Food

Errors in the cooling process can lead to a matte, grey surface (caused by uncontrolled crystallization of cocoa butter) and this can affect the melting texture – thus the taste – as well as the break.

The task of bringing the medium “chocolate” to the right temperature within the right time period is made that much more difficult due to one factor: If the chocolate is used as a coating for bars or pralines, it needs to set quickly and smoothly on the product. This requires a relatively low target temperature so that the chocolate becomes very viscous. In this case, its dynamic viscosity can be up to 10,000 Centipoise.

That is why the heat exchangers used in the temperature controllers for chocolate coatings must meet the following specifications:

• High pressure resistance due to the highly viscous medium
• Low difference between the temperature of the target and the cooling medium to prevent the crystallization of the cocoa butter in the heat exchanger.
• Compact design, i.e., the highest possible thermal performance per surface area

Size and cost advantages of plate heat exchangers

The requirements listed above can be met with shell & tube heat exchangers, for example. Gasketed plate heat exchanges represent a less expensive and compact alternatives to this. However, several aspects must be taken into account for plate selection and heat exchanger design due to the viscosity of the sweet medium.

Design for temperature controllers

Depending on the process, the flow control design of the heat exchanger is either one-way or two-way. A one-way design is sufficient if the heating and cooling takes place one after the other. However, if the product is heated and cooled continually at the same time, then two passes are critical.

In both cases, the sensitive cooling process typically takes place using warm water at approx. 29 C. The water should cool the chocolate mass down from 45°C to 30°C. This small difference in temperature ensures that the product cannot be “undercooled” at any point as this could lead to the crystallization of the cocoa butter contained therein. In order to provide the required cooling capacity for the small temperature difference, approx. ten times more water than chocolate flows through the heat exchanger.

Slow media flow on the sweet side

The channels on the intake of the plates have asymmetrical flow channels to ensure an even flow for the heat exchanger plate at flow speeds of sometimes only 0.02 m/s in the case of chocolate. In cases where the chocolate travels a longer distance, the channels are wider so that the flow resistance of all channels is identical. This results in an even flow for the entire cross section, optimizing the heat transfer and preventing product deposits. This, in turn, allows for longer production times between cleaning intervals.

The plates from either the NT or the NL series models are suitable for this application. Both are suitably pressure-resistant – this is important because a high pressure must be built up in order to be able to convey the viscous chocolate mass and this pressure is often up to 10 bar for two-stage heat exchangers. Both plate types have a herringbone corrugation and medium-sized gaps. Narrower gap sizes would certainly improve the heat transfer, but cannot be used due to the high viscosity. Therefore, the NL plates, for example, have a gap size of approx. 4 mm. This results in an ideal compromise between high efficiency and low pressure loss.

Plate heat exchangers have become common

Temperature controllers with plate heat exchangers are currently being used in many plants in the confectionary industry. Gasketed plate heat exchangers transfer heat in most applications more efficiently than shell & tube heat exchangers. This is mainly due to the size of the corrugation pattern of the plates which creates a type of turbulence and thus encourages the heat exchange. They use only 20% to 50% of the space that comparable shell & tube heat exchangers would require and they are also much more lightweight.