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issue: October 2004 APPLIANCE Magazine

Engineering Heat Exchangers
Considerations in Designing Next-Generation Finned, Copper Tube Heat Exchangers


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by Hemant Kale, PE, president, Thermorise Inc.

Through the increasing regulation of appliances to meet energy-efficiency standards worldwide, this paper reports on innovations in experimental coil configurations that can reduce fan and motor size in HVAC/R appliances—and help OEMs save on materials costs.

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.

   

Figure 1. CLICK for larger graphic.
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

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.

 

About the Author

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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