Method and apparatus for retention of a refrigerant fluid in a refrigeration enclosure

The method of the invention is based on the comparison of local vapor concentrations at the inlet and outlet refrigeration ports and taking action based on that comparison. Control apparatus incorporating the invention is installed inside a refrigeration enclosure, adjacent to a port, preferably at the lowermost port. If the enclosure contains multiple ports at similar height, then each port has a form of the control apparatus attached to it. The control apparatus adjusts a flow of vapor leaving the interior of the enclosure. The control apparatus includes a duct assembly and a blower system. The bottom portion of the duct assembly is a tunnel enclosure through which a conveyor belt passes. Connected to an inside edge of the tunnel enclosure is a duct that extends upward from the conveyor belt. A blower for this duct either sucks vapor away from the conveyor belt or blows vapor from the enclosure interior toward the belt. Regardless of the flow direction, a vapor curtain forms inside the tunnel enclosure and represents a transitional region from all vapor to all air. Control of the blower for the duct assembly is based on vapor concentrations in the tunnel enclosures adjacent to each port. A microprocessor compares measured concentration levels and alters the blower motor frequency in such a manner as to minimize the difference in concentration levels at each port.

FIELD OF THE INVENTION 
This invention relates to a method and apparatus for improving the overall 
efficiency of a refrigeration enclosure and, more particularly, to an 
improved method and apparatus for retaining a refrigerant fluid within the 
refrigeration enclosure. 
BACKGROUND OF THE INVENTION 
In operating cryogenic refrigeration equipment, effort must continually be 
expended to minimize the amount of air that enters the equipment during 
operation. In such equipment, the refrigerant is a cryogenic fluid that is 
vaporized during the refrigeration process. Characteristically, air 
manages to enter the refrigeration enclosure through ports which allow 
product to pass into and out of the enclosure. Typically, the air is much 
warmer and contains a considerable amount of moisture relative to the 
environment inside the enclosure. Moreover, the moist air can be thought 
of as a contaminant in the sense that it reduces the purity level of the 
vapor inside the enclosure. 
There are a number of reasons to minimize air infiltration: refrigeration 
efficiency, economics and capacity to recycle the vaporized refrigerant. 
Refrigeration efficiency is defined as the quantity of heat removed from a 
product being cooled, compared to the amount of refrigeration being 
expended by the cryogen. When moist air enters a refrigeration enclosure, 
it will necessarily be cooled to the current temperature therewithin. 
Cooling of air instead of product decreases the cooling potential of the 
refrigerant, and hence decreases refrigeration efficiency. Additionally, 
freezing of the water vapor can potentially lead to a damaging build-up of 
ice inside the enclosure. Ice build-up can become severe enough to require 
a stop in the production line of the product being cooled, to allow for a 
thawing period. Clearly, the cooling potential of the refrigerant is 
reduced or lost during this thaw cycle. The net result is a higher cost in 
operation. 
To recycle the refrigerant, the best approach is to start with the purest 
stream possible from the source, which in this case is a refrigeration 
enclosure. The economics associated with recycling a refrigerant are 
greatly impacted by relatively small changes in the purity of the vapor 
inside the enclosure. Hence, the greatest economic advantage is achieved 
when air infiltration is minimized. 
Minimization of air infiltration is important in both tunnel and spiral 
refrigerators. Typically, a tunnel refrigeration enclosure has an entrance 
port to allow product to enter the enclosure, an exit port to allow 
product to leave the enclosure, and a flat conveyor in-between. A spiral 
refrigeration enclosure has similar porting, except that the ports are at 
different heights relative to the base of the refrigeration enclosure. 
Inside the enclosure, the conveyor follows a spiral or helical pattern 
between the ports. 
U.S. Pat. No. 3,728,869 to Schmidt describes the recycling of cryogenic 
vapors from an enclosure (primarily a spiral refrigerator). The pressure 
within the refrigeration enclosure is kept above atmospheric pressure to 
minimize air and other contaminant infiltration, and pressure and 
gravitational effects cause a flow thereof from each refrigeration port. 
The exiting vapor is collected in adjacent vestibules or spillover boxes 
in such a manner as to form a vapor barrier above the vestibule. Air 
infiltration is prevented by a vapor dam. Vapor is removed from the bottom 
of a vestibule by a piping network driven by a blower system. Control of 
vapor removal is through motorized on/off dampers in the ducting leading 
away from the vestibules. 
U.S. Pat. No. 4,356,707 to Tyree et al., describes several refrigeration 
enclosure designs which utilize both mechanical and cryogenic 
refrigeration. A spiral refrigeration enclosure using a cryogenic 
refrigerant is described wherein diluting chambers are positioned adjacent 
the refrigeration ports. The concern at a lower port is to minimize 
outflow of the denser-than-air cryogen vapor from the refrigeration 
enclosure. A chamber adjacent to the lower port includes several baffles 
and a blower system operated at a constant frequency. Vapor is retarded 
from leaving the refrigeration enclosure by sucking a portion of the vapor 
from a dilution chamber and redirecting it back into the enclosure. The 
remaining portion of the vapor exits through the refrigeration enclosure 
opening and dilutes any air trying to enter the enclosure. Side vanes, 
manually positioned, are used to balance flow across a conveyor belt. 
Variable fan speed control has been employed in the prior art as a means to 
prevent premature spillover of cryogenic vapor from a refrigeration 
enclosure or to prevent air from entering. In U.S. Pat. Nos. 4,528,819 
(Klee) and 4,800,728 (Klee), the concern is how to prevent loss of cryogen 
vapor from a refrigeration enclosure or air infiltrating into the 
enclosure. A temperature sensor is used to indicate whether cryogenic 
vapor is leaving the enclosure or air is entering the enclosure. Coupled 
to the temperature sensor is a blower system. In U.S. Pat. No. 4,528,819, 
the blower is on the exhaust line of the refrigeration enclosure. In U.S. 
Pat. No. 4,800,728, the blower mechanism is internal to the refrigeration 
enclosure and is part of the circulation system. 
Other methods have been employed to minimize the vapor leaving a 
refrigerator or the surrounding air from contaminating the interior of a 
refrigeration enclosure. U.S. Pat. No. 4,947,654 (Sink et al.) describes 
atmosphere control within spiral refrigerators and tunnel refrigerators. 
For spiral refrigerators, an improvement to the dilution system discussed 
in U.S. Pat. No. 4,356,707 is disclosed. The blower or blowers of the 
dilution system are no longer operated at a fixed frequency, but control 
of the blower system is now coupled to the cryogen injection rate. The 
primary sensing device can be either a temperature sensor inside the 
enclosure or a pressure sensor in the liquid supply line that feeds the 
cryogen injectors. The exit port can have a similar system to prevent air 
infiltration by letting a small amount of vapor exit through the port. For 
a tunnel refrigerator, similar means are discussed for minimizing air 
infiltration and reducing the premature loss of vapor from the 
refrigerator. 
U.S. Pat. No. 4,955,206 (Lang et al.) discusses a variable speed control 
method for maintaining the environment within a refrigerator. For a tunnel 
refrigerator, maintenance of the internal environment is enhanced by the 
addition of a photocell transmitter and receiver sensor system located 
outside the entrance port and a baffle-linkage scheme surrounding one of 
the internal axial fans. The sensor system provides control information 
based upon how much vapor is leaving the refrigerator. If an excessively 
high level of vapor is escaping, the baffle-linkage system directs flow 
away from the port. If the opposite is true, the baffle-linkage responds 
by directing vapor toward the port. In a spiral refrigerator, the dilution 
blower system is coupled to the photocell sensing system and is not 
dependent on the injection rate. In both refrigerator configurations, the 
blower systems have either variable or single speed drives. 
Another method for maintaining cryogenic purity inside a refrigeration 
enclosure is by employing a controlled evacuation system on the enclosure. 
U.S. Pat. No. 5,186,008 (Appolonia et al.) discusses a method for 
controlling an amount of vapor extracted from an enclosure as part of a 
recycle effort. For a spiral refrigeration enclosure, the locations of 
suction are at an upper vestibule and at the bottom of the refrigeration 
enclosure. For the bottom suction location, the amount of vapor leaving 
the enclosure is a constant ratio relative to the injection rate. The 
remaining portion of vapor resulting from injected cryogen exits through 
the entrance and exit ports. Sufficient suction needs to be applied at the 
upper vestibule to minimize gravitational effects on the vapor flow 
leaving through the lower port and to prevent air infiltration in the 
upper port. Hence, the pressure in the upper vestibule region is required 
to be the lowest pressure relative to the refrigeration enclosure and the 
surrounding atmosphere. 
It is an object of this invention to provide an improved apparatus and 
method for minimizing escape of refrigerant vapor from a refrigeration 
enclosure and inlet of air into the enclosure. 
SUMMARY OF THE INVENTION 
The method of the invention is based on the comparison of local vapor 
concentrations at inlet and outlet refrigerator ports and taking action 
based on that comparison. Control apparatus incorporating the invention is 
installed inside a refrigeration enclosure, adjacent to a port, preferably 
at the lowermost port. If the enclosure contains multiple ports at similar 
height, then each port has a form of the control apparatus attached to it. 
The control apparatus adjusts a flow of vapor leaving the interior of the 
enclosure. The control apparatus includes a duct assembly and a blower 
system. The bottom portion of the duct assembly is a tunnel enclosure 
through which a conveyor belt passes. Connected to an inside edge of the 
tunnel enclosure is a duct that extends upward from the conveyor belt. A 
blower system for this duct either sucks vapor away from the conveyor belt 
or blows vapor from the enclosure interior toward the belt. Regardless of 
the flow direction, a vapor curtain forms inside the tunnel enclosure and 
represents a transitional region, from all vapor to all air. To assist the 
formation of the vapor curtain, a further suction duct assembly is 
connected to the outer edge of the tunnel and spans the conveyor belt. 
This duct draws the exiting vapor toward the top of the tunnel enclosure. 
Hence, a major portion of the vapor gets directed back into the 
refrigeration enclosure while a small amount of vapor leaves the enclosure 
to prevent air contamination. A gas analyzer is used to measure the vapor 
concentration level in the tunnel. 
Control of the blower for the duct assembly is based on vapor 
concentrations in the tunnel enclosures adjacent to each port. At regular 
intervals, the vapor concentration level at each port is measured. A 
microprocessor compares the measured concentration levels and alters the 
blower motor frequency in such a manner as to minimize the difference in 
concentration levels at each port. 
In a preferred embodiment a vapor curtain balance is established. By 
maintaining a vapor curtain balance, a relatively high purity vapor stream 
can be withdrawn from the enclosure through a third port without affecting 
the vapor curtain balance of the refrigeration enclosure. Internal blowers 
within the enclosure can advantageously provide circulation and mixing of 
the vapor throughout the enclosure to minimize stratification of the vapor 
and permit removal of a high purity vapor stream from any point within the 
refrigeration enclosure.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIG. 1, a refrigeration enclosure includes insulated walls, 
base, top and an interior volume 12. One or more circulating fans 13 (or a 
center cage fan 11 as shown in FIG. 2) are positioned about interior 
volume 12. A conveyor belt 16, having a helical or spiral pattern, 
transports product through refrigeration enclosure 10. The product to be 
cooled passes through a lower port 14 and exits refrigeration enclosure 10 
through an upper port 15 or vice versa. While the following discussion is 
specific to a spiral refrigeration enclosure, other refrigeration designs 
such as a tunnel configuration can utilize the invention. 
A set of injectors and associated piping (not shown) deliver a cryogenic 
fluid (e.g. carbon dioxide, nitrogen, etc.) into the volume 12. The 
refrigeration control system is temperature based and provides a signal to 
a modulating valve on the incoming cryogenic feed line to deliver the 
amount of cryogen fluid necessary to reach a given temperature within the 
enclosure. 
Referring to FIG. 3, refrigeration enclosure 10 includes a duct assembly 17 
at inlet port 14. Duct assembly 17 provides a means to suck refrigerant 
vapor away from conveyor belt 16. Duct assembly 17 spans conveyor belt 16. 
Inlet port 14 of enclosure 10 couples to a low clearance outer tunnel 20 
and an under-the-belt flat plate 35. The upper run of conveyor belt 16 
passes over flat plate 35 and through outer tunnel 20 while the lower run 
of belt 16 does not. Baffle 32 lies between the upper and lower runs of 
conveyor belt 16 to prevent premature egress of vapor from the enclosure. 
Duct assembly 17 opens into interior volume 12 at aperture 21, which is the 
leading edge of an inner tunnel 22. At the junction of tunnels 20, 22 is a 
vertical duct 23. The bottom edge of vertical duct 23 has the lowest 
clearance relative to the conveyor belt. Outer tunnel 20 connects to 
vertical duct 23 slightly above the bottom edge to establish a small 
retention cavity along the top of outer tunnel 20. The retention cavity 
functions to dilute the air that gets into outer tunnel 20 to minimize air 
contamination reaching the interior of enclosure 10. Under-the-belt plate 
35 extends from slightly beyond the enclosure 10 to at least slightly 
beyond vertical duct 23 so that plate 35 extends to the extreme edge of 
tunnel 20, and to about the edge of tunnel 22. 
The second end of vertical duct 23 attaches to a ninety degree bend 24, 
which allows a transition in the width of the ducting. Attached to the 
second end of bend 24 is a horizontal duct 25 which spans belt 16 but is 
wider than vertical duct 23. Horizontal duct 25 terminates at a plate 34 
having dimensions similar to horizontal duct 25. Plate 34 includes 
openings to accommodate fans 26. It is preferred that two, multiple 
bladed, center hub blower fans 26 be mounted side by side. Fans 26 are 
driven by motors 27 externally mounted to refrigeration enclosure 10. The 
vapor that exits horizontal duct 25 impacts the enclosure wall in region 
28 and is dispersed into interior 12. Baffles can be used to direct the 
vapor flow upwards (relative to horizontal duct 25), downwards, or 
sideways back into interior volume 12. 
As stated above, the present embodiment sucks refrigerant vapor away from 
conveyor belt 16, as indicated by the arrows in FIG. 3. A small portion of 
the vapor leaves enclosure 10 through outer tunnel 20, while the major 
portion of the vapor is redirected into interior volume 12 of enclosure 
10. The vapor that escapes through lower port 14 is collected in a 
spillover box 31. Spillover box 31 is cleared by an exhaust system 
schematically represented by external vertical duct 33. In this fashion, 
vapor is exhausted out of the room containing enclosure 10 and away from 
personnel. 
As shown in FIG. 4, it is preferred to locate a spillover baffle 38 within 
spillover box 31 under the lower run of conveyor belt 16, that extends 
horizontally from the outside wall of enclosure 10 to slightly beyond 
conveyor belt roller 29 and then vertically to slightly below the upper 
run of conveyor belt 16. Spillover baffle 38 collects vapor escaping from 
inlet port 14 to create a further barrier against the outflow of vapor. 
In an alternative embodiment, as shown in FIG. 5, spillover box 31 contains 
a spillover baffle 42 that extends along the contour of the lower run of 
conveyor belt 16 and around conveyor belt roller 29 to slightly below the 
upper run of conveyor belt 16. In this alternative embodiment, a roller 
baffle 41 is located adjacent to conveyor belt roller 29 in between the 
upper and lower runs of conveyor belt 16. Roller baffle 41 and spillover 
baffle 42 create further barriers against the outflow of vapor. 
Outlet port 15 of refrigeration enclosure 10 is also locally modified by 
additional duct work (see FIG. 6). Outlet port 15, like inlet port 14, has 
a tunnel-shaped enclosure 49 formed by several interconnecting pieces to 
impede air entering and vapor escaping. An under-the-belt plate 50 begins 
just forward of conveyor belt roller 59 and extends into refrigeration 
enclosure 10. Tunnel side pieces (not shown) begin at the edge of the 
refrigeration wall and extend into the enclosure. The interior edge of 
under-the-belt plate 50 and the interior edge of the side pieces should 
form a common edge inside the enclosure 10. Top 51 of tunnel 49 is the 
refrigeration enclosure ceiling. A vertically positioned baffle 52 that 
spans conveyor belt 16 is also affixed to refrigeration ceiling 51. The 
clearance between baffle 52 and conveyor belt 16 is determined by the 
product being cooled and preferably is adjustable. Baffle 52 should be 
contained by the sides of tunnel 49, but does not have to be located at 
the interior edges of the side pieces. 
The position of outlet port 15 is determined by the height of over-the-belt 
pickup unit 53. Like the vertically positioned baffle 52, the clearance of 
over-the-belt pickup unit 53 off conveyor belt 16 is determined by the 
product to be cooled. With this tunnel configuration, a retention cavity 
is formed similar to the one in outer tunnel 20 near lower port 14. 
Additional baffles 54 are placed between the upper and lower layers of 
conveyor belt 16 to minimize the inlet of air and the outlet of vapor. A 
spillover box 55 collects the vapor exiting from outlet port 15 and is 
exhausted via duct 56. 
As shown in FIG. 7, at outlet port 15, as with inlet port 14, it is 
preferred to locate a spillover baffle within the spillover box. A 
spillover baffle 57 is located within spillover box 55 under the lower run 
of conveyor belt 16 extending horizontally from the outside wall of 
enclosure 10 to slightly beyond conveyor belt roller 59 and then 
vertically to slightly below the upper run of conveyor belt 16. Spillover 
baffle 57 collects vapor exiting from outlet port 15 creating a further 
barrier against the outflow of vapor. 
In an alternative embodiment, as shown in FIG. 8, spillover box 55 contains 
a spillover baffle 62 that extends along the contour of the lower run of 
conveyor belt 16 and around conveyor belt roller 59 to slightly below the 
upper run of conveyor belt 16. In this embodiment, a roller baffle 61 is 
located adjacent to conveyor belt roller 59 in between the upper and lower 
runs of conveyor belt 16. Roller baffle 61 and spillover baffle 62 create 
further barriers against the outflow of vapor. 
It is preferred that both ports (14 and 15) have an over-the-belt pick-up 
unit (30 and 53). Each over-the-belt pickup unit (30 and 53) has a 
positive seal to adjacent tunnels (20 and 49) as depicted in FIGS. 3 and 
6. Suction for over-the-belt pickup unit (30 and 53) is provided by the 
exhaust system, here shown as external ducts 33 and 56. The function of 
this pickup unit is two-fold. First, it minimizes the amount of air that 
enters into a tunnel. Second, it tends to cause any exiting cryogenic 
vapor to raise off of conveyor belt 16 and combats the effect of gravity 
on the vapor. By keeping the vapor level as high as possible in a tunnel, 
any air that enters a tunnel is diluted. In addition, an over-the-belt 
pickup unit has been found to minimize vapor stratification inside a 
tunnel. 
A control procedure for refrigeration enclosure 10 is based on monitoring 
vapor concentrations near each of ports 14 and 15. The sensing system 
includes gas analyzers to monitor the vapor concentration in each tunnel 
configuration. Therefore, outer tunnel 20 has a sensor port 40 and upper 
tunnel 49 has a sensor port 60. In general, the preferred sensor location 
is inboard from the leading edge of the over-the-belt pickup units 30 and 
53. The control procedure is based on the difference in cryogenic gas 
concentrations between the two tunnels. For comparison purposes, it is 
preferred to use a single analyzer for monitoring both locations. 
Therefore, an appropriate network of pipe/tubing, automatically controlled 
valves and a timing device are required (not shown). 
FIG. 9 illustrates a microprocessor 81 which provides a means to control 
the timing of the valves as required to obtain acceptable readings from 
each location using gas analyzer 80. An algorithm based primarily on the 
difference in concentrations in each tunnel provides a frequency setting 
signal to variable speed drive 82, which drives fan motors 27. The 
algorithm optimizes frequency control to the extent that a minimum is 
achieved in the difference in concentrations. A predetermined setpoint 
pattern is not used. Essentially, the correction to the frequency of 
variable speed drive 82 is based on the magnitude of the difference in 
concentrations. The larger the difference, the greater the correction to 
the frequency level. 
The algorithm essentially has two modes: a near steady state condition or 
non-steady state. For near steady state conditions, the control algorithm 
is an endless loop that does the following: collects vapor concentration 
samples from each tunnel following a predetermined time interval, compares 
the samples collected, and corrects fan frequency based on the difference 
in the samples. For non-steady state conditions, such as during a cool 
down of refrigeration enclosure 10, the fan frequency is corrected as a 
function of the rate of change of the injection rate and/or the rate of 
change of the refrigeration enclosure temperature. 
Duct assembly 17, as adjusted by the control system, establishes a vapor 
curtain in outer tunnel 20. The term vapor curtain is defined here to mean 
a vapor front where transition occurs from all vapor (concentration level 
of interior volume 12) to all air. The thickness of this front is not 
critical, except that it needs to be contained in outer tunnel 20. If the 
front resides outside port 14, then blower motors 27 are not rotating fast 
enough. If no vapor front forms in tunnel 20, then the motors 27 are 
rotating too fast. 
The key to maintaining a high purity level inside enclosure 10 is the 
establishment of a vapor curtain or front in outer tunnel 20. This is only 
successful if the upper outlet port 15 and the lower inlet port 14 are in 
gaseous communication with each other. When a vapor curtain forms in 
tunnel 20, a vapor front is also formed in tunnel 49. 
Extraction of Vapor Stream for Recycling 
The present invention permits withdrawal of a high purity vapor stream, 
assuming a vapor curtain balance system is in place and operating. Within 
refrigeration enclosure 10, the internal blower system (fan units 13 or 
center cage fan unit 11), provides a well mixed environment. Since a 
thoroughly mixed environment is contained within the enclosure 10, a high 
purity vapor stream can be withdrawn from interior volume 12 anywhere on 
or within enclosure 10. Accordingly, the high purity vapor stream may be 
withdrawn from any location including, for example, at or near an exterior 
wall of enclosure 10, at or near ports 14 or 15, and at or near the center 
of interior volume 12. Such a high purity vapor stream may be removed as a 
controlled exhaust, and then liquefied and reintroduced into enclosure 10. 
Referring to FIG. 10, a withdrawal port 101 includes ducting 102 that is 
sealed to the insulated ceiling of enclosure 10. A plate 100 is located 
below the lower end of duct 102 to protect the duct during cleaning of the 
enclosure. The opposite end of duct 102 is connected to an isolation valve 
103. Additional ducting 104 is connected to the opposite end of isolation 
valve 103. The downstream end of duct 104 connects to a blower housing 
105, which is driven by a motor 106. 
The duct assembly extending from withdrawal port 101 to blower housing 105 
will contain a vapor having a subatmospheric pressure and therefore, 
proper sealing of the ductwork is required. Connected to the outlet of 
blower housing 105 is additional ducting 107 which terminates at an 
isolation valve 108. Beyond isolation valve 108 is a refrigeration system 
120 to liquefy the vapor stream for recycling purposes. 
Ducting 107 downstream of blower housing 105 contains a number of devices 
including a static pressure sensing location 110, a temperature indicator 
111, a gas flow metering device 113, and a modulating valve 109. 
Modulating valve 109 is required to permit, when necessary, a portion or 
all of the vapor stream to be diverted away from refrigeration system 120. 
Static pressures are used to monitor the operational characteristics of 
blower housing 105 (via readings from pressure sensor 115 in ductwork 104 
upstream of blower housing 105 and from pressure sensor 110 in ductwork 
107 downstream of blower housing 105). Upstream of blower housing 105 is a 
gas analyzer 112. 
Extraction of vapor from enclosure 10 is precisely controlled and is 
dependent on the control system for the vapor curtain balance system. Like 
the control procedure for the vapor curtain, control of the extraction of 
a vapor stream (recovery line blower motor frequency) from enclosure 10 is 
based on a comparison in gas concentrations inside the tunnels (20 and 49) 
and vapor stream in the recovery line ductwork 104. The tunnel 
concentration value can be either an average value of the monitored 
concentrations at each sensor (40 and 60) or a single measurement taken at 
either sensor. 
The underlying principal of the control procedure is to maintain the 
highest concentrations in the recovery line, and, secondarily, to maximize 
the flow of extracted vapor without collapsing the vapor fronts that are 
established in the tunnels. Testing has shown that control of the recovery 
line blower motor frequency can be achieved with considerable difference 
in the concentration values, on the order of 10 percent to 50 percent. 
Hence, the control procedure monitors the concentrations and maintains the 
difference between the concentrations within a predetermined maximum 
offset value. The correction to the blower frequency is based on the 
magnitude of the difference in concentrations and how close the maximum 
offset value is being satisfied. A decrease in a tunnel gas concentration 
will obviously occur before the recovery line concentration decreases. 
Furthermore, a significant reduction in the injection rate is used to 
indicate that the concentration level in enclosure 10 is expected to 
decrease. 
There are three control modes (see FIG. 10). Mode one has first isolation 
valve 103 on the recovery line closed. This condition is the same as if 
the recovery line was not attached to the enclosure. The recovery line 
control system is essentially idle. 
All sensors 110, 111, 112, 113 and 115 on the recovery line are monitored 
by microprocessor 81 (See FIG. 11). Microprocessor 81 provides control 
signals to modulating valve 109 and to variable speed drive 130, which 
operates the blower motor 106 at the correct frequency. 
Mode two has first isolation valve 103 open, but second isolation valve 108 
closed. In this mode, the recovery line behaves like an exhaust line, as 
all vapor leaving through withdrawal port 101 exits the recovery line 
through modulating valve 109. This condition will occur if there is a 
sudden problem with refrigeration system 120. By quickly redirecting the 
flow, impact on the environment of interior volume 12 is kept to a 
minimum. 
Mode three, the typical mode of operation, occurs when both isolation 
valves 103 and 108 are open. In this mode, vapor is withdrawn from 
enclosure 10 and is sent to refrigeration system 120. Again, the objective 
of the control procedure is to maximize the vapor concentration level in 
the recovery line. 
The recovery line control procedure is dependent on the vapor curtain 
control procedure. When the recovery line is operational, the vapor 
balance curtain scheme essentially balances the refrigeration to provide a 
fixed loss of vapor out of refrigeration enclosure ports 14 and 15. The 
remaining portion of vapor exits enclosure 10 through the recovery line. 
If the extraction rate from enclosure 10 is too great, the vapor balance 
system indicates an upset by an increase in air infiltration since too 
much vapor is being removed. If the extraction rate is too low, the 
recovery line system is not optimized and flow through blower housing 105 
needs to be increased. If the flow in the recovery line meets the capacity 
of the refrigeration system, refrigeration system 120 is maximized and any 
excess vapor flows through ports 14 and 15 providing additional support to 
the vapor curtains. 
Tunnel Refrigeration Enclosure Configuration 
A tunnel refrigeration enclosure operates in a similar manner to a spiral 
refrigeration enclosure to maintain the interior environment at high vapor 
concentrations. The main difference is that tunnel enclosure ports are 
typically at the same height relative to the base of the refrigeration 
enclosure. As a result, gravitational effects are not as prevalent in a 
tunnel refrigeration enclosure as they are in a spiral arrangement. The 
present invention controls the inlet of air into the tunnel refrigeration 
enclosure by allowing at least a small portion of vapor to leave each 
tunnel port. 
A tunnel refrigeration enclosure 200 is shown in FIG. 12. For exemplary 
purposes, product enters enclosure 200 through port 201 and exits the 
refrigeration enclosure through port 202. Product is transported through 
enclosure 200 on a conveyor belt 203. A cryogenic fluid enters the 
refrigeration enclosure via an injection system 204. The amount of cryogen 
being delivered inside enclosure 200 is based on a temperature control 
method in conjunction with a modulating valve on the injection line and is 
known to those skilled in the art. 
Additional ducting and blower systems are provided adjacent to each 
refrigeration port to control and minimize air infiltration into the 
refrigeration enclosure and uncontrolled outlet of vapor from enclosure 
200. The principle involved is similar to the method employed for the 
spiral refrigeration enclosure described above. At inlet port 201, a 
ductwork configuration and multiple fans 210, each driven by its own motor 
211 are positioned. Vapor is directed as shown by arrow 212. Vapor is 
drawn into duct assembly 213, which has at least one bend. Duct assembly 
213 can have multiple bends, and must span the width of conveyor belt 203. 
The bottom portion of duct assembly 213 directs this vapor to impact upon 
vapor trying to leave enclosure 200 through the enclosure port. A vapor to 
air front forms in tunnel enclosure 214 or just beyond the tunnel. 
Tunnel enclosure 214 rests on a base plate 216 to control how the vapor 
exits the port. At the leading edge of tunnel 214 is an over-the-belt 
pickup unit 231 which aids in minimizing air infiltration. The pickup unit 
231 is of similar design to the unit 30 used on the spiral enclosure. 
Vapor exiting from inlet port 201 is collected in a spillover box 217 and 
is exhausted via duct 230. A gas sensor 215 is used to monitor vapor 
concentration inside tunnel 214. Gas sensor 215 is preferably located on 
the inside of the leading edge of over-the-belt pickup unit 231. 
A similar configuration is required at outlet port 202. Adjacent to opening 
202 are positioned ductwork and multiple fans 220, each driven by its own 
motor 221. Vapor is directed as shown by arrow 222. Vapor is drawn into 
duct assembly 223, which has at least one bend. Duct assembly 223 can have 
multiple bends, and spans the width of conveyor belt 203. The bottom 
portion of duct assembly 223 directs this vapor to impact upon vapor 
trying to leave enclosure 200 through outlet port 202. As with port 201, 
transition tunnel 224 rests on a base plate 226 to control how the vapor 
exits the port. A gas sensor 225 is used to monitor vapor concentration 
inside tunnel 224. At the edge of tunnel 224 is an over-the-belt pickup 
unit 241. The vapor exiting from outlet port 202 is collected in spillover 
box 227 and is exhausted via duct 240. 
As mentioned above, vapor concentration is monitored in each tunnel, 214 
and 224. A microprocessor-based device 281 (see FIG. 13) provides a means 
to control timing of valves of a piping network (not shown) to obtain 
acceptable readings from each location, using a single gas analyzer 280. 
The control algorithm is based on the difference in concentrations in each 
tunnel as discussed above with respect to the spiral refrigeration 
enclosure. The difference in tunnel concentrations is to be minimized to 
maximize the concentration inside enclosure 200. Since both tunnel ports 
201 and 202 include a duct apparatus, one blower system is operated at a 
fixed frequency while a second blower system has a controlled variable 
frequency. The fixed frequency blower system simulates the gravity head 
that naturally occurs in a spiral refrigeration enclosure. By measuring 
the difference in the port concentrations 215 and 225, the variable speed 
blower is adjusted accordingly. 
For example, consider port 201 with a variable speed blower system 211 and 
port 202 with the fixed frequency blower system 221. If sensor 215 reads a 
higher concentration relative to sensor 225, the frequency of the blower 
will be increased. If sensor 215 reads a lower concentration relative to 
sensor 225, the frequency of the blower will be decreased. The size of the 
correction to the variable speed blower system is based on the magnitude 
of the difference in concentrations. The larger the difference, the 
greater the correction to the blower motor frequency. 
Like the spiral enclosure vapor balance control method, the tunnel 
enclosure algorithm essentially has two modes. For near steady state 
conditions, the control algorithm is an endless loop that does the 
following: collects vapor concentration samples from each tunnel following 
a predetermined time interval, compares the samples collected, and 
corrects blower frequency based on the difference in the samples. For 
non-steady state conditions, such as during a cool down of enclosure 200, 
the blower frequency is corrected as a function of the rate of change of 
the injection rate and/or the rate of change of the refrigeration 
enclosure temperature. 
The extraction of vapor from enclosure 200 for recycling purposes is 
similar to the method used with a spiral refrigeration enclosure. The key 
objective, as with the spiral enclosure, is to maintain high purity levels 
within the enclosure. Hence, both vapor curtains need to be operational to 
successfully extract a high purity vapor stream from enclosure 200. The 
withdrawal port for recovery line 250 can be located anywhere on enclosure 
200, with the top or bottom surface of enclosure 200 being preferred. The 
operation of the recovery line system discussed earlier for spiral 
enclosures is identical for tunnel refrigeration enclosures. On FIG. 12, 
this scheme has been designated by isolation valve 103, corresponding to 
the initial valve of the recovery line system as shown in FIG. 10. 
A number of alternative configurations may be employed to meet the 
objective to minimize air infiltration and uncontrolled outlet of vapor. 
The following embodiments pertain primarily to spiral refrigeration 
enclosures, but can also be incorporated into other enclosures, such as 
tunnels. The discussion initially considers alternative designs for 
ductwork 17 adjacent to input port 14. Alternative duct geometries and 
control methods are then presented, followed by alternatives for the 
extraction of a vapor stream from an enclosure. 
A major objective of duct assembly 17 is to establish a uniform vapor flow 
pattern across the width of conveyor belt 16. Primarily, the means to 
develop a vapor curtain requires the use of axial fans, in which the vapor 
flows through the blades in a direction parallel to the shaft axis of the 
fan motor. However, axial fans induce considerable swirl into the flow 
entering, passing through, and exiting duct assembly 17. Straightening 
vanes, baffles, and curvature or shape of the duct can minimize the swirl 
effect on the flow along the conveyor belt in outer tunnel 20 adjacent to 
port 14. 
A centrally placed baffle was inserted into horizontal duct 25 (see FIG. 3) 
to minimize the upstream effects of the axial fans when implementing the 
suction method. The baffle extended from top to bottom of duct 25 and 
split the duct into two smaller rectangular ducts. Testing with and 
without the baffle indicated that its effect on the flow was marginal, but 
certainly did not produce a negative effect. A horizontal baffle spanning 
the duct and placed at the shaft height was also investigated. Like the 
vertical baffle, the effect on the flow in outer tunnel 20 was minimal. 
Similar baffling can be inserted into vertical duct 23. Again, the purpose 
is to disrupt the large-scale vortical flow pattern observed to form in 
the duct assembly. Two or more vanes can be placed inside vertical duct 23 
to act as flow straighteners. Also, baffling, parallel to the conveyor 
belt path, in vertical duct 23 has been used as blockage to impede vapor 
pickup off the conveyor belt in an attempt to tune specific flow regions 
inside outer tunnel 20 to achieve a balanced flow. However, cost 
considerations and cleaning issues were strong enough factors to render 
the baffle solution less preferred. 
While preferred embodiments are shown in FIGS. 3, 4 and 5, an alternative 
design to achieve lift off suction is shown in FIG. 14. The primary 
difference between the two designs is the duct configuration at inlet feed 
21 and the flow coming from fan outlet 28. By comparing FIG. 3 and FIG. 
14, the design of FIG. 14 has an angular baffle 300 replacing inner tunnel 
22 and part of vertical duct 23. Note, inner tunnel 22 has been moved 
further into enclosure 10 along the conveyor belt pathway and remains the 
leading edge of the inlet to the duct assembly. A gap 303 exists between 
the trailing edge of inner tunnel 22 and the leading edge of angular 
baffle 300. In addition, the under-the-belt plate 35 has been extended to 
yield a common edge with inner tunnel 22. Also, gap 303 exists only in the 
horizontal plane parallel to the conveyor belt path. The side wall height 
of inner tunnel 22 has been extended to join the side wall defined by the 
termination of angular baffle 300. 
Using the geometry of FIG. 14, inner tunnel 22 acts as a conditioning 
tunnel for the vapor trying to leave the enclosure along conveyor belt 16. 
The vapor that is sucked up the ductwork leaves the fan region through 
ducting 301 and is directed to interior volume 12 of enclosure 10. When 
the combination of under-the-belt plate 35, conditioning tunnel 22, and 
gap 303 are not present, performance degrades and control of the vapor 
leaving the enclosure is poor. A variation of this configuration is that 
angular baffle 300 may contain ports with covers that can be adjusted to 
allow different suction patterns to develop. The ports may or may not be 
equally spaced across the span of the conveyor belt and are used to 
balance the flow in outer tunnel 20. Linkage can be connected to the ports 
to provide manual or motorized adjustment, without requiring access to the 
interior of the enclosure. 
For duct assembly 17, the preferred configuration includes two fans and two 
motors for a spiral refrigeration enclosure. The fans are axial and have a 
multiple bladed pattern and a large center hub. For some duct geometries, 
the preferred blade style is centrifugal. However, due to a centrifugal 
fan becoming unbalanced when icing occurs on the blades, axial fans are 
used for this invention. For two fans mounted side by side, there is a 
preferred rotational direction for each fan when employing the suction 
method. There are three possible configurations for two fans: both fans 
rotating in opposite directions with the common flow region upward between 
the two fans, two fans rotating in opposite directions with the common 
flow region downward between the two fans, and both fans rotating in the 
same direction. This last configuration is the most preferred. 
In addition to testing with multiple bladed fans, testing was completed 
with one larger two bladed fan. The associated duct work was modified to 
handle the larger opening required and is shown schematically in FIG. 15. 
When using a single two bladed fan to develop suction inside the duct, 
testing revealed that limitations due to the duct/shaft geometry produced 
an inward flow along the motor shaft originating at the discharge of the 
fan. This adverse flow condition was minimized by installing a circular 
disk on the motor shaft to inhibit inward flow. A single, two bladed fan 
would be expected to be an acceptable alternative to the two fan approach 
when utilizing the preferred duct geometry. 
To achieve a balanced flow in the outer tunnel 20 along the conveyor belt, 
other duct shapes were investigated where the duct geometry was designed 
to smooth out the vapor flow inside outer tunnel 20. One viable 
alternative is to have the duct contained within the enclosure. Two 
variations of internal ductwork were investigated and are shown in FIG. 
15. As in the preferred design, vapor is sucked away from the conveyor 
belt through duct 401 and discharged at fan 403. In one variation, as 
shown by the solid lines in FIG. 15, the flow is turned twice in duct 401 
in an effort to smooth out the vapor flow adjacent to conveyor belt 16. A 
second variation, as shown by the dashed lines in FIG. 15, turns the flow 
in duct variation 402 three times to reduce the swirl effect. The 
advantage to increasing the number of bends is to achieve greater 
reduction in the swirl effects produced by the fan. However, the greater 
the number of bends, the higher the horsepower that is required to move an 
equivalent amount of vapor. 
The major disadvantage with an internal duct design as depicted in FIG. 15 
involves cleaning and an ability to verify the duct integrity prior to 
cooling refrigeration enclosure 10 on a consistent basis. As shown in FIG. 
16, the cleaning issues can be readily addressed by installing a major 
portion of duct assembly 501 external to the enclosure. Basically, duct 
501 functions the same way as the one shown in FIG. 15. While cleaning 
concerns are reduced, the external portion of the duct presents different 
issues. First, the wall of the duct needs to be insulated or refrigeration 
efficiency of the enclosure decreases. In addition, duct 501 can 
potentially be on the suction side of the fan and the susceptibility to 
air infiltration increases. The motors are positioned most favorably when 
they are closest to the conveyor belt. On the other hand, the duct 
assembly has to be of sufficient height to minimize the swirl effect from 
the fan(s) on the flow inside tunnel 20. 
A further embodiment of the invention blows vapor along the conveyor belt 
as opposed to sucking vapor away from the conveyor belt, as previously 
discussed. With reference to FIG. 17, two possible duct geometries are 
shown. The key to making the method successful is to push sufficient vapor 
down duct 601 to block the vapor trying to exit through lower port 14 due 
to gravitational effects. As with the suction method, multiple bends in 
duct 602 are preferred to minimize swirl in the flow adjacent to conveyor 
belt 16. At the base of the duct assembly, a flat adjustable plate 605 
forms the top of outer tunnel 20. An important parameter appears to be the 
extent of insertion of flat plate 605. In addition, at aperture 21, the 
height of the upper leading edge of duct 601 (602) from the conveyor below 
also influences the development of the vapor curtain. Observations made 
during testing of blowing vapor along the conveyor belt indicated that 
this method is less efficient than the suction method in a spiral 
refrigeration enclosure. However, tests completed with a model of 
configuration 601 revealed that control of the fan motor frequency can be 
derived from a pressure sensor as well as a gas analyzer. 
The preferred control method is a self regulating system based on the 
difference in concentration in the tunnels that are adjacent to each of 
the refrigeration enclosure ports. The placement of the gas monitoring 
device needs to be a sufficient distance away from a port to prevent 
periodic room air currents from influencing control of the vapor curtain 
balance system. 
Besides using vapor concentrations for control information, other possible 
control parameters can be used. In particular, pressure sensors can be 
used to give an indication of how well the vapor curtain is forming. 
Pressure control is based upon static pressure within the refrigeration 
enclosure compared against a setpoint pressure. The setpoint is 
empirically established for a given temperature within the enclosure. 
Blower speed is adjusted as necessary to maintain the desired setpoint 
pressure for the selected enclosure temperature. When pressure control is 
used, it is preferred that measurements of static pressure are made in two 
locations and a differential pressure is calculated for comparison with 
the setpoint pressure. Static pressure measurements are made at or near 
the vapor curtain. Referring to FIG. 3, static pressure is preferably 
measured in duct 17 at locations 18 and 19. 
This invention also permits the vapor curtain to be manually controlled by 
an operator. The operator becomes equivalent to the microprocessor and 
takes action based upon reading the difference in vapor concentrations 
measured in each tunnel. An experienced operator can set the controls for 
the vapor curtain based on visual indicators inside the tunnels, such as 
streamers or vapor cloud (formed by condensing moisture of the 
infiltrating air meeting the exiting vapor stream inside the outer 
tunnel). The operator will adjust the frequency signal of a variable speed 
drive, which is connected to the blower motors. The disadvantage of manual 
control is that an operator is required whenever the enclosure is running. 
Control of the vapor being removed from the enclosure is also automatically 
controlled based on maximizing the vapor concentration in the recovery 
line and the enclosure. However, alternate indicators can also be used. 
For example, the flow rate inside the recovery line can be measured and 
used to control the frequency drive for the recovery line blower system 
based on a fixed loss of vapor through the refrigeration ports. In 
addition, differential static pressure can be used as an indication of how 
well the blower is operating. The advantage in using pressure measurement 
is that the reading is static and is therefore, less susceptible to 
freezing. The frequency drive for the recovery line blower system can also 
be operated in a manual mode. As with vapor curtain manual control, the 
operator will base decisions on the indicator method being utilized to 
sense and control the flow activity inside the recovery line. 
An alternative control method to achieve high vapor concentrations in 
tunnel configuration 200 (see FIG. 12) is as follows. First, duct 
assemblies 213 and 223 are modified from the shape shown in FIG. 12 by 
adding additional curvature to the duct assembly. For this case, the vapor 
to air front forms in outer tunnels 214 and 224. The second change is to 
replace the fixed frequency blower system with a controlled, variable 
frequency drive system. Now, both blower systems are controlled by 
microprocessor 281. However, the frequency of blower systems 211 and 221 
may or may not be running at the same frequency. 
For this system, control is based principally on an overpressure-like 
condition in enclosure 200 due to the vaporizing liquid refrigerant, 
rather than maintaining a difference in concentrations in the tunnels 
adjacent to each port, per se. However, both tunnels are monitored and 
corrective action is taken when vapor concentrations change. For example, 
if vapor concentration is decreasing in the tunnels, the frequency of both 
blower systems is increased. This control method is more expensive than 
the method described with respect to the preferred configuration since 
additional ducting and a possible second variable speed frequency drive 
are required. 
The advantages derived through use of the invention will now be considered. 
Vapor purity levels in a refrigeration enclosure is kept relatively high, 
as air does not readily enter the enclosure. Low air entrainment into the 
enclosure yields a more efficient operation since refrigeration is not 
being expended in cooling the incoming air. Moreover, low air infiltration 
into an enclosure permits a vapor stream having high purity level to be 
extracted from the enclosure in a controlled manner for recycling 
purposes. 
One aspect of this invention is the improvement gained through the 
installation of the control apparatus incorporating the invention near a 
port of the refrigeration enclosure, preferably at the lowermost port. In 
particular, the means to redirect the vapor trying to leave the enclosure 
has been improved. For a spiral refrigeration enclosure, the prior art has 
utilized fans and ducting to redirect vapor back into the interior of the 
enclosure, but these systems had limitations in that the exiting vapor 
flow was manually controlled through use of sliding vanes. The net result 
was an uneven flow pattern for the vapor stream exiting the refrigeration 
through the conveyor port. Such a condition required higher flow rates to 
prevent air infiltration. This invention employs a duct assembly and fan 
system that draws vapor smoothly away from the enclosure port and 
redirects it to the interior of the enclosure. As mentioned above, a small 
amount of vapor leaves through the enclosure port to prevent air 
infiltration. The reduced vapor flow rates through the enclosure ports 
become important when the enclosure is part of a recycling system. 
The present invention improves on the prior art in the control scheme 
employed to balance the vapor contained in the enclosure. Prior art 
systems have used blower systems driven at constant frequency or variable 
drives. In addition, blower frequency has been tied to injection rate. One 
limitation to this method is lack of control when there is no injection 
that results in a subsequent loss of the refrigeration capacity. When 
blower frequency is tied to a system based on sensing a visible vapor 
cloud, control becomes dependent on the local relative humidity level. 
Rooms with low humidity and dry products to be cooled would not have 
effective control. Temperature sensing has been successfully used in 
maintaining vapor balance control, so this option is not available per se. 
None of the mentioned control schemes communicate information from both 
refrigeration enclosure ports to provide an indication of inflow of air or 
an outflow of vapor. 
The present invention employs a control system utilizing gas analyzers to 
provide an indication of how well vapor is being contained in the 
enclosure by monitoring concentrations at both ports. Moreover, the 
present invention does not have a setpoint based control scheme or a 
predetermined pattern for the blower frequency. Instead, the blower system 
responds to purity levels inside the enclosure to achieve optimum 
frequency. 
The present invention further improves on known systems for the recycling 
of cryogenic vapors. Prior art methods require the generation of 
sufficient suction pressure at the upper vestibule, which is to be at a 
pressure level below the lowest pressure in the refrigeration enclosure as 
well as below atmosphericpressure. Testing of such methods have shown that 
the amount of makeup air taken from the room is considerable with such a 
method. 
The economic advantage of the present invention is that the controlled 
extraction of a vapor rich stream does not require large amounts of makeup 
air and in fact, should reduce the amount of makeup air required in 
recycle applications. This reduction in makeup air is a cost advantage. 
The control scheme of the present invention for recycle applications 
provide an additional advantage. For example, in U.S. Pat. No. 5,186,008, 
the amount of vapor withdrawn for recycling purposes is a constant times 
the injection rate. This implies that the vapor losses from the enclosure 
fluctuate at a constant times the injection rate. Hence, the vapor losses 
from the enclosure vary with injection rate. 
In the present invention the vapor losses from an enclosure are essentially 
fixed at some value for a given application. Therefore, the flow of the 
vapor stream being recycled is not a fixed ratio of the injection rate. 
The advantage of this control method is more flexibility to define the 
acceptable range of gas concentrations for a recycle system to be 
economically feasible. 
It should be understood that the foregoing description is only illustrative 
of the invention. Various alternatives and modifications can be devised by 
those skilled in the art without departing from the invention. 
Accordingly, the present invention is intended to embrace all such 
alternatives, modifications and variances which fall within the scope of 
the appended claims.