Heat exchanger for wafer cream
The Filling Makes the Difference. When it comes to filled wafers and ice cream wafers, the cream makes the crucial difference. The delicate interlayer made of nougat or vanilla cream, for example, imparts the pastry with its own individual taste and requires a special production process. This is because a thin layer needs to be applied with precisely the right thickness in order to fit optimally between the wafer layers. The cream must be flowable, but not too runny because this could macerate the pastry and then its thickness could become difficult to control. On the other hand, if it is too firm, the coating becomes difficult to distribute and any successful application would require a higher pressure.
Cost advantages of gasketed plate heat exchangers
In order for the wafer cream to take on the ideal consistency after stirring, it must be cooled down from approximately 42 C to 28°C via a heat exchanger. Gasketed plate heat exchangers have proven themselves for this task as a cost-effective alternative to shell & tube heat exchangers, for example their design is absolutely crucial because the following characteristics require a meticulous concept:
- The cream is highly viscous at the temperature at which it is applied. Depending on the recipe, the dynamic viscosity can be up to 5,000 Centipoise. When the viscosity increases, so too does the pressure loss in the heat exchanger and, therefore the required operating pressure.
- The medium to be cooled passes through the heat exchanger at a low speed. Therefore, many methods that prove effective for low viscosity media to ensure an even flow for the heat exchanger plate do not work in this case.
- The target temperature must be reached within a specific time period and must be maintained precisely. A partial “undercooling” of the cream in sections of the heat exchanger could lead to a crystallization process of the cocoa butter, for example, which would be a quality defect.
Selection of suitable heat exchanger plates
When dealing with heat exchanger plates, you need to find the optimal compromise between effective heat transmission and low pressure loss. When selecting the gap width and corrugation, you don’t need to take into account particle size because the wafer cream is a fluid without particles.
NL plates with a herringbone corrugation and a gap width of approximately 4mm have proven themselves as a suitable choice for many pasty media. Compared to the VT series plates used before, they offer higher efficiency and enable improved distribution of the media in the gap. Even a shear-sensitive, viscous product like yogurt, for example, can pass through the NL plates without affecting its sensitive structure. The NL plates have been tested for operating pressures up to 25bar so that the design can cope with the high pressures that inevitably occur when dealing with viscous media.
Maximum efficiency and minimum cleaning requirement
The even distribution of the medium over the entire plate width is desired to ensure an effective cooling of the cream. This is achieved by the triggering of turbulences for runny media. However, this turbulence effect fails to appear for the low flow speeds of the cream of approximately. 0.02 to 0.04m/s that occurs in the channels forming between the plates. In order to achieve an even distribution, the corrugation has different, asymmetrical flow channels at the intake of the NL plates. The channels at the edges through which the product travels a greater distance are slightly bigger so that the medium is distributed evenly, ultimately achieving the same product volume flow at all locations of the plate cross section according to the distribution matrix. This supports an even distribution when the product is also fed through the heat exchanger from the bottom to the top. The flow through the entire cross section not only optimizes the heat transfer, but the homogenous product distribution also prevents deposits, which allows for longer operating times between cleaning cycles when compared to older models.
Single-stage design for low pressure losses
When considering pressure losses, you also have to ensure that the cooling process can take place as much as possible without redirection in one path. Therefore, a relatively large heat transfer surface is required because the cooling water is only about 1 K colder than the target temperature, depending on the composition of the cream. This low temperature difference ensures that no crystallization takes place due to extreme cooling. Approx. 10 to 20 times the amount of water is used for each cubic meter of cream that flows through the heat exchanger to achieve the desired cooling performance for this low temperature difference.
Successful recipe for cream and cooling
Kelvion heat exchangers with their NL80M plate, for example, have been proving their knack for cooling at an Italian wafer manufacturer since 2017, where creams of various flavors are cooled to temperatures of between 28 C to 25 C, whereby the media flow at different speeds depending on the target temperature and viscosity. Due to their excellent performance, the manufacturer has decided to equip additional production lines in 2021 with gasketed plate heat exchangers using NL plates.