Skewed turn coiled tube heat exchanger for refrigerator evaporators

A heat exchanger for refrigerator evaporators of the type consisting of helically coiled refrigerant-carrying tubing with radially inward extending fins formed along the length of the coiled tubing, with the air to be refrigerated directed across the axis of the coil turns. The coil turns are skewed from the helix angle to expose a greater proportion of the fins into the air flow path between the coil turns so as to increase the air flow contact with the fins. The skewing is created by relatively offsetting opposite portions of the coil turns along the air flow path across the helical coil to increase the obliqueness of a portion of each coil turn with respect to the direction of air flow. The tube coil includes sections folded into a side-by-side relationship, with the skew direction of the coil sections placed in reverse orientations to increase the length of the flow path of the air circulated through the coils and to position gaps between each of the coil turns in either section opposite the areas in the other coil section occupied by radial fins.

BACKGROUND DISCUSSION 
U.S. Pat. No. 3,766,976 discloses a heat exchanger for refrigerator 
evaporators in which tubing is wound in a circular or roughly elliptical 
helix and along which is formed an array of fins extending radially inward 
from the tube windings in the coil to enhance the transfer of heat of the 
tube coil with the air circulated across the coil. The coil is formed in a 
pair of sections, placed in side-by-side relationship in a compartment 
provided within the refrigerant cabinet, and air to be refrigerated is 
directed over the coils in a direction transverse to the axis of the 
coils. The air passes between the turns of the windings over some of the 
fins, such that the refrigerant circulated within the tubing absorbs heat 
and cools the air. 
As discussed in detail in that patent, this heat exchanger configuration 
produces efficient heat transfer, while being relatively resistant to 
frost blockage and can be manufactured at low cost. 
In addition the absence of external fins allows ready handling of the unit 
during assembly without likelihood of damaging the relatively fragile 
fins. 
While evaporators of this type have come into use in recent years, the 
traditional heat exchanger for these applications comprised a 
plate-on-tube type heat exchanger in which loops of refrigerant-carrying 
tubing are joined by an array of parallel fins. The air to be refrigerated 
is passed over the tubing and between the fins to cause the refrigerant to 
absorb heat from the air and refrigerate the air. In this configuration, 
air flow is directed in a parallel direction to the tubing fins. 
In both heat exchanger configurations, the fins are of course provided to 
enhance the transfer of heat between the air and the heat exchanger tubing 
carrying the refrigerant. The fins are of relatively low thermal mass and 
great surface area and are so able to rapidly absorb heat and conduct it 
into the tubing. In the plate type heat exchanger, the air may be directed 
in a parallel direction to the fins as noted and good air flow is afforded 
over the fin structure and is well distributed across the fin array. The 
plate-on-tube design, as noted in the specification of the 
above-referenced patent, has the advantage that the fins are relatively 
closely disposed and subject to ice bridging and are somewhat more costly 
to manufacture. 
While the coiled tube evaporator is highly successful, several factors in 
regards to the mode of air contact with the fins could allow improved 
efficiency of the unit. 
Firstly, the fins extend radially inward from the tube windings and the air 
flow being at a direction transverse to the coil axis, the leading section 
of each turn of the coil shadows a substantial proportion of the fins 
extending radially inward from the turn section. That is, it blocks the 
air flow from passing over the portions of the fins closely adjacent the 
section of the tube. 
In the particular design described in the above-referenced patent, the fins 
extend radially inward, but there is a central opening between opposing 
fins in each turn to prevent total ice blockage. The opening is resistant 
to total bridging by ice build-up due to the relatively large size of the 
opening. However, this opening falls into the gap between windings such 
that the air tends to bypass the shadowed portions of the fins and pass 
through the central region intermediate the fin array to further reduce 
the degree of contact with the fins. 
In addition, the air flow is more nearly parallel to the direction of the 
array of the fins since the tube windings are at a relatively steep angle 
with respect to the coil axis, which causes shadowing of the fins by other 
fins in the array. 
The relatively steep angle of the tubing coil turns with respect to the 
coil axis for closely wound coils also creates gaps between the coil 
windings which, when viewed in the direction of air flow, are unoccupied 
by fins. This allows a substantial proportion of the air to pass over the 
coil windings without passing over any fins, to further degrade the 
efficiency of heat transfer. 
While the obliqueness of the coil turns to the direction of the air flow 
could be increased by increasing the helix angle or the angle at which the 
tubing is wound into the helix, the helical angle is generally determined 
by the number of coil turns and the available space within which the coil 
is to be mounted. 
It is accordingly an object of the present invention to provide an improved 
tube coil type heat exchanger in which the air flow contact with the 
radially extending fins in passing across the coils is increased. 
It is yet another object of the present invention to provide such a coiled 
tube heat exchanger in which the proportion of the air flow passing 
between the gaps in the coil windings without contacting the fin array is 
reduced. 
It is still another object of the present invention to provide such a 
coiled tube heat exchanger of improved efficiency without increasing 
substantially the manufacturing costs to preserve the advantages of the 
coiled tube configuration while improving its performance. 
It is still a further object of the present invention to reduce the degree 
of shadowing by the leading portions of the tube windings and by the fins 
in the array which increases the proportion of air flow in contact with 
the fin array. 
It is yet another object of the present invention to provide a coiled tube 
evaporator with an increased air flow contact with the fin array which 
does not substantially increase the space required to house the 
evaporator. 
SUMMARY OF THE INVENTION 
These and other objects of the present invention, which will become 
apparent upon a reading of the following specification and claims, are 
accomplished by offsetting opposite portions of the turns of a coiled tube 
evaporator along the direction of the air flow across the coil such that 
the resultant skewing of the coil turns disposes portions thereof with 
increased obliquity. This improves the angle of the attack of the air flow 
through the fin array and exposes a greater part of the fin array surfaces 
within the spaces between the coil windings presented to the air flow in 
passing across the coil. 
The coil is formed in side-by-side sections which are reversely wound with 
respect to each other such that the angle of the oppositely skewed 
windings produces a herringbone pattern which increases the path length of 
the air flow in passing across both coil sections. In addition, the 
reverse winding positions finless gaps between the coil windings in each 
coil section opposite regions occupied by fins to further eliminate gaps 
through the side-by-side coil section combination through which air may 
flow without contacting the fin surfaces. 
The offsetting of the coil turn portions results in a modest increase in 
the length of the coil while greatly increasing the proportion of fins 
encountered by the air in flowing across the coils, while the air flow 
across the fin array enhances the overall heat coefficient between the air 
and the fins in the array.

DETAILED DESCRIPTION 
In the following detailed description, certain specific terminology will be 
utilized for the sake of clarity and a particular embodiment described in 
accordance with the requirements of 35 USC 112, but it is to be understood 
that the same is not intended to be limiting and should not be so 
construed inasmuch as the invention is capable of taking many forms and 
variations within the scope of the appended claims. 
Referring to the drawings and particularly to FIG. 1, a coiled tube 
evaporator 10 according to the above-referenced U.S. patent is depicted in 
simplified form. In this configuration, the tubing 12 within which the 
refrigerant is circulated is wound into helical coil sections 14 and 16, 
both of which are wound in the same direction. An intermediate section 18 
of the tubing interconnects both coil sections such that the refrigerant 
is circulated through both coil sections 14 and 16. 
The helical angle of both coil sections 14 and 16 is the same and is 
determined by the lead of the helix. The tube thus is wound at a constant 
inclination to the axis of the helix in accordance with conventional 
design. Such coils being wound with a constant helix angle, there is a 
resultant symmetrical disposition of each of the windings with respect to 
the direction of air flow indicated by the arrows in FIG. 1. 
The tube 12 is adapted to carry refrigerant received from the condenser via 
connections with lines 20. 
The tube 12 is provided with a fin array comprising fins 22 (FIG. 5) which 
extend inwardly along the general direction of the coil radius of the tube 
12 at the point whereat each fin is attached to thus be directed toward 
the interior of each of the coil sections 14 and 16. The fins 22 are 
formed from a flange integral with the tube 12 and are split from the 
flange and bent to provide a fanning of the fins 22 to insure that they 
are not aligned in the array. 
Accordingly, a fin array is provided by the radially inwardly extending 
fins 22 along the length of the tube 12. 
As indicated by the arrows in FIG. 3, the tubing 12 tends to shadow the 
root sections of the fins 22 since the leading portion of the tube 12 is 
interposed between the fins 22 and the air flow into the coil sections 14 
and 16. 
In addition, as seen in FIG. 5, the inclination of the fins 22 and the 
positioning of the coil turns is such that opposed very shallow roughly 
conical regions are established by the fin array which leave interposed 
gaps 26 at the top and bottom of the coil sections as viewed in the 
direction from which the air flow occurs. These gaps are not occupied by 
fins in the fin array and thus allow air flow through the coils without 
coming into contact with the fin array or even the tubing 12 surfaces. 
Referring to FIG. 2, the skewed coil heat exchanger 29 similarly includes a 
helically coiled tubing 28 having spaced coil turns, and within which is 
circulated the refrigerant via connections 30 and 32. In similar fashion, 
the coil is provided in two different coil sections 34 and 36 which are 
positioned in side-by-side relationship. However, the respective coil 
sections 34 and 36 are oriented such as to be reversely wound for a reason 
to be described hereinafter. 
As can be seen in FIG. 2, the skewed coil heat exchanger 28 differs from 
the coil sections 14 and 16 further in that each of the turns of the coil 
sections 34 and 36 has been axially offset on opposite sides thereof; that 
is, along the direction of air flow. This offset is produced by a relative 
displacement of the opposite points on each coil turn as indicated in FIG. 
4. 
A pair of tools 38 and 40 having a pitch equal to the pitch of the coiled 
tubing 28 are engage with the coil and are then relatively displaced to 
produce an offsetting of opposite portions of the coiled tubing 28. The 
points of engagement of the tools 38 and 40 are on the opposite sides 
across which the air flow is to occur, such that the intermediate or 
crossover portions 42 are increased in inclination with respect to the 
direction of air flow while the intermediate portions 44 of the coil 
windings are reduced or reversed, depending on the degree of skew of the 
reforming of the coiled tubing. Thus, the coil turn portions 42 are 
increased in obliqueness by being turned into the air flow which cause a 
greater proportion of the fins 22 to be exposed to the air flow in flowing 
across the coil sections 34 and 36. 
The coil turn portions 44, depending on the degree of skew, may be reduced 
in inclination, unless the degree of offset is sufficient to create a 
reverse inclination from the original helix angle. 
However, any such reduction in inclination has a relatively minor effect 
since at relatively steep angles, i.e., at angles of inclination 
relatively slightly inclined from the direction of air flow, a change in 
the angle is of much less effect on the degree of exposure of the fins 22 
than the greatly increased exposure created by the much greater change in 
inclination of the portion 42. This is because there is a sine function 
relationship between the angle of inclination of the tubing with respect 
to the air flow direction and the extent of exposure of the radially 
inward fins 22. Since the angle of inclination is relatively steep, the 
relatively slight changes of inclination of the sections 44 have little 
effect in reducing the exposure of the fins on this portion of the coil 
turns. 
This increase in inclination of section 42 also improves the angle of 
attack of the air flow in the sense that the direction of the air flow in 
passing through the fins 22 formed on portion 42 of the coil turn is more 
nearly transverse or across the fin array. 
In addition, the fins attached to the section 42 are moved to a greater 
degree out of alignment along the direction of air flow with the tube 
sections 46 such that the fins 22 secured to the portions are much less 
subjected to shadowing by tube lead portions 46. The increased inclination 
also reduces the shadowing effect on the fins 22 in the array along the 
portion 42 of each winding. 
With reference to FIGS. 5, 6 and 7, it may be further seen that the gaps 26 
"seen" by the air flow across the coil, which were present in the 
conventionally wound coil intermediate successive windings, have been 
greatly reduced at the top or bottom of the coil by now being occupied by 
the crossover portions 42 of each winding. It can also be seen by 
reference to FIGS. 6 and 7 that a reverse winding of the coil sections 34 
and 36 locates the crossover portion 42 at the top and bottom of the coil 
sections 34 and 36, respectively, such that there is substantially no gap 
remaining which is unoccupied by either tubing 12 or fins 22 in the array, 
as viewed from the direction of the air flow. Thus, the air cannot "see" 
through the coil sections 34 and 36, causing a reduction in the proportion 
of air flow passing through such openings without coming into the fin or 
tube surfaces of the heat exchanger. 
The "herringbone" pattern created by the oppositely extending skew of the 
coil sections 34 and 36 tends to produce a longer flow path and a longer 
time duration of a given air mass within the heat exchanger since the flow 
is first directed at one inclination and thence in a reverse direction and 
at a greater inclination to the coil axis than a conventionally wound 
coil. 
The overall envelope required to house the coil sections 34 and 36 is only 
modestly increased as the increased dimension is equal to only double the 
distance of offset of a single coil turn. 
This increase in efficiency of heat transfer will thus allow either a 
reduced power consumption and overall efficiency of the unit or will allow 
the use of a smaller heat exchanger with fewer tubing turns. 
Referring to FIGS. 8 and 9, the installation of the heat exchanger 
according to the present invention, into the evaporator chamber 48 of a 
refrigerator 50 is illustrated. The region above the evaporator chamber 48 
is a food storage compartment of the refrigerator, the air within which is 
to be refrigerated to maintain the temperature of the storage compartment 
at the appropriate low temperature. 
The air within the storage compartment 52 is drawn in through inlets 54 as 
shown, into the evaporator chamber 48. The air is drawn into the chamber 
48 by means of a circulating fan 54 such that the air is drawn across the 
coil sections 34 and 36 in a direction across the axis of the coil tubes 
and thence recirculated into the refrigerator through outlet passages 58 
and 60. 
The coil sections 34 and 36 are positioned over a drain pan 62 which is 
adapted to collect water during the defrost cycle with a drain opening 64 
provided to direct the water to a removable pan. A defrost heating element 
66 is also provided for intermittent defrost cycles. 
The evaporator chamber 48 is defined by upper walls 68, lower walls 70 and 
end walls 72 to confine the air flow and direct it across the coil 
sections 34 and 36 in a direction transverse to the axis of the coil 
sections 34 and 36. The coil sections 34 and 36 may be somewhat flattened 
as shown in FIG. 9 to provide a roughly elliptical shape in order to 
accommodate them to a relatively shallow chamber 48 dimension. 
Accordingly, it can be seen that the offsetting of the opposite portions of 
each of the convolutions or turns of the coil sections 34 and 36 greatly 
improves the thermal contact of the air in passing over the coils and 
improves the mode of contact of the air flow with the heat exchanger in 
that a much larger proportion of the air flow comes into contact with the 
fin surfaces in the fin array. Further, the air in flow direction in 
passing over the fin array is in a more oblique direction to enhance the 
temperature differential maintained during the transfer of heat to thereby 
enhance the transfer of heat. In addition, the gaps between windings, 
which were present in the above-described conventionally wound helical 
coil, have been largely eliminated. 
The reverse windings of the side-by-side coil sections 34 and 36 produces a 
longer time of contact of the air within the heat exchanger by increasing 
the length of the flow paths by reversely skewed coil turns and at the 
same time places the crossover portions of each loop on opposite sides of 
each other such as to eliminate aligned gaps in each coil section. 
This improved efficiency has been achieved by a very simple manufacturing 
procedure, i.e., merely offsetting opposite portions of each of the coil 
windings, deforming the coil turns into an offset or skewed configuration. 
This increases the size of the coil, but modestly. 
The shadowing effect has been greatly reduced by moving those portions of 
the coil turns directly behind the leading coil turn portions ahead of the 
crossover section such as to provide greater contact of the air with the 
portions of the fin array which were shadowed by the prior art design. 
The particular skew angle or offset depends on the spacing of the coils and 
the degree of clearance required for the fin array to prevent ice 
bridging. That portion of the coil winding which is reduced in inclination 
is displaced to be nearly parallel or reversely inclined with respect to 
the air flow direction. This will produce an angle of inclination of the 
turn portions 42 at an angle of 20.degree. to 30.degree. from the 
direction of air flow, i.e., normal to the coil axis. However, any degree 
of offset will improve the efficiency of the unit. 
The heat exchanger of the present invention is particularly adapted to 
refrigerator evaporator coils in which air is cooled by passing it over a 
tubing condensed refrigerant which is allowed to vaporize in the 
evaporator coils and thus to absorb heat from the air. This is because 
this heat exchanger configuration resists ice bridging and may be 
manufactured at low cost. 
However, the invention is not limited to such application and may be 
utilized for either the heating of fluids, as well as the cooling of 
fluids, between a first fluid circulated within the tubing and a second 
fluid which is directed over the coil, with the second fluid directed over 
the coil windings in a direction transverse to the coil axis. 
It is noted that the skewed coil configuration produces a coil turn in 
which portions of the turn extend at variable helix angles, such that 
portions of each turn are inclined to the air flow direction (normal to 
the coil axis) at varying degrees of obliqueness. 
Hence, the offsetting described is in the sense that the portions of the 
coil turns are displaced from their positions assumed during forming of 
conventionally constant helix angle coils. As noted, conventionally wound 
coils are wound at constant helix angles such that for a given number of 
turns or coil spacing, there is a "natural" helical angle and the turns of 
the coil advance at this inclination with respect to the axis of the coil. 
The present invention lies in the offsetting of opposing portions of each 
turn along the direction of air flow such as to skew these portions from 
the "natural" helical angle of the coil and produce turn portions varying 
from the helix angle, one of which is inclined at increased obliquity to 
that angle, the other portion being moved such that the adjacent section 
is reduced or reversed in inclination.