High speed freezing system

A fast-freeze system comprises an insulating housing for a product conveyor of the vertical helix or spiral-type wherein the upper and lower loops (or tiers) respectively, of the helix discharge or receive the product, as the case may be, at product openings (or ports) in the housing; a multiple-spray nozzle array on a header receives a regulated supply of liquid nitrogen according to a preset freezing temperature through a modulating servo-valve control; the header subtends a restricted sector only of an upper conveyor tier for producing heat transfer by direct liquid nitrogen-to-product contact to establish a single very cold zone substantially in advance of the discharge port, and the exhaust nitrogen vapor is discharged by blower action through the product entrance port.

BACKGROUND OF INVENTION 
Fast-freeze systems for processing food products, such as meats, fruits, 
vegetables, etc. have been extensively used in the food industry for many 
years in order to preserve and store for economical year-round use 
seasonal foods and those whose production is subject to variables of 
economic conditions. In addition to storage, an important commercial 
consideration requires that the natural flavor, juices, nutrients, color 
and appearance of the foods be preserved to the extent practically 
possible. In this respect, high rate of freezing is an important 
consideration. 
The prior art systems generally available for this purpose have significant 
disadvantages that the present invention aims to overcome; for example, in 
the well-known cold air-blast system, the freezing process is not 
sufficiently fast for avoiding some dehydration, with corresponding 
decrease in quality of the frozen product. Spraying of liquid carbon 
dioxide into the freezing compartment where it is vaporized and circulated 
around the food products, has also been used and found in some instances 
to lack sufficiently high freezing rate for desired production. In another 
known system wherein freezing takes place within a so-called 
liquid-nitrogen tunnel, the operation is generally more expensive due to 
the cost of the storage and transfer piping required to handle it; also, 
production time may be lost while waiting for cool-down of the tunnel. 
Other freezing systems using nitrogen in lieu of carbon dioxide spray have 
been tried but have not to the best of applicant's knowledge been 
successful for obtaining satisfactory freezing due in general to improper 
techniques in handling the nitrogen spray. 
The present invention is concerned with a highly efficient and high-speed 
freezing system that utilizes liquid nitrogen spray to advantage, and is 
economical both in equipment cost, operating cost and system maintenance. 
SUMMARY OF INVENTION 
In accordance with the invention, high speed and efficient food freezing is 
economically achieved in a new and improved system wherein the principal 
features of several component systems, namely (1) a freezing compartment 
with a spiral-type conveyor system having multiple tiers, as in a vertical 
helix, (2) a liquid nitrogen (LQN) supply with a feed system regulated by 
a temperature sensor located appropriately in the freezing compartment, 
and (3) a multiple spray array and header connected to the LQN feed 
system, are advantageously combined within an insulating housing or 
freezing compartment in unique manner for efficient high-speed freezing. 
In particular, the header which is formed as a loop or equivalent, has a 
multiple array of spray jets that subtend and impinge upon an arcuate 
sector of a single tier of the conveyor to produce a single very cold 
zone, preferably at the upper tier of the conveyor and significantly in 
advance of the product discharge port. In this arrangement, LQN spray not 
falling directly on the product itself, continues falling through the 
usual mesh-type conveyor belt onto the product in the next lower tier, 
thereby completely and efficiently utilizing the heat of vaporization 
factor for rapid heat transfer and progressive cooling of the product; in 
addition, the now vaporized cold nitrogen gas (N.sub.2) further chills the 
oncoming product on the lower tiers as the N.sub.2 is moved downward 
through the conveyor by conventional fan or blower action. The N.sub.2 
under blower pressure finally exhausts through the product entrance port, 
thereby efficiently serving to pre-cool the entering product. 
A principal object of the invention therefore is to provide a new and 
improved freezing system and process using liquid nitrogen spray for 
freezing food products, that has exceptionally fast freezing capability 
for obtaining improved quality of the frozen product, and that is 
efficient and economical as regards production time, operation and 
maintenance. 
Other objects, features and advantages will appear from the following 
description with reference to the accompanying drawings.

DESCRIPTION OF PREFERRED EMBODIMENT 
FIG. 1 shows the upper part of the freezing compartment and apparatus that 
contains essentially the basic components of the invention, the lower 
remainder of the housing and apparatus being apparent from the section 
view, FIG. 2. The basic components comprise an insulating housing 10 
forming a freezing compartment 12 within which is mounted a food transport 
or conveyor equipment generally indicated at 14. The insulating housing 
with suitable access doors, etc. and the conveyor can in general be of the 
construction disclosed in U.S. Pat. No. 3,733,848, granted to Duron et al, 
May 22, 1973 for "FREEZING SYSTEM". In the preferred form of the invention 
lower section of the compartment housing (not shown) has a product 
entrance opening or port 16 opposite a conveyor tier (as indicated in FIG. 
2) and the upper section shown in FIG. 1 has a product discharge opening 
or port 18 at the opposite side of the compartment through which the 
product from the corresponding conveyor tier passes. 
The conveyor is preferably of the spiral type in the configuration of a 
vertical helix. As indicated in FIG. 1, a plurality of ascending flights 
or tiers 20 are formed by a continuous belt 22 that carries the product to 
be frozen upwardly along the spiral from the entrance port 16 to the 
discharge port 18. The conveyor belt 22, which may advantageously be of 
stainless steel mesh, is mounted, driven and guided within the housing 10 
in any suitable manner, such as disclosed for example in U.S. Pat. No. 
3,733,848 above. In the preferred embodiment shown, the entrance port 16 
opens onto the ramp of the lowest tier of the conveyor, FIG. 2, and the 
upper tier 24 discharges at its ramp 24' the product through the port 18. 
Freezing of the product is accomplished by LQN spray that is directed by a 
multiple spray-nozzle array on a header 26 from above directly onto the 
product moving along the upper tier 24 as indicated in FIG. 1. The header 
which in the present example defines a double loop, FIG. 2, has a 
comparatively large number of jet spray nozzles 28 that are compactly 
arranged for insuring that the respective spray patterns combine to cover 
and to concentrate a comparatively large amount of LQN spray per unit area 
on the tier belt beneath the header. 
A vent nozzle 27 functioning as a gas relief valve is mounted on the upper 
side of the header at each end thereof to prevent accumulation of N.sub.2 
vapor in the header due to heat inleak. This provides for (1) equalization 
of pressure at the respective spray nozzles, (2) liquid phase flow only of 
LQN through the nozzles for true liquid-to-product heat transfer, and (3) 
efficient and uniform utilization of excess LQN for the product on the 
next following tier below. 
As shown, the spray header 26 covers but a minor portion only of the upper 
tier 24, so that the area-concentrated LQN spray produces a single very 
cold zone solely at that portion. In the example disclosed, the header 
subtends approximately a 90.degree. sector of the tier 24, this sector 
being located about 180.degree. (or 1/2 revolution) in advance of the 
discharge port 18. This provides for practical temperature equalization 
before product discharge. The cold zone is remotely located from 
lubricated bearings, etc. of the spiral conveyor system that may be 
adversely affected by very low temperatures. The direct LQN-to-product 
contact provides for optimum temperature differential and heat transfer 
coefficient. 
The regulation of the LQN spray is advantageously in response to a signal 
from a temperature sensor 30 that is positioned at an appropriate place in 
the freezing compartment, for example, about 45.degree. in advance, i.e. 
upstream with reference to the product flow, of the cold sector with the 
probe preferably above the upper tier belt as indicated in FIG. 1. The 
temperature signal, which is with reference to a preset temperature value, 
is used for electronic control in known manner of an electric servo-motor 
32 that operates a modulating valve 34 according to the magnitude and 
sense of the signal. The valve 34, which controls the LQN supply line 36 
between a suitable LQN source at 38 and the loop header 26, regulates the 
amount of LQN spray delivered (according to the temperature setting) to 
the cold sector described above. 
It will be apparent that some of the concentrated LQN spray will fall 
directly onto the mesh tier belt beneath the compact loop header 26 rather 
than onto the product; in this case the LQN spray, which tends to flood 
the limited area beneath the header, will fall through the belt onto the 
product on the next lower tier or tiers, thereby completing optimum heat 
transfer by LQN-to-product contact. Accordingly, only the sensible heat of 
N.sub.2 vapor is being transferred to the product in the remaining lower 
tiers of the freezer by means of the usual circulating fans or blowers. 
This insures temperatures that are compatible with the spiral conveyor 
equipment, especially at the lower part of the housing containing the 
mechanical drive gear, etc. 
The exhaust N.sub.2 under blower pressure is discharged only through the 
product entrance port 16 as mentioned above, thereby precooling the 
incoming product and reducing the so-called "residence" temperature. In 
practice, the N.sub.2 is discharged at about 0.degree. F. This 
comparatively high temperature clearly indicates that a high degree of 
refrigeration is obtained from the LQN and N.sub.2 phases, that in turn 
results in reduced consumption of LQN. 
For facilitating N.sub.2 exhaust solely through the entrance port 16, FIG. 
2, and producing progressive cooling of the product from the moment it 
enters the freezing compartment, the exhaust N.sub.2 vapors are directed 
in counterflow to the incoming product flow by means of an exhaust fan 
(not shown) through a stainless steel sheet metal channel 40 that 
envelopes the comparatively straight section of the conveyor belt ramp 
leading from the port 16. 
In summary, the present invention is found to be an improved and highly 
efficient fast freezing system that combines the inherent advantages of 
the spiral type product conveyor with an intense and direct application of 
LQN spray to the product within a limited sector only of a single upper 
tier. This LQN spray application materially in advance of the product 
discharge, produces the single and only intense cold zone in the freezing 
compartment. The use of any excess LQN spray for directly cooling the 
product on the next following tier or tiers, together with the efficient 
use of the sensible heat in the remaining N.sub.2 vapor before exhaust 
from the freezing compartment significantly increases the efficiency of 
the refrigeration process thereby utilizing to an optimum extent the LQN 
supply. High efficiency is further insured by the use of a proportional 
flow valve control of modulated type that is responsive to a predetermined 
temperature sensor signal. 
Having set forth the invention in what is considered to be the best 
embodiment thereof, it will be understood that changes may be made in the 
system and apparatus as above set forth without departing from the spirit 
of the invention or exceeding the scope thereof as defined in the 
following claims. For example, the LQN spray header is not necessarily 
located above the top conveyor tier, and may in fact be located above a 
lower tier where efficient LQN-to-product heat transfer is achieved.