The present invention relates to a plant for heat treat¬ ment of a liquid food product.

A plant of this kind which usually is used for pasteurizing of food products, as milk, comprises a pasteurizer with one or several regenerative sections, a heating section, a holding section in which the food product is kept at a predetermined temperature during a predetermined time and a cooling section.

When a food product is to be treated in the plant it is firstly brought to pass one or several regenerative sections where it is heated by already heat treated product, then the heating section where it is heated to the desired heat treatment temperature - when pasteu¬ rizing 72-75°C - by a medium giving off heat and then kept in the holding section at this temperature during a predetermined time. After the holding section the product is again brought to pass the regenerative section or sections in order to give off obtained heat to entering product. As an ending step the product is cooled with a cooling medium to the desired storing temperature in the cooling section. In order to illus¬ trate the way the product takes through the regenerative sections the expression "upstream" is used for the passage up to the heating section and "downstream" for the passage away from the heating section.

A plant of this kind also comprises a temperature transmitter connected to a control unit which is arranged at the outlet from the holding section. A first flow diversion valve is arranged downstream of the temperature sensor. This valve is arranged to be movable

between a first position in which the food product is transported further to the downstream side of the regenerative section (return side) and a second position in which the food product is returned to the inlet of the pasteurizer. In order to ensure that the pressure is higher in the heat treated product than in not treated product a pressurizing means is arranged downstream of the diversion valve.

Pasteurizers of this kind have been available on the market during several years. The pressure increasing means - usually a booster pump - has been prescribed in laws regulating pasteurization in many countries.

Within EC rules are to be enforced which demand that the products which have been heat treated in order to obtain a pasteurization should be truly pasteurized. The product should be subjected to a correct temperature treatment and unpasteurized product may not leak into the pasteurized product neither through leaking valves nor during the passage through the pasteurizer.

In order to meet these demands it is proposed according to the invention that a plant of the kind described above should be equipped with pressure transmitters at least at the product inlet prior to the first regenerative section on the upstream side of the pasteurizer and downstream of the regenerative section on the downstream side of the pasteurizer (alternatively after the cooling section). These pressure sensors are connected to the control unit mentioned earlier. Downstream of the cooling section there is arranged a constant pressure valve and after this valve in the flow direction there is a second flow diversion valve. This second diversion valve is movable between a first

position in which the food product is transported away from the plant and a second position in which the food product is returned to the inlet of the pasteurizer. A further conduit for transport of the food product is arranged between the first diversion valve and the outlet from the constant pressure valve. If the control unit receives a signal that erroneous pressure conditions are sensed by the pressure sensors or if too low a temperature is sensed by the temperature sensor, the first diversion valve is actuated to move to its second position in which heated food product is returned to the inlet of the pasteurizer and is brought to pass it afresh. Simultaneously the pressurizing means forces the amount of heat treated food product which at the change-over of the diversion valve is situated in the regenerative section and in the cooling section of the pasteurizer to circulate in a circulation path over the downstream side of the regenerative section in such a way that the pressure of this amount of heat treated food product is maintained.

With a plant designed in this way it is possible to ensure that the desired pressure difference between the downstream side of the regenerative section which is passed by heat treated (pasteurized) product and its upstream side where not yet treated product is passing, is maintained even when the product flow to the down¬ stream side of the regenerative section falls away when the first diversion valve returns the product to the inlet of the pasteurizer following for example an insufficient heating in the heating section.

The further transport conduit which is taken in use in order to create the circulation path is during normal operation closed by means of shut-off valves, which open

simultaneously as the first diversion valve takes its second position.

In a plant according to the invention a self-cleaning centrifugal separator may be connected to the upstream side of the regenerative section. The food product is led to the centrifugal separator after a preheating in the first regenerative section and after clarification and a possible separation of heavier or lighter fraction the whole or the largest part of the food product is returned to the second regenerative section. Also the centrifugal separator is a part of the circuit which is obtained over the pasteurizer when the first diversion valve returns food product directly to the inlet of the pasteurizer.

The first as well as the second diversion valves consist with advantage of mixproof three-way-valves approved for food.

If the food product which is to be treated in the plant consists of milk, equipment as well as for standar¬ dization as for homogenization of the milk may be connected between the outlet from the centrifugal separator and the inlet of the second regenerative section.

The plant according to the invention is described further with reference to the attached drawing which schematically shows an embodiment of a milk pasteurizer chosen as an example only.

The plant comprises a balance tank 1 for untreated product in this case milk. The product is transported by a pump 2 to a pasteurizer 3 which consists of a plate

heat exchanger. This comprises four sections 4, 5, 6 and 7. The product passes at first a regenerative section 5 where it is heated by earlier treated product and is then led further to a centrifugal separator 8 where a cream fraction 9 is separated off. If it is suitable the plant is provided with equipment for standardization and homogenization of the milk. After separation of the cream the milk is led to a second regenerative step 6. Also in this step the heating of the incoming product takes place by means of already heat treated milk. From the regenerative section the milk is transported to the heating step 7 where it is heated to a predetermined temperature by means of hot water or steam 9. In order to obtain the necessary holding time at the predeter- mined temperature the milk is brought to pass a holding cell 10. A temperature sensing means 11 senses the temperature of the milk and sends this information to a control unit which is not shown in the drawing. A first diversion valve 12 is movable between a first position in which the milk is transported further to the regene¬ rative section 6 and a second position in which the milk is returned to the balance tank 1.

In the conduit between the first diversion valve 12 and the regenerative section 6 there is a booster pump 13. From the regenerative section 6 the milk is transported to the regenerative section 5 and the cooling section 4 where it is cooled with ice water 14 to a suitable storing temperature.

In the conduit from the cooling section 4 there is a constant pressure valve 15. A second diversion valve 16 is connected to the constant pressure valve. This second diversion valve is movable between a first position where pasteurized milk is transported out from the plant

and a second position where the milk is returned to the balance tank 1.

By arranging the pressurizing pump prior to the inlet to the downstream side of the second regenerative section 6 and by arranging a constant pressure valve 15 after the inlet from the cooling section it is ensured that the pressure of the heat treated product is higher than the pressure of not yet heat treated product. If a leakage should occur between the plates in the pasteurizer there is no risk that untreated product may enter and be mixed with the treated product.

In order to control that the desired pressure conditions are obtained in the pasteurizer, pressure sensors 17,

17 ' are arranged both at the inlet to the first regene¬ rative section 5 on the upstream side and at the outlet of the same regenerative section 5 on the downstream side. These pressure sensors 17, 17' are connected to the control unit not shown in the drawing and the pressure sensor 17 ' shall always sense a higher pressure, for example 0,5-0,8 bar higher than pressure sensor 17.

If the temperature sensor 11 indicates that the milk has been heat treated to an insufficient extent (too low a temperature) the control unit gives a signal to the first diversion valve 12 to change its position in such a way that the milk via the conduit 20 is returned to the balance tank 1. In order to ensure that the desired pressure conditions are obtained in the regenerative sections 5 and 6, a conduit 21 is opened, which during normal operation is kept closed by cut-off valves not shown, and a circulation path is created over the diversion valve 12, the pump 13, the regenerative

sections 6 and 5, the constant pressure valve 15 and the conduit 21.

Also in this operation conditions the pump 13 and the constant pressure valve 15 consequently provide for that the desired pressure difference against untreated milk is maintained in the regenerative sections.

The milk which is returned to the ^' balance tank is again transported to the pasteurizer and when a sufficiently high temperature is sensed by the temperature sensor 11 the diversion valve 12 is switched such that the milk again passes the regenerative section.

The corresponding switch between normal operation and pressure maintaining circulation pass may take place if the pressure sensors indicate that the desired pressure conditions in the pasteurizer differ from the desired value.

In order to further increase the safety the pasteurizer is with advantage provided with pressure sensors 18, 18' also at the inlet to the second regenerative section 6 on the upstream side and at the outlet from the same regenerative section 6 on the downstream side and with pressure sensors 19, 19' at the outlet from the regene¬ rative section 6 on the upstream side and inlet to the regenerative section 6 on the downstream side.