Evaporative heat exchanger with elliptical tube coil assembly

The present invention relates to a coil assembly for use in an evaporative parallel flow or counterflow heat exchanger wherein the heat exchanger comprises a conduit oriented in a vertical direction through which external heat exchange fluids flow in a generally vertical direction, the coil assembly being mountable within the conduit, the coil assembly comprising inlet and outlet manifolds and a plurality of tubes connecting the manifolds, the tubes including bights and segments extending generally horizontally across the conduit and connected to at least one bight, the bights being oriented vertically and connecting segments of the tube at different levels within the conduit, the bights of adjacent tubes being in contact with each other, the segments having a generally elliptical cross sectional shape such that the segments of adjacent tubes are spaced from each other in a direction generally normal to the flow direction. The elliptical segments may be angled in the same or opposition directions as long as the spacing is maintained between the segments of adjacent tubes. The bights may have a circular or elliptical cross section.

BACKGROUND OF THE INVENTION 
1. Field of The Invention 
The present invention relates to a coil assembly for use in an evaporative 
heat exchange apparatus in which the coil assembly is to be mounted in a 
vertically oriented duct or conduit of a duct or conduit of the apparatus 
in which heat exchange fluids, typically a liquid, usually water, and a 
gas, usually air, flow externally through the coil assembly to cool or 
condense a heat transfer fluid passing internally through the tubes of the 
coil assembly. More particularly, the coil assembly of the present 
invention is most effectively mounted in a counterflow evaporative heat 
exchanger so that water flows downwardly and externally through the tube 
assembly while air travels upwardly and externally through the coil 
assembly. 
The coil assembly of the present invention can be used also in a parallel 
flow evaporative heat exchanger in which the air travels in the same 
direction over the coil assembly as the water. The evaporation of the 
water cools the coil assembly and the internal heat transfer fluid inside 
the tubes forming the coil assembly. 
In accordance with the present invention, the coil assembly comprises an 
array of closely packed serpentine tubes in which the tubes have two 
different cross sectional dimensions, preferably when viewed in a 
horizontal plane. Each tube comprises a plurality of two different types 
of portions, "segments" and "bights" The "segments" are generally straight 
tube portions which are connected by the "bights", which are the curved 
portions, sometimes referred to as return bends, to give the tube its 
serpentine structure. In the preferred embodiment of the coil assembly of 
the present invention, the segments of each tube are generally elliptical 
in cross section and the bights are generally circular in cross section. 
The generally horizontal diameter of the elliptical segments is smaller 
than the generally horizontal cross sectional dimension of the generally 
circular bights. If desired, the bights can have an elliptical cross 
section, so long as the generally horizontal cross sectional dimension of 
the segments is less than the generally horizontal cross sectional 
dimension of the bights. In view of these different cross sectional 
dimensions, segments of adjacent tubes are always spaced from each other 
even though the bights of adjacent tubes are in contact with each other. 
The segments are preferably arranged in generally horizontal rows 
extending across the flow path of the air and water which flow externally 
through the coil assembly, whether the air and water are in counterflow or 
in parallel flow. 
The coil assembly of the present invention provides a number of significant 
advantages. It allows for freer flow of air externally through the coil 
assembly at lower fan horsepower. It also allows higher spray water flow 
rates externally over the coil assembly, and thus, higher thermal 
capacity, without adversely affecting the airflow. It provides for a 
maximum amount of coil heat transfer surface area within a given coil 
assembly volume. As a result, the coil assembly provides greater heat 
transfer capacity. Further, the coil assembly is easy to manufacture and 
is stronger and more rigid than other designs. 
2. Description of The Prior Art 
U.S. Pat. Nos. 3,132,190 and 3,265,372 disclose one type of counterflow 
evaporative heat exchange apparatus in which a coil assembly is mounted in 
a duct with water sprayed externally downwardly over the coil assembly 
while air is blown upwardly through the coil assembly. These patents are 
typical of prior art coil assemblies which will be referred to herein as 
"tight packed" coil assemblies. In such tight packed coil assemblies, the 
tubes forming the coils extend in a vertical plane between upper and lower 
inlet and outlet manifolds in a serpentine manner in which the tubes also 
extend generally horizontally across the conduit or duct in which the coil 
assembly is mounted. To maximize the surface area of the tubes being 
subjected to the external air and water contact, the tubes of the coil are 
tightly packed together and are in contact with adjacent tubes at the 
bights and, because the segments and bights have the same cross sectional 
dimension and shape, they are not spaced apart from each other laterally 
throughout the entire length of the tube segments. The segments are offset 
from each other vertically by placing alternate coil circuits at different 
levels. The open space between two tubes on the same level is equal to the 
width of the tube in between them. It can be said that a tight packed coil 
assembly has essentially a 50% open area on each generally horizontal 
level of segments. 
A tight packed coil assembly has the maximum number of tubes that can be 
built into any given unit width to provide what was thought to be the 
maximum amount of surface area for a coil assembly for that width. Because 
of the high number of tubes, the tight packed coil assembly has a 
relatively low flow of internal fluids flowing within each tube of the 
coil assembly and a low pressure drop through the interior of the tubes. 
The airflow pressure drop of the air travelling externally through the 
coil assembly is relatively high because the tubes are tightly packed 
together. The external air and water flow through the 50% open area. Spray 
water flowing down over the coil assembly in a direction opposite the 
airflow, that is, countercurrent to the airflow, restricts the flow of air 
to such an extent that the amount of spray water flowing has to be limited 
as a practical matter to be just enough to wet the coil assembly, but not 
so much that the airflow rates are adversely affected. Typically, this 
water flow rate has been limited to values of 11/2 to 3 gallons per minute 
(gpm) per square foot of plan area. Even for parallel flow equipment, 
where the external air and water flow in the same direction, the 50% open 
area is still quite restrictive. Similar to counterflow equipment, water 
flow rates had to be limited so as not to adversely affect the airflow. 
In a effort to improve the heat exchange fluid flow characteristics and 
heat transfer results, another system was developed and is disclosed in 
U.S. Pat. No. 4,196,157. The coil assembly used in this system will be 
referred to herein as a "spaced tube" coil assembly. With a spaced tube 
coil assembly, the tubes forming the coils have serpentine circuits 
extending between an upper inlet manifold and a lower outlet manifold 
while also extending generally horizontally across the duct or conduit of 
the evaporative heat exchanger in which the coil assembly is mounted. 
However, rather than packing the tubes so tightly that they contact each 
other, spacers are used so that laterally adjacent tubes are spaced apart 
from each other along the entire length of tubes, that is, at both the 
bights and segments, by a distance comprising a narrow critical range. As 
in the tight packed coil assemblies, in the spaced tube coil assemblies, 
the segments are offset from each other vertically by placing alternate 
coil circuits at different levels. Thus, to provide the efficient heat 
transfer characteristics disclosed in the patent, the tubes of the spaced 
tube coil assembly must be spaced apart from each other by an amount such 
that the space between adjacent tube segments at each horizontal level is 
greater than the diameter of the tubes but is less than twice the tube 
diameter. In this type of coil, the open area at any horizontal level 
could range from slightly greater than 50% to a maximum of 67% and in 
practice has been approximately 55%. 
The spaced tube coil assembly provides certain advantages in counterflow 
and parallel flow heat exchangers compared to the tight packed coil 
assembly. The open spaces between the laterally adjacent tubes results in 
a lower pressure drop requiring a lower fan horsepower to move equal 
amounts of air externally through the coil assembly than if a tight packed 
coil assembly were used. It allows the spray water flow to be increased 
somewhat without an adverse performance penalty on the air fan system. 
Despite the claimed improvement in counterflow evaporative heat exchange 
systems using the spaced tube coil assembly compared to a tight packed 
coil assembly, there are limitations associated with the spaced tube coil 
assembly. There is a penalty for the tube spacing in that approximately 
20% fewer tubes, and therefore, approximately 20% less surface area, can 
be built into a given unit width. This results in an approximate 20% 
higher flow per tube and a corresponding approximate 40% higher pressure 
drop of fluid flowing internally within the coil assembly. What has been 
gained by the use of lower fan horsepower and improved airflow externally 
through the coil is offset by the loss in heat transfer surface area. 
Nevertheless, in practice, systems employing the spaced tube coil assembly 
have demonstrated capacities almost the same as the systems using the 
tight packed coil assembly. The primary advantage of using a spaced tube 
coil assembly has become a cost savings to the manufacturer due to the 
fewer number of tubes required. 
With the present invention, the advantages of the large amount of surface 
area of the tubes in a tight packed coil system are combined with the 
enhanced external air and water flow characteristics of a spaced tube coil 
assembly to provide a significant increase in heat exchange capacity in an 
evaporative heat exchanger as compared to equipment of the same size using 
either a tight packed coil assembly or a spaced tube coil assembly. The 
present invention results in a real advantage both to the manufacturer of 
the equipment and the customer by increasing the capacity of a unit of 
given dimensions. 
SUMMARY OF THE INVENTION 
One aspect of the present invention includes a coil assembly for use in an 
evaporative heat exchange apparatus in which external heat exchange fluids 
flow externally through the coil assembly in a flow direction generally 
normal to a major plane of the coil assembly, the coil assembly comprising 
inlet and outlet manifolds and a plurality of tubes connecting the 
manifolds, the tubes having a plurality of segments and a plurality of 
bights, the bights being oriented in planes parallel to the flow 
direction, the segments of each tube connecting the bights of each tube 
and extending between the bights in a direction generally normal to the 
flow direction, the bights of each tube being in contact with the bights 
of adjacent tubes, the segments having a generally elliptical cross 
sectional shape such that the segments of adjacent tubes at the same level 
in the coil are spaced from each other in a direction generally normal to 
the flow direction. This spacing does not adversely block and actually 
enhances the flow of the external heat exchange fluids externally through 
the coil assembly. 
More particularly, the present invention is directed to a coil assembly for 
use in an evaporative heat exchanger, preferably a counterflow or parallel 
flow heat exchanger wherein the heat exchanger comprises a conduit 
oriented in a vertical direction through which external heat exchange 
fluids flow in a generally vertical direction, the coil assembly being 
mountable within the conduit, the coil assembly comprising inlet and 
outlet manifolds and a plurality of tubes connecting the manifolds, the 
tubes including bights and segments extending generally horizontally 
across the conduit and connected to at least one bight, the bights being 
oriented vertically and connecting segments of the tube at different 
levels within the conduit, the segments of adjacent tubes being staggered 
and spaced vertically with respect to each other to form a plurality of 
staggered levels in which every other segment is aligned in the same 
generally horizontal level, the bights of adjacent tubes being in contact 
with each other and having a cross sectional horizontal dimension, the 
segments having a generally elliptical cross sectional shape such that the 
segments of adjacent tubes at the same level are spaced from each other by 
an amount greater than the horizontal cross sectional dimension of the 
bights. The flow of the external heat exchange fluids externally through 
the coil assembly is enhanced by this spacing. 
The present invention also includes evaporative heat exchange apparatus 
employing the novel coil assembly summarized above and explained in detail 
hereinafter. 
As used herein, the term "generally horizontal" and equivalent terms mean 
that the segments or other components of the present invention described 
as being generally horizontal may be inclined upwardly or downwardly 
within a few degrees. Thus, for example, the segments of a tube typically 
are inclined downwardly between the bottom of one connecting bight to the 
top of a bight connected to the other end of the segment. As used herein, 
the "generally horizontal" includes the angle of inclination of the tube 
segments between the bights. 
As used herein, a "major plane" of the coil assembly means planes generally 
parallel to those planes containing each level of tube segments within the 
coil assembly. In the preferred embodiments illustrated in the drawings, 
for example, the major plane of the coil assembly is generally horizontal. 
It is preferred that the distance between the centerline of adjacent bights 
substantially equals the cross sectional horizontal dimension of the 
bights and that the space between segments of adjacent tubes at the same 
level is between about 1.1 and about 1.5, and most preferably, about 1.2, 
times the horizontal cross sectional dimension of the bights. Preferably, 
the spacing between the segments results in an open area at any horizontal 
level of about 55% to about 75%, and most preferably, about 60%. 
The coil assembly of the present invention provides the following 
advantages compared to the prior art in addition to those discussed above. 
The use of the present invention increases the net amount of heat transfer 
in an evaporative heat exchanger compared to the prior art; not the heat 
transfer per unit area of tube surface, but the total heat transfer. As a 
result, the operating cost per unit of heat transferred is reduced 
significantly by the present invention compared to the prior art. Since 
the segments of the tubes between the bights comprise most of the surface 
area of the coil assembly, the generally elliptical cross sectional area 
of the segments having their major axes oriented vertically gives more 
open space between the tubes for airflow and spray water flow than the 
tight packed coil assembly. Moreover, the spacing of the elliptical 
segments of the serpentine circuits of the tubes would be defined by the 
degree of the ellipse and by virtue of the contact of the laterally 
adjacent bights. This provides the same high number of tubes per unit 
width as in the tight packed coil assembly and the same high coil surface 
area per coil assembly plan area as in the tight packed coil assembly. 
Although there would be a slight loss of flow area internally within the 
tubes due to the ellipse (on the order of about 5-10%) that would result 
in an increased pressure drop of about 10% to about 20% over the same type 
of system using a tight packed coil assembly. However, the present 
invention would have about 20% to 30% less pressure drop than the spaced 
tube coil assembly. The overall performance of the coil assembly of the 
present invention is improved significantly because of the spaced 
segments. 
The 20% increase in space between tube segments at the same horizontal 
level of adjacent segments of the coil assembly compared to the tight 
packed coil assembly provides lower resistance to airflow and water flow 
and also makes it easier to clean the coil assembly. Surprisingly, it has 
been found that the static pressure resistance to external airflow with 
the present invention is even lower than it is in the spaced tube coil 
assembly of the prior art where there is equal open space between lateral 
tubes in the two systems. This occurs even when using higher spray water 
flow rates over the coil in the present invention. Higher spray water flow 
rates are desirable because they result in increased thermal capacity. 
This is because of improved air and water contact and improved contact of 
the tube surface with larger amounts of cooling water. It has been found 
that even at water flow rates up to 8 gpm per square foot of plan area, 
the present invention shows increased thermal capacity compared to the 
spaced tube coil assembly which, in practice, is limited to 4.5 gpm of 
water per square foot of plan area. 
The thermal performance of any evaporative cooling device such as this is 
dependent upon its ability to thoroughly mix the air and water flow 
streams. The object of an evaporative cooler is to expose as much surface 
area as possible of the evaporating water to the air, thereby bringing as 
much of the air as possible to its saturation point. In this invention, 
large amounts of both the air and water are mixed turbulently inside the 
device in the region of the coil and provide for improved thermal 
performance. 
Also, the thermal performance of an evaporative cooler depends upon its 
ability to transfer heat from the internal heat fluid flowing inside the 
heat exchanger, coil assembly to the external heat exchange fluids (air 
and water). The amount of heat transferred is a function primarily of the 
coil assembly surface area but the geometry and construction of the coil 
assembly plays an essential part in the turbulent mixing of the air and 
water, as well. 
The prior art, using round tubes or tubes of generally equal cross 
sectional dimensions at the bights and segments have been unable do both, 
that is, to provide a maximum amount of heat transfer surface area and to 
provide for good turbulent mixing of large amounts of air and water 
flowing externally through the coil assembly. 
The prior art spaced tube coil assemblies allow the mixing of larger 
amounts of air and water, but require a coil tube constructed with a 
greater percentage of open plan area at the expense of lower coil surface 
area. With the present invention, the surprising result of less resistance 
to the airflow and the spray water flow has allowed the use of higher 
spray water flows that provide additional thermal capacity compared to the 
prior art systems. This is especially important for propeller fan units 
which are generally less capable of handling high static pressures and 
have improved efficiency when the static pressure is reduced. 
The open area, that is, the spaces between the segments of adjacent tubes 
at the same horizontal level in the present invention, may be tuned to a 
particular fan's characteristics by varying the degree of the elliptical 
cross sectional shape of the segments, the angle of the elliptical 
segments and the spray water flow rate, thereby allowing the fan to 
operate at its most efficient point. 
Since a tube with an elliptical cross sectional shape will have less flow 
area than a tube having a circular cross sectional area of the same 
circumference, the flow velocity inside a tube with elliptical segments 
will be higher than that of a tube having circular segments. This is also 
an advantage in that higher velocities within the tube increase the 
turbulence and the internal film heat transfer coefficient, and thus, the 
thermal performance of the coil assembly, as compared to the tight packed 
coil assembly using tubes having a uniform circular cross sectional shape. 
The coil assembly of the present invention can be applied to both 
counterflow and parallel flow evaporative heat exchangers. In both of 
these designs, performance is maximized by providing the greatest amount 
of water or other liquid and the greatest amount of air or other gas (the 
external heat exchange fluids) in intimate and efficient contact with each 
other and in contact with the greatest amount of coil surface area. 
The manufacture of the coil assembly of the present invention is easier 
than the construction of the prior art spaced tube coil assemblies. No 
special spacers are required to maintain a critical spacing between tubes. 
This eliminates the special handling required during the preliminary 
processing and assembly of the units. By tightly packing together the 
bights in the present invention, the novel coil assembly is much more 
rigid than the prior art spaced tube coil assemblies. The compound 
curvature of the tightly packed bights makes the coil assembly of the 
present invention very strong. 
In summary, the present invention provides for improved airflow 
characteristics without losing any surface area or tubes. The coil 
assembly of the present invention permits even higher spray water flow 
over the coil and higher thermal performance without penalizing the fan 
performance. The pressure drop of fluid flowing in the interior of the 
coils has increased, but by much less than half of the increase of the 
spaced tube coil assembly as compared to the tight packed coil assembly. 
All of these benefits combine in this invention to produce a unit with 
greater thermal capacity than other designs, and it is able to fit in a 
smaller space than prior art spaced tube coil assemblies with the same 
number and size of tubes with the same spacing between segments. The lower 
space requirements are very important because of end user construction 
costs and building volume that could be used for more important income 
producing purposes.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
With reference to the drawings, wherein reference numerals and reference 
letters represent like elements, there is shown in FIG. 1 a first 
embodiment of an evaporative heat exchanger 10 built in accordance with 
the present invention. Heat exchanger 10 includes a generally vertical 
duct or conduit 12 typically made of galvanized sheet metal. A coil 
assembly of the present invention 14 is mounted in conduit 12 in any 
suitable manner such as by being bolted to support brackets 16. Although 
conduit 12 is shown as being oriented in a vertical direction, which is by 
far the most typical case, conduit 12 could be oriented in any other 
direction, as long as coil assembly 14 is mounted within the conduit such 
that the major plane of the coil assembly is generally normal to the flow 
direction of external heat exchange fluids flowing externally through the 
coil assembly. Preferably, the major plane, represented by a plane resting 
on the top of coil assembly 14 or on the second level of segments within 
the coil assembly, is generally horizontal. 
A blower assembly 18, which may be a centrifugal blower as illustrated or a 
propeller type fan (not illustrated), blows a gaseous heat exchange fluid, 
typically air, into conduit 12 and externally through coil assembly 14. If 
desired, instead of having a forced draft blower system, in which the fan 
or blower is mounted at the bottom of conduit 12, the system could be an 
induced draft unit in which the blower or fan is mounted on the top of the 
unit. 
An external heat exchange liquid 20, typically water, is sprayed in a 
direction counter to the flow of the air by spray assembly 22 externally 
through coil assembly 14. Although the external heat exchange fluids could 
be gases and liquids other than air and water, this invention will be 
described hereinafter by referring to air and water as exemplary of any 
other suitable fluids. Water 20 thereby coats the surfaces of the tubes 
forming the coil assembly. As the air travels externally through the coil 
assembly, the water is evaporated, thus cooling the surfaces of the tubes, 
and by conduction, cooling the internal heat transfer fluid flowing within 
the inside of the tubes. Thus, heat is exchanged among the air and water 
and the internal heat transfer fluid. 
Water 20 flows downwardly through conduit 12 into a sump area 24 where it 
can be recycled to spray assembly 22 or discharged. The air laden with 
mist travels through a drift eliminator assembly 26 which removes most of 
the mist from the air before it exits from the heat exchanger as indicated 
by the arrows above the heat exchanger. Any suitable drift eliminators may 
be used, although the preferred drift eliminators are those disclosed in 
U.S. Pat. No. 4,500,330, assigned to the assignee of the present invention 
and application. 
If it is desired to use the coil assembly of the present invention in a 
parallel flow heat exchanger, in which the air flows in the same direction 
as the water, one skilled in the art would be able to modify apparatus 10 
readily. For example, a blower could be mounted on the top of conduit 12 
to blow air downwardly through the coil assembly and drift eliminators 
could be located below the level of the coil assembly. Many other 
modifications are possible and it is not believed necessary to describe 
them since they would be readily apparent to one of ordinary skill in the 
art. 
FIG. 2 illustrates an alternate embodiment of a counterflow evaporative 
heat exchanger 30 in accordance with the present invention. Heat exchanger 
30 includes a duct or conduit 32 in which is mounted in any suitable 
manner a coil assembly 34 according to the present invention. Air or other 
gas is blown upwardly through the coil assembly, and then through first 
and second stages 36, 38, respectively, of contact bodies, sometimes 
called wet deck fill, which further enhances the heat transfer between the 
water and the air. Although two decks of contact bodies are shown, one 
deck or level may be sufficient in many instances. Also, the wet deck fill 
may be placed below the coil assembly instead of above it, if desired. As 
indicated by the absence of any contact bodies in FIG. 1, the use of 
contact bodies is optional. Contact bodies of the type suitable for use in 
heat exchanger 30 are well known to those of ordinary skill in the art. 
However, it is presently preferred to use contact bodies, of the type 
disclosed in U.S. Pat. No. 4,579,694, assigned to the assignee of the 
present invention and application. 
Water 40 is sprayed by spray assembly 42 through the contact bodies 36 and 
38 and onto the surfaces of coil assembly 34 where the evaporative heat 
exchange takes place as discussed above. The water then is collected in a 
sump (not shown) as described above and mist laden air passes through a 
drift eliminator assembly 44 as it exits the heat exchanger. The apparatus 
of FIG. 2 could also be modified readily to operate in a parallel flow 
manner instead of a counterflow manner. 
The details of the coil assembly of the present invention will now be 
described with initial reference to FIGS. 3 and 4 showing, in essence, a 
partial plan view of coil assembly 34 in FIG. 3 and a partial sectional or 
side view of coil assembly 34 in FIG. 4. 
Coil assembly 34, which is constructed in a manner substantially identical 
to coil assembly 14 of FIG. 1, comprises an upper inlet manifold 46 and a 
lower outlet manifold 48 which extend generally horizontally across the 
interior of conduit 32. The manifolds are mounted on an interior side wall 
of conduit 32 by a pair of brackets 50 and 52. The brackets may be 
supported by or attached to brackets such as brackets 16 illustrated in 
FIG. 1. An inlet conduit 54 extends through the side wall of duct or 
conduit 32 and communicates with the upper inlet manifold 46. Likewise, an 
outlet conduit 56 extends through the side wall of duct or conduit 32 and 
communicates with the lower, outlet manifold 48. The fluid conduits are 
connected to a source of an internal heat transfer fluid to be cooled or 
condensed, for example a refrigerant from a compressor in an air 
conditioning system (not shown). 
Bights 62 of coil assembly 34 are supported by horizontally extending 
support rods 64 and 66. Support rods 64 are mounted between brackets 70 
and 72 that are attached to the side wall of the duct or conduit 32 
opposite the side wall on which the manifolds are mounted. Support rods 66 
which are located between upper and lower manifolds 46 and 48 are 
supported by the same brackets 50 and 52 by which the manifolds are 
mounted to the side wall of duct or conduit 32. 
A plurality of tubes designated generally as 58 are connected to manifolds 
46 and 48 after extending generally horizontally back and forth across 
conduit 32 in a serpentine manner. Tubes 58 have a plurality of generally 
straight segments 60 connected to and extending between the plurality of 
bights 62. As indicated in FIGS. 3 and 4, bights 62, and therefore, tubes 
58, are oriented in a vertical direction which corresponds to the 
direction of the flow of the air and water flowing externally through the 
coil assembly. Adjacent tubes, for example, tubes 58a and 58b in FIGS. 3 
and 4 preferably are arranged in alternately vertically offset arrays, 
such that the segments of every other tube are generally aligned in the 
same horizontal plane, but above or below the next adjacent tube. Thus, 
for example, as best illustrated in FIG. 4, segment 60a of tube 58a is 
located above segment 60b of tube 58b. As illustrated in FIG. 4, the 
vertical spacing of the tubes preferably is such that the vertical spaces 
between the segments of adjacent tubes are substantially equal. 
While the number of segments and bights depend upon the overall design of 
the heat exchange system, typically, the coil assembly of the present 
invention includes between 3 and 11 bights 62 which are connected to 
between 4 and 12 segments 60. Also, in a typical counterflow evaporative 
heat exchanger with cross sectional dimensions of about 57 inches by 
twelve feet, 53 tubes with an outside diameter of 1.05 inches could extend 
across the duct or conduit. So that a coil assembly having tubes with the 
maximum amount of surface area per any given cross sectional area of the 
duct or conduit can be attained, the tubes are arranged such that the 
bights 62 contact each other. This is best illustrated in FIG. 3 where 
bights 62c, 62d, 62e and 62f clearly contact each other. Thus, the bights 
of the coil assembly of the present invention are in a tight packed 
arrangement, substantially identical to the bights in a prior art tight 
packed coil assembly. 
Unlike the prior art tight packed coil assembly, however, the coil assembly 
of the present invention is constructed to provide for spaces between 
adjacent segments 60 of adjacent tubes 58 at different levels. These 
spaces are clearly illustrated in FIG. 3 as being between segments 60c, 
60d, 60e and 60f of tubes 58c, 58d, 58e and 58f, respectively. More 
importantly, adjacent segments at the same horizontal level are spaced 
laterally from each other by a greater distance than segments of tubes in 
the prior art tight packed coil assembly. The increased spacing between 
adjacent segments at the same horizontal level can be seen with reference 
to FIGS. 3 and 5, and specifically, such spacing is represented by the 
spacing between segments 60c and 60e of tubes 58c and 58e, respectively, 
at a higher horizontal level, and by the spacing between segments 60d and 
60f of tubes 58d and 58f, respectively, at a lower horizontal level. 
By virtue of the spaced adjacent segments at different levels and the 
increased spacing between adjacent segments at the same horizontal level, 
the coil assembly of the present invention has some similarity to the 
prior art spaced tube coil assembly. However, as explained herein, the 
coil assembly of the present invention is even more efficient than the 
prior art spaced tube coil assembly and provides some surprising and 
unexpected advantages. 
The spacing of the segments in the coil assembly of the present invention 
is achieved by virtue of making the tubes with two different cross 
sectional transverse (preferably horizontal) dimensions, whereby such 
cross sectional transverse dimension of the segments is less than that of 
the bights. To provide for efficient heat transfer between the external 
and internal fluids as explained above, segments 60 have a generally 
elliptical cross sectional shape whereby the segments of adjacent tubes at 
the same level are spaced apart from each other due to their elliptical 
shape by an amount greater than the cross sectional transverse dimension 
of the bights, which may have a generally circular or generally elliptical 
cross sectional shape, such that the flow of the air and water externally 
through the coil assembly is not adversely affected. The major axis of 
each tube segment 60 preferably is oriented in a vertical plane. However, 
as explained below in detail, the major axis of the ellipses may be 
oriented at varying angles at random with respect to the vertical plane 
and may even be skewed at opposite angles in adjacent tubes as long as a 
space is maintained between adjacent tubes in a direction transverse to 
the flow direction of the air and water externally through the coil 
assembly. Tubing having segments with an elliptical cross sectional shape 
can be formed readily by techniques well known to those of ordinary skill 
in the art. 
Further details of the coil assembly of the present invention, and 
particularly the characteristics of the present invention compared to the 
prior art, will be described with respect to FIGS. 5-7. 
FIG. 5 illustrates a first and presently preferred embodiment of a portion 
of a coil assembly taken along line 5--5 of FIG. 3. For the purpose of 
clarity, support rod 64 has been eliminated in FIG. 5. FIG. 5 illustrates 
four adjacent tubes 58c, 58d, 58e and 58f Which include segments 60c, 60d, 
60e and 60f, respectively, as well as bights 62c, 62d, 62e and 62f, 
respectively. In the embodiment of FIG. 5, bights 62 have a generally 
circular cross sectional shape, at least where they join segments 60. Each 
of the tubes 58 at bights 62 has a diameter of X. Since the bights are in 
contact with each other, the distance D between the centerlines of the 
bights of adjacent tubes, for example tubes 58c and 58d and tubes 58d and 
58e, each equals the diameter X. Thus, the distance between the 
centerlines of adjacent tubes on the same horizontal level, namely, tubes 
58c and 58e or tubes 58d and 58f, equals two times D, or 2X. 
Also as illustrated in FIG. 5, the segments of adjacent tubes at the same 
level have an open space S between them by virtue of the elliptical shape 
of the segments which are automatically spaced from each other. Because 
the dimension of the minor axis Y of the elliptical segments 60 is less 
than the diameter X of the bights 62, the open space S=2D-Y and is greater 
than X. The minor axis of the ellipse corresponds to the transverse cross 
sectional dimension in a direction transverse to the flow direction of the 
water and air externally through the coil assembly and transverse to the 
longitudinal axis of the segment. It is preferred that this dimension, and 
specifically the minor axis, have a length or dimension Y of about 0.5 to 
about 0.9 times, and most preferably, about 0.8 times the diameter X of 
the bight. Thus, using the foregoing formula, the space S between segments 
of adjacent tubes of the same level in a horizontal direction preferably 
is between about 1.1 and about 1.5 times the diameter or dimension X. 
The larger space between segments of adjacent tubes at the same level 
allows for more efficient airflow between the tubes of the coil assembly, 
providing for more efficient evaporation and better thermal performance 
and efficiency than if there were smaller spaces between the segments of 
adjacent tubes at the same level as in the tight packed coil assembly of 
the prior art. The larger space between the segments of adjacent tubes in 
the same level provides for more efficient (eased) airflow between the 
segments of the coil assembly. A possible concern, however, is that the 
eased airflow is streamlined, less turbulent and even bypasses the tube 
segments completely. This would result in a loss of heat transfer 
capacity. However, surprisingly, this does not occur. The open space 
between the tube segments, the high coil surface area and the higher spray 
water flow rates combine to improve the evaporation and thermal 
performance over the tight packed coil assembly of the prior art. A 
typical prior art tight packed coil assembly is illustrated in FIG. 6 for 
purposes of comparison with FIG. 5. 
With reference to FIG. 6, the tight packed prior art coil assembly includes 
tubes 78 having segments 80 and bights 82. It is clear from FIG. 6 that 
the tubes used in the prior art tight packed coil assembly have a uniform 
cross sectional shape with a uniform cross sectional dimension throughout 
the length of each tube. Thus, the cross sectional dimension of segments 
80 equals the cross sectional dimension of bights 82, namely, the diameter 
of the tube, represented as X.sub.1. This distance D.sub.1 between the 
centerlines of adjacent tubes 78a and 78b or between adjacent tubes 78b 
and 78c is equal to the diameter or distance X.sub.1. Accordingly, the 
distance between the centerlines of two segments on the same level, namely 
segments 80a and 80c, equals two times D.sub.1, which equals two times 
X.sub.1 or twice the diameter of the tubes, since they are packed as 
tightly as can be. In this case the open space between tubes at the same 
level S is always equal to 2D.sub.1 -X.sub.1, which equals D.sub.1. 
FIG. 7 illustrates a portion of a prior art spaced tube coil assembly for 
the purpose of the comparison with FIG. 5 illustrating the present 
invention and FIG. 6 illustrating the tight packed coil assembly. The 
spaced tube coil assembly illustrated in FIG. 7 includes a plurality of 
tubes 88 having segments 90 and bights 92. Adjacent tubes are spaced from 
each other laterally by spacer rods 94. Thus, bights 92 of adjacent tubes 
88 are not in contact with each other as in the present invention or as in 
the prior art tight packed coil assembly. As with the prior art tight 
packed coil assembly, tubes 88 of the prior art spaced tube coil assembly 
have a uniform cross sectional shape, generally circular, having a cross 
sectional dimension X.sub.2, corresponding to the diameter of the tube. 
Spacer rods 94 space adjacent tubes from each other by a distance R. 
Accordingly, the distance D.sub.2 between the centerlines of the segments 
of adjacent tubes, such as segments 90a and 90b or segments 90b and 90c, 
is equal to the distance X.sub.2 plus R. Therefore, the distance between 
segments of adjacent tubes at the same level, namely the distance S.sub.2 
between segments 90a and 90c, is 2D.sub.2 -X.sub.2, or X.sub.2 +2R. 
It should be clear from the foregoing and a review of FIGS. 5-7 that for 
the same size tubing, D.sub.2 is greater than D. Accordingly, more tubes 
can be used in a coil assembly having a given width than could be used in 
the prior art spaced tube coil assembly illustrated in FIG. 7, assuming 
that the tube diameter of the bights is the same (that is, where 
X=X.sub.2) This results in the significant advantages of the present 
invention over the prior art as discussed above, namely, it achieves 
higher thermal performance with 20% more coil surface area and 20-25% 
lower internal pressure drop. 
It is presently preferred that the tubes used in a coil assembly of the 
present invention have bights with a circular cross sectional shape. 
Nevertheless, the present invention is not limited to tubing having a 
circular cross section. Rather, coil assemblies according to the present 
invention can be made from tubing of any cross sectional shape, as long as 
the cross sectional dimension of the segments corresponding to dimension Y 
of FIG. 5 is less than the cross sectional dimension of the bights 
corresponding to dimension X of FIG. 5. 
FIG. 8 illustrates another embodiment of the present invention in which the 
tubing has an elliptical cross sectional shape such that the major axis of 
the ellipse in the segments and at the bight where the bights are joined 
with the segments is parallel to the direction that the air and water 
flows externally through the coil assembly. 
The coil assembly of FIG. 8 includes an array of tubes 98 having segments 
100 and bights 102. Bights 102a, 102b and 102c of tubes 98a, 98b and 98c, 
respectively, are in contact with each other. Bights 98 have a cross 
sectional dimension X.sub.3. Segments 100 have a cross sectional dimension 
Y.sub.3. The distance D.sub.3 between adjacent tubes 98, such as the 
distance between the centerlines of tubes 98a and 98b or tubes 98b or 98c 
substantially equals the dimension X.sub.3, since the bights are in 
contact. Thus, the distance between the centerline of segments of adjacent 
tubes on the same level, namely segments 100a and 100c, equals two times 
D.sub.3 which equals 2 times X.sub.3. The space S.sub.3 between adjacent 
segments at the same level, namely the space between segments 100a and 
100c, equals 2X.sub.3 -Y.sub.3, which is greater than X.sub.3. 
Although the segments may be flattened as indicated in FIG. 8 to almost any 
extent, as a practical matter, due to the trade off between performance 
which may be adversely affected by restricting the flow of the internal 
heat transfer fluid inside the tubing of the coil assembly and the 
increase in performance by increasing turbulence within the tubes and the 
increased water coating and airflow externally through the coil assembly, 
the dimension Y.sub.3 should be no less than 0.5 times the dimension 
X.sub.3. Preferably, the dimension Y.sub.3 equals 0.8 times X.sub.3. These 
are the same preferred relationships which applied with respect to the 
embodiment illustrated in FIG. 5. Thus, in the presently preferred 
embodiment of FIG. 8, Y.sub.3 equals 0.8 times X.sub.3. 
By using tubes having an elliptical cross sectional shape, such as that 
illustrated in FIG. 8, the effect is to provide even more tubes with even 
more total surface area to be built into a coil assembly having a given 
total plan area. In the past this was thought to be impossible and 
impractical and is contrary to the teachings of U.S. Pat. No. 4,196,157. 
However, this invention has made it both possible and practical to 
achieve. This feature of additional surface area would be particularly 
useful in applications demanding more surface area, such as laminar flows 
or intermittent dry operation cycles. 
FIG. 9 illustrates yet another embodiment of a coil assembly according to 
the present invention in which the major axis of the elliptical segments 
of the tubes are angled with respect to the flow direction of the external 
heat exchange fluids passing through the coil assembly. FIG. 9 illustrates 
a particular preferred embodiment of such a coil assembly having angled 
elliptical segments, in which the major axes of the elliptical segments on 
adjacent tubes at different levels are angled in opposite directions with 
respect to each other and with respect to the vertical plane, which 
represents the most common flow direction for the external air and water 
through the coil assembly. 
The coil assembly of FIG. 9 includes tubes 108 having segments 110 and 
bights 112. The tubes in the area of the bights, and particularly in the 
areas where the bights join the segments, may have any suitable cross 
sectional shape, but a circular cross sectional shape is illustrated in 
FIG. 9. Bights 112a, 112b and 112c of tubes 108a, 108b and 108c, 
respectively, are in contact with each other. The tubing has a diameter or 
cross sectional dimension X.sub.4 in the area of the bights particularly 
where the bights join the segments. The angled elliptical segments at the 
same level, for example segments 110a and 110c, are spaced apart a greater 
distance than the diameter or cross sectional dimension X.sub.4 of the 
bights. Y.sub.4 is the cross sectional dimension of the angled elliptical 
segments 110. 
As in the other embodiments of the present invention, the distance D.sub.4 
between the centerlines of adjacent tubes 108, such as the distance 
between the centerlines of tubes 108a and 108b or tubes 108b and 108c 
equals the distance X.sub.4. The distance between the centerlines of 
segments of adjacent tubes at the same level, namely, the distance between 
the centerlines of segments of 110a and 110c, is two times D.sub.4. The 
space S.sub.4 between segments of adjacent tubes at the same level, 
namely, between segments 110a and 110c is 2X.sub.4 -Y.sub.4. 
The major axes of the elliptical segments can be angled up to 45 degrees on 
either side of a vertical plane corresponding to the flow direction of the 
external fluids through the coil assembly. Angles of up to 40 degrees on 
either side of the vertical plane are preferred, such that the angle of 
the major axis of elliptical segment 110a may be at 40 degrees, while the 
angle of the major axis of elliptical segment 110b of the adjacent tube is 
at 320 degrees from the same vertical plane. 
As the major axes of the elliptical segments of the tubes are oriented at 
greater angles approaching right angles away from the vertical plane, they 
will cause increased turbulence in the air and water flows. The angled 
segments present more tube surface area to the air and water flow streams, 
but they also reduce the space S.sub.4 between segments at the same level 
and may restrict the airflow. It is believed that the trade-off between 
the improved turbulence and the reduced airflow would be favorable as long 
as the space S.sub.4 is maintained greater than X.sub.4. 
The present invention may be embodied in other specific forms without 
departing from the spirit or central attributes thereof and, accordingly, 
reference should be made to the appended claims, rather than to the 
foregoing specification as indicating the scope of the invention.