For the past several decades, little has changed in the design of finned-tube,
forced-air heat exchangers. These exchangers, commonly called “coil,” are
routinely used in most HVAC/R products that are being produced throughout
the world. Most consist of copper tubing equipped with pressed-on, thin-plate
aluminum fins. The fluid is carried throughout the tubing as air flows
through the fins, enhancing the heat transfer exchange.
Most innovation in coil design, though modest, has occurred in the formation
of thin-plate aluminum fins. Such fins are provided with corrugations or
sine-wave forms, or have small piercings or lancings, to further embellish
heat transfer. Although these types of fin formations do increase heat
transfer, they also create air turbulence. The added air turbulence heightens
the air resistance, requiring more fan power. In some cases, such as with
lancings, fin formation tooling is more expensive to purchase and maintain.
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Figure 1. CLICK for
larger graphic. |
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For forced-air heat exchangers to operate more efficiently and
cost effectively, coils should ideally transfer (reject or absorb)
a maximum amount of heat, with a minimal amount of resistance to
both the air flowing through the fins and the fluid pumping through
the tubes. The greater the resistance, the more power that is needed
to move the air or fluid. Most of today’s coil designs reduce
air resistance primarily by decreasing the number of tube rows. |
A New Approach to Coil Design
Heat transfer is a function of many variables, one of which being the real-time
contact between two heat-exchanging media (air and fluid). The longer the
duration of real-time contact between the two, the greater the amounts
of heat transfer. Prior designs of forced-air, finned-tube heat exchangers
minimized the dwell time, or real-time contact between air and fluid, by
reducing the number of tube rows and/or by tightening up the tube spacing.
After years of extensive research, a new aluminum-finned, copper- tube
heat exchanger has been developed based on a unique configuration of tubes,
fins, rows, air velocity, and fin style. Its primary applications are as
an evaporator and condenser in air-conditioning and refrigeration products.
The deep-coil configuration takes an exact opposite approach to conventional
coil design by significantly enlarging the tube spacing, increasing the
tube rows, and drastically reducing fin density—all of which are
done concurrently.
Through this process, the conventional coil—comprising tight tube
spacing, high fin density, fewer tube rows, and shortened air path—is
transformed into a deep-coil configuration, comprising large tube spacing,
low-fin density, more tube rows, and a longer air path. As shown in Figure
1, both the air path and dwell time is doubled. Other adjustments, such
as further lengthening of the air path, increasing the dwell time, and/or
adding fin formations, can also be made to optimize the deep-coil’s
performance. Both the air path and dwell time can be increased even more
when developing the deep-coil configuration for commercialization to achieve
even greater energy efficiency.
Verifying Prototype Performance
As deep-coil prototypes were developed, comparative testing began in 1997
and as recently as May 2004, to determine the performance of deep-coil
configurations and equivalent conventional coils.
As shown in Figure 1, Coil A represents the prototype deep-coil configuration;
Coil B represents the equivalent conventional coil design. The tube spacing
in Coil A is 2-by-1 in, with a fin density of 6 fpi; whereas the tube spacing
in Coil B is 1- by 1-in, with a 12-fpi fin density. Both representations
used the exact quantity and quality of heat transfer material, as well
as the same circuitry. Other details not mentioned here but included in
Figure 1, such as the face area, tube thickness, etc., were also the same
between the test samples. Both samples were tested under the same conditions,
which were 3-gpm water flow, 135º F entering water, and 95º F entering
air.
Table 1 provides the actual heat transfer data. Coil A’s heat transfer
capacities are approximately 15 percent less than those of Coil B, most
likely attributed to deficiencies in the deep-coil prototype’s construction.
The tooling required for Coil A, namely the fin dies with 2- by 1-in spacing
and low-fin density, was not available at any coil shop. Therefore, Coil
A was constructed using patched-up subassemblies. Further, the fins in
Coil A did not have a full fin collar, as would be the case with proper
commercial tooling. Save for these deficiencies, the heat-transfer capacities
of the deep-coil configuration will very likely match that of conventional
coil. However, the deep-coil configuration still provides air resistance
that is half that of conventional coil.
| CFM |
Velocity |
COIL A (DEEP) |
COIL B (Conventional) |
| BTUs |
Air
Press. Drop, "wg |
BTUs |
Air
Press. Drop, "wg |
| 1,000 |
250 |
22,108 |
0.026 |
25,516 |
0.057 |
| 1,200 |
300 |
23,980 |
0.035 |
28,136 |
0.087 |
| 1,400 |
350 |
26,212 |
0.044 |
29,953 |
0.105 |
| 1,600 |
400 |
27,868 |
0.053 |
31,939 |
0.132 |
Table
1. Comparative test data for deep-coil prototype
and equivalent conventional coil
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Per the fan laws, fan BHP = (CFM x S.P. x Sp. Gr.)
/ 6356 x fan efficiency), the fan BHP is directly proportionate to the
air resistance.
Therefore,
if the air resistance can be reduced by half, so can the fan BHP, resulting
in a smaller-sized fan and motor in commercial HVAC/R products.
The deep-coil (Coil A) heat transfer capacities in Table 1 can be brought
up to the level of conventional coil in other ways as well. For example,
by changing Coil A’s fin density to 7 fpi, the equivalent of 14 fpi
in Coil B, the air resistance will increase disproportionately higher than
that of the heat transfer capacity. Based
on computer projections in Coil B, air resistance will increase by 15 percent,
while the heat transfer will increase only by 8 percent. The deep-coil
configuration’s open space format allows the increase in air resistance
to be far less than that in the heat transfer capacity, relative to the
conventional coil’s tight space format. Fin formations, such as corrugation
or sine-wave forms, can also increase deep-coil, heat-transfer capacity
to match or exceed that of Coil B, and still reduce air resistance by more
than 50 percent.
During prototype testing, only one of the two tube spacings was changed
from 1 in to 2 in, mainly because of tooling constraints. Also, potential
commercial optimization of the deep-coil configuration is not represented
in the test
data. Further optimization can be achieved by increasing one or both of
the tube levels beyond those used in the prototype test sample, or by concurrently
adjusting or reducing fin density and/or the tube row count to a suitable
level.
The basic difference between Coil A and Coil B is that the same amount
of heat transfer material is being distributed just as uniformly and evenly
in twice the amount of cubic feet of space, reducing air resistance. Because
the dwell time is also doubled, it will enhance heat transfer. Test data,
as illustrated in the Table 1, also shows evidence of enormous reductions
in air resistance, but fails to show any gains in heat transfer, most likely
caused by deficiencies in the deep-coil prototype’s construction.
When commercial tooling is used, it is expected that the heat transfer
capacities will match, if not exceed those used in conventional coil.
Potential Benefits
Although early comparative test data indicates clear reductions in air
resistance, it is fully expected that deep-coil configuration will reveal
other potential advantages to OEMs, once proper tooling is achieved, including:
- 20-
to 25-percent more heat transfer capacity for the same amount of
heat transfer surface and fan power, or more than 50-percent reduction
in fan
power for the same amount of heat transfer capacity and surface;
- smaller
footprint in some configurations;
- significantly reduced incidence
of moisture blow-over in the air stream;
- substantially reduced incidence
of icing attributed to low fin density and high tube spacing format;
- potentially
a high level of refrigerant cycle stability over a wider range of
ambient temperatures, made possible due
to more heat transfer
capacity
while circulating the same amount of refrigerant throughout the system;
- possible
elimination or reduction in the size of the refrigerant receiver
by achieving
increased heat transfer capacity without increasing
the amount
of refrigerant being circulated through the system; and
- reduced need
for pressure control valves in the refrigerant circuitry.
Potential Applications
By reconfiguring conventional coils, deep-coil configuration can accommodate
almost any coil application, including:
- refrigerant or water/glycol coils
that dissipate heat to the atmosphere directly without external
ducts, including refrigeration/air-conditioning
units, outdoor heat pump units, and dry fluid coolers;
- sanitary environments,
such as food processing, pharmaceutical manufacturing, and clean
rooms; and
- applications where low moisture blow-over is essential.
Conclusion
The deep-coil configuration is a breakthrough concept in a mature technology
that can be used as a cost-effective means to improve SEER. Because it
does not alter current refrigeration cycle design, it can be applied as
a “patch” to readily upgrade HVAC/R appliances currently in
production. Also, because deep-coil configuration is not a new scroll compressor
or other complex product, it does not require unproven or new tooling,
materials, or processes. After a true prototype is developed and further
testing is conducted, deep-coil configuration can be ready for commercialization
within 12 months.
The primary difficulty of the deep-coil figuration prototypes used in initial
comparative testing of this concept was the lack of proper tooling. Computer
models for coil selection are based on empirical data for conventional
coil and can’t be used to extrapolate deep-coil performance, nor
can the deep-coil configuration’s inherent energy-saving benefits
be derived from theoretical analysis. Only actual testing of true prototypes
will allow development of the databases needed to design deep-coil configurations
for real-life applications or prepare the appropriate coil selection software.
Therefore, the early test data provided here is only meant to prove the
validity of the deep-coil concept.
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About the Author
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Hemant
D. Kale, P.E, has more than 35 years of experience
in all facets of the HVAC/R industry. He has a B.S.
degree in mechanical engineering from the University
of Bangalore and holds many patents in the heat transfer
field in the U.S. and Canada. Prior to founding Thermorise
Inc., he worked at Intertek Testing Services, Underwriters
Laboratories Inc., and Lennox. |
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