RU2293262C2 - System for distributing air for fast freezing - Google Patents

Abstract

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

Description

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

High freezing rates can be achieved either only due to a significant decrease in the temperature of the cooling medium, or in combination with an increase in the flow rate of the cooling medium on the surface of the cooled object. Moreover, an increase in the flow rate is a very promising direction, since it leads to a significant increase in the heat transfer coefficient on the surface of the cooled object.

A device for the quick freezing of dough semi-finished products is described in Russian patent No. 2189750 "A device for in-line freezing of dough semi-finished products" [1], which contains pre-cooling, quick freezing and glazing chambers, as well as a transport device common to all chambers. The chambers are interconnected by air lines, in which fans are installed for supplying and removing air.

A 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 products") [2]. The apparatus contains a heat-insulated chamber, air coolers, a mesh conveyor, a device for forming a product, which is installed at the entrance to the chamber. The upper branch of the mesh conveyor from below and from above is enclosed by air ducts with nozzles that supply air jets from below and above perpendicular to the grid surface.

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

Known tunnel type system with a moving conveyor for quick freezing of meat products, described in US patent No. 5551251 [4]. To ensure quick freezing use air jets coming from above and below the products located on a moving conveyor. Jets of air are formed in plates with holes staggered.

Closest to the proposed air distribution system for high-speed freezing is a cooler or freezer with multiple channels having characteristic slots for the exit of air flow or coolant entering a moving conveyor with products (see US Patent No. 6557367) [5]. The cooled air is supplied by the fan along the air ducts and leaves perpendicularly through the slots. The recommended temperature of the supplied chilled air is -48 ° C / -49 ° C.

A common drawback of all the above solutions is the high energy consumption of cooling devices and the complexity of the design of ducts subjected to significant hydraulic loads. The problem solved by the invention is to constructively ensure high speeds of cooling air on the surface of the cooled object while providing hydraulic resistance in the air distribution device.

The technical result, which consists in eliminating these shortcomings in the air distribution system for high-speed freezing of substances placed in a container consisting of a source of cooled air, a fan and air ducts, is achieved by cooling the container with cold air flow through a distributor with slots and jumpers that separate the air jets directed and leaving from all sides except the top. The width of the slotted channels for incoming and outgoing air is from 3 to 8 mm.

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

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

So, in the most common case, it is necessary to ensure rapid cooling of the parallelepiped-shaped container from a temperature of + 30 ° C to -40 ° C. The substance used for cooling is air at a temperature of -100 ° C. The main task for the calculation is to choose the optimal design that allows you to organize the air flow so that the heat transfer coefficient is evenly distributed on the cooled surface, the heat transfer coefficient is more than 150 W / m ² K and the hydraulic resistance arising in the structure remains in such a range that one of the standard fans could be used.

To achieve the desired result, it is required to carry out calculations of air flows in a number of designs and to calculate the parameters of conjugate heat transfer, allowing to determine the heat transfer coefficient on the cooled surface. For this, in practice, usually numerical calculations using differential equations are not carried out, but difference estimates are used on a pre-formed grid.

In this option, to select the optimal design, they use the solution of the Navier-Stokes system of equations together with the energy equation to determine the velocity, pressure and temperature fields in a two-dimensional formulation. In some structural elements, velocities can reach significant values, which leads to the development of turbulent flows. Therefore, to take into account the possible effect of turbulence on flow characteristics and heat transfer, a standard K-ε turbulence model is used.

When sampling the equations, a 252 × 40 grid is used with the dimensions of the computational domain 0.126 × 0.01 m. For a joint solution of the system of equations, it is advisable to use the PHOENICS package (see www.cham.co.uk/phoenics/d_polis/d_info/phover.htm) [ 6].

, q - плотность потока тепла на охлаждаемой поверхности, Т_surf -локальная температура поверхности, T_air - температура воздуха в воздушном коллекторе.The heat transfer coefficient was determined by the formula

, q is the heat flux density on the cooled surface, T _{surf is the} local surface temperature, T _air is the air temperature in the air manifold.

When implementing various design options, an analysis of their advantages and disadvantages was carried out.

The essence of the proposed solution and the methodology for assessing the optimality of various options is explained with the use of the following graphic materials:

Figure 1. Temperature distribution (in ° C) during air flow A along a flat plate with a gap of 10 mm.

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

Figure 3. Distribution of air pressure (in Pa) with periodic air supply A in the slots of the plate.

Figure 4. Temperature distribution (in ° C) with periodic air supply A in the slots of the plate.

Figure 5. Schematic diagram of the air distributor in a periodic design with jumpers, where

1 - The surface of the container;

2 - Spacer;

3 - Jumper;

4 - Slit.

6. Air pressure distribution (in Pa) in the periodic design of the air distributor with jumpers.

7. Temperature distribution (in ° C) with periodic air supply A in the slots of the plate with jumpers.

Fig. 8. Velocity distribution (in m / s) with periodic air supply A in the slots of the plate with jumpers.

Fig.9. The appearance of the air diffuser from above with periodic air supply into the slots of the plate with jumpers.

Figure 10. The appearance of the air distributor from the side of the supply and exhaust air slots during periodic air supply A to the slots of the plate with jumpers, where C are the welded rods.

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

The calculation results showed that for average heat transfer coefficient of about 100 W / m ² K is necessary that air flowed around surface at a speed of 28 m / s. In this case, the heat transfer coefficient is distributed extremely unevenly on the surface of the plate, the maximum is observed in the initial part of the plate (275 W / m ² K), and in the final part the heat transfer coefficient is much lower (82 W / m ² K). In the problem posed, it is necessary that the heat transfer coefficient be distributed evenly or its maximum would be in the center of the streamlined surface. In addition, at such high flow rates, the air flow rate (on all cooled surfaces) is 444 m ³ / h with a hydraulic resistance of the gap (1 cm), in which a flow of 438 Pa occurs.

Conclusion: flow-hydraulic characteristics are quite achievable, but the heat transfer coefficient is lower than necessary under specified conditions.

Air supply in the center of the plate. To increase the heat transfer coefficient in the center of the cooled surface, a symmetric flow pattern is considered. The design and calculation results are shown in Fig.2. The gap between the planes and the cooling surface is 2 mm.

Figure 2 shows the velocity vectors and temperature distribution. Calculations of flow in this design has shown that even at relatively low cost (about 160 m ³ / hour) is required to overcome the considerable hydraulic resistance (about 500 Pa) to provide a heat transfer coefficient of 128 W / m ² K. Attempts to increase the heat transfer coefficient by decreasing the clearance or increase in speed, lead to a significant increase in hydraulic resistance.

Periodic design. At the next stage, options for periodic supply of cold air to the surface to be cooled were considered. The results of calculating the distributions of pressure, speed and temperature for this design option are presented in FIGS. 3 and 4.

The calculation results showed that this design option can be used for cooling purposes. With a flow rate of about 200 m ³ / h and a hydraulic resistance of 170 Pa, this design (with a gap of 2 mm) provides a heat transfer coefficient of 156 W / m ² K. Decreasing the gap to 1 mm leads to an increase in hydraulic losses to 400 Pa and to a slight increase in the heat transfer coefficient only up to 161 W / m ² K. A change in the geometric dimensions of the structure (the dimensions of the cold air supply and exhaust sections) led to an insignificant change in the heat transfer characteristics.

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

In this design, as in the previous case, the air supply and exhaust is carried out periodically, and the gap, in this specific example, is 1 mm. In this case, cold air blows the container from five sides, except for the upper surface. The presence of jumpers that separate the air jets directed at the container and leaving, increase the efficiency of the cooling system, not allowing the flows to mix with different temperatures. The permissible width of the slotted channels for incoming and outgoing air is from 3 to 8 mm, and the distance between the surface to be cooled and the air distributor should be in the range from 0.8 to 2 mm.

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

Analysis of the calculation results showed that the heat transfer characteristics of the latter design are even better than the previous one. So at a rate of 100 m ³ / h 140 Pa hydraulic resistance of the heat transfer coefficient is 151 W / m ² K. The increase in flow rate 2 times leads to an increase of hydraulic resistance to 500 Pa, and the heat transfer coefficient of 185 W / m ² K.

Thus, the studies showed that it is most advisable to use a design option for supplying cold air to the surface to be cooled, shown in Figure 5, that is, a periodic design with jumpers. In this case, the optimal width of the slots supplying and removing air was 5 mm, and the optimal distance from the surface of the air distributor with slots to the surface of the cooling object was 1 mm.

The appearance of such a distributor is shown in Figs. 9 and 10. In order to ensure a distance between the surface to be cooled and the air distributor in the range from 0.8 to 2 mm, it is advisable to use at least three welded rods placed across the distributor slots. This allows not only to provide the required clearance, but also to securely fasten all five distributors to the sides of the container with one fastener.

Claims (4)

1. The air distribution system for high-speed freezing of substances placed in a container consisting of a source of chilled air, a fan and air ducts, characterized in that the container is cooled by cold air flow through a distributor with slots and jumpers that separate the air jets directed to the container and leaving all sides except the top surface. 2. The air distribution system according to claim 1, characterized in that the width of the slotted channels for incoming and outgoing air is from 3 to 8 mm. 3. The air distribution system according to claim 1, characterized in that the distance between the surface to be cooled and the air distributor is from 0.8 to 2 mm. 4. The air distribution system according to claim 1, characterized in that at least three rods placed across the slots of the distributor are included in its design.

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