Cost-Saving, Energy-Efficient, Fin-Tube Coil Explained
August 10, 2009
The need for energy-efficient, practical heat exchange coils for HVAC systems has been well known, but one of its potential solutions is just starting to get more recognition. The Deep fin-tube coil was invented by engineer Hemant D. Kale, P.E., of Thermorise.
The idea evolved from an examination of the basic fundamentals of heat transfer, he said. “I felt that allowing more dwell time for two heat exchanging media was an important consideration. The longer the dwell time, the more complete would be the heat transfer.”
“Dwell time” is the length of real time at which the two heat exchanging media stay in contact, he explained. “However, the more efficient coils of today (namely one row, up to 26 fins-per-inch fin density, large square foot face area) minimized the dwell time to the extreme.”
His Deep coil “goes to a considerable extent in the opposite direction.” The coil uses multiple rows (six, seven, eight, or more); six, seven, or eight fins-per-inch fin density; and a small square foot face area.
HEAT TRANSFERABILITYWith his extensive professional background in heat transfer, Kale knew that to be cost effective and energy efficient, a forced-air, fin-tube heat exchanger should ideally transfer all available heat (absorb or reject) with a minimal amount of resistance to air movement over the tubes, as well as fluid movement within the tubes.
The lower the air-side resistance, he reasoned, the lower the fan motor power consumption would be. Similarly, the less the fluid resistance inside the tubes, the lower the fluid pump/compressor power consumption would be.
With his Deep coil, Kale said he is attempting to achieve these same goals by reducing fin density, increasing tube spacing, and increasing the tube rows. “The combined result of reduced fin density and the increased fin spacing provides for reduced air-side resistance,” he explained, “allowing for much smaller fan power.
“By increasing the tube rows, we increase the real-time contact between the two heat exchanging media, namely fluid inside the tubes and air flowing over the tubes.”
PROTOTYPESUsing this principle, Kale made production-quality prototype samples. These required custom-designed tooling, particularly the fin dies. “Four samples were made, each 16 by 36 inches (four square feet) face dimension, but with some variations such as fin density or number of rows,” Kale said. The samples were tested at Intertek testing laboratories, Cortland, N.Y.
Table 1 shows these coils designated as D1, D2, D3, and D4. The equivalent current-design coils are designated T1, T2, T3, and T4. The equivalent current-design thin coil has the same total length of copper tubing and fin square footage. Both coils use 0.016-, 3/8-inch copper tubes, and 0.008-inch-thick aluminum fins.
Tests were conducted at 135°F inlet water, 3 gpm, 95° dry bulb inlet air. Table 1 compares actual data recorded on Deep coils, with the equivalent thin coil data obtained from coil-selection software.
“In our data, the heat transfer capacities for the Deep coil are about 15-20 percent below the equivalent thin coil,” said Kale. “For example, coil D4 has 29,999 Btu, as compared to 36,663 Btu for coil T4. This can be largely corrected by fin formation such as corrugation or sine wave to the Deep coil D4, which has flat fins. This will increase both the heat transfer and the air resistance, but will still provide substantial energy savings without adding more fins. In our table, the fan BHP [brake horsepower] per 10,000 Btu for Deep coil is only about 30-34 percent of the equivalent thin coil.”
The fan power reduction of over 20 percent comes from reduction in the air-side resistance, he said. “The fluid pump/compressor power reduction comes from reduction in the tube resistance. The Deep coil has much larger tube spacing, for example, 2 by 1 inch as compared to most common 1 by 0.866 inch for thin coils. Due to this larger spacing, the return bends are, in this case, 2 inches in diameter as compared to 1 inch in diameter,” Kale said.
“The much softer return bends, due to the large turning diameter, allow for a reduction in fluid resistance of 20 percent plus.”
The reduced fan power (reduced fan motor hp), and the reduced pump/compressor power (which is reduced pump/compressor motor hp) allows for a lower OEM cost and, concurrently, allows for lower operating costs to the consumer,” Kale explained.
In a graphical representation of the empirical test data for test samples D1 and T1, the pattern repeats for all four coil samples, he added. “The heat transfer seems to plateau beyond 500 fpm velocity. That is, any increase in velocity will cause a disproportionate increase in the fan power.” Translation: With the Deep coil, the threshold velocity can be increased beyond 500 fpm to, say, up to 700 fpm. “This is significant,” said Kale. “It will allow a design engineer to shrink the footprint of the end product.”
In another graphic representation of comparison between Kale’s design and the equivalent thin coil, Kale said, test data on both coils are empirical. The equivalent thin coil has the same tube and fin material, both type and quantity.
The Deep coil is particularly effective in outdoor condensers that use propeller fans, Kale said. The propeller fans are sensitive to static pressure. The coil reduces the air resistance (static pressure) significantly, he explained. “The current one-row, 24-fins-per-inch, U-shaped coil can be converted to a multiple-row, low-fin-density, slab coil design with low static pressure.” A large 6- or 8-square-foot, one-row, 24-fins-per-inch coil can be converted to a Deep coil with a face area in the neighborhood of 2 to 3 square feet, he said, and reduce the overall footprint of the unit.
Contact Hemant D. Kale, P.E., at 315-416-0780 or email@example.com, or visit www.thermorisecoil.com. Thermorise Inc. is a newly formed corporation for the purpose of manufacturing the Deep coil. The New York State Energy Research & Development Agency (NYSERDA) has provided some money to defray the costs.
Publication date: 08/10/2009