product tubes to be taken out of the shell by unscrewing the end bolts. This makes the unit strippable for inspection.
The floating design absorbs thermal expansion and the product tube bundles in the shell can be changed, allowing different combinations to be used for different applications.
The monotube is a version with only one inner tube, which will permit particles with a diameter up to 50 mm to pass. Multi/mono tubes are well suited for processes operating at very high pressures and high temperatures.
Scraped-surface heat exchanger
The scraped-surface heat exchanger, figure 6.1.20, is designed for heating and cooling viscous, sticky and lumpy products and for crystallisation of products. The operating pressures on the product side are high, often as much as 40 bar. All products that can be pumped can therefore be treated.
A scraped surface heat exchanger consists of a cylinder (1) through which the product is pumped in countercurrent flow to the service medium in the surrounding jacket. Exchangable rotors (2) of various diameters, from 50.8 to 127 mm, and varying pin/blade (3) configurations allow adaptation to different applications. Smaller diameter rotors allow larger particles (up to 25 mm) to pass through the cylinder, while larger diameter rotors result in shorter residence time and improved thermal performance.
The product enters the vertical cylinder through the lower port and continuously flows upwards through the cylinder. At process start-up, all the air is completely purged ahead of the product, allowing complete and uniform product coverage of the heating or cooling surface.
The rotating blades continually remove the product from the cylinder wall, figure 6.1.21, to ensure uniform heat transfer to the product. In addition, the surface is kept free from deposits.
The product exits the cylinder via the upper port. Product flow and rotor speed are varied to suit the properties of the product flowing through the cylinder.
At shut-down, thanks to the vertical design, the product can be displaced by water with minimum intermixing which helps assure product recovery at the end of every run. Following this, completely drainage facilitates CIP and product changeover.
As mentioned above, rotor and blades are exchangeable, an operation which is possible owing to the automatic hydraulic lift that facilitates raising and lowering the rotor/blade assembly, figure 6.1.22.
Typical products treated in the scraped-surface heat exchanger are jams, sweets, dressings, chocolate and peanut butter. It is also used for fats and oils for crystallisation of margarine and shortenings, etc.
The scraped-surface heat exchanger is also available in versions designed for aseptic processing. Two or more vertical type scraped-surface heat exchangers can be linked in series or parallel to give a greater heat transfer surface depending on the processing capacity required.
