Method and apparatus for the control of the flow rate of the refrigerating fluid in ice cream making machines

#The present invention relates to a method for the control of the flow rate of the refrigerating fluid in ice cream making machines and an apparatus for carrying out this method.

#As is known, ice cream making machines basically comprise a freezing cylinder equipped with rotating blades connected to a system for monitoring the ice cream hardness, and a vapour-compression refrigerating circuit carrying a refrigerating gas supplied automatically to this cylinder through a thermostatic expansion valve.

#Considerable irregularities occur with these machines towards the end of the freezing operation, when the product is at a temperature close to the desired final temperature. These irregularities are a consequence of the fact that the thermal power at the evaporator, that is the freezing cylinder, decreases progressively during the freezing process as the fraction of frozen water present in the mixture contained inside the cylinder rises. This decrease in the thermal power at the evaporator triggers a progressive reduction in the rate of flow of refrigerating gas and a consequent reduction in the evaporation pressure and in the corresponding saturated evaporation temperature. This temperature drop in the evaporator causes a significant hardening of that part of the mixture which is immediately in contact with the inner walls of the cylinder, which makes a large amount of power necessary to scrape the product off these walls.

#Other objects and advantages of the present invention will be understood more clearly in the course of the following description, which is given by way of example, no limitation being applied, and refers to the single attached drawing, in which:

#FIG. 1 is a block diagram of an ice cream making machine comprising a freezing cylinder, upstream of which hot gases are injected in accordance with the method and by the apparatus of the present invention.

#Referring to the single figure of the drawing, the number 1 denotes a freezing cylinder, or evaporator, into which is introduced a mixture of known components which by means of operations of aeration and stirring and cooling gives rise to the final product, namely the ice cream. This cylinder 1 contains a screw 101 for stirring the mixture, mounted on a rotary shaft 102 connected to a system 2 for monitoring the ice cream hardness. Illustrated upstream of the cylinder 1 is a thermostatic expansion valve 3 that controls the injection of refrigerating fluid into the evaporator 1 on the basis of a comparison of two temperatures measured in a main flow circuit P carrying the said refrigerating fluid: temperature T1 or saturated evaporation temperature, sensed by the thermostatic valve 3 from the pressure P1 of evaporation, and temperature T2 at the outlet, sensed by a dedicated sensor 4 connected to the latter valve. Continuing along this main circuit P, which is effectively indicated by the outermost lines in the figure, upstream of the said thermostatic valve 3 the following are illustrated: a solenoid valve 5, a flow indicator 6, a filter 7, a condenser 8 and a compressor 9, the latter having two corks 10 and 11 for shutting off the gas on the delivery and intake sides. The said compressor 9 is connected, in a known manner, to the said ice cream hardness monitoring system 2. In this main refrigerating fluid flow circuit P, a tapping-off line D is inserted between the compressor 9 and the condenser 8 and takes a flow of gaseous fluid which it transfers to a point downstream of the thermostatic expansion valve 3. This line D is equipped with a pulse solenoid valve 12 whose operation is controlled by a control device 13 communicating with a pressure sensor 14 positioned downstream of the evaporator 1.

#The machine illustrated in the diagram shown in the figure works as follows: the mixture which will be turned into the ice cream is placed in the evaporator 1, the inner walls 103 of which are visible, and is stirred by the screw 101; the circuit P is carrying a certain quantity of refrigerating fluid which is compressed and heated (compressor 9), condensed and cooled (condenser 8), filtered (filter 7) and sent to the freezing cylinder 1 in appropriate quantities controlled by the expansion valve 3. Inside this cylinder the stirring screw 101 is active until the mixture has reached the desired final hardness and temperature: basically, the ice cream hardness monitoring system 2 senses the load on the shaft 102 as it turns the said screw 101 and, when the said load reaches a predetermined value, stops the compressor 9. The problem, as mentioned earlier, is that because of the fraction of frozen water that forms on the inner walls 103 of the cylinder 1, the monitoring system 2 senses a load which in reality does not correspond to the actual hardness of the ice cream. The behaviour of the thermostatic valve 3 is also very irregular because, when the compressor is stopped, the temperatures T1 (relative to the pressure P1 in the evaporator 1) and T2 at the outlet of the evaporator 1, exhibit large variations.

#The line D for tapping off refrigerating fluid is therefore used to take the correct flow rate of gas leaving the compressor 9, while it is at a relatively high temperature, and conveys it through the solenoid valve 12 to a point downstream of the thermostatic valve 3: in this way the said valve 3 will sense a higher temperature T2 and consequently send a greater quantity of fluid to the evaporator 1, increasing the heat load in the latter and eliminating unwanted stoppages of the compressor. The flow of gas tapped off along the line D is monitored by the sensor 14 and controlled by the device 13 from the temperature-pressure values on the intake side of the compressor 9. Once the correct temperature-pressure values are determined for the intake, a calculation algorithm is used to regulate the degree of opening of the solenoid injection valve 12, which will therefore send a precise flow of hot gases to a point downstream of the thermostatic valve 3.