EP0621740B1 - Method for uniformly heating, pasteurizing and sterilizing of a product by micro-waves - Google Patents

Description

The invention relates to a method and a device as described in the preamble of the first claim.

A generic method is known from the applicant's DE-PS 38 30 867. A pulse-like, intermittent feeding of the microwave energy required for heating to a certain temperature ensures that the product surface does not heat up undesirably. Different forms of microwave power are used. A rectangular form of performance has the greatest influence on the product surface. But even with trapezoidal or sinusoidal waveforms, useful results are achieved with a pulse operation of the system.

The long settling range per pulse has proven to be particularly disadvantageous with regard to the heating-up time when using systems operating at 2,450 gigahertz, so that for each energy feed-in pulse relatively little energy can be fed in, because a larger amount of energy per pulse leads to damage to the product surface.

A pulsed, intermittent energy feed into the products to be treated makes sense, because otherwise the microwave energy fed in (power x time) is too high in relation to the temporal absorption capacity of the product, i.e. there is a critical increase in the temperature of the product surface. A pulse-like energy coupling means an energy supply with a certain length of time and repetition.

It is the object of the invention to achieve an improvement in the pulse-like feeding of the microwave energy into products to be treated. The aim is to achieve an increase in the absorbed energy in the product in a shorter time, with higher output and thus an increase in the belt speed or the output of a continuous system for heating, pasteurizing and sterilizing food in packaged or loose form without any critical signs of overheating occur on the product surface. The aim is to increase the amount of energy absorbed by the product in a certain time without damage.

The object is achieved by the features described in the characterizing part of the claims in conjunction with the features of the preamble.

An energy pulse with an almost rectangular power profile is used for rapid heating of the products, which results in a considerable increase in the amount of energy absorbed without causing an excessive increase in the temperature of the product surface or inside the product.

It is thus possible to use a magnetron of e.g. 1.2 kW nominal power output power pulses in the range of microseconds that are greater than 1.2 kW.

In the case of control electronics according to the prior art, due to the analog mode of operation, a slope of t (a)> = 80 to 200 milliseconds should not be fallen below the rising edges during power on. A difficulty here is that when transferring energy into a product that is in a package of 1 <80 mm and that is at a conveying speed of v> = 6 m / min in the available time of tmax = <800 Moved milliseconds, parts of the product during the flat rising power settling range cannot be loaded with the full power required per se. The minimum pulse duration achievable in accordance with the invention in the micro to millisecond range with extreme steepness thus enables, among other things. a significantly higher conveying speed than is the case according to the prior art under the most favorable conditions.

In order not to be limited according to the invention with the throughput capacities of m> 2,500 kg / h required by design and process technology (system width, conveying speed), it is therefore advantageous to use the faster technology according to the invention. The large steepness that can be achieved also results in a significantly better efficiency.

• 1 . Die Impulsleistung des Magnetrons kann ein Vielfaches der Dauerstromnennleistung betragen, wenn die mittlere Leistung die leistung nicht überschreitet.
• 2. Durch die erzielbare große Flankensteilheit des Leistungsimpulses, steht mit Einschaltung der Leistung sofort die gesamte Leistungshöhe für den Erwärmungsprozeß im Produkt zur Verfügung.
• 3. Überraschender Weise ergibt sich durch die so geformten Leistungsimpulse im Millisekundenbereich eine größere Eindringtiefe der auf das Produkt übertragenen Energie, wie auch ein schnelleres Auf- heizen des Produktes mit geringerem Temperaturgradienten in Eindringrichtung.
• 4. Es findet ein relativ geringerer Leistungsumsatz an der Produktoberfläche und damit eine Reduzierung der überhitzung der Produktoberfläche statt.
• 5. Durch die Anhebung der Steilheit der Anstiegsflanken ist eine größere Fördergeschwindigkeit des zu behandelnden Produktes ohne kritische Überhitzung der Produktoberflächen oder beispielsweise bei Kartoffeln ohne kritische Überhitzung des Inneren möglich (volle Leistungsabgabe bei Millisekunden-Impulsen).

It has been shown that, for example, in the treatment of food, the use of impulses which are as short as possible and of great power is of crucial importance. If, instead of the previously used 100 Hz system clock, a frequency is used which has a greater steepness of the power pulse in the range of, for example, a few microseconds permits, for example a 50 kHz system clock, power pulses with an almost rectangular shape with pulse lengths of certainly from 1 millisecond can be represented without an upper time limit. In particular, the advantages described below can be used in this way when operating the microwave generators (magnetrons) controlled in this way.

• 1 . The magnetron's pulse power can be a multiple of the continuous current rating if the average power does not exceed the power.
• 2. Due to the achievable steepness of the edge of the power pulse, the entire power level is immediately available for the heating process in the product when the power is switched on.
• 3. Surprisingly, the power pulses shaped in this way in the millisecond range result in a greater depth of penetration of the energy transferred to the product, as well as faster heating of the product with a lower temperature gradient in the direction of penetration.
• 4. There is a relatively lower power conversion on the product surface and thus a reduction in the overheating of the product surface.
• 5. By increasing the steepness of the rising flanks, a higher conveying speed of the product to be treated is possible without critical overheating of the product surfaces or, for example, in the case of potatoes without critical overheating of the inside (full power output in the case of millisecond pulses).

The invention is further explained below with reference to the drawings.

Fig. 1 und Fig. 3

stellen jeweils das 50/100 Hz-Taktsystem gemäß dem Stand der Technik dar.

Fig. 2 und Fig. 4

zeigen ein erfindungsgemäßes Taktsystem, welches mit 50 kHz arbeitet.

Show it:

1 and 3

each represent the 50/100 Hz clock system according to the prior art.

2 and 4

show a clock system according to the invention, which operates at 50 kHz.

Fig. 1:

50/100 Hz Systemtakt
Die aus der Netzfrequenz über Brückengleichrichter gleichgerichteten 50 -> 100 Hz-Impulse (1) haben nach integrierender, elektronisch geregelter Gleichrichtung (100 Hz) eine entsprechend langsame Anstiegsflanke und eine Restwelligkeit (=nach el. Glättung=).

Fig. 2:

50 kHz Systemtakt
Die zu 1 äquvalenten Impulse 2 haben aufgrund des hochfrequenten Systemtaktes in der zeitlichen Darstellung (Abszisse) eine nahezu ideale Rechteckform mit einer rechtwinkeligen Impulsamplitude (A2a, B2a, C2a).
Aufgrund dieser rechtwinkeligen Impulsform, d.H. aufgrund der großen Flankensteilheit, ist man in der Lage, das mehrfache der Dauerstromnennleistung einzustellen, solange diese leistungsmäßige Übersteuerung nicht länger als 50 bis 80 Millisekunden anhält und danach eine Impulspause folgt, die in der mittleren Leistung den Nennwert von (hier) 1,2 kW nicht überschreitet (A2b, B2b, C2b).

Fig. 3:

50/100 Hz Systemtakt
Die mehr oder weniger gleichmäßigen Impulse 1 (A1a) ergaben nach der integrierenden Leistungsregelung eine wellige Konstantleistung mit flacher Anstiegsflanke.

Fig. 4:

50 kHz Systemtakt
Mit den kurzen Leistungsimpulsen (A2a) kann man mit einer hohen Leistung in das zu behandelnde Produkt eindringen, ohne eine Überhitzung zu erzeugen. Ebenso ist es aber auch möglich, die gleiche Energie (P x t) als niedrige Konstantleistung in rechtwinkeliger Form (steile Impulsflanken, Welligkeit der Amplitude (A2b)). Eine entsprechende Leistungsform zu Fig. 3, Alb ist gleichermaßen möglich, jedoch ohne die nicht erwünschte Welligkeit (2).

Representation of the described difference between the 50/100 Hz and the 50 kHz system clock pulse technology:

Fig. 1:

50/100 Hz system clock
The 50 -> 100 Hz pulses (1) rectified from the mains frequency via bridge rectifiers have an appropriately slow rising edge and a residual ripple (= after el. Smoothing =) after integrating, electronically controlled rectification (100 Hz).

Fig. 2:

50 kHz system clock
The pulses 2, which are equivalent to 1, have an almost ideal rectangular shape with a rectangular pulse amplitude (A2a, B2a, C2a) due to the high-frequency system clock in the temporal representation (abscissa).
Due to this right-angled pulse shape, i.e. due to the large slope, it is possible to set a multiple of the continuous current nominal power as long as this power override does not last longer than 50 to 80 milliseconds and then follows a pulse pause that in the mean power the nominal value of ( here) 1.2 kW does not exceed (A2b, B2b, C2b).

Fig. 3:

50/100 Hz system clock
The more or less uniform pulses 1 (A1a) resulted in a rippled constant power with a flat rising edge after the integrating power control.

Fig. 4:

50 kHz system clock
With the short power pulses (A2a) you can penetrate the product to be treated with high power without causing overheating. However, it is also possible to use the same energy (P xt) as a low constant power in a rectangular shape (steep pulse edges, ripple in the amplitude (A2b)). A corresponding form of performance to Fig. 3, Alb is equally possible, but without the undesirable ripple (2).

Claims (1)

1. Method of uniformly and rapidly heating by means of microwaves supplied to the products in a pulse-like manner, wherein the products to be treated, such as chemical or pharmaceutical products or ready-to-serve meals for example, are guided through a treatment chamber in open or sealed containers, which are permeable to microwaves, on a continuously operating conveyor system, inlet ducts of microwave generators being orientated vertically or inclinedly relative to the conveyor belt and being disposed in said treatment chamber, the energy outputs of said generators being effected in a pulse-like manner and being controllable by a computer,
   characterised in that
   the system is operated

   a) with a microwave power profile, which shortens the inward and downward oscillation ranges of the microwave pulses,

   b) with an operating frequency of between 20 and 100, preferably between 40 and 80, and especially 50 kHz, said frequency having an edge steepness of less than 1 millisecond,

   c) with pulses having a largely rectangular amplitude shape and a length in the micro- to millisecond range.

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