Dairy Processing Handbook/chapter 6.1 87

through holes in the corners of the plates.Varying patterns of open and blind holesroute the liquids from one channel to thenext.

Gaskets round the edges of the platesand round the holes form the boundaries ofthe channels and prevent external leakageand internal mixing.

Flow patternsThe product is introduced through a cor-ner hole into the first channel of the sec-tion and flows vertically through the channel. It leaves at the other endthrough a separately gasketed corner passage. The arrangement of thecorner passages is such that the product flows through alternate channelsin the plate pack.

The service (heating or cooling) medium is introduced at the other end ofthe section and passes, in the same way, through alternate plate channels.Each product channel consequently has service medium channels on bothsides.

For efficient heat transfer the channels between the plates should be asnarrow as possible; but both flow velocity and pressure drop will be high if alarge volume of product must pass through these narrow channels. Neitherof these effects is desirable and, to eliminate them, the passage of theproduct through the heat exchanger may be divided into a number of paral-lel flows.

In figure 6.1.16 the blue product flow is divided into two parallel flowswhich change direction four times in the section. The channels for the redheating medium are divided into four parallel flows which change directiontwice.

This combination is written as 4 x 2 / 2 x 4, i.e. the number of passestimes the number of parallel flows for the blue product over the number ofpasses times the number of parallel flows for the red service medium. Thisis called the grouping of the plates.

Tubular heat exchangersTubular 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 contactpoints in the product channel and can thus han-dle products with particles up to a certain size.The maximum particle size depends on the dia-meter of the tube. The tubular heat exchangercan also run longer between cleanings than theplate heat exchanger in UHT treatment.

From the standpoint of heat transfer the tubu-lar heat exchanger is less efficient than a plateheat exchanger.

Tubular heat exchangers are available in twofundamentally different types; multi/mono chan-nel and multi/mono tube.

Multi/mono channelThe heat transfer surface of a multichannel tubu-lar heat exchanger, shown in figure 6.1.18, con-sists of straight tubes of different diameters con-centrically located on a common axis by headers(1) at both ends. The tubes are sealed againstthe header by double O-rings (2), and the whole assembly is held to-gether by an axial compression bolt (3).

The two heat exchange media flow in countercurrent in alternateannular channels between concentric tubes. The service medium is always

Fig. 6.1.16 The system of parallel flowpattern for both product and heating/cooling medium channels. In this examplethe combination is written 4 x 2 / 2 x 4.

Fig. 6.1.17 The tubular heat exchangertubes are assembled in a compact unit.