US20090064690A1

US20090064690A1 - System and method for cryogenic enhancement to mechanical freezers

Info

Publication number: US20090064690A1
Application number: US11/899,598
Authority: US
Inventors: John Martin Girard; David Joseph King; Theodore Hall Gasteyer, III; John Nicholas Hryn; Raymond James Schaeffer
Original assignee: Praxair Technology Inc
Current assignee: Praxair Technology Inc
Priority date: 2007-09-06
Filing date: 2007-09-06
Publication date: 2009-03-12
Also published as: CA2698262A1; EP2191213A2; CN101842650A; WO2009032997A2; WO2009032997A3

Abstract

A cryogenic enhanced mechanical freezer system and method are provided. The disclosed system includes a mechanical freezer compartment adapted for freezing food products, a cryogen injection system adapted to directly inject cryogen into the mechanical freezer compartment, an exhaust system adapted to rapidly evacuate cryogen vapors from the freezer compartment; and a freezer control system adapted to operatively control the exhaust system and cryogen injection system in response to selected inputs. The selected inputs may include user inputs as well as inputs received from a temperature sensor disposed in the freezer compartment and a gas analyzer adapted to monitor concentrations of selected gases within the freezer compartment. Access to the freezer compartment is prevented during cryogen injection and when the atmosphere in the freezer compartment is not safe as determined by the gas analyzer.

Description

FIELD OF THE INVENTION

The present invention relates to refrigeration and freezing systems, and more particularly to a system and method for the cryogenic enhancement of mechanical freezers with liquid nitrogen or carbon dioxide.

BACKGROUND OF THE INVENTION

In typical mechanical refrigeration or freezing systems there are inherent performance limitations on the ability of the freezer systems to cool foodstuffs. One such limitation arises when excessive ice forms on the refrigeration coils. Excessive ice on the coils requires the freezer system to be defrosted on a regular basis which interrupts and adversely impacts food production. Another common freezer performance limitation occurs when the condenser unit in the mechanical freezer system is operating at full capacity and the freezer system cannot adequately reject any excess heat from the condenser unit. Such equipment issues often result in limitations of the refrigeration potential for the freezer system which, in turn, is often a limiting factor in the food production line.
Prior art attempts to solve these problems of system inefficiency involved the use of a combination freezer that employs both cryogenic refrigeration techniques in conjunction with mechanical refrigeration. In such prior art cryogenic enhancement systems, a cryogenic freezer and a mechanical freezer are typically disposed into a single enclosure with each freezer operating independently. See for example, U.S. Pat. Nos. 4,856,285 (Acharya et al.); 4,858,445 (Rasovich) and 5,170,631 (Lang et al.).
For example, in both the Acharya et al. and Rasovich disclosures, the cryogen freezer is segregated from the mechanical freezer and any cryogenic vapor present in the combination freezer is used indirectly to supplement the mechanical refrigeration. Because the cryogenic vapors are used to effect cooling in an indirect manner, the cryogenic vapors and the mechanically refrigerated air are controlled and managed separately.
The Lang et al. prior art system uses cryogen vapor in a direct refrigeration capacity, but the cryogenic freezer is also spatially and operationally segregated from the mechanical freezer which keeps the cryogenic vapors and the mechanically refrigerated air separate. Moreover, the Lang et al. reference does not address the need to safely manage the hazardous atmosphere that arises when cryogenic vapors and mechanically refrigerated air is mixed.
What is needed therefore, is a safe and relatively low cost, integrated cryogenic enhancement to mechanical freezer systems that would combine cryogenic vapors and mechanically refrigerated air to effect the freezing of foodstuffs. The combined effect of cryogenic vapor and mechanically refrigerated air operates to increase freezer capacity and freezer performance over a pure mechanical freezer without costly modification to components within the mechanical freezer system.

SUMMARY OF THE INVENTION

The present invention may be characterized as a cryogenic enhanced mechanical freezer system comprising: a mechanical freezer compartment adapted for freezing food products; a cryogen injection system adapted to directly inject cryogen into the mechanical freezer compartment; an exhaust system in communication with the freezer compartment and adapted to rapidly evacuate cryogen vapors from the freezer compartment; and a control system adapted to control the exhaust system and cryogen injection system in response to selected inputs. The selected inputs may include user inputs as well as inputs received from a temperature sensor disposed in the freezer compartment and a gas analyzer adapted to monitor gas concentrations within the freezer compartment.
In another aspect, the invention is characterized as a method of freezing foodstuffs in a mechanical freezer system comprising steps of: (a) mechanically refrigerating a freezer compartment; (b) sensing the temperature in the freezer compartment; (c) selectively injecting a cryogen directly into the freezer compartment based on the temperature of the freezer compartment to cryogenically enhance the freezing of the foodstuffs; (d) monitoring the concentrations of selected gases within the freezer compartment; (e) exhausting a portion of the cryogen vapors from the freezer compartment during injection of the cryogen; and (f) rapidly evacuating the cryogen vapors from the freezer compartment when access to freezer compartment is required.

BRIEF DESCRIPTION OF THE DRAWINGS

While the specification concludes with claim distinctly pointing out the subject matter that applicants regard as their invention, it is believed that the invention will be better understood when taken in connection with accompanying drawings in which:

FIG. 1 is a schematic view of the present cryogenic freezer system in accordance with an embodiment of the invention;

FIG. 2 is a schematic view of a control system suitable for use with an embodiment of the invention;

FIG. 3 is a detailed view of the preferred door interlock scheme used in an embodiment of the invention; and

FIG. 4 is a flowchart delineating the preferred steps involved in the safe operation of cryo-boost enhancement

DETAILED DESCRIPTION

The following description sets forth the best mode presently contemplated for practicing the present system and method for cryogenic freezer enhancement. It is not to be taken in a limiting sense, but rather should be read in conjunction with the appended claims.
With reference to FIG. 1, there is shown an embodiment of the present cryogenic freezer enhancement system 10. As seen therein, the cryogenic freezer enhancement system 10 includes a mechanical-based freezer compartment 12 having a door 14 or other means for access and egress to the interior of the freezer compartment 12. The freezer compartment 12 may include various conveyors or other structures disposed therein for containing food product. The freezer system 10 further includes a cryogen injection subsystem adapted to introduce a cryogen into the freezer compartment 12 to treat the food product as well as an exhaust subsystem and a freezer control subsystem.
In the illustrated embodiment, the cryogen injection subsystem preferably includes a source of cryogen 21, such as liquid nitrogen or carbon dioxide, contained within a storage vessel 23 and coupled via a cryogen circuit 24 to cryogen nozzles 25 disposed within the freezer compartment 12. A plurality of valves, such as shut-off valves 26, control valves 27, safety relief valves 28, etc. are disposed along the cryogen circuit and operatively coupled to the freezer control panel 42. The direct injection of the cryogen into a mechanically refrigerated freezer is an efficient and cost effective means to improve overall freezer performance relative to production capacity. The refrigeration capacity of the cryogen, as applied directly to the mechanically refrigerated space in the freezer compartment, works in conjunction with and boosts the mechanical refrigeration capacity.
The preferred process to boost the refrigeration capacity in a mechanical freezer with a cryogen, such as liquid nitrogen or carbon dioxide is straight-forward. The present cryo-boost process contemplates the mechanical refrigeration system to operate at the optimum conditions to maximize efficiency, which is typically at a prescribed set point temperature. The cryo-boost process is preferably operated at a temperature control point slightly warmer than the mechanical refrigeration system set point temperature. Setting the cyro-boost temperature control point slightly warmer than the mechanical refrigeration system set point temperature, the demand for injection of nitrogen or carbon dioxide into the freezer compartment 12 is controlled without adversely influencing the operation of the mechanical refrigeration system. The warmer temperature control point of the cryo-boost process ensures the mechanical refrigeration system operates at or near maximum efficiency. Nitrogen or carbon dioxide is added only when the refrigeration demand is near or exceeds the mechanical refrigeration capacity. When the refrigeration demand diminishes, the cryogen flow will terminate before the mechanical refrigeration system unloads since the cryo-boost control temperature is warmer than the mechanical refrigeration temperature set point.
In the illustrated embodiment, the exhaust subsystem preferably includes a primary exhaust duct 31 and primary exhaust blower 33 adapted to collect and discharge expended cryogen vapors during the operation of the freezer. The illustrated exhaust subsystem further includes an auxiliary exhaust duct 35 and auxiliary exhaust blower 37 adapted for the rapid evacuation of the cryogen vapor and refrigerated air within the freezer compartment upon activation. Activation of the auxiliary exhaust is preferably initiated as a command from the controller in response to various user inputs or automatic safeguards programmed into the controller.
The rapid exhaust blower 37 is preferably sized to evacuate the atmosphere within the freezer compartment 12 in a few minutes, preferably less than four minutes, and replace the same with a source of make-up air. From a safety standpoint, evacuation of the freezer compartment atmosphere should require a 4 to 5 times volume exchange to ensure a safe and breathable atmosphere in the freezer compartment For example, a 2000 cubic foot freezer preferably employs an auxiliary exhaust blower sized at about 4000 cubic feet per minute. The rapid exhaust blower is activated and preferably runs for 2 minutes in order to achieve the 4 times volume exchange. Preferably, a make-up air port 39 connected to a source of make-up air is also included as part of the exhaust subsystem for use when the rapid exhaust blower 37 is activated. Additional safety precautions may be invoked that keep the rapid exhaust blower 37 or the primary exhaust blower 33 or both operating whenever the door 14 to the freezer compartment 12 is open allowing access to the freezer compartment.
Both the primary exhaust and the auxiliary rapid exhaust are adapted to prevent most of the cryogenic vapors injected into the freezer compartment 12 from filling the process area 50 surrounding the freezer compartment 12. Both the primary exhaust duct 31 and the auxiliary exhaust duct 35 carry away and vent the evacuated streams out of the freezer compartment 12 and away from the process area 50. Ideally, the location of the exhaust pickups for the exhaust subsystem is proximate the floor or bottom of the freezer compartment 12 as both preferred cryogens, namely nitrogen and carbon dioxide, are denser than air at temperatures near −30° F. and tend to settle toward the bottom of the freezer. To maximize the effectiveness of the rapid exhaust in replacing the air within the freezer compartment, the make-up air preferably should enter from a duct 39 or port at the top of the freezer compartment 12 while the colder cryogenic vapors exit from the lower portion of the freezer compartment 12.
The freezer control subsystem preferably includes a microprocessor-based control panel 42 adapted to receive a plurality of inputs including, as a minimum, a temperature sensor input and a gas analyzer input as well as various user inputs and control the operation of the cryogen enhanced mechanical freezer system. As seen in FIG. 1, a temperature sensor 44 and a gas analyzer 46 are coupled as inputs to the freezer control panel. Other inputs representing or corresponding to the freezing or refrigeration parameters are also used in the present freezer control subsystem. Such other inputs may include, for example, characteristics of the cryogen used, characteristics of the product being treated, or operational characteristics of the processing area, associated equipment and components thereof.
The freezer control subsystem operatively controls the flow of cryogen in the cryogen circuit 24 and injection of the cryogen into the freezer compartment 12 by controlling selected valves 26,27,28 in response to the various inputs, identified above. In the preferred embodiment, the amount of cryogen injected into the freezer compartment 12 is based on the temperature in the freezer compartment 12, as ascertained by the temperature sensor 44, such that the efficiency of the mechanical refrigeration system is optimized. This cryo-boost enhancement is preferably a user initiated or selected process that can be turned on or off depending on the refrigeration needs for the product within the freezer compartment. For example, during high production runs or during summer months when temperatures of the processing area have increased, supplemental refrigeration capacity via cryogen enhancement or cryo-boost is both useful and cost-effective to achieve the desired temperature of the product.
The freezer control subsystem also controls the rapid exhaust and other safety features of the cryogen enhanced freezer system 10 in response to selected inputs and operating parameters. When the cryo-boost feature is activated, cryogen injection via nozzles 25 into the freezer compartment 12 is precisely controlled based, in part, on the temperature insider the freezer compartment 12. The cryo-boost enhancement process remains active until the control system or operator turns off the cryo-boost enhancement or someone attempts to enter the freezer compartment 12. Upon either of these events, a rapid exhaust blower 37 is activated to purge the atmosphere inside the freezer compartment 12 within a short period of time, preferably in about two minutes or less.
In the present embodiment, access into the freezer compartment 12 is via the door 14 or other egress is strictly controlled. When the cryo-boost enhancement is activated, the doors 14 to the freezer compartment 12 are inaccessible. More specifically, access to the freezer compartment 12 from the outside is prevented by covering and securing the door handles with appropriate interlocks 48 when the atmosphere within the freezer compartment 12 is oxygen-deficient.
In addition, a gas analyzer 46 operatively coupled to the freezer control panel 42 and the door interlocks 48 is also provided. In this manner, gas analyzer is used as an input to control (e.g. stop) the rapid exhaust as well as to limit access to the freezer compartment 12 via the door 14. In the preferred embodiment, access to the door 14 to the freezer compartment 12 remains inaccessible so long as the gas analyzer 46 indicates the presence of an oxygen-deficient atmosphere inside the freezer compartment 12. In the liquid nitrogen embodiment of the cryo-boost enhancement process, the gas analyzer 46 is preferably an oxygen gas analyzer which will allow access to the freezer compartment 12 only when the cryo-boost enhancement is deactivated (i.e. OFF) and there is sufficient oxygen in the freezer compartment 12 to allow a safe breathing environment. Alternatively, in the carbon dioxide embodiment of the cryo-boost enhancement process, the gas analyzer 46 is a carbon dioxide gas analyzer which will deny access through the door 14 of the freezer compartment 12 when the cryo-boost enhancement process is activated (i.e. ON) or until the concentration of the carbon dioxide is below a safe threshold level.
Turning to FIG. 2 there is shown another schematic of an embodiment of the cry-boost enhancement highlighting the freezer control aspects of the system and process. As seen therein, control panel 42 is adapted to receive various inputs including, as a minimum, selected user inputs 69; temperature sensor inputs 67; and gas analyzer inputs 68. As indicated above, the user inputs 69 represent or correspond to the freezing or refrigeration parameters such as, temperature, flow and type of cryogen, refrigeration requirements of the product being frozen etc. The preferred control panel 42 is a microprocessor based control unit having adapted for receiving selected inputs and generating a series of commands or instructions 61,62,63,64,65,66 to operate the cryo-boost enhancement process. The control panel 42 also preferably includes appropriate user interfaces such as a display screen and operator buttons (not shown) as well as suitable data and instruction storage means.
Using one or more of the above identified inputs, the freezer control subsystem operationally controls using output commands 61,62,63,64,65,66 the primary exhaust blower 33; the rapid exhaust blower 37; a cryogen injection; the make-up air replenishment; and the interlocks 48 on the freezer doors 14 via commands or outputs from the control panel 42. Control of the cryogen injection is operatively controlled by controlling the flow of cryogen 21 from the source of cryogen 23 through a cryogen circuit 24 that includes plurality of flow control and safety valves 26, 27, 28 and to nozzles 25 adapted to inject the cryogen into the freezer compartment proximate the food product to be treated. Control of the cryogen and air flow out of the freezer compartment 12 is governed by the primary exhaust pickup 32; primary exhaust duct 31; primary exhaust blower 33; the rapid exhaust pickup 34; rapid exhaust duct 35; and rapid exhaust blower 37. Make-up air to replace the atmosphere vented via the exhausts is operatively controlled by governing the flow through the intake duct 39 or port.
Knowing the status and condition of the freezer door position is important for safety considerations. Such status and conditions include whether the freezer door 14 is open or closed and whether the freezer door 14 is locked or unlocked. FIG. 3 depicts an embodiment of the interlocks 48 used in conjunction with the above-described cryogenic enhanced mechanical freezer system. In the illustrated embodiment, a proximity switch 52 is disposed on each freezer door 14 and coupled as an input to the freezer control panel 42. The proximity switch 52 monitors the position of the freezer door 14 to the freezer compartment 12.
The door interlock 48 includes a stainless steel cover 53 securely attached to the freezer proximate the door handle 54 with one or more guides 55 and a retaining mechanism. The cover 53 is adapted to move between a first position that shrouds the door handle 54 and a second position exposing the door handle 54. In the unlocked position, the cover 53 easily slides, rotates or otherwise exposes the door handle 54 to allow access to the door handle 54 by a person desiring entry to the freezer compartment 12. However, in the locked position, the cover 53 is retained in a position that prevents access to the door handle 54 by a person thereby preventing access or entry to the freezer compartment 12.
In the illustrated embodiment, the retaining mechanism comprises a small pneumatic cylinder 56 operatively actuated by commands from the freezer control panel 42. When the pneumatic cylinder 56 is actuated, a rod 57 extends and engages the cover 53 to physically lock the cover 53 over the door handle 54 and prevent access to the freezer compartment 12. When the atmosphere in the freezer compartment 12 is safe, the freezer control panel 42 de-energizes the pneumatic cylinder 56 and retracts the rod 57 allowing the cover 53 to move and expose the door handle 54.
Turning now to FIG. 4 there is shown a flowchart broadly characterizing the control logic involved in the safe operation of cryo-boost enhancement process. A preliminary step in the depicted control process is to establishing the operating parameters, temperature control points, and temperature set points for the freezer (Block 81). Such operating parameter setting is preferably accomplished via user inputs to the control panel of the freezer system. The cryo-boost control process involves operating the mechanical freezer while selected operating conditions, namely temperatures are monitored (Block 82). During such freezer operation, the microprocessor based control unit continually inquires whether the conditions exist for activation of the cryo-boost enhancement (Block 83). If activation of cryo-boost is not desired, the freezer control subsystem continues to monitor operating conditions. On the other hand, if operating conditions dictate that the cryo-boost enhancement process is to be activated, the freezer control subsystem activates the door interlocks and initiates the cryogen injection (Block 84). Upon initiation of the cryo-boost enhancement, the standard or primary cryogen exhaust subsystem is activated (Block 85) and the gas analyzer is tested to confirm proper operation thereof (Block 86).
The freezer control subsystem then proceeds to monitor the operating conditions of the freezer as well as the cryogen injection subsystem (Block 87). During such cryo-boost enhancement operation, the microprocessor based control unit continually inquires whether the conditions exist for stopping or de-activating the cryo-boost enhancement (Block 88). If operating conditions dictate continued operation of the cryo-boost enhancement, the freezer control subsystem continues to monitor the operating conditions of the freezer and cryogen injection subsystem (Block 87). However, if operating conditions dictate stopping the cryo-boost enhancement, the freezer control subsystem ceases further injection of cryogen and initiates the quick exhaust (Block 89). During the shut-down of the cryo-boost enhancement process, the freezer control subsystem continues to monitor the operating conditions of the freezer (Block 90) including appropriate temperatures and selected gas concentrations within the freezer compartment, as determined from the gas analyzer. Only when the atmosphere within the freezer compartment has been determined to be safe (Block 91), the freezer control subsystem proceeds to shut down the quick exhaust (Block 92), disables the door interlocks (Block 93) and turns off the standard or primary cryogen exhaust (Block 94). At this point the freezer compartment is accessible (Block 95).
Although the present control scheme is primarily concerned with controlling the cryogen injection, the auxiliary exhaust, and limiting access to the freezer compartment during cryogen enhancement operations, it is also contemplated that the present control scheme can be employed to concurrently control other equipment, such as transition fans, product conveyors, etc. In addition, control parameters other than operating temperature of the freezer may be used to control cryogen injection and exhaust subsystems during cryogen enhancement operations.
As described herein, the present system and method for cryogenic enhancement to mechanical freezer systems provides a safe and low cost alternative that improves the performance and efficiency of the mechanical freezer system. One of the many advantages the present system and method has over prior art cryogenic enhancements to mechanical freezer systems is the disclosed safety features that prevent access or egress into the freezer compartment when the cryo-boost is activated and/or until the atmosphere within the freezer compartment is identified as safe and breathable. Another safety-related advantage embodied in the present system is realized by coupling the control of the rapid exhaust to atmosphere monitoring within the freezer compartment.
While the present invention has been described with reference to a preferred embodiment, numerous changes, additions and omissions may be made without departing from the spirit and scope of the present invention, as defined by the appended claims.

Claims

1. A cryogenic enhanced mechanical freezer system comprising:

a mechanical freezer compartment adapted for freezing food products;

a cryogen injection system adapted to directly inject cryogen into the mechanical freezer compartment;

a temperature sensor disposed in the freezer compartment and adapted to ascertain the temperature within the freezer compartment;

a gas analyzer disposed in operative association with the freezer compartment and adapted to monitor concentrations of selected gases within the freezer compartment;

an exhaust system in communication with the freezer compartment and adapted to rapidly evacuate cryogen vapors from the freezer compartment

a freezer control system adapted to control the exhaust system and cryogen injection system in response to selected inputs including inputs received from temperature sensor and gas analyzer.

2. The system of claim 1 wherein the exhaust system further comprises a first exhaust adapted to remove cryogen vapors injected into the freezer compartment when the cryogen injection system is active and an auxiliary exhaust adapted to rapidly evacuate cryogen vapors from the freezer compartment when the cryogen injection system is inactive.

3. The system of claim 1 further comprising a cryogen source in flow communication with the cryogen injection system wherein the cryogen is carbon dioxide and the gas analyzer monitor concentrations of carbon dioxide within the freezer compartment.

4. The system of claim 1 further comprising a cryogen source in flow communication with the cryogen injection system wherein the cryogen is nitrogen and the gas analyzer monitor concentrations of oxygen within the freezer compartment.

5. The system of claim 1 further comprising freezer compartment interlocks adapted to prevent access into the freezer compartment when the cryogen injection system is active.

6. The system of claim 1 further comprising freezer compartment interlocks coupled to the freezer control system, the freezer compartment interlocks adapted to prevent access into the freezer compartment in response to inputs from the gas analyzer.

7. The system of claim 6 wherein the freezer compartment interlocks further comprises:

a door handle cover moveably attached to the freezer compartment proximate a door handle, the door handle cover adapted to move between a first position that shrouds the door handle and a second position exposing the door handle; and

a retaining mechanism that is adapted to lock the door handle cover in the first position in response to inputs from the gas analyzer.

8. The system of claim 1 wherein the cryogen injection system operates in response to inputs received from the temperature sensor and at a temperature control point that is warmer than a temperature set point associated with the mechanical freezer system.

9. A method of freezing foodstuffs in a mechanical freezer system comprising the steps of:

mechanically refrigerating a freezer compartment;

sensing the temperature in the freezer compartment;

selectively injecting a cryogen directly into the freezer compartment based on the temperature of the freezer compartment to cryogenically enhance the freezing of the foodstuffs;

monitoring the concentrations of selected gases within the freezer compartment;

exhausting a portion of the cryogen vapors from the freezer compartment during injection of the cryogen; and

rapidly evacuating the cryogen vapors from the freezer compartment when access to freezer compartment is required.

10. The method of claim 9 wherein the cryogen is carbon dioxide and the step of monitoring concentrations of selected gases within the freezer compartment further comprises monitoring concentrations of carbon dioxide within the freezer compartment.

11. The method of claim 9 wherein the cryogen is nitrogen and the step of monitoring concentrations of selected gases within the freezer compartment further comprises monitoring concentrations of oxygen within the freezer compartment.

12. The method of claim 9 further comprising the step of interlocking any entry into the freezer compartment to prevent access into the freezer compartment when the cryogen is being injected into the freezer compartment.

13. The method of claim 9 further comprising the step of interlocking any entry into the freezer compartment to prevent access into the freezer compartment based on concentrations of selected gases within the freezer compartment.

14. The method of claim 9 wherein the step of mechanically refrigerating a freezer compartment is based on a temperature control point and injection of cryogen is initiated at a temperature control point that is warmer than the temperature set point.