Note: Descriptions are shown in the official language in which they were submitted.
113346~
HEAT EXCHANGE SYSTEM
Back round of the Invention
g
This application is a division of Cdn. Ser. No. 359,709,
filed September 5, 1980, which is a division of Cdn. Ser. No.
273,487, filed March 8, 1977.
Ventilating systems for enclosed spaces in which a heat
exchanger is placed between the supply air stream and the exhaust
air stream to take advantage of the temperature difference, for
the purpose of conserving the energy required for either heating
or cooling, are known. For example see U.S. Patents No.
1,725,906; 2,206,858; and 2,019,351. Carrying the concept a step
further, it is even known that heat pipes may be used in such
heat exchange systems as described and illustrated in U.S. Patent
No. 3,788,388.
Such known systems, however present various problems.
For example, most of these systems include a plurality of straight,
individually sealed and charged pipes which carry a working fluid.
Such a system is expensive and complicated. The problems are even
further magnified if it is desired to control the rate of heat
recovery in response to the temperature of the ventilated space.
This is known as "modulating". Heat pipe devices normally
transfer heat at a rate solely proportional to the temperature
difference between warm and cold ends. Modulating this heat
transfer rate has required complicated apparatus.
Further, when the supply air reaches a temperature below
a prescribed point, the outgoing air stream will reach a temper-
ature where moisture will precipitate and condense on the pipes.
If the surface temperature of the pipes is below freezing, a
forming and buildup of frost thereon will occur with undesirable
results.
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SUMMARY OF THE INVENTI~N
In this divisional application, the invention
broadly pertains to a heat exchange system of the type includiny
a plurality of rows of heat exchange tubes containing a working
fluid of the type having a liquid phase and a vapor phase within
system operating temperatures, and wherein the heat exchange
tubes have one end thereof in communication with a supply air
stream and the other end thereof in operative communication
with an exhaust air stream. The improvement comprises a control
means for deactivating some of the heat exchange
tubes, the control means being operated in response to the
buildup of frost on certain of the heat exchange tubes to
selectively deactivate at least one selected row, but not all
rows, of tubes in the system.
In one preferred embodiment the control means includes
a pressure switch in communication with the working fluid in
each of the rows of heat tubes, the pressure switch deactivating
the corresponding row periodically when the pressure of the
working fluid falls to a prescribed level.
In another preferred embodiment the control means
comprises a pressure switch in operative communication with the
exhaust air stream, the pressure switch being activated to
deactivate selected rows of heat tubes in response to a pressure
drop of a prescribed level in the exhaust air stream.
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More particularly, the present invention as disclosed
is directed to a heat exchange system applicable for use with a
heated and/or cooled enclosed space, hereinafter referred to as
the "enclosed space", in which ventilation is also required, such
as, for example commercial and industrial buildings. The system
described herein includes one or more banks of heat exchange pipes,
each bank including a plurality of horizontally extending, verti-
cally spaced heat exchange pipes with one end in contact with a
supply air stream and the other end in contact with an exhaust
air stream. In each bank the individual pipes or pipe sections
are connected by U-shaped end sections, so that the entire bank
forms a continuous, sinuous passageway. The end sections of each
pipe are of a smaller effective diameter than the central portions
thereof, and are preferably offset upwardly from the central axis
thereof, so that a weir or dam is formed to retain a specified
amount of working fluid in each pipe section. mis results in
considerable economies in that each individual pipe does not have
to be charged and maintained separately. It also permits the
system to be more compatible with a control means as will be dis-
cussed hereinbelow.
The working fluid within the pipe is of a type whichabsorbs heat from the warmer of said air streams thus effecting
vaporization of the fluid, the vaporized fluid flowing to the
cooler end of the tube (due to a slight pressure differential
between the warm and cold end). As the vaporized fluid gives up
heat to the cooler air stream, it condenses in the tube and flows
back to the warm end by gravity again to be vaporized, thus
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completing the cycle. This is known commonly as a "thermosiphon
system". A novel control means includes a working fluid reser-
voir which is connected to the supply end of the lowest tube of
each bar.k and in which the working fluid begins condensing and
collecting as soon as the surface temperature of the reservoir is
cooler than the working fluid in that bank.
It is known that heat pipes can be "turned off" by
keeping the working fluid out of contact with the wicking portion
of a heat pipe. The present invention accomplishes this by means
of the selectively heated reservoir which is positioned in the
supply stream, which in cold weather is the coolest point of the
system. In cold weather, the working fluid naturally tends to
migrate to the reservoir, deactivating the heat pipes. As long
as the reservoir is heated, working fluid returns to the heat
pipes, reactivating them.
In warm weather, when supply air is warmer than exhaust
air, the reservoir stays empty of li~uid, and the heat exchange
tubes stay fully active. During this time the enclosed space
must, of course, be refrigerated to benefit from this systeln.
During intermediate weather seasons, particularly in
commercial office buildings, hospitals, and schools, the more
moderate daytime temperatures combined with the heat caused by
sunshine on the windows and the interior lighting create con-
ditions in which the heat exchange system must often be operated
at less than full capacity. A modulated control, where the heat
exchange rate varies gradually, is preferable to a system which
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is either completely "on" or completely "off". The present
invention accomplishes such a modulated control simply, with a
minimum of expense, and without moving parts as in other known
systems. Merely turning the reservoir heating coil on or off
causes the system to gradually increase or decrease in efficiency
in a surprisingly simple and economic manner.
To remove the f~ost mentioned hereinabove, which ac-
cumulates in cold weather on the last few rows or banks of the
exhaust air stream, these rows are warmed up at regular intervals,
or responsive to prescribed conditions, and the frost melted.
Toward this end, such rows of the system are merely deactivated
for a brief prescribed period during which very little hea~ is
transferred, so that the overall heat recovery effectiveness of
the unit drops causing the temperature on the exhaust side of
the rows with frost thereon to rise. The frost will begin to
melt. As soon as the frost is melted or after a prescribed
time period, the deactivated rows are reactivated and the unit
resumes full recovery.
It is therefore an object of the present invention to
provide a heat exchange system which utilizes heat exchange tubes
arranged in a new and novel way.
It is further an object of the present invention to pro-
vide a system of the type described wherein the liquid level of
the wor~ing fluid in each horizontal tube of the system may ba
controlled.
~ 33464
It is yet a further object of the present invention to
provide a heat exchange system of the type described wherein the
operation of the system may be controlled responsive to the tem-
perature in a space served by the supply air stream.
Another object of the invention is to provide a heat
recovery system which may be modulated during cool weather to
operate at less than full efficiency in a simple, economic, and
novel manner.
Still another object of the present invention is to
provide a heat exchange system of the type described which includes
an auxiliary frost control system for operating one or more banks
of heat exchange tubes responsive to the development of frost
forming conditions.
Other objects and a fuller understanding of the present
invention will become apparent from reading the detailed descrip-
tion of the preferred embodiment along with the accompanying
drawings in which:
Figure 1 is a perspective view of the heat exchany~
unit with certain parts broken away for the sake of clarity;
Figure 2 is a sectional view taken substantially along
lines 2-2 in Figure 1, and illustrating a cross section of one
of the heat exchange tubes;
Figure 3 is a schematic representation of a heat ex-
change system without the frost control unit, illustrating }-ow
frost can build up on the heat exchange tukes in the exhaust
path;
1~33464
Figure 4 is a schematic representation similar to
Figure 3, except showing the resulting system after the frost
control circuit is activated; and
Figure 5 is an electrical schematic illustrating the
defrost control for the heat recovery coils, with Figs. 1 and 2.
Referring now to the drawings, and particularly to
Figures 1 and 2, there is shown a heat exchange system or apparatus
10 which includes a plurality of banks 12,14 and 16 of heat ex-
change tubes extending between a supply air conduit 18 and an ex-
haust air conduit 20. Each bank 12, 14 and 16 as illustrated in
Figure 1, includes a plurality of vertically spaced, horizontally
extending, linear sections or tubes 22, 24, 26, 28 in heat exchange
relationship with fins 29 which extend transversely to the tubes.
It should be recognized that the banks may be arranged either in a
vertical plane/ or the top or bottom may be tilted upstream or
downstream as much as 80 from vertical for convenience in duct
installation, which change will have no effect on the operation
thereof.
One end of the upper linear section 22 is connected to
the corresponding end of the next lowest linear section 24 by a
U-shaped end section 30. The opposite end of linear section 24 is
connected to the corresponding end of linear section 26 by a
second U-shaped end section 32. Section 34 similarly connects the
other end of linear section 26 with the corresponding end of sec-
tion 28. A cap 36 seals the free end of the uppermost linear
section 22.
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~33464
As illustrated by Figures 1 and 2, the diameter of
end sections 30, 32 and 34 is smaller than the corresponding
diameter of linear sections 22, 24, 26, 28. Additionally, at
least at the exit end of the linear sections, the connectors
join the tubes at a point slightly above the axis of the linear
tube sections. The "exit end" of the linear sections are the
ends adjacent the upper end of the U-shaped connector through
which working fluid passes on its way to the next lowest linear
section. A series of weirs or dams is thus provided at the ends
of the linear sections, so that the level of the working fluid
may be controlled. It is preferable, as illustrated in Figure
2, that the liquid level be maintained at approximately 1/3 the
total capacity of each linear section. Obviously, other systems
of weirs or dams could be used, at differing levels, depending
on the amount of working fluid desired to be maintained in each
tube section. Thus, the ends of each pipe are of a smaller di-
ameter than the central portions thereof and at least at the
exit end thereof are offset axially thereabove, so that a weir
is formed to maintain a specified amount of working fluid in each
pipe,.while the overflow is conducted to the next lower tube
section. Such an arrangement results in considerable economies
in that each individual pipe does not have to be charged and
maintained. It also permits the system to be more adaptable to
a control means such as will be discussed hereinbelow.
At the end of the lowermost linear section 28, opposite
the end to which connector 34 is coupled, a reservoir ~0 of such
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li33~6~
capacity as to contain all of the working fluid in bank 12 is
connected by means of a liquid trap 42. Reservoir 40 is placed
in a shunt chamber 44 adjacent supply air conduit 18 through
which a small portion of the supply air stream is directed.
~eater coils 46 are positioned to heat reservoir 40 and are
controlled by a thermostat 48 placed in the "enclosed space".
Whenever supply air is cooler than exhaust air, the
reservoir for the working fluid is in the cooler air stream, and
the working fluid will tend to migrate into the reservoir. With-
out heating of the reservoir, it cannot escape back into thesystem. Thus the system may be completely deactivated after
heater coils 46 are deactivated for a period of time.
When the thermostat 48 in the enclosed space calls
for heating, as is normal in cool weather operation, heaters 46
on the reservoirs are activated. The working fluid will gradually
begin to vaporize, increasing the pressure within the reservoir
above that within the heat tubes, forcing the liquid into the
lowest heat tube where it vaporizes and redistributes to all
tubes. As long as heaters 46 are activated this thermosiphonic
condition is either at or building toward peak efficiency. When
the thermostat 48 within the "enclosed space" is satisfied,
heaters 46 are deactivated and the vaporized working fluid will
begin collecting in the reservoir, thus slowly decreasing the
efficiency of the system. As the temperature in the served space
again falls below the "set point", heater 46 will again be acti-
vated to increase efficiency. The working fluid collected in
1133464
the reservoir will begin working its way back into the system
along the sinuous path of the bank of heat tubes.
In reality, during cool or intermediate seasonal opera-
tion, the system will not operate at either full or mlnimal ef-
ficiency, but will fluctuate so that the temperature within the
served space will average at the "set point" (thermostat setting).
Heat exchange tubes will be neither completely full nor completely
empty. This is known as "modulated control" in which heat trans-
fer effectiveness reaches equilibrium at a point just sufficient
to ~atisfy the temperature requirement in the served space. The
present system is able to thus modulate the system through varying
rates of efficiency, rather than by an on-off system control.
Such modulation by other known means are more complicated and
expensive, and generally require moving parts. It should be
recognized that preferably each bank of heat tubes will be pro-
vided with its own reservoir and heater, so that each bank may
be individually controlled, and while some banks are operating,
others may be deactivated for frost control as discussed herein-
below.
The working fluid may be selected from any of several
types, such as, for example, fluorinated hydrocarbons, water,
glycol, Dowtherm (trademark of Dow Chemical). Also, it is pref-
erable that the inner walls of each heat exchange tube be provided
with small, peripherally extending circular or spiral grooves,
not to act as a longitudinal wick for the ~orking fluid, but
rather to spread the working fluid entirely around the wall at
the hot end of the heat exchange tube to facilitate vaporization,
--10--
and to provide a larger surface area at the cool end on which
the vaporized working fluid may condense and drain down the
inner wall to the fluid therebelow. Although not described
spéclflcally, the banks 14,16 of heat tubes are arranged and
operate similarly to the manner described for ban~ 12.
Turning now to Figures 3 and 4, there is provided a
frost control system, which may best be understood by consider-
ing the temperature and humidity o~ air as it flows through
the heat exchange system in cold weather. For example, Figure
3 show6 a unit operating wherein the supply is at 0F. and 80~
relative humidity. For purposes o~ this discussion it is assumed
that a heat recovery effectiveness of 70% is achieved.
It is well known that warm air can hold much more
moisture than cold air. For example, at 70F., saturated air
~100% relative humidity) contains about .016 pounds water per
pound of air, while at 0F., saturated air contains less than
.001 pounds water per pound air. Thus, air is cooled, its rela-
tive humidity increases, because the amount of water in the
air remains constant while the amount it could possibly hold
decreases.
Turning now to specifics, the exhaust air leaving the
enclosure at 70F., 40% relative humidity, reaches 100% relative
humidity when it is cooled to 45F. If that air is cooled
further, it becomes supersaturated, and moisture will precipitate
therefrom. In the example of Figure 3, moisture is precipitated
on the three banks of heat exchange tubes nearest the outside
in the exhaust air stream, since the air at those tubes is chilled
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~33464
to less than 45F. Thus moisture is condensed on the surfaces
of the fins and tubes it passes over in this area. On the other
side or in the supply air stream, as the cold air is warmed, it
decreases in relative humidity because its absolute water content
is unchanged while its moisture capacity at saturation increases.
Therefore the relative humidity of the supply air stream decreases
rom 80~ to 10~, even though no moisture is added or lost on the
supply air stream side.
Figure 3 illustrates the approximate air temperatures
which will exist between each row or bank of tubes during opera-
tion at certain typical conditions. The surface temperature of
the tubes in a given row will, because of the extremely effective
heat transfer of the heat exchange tube, be nearly uniform through-
out its length. The surface will reach e~uilibrium at a mean
value between the air temperatures it is exposed to. Thus, row
number 1, which is the upper row in Figure 3, will reach a mean
surface temperature of (0+8+32.2+37)/4=19.3F. Similarly the
surface temperature of rows 2 and 3 will be 25.5F. and 31.5F.
respectively.
As mentioned hereinabove, when the temperature of the
exhaust air stream falls below 45F., moisture will begin to
condense on the heat exchange tube surface. In observing Figure
3, it will be noticed that this will occur on rows 1, 2 and 3
on the exhaust side of the unit. Since these tubes are at a
mean surface temperature of less than 32F., the condensate will
freeze, or precipitate as frost, rather than water. The frost
will adhere to the tubes and gradually build up in thickness,
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1133464
increasing the air pressure drop through the coil and de~ra~in~
its heat exchange performance, since frost has considerable thermal
resistance.
In order to remove the frost which accumulates on rows
1, 2 and 3, it is necessary to warm up those rows at periodic
intervals, or stated otherwise, melt the frost which will quickly
drain off. To accomplish.this end, rows 1, 2 and 3 are deactivated
for a brief period, as explained hereinabove by turning off heaters
46 associated with these rows and by drawing off the working fluid
from the heat pipes into the reservoirs 40.
With rows 1, 2 and 3 deactivated, very little heat is
transferred in those rows, so that the overall-heat recovery
effectiveness of the unit drops to about 45~. The resultin~
temperature levels are shown in Figure 4. It can be seen that
42.8F. air will flow across rows 1, 2 and 3 on the exhaust side,
which is warm enough to melt the frost on those rows. As soon
as the frost is melted, the deactivated rows are reactivated by
turning on the heaters for the respective reservoirs, so that
the unit will resume heat recovery.
There are several possibilities for controlling the
deactivation of selected rows. For example, the heat pipe fluid
vapor pressure may be sensed by a sensor in each bank. For a
given working fluid, there is a fixed working relationship be-
tween vapor pressure and temperature. Thus, when the vapor
pressure corresponds to a temperature below 32F., a pressure
switch PSl, PS2, or PS3 (Figure 5) is activated. During the time
1133464
when one of pressure switches PSl, PS2, or PS3 is closed, a
time delay relay TR will periodically close the corresponding
set of contacts TRl, TR2, or TR3. Since contacts TRl, TR2, and
TR3 are arranged in parallel with pressure switch~s PSl, PS2,
and PS3 respectively, the closing of both switches for one bank
will deactivate the corresponding heater coil Hl, H2, or H3.
These heaters correspond to heater 46 in Figure l, and control
the working fluid. Time delay relay T~ controls t~le defrost
cycles for all rows, whereby if more than one row is calling for
defrost, all rows are defrosted at the same time. A circuit to
accomplish this possibility is illustrated in Figure 5. A typi-
cal timing cycle for a time delay relay TR might be two hours
closed, five minutes open.
Secondly, it is possible to sense the air pressure
drop across the bank of heat exchange tubes in the exhaust stream.
If the pressure drop increases above a preselected level, indica-
ting frost buildup, the first bank or banks (nearest the outdoor
side), are deactivated for a given length of time, or until the
pressure drop returns to a lower selected level. Deactivation
can be accomplished by opening a circuit to the electric heaters
46 on reservoirs 40, which permits cold outdoor ai~ to cool the
reservoir to the point that it collects the working fluid.
It is also possible to sense the heat pipe fluid tem-
perature or the surface temperature instead of pressure drop and
activate and deactivate the heating coil in response thereto as
a further means for controlling the defrost cycle.
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~133464
In the drawings, six banks of heat tubes are provided,
however this number can be altered as desired. Also it should
be understood that although a preferred embodiment of the in-
vention has been described in detail, other changes, alterations,
and modifications might be made without departing from the spiri~
and scope of the invention, which is defined by the following
claims.
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