Heat exchanger slab assembly having improved condensate retaining features

A condensate retaining apparatus for use with a refrigeration heat exchanger having at least two plannar slabs mounted in an air duct. Each slab has an upper and lower end and forms an oblique angle with respect to the direction of air flow and further includes a coil for conducting a fluid and a plurality of form defining channels for conducting condensate toward the lower end of the slab. The slabs are mounted so that the lower end of each overlying slab is offset inwardly from the upper end of the nearest underlying slab with each pair of adjacent slab ends defining an apex of the heat exchanger. The apex is covered to prevent condensate from being entrained in that air flow and as well as being shielded from the air flow.

BACKGROUND OF THE INVENTION 
The present invention relates to heat exchangers and fan coils of the type 
used in refrigeration and air conditioning systems, and is directed more 
particularly to a heat exchanger slab assembly that has improved 
condensate retention properties. 
All refrigeration and air conditioning systems intake relatively warm air 
that has an unknown moisture content and discharges air at a reduced 
temperature. In the process, intake air is passed over fan coils or other 
heat exchangers which carry refrigerant liquids, such as ammonia or water, 
which have a temperature lower than that of the intake air. As this 
occurs, the moisture in the air condenses on the fins of the fan coils or 
heat exchangers, and forms droplets of water that eventually become large 
enough to flow under the force of gravity. This condensate water then 
flows along the surface of the fins until it reaches a pan or tube from or 
through which it can be drained off. 
An important consideration in the handling of condensate is the need to 
prevent it from being blown off of the fins and entrained in the air 
flowing out of the heat exchanger or fan coil. This is because such 
entrained moisture flows through the duct system of the space to be 
cooled, where it can cause moisture damage, rot and mildew. The problem of 
preventing condensate from flowing off of the fins of heat exchangers and 
fan coils is complicated by the fact that, in order to provide the maximum 
possible surface area for heat exchange, heat exchangers and fan coils are 
often made up of two or more generally planar heat exchanger 
subassemblies, commonly referred to as slabs, which have their planes 
oriented obliquely with respect to the direction of air flow and which, 
together, occupy the height and width of the duct within which they are 
located. In one configuration, known as an "A coil", two slabs are formed 
into an A or V shaped slab assembly the apex of which points either 
upstream into or downstream from the air flow. In another configuration, 
known as an "N coil" three slabs are formed into and N or Z shaped slab 
assembly having a first apex that points upstream and a second apex which 
points downstream. 
The retention of condensate in multi-slab slab assemblies is relatively 
straightforward in heat exchangers in which the slabs are mounted 
vertically with one slab behind another, i.e. in fluidic series with one 
another with respect to the air flow through the duct. This is because, in 
such slab assemblies, condensate that flows down the fins of such slabs 
under the force of gravity remains in parallel streams which do not cross 
from slab to slab and which empty into a common catchment tray and from 
there directed into a drain for disposal. The problem with this vertical 
orientation is that the downstream ones of the slabs are in the thermal 
shadow of the upstream slabs and therefore exchange heat less efficiently. 
In the case of multi-slab heat exchangers in which the slabs are mounted 
horizontally with one slab above or below another, i.e., in parallel 
relationship with respect to the air flow through the duct, the processing 
of condensate is more difficult. This is because condensate flowing along 
the fins of such slab assemblies under the force of gravity flows in 
streams that must cross from one slab to another before reaching their 
catchment tray, and because condensate is more easily blown off of the 
slabs as it crosses from one slab to another, i.e., when it is in 
proximity to the apexes of such slab assemblies. 
Prior to the present invention, the problem of retaining condensate within 
horizontally oriented slab assemblies was dealt with in one of two ways. 
One of these was to include splitter plates between adjacent slabs. These 
splitters served to intercept and collect condensate that was blown off of 
the overlying slabs at the apexes of the slab assembly and direct it 
downwardly onto the underlying slab thereof. The problem with this 
solution is that condensate flowing along the surface of the splitter 
moves toward the edges of the underlying slab, where it is directed into a 
relatively small number of the fins thereof. Once there, it causes the 
condensate carrying capacity of these outer fins to be exceeded, thereby 
allowing condensate to blow off of the slab assembly and become entrained 
in the air flow leaving the heat exchanger. 
Another solution to the problem preventing condensate blow off included the 
provision, for each overlying slab, of a separate, direct drainage path to 
the common catchment tray. This solution had the advantage that it 
eliminated the need for slab-to-slab flow of condensate, but it made the 
slab assembly and the heat exchanger of which it was a part considerably 
more complex and expensive. This complexity and expense is compounded by 
the fact that, for safety reasons, all drainage paths must be provided 
with a parallel, redundant drainage path that protects the building in 
which it is used from being flooded as a result of the blockage of any 
single drainage path. 
Thus, prior to the present invention, there has existed a need for a 
condensate retaining apparatus which is effective, but which also is both 
simple and inexpensive. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, there is provided an improved 
condensate retaining apparatus which is both simple and inexpensive, and 
which provides the desired condensate retention without using splitter 
plates, and without requiring that individual drainage paths be provided 
for each slab. 
Generally speaking, the present invention is based in part on the discovery 
that, if it can be assured that the flow of condensate from an overlying 
slab can be distributed relatively evenly across among the channels 
defined by the fins of the adjacent underlying slab, blow off can be 
prevented without providing separate, individual drainage paths for each 
slab. This is because, if this condition is met, the channels of the 
underlying slab are prevented from having their condensate carrying 
capacities exceeded and thereby allowing condensate to escape from the 
slab assembly as a whole. The present invention is also based in part on 
the discovery that the desired relatively even distribution of the flow 
from an overlying slab to an underlying slab can be assured by so 
positioning the slabs that the lower tips of the fins at the lower end of 
each overlying slab are offset with respect to the upper tips of the fins 
at the upper end of the adjacent underlying slab. Because of this offset 
positioning, the force of gravity is made to oppose the tendency of 
condensate to blow off of the slab assembly as it flows from one slab to 
another. 
In the preferred embodiment, this positional relationship is established by 
introducing a suitable offset between the longitudinal and vertical 
positions of the ends of the slabs that lie at the apexes of the slab 
assembly. As used herein, the terms "longitudinal" or "inward" and 
"outward" refer to directions that lie along the direction in which air 
flows through the passage in which the heat exchanger is located. The 
terms "vertical" or "upward" and "downward" refer to those directions that 
are perpendicular to both the direction of air flow, and to the surface of 
the earth, while the terms "horizontal" and "lateral" refer to those 
directions that are perpendicular to the direction of air flow, but 
parallel to the surface of the earth. 
In both preferred and non-preferred embodiments, the condensate retaining 
apparatus of the invention includes suitable apex end covers, shields or 
similar structures for preventing condensate from blowing off of the slab 
assembly at any of the numerous, horizontally distributed points at which 
condensate flows downwardly out of channels defined by the fins of the 
overlying slab and into the channels defined by the fins of the underlying 
slab. These structures have cross-sectional shapes that conform to the 
cross-sectional shapes of the apexes which they cover, including their 
respective longitudinal and vertical offsets, and cover at least the 
portions of the slab ends that are in immediate proximity to the locations 
at which condensate flows from slab to slab. These structures also 
preferably have end portions which extend to and over at least the 
fluidically downstream or trailing edges of the slab ends, thereby 
eliminating the tendency of condensate to become trapped along these edges 
by the pressure of the air flowing thereby. In slab assemblies through 
which air must be able to flow bidirectionally, these end portions 
preferably extend to and over both the leading and trailing edges of the 
slab ends. 
The offset positioning contemplated by the present invention is compatible 
with a variety of different spatial relationships between the ends of the 
slabs which define the apexes of the slab assembly. The adjacent slab ends 
may, for example, be in such critical proximity to one another that the 
tips of the fins of the overlying slab are in contact with the upper edges 
of the respective fins of the underlying slab, a spatial relationship 
which facilitates and optimizes the retention of condensate during slab to 
slab transfer. The adjacent slab ends may, on the other hand, be so 
positioned that the fins of the over and underlying slabs become 
interleaved, a spatial relationship which also facilitates and optimizes 
the retention of condensate during slab to slab transfer. Other, less 
critical spatial relationships, such as those in which a substantial air 
gap exists between the fins of the over and underlying slabs may also be 
used, however, provided that the offsets of the present invention are 
used. It will be understood that all of these spatial relationships, and 
those variants thereof that would be apparent to those skilled in the art, 
are within the contemplation of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1A, there is shown the blower portion 10A of a heating 
and cooling unit (not shown) having an inlet 10A1 which is connected to 
receive intake air through an air inlet duct 12A, and an air outlet 10A2 
which is connected to discharge air through an air outlet duct 14A. 
Connected between blower intake 10A1 and inlet duct 12A is a heat 
exchanger 20, commonly referred to as a "furnace coil", through which air 
flows longitudinally from right to left, as indicated by the arrow labeled 
F in FIG. 1A. When used in the location shown in FIG. 1A, heat exchanger 
20 is commonly described as being in the "horizontal furnace right" 
position. The blower, ducts and heat exchanger shown in FIG. 1B are 
similar to those shown in FIG. 1A, like functioning parts being similarly 
numbered (except for a difference in postscript), except that heat 
exchanger 20 is the position commonly described as "horizontal furnace 
left". Because heat exchanger 20 operates in the same way, without regard 
to the direction of air flow therethrough, the heater exchangers of FIGS. 
1 A and 1 B are identical, as indicated by the absence of a postscript 
after the label 20. 
When the heating and cooling unit operates in its heating mode, heat 
exchanger 20 is unused and no refrigerant is pumped therethrough. When the 
heating and cooling unit is operating in its cooling mode, however, a 
refrigerant liquid such as water or ammonia is pumped through heat 
exchanger 20 to cool the air entering the blower unit. As this air is 
cooled, a considerable amount of moisture condenses therefrom. In order to 
retain and drain off this condensed water or condensate, heat exchanger 20 
makes use of the offset construction of the invention. This offset 
construction will now be described with reference to FIGS. 2 , 3 and 5. 
Referring to FIG. 2, there is shown an enlarged, oblique exterior view of 
heat exchanger 20 of FIGS. 1A and 1B, shown as it looks when not mounted 
within a duct system such as that shown in FIG. 1. As shown in FIG. 2, 
heat exchanger 20 includes an open ended flow through housing 22 having an 
intake opening 22 and an outlet opening, not visible in FIG. 2. Housing 22 
encloses a heat exchanger slab assembly 30, best shown in FIG. 3, which 
includes three finned heat exchanger slabs 31, 32, and 33, which are 
generally planar in shape and have planes that form oblique angles with 
the direction of air flow shown by arrow F. Each of these slabs includes a 
pair of spaced apart redundant refrigerant coils (not shown) which are 
surrounded by an array of parallely disposed, closely spaced fins, which 
are often referred to collectively as a fin pack. The upper edges of a few 
representative ones of the fins of the fin packs of slabs 31 and 33 are 
labeled 31F and 33F, respectively in FIG. 2. As will be explained more 
fully later, the invention is suitable for use with slab assemblies having 
two or more slabs which are disposed above or below one another. 
Slab assembly 30 of FIG. 2 also includes a catch pan 34 which lies below 
slabs 31-33, and serves to catch and collect condensate running downwardly 
through and from these slabs and direct the same to a pair of redundant 
drain lines 35 and 36, the latter of which is located slightly above the 
other for safety reasons. Refrigerant is supplied to the two redundant 
coils of each of the slabs through a refrigerant input line 37 and through 
a refrigerant distribution head and piping not visible in FIG. 2. 
Refrigerant flowing out of the slabs is directed into a refrigerant output 
manifold 38, best shown in FIG. 3, from which it exits housing 22 through 
a pipe 39. 
Referring to FIG. 3, there is shown a side view of slab assembly 30, from 
which the refrigerant inlet system and the coil ends have been omitted for 
the sake of clarity. In FIG. 3 slabs 31 through 33 are seen end on, and 
are shown in dotted lines because they lie behind end plates that are used 
to secure them to the support structure or frame 40 which holds them in 
the desired positions relative to one another and relative to catch tray 
34. This support structure includes a number of vertical plates, such as 
plate 41, and connecting straps, such as strap 42, which are connected 
together by bolts or other suitable fasteners. Since the support 
structures of slab assembly 30 are of a type familiar to those skilled in 
the art, they will not be further described herein. 
Slab assemblies having configurations of the general type shown in FIG. 3 
are referred to as N coils because they include three slabs which are 
disposed one above or below the other in a generally N shaped 
configuration with their ends defining two apexes A1 and A2. The present 
invention may, however, be practiced with any number of slabs that is 
greater than two and includes at least one apex. It may, for example, 
include two slabs which are arranged in an A or V shaped configuration and 
have a single apex, as shown in FIG. 4. It may also include four slabs 
which are arranged in a W shaped configuration and have three apexes. 
As condensate forms on the fins of slab assemblies such as that shown in 
FIG. 3, it flows downwardly along the slab through channels defined by 
adjacent pairs of the fins thereof. For each overlying slab, such as slab 
33, this downward flow continues until the condensate reaches the 
lowermost end thereof, in this case end 33L. As it flows off of this end 
it enters the immediately adjacent underlying slab, slab 32, at the 
uppermost end 32U thereof. It then flows downwardly along slab 32, where 
it merges with the liquid which first condensed on that slab, and then 
flows toward the lowermost end 32L thereof. This flow then continues from 
slab to slab, growing ever larger in magnitude, until it eventually 
reaches catch tray 34 and flows out of the heat exchanger for disposal. 
Within each slab, there is little tendency for the condensate that first 
condenses on that slab to flow laterally across the slab. This is because 
the fins define channels that disfavor flow in that direction. As 
condensate flows from an overlying slab to an underlying slab, however, it 
may not do so uniformly across the width of the slab. If such a lateral 
non-uniformity of flow does occur for any reason, the condensate carrying 
capacity of the channels between the fins can be exceeded, causing 
condensate to be blown off of the slab assembly. Condensate can also be 
blown off of the slab assembly at the apexes, where condensate must cross 
from one slab to another. Since condensate that blows off of the slab 
assembly can become entrained in the flow of air through the blower and 
cause damage to the space to be cooled, it is important to prevent blow 
off from occurring. 
Prior to the present invention, one approach to the problem of retaining 
condensate within a heat exchanger involved providing each slab with its 
own separate drain path and thereby preventing slab to slab flow 
altogether. This approach was relatively complex and costly, however, 
particularly since safety considerations require that all drain paths be 
made redundant. Another approach involved equipping each apex of a slab 
assembly with apex baffle and splitter plates. An example of an A coil 
that is equipped with an apex baffle plate and a splitter plate is shown 
in FIG. 4. 
Referring to FIG. 4, there are shown an overlying slab 45 and an underlying 
slab 46 which are mounted in conventional positions relative to one 
another, i.e., with the uppermost and lowermost tips of their fins aligned 
and almost touching at an apex B. As is best seen in the simplified 
exploded view of apex B that is shown in FIG. 4A, the ends of plates 45 
and 46 are separated by a splitter plate 47 and covered by a baffle plate 
48. In operation, baffle plate 48 serves to prevent condensate from being 
blown off of the slab ends, and splitter plate 47 serves to receive the 
condensate flowing off of overlying slab 45 and distribute the same over 
the upper surface of underlying slab 46. 
In spite of the apparently foolproof character of the above-described 
design, it does not solve the problem of condensate blow off. This is 
because, as was discovered during the making of the present invention, the 
air flowing through the slab assembly of FIG. 4 tends to flow toward the 
lateral ends of the slabs. This causes more condensate to be directed into 
the channels between the fins at the ends of the underlying slab than into 
the channels between the fins in the interior of that slab. This, in turn, 
caused the condensate carrying capacity of the endwardly disposed channels 
to be exceeded and resulted in condensate blow off It also had the 
undesirable effect of making an inefficient use of the underlying slab 
because it allowed a substantial portion of the heat transfer capacity of 
that slab to be unused. 
In accordance with the present invention, it has been discovered that the 
blow off characteristics of slab assemblies can be improved by eliminating 
the splitter plate entirely and introducing a longitudinal and/or vertical 
offset between the slab ends of each apex of the slab assembly. By 
introducing these offsets, and by positioning adjacent slabs in relative 
proximity to one another, as shown in FIGS. 3 and 5A through 5C, there is 
created a condition under which the force of gravity causes the condensate 
flowing from one slab to another to distribute itself approximately 
uniformly across the entire widths of the slabs. This tendency of the 
condensate to distribute itself uniformly has been found to be 
sufficiently strong that it is able to overcome the force of the air flow 
through the heat exchanger, which tends to concentrate condensate flow 
towards the lateral ends of the slabs. 
Referring to FIG. 5A, which is an enlarged, simplified fragmentary view of 
apex A2 of FIG. 3, there is shown an apex that is constructed in 
accordance with the preferred embodiment of the slab assembly of the 
invention. Apex A1 will be understood to be identical to apex A2, except 
that it faces in a direction opposite to that of apex A2. Since the effect 
of these apexes on the flow of condensate from slab to slab is similar, 
only one of these apexes will be described in detail herein. As shown in 
FIG. 5A, overlying slab 32 is displaced with respect to underlying slab 31 
by a longitudinal offset L2 and by a vertical offset V2. Longitudinal 
offset L2 should be large enough to assure that condensate which flows off 
of slab end 32L encounters a downwardly sloping gradient that extends for 
a fluidically significant distance both outwardly and inwardly of the 
point at which the slabs are closest to one another. As used herein, the 
term "inward" means the longitudinal direction that extends toward the 
center of the slab assembly as a whole, and the term "outward" refers to 
the longitudinal direction that extends away from that center; neither 
term is related to the direction of air flow through the heat exchanger. 
It will be noted in this connection that the overlying ones 33 and 32 of 
the slabs at apexes A1 and A2, respectively, are both offset inwardly with 
respect to their underlying slabs 32 and 31, respectively. 
While the size of the offset L2 can have a variety of different values, its 
most suitable values are those which are in the range of from about 20% to 
about 80% of the height H of the fin packs of the slab assembly. In 
embodiments of the type shown in FIG. 5A, i.e., embodiments in which the 
tips of the fins of the adjacent fin packs are in actual or very near 
contact, vertical offset V2 is a derivative quantity the magnitude of 
which is determined by the size of longitudinal offset L2 and the angle 
between the slabs and the direction of air flow. 
Because the ends of the slabs are defined by an array of parallely disposed 
fins having individual positions that vary somewhat from slab to slab, it 
is not to be expected that the fins of adjacent slabs will be in actual 
contact with one another or even be in registry or aligned with one 
another. It has been found, however, that no such actual contact between 
or registry of the fins is necessary to produce the results contemplated 
by the present invention. As a result, the slab assembly of the invention 
may be manufactured easily and quickly without the necessity of 
maintaining precise alignments or tight tolerances either within the slabs 
or between the slabs. 
Also included in the apex of the embodiment of FIG. 5A is a plate-like apex 
cover or shield 50 which has a generally zig-zag shaped cross-section and 
extends across at least that part of the slab assembly where the adjacent 
slabs are in immediate proximity to one another. Shield 50 serves as a 
physical barrier that prevents any condensate that manages to escape from 
the point of transfer P between the slabs from being blown off of the slab 
assembly and entrained in the flow of air through the heat exchanger. As a 
practical matter, however, shield 50 serves the function of a backup since 
the relative locations of the slab ends are themselves highly effective in 
retaining condensate within the slab assembly. In the preferred 
embodiment, shield 50 extends not only across the parts of the slab 
assembly that are in proximity to point of transfer P, but also up to and 
over the non-adjacent or distal edges 31 D and 32D of slabs 31 and 32, 
respectively. This coverage of the distal edges is desirable because, in 
the absence of coverage, these distal edges can act as fluidic cul de sacs 
from which condensate must flow against the force of the air flow through 
the heat exchanger in order to ultimately reach the catch tray. Thus, 
shield 50 ideally has a shape that conforms to the shape of the apex with 
which it will be used and extends to and over the non-adjacent edges of 
the slabs with which it is used. 
Referring to FIG. 5B, there is shown an enlarged fragmentary view of the 
apex of a second embodiment of a slab assembly constructed in accordance 
with the present invention. This embodiment is similar to that of FIG. 5A, 
like functioning parts being similarly numbered, except that its vertical 
offset V2' is sufficiently larger than that of the embodiment of FIG. 5A 
that the overlying and underlying slabs overlap one another and establish 
an interleaved relationship between their respective fins. This embodiment 
operates in generally the same way as the embodiment of FIG. 5A, but has 
the advantage that it allows much of the condensate from an overlying slab 
to be introduced or injected into the interior of the underlying slab 
before it loses contact with the overlying slab, thereby positively 
preventing condensate from escaping from the slab assembly as it flows 
from slab to slab. Because the fins of this embodiment must be properly 
aligned during assembly, however, this embodiment is more difficult to 
assemble. As a result, the embodiment of FIG. 5B is not the preferred 
embodiment of the present invention. 
Referring to FIG. 5C, there is shown an enlarged fragmentary view of the 
apex of a third embodiment of the slab assembly of the invention. This 
embodiment is similar to those of FIGS. 5A and 5B, like functioning parts 
being similarly numbered, except that its vertical offset V2" is 
sufficiently smaller than that of FIG. 5A that an open space or gap is 
created between the overlying and underlying slabs. This embodiment 
operates in generally the same way as the embodiments of FIGS. 5A and 5B, 
but has the advantage that it permits the use of slab components that have 
looser tolerances, and can be more easily assembled than either of the two 
already described embodiments. It does, however, have the disadvantage 
that it makes the flow of condensate from the overlying slab to the 
underlying slab more subject to being affected by the force of the air 
flowing through the heat exchanger, particularly if the rate of flow of 
the condensate is small enough and the separation between the slabs is 
large enough to allow droplets of condensate to form in the gap. As a 
result, the maximum size of gap that permits the slab assembly to operate 
in the manner contemplated by the present invention cannot be stated in 
absolute terms; it must be determined on an application by application 
basis. 
The embodiments of FIGS. 5A through 5C are the principal embodiments for 
slabs that have fins with rectangular shapes. If the fins of the slabs 
have non-rectangular shapes, e.g., parallelograms or closed figures having 
more than four sides, many additional embodiments of the invention are 
possible. Simplified representations of a few of the additional 
embodiments that are made possible by non-rectangular fin shapes are 
included in FIGS. 6A through 6D. Because these embodiments are so similar 
in principle to those already described, they will not be individually 
described herein, but will nevertheless be understood to be within the 
contemplation of the present invention. 
While the present invention has been described with reference to a number 
of specific embodiments, it will be understood that the true spirit and 
scope of the present invention should be determined only with reference to 
the appended claims.