23 August 2012
Leland E. Teschler
It is well known that small bodies are easier to cool than larger ones. With that in mind, consider what happens as you reduce the diameter of a tube and lengthen it: The surface-to-volume ratio rises as the diameter shrinks.
These are among the reasons the International Copper Association has been highlighting the use of smallerdiameter, round copper tubes in air-conditioning and refrigeration (ACR) applications. The tubes have a special construction that incorporates inner grooves. Rifling or grooving the inside surface of the tube boosts its surface area. The effect is more pronounced in small-diameter tubes — those smaller than the typical 3/8-in. tubing used in heat exchangers — because more surface area is available to groove for a given volume.
Interest in ACR product design has intensified in recent years and new designs of heat pumps, air conditioners, and refrigerators are flourishing. ACR manufacturers have become highly motivated to reduce the cost of their products and one way to reduce costs is to reduce the materials content.
All else being equal, the highest coefficients of performance (COPs) comes from more-efficient cooling of refrigerant in the condenser and more-efficient warming of refrigerant in the evaporator. Thus, energy-efficient coils made from smaller-diameter, inner-grooved copper tubes can help boost the efficiency of ACR units in the new climate of innovation.
Microgroove technology has been quietly developed in China over the past half decade, especially for the highly competitive room-A/C or window-A/C marketplace, product categories based on high volumes and low prices. With much manufacturing based in China, U.S. designers and manufacturers may not fully grasp this important innovation, though it was strongly supported by the global copper industry. And yet, there are many manufacturers in the U.S. and Europe who also could benefit from the competitive edge offered by MicroGroove technology in a wide range of heat-exchanger applications for commercial buildings. Applications in commercial refrigeration can have wide-ranging effects on the industry and help to reduce energy consumption. This innovative technology is now ready for use. MicroGroove technology can serve as the foundation for a whole new generation of innovative ACR products.
Scaling of tube diameters
Improved heat transfer through smaller-diameter tubes relates to scaling. The enclosed-volume of a tube increases in proportion to the square of the radius (V = πR2L). Meanwhile, the surface area scales in direct proportion to the radius (A = 2 πRL). Consequences of this scaling include:
• If one doubles the tube diameter, the surface area doubles while the volume rises fourfold.
• If one halves the diameter and doubles the length, the surface area stays constant while the tube encloses half the volume.
• If one halves the diameter and quadruples the length, the enclosed volume remains the same while the surface area doubles.
The accompanying table demonstrates scaling by comparing how some geometric quantities change as others vary or are held constant. The upshot is that reducing the tube diameter has a dramatic effect on heat transfer between the refrigerant and the inside tube wall as described by the local heat-transfer coefficient (HTC), .
Data from laboratory experiments illustrate how the HTC rises as tube diameter shrinks. Local HTCs were measured under tightly controlled laboratory conditions in a series of experiments at the Institute of Refrigeration and Cryogenics at Shanghai Jiao Tong University (SJTU).
Surface enhancements, or inner grooves on the inside surface of the tubes, can also significantly boost the local heat-transfer coefficient. The “boundary layer” is an important quality of fluid flow, and it has a major effect on heat transfer as well. Surface enhancement through grooving promotes turbulence in the flow and reduces the thickness of the boundary layer. The result is more effective heat transfer. Smaller-diameter tubes also contribute to the turbulence.
Besides reducing the local heat-transfer coefficient, a reduction in tube diameter makes tubes stronger, a fact well known from pipe engineering.
No surprise that manufacturers of window-type and other residential air conditioners were among the first to adopt MicroGroove technology. Better heat-transfer coefficients and the ability to use thinner-tube walls saved them money on materials. There were additional savings because MicroGroove tubing can handle a given cooling load while using less fin material and refrigerant.
The International Copper Association supports several research initiatives exploring design techniques for coils made from smaller-diameter copper tubes. Initially, ICA cooperated with several OEMs on projects that demonstrated the technology. These basically swapped smaller-diameter tubes for larger-diameter tubes in existing coil designs. Even without optimizing the tube spacing and fin design, the smaller tubes brought significant savings.
More recently, computer simulations and design case studies illustrate how to design with smaller-diameter copper tubes. New research results were presented at two international conferences this summer, including the Tenth Gustav Lorentzen Conference on Natural Refrigerants on the campus of Delft University of Technology in the Netherlands in June; and the Fourteenth International Refrigeration and Air Conditioning Conference at Purdue University in July.
The design and optimization of heat exchangers requires the use of computational-fluid-dynamics (CFD) methods to analyze the airflow around the tubes and fins. It also involves computer simulations of refrigerant flow and temperatures inside the tubes.
One example of a residential air-conditioning product containing MicroGroove Technology is the Chigo split system, which provides a cooling capacity of 2,500 W and a coefficient of performance (COP) of 3.2. A/C manufacturing giant Chigo, City of Industry, Calif., reduced the tube weight in this air-conditioning system by 30% simply by switching to smallerdiameter copper tubes in the evaporator and condenser coils. Chigo reduced the tube diameter from 9.52 to 5 mm in the condenser and from 7 to 5 mm in the evaporator. (Note: A COP of 3.2 is equivalent to an energy-efficiency ratio (EER) of 10.9. The EER in units of Btu/hr/W is obtained from the COP in units of W/W through multiplication by 3.1413 because 1 W = 3.413 Btu/hr.)
In addition, use of smaller copper tubes promotes the use of eco-friendly refrigerants. R290 is considered an eco-friendly natural refrigerant because it has zero ozone-depletion potential and virtually zero globalwarming potential. Moreover, it does not give out any toxic decomposing agents on combustion and is compatible with the materials and lubricants used in air conditioning.
Shanghai Jiao Tong University Institute of Refrigeration and Cryogenics scientist Prof. Guoliang Ding presented an R290 case study at the 2012 Lorentzen Conference with coauthors from SJTU, the International Copper Association, and the Guangdong Kelon Air Conditioner Co. Ltd. in China. In this case study, designers were able to reduce refrigerant charge compared to systems using 7 or 9.52-mm-diameter tubes; and the cooling capacity got better through optimization of the heat exchanger.
All in all, smaller-diameter copper tubes are essential for improving the efficiencies of ACR products.