RU2036396C1 - Method and device for freezing fruit and vegetables - Google Patents

Abstract

FIELD: refrigerating engineering. SUBSTANCE: products are placed on conveyer and are partially frozen in quasi-liquid layer at alternating of zones with jets of air of increased pressure and zones without such jets along length of conveyer; final freezing is effected when product is laid on conveyer belt in dense layer and air is fed onto it in descending flow. Device for realization of this method has heat-insulated chamber divided by partitions into preliminary freezing chamber and final freezing chamber, conveyer for products, air coolers arranged under conveyer, air distributing system located under conveyer in preliminary freezing section and provided with row of cells and swivel blades mounted under cells for closing them in turn. Fans of air coolers are mounted under conveyer for feeding air to first section and above conveyer for feeding air to second section. EFFECT: enhanced efficiency. 3 cl, 5 dwg

Description

 The invention relates to refrigeration and can be used on refrigerators in the food industry.

 Known methods and devices for freezing fruits, vegetables and semi-finished products.

 The prototype is a quick-freezing apparatus for freezing small-sized products, including a heat-insulated chamber with loading and unloading pipes, an air cooler installed in the chamber, a food conveyor above it with a flexible perforated tape, suction and discharge fans, a rigid grill fixedly mounted under the upper branch of the conveyor, and block of partitions mounted above the upper branch of the conveyor belt with the possibility of moving them in a horizontal plane and rota each about a horizontal axis.

 In addition, in existing freezers, the freezing process is carried out in two stages by freezing on the first conveyor belt in a thin fluidized bed and freezing on the second belt in a fluidized or dense layer. In this case, the presence of two conveyors and, accordingly, two drives to them. Freezing and freezing are carried out in an upward flow of cold air.

(1) где G производительность скороморозильного аппарата, кг/ч;
τ₁ время подмораживания, ч;
Н_о высота неподвижного слоя продукта на первой транспортерной ленте, м;
ρ_п насыпная плотность продукта, кг/м³.The area of the first supporting grid of the conveyor of the freezing apparatus is determined by the dependence
F

(1) where G is the capacity of the freezer, kg / h;
τ ₁ freezing time, h;
N _{about the} height of the fixed layer of the product on the first conveyor belt, m;
ρ _p bulk density of the product, kg / m ³ .

From equation (1) it can be seen that the area of the supporting grid can be reduced (and therefore the dimensions of the apparatus) by reducing the freezing time τ ₁ or by increasing the thickness of the product layer Н _о .

The reduction in freezing time at a constant temperature of cooling air is associated with an increase in the air velocity ω (m / s), as a result of which the power consumption of fans of the freezing apparatus increases by a factor of ³ .

With an increase in H _{about, the} power to the electric motors of the fans of the freezing apparatus increases in direct proportion to the thickness of the fixed layer.

 Thus, the second way to reduce the overall dimensions of the device (reducing the area of the supporting grid) is more rational.

 The freezing apparatus has inherent disadvantages associated with the device of a fixed grate under the upper branch of the conveyor, which causes some unproductive loss of air flow pressure, and the device of the block of rotary partitions mounted above the upper branch of the conveyor belt with the possibility of moving them in the horizontal plane complicates the operation of the device.

 So, when the conveyor is temporarily stopped, product agitators above the conveyor grid can freeze into the product, and then, when the conveyor resumes operation with the same product (in the form of a frozen plate), it can be destroyed. In addition, when freezing delicate berries such as strawberries or raspberries, the introduction of foreign moving objects into the fluidized bed will violate their integrity, which will affect commercial quality.

 Freezing fruits and vegetables in a thin layer on the first transporter in ZFT-1 devices has the disadvantage that due to breakthroughs of jets through a thin layer, not all of the product is fluidized and some of it may freeze to the grid. Mixing of the product on the entire conveyor grid with high-pressure jets is not provided.

 Typically, freezing on the second conveyor is also carried out in the fluidized bed.

 It should be noted that the transition from a dense layer (by increasing the velocity of the ascending air flow) to a fluidized bed for small-fruited raw materials is associated with a sharp decrease in the heat transfer coefficient, which lengthens the freezing time and contributes to an increase in energy costs for fans of the freezing device. It is not possible to increase the air velocity above critical (the beginning of fluidization in a dense layer) with an upward flow of air.

Thus, existing methods and devices for freezing fruits and vegetables have the following disadvantages:
design complexity and increase in energy costs;
the presence of two conveyors and, accordingly, two drives to them;
lack of product mixing on the entire first conveyor grid with high-pressure air jets;
the impossibility of more intense freezing of fruits and vegetables on the second conveyor in a dense layer.

 The purpose of the invention is the elimination of these disadvantages.

 The goal is achieved in that freezing in a fluidized bed is carried out by alternating along the length of the conveyor zones with jets of increased air pressure with zones without such jets, and freezing is carried out when the product is located on the conveyor belt in a dense layer and air is supplied to it in a downward flow.

 The intensification of the freezing process is carried out at an air flow rate above the critical fluidization velocity.

 In addition, in the device, the chamber is divided into a freezing section and a freezing section with a vertical transverse partition and a movable partition mounted above the conveyor upper branch, while the air cooler fans are installed with the possibility of supplying air in the first section under the conveyor, and in the second section on the conveyor from above, and the system air distribution is located under the conveyor in the freezing section and contains rows of cells and rotary blades mounted under the cells with the possibility of their oocherednogo overlap.

 To prevent freezing of fruit and vegetable raw materials to the conveyor mesh in the freezer when it is frozen, while reducing energy costs, the process is carried out in two stages.

At the first stage, in the fluidized bed, fruits or vegetables are frozen, during which they are cooled and a thin frozen layer is formed on the surface. This prevents their further clumping and freezing to the conveyor grid. Subfreezing may take the cold air temperature range minus 20 to minus 30 ^C. If necessary intensive mixing subfreezing produce raw materials, which is achieved by increasing the air velocity in separate, alternating, plots. So, the critical mass velocity of the beginning of fluidization for fruits and vegetables is approximately, kg / (m ² ˙ s): Green peas 2 Cherries 3 Plums 4 Apricots 4.7
In order to achieve normal fluidization, the air filtration rate should exceed approximately 2 times the critical rate. When partial overlap of blades living section cells below the upper branch of the grid conveyor air flow (jet speed) will reach for green peas ρω = 7 kg / (m ² ˙ s), to cherries 9 kg / (m ² ˙ s), drain 12 kg / (m ² ˙ s) and apricots 14 kg / (m ² ˙ s) (ρ, kg / m ³ air density, ω, m / s air speed).

The second stage is freezing, carried out in a dense layer at the above temperatures and washing the fruits or vegetables with a downward flow of air. The mass filtration rate, depending on the required duration of the freezing process, can vary in the range, kg / (m ² ˙ s): green peas 2-4; cherries 3-5; plums 3-6; apricots 3-6. To ensure the freezing of these fruits and vegetables, it is necessary to install fans with adjustable flow and pressure.

 In FIG. 1 shows the proposed device for freezing, a longitudinal section; in FIG. 2, section AA in FIG. 1; in FIG. 3 section BB in FIG. 1; in FIG. 4, section BB in FIG. 3; in FIG. 5 section GG in FIG. 4.

 The device includes a heat-insulated circuit 1, a fixed transverse partition 2 of the chamber, a rotary transverse partition 3, a fixed air distributor 4, a loading hopper 5, a rotary blade of the air distributor 6, a conveyor mesh belt 7, fin-tube air coolers 8 and 11, the device body 9, fans 10, unloading tray 12, rods 13 and 14 and the drive drum 15.

 The device operates as follows.

 The product is fed into the loading hopper 5, from where it enters the cold circuit on the conveyor mesh belt 7. Having reached the wall of the device 9, under the influence of cold air jets, the fruit and vegetable raw materials are fluidized and moved in the form of a fluidized bed to the movable transverse partition 3. Through the formed hole in the fixed transverse partition 2 fruits or vegetables are poured onto the second part of the conveyor. On the first part of the conveyor, the fruits are frozen and dried, as a result of which they cannot freeze to the net and freeze into lumps when frozen on the second part of the conveyor. Frozen fruits or vegetables through the discharge tray 12 are transferred to the packaging. The conveyor grid is driven through the drive drum 15.

The regulation of the height of the fluidized bed on the first part of the conveyor (freezing) is carried out by changing the lifting height of the rotary partition 3
On the first part of the conveyor, fruits or vegetables are frozen in an upward flow of air. The air is sucked in by the fan 10 from the chamber and blown through the finned-tube air cooler, inside which the refrigerant boils. Having passed the air cooler, the air is cooled and then enters the air distribution device. The air distribution device is designed for uniform distribution of air flow over the entire area of the supporting branch of the first part of the conveyor.

 The air distribution device consists of two parts: fixed 4 and movable 6. The fixed part of the air distribution device consists of square cells (in plan) (Fig. 4 a) formed from sheet material, and the moving part consists of rotary blades 6 attached to hinges to lower stationary part (Fig. 5 b, c, d). In the lower part of the rotary blades 6 on the hinges are attached rods 13 and 14 (Fig. 5 d, e). By moving the rods to the left or to the right, it is possible to overlap (alternating) part of the cross-section of the stationary air distributor. In this case, the even and odd blades always turn in the opposite direction, thereby closing or opening part of the channels.

 In FIG. 5 b, c and d show possible variations in the operation of the air distribution device.

 In the embodiment shown in FIG. 5c, the cross-section of the air distribution device is halved, as a result, the speed and pressure of the air increase, which contributes to a more intensive fluidization over these channels. Then, turning the rotary blades (Fig. 3 g), open the other channels, and close the first ones. Thus, providing the drive rods 13 and 14, it is possible to continuously or periodically mix fruits or vegetables on the first part of the conveyor mesh with air jets. Cold air, passing through the product layer on the first part of the conveyor, is heated and sucked in again by fans.

 On the second part of the conveyor belt, fruits or vegetables are frozen in a dense layer. Cold air after the air cooler 11 is sucked in by the fan 10 and is supplied to the upper part of the chamber, and then passes through the product from top to bottom (downward air flow).

 The insulated air stream (after the product layer) enters the air cooler 11, where it is cooled. In this case, at an air speed above the critical fluidization velocity, the product will not be fluidized, which will contribute to the intensification of freezing at a lower energy cost.

 To prevent mixing of air flows in the freezing zone and in the freezing zone, the chamber is divided by a partition 2.

 Using the method and device will reduce energy and capital costs.

Claims (3)

 1. A method of freezing fruits and vegetables, which involves moving them on a conveyor and supplying cold air products to the layer to ensure they are frozen in the fluidized bed and further frozen, characterized in that the freezing in the fluidized bed is carried out by alternating zones along the conveyor with increased pressure jets air with zones without such jets, and freezing is carried out when the product is located on the conveyor belt in a dense layer and air is supplied to it in a downward flow.  2. The method according to claim 1, characterized in that the freezing is carried out at an air flow rate above the critical fluidization rate.  3. A device for freezing fruits and vegetables, containing a thermally insulated chamber, a food conveyor, air coolers below it with finned tube heat exchangers and fans, an air distribution system, a loading hopper, an unloading chute, characterized in that the chamber is divided into a freezing section and a freezing section a vertical transverse partition and a movable partition installed under the upper branch of the conveyor, while the fans of the air cooler are installed with possible air supply in the first section under the conveyor, and in the second section on the conveyor from above, and the air distribution system is located under the conveyor in the freezing section and contains rows of cells and rotary blades installed under the cells with the possibility of their alternate overlapping.

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