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Ice Cream Freezers

Ice cream is a well-liked dairy product. It is always consumed in frozen condition.The freezing temperature of an ice-cream mix is very low due to the ingredients like milk sugar, salts and cane sugar, etc. Therefore it is required to be cooled to a very low temperature for freezing. Thus, manufacturing of ice cream is not possible without the refrigeration/ cooling application. An ice-cream freezer has the following functions during manufacturing of ice cream.

1. First, It must cool the mixture of ice-cream ingredients to the proper low temperature for handling in packages/containers. Thereafter, it must cool the packaged ice cream to a low enough (such a low) temperature so that the ice cream hardens properly with smooth consistency and uniform ice-crystals.

2. Alongwith cooling, it must mix a predetermined amount of air uniformly into this ice cream in such a manner that it gives the ice cream the proper swell or overrun.

3. If required, It must perform the mixing of fruits and flavour into ice cream while it is being frozen.

Here we will discuss only the first major function of ice-cream freezing equipment,i.e., cooling and freezing of ice-cream mix. The extraction of heat from ice-cream mix takes place in three different steps. First is the extraction of sensible heat of liquid mix so that its temperature lowers down to the freezing temperature. Second step is extraction of latent heat of liquid mix at freezing point so that it becomes semi-frozen slush. And third is the extraction of sensible heat of this slush so that its temperature further lowers down below the freezing temperature and it become hard ice cream. Depending on the amount of ice cream to be manufactured and so the amount of heat extraction, the freezing equipment is designed. Freezers of the commercial type may be classified as batch or continuous, depending upon whether the ice cream is made in batches or is ejected in a continuous stream.

Depending on the type of cooling/ method of refrigeration, these are also classified as brine type or direct expansion type. In the brine type, the brine is cooled to a required low temperature by a conventional refrigeration plant and is supplied to the ice-cream freezer for extracting heat of ice-cream mix. This is the indirect method of cooling. Brine is simply a solution of salt in water and can be cooled to quite low temperature less than 0 o C without freezing. So, brine is used in place of water in case of ice-cream manufacturing application. In the direct type, the refrigerant is directly expanded in the suitably designed ice-cream freezer where it evaporates and extracts heat from ice-cream mix. The various types of ice-cream freezer/ equipment are explained here.

i. Batch Freezer

It consists of a double wall cylindrical drum of varying capacity. The inside wall of freezing cylinder is made of steel or copper with a stainless steel liner. The outer wall is insulated with cork and covered with airtight metal housing. The narrow space between the inside and outside wall is called jacket through which either chilled brine or refrigerant flows. Due to narrow passage in jacket, the brine will flow with high speed and provide good heat transfer. At a time one batch of ice- cream mix is put in the drum and cooled by the flow of brine or expansion of refrigerant in the jacket. Inside the cylinder a dasher is fitted which rotates by a direct motor drive. The dasher has scrappers, which scrap the frozen ice cream from the refrigerated inside surface of cylinder. A beater is also provided for whipping/ mixing of air into the ice cream so that it becomes fluffy.

Alongwith the freezing cylinder, certain other accessories are also used in batch freezers. The batch-measuring device measures the charge of mix. The draw-rate indicates the stiffness of the ice cream by means of load on the dasher motor.The ice-cream mix is required to be cooled to a certain temperature depending on the level of stiffness required. Therefore, some means of controlling the cooling and stopping during the whipping and unloading period must be provided.

ii. Refrigeration Control

In case of brine cooling, control is very much easy by simply controlling the flow rate of brine through the cylinder with the help of a valve. But in case of direct expansion of refrigerant usually ammonia, two principal types of controls are used  These are York-type refrigerating system and the creamery - package type refrigeration system. Both the systems control the flow of ammonia vapors from accumulator to the suction of compressor.

a) In the York-type system, the accumulator with low side float control valve is situated at a higher level than the freezing cylinder. Due to float valve, a fix level of liquid ammonia is maintained in the accumulator. The bottom of accumulator is connected to the bottom of freezing cylinder with a pipe. In the same way, top of freezing cylinder is also connected to the top of accumulator through a pipe and control valve. When this valve is open, the liquid ammonia filling in the freezing cylinder being at a lower level takes heat from the ice-cream mix and produce cooling. On absorbing heat liquid ammonia converts into vapours, which lift through control valve, and accumulates in the evaporator. From the accumulator, compressor sucks these vapors. When the control valve is closed, the ammonia vapours formed in the jacket of freezing cylinder cannot lift up to the accumulator and also cannot pass through the bottom because of lighter weight. In this way, the ammonia gas collects in the jacket and push back the liquid ammonia from jacket thus stops the cooling very quickly. Thus control valve is open when freezing is to start and closed when freezing is to be stopped during whipping.

b) In the creamery package type refrigeration system, The liquid ammonia is filled in the freezing jacket and accumulator in the same way as in York type system. But here in this case, the control valve is provided in the passage between accumulator and compressor. When the valve is opened, it allows the compressor to reduce pressure in the accumulator and hence in the freezing jacket. By reduction in pressure more and more liquid ammonia will evaporate to a low temperature and produce more cooling. When the valve is closed, it does not allow the compressor to suck ammonia vapors and thus builds up the pressure in the refrigerator jacket.Due to higher pressure, the saturation point increases and the rate of evaporation and hence cooling lowers irrespective of that the refrigeration jacket is still filled with liquid ammonia. This control method does not stop the freezing so quickly as the first method described. Hence the control valve is shut off in advance so that the cooling reduces at proper time. The cooling goes on reducing but does not stop.However it may be beneficial sometimes in the ice cream processing.

iii. Continuous Freezer

In the continuous freezer, the ice-cream mix is continuously fed or pumped under pressure from one end and we get ready ice-cream from other end continuously.In this way here ice cream flows under pressure. During flowing through the freezer, it undergoes all the processes, i.e., freezing, whipping of air, mixing of various fruits, etc. The continuous freezer is desired due to its various benefits like it saves space, labour and cost of operation. Moreover in continuous freezer, more uniform results are obtained. Due to continuous flow of ice-cream mix, heat transfer is also improved and we get positive control on the percentage of air and mix. The system of refrigeration is almost of the same type. Generally ammonia is used as the refrigerant, which is directly expanded in the jacket of freezing cylinder.Depending on the feeding arrangement of ammonia, two types of continuous freezers are as described below:

a) Vogt Freezer using Jet type Ammonia System

The freezing cylinder alongwith ammonia refrigeration system . A jacketed tube type-freezing cylinder is situated above the accumulator. In the accumulator, the level of liquid ammonia is maintained with the help of float control valve. From the liquid ammonia feed line, liquid ammonia is fed to the jacket of freezing cylinder by the jet action through a nozzle. It expands in the jacket of freezing cylinder tube and freezes the ice cream mix flowing inside the freezing tube. The metered quantity of air mix enters from one end by the action of pump and frozen ice cream comes out from the other end continuously. After producing cooling the ammonia vapours drops back in the accumulator. From the vapour zone of accumulator, the compressor again sucks these vapours. Below the jet a spring- loaded quick shut-off valve is provided which when operated quickly stops the ammonia supply and hence cooling in freezing cylinder.

b) Creamery Package Continuous Freezer

The freezing cylinder of this freezer is like that of a batch freezer. The size of freezing cylinder is large and it is connected to an accumulator fixed at a higher level, which maintain the liquid ammonia always filled in the freezing cylinder. The accumulator is connected to the compressor through a backpressure valve. However, the system of feeding the ice-cream mix is made continuous i.e. it enters from one end and exit from the other end by the force of a metering type pump. The metering type pump can also control the flow rate of mix. Air is also fed continuously along with mix through a control valve. The working of this type of freezer is automatic i.e. the ammonia is controlled automatically depending on the quantity and quality of mix and cooling required.

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