US20120272664A1

US20120272664A1 - Electrostatically charged cryogen fog

Info

Publication number: US20120272664A1
Application number: US13/094,879
Authority: US
Inventors: Michael D. Newman
Original assignee: Linde GmbH
Current assignee: Linde GmbH
Priority date: 2011-04-27
Filing date: 2011-04-27
Publication date: 2012-11-01

Abstract

An apparatus for providing electrostatically charged cryogen fog to a product includes an injection tube having a chamber therein for receiving a cryogenic substance introduced into the chamber and to be provided to the product, a first inlet in communication with the chamber for providing the cryogenic substance to the chamber, an electrostatic device in communication with the chamber for charging the cryogenic substance with a charge opposite to a charge of the product, and an outlet in communication with the chamber and through which the charged cryogenic substance is exhausted to be attracted to the product for adhesion thereto. A method of providing an electrostatically charged nitrogen fog to a product is also provided.

Description

BACKGROUND

The present embodiments relate to chilling or freezing of products such as food products, with a cryogen substance.
In known cryogen food freezing systems where nitrogen is used, liquid nitrogen is sprayed into a freezing chamber where the products are disposed to provide refrigeration for the products. The spray of liquid nitrogen is directed onto the warm surface of the incoming food product so that a phase change (heat of vaporization) occurs at the surface of the product. This evaporative cooling effect creates extremely high heat transfer coefficients for the food product to chill or freeze same.
Unfortunately, it is extremely difficult if not impossible to have an acceptable proportion of the liquid cryogen contact the food product, as the cryogen upon discharge from the nozzles is sprayed into the chamber toward the food products positioned on a conveying belt where 25-60% of liquid cryogen is lost. A large proportion of the liquid nitrogen contacts the belt, instead of the food product, and indeed passes through the mesh belt or contacts other areas of the spray chamber in which the food product is being conveyed. This results in a waste of refrigeration which would otherwise have been achieved during the heat of vaporization of the liquid nitrogen.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present embodiments, reference may be had to the following drawing figures taken in conjunction with the description of the embodiments, of which:

FIGS. 1 and 2 show an apparatus and method for providing an electrostatiscally charged nitrogen fog.

DETAILED DESCRIPTION OF THE INVENTION

The present embodiments provide an apparatus and method for coating a product, such as for example, a food product, with a cryogen, such as for example a nitrogen (N₂) or carbon dioxide (CO₂) fog using an electrostatic charge. When, for example, liquid nitrogen is atomized either by electrostatic atomization or by an ultrasonic fog generator, surfaces of the nitrogen droplets can be charged by exposure to an electrostatic field. The product, and by way of example a food product, can be grounded or given an opposite charge which will attract the atomized nitrogen droplets, thereby allowing for a much greater proportion of the cryogenic spray to be deposited onto the food product which will maximize overall evaporative heat transfer at the food product. In effect, the opposite charges of the atomized spray and the food product provide for attraction of the spray to the food product to better control the amount of cryogenic spray used, and to be confident that any nitrogen spray used will indeed contact the food product and not be wasted by adhering or passing through the conveyor belt or other surfaces of the conveyor chamber through which the food product is being transferred for chilling or freezing.
Using a food product for example, electrostatic atomization exposes the liquid cryogen to an intense electric field between a charged atomizer and a grounded food product. The charge transfers to the liquid cryogen, and repulsive forces between the atomizer and the liquid cryogen force droplets of the cryogen from the atomizer and direct them toward the food product. An energy source for the electrostatic atomization is an electric charge received by the liquid cryogen. Particle size and electrostatic atomization of the liquid cryogen is a function of three (3) factors, that is (i) electric field strength, (ii) liquid flow rate, and (iii) fluid properties (including electrical properties of the fluid).
Referring to FIGS. 1 and 2, and using nitrogen (N₂) for the cryogenic liquid by way of example only, an apparatus for generating an electrostactically charged nitrogen fog is shown generally at 10 and includes a liquid nitrogen (N₂) injection tube 12 or nozzle The injection tube 12 is connected to a DC voltage generator 16. A conveyor belt 18 is spaced apart from and disposed beneath the outlet 14 of the injection tube 12, and provides a surface 20 upon which a product 22, such as a food product for example, is disposed for transport beneath the injection tube 12. The conveyor belt 18 is fabricated from metal and may be solid or of the mesh-type. A ground wire 24 is in contact with the conveyor belt 18 to ground said belt, including the belt surface 20.
The injection tube 12 includes an internal chamber 26 accessible by an inlet 28 or inlet port. Droplets 30 of nitrogen are produced external to the injection tube 12 and provided to same via the inlet 28 as shown by arrow 32. The injection tube 12 also includes another inlet 34 or inlet port in communication with the chamber 26 so that gaseous nitrogen can be introduced into the chamber as indicated by arrow 36. The gaseous nitrogen is a carrier gas 35 for the droplets 30. If carbon dioxide is used for the droplets 30 then the carrier gas will also be carbon dioxide. The carrier gas 35 can also be provided to atomize the droplets 30 introduced into the injection tube 12 upstream of the outlet 14.
The DC voltage generator 16 has a portion 17 extending into the internal chamber 26. The portion 17 can be of any length and therefore have any amount of surface area. However, by maximizing the surface area of the portion 17 for exposure to the cryogen droplets this will increase the number of charged particles 40 for adhering to the product 22. The injection tube 12 includes a side wall 38 which is insulated or vacuum jacketed. The outlet 14 of the injection tube 12 may be in registration with the product 22 passing beneath the outlet 14 as further discussed below.
Atomization of the liquid cryogen, such as liquid nitrogen, can occur in a plurality of ways. Such atomization can occur by (1) electrostatic atomization wherein the electric field created by the DC generator actually atomizes the liquid nitrogen into droplets to be attracted to the product 22, or (2) the liquid nitrogen can be atomized upstream of the outlet 14 and upon entry into the chamber such atomized particles are charged prior to being sprayed from the outlet 14 onto the product 22.
The nitrogen droplets 30 introduced through the inlet 28 into the chamber 26 are “un-charged” nitrogen particles. Upon exposure to the high voltage DC current provided by the generator 16, the nitrogen particles receive a charge transfer and become negatively charged particles 40. Free ions 42 are also provided upon discharge from the outlet 14.
Because the conveyor belt 18 is grounded, the food product 22 is also grounded. Accordingly, because the food product 22 is disposed closer to the outlet 14 of the injection tube 12 than is the conveyor belt 18, the charged nitrogen particles 40 in an atomized state will first adhere to the food product before adhering to the underlying conveyor belt. In effect, the food product 22 becomes a magnet for the charged nitrogen particles 40 which are attracted to the food product to adhere to the food product's exterior surface. Therefore, a majority of the atomized charged nitrogen particles 40 adhere to the exterior surface of the food product 22 as opposed to the underlying conveyor belt 18. There is therefore less waste of the nitrogen particles and a greater percentage of same is used for heat transfer effect at the food product 22.
The embodiments provide for an increase in overall heat transfer effect of the food freezing process and provide for a more efficient use of the atomized cryogen. It is also possible to use liquid carbon dioxide (CO₂) instead of liquid nitrogen with the present embodiments.
It will be understood that the embodiments described herein are merely exemplary, and that one skilled in the art may make variations and modifications without departing from the spirit and scope of the invention. All such variations and modifications are intended to be included within the scope of the invention as described and claimed herein. Further, all embodiments disclosed are not necessarily in the alternative, as various embodiments of the invention may be combined to provide the desired result.

Claims

1. An apparatus for providing electrostatically charged cryogen fog to a product, comprising an injection tube having a chamber therein for receiving a cryogenic substance introduced into the chamber and to be provided to the product, a first inlet in communication with the chamber for providing the cryogenic substance to the chamber, an electrostatic device in communication with the chamber for charging the cryogenic substance with a charge opposite to a charge of the product, and an outlet in communication with the chamber and through which the charged cryogenic substance is exhausted to be attracted to the product for adhesion thereto.

2. The apparatus of claim 1, wherein the product comprises a food product.

3. The apparatus of claim 1, wherein the cryogenic substance comprises liquid droplets selected from carbon dioxide and nitrogen.

4. The apparatus of claim 1, wherein the injection tube further comprises a second inlet in communication with the chamber for providing a carrier gas to the cryogenic substance to be exhausted at the outlet.

5. The apparatus of claim 4, wherein the carrier gas comprises a gaseous equivalent of the cryogenic substance.

6. The apparatus of claim 1, wherein the electrostactic device comprises a portion disposed in the chamber, the portion having a surface area exposed to the cryogenic substance.

7. The apparatus of claim 1, wherein the injection tube comprises insulation at an exterior surface thereof.

8. The apparatus of claim 1, wherein the injection tube comprises a nozzle.

9. A method of providing an electrostatically charged cryogen fog to a product, comprising charging a cryogenic substance with a first charge, providing a product with a second charge opposite to the first charge and in relative proximity to the cryogenic substance, repulsing the cryogenic substance with the first charge to be attracted to the product having the second charge, and attracting the cryogenic substance to the product for adhering thereto.

10. The method of claim 9, wherein the product comprises a food product.

11. The method of claim 9, wherein the cryogenic substance comprises liquid droplets selected from carbon dioxide and nitrogen.

12. The method of claim 9, further comprising providing a carrier gas to the cryogenic substance for carrying the cryogenic substance from the charging to the adhering of the cryogenic substance to the product.

13. The method of claim 12, wherein the carrier gas comprises a gaseous equivalent of the cryogenic substance.

14. The method of claim 9, further comprising atomizing the cryogenic substance after repulsing the cryogenic substance.

15. The method of claim 9, further comprising atomizing the cryogenic substance before charging the cryogenic substance.