Researchers in the copper industry believe they have discovered a new way to improve heat exchanger performance, using small diameter tubes and ‘microgrooves’. RAC reports.
Work by the International Copper Association has resulted in new high performance heat exchanger coil technology from a potent combination of smaller diameter copper tubes and ‘microgrooves’ – grooved internal surfaces
Following a number of years of research, the ICA believes it can now offer viable HVACR products that benefit from the fact that heat transfer from the refrigerant to the tube wall is more effective inside copper tubes with diameters smaller than the norm – of the order of 4-5 mm, as opposed to the more conventional 6.5-9.5 mm diameter. Consequently, coils can be made less bulky and in turn air conditioners and refrigeration equipment can be made smaller and lighter for the same performance.
Other advantages of having smaller diameter tubes, including obviously lower overall copper costs, lower shipping weight and the need for reduced refrigerant charge.
Researchers found that by reducing tube diameter from the conventional 9.5 mm to 5 mm resulted in a 15 per cent higher heat exchange coefficient, when all other variables were equal. In addition, use of the smaller diameters in heat exchangers could save 42 per cent in copper usage. Refrigerant charge can be reduced by 15-20 per cent. At the same time, the smaller diameter tube is better able to accommodate increased refrigerant pressures.
The disadvantage of having smaller tubes is that more, shorter, lengths are required in each heat exchanger to manage pressure drops, but the ICA says that the heat exchange improvement more than offsets any additional tube weight.
One of the most interesting recent developments is the performance of ‘microgroove-enhanced’ tubes. Laboratory tests were conducted on small-diameter copper tubes with and without enhanced inside surfaces. The results prove that grooving the inner surfaces greatly increases the local heat transfer coefficient inside such “microgroove” tubes.
“The inner grooves ‘stir’ the refrigerant as it passes through the tubes,” says Wenson Zheng, Deputy Director of Technology for ICA. “Such stirring is crucial to efficient heat transfer.” “They are at the sweet spot in coil design. Heat-transfer coefficients are high, yet pressure drops are still manageable. With all other factors being equal, very pronounced increases in the local heat transfer coefficient are plainly evident for the tubes with micro-grooving compared to smooth tubes,” he says..
The small diameter revolution is based on ongoing research from China-based consortium into such subject matters as tube circuitry, fin design and the measurement of heat transfer coefficients. Consortium members include the ICA, two universities and at least five air-conditioning OEMs.
The laboratory experiments were performed at the Institute of Refrigeration and Cryogenics at Shanghai Jiao Tong University (SJTU) in Shanghai, China.
“Conventional copper tubes have set the standard for coils for decades,” says Nigel Cotton, OEM Team Leader for the ICA, “But these results demonstrate the feasibility of making residential air-conditioners and refrigeration products with small diameter copper tubes. They offer all of the advantages of conventional tubes, while being more economical because of the higher heat-transfer coefficients. They set the stage for high-volume production of a new generation of economical, energy-efficient air-conditioning products. “
OEM members of the research consortium have already designed consumer products, such as room air conditioners, using the combination of small tube copper with microgrooves, and these products are being introduced into global markets in North America, Europe and Asia.
Full test papers and other technical materials can be viewed at www.microgroove.net.