The present invention relates to a method and an arrangement for enforced heat transmission between an alimentary body, solid or liquid, and an ambient gas. In particular, the invention relates to heat transmission from relatively small solid alimentary bodies, which occur in large amounts, and where it is desirable to fluidiz the alimentary bodies constituting a product flow in order thus to improve both the heat transmission and the motion of the alimentary bodies. The enforced heat transmission is achieved in that the ambient gas is set in oscillatory motion which is generated by a standing sound wave of low frequency and in that the alimentary bodies are placed in that part of the sound wave where the oscillatory motion is greatest.

A fundamental problem in cooling/freezing alimentary bodies, i.e. products intended as food for human beings or animal fodder, is that the transferred thermal effect per surface unit from the alimentary body to a gas flow enveloping the alimentary body will be low at low gas flow rates. In order to transfer large thermal effects, high gas flow rates are required, which implies that a large air flow will be necessary. At the same time, however, the temperature rise in the air will be slight. The large flow entails that cooling/freezing will be expensive and, in consequence of the slight temperature rise, the energy in the heated air can seldom be utilized.

Depending on, among other things, the feeding temperature of the alimentary bodie their consistency, thickness etc., variations in transit time must be foreseen in order achieve the desired outlet temperature of the alimentary bodies. The transit time, i.e the time during which the alimentary bodies are present in the cooling/freezing chamber, is generally controlled by the feed rate of the product flow, so that a low fe rate result in a longer transit time than a high feed rate would.

It is previously known from V. B. Repin, "Heat exchange of a cylinder with low-frequency oscillations", Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi, No. 5, pp.

67-72, September-October 1981 , that heat transmission may be improved by generating a sonic field in the gas. It is also previously known that it is advantageo if such a sonic field is of low frequency.

It will be obvious from the two parameters sound pressure and particle velocity in sonic field that it is the particle velocity which provides the enforced heat transmis It is also obvious that the heat transmission increases with increasing particle velocities. The reason why the prior-art method of employing low-frequency sound heating or cooling of bodies has not hitherto enjoyed any practical importance is t there have not been any usable method or apparatus for generating sound with a sufficiently high particle velocity throughout the entire surface of the body intende be cooled, or alternatively, heated.

The object of the present invention is to solve the above mentioned problem and t attain a method and an arrangement for achieving enforced heat transmission by transferring high thermal effect per surface unit from a alimentary body to ambient gas, especially for applications in which the alimentary body consists of a quantity small solid alimentary bodies, for example granules or pellets or drops. Instead of increasing the heat transmission by passing the gas over the surface of the alime body at high speed, the enforced heat transmission is achieved by imparting to th ambient gas a low frequency oscillation. For the purposes of clarifying the present invention, different embodiments thereof regarding to cooling/freezing will be described.

The nature of the present invention and its aspects will be more readily understoo from the following brief description of the accompanying drawings.

In the accompanying drawings:

Fig. 1 shows a solid body in a constant velocity airflow; Fig. 2 shows a solid body in an air flow which is exposed to an infrasonic fie

Fig. 3 shows an embodiment of an arrangement according to the invention; Fig. 4 shows another embodiment of an arrangment according to the invent

Fig. 5 shows a third embodiment of the invention which can be used in an installation for cooling alimentary bodies;

Fig. 6 shows a fourth embodiment of an arrangment according to the inventio which is particularly suitable for cooling of alimentary bodies after, for instanc blanching;

Fig. 7 shows a cross section view of a freezer including the invention; Fig. 8 shows another sectional view of the freezer in Fig. 7, which illustrate, in particular, the path of the track and the cooling pipes; Fig. 9 shows an elevational view of a freezing chamber of the freezer shown i Figs. 7 and 8.

As mentioned above, enforced heat transmission may be achieved between the surface of a alimentary body and an ambient gas if the gas is influenced so as to reciprocate with the aid of a standing sound wave generated in the gas. Fig. 1 show a solid alimentary body at a temperature Tg which is exposed to an air flow. A parti of the air flow is marked as a dot and the position of the air particle at various points time is marked by X Xγ. The temperature of the air flow is T- _j before it has passed t alimentary body, and T2 after the alimentary body has been passed. Fig. 2 shows t same solid alimentary body which is exposed to the same air flow, but under the influence of infrasonic sound. Even here the position of the air particle at different points of time is marked by t- _j -tγ. As will be apparent from Fig. 2, because of the pulsating air low generated by the low frequency sound, each air particle which passes the solid alimentary body, will pass not just once but a plurality of times. If th alimentary body is at a higher temperature than the airflow, the air particle will absor more and more heat each time it passes the solid alimentary body, and the temperature of the alimentary body will be correspondingly reduced. Enforced heat transmission will thus be obtained.

In certain parts of the standing sound wave, the velocity of the oscillating motion of t gas, the so-called particle velocity, is high, while the pressure variations, the so-call sound pressure, are slight. In other parts, the pressure variations are great while the

velocity of the oscillating motion is low. At a certain point, both the particle velocity a the sound pressure will thus vary with time and, under ideal conditions, will describe sinusoidal oscillatory motion. The highest vafue of the particle velocity and the soun pressure, respectively, is indicated by the amplitude of each respective oscillatory motion. As a rule, the amplitude of the particle velocity assumes a maximum value, has a so-called particle velocity anti-node, at the same time as the amplitude of the sound pressure assumes a minimum value, i.e. has a so-called sound pressure nod

It is desirable, in accordance with the foregoing, that the particle velocity assumes a value as high as possible in order to obtain that maximum enforced heat transmission. In a standing sound wave, there may be several points where the particle velocity amplitude assumes its maximum level. In a standing sound wave whose length corresponds to a quarter or a half wavelength, or alternatively a part a quarter or a half wavelength, the amplitude of the particle velocity has a maximu only at one point. In order to obtain an enforced heat transmission as high as possible, the surface from whence the heat transmission is to take place should therefore be sited at a position as close to the particle velocity anti-node as possibl

In the method according to the present invention, enforced heat transmission betw an alimentary body, solid or liquid, and a gas, as shown in Fig. 2, is realized in that standing, low-frequency sound wave is generated in a closed, br in any case acoustically virtually closed, sound resonator. The term low-frequency sound is her taken to mean sound at a frequency of 50 Hz or lower. The reason why frequencie above 50 Hz are less interesting is that such a closed half-wave resonator has suc small dimensions at high frequencies that the whole arrangment will be uninterestin due to the low capacity of the arrangement. Since possible disruptive sound fades lower frequencies, a frequency of 30 Hz or lower should preferably be used. At this frequency, disturbances may be considered as very slight. The sound resonator is preferably of a length corresponding to a half wavelength of the generated low-frequency sound, but other designs of the sound resonator are also possible. T sound wave is obtained in that air pulses are generated by a so-called exigator located at a sound pressure anti-node in the resonator. The term exigator is here

employed to indicate that part of a generator for low-frequency sound which generates a particle velocity in one point in a resonator where a high sound pressur prevails, see for example Swedish patent No. 446 157 and Swedish patent applications Nos. 8306653-0, 8701461-9 and 8802452-6. Somehwere in the resonator a particle velocity anti-node will occur and here the alimentary body is supplied which is to be exposed to enforced heat transmission. When freezing alimentary bodies susceptible to fluidization, that is the alimentary body in question i provided as granules, pellets or similar which in their fluidized state would have an angle of repose (X = 0°, then the total heat transmission from the alimentary bodies i their fluidized state will increase due to the relativ mutual separation of the individua alimentary bodies. Thus it is apparent that the fluidizing property of the particle velocity of the sound has a favourable influence on the heat transmission.

In the case that the alimentary body in question, which constitutes an obstacle to th sound, becomes all too large, this is revealed in that the sharpness of the resonanc of the resonator becomes poorer, which means that the ratio between the amplitude of the particle velocity in the anti-node and that in the node decreases. In a condition with large losses there is therefore no reason to generate the standing sound wave with the aid of a long resonance tube. By placing the exigator closer to the particle velocity anti-node the resonance tube can be shortened.

When practically designing the sound resonator there are several possibilities. Examples of different designs, i.e. embodiments, are illustrated in Figs. 3-9, the principles of which are briefly described hereinafter. In all cases an acoustically closed system is aspired to. Fig. 3 shows a generator for low-frequency sound with exigator 1 and a resonator 2 with a length corresponding to a half of a wavelength of the generated low-frequency sound. A particle velocity anti-node occurs in an area close to the centre of the resonator and consequently the substance which is to be ^■ exposed to enforced heat transmission is supplied just above the centre of the resonator and drained just below the centre thereof. Fig. 4 shows a resonator which functions in the same manner as the resonator in Fig. 3 with the difference that the lower half of the resonator has been replaced by a resonator of Helmholtz type. Her

there is thus a tube resonator 3 with a length corresponding to a quarter of a wavelength combined with a Helmholtz resonator 4 which is so dimensioned that it tuned for the same resonance frequency as the tube resonator, implying that the t resonator and the Helmholtz resonator in this case jointly form a resonator. In Fig. the Helmholtz resonator of Fig. 4 has been given a funnel shape so that the substance which is to be exposed to enforced heat transmission is collected by th Helmholtz resonator 10 and, through an opening in its bottom, is passed on. Fig. 6 shows another embodiment in which two resonators 30, 31 each with a length corresponding to a quarter of a wavelength, have been placed side by side so that their open ends are communicating. Two exigators 32, 33 generate a standing so wave of the same frequency in each resonator. By permitting these exigators to operate in counterphase, there is generated one single common standing sound wave. In principle, this joint resonator functions in the same manner as a half-wav resonator.

In case a sound resonator has irregular shape the appearance of the amplitude of particle velocity is influenced so that the original sinusoidal wave becomes difficult recognize. The volume velocity of the sound, however, is not influenced in the sa way and instead it retains its sinusoidal shape, which in periodicity coincides with t amplitude of the particle velocity. In case of a sound resonator of irregular shape it may thus be more appropriate and easier to identify the area where the largest he transmission can be obtained as the area where the volume velocity has an anti-node.

The invention will now be described in greater detail with reference to different embodiments which concern cooling/freezing.

Fig. 5 illustrates an arrangement for freezing alimentary bodies, such as green peasmay for example an infrasound generator of the type described in the Swedis patent application 8802452-6 can be used. This comprises a tube resonator 11 w preferably has a length equivalent to a quarter of a wavelength, at one end of whi an exigator 12 is mounted. At its other end it is provided with a diffusor 13 which i

directly mounted on a freezing chamber 14 through the upper end of which aliment bodies 15 as granules are supplied via a supply pipe, 16. Together with the diffusor the freezing chamber and the Helmholtz resonator 10, the tube resonator form a resonator corresponding to a half-wave resonator. The diffusor and the freezing chamber are situated within an area in which a volume velocity anti-node occurs. T alimentary bodies 15 fall by gravitation down through the freezing chamber 14. This furnished with a plurality of inclined obstacles 17 which momentaneously catch the alimentary bodies so that the transport time of the alimentary bodies through the ar with a high volume velocity is prolonged. The obstacles consist preferably of trays fitted with nets, but the obstacles may also have other designs which permit air to pass through them while the alimentary bodies are unable to pass through them, e. pipes, beams or similar. At the lower end of the freezing chamber there is provided the Helmholtz resonator 10 which functions like a funnel and catches the alimentary bodies for further transport to a container. At the upper part of the Helmholtz resonator, cooling air is supplied by a fan through a duct 18. This air rises up throug the freezing chamber and is heated by the alimentary bodies. The heated cooling ai is discharged through a duct 19.

Enforced heat transmission is obtained between the alimentary bodies and the gas, this case air, influenced by low-frequency sound. When the alimentary bodies are caught by the trays the air motion generated by the sound accomplishes fluidization the alimentary bodies.

Fig. 6 shows another embodiment for cooling/freezing of particleshaped alimentary bodies after, for instance, blanching. The arrangment consists of two resonators 30, 31 both of which have a length corresponding to a quarter of a wavelength. Located at the upper end of each respective resonator is an exigator 32, 33 which even in thi case may appropriately be of the type described in the Swedish patent application 8802452-6 or equivalent. These two exigators 32, 33 are driven by a common moto 34, such that they operate in counterphase with each other. By this means a single common standing soundwave is generated in the two resonators, which are situated side by side so that their open ends 35, 36 are communicating through a joint space

37. In the lower part of each respective resonator and in the proximity of the joint space 37 an zone is obtained displaing a volume velocity anti-node which constitut the actual cooling/freezing chamber. Installed in the cooling/freezing zone are obstacles in the form of pipes 38, 39 which are conveyed to and fro several times within the cooling/freezing zone and thus form two pipe systems. A cooling agent such as water, ammonia, freon or similar, flows through these pipes. The particle-shaped alimentary bodies to be ooled/frozen are supplied to the arrangement from above through a pipe 40 which has two branches 41 , 42, emanating immediately above the two pipe systems. The supplied particle-shaped alimentary bodies pass slowly down through the pipe systems due to gravitation a are cooled/frozen during this passage. The outside of the pipe systems thus constitutes a convection surface so that heat transmission takes place first betwee the particle-shaped alimentary bodies and the air inside the resonators and therea between the air and the convection surface. The heat absorbed by the coolant is subsequently led off to be used, for example for heating alimantary bodies in a blanching device. When the particle-shaped alimentary bodies have been cooled/frozen and have passed the obstacles 38, 39 they are collected and remov through a pipe 43 situated at the lower portion of the joint space 37.

Fig. 7 shows a preferred embodiment of a freezer 52 according to the invention, w comprises an insulated housing 56 supported by a stand 54. The insulated housin 56 encloses a freezing chamber 58 with an open upper end which is connected wit a tube resonator 60. Via the tube resonator 60, infrasonic soundwaves, generated an exigator 62 enter the upper portion of the freezing chamber where they are dispersed into the entire freezing chamber 58 by means of a wave divider 63 in th shape of a pyramid.

In the preferred embodiment of the freezer according to the invention, as shown in Fig. 7, the upper portion of the freezing chamber 58 is, furthermore connected to a duct 64 which emanates, via the passing of a fan 66, into the lower portion of the freezing chamber 58. It is preferable that also the fan duct 64 is provided with heat insulation.

Figs. 7 and 8 illustrate particularly well the inclination of the perforated obstacles or tray bottoms 68, 70, 72, 74, 76, 78, 80, 84, 86, 88. The uppermost tray bottom 68 is communicating with a product inlet 90 (see Fig. 9), while the lowermost bottom is communicating with a product outlet 92. Alimentary bodies introduced via the produ inlet 90 slide down the tray bottom until they reach the edge of the tray bottom in question, then they fall onto the the next, adjacent tray bottom situated at an angle virtually 90° to the previous tray bottom. The infrasonic soundwaves oriented virtual transversally across the respective tray bottom thus contribute to enhanced heat transmission and a more speedy heat transport between the alimentary bodies and cooling agent circulating inside pipes of cooling batteries 98, 100, 102, 104, 106, 10 110, 112, 114, 116, 118, 120 arranged underneath the respective bottoms. At 92, th completely or partially frozen alimentary bodies will finally leave the freezing chamb 58 for further treatment such as packing, storing etc.

Fig. 9 shows a top view of the freezing chamber 58 of the freezer 52 shown in Figs. and 8. At the upper side-surface in Fig. 9, the product inlet 90 is shown, and at the side-surface to the right in Fig. 9, the product outlet 92 is shown. The flow direction the products is indicated with arrows. The essentially rectangular, perforated tray bottoms 68, 70, 72, 74 form a helical curved track down which the products to be frozen are transported. The tray bottoms may be perfectly rectangular in which case they are connected, in the corner areas, to corner sections 69, 71 , 73, but they ma as well have oblique corners and in that case the comers of two adjacent tray botto parts have ends cut at an angle of 45° in order to form a right angle when mounted together. In another embodiment, the tray bottoms are furthermore adjustable with view to change the pitch angle of the helical curved track.

Moreover, as is shown in Fig. 8, the cooling batteries 98, 100, 102, 106, 108, 110, 114, 116, 118 are arranged immediately below the respective tray bottom 68, 70, 72 76, 78, 80, 84, 86, 88. Each and everyone of these is preferably assembled of a plurality of cooling pipes which are illustrated with a "+"-sign in the sectional views o Figs. 7 and 8. The cooling battery 98, installed underneath the bottom 68 which is communicating with the product inlet 90, has an outlet 122 for a cooling agent

arranged at one of its ends (see Fig. 7), while the other end of the cooling battery i connected with a cooling battery 100 situated below the next, adjacent tray bottom (see Figs. 8 and 9). A cooling battery 89, shown in Fig. 8 but only schematically wit dashed line, and located underneath the tray bottom being connected to the produ outlet 92, is connected to both a preceeding cooling battery and an inlet 124 for a cooling agent. The connections between the individual cooling batteries may be flexible to allow adjustment of the inclination of the cooling batteries in accordance with the inclination of the respective tray bottom.

During operation, the products that are to be frozen are fed into the freezer 52 thro the product inlet 90. The products slide down the inclined tray bottoms and are fro gradually. The standing soundwave inside the freezing chamber 58 accelerate the freezing process through quicker heat transport between the alimentary bodies an the cooling batteries. The frozen alimentary bodies are eventually discharged thro the product outlet 92 for further treatment, such as packing, storing etc.

When freezing alimentary bodies susceptible to fluidization, it is especially prefera to utilize the infrasonic sound to transfer the bodies into the fluidized state. The alimentary bodies to be frozen are thus transferred into fluidized state while passin into the freezing chamber 58 via the product inlet 90, due to the fact that the freezi chamber is being furnished with infrasonic sound generated by the exigator 62; an they will preferably remain in this state until they leave the freezer via the product outlet 92. Alternatively, the above mentioned tray bottoms may be essentially horizontal with a difference in level between two adjacent tray bottoms. If a "dead zone", i.e. a zone within which no fluidization takes place thus causing non-fluidize alimentary bodies to build up a wall, is allowed at the end of each respective tray bottom then a fluidization zone is created above this tray bottom and between the ends of the tray bottom. By continuously feeding alimentary bodies, the fluidized alimentary bodies will overflow the wall and reach the next fluidization zone at a distance below the previous, until the alimentary bodies leave the freezer through product outlet 92.

If the speed of the product flow, during fluidization on substantially horizontal tray bottoms, should be too low, it is preferred to direct the infrasonic soundwaves in s a way that they form an acute angle with the tray bottom, the horizontal compone the infrasonic soundwaves thus increasing the speed of the product flow. When freezing alimentary bodies with difficult properties regarding the freezing process, sticky products, it is preferredle to operate the fan 66 which is located in the fan du 64 between the top of the freezing chamber and the lowermost part of the freezing chamber in order to accomplish the intended technical effect.

It has been shown that the introduction of infrasonic sound technique in connectio with a freezer increases the heat transmission and the heat transport between alimentary bodies to be frozen and cooling batteries, and that this technique may eliminate the use of fans, especially if the cooling batteries are installed in close proximity of the product flow and under the condition that the intensity of the infras sound is sufficiently high. The result is quicker freezing and an increased efficienc rate in particular resulting from the fact that there are no moving parts inside the freezer, such as fans, that need maintenance, defrosting etc. Λ special advantage the infrasonic sound technique is that the infrasonic sound may be utilized for fluidization of products susceptible to fluidization, which increases the productivity even more.

Even if the infrasonic sound technique here has been described in relation to a ne and preferred freezer, it is also applicable to cooling and other kinds of processes alimentary products where enforced heat transmission is desirous, for example heating such as frying, cooking, drying etc.