Improving heat exchanger performance is one part — a critical one — in enhancing the overall system performance in HVACR systems.

One way to do it is to think small — as in smaller tubes in fin and tube heat exchangers and using small-diameter copper tubes.

Those ideas comprised two of five papers presented during a two-hour session at the 14th International Refrigeration and Air Conditioning Conference this summer at Purdue University. It took place on the West Lafayette, Ind., campus along with sessions that were part of the 21st International Compressor Engineering Conference and 2nd International High Performance Building Conference.

Here, based on presentations made at the HX Performance and Enhancement session, is some of the latest thinking on heat exchangers.

Smaller Tubes

The principle of designing fin-and-tube heat exchangers with smaller tubes was the focus of a joint paper from the Institute of Refrigeration and Cryogenics and the Shanghai Jiao Tong University, both in Shanghai, China, as well as the International Copper Association’s Shanghai office.

“The principle includes designing of fin configuration and designing of refrigerant circuits,” it was noted. “In the design principle, the suitable fin configuration for 5-mm diameter tubes is designed by Computational Fluid Dynamic- [CFD] based method, and the refrigerant circuit with 5-mm diameter tubes is designed by simulation-based method.”

The researchers said that to verify the results of designing, “experiments on air conditioner units are carried out. The experimental results confirm the design principle, and indicate that the optimal air conditioner with smaller diameter tubes has better performance and lower refrigerant charge, which will promote the application of (flammable natural) refrigerants, such as R-290.”

Optimized A/C Coils

Excel Consulting Group of Palatine, Ill.; the Copper Development Association (CDA Inc.); and Optimized Thermal Systems of College Park, Md., teamed up for a report titled “Simulation-Based Comparison of Optimized AC Coils Using Small Diameter Copper and Aluminum Micro Channel Tubes.”

“Demands for higher energy efficiencies in both residential and commercial refrigeration and air conditioning systems have resulted in a trend toward heat exchanger designs that are more compact with higher capacities for heat transfer,” the report said in its opening. “Traditional copper tube-aluminum fin coil manufacturing technology remains prevalent throughout the industry and, when modified for smaller-diameter copper tubes of 5 mm or less, significant improvements in heat transfer can be achieved. When coupled with internal enhancements to the copper tubes such as microgrooves, coil designs can be smaller, more efficient, and less costly.”

The researchers used a commercially available heat exchanger design, simulation software, and CFD modeling to compare optimized 3-ton air conditioning condenser coils manufactured with small-diameter internally enhanced copper tubes against condensers with aluminum micro-channel tubes.

“Simulated operating conditions are held constant, including refrigerant inlet pressure and temperature, as well as air flow rate and inlet temperature. Comparisons of material consumption, refrigerant charge, volume, and heat transfer performance are demonstrated. It was found that using internally enhanced copper tubes with a diameter of 5 mm, condenser coils can be designed to operate with less refrigerant charge and have the potential to be lighter and more compact than commercially available, optimized aluminum coil designs with microchannel tubes.”

Fin Efficiency

“Evaluation of Fin Efficiency and Heat Transfer Coefficient of Heat Exchanger Having Plate Fins” was the topic of a paper from Kunsan National University, School of Mechanical and Automotive Engineering of Gunsan, Jeonbuk, South Korea.

“This study discussed the estimation of the fin efficiency and the pure-heat transfer coefficient in the heat exchanger,” the researchers said. “One hundred twenty cases of plate fins having known heat transfer coefficients were tested numerically to investigate the validity of the previous classical theory on the fin efficiency.

“The conventional theory on the fin efficiency was only useful when the value of NTUƒ [number of heat transfer units for fin] was near zero. However, it was not useful at high NTU ƒ and low fin efficiency in the heat exchanger.

“A new definition of fin efficiency and a model for pure-heat transfer coefficient are suggested, which are applicable to the heat exchanger. The present model reduced error greater than the classical model in the estimation of the pure-heat transfer coefficient at 0 < mL < 2, 0 < NTUƒ < 2.5.”

Note: L refers to fin length and m refers to parameter in theoretical fin efficiency.

Wicking Structures

The issue of dual-mode wicking structures for enhanced evaporative heat transfer was presented by researchers from the Pacific Northwest National Laboratory in Richland, Wash.

“This paper described unique structures for processing gas-liquid mixtures and reported on thermo-fluid characteristics of a dual-mode wick device,” the research said.

“A two-phase heat transfer coefficient exceeding 25,000 W/m2K was obtained with a dual-mode wick device. Scaling calculations were presented supporting a thin film heat transfer model with a film thickness comparable to the liquid channel effective pore diameter.

“In comparison to an empty channel device, the dual-mode wick device enabled:

• “Stable operation (without pressure and temperature oscillations) and generally self-regulating due to capillary action;

• “Enhanced heat transfer and hence reduced superheat requirement for evaporative heat exchanger applications; and

• “Complete vaporization of liquid to vapor at lower superheat.”

Micro Fins

“The Effect of Micro Fins on Heat Rejection in Desuperheating, Condensation in Superheated Region and Two Phase Zone” was the topic of a joint project of the University of Illinois Mechanical Science and Engineering ACRC West Green of Urbana, Ill.; Creative Thermal Solutions Inc. of Urbana; and Kyushu University, Interdisciplinary Graduate School of Engineering Science of Kasuga, Fukuoka, Japan.

“Typical condenser design model simply divides the heat rejection process into de-superheating, two-phase, and subcooling,” the report said. “By neglecting condensation in presence of superheated vapor, error can be introduced.”

To address this, the researchers said they used “experimental data on superheated CO2 and R-410A flow in horizontal micro fin tubes of 6-mm inner diameter at reduced pressure from 0.55 to 0.95.

“Gradually increasing heat transfer coefficient, from when tube wall reaches saturation temperature, demonstrates the criteria of condensation occurrence and the proposed heat transfer model.”

They further noted, “Heat rejection heat transfer coefficient data of flowing CO2 and R-410A from superheat to two-phase zone in horizontal micro fin tubes has been experimentally investigated. The main findings:

• “In the superheat zone, experimental heat transfer coefficient starts gradually deviating from the correlation proposed for single-phase cooling from when tube wall reaches saturation point. This identifies condensation occurrence in the presence of superheated vapor at exactly the same criteria as smooth tubes.

• “A predicting correlation considering condensation superheated vapor was derived from a heat balance in superheated vapor and coexisting condensate. The predicting correlation evaluates the liquid properties at film temperature, was validated with experimental results at the reduced pressure ranging from 0.55 to 0.95.

• “Condensing occurrence in superheat zone affects condenser sizing. This was demonstrated with the case study of a typical cross-fin-tube heat exchanger using micro fin tubes for R-410A at 2.7 MPa.

• “To consider the condensation occurrence in the superheat zone becomes more important for condenser designing as approaching the critical point, because the degree of superheat at compressor discharge becomes higher and the latent heat becomes smaller.”

Publication date: 11/19/2012