According to the International Copper Association, research presented this summer at international conferences on air conditioning and refrigeration (ACR) points to a renewed interest in copper as a coil material for ACR products.
One of the primary motivations behind the new research is the rise in the use of smaller-diameter copper tubes in the round tube, plate fin (RTPF) coils. Traditional RTPF coils widely used in ACR products typically rely on tubes with diameters of 9.52 mm, or 3/8 inch diameter. Hence, there is a demand for data on smaller diameter tubes, which is driving new research.
"Smaller diameter copper tubes offer many advantages with respect to higher efficiencies and less usage of material," says Nigel Cotton. "It's an exciting time for researchers to work on new eco-friendly core designs made possible by MicroGroove copper tubes."
As evidence of the renewed interest in copper, copper tubes figured prominently in heat exchanger (HX) research papers presented at the Purdue Conferences this summer in West Lafayette, Indiana. Copper tubes were discussed in sessions on HX frosting; HX performance and optimization; heat transfer; HX modeling; HX maldistribution and fouling; HX wetted; and HX performance and enhancement. Several papers focused specifically on the properties of smaller diameter copper tubes in particular. Also, several papers compared the performance of copper tubes with aluminum multichannel. In many cases, superior performance was obtained from the copper tubes, especially with regard to maldistribution of refrigerant flow, drainage, frosting, wetting and deluge cooling.
A snapshot of current research activities into the properties and performance of copper tubes in ACR applications is provided by a sampling of the papers presented this past summer.
In two outstanding papers, Pega Hrnjak and Chieko Kondo carefully examined the condensation behavior of R410a and R744 in 6.1 mm copper tube with and without inner fins. They found that a very thin film of condensate forms even in the desuperheating zone and that this film affects heat transfer in that zone. (ID 2503 and 2566).
Smooth versus Inner Grooves
Ryuhei Kaji of Daikin Industries compared three types of copper tubes, including smooth and inner grooved tubes, by viewing R744 flow inside the tubes through glass. It was found that inner grooving can be effective in removing oil away from the inner surface of the tubes and thus enhancing performance. (ID 2347)
Oval Shapes with Inner Micro-Fins
In research from South Korea, condensation heat transfer coefficients (HTCs) and pressure drops were measured for 7-mm diameter copper tubes. HTCs were measured for various aspect ratios of oval-shaped tubes and oval shaped tubes with inner microfins. (ID 2580)
Frosting and Drainage
Newly tenured professor Lorenzo Cremaschi compared frosting and drainage of coils made with smaller diameter (5 mm and 7 mm) and conventional diameter (9.5 mm) copper tubes with microchannel tubes. The copper tubes showed excellent water drainage and good performance in frosting operating conditions compared to the microchannel tubes. (ID 2193)
Oil Foam and Boiling Flow
Seongho Kim and Professor Hrnjak observed through a visualization experiment on 11.2 mm diameter copper tubes that oil-induced foam can nucleate boiling in R744 and enhance HTCs under conditions where convective boiling is dominant. Inner grooves obviate the need for such foam enhancement. (ID 2178)
Interlaced or Face Split
Martin Ryhl Kaern examined airflow in residential air conditioners. He modified the U-bend connections of a 17.58 kW evaporator, reconfiguring the RTPF tube circuitry of the evaporator from interlaced to face split. Tube diameters were 9.52 mm. Performance is better for face-split circuitry if the refrigerant flow can be controlled in each circuit. Coauthored by Thomas Tiederman from Danfoss in Offenbach, Germany, this paper references Kaern's 2011 doctoral thesis from Technical University of Denmark is referenced. (ID 2178)
Deluge Water Cooling
Researchers from the University of Maryland developed apparatus to examine the cooling capacities and airside pressure drops for a round-tube heat exchanger with louver fins and a frontal area of one-quarter of a square meter. The tube diameters were 10.6 mm. The heat exchanger was tested for wet and dry cases; for two air flow rates; and for angles-to-the-vertical of 0 and 21 degrees. According to the authors, the experimental facility could be used for more comprehensive studies. (ID 2331).
An exotic new fin design consists of radial fins with bases attached to the round copper tubes and a peripheral, hexagonal fin at a distance from the tube. The honeycomb arrangement allows for ice to form around the tube while still allowing for airflow around the tubes and heat conduction through the radial fins. (ID 2143).
Professor Guoliang Ding from Shanghai Jiao Tong University (SJTU) was a coauthor of a research report delivered by Ji "Kerry" Song from the Shanghai Office of the International Copper Association. Conference attendees were treated to illustrations of new software programs for optimizing coils with smaller diameter copper tubes. The software was developed by ICA in cooperation with consortium of OEMs representing the majority share of production or room air conditioners globally. Kerry described a step-by-step procedure for optimizing heat exchanger design and illustrated the principles with case studies. (ID 2223)
MicroGroove Meets Microchannel
Until now, there have been few studies comparing smaller-diameter copper tubes directly with aluminum microchannel technology. For that reason, ICA sponsored a research project with Exel Consulting and Optimized Thermal Systems to allow for meaningful comparisons of the performance of these disparate systems. The method of comparison is simple: A search was made for a state-of-the-art, best-in-class brazed aluminum multichannel (BAM) heat exchanger. The performance specifications were then identified and set as a target for the RTPF heat-exchanger with smaller diameter copper tubes. The design space was searched for candidate RTPF designs that met the performance specification. The simulations were performed at Optimized Thermal Systems, College Park, Maryland. (ID 2464).
The attached Table lists a selection of papers from the conference and provides links to them online. The interested reader is encouraged to read the original papers and join our discussion in the MicroGroove group on Linked In.
"Original and creative work continues to be produced by university and industrial laboratories globally," says Nigel Cotton. "The increased research on smaller diameter copper tubes is clearly in evidence this year as the copper industry has demonstrated a path toward high efficiency and reduced materials use, as well as options for using low GWP refrigerants such as propane and R744."
For more information visit www.copperinfo.com.