System for distributing air for fast freezing

FIELD: refrigeration or cooling.

SUBSTANCE: system comprises source of cooled air, fan, and air ducts. The container is cooled by cold air flowing through the distributor with slots and cross-pieces that separate the air jets.

EFFECT: enhanced rate of cooling.

3cl, 10 dwg

 

The invention relates to refrigeration and can be used in devices for air distribution systems for fast and ultra-fast freezing of food products and other objects.

High freezing rate can be achieved either due to a significant temperature drop of the cooling medium, either in combination with an increase in the flow rate of the cooling medium on the surface of the cooled object. The increase of the flow velocity is very promising, because it leads to a significant increase of heat transfer coefficient on the surface of the cooled object.

A device for the quick freeze test semi-finished products described in the Russian patent No. 2189750 "Device for in-line freezing test semi-finished products" [1]containing chamber cooling quick freezing and glazing, as well as the same for all cameras conveying device. Cameras are connected air lines, have fans for supply and exhaust air.

Known apparatus for use in the food industry for the production of frozen flat food products (see Russian patent No. 2215248 "Freezing apparatus for freezing flat food") [2]. The apparatus includes a cooking and heating fuel is aerovane camera air coolers, mesh conveyor, the apparatus for forming the product, which is installed at the entrance to the chamber. The upper branch of the mesh conveyor top and bottom covered by ducts with nozzles, which serves a jet of air from below and from above perpendicular to the surface of the grid.

Some studies of heat-mass transfer and flow characteristics of air at the exit of the guide channels with and without the use of drainage canals when blowing plate with naphthalene coating (see Dong-Ho Rhee, Pil-Hyun Yoon, Hyung Her Cho. Local Heat/Mass Transfer and Flow Characteristics Array of Impinging Jets with Effusion Holes Ejecting Spent Air / International Journal of Heat and Mass Transfer 46 (2003) 1049-1061) [3]. In the described constructions guides and idler channels are alternately staggered at different distances.

A well-known system of tunnel type with a moving conveyor for quick freezing of meat products described in U.S. patent No. 5551251 [4]. To ensure rapid freezing using jets of air entering at the top and bottom products located on a moving conveyor. Jets of air are formed in the plates with the holes placed in a checkerboard pattern.

Closest to the proposed distribution system for high-speed freezing is cooler or freezer with megachile the governmental channels, with a distinct gap for the flow of air or coolant coming on a moving conveyor products (see U.S. Patent No. 6557367) [5]. The cooled air is supplied by fan along vostokovednyh channels and extends perpendicularly through the cracks. The recommended temperature of supplied cooled air is -48°/-49°C.

A common drawback of all the above solutions is the high energy intensity of the cooling devices and the complexity of the design of ducts exposed to significant hydraulic loads. The problem solved by the invention consists in a constructive ensuring a high velocity of the cooling air on the surface of the cooled object while ensuring that the flow resistance in the air-distributing device.

The technical result consists in the elimination of these shortcomings in the distribution system for high-speed freezing substances placed in the container, consisting of a current source of chilled air fan and air ducts, is achieved by the fact that the container is cooled by a cold air flow through the dispenser cracks and ridges that separate air jet directed and stretching on all sides except the top. Width of narrow channels to do what it and the outgoing air is 3 to 8 mm

The distance between the cooled surface and air raspredeliteli is from 0.8 to 2 mm.

The system has at least three rod placed across the slots of the distributor.

So, in the most common case, it is necessary to provide rapid cooling of the container in the form of a parallelepiped with a temperature of +30°C up to -40°C. the drug Used for the cooling air at a temperature of -100°C. the Main task is to select the optimal design that allows you to organize the flow of air to the heat transfer coefficient is distributed evenly on the surface to be cooled, the value of the coefficient of heat transfer was more than 150 W/m2And hydraulic resistance arising in the design remained within such limits, so you can use one of the standard fans.

To achieve the desired result, you need to calculate currents of air in a number of designs and calculate the parameters of the conjugate heat transfer, allowing to determine the heat transfer coefficient on the surface to be cooled. To do this, in practice, usually do not carry out numerical calculations using differential equations, and use differential assessments on a pre-formed grid.

In the present embodiment, for selecting the optimal design, the functions use the solution of Navier-Stokes equations together with the energy equation to determine the velocity fields, pressure and temperature in the two-dimensional formulation. In separate elements of the design speed can reach significant values, which leads to the development of turbulent flows. Therefore, consideration of the influence of turbulence on the flow characteristics and heat transfer using standard K-ε turbulence model.

For discretization of the equations using the grid 252×40 with the size of the computational region 0,126×in 0.01 m To jointly solve the system of equations it is advisable to use the package PHOENICS (see www.cham.co.uk/phoenics/d_polis/d_info/phover.htm) [6].

The heat transfer coefficient was determined by the formula, q is the density of heat flux on the surface to be cooled, Tsurflocal surface temperature, Tair- the temperature of the air in the air manifold.

When implementing the various design options analyzed their advantages and disadvantages.

The essence of the proposed solution and methodology for assessing the optimality of the different options is illustrated with the involvement of the following graphics:

Figure 1. Temperature distribution (in ° (C) when the air flow along a flat plate with a clearance of 10 mm

Figure 2. Temperature distribution (in ° (C) when the supply air And in the center of the plate (inlet diameter 20 mm) with a gap of 2 mm

Figure 3. The distribution of air pressure (in PA) is periodic ri air And on the slit plate.

Figure 4. Temperature distribution (in ° (C) at periodic air And in the slit plate.

Figure 5. Schematic diagram of the distributor in periodic structures with bridges, where

1 - the surface of the container;

2 - Spacer item;

3 - Jumper;

4 - Crack.

6. The distribution of air pressure (in PA) in the periodic structure of the diffuser with jumpers.

7. Temperature distribution (in ° (C) in case of periodic air supply And the slit plate tabs.

Fig. Velocity (m/s) during periodic air supply And the slit plate tabs.

Fig.9. The appearance of the diffuser from the top to the periodic supply of air in the slit plate tabs.

Figure 10. The appearance of the diffuser with side inlet and outlet air gaps for periodic air And slit plates with ridges, where - welded bars.

Flat plate. As the starting point of the study was considered heat transfer in air flow around a flat plate. The temperature distribution is presented in figure 1.

The calculation results showed that to ensure the average heat transfer coefficient of about 100 W/m2K it is necessary that the air will flow around the surface with a speed of 28 m/s the coefficient of talaud is Chi distributed very unevenly over the surface of the plate, a maximum is observed in the initial part of the plate (275 W/m2K), and in the end part of the heat transfer coefficient is much smaller (82 W/m2K). In the set the same task, it is necessary that the heat transfer coefficient was distributed evenly or at most it would be in the center of the streamlined surface. In addition, the high velocity flow of air consumption (at all refrigerated surfaces) is 444 m3per hour when the hydraulic resistance of the gap (1 cm), which occurs for equal 438 PA.

Conclusion: expendable-hydraulic characteristics are achievable, however, the heat transfer coefficient is lower than required under specified conditions.

The air flow in the center of the plate. To increase the heat transfer coefficient in the center of the surface to be cooled considered symmetric pattern of the flow. The design and calculation results are shown in figure 2. The gap between the planes and the surface cooling is 2 mm.

Figure 2 shows the velocity vectors and temperature distribution. Calculations of the flow in this design showed that even at relatively low cost (about 160 m3per hour) is required to overcome significant hydraulic resistance (500 PA) for providing the heat transfer coefficient 128 W/m2K. Attempts to increase the ratio th the recoil due to the reduction of the gap or increase the speed lead to a significant increase in hydraulic resistance.

Periodic design. In the next step, reviewed options periodic flow of cold air to the cooling surface. The results of the calculation of the distributions of pressure, velocity and temperature for this version of the design presented in figure 3 and 4.

The calculation results showed that this variant can be used for cooling purposes. At a cost of about 200 m3/h and the hydraulic resistance of 170 PA this design (with a gap of 2 mm) provides the heat transfer coefficient 156 W/m2K. the Reduction of the gap up to 1 mm leads to an increase of hydraulic losses of up to 400 PA, and to a slight increase in the heat transfer coefficient up to 161 W/m2K. Changing the geometric dimensions of the structure (size of areas of cold air supply and Sewerage) led to minor changes in the characteristics of heat transfer.

Periodic design with jumpers. At the last stage of the research were calculated parameters of heat transfer in the construction shown in Figure 5.

In this construction, as in the previous case, the supply and exhaust air is carried out periodically, and the size of the gap, in this particular example, is 1 mm When the cold air blows container with five sides, except in the RNA surface. The presence of a jumper, which is separated by an air jet directed on the container and stretching, improve the efficiency of the cooling system, not allowing the mixed streams with different temperatures. Allowable width of narrow channels for incoming and outgoing air is 3 to 8 mm, and the distance between the cooled surface and the air distributor must be in the range of from 0.8 to 2 mm.

The results of the calculation of the fields of pressure, temperature and speed are presented on Fig.6, 7 and 8, respectively.

Analysis of the calculation results showed that the heat transfer characteristics of the latest design even better than the previous one. So at the rate of 100 m3per hour and hydraulic resistance of 140 PA heat transfer coefficient is 151 W/m2K. the Increase in consumption of 2 times leads to an increase in hydraulic resistance up to 500 PA, and the heat transfer coefficient up to 185 W/m2K.

Thus, studies have shown that the most appropriate use of the design flow of cold air to the surface to be cooled, is depicted in Figure 5, that is, a periodic structure with jumpers. The optimal width of the slits, the supply and exhaust air, was 5 mm, and the optimal distance from the surface of the diffuser with slits to the object surface cooling status is usually 1 mm

The appearance of such a dispenser is shown in figures 9 and 10. To ensure that the distance between the cooled surface and the air distributor in the range from 0.8 to 2 mm, it is advisable to use at least three welded rod placed across the slots of the distributor. This allows not only to provide the required clearance, but also securely attach all five distributors to the sides of the container with one clasp.

1. Distribution system for high-speed freezing substances placed in the container, consisting of a source of chilled air fan and air ducts, characterized in that the container is cooled by a cold air flow through the dispenser with cracks and ridges that separate air jet directed on the container and leaving on all sides except the top surface.

2. Distribution system according to claim 1, characterized in that the width of narrow channels for incoming and outgoing air is 3 to 8 mm

3. Distribution system according to claim 1, characterized in that the distance between the cooled surface and the air distributor is from 0.8 to 2 mm.

4. Distribution system according to claim 1, characterized in that its design included at least three rod placed across the slots of the distributor.



 

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