- What are the major benefits of economical, eco-friendly copper tubes?
Energy efficiency. Reducing the diameter of copper tubes in coils provides an economical path to energy efficiency for air-conditioning and refrigeration (ACR) products. System energy efficiency could also be improved by using a larger number of conventional tubes but then a penalty would be paid in terms of the increased weight of tube material and fin material as well as increased refrigerant volume.
Less material. Tube-diameter reduction results in more effective heat transfer and allowing for smaller, lighter coils, or more heat transfer or greater efficiency at the same coil size. Smaller evaporator and condenser coils allow for smaller overall product dimensions and consequently easier storage and transport, easier handling during installation and a smaller footprint at the point of use.
Less refrigerant. A dramatic reduction in refrigerant volume is a further benefit of economical, eco-friendly copper tubes. The smaller internal volume of the coils means that less refrigerant is necessary to charge the coil. Less refrigerant means less expense.
Durability. Coils made of copper tubes and aluminum fins (CTAF) or copper tubes and copper fins (CTCF) are durable and dependable. They set the industry standard for corrosion resistance and long, reliable service life. Advanced coatings and surface treatments continue to improve durability in the harshest environments. The industry has found it difficult to improve upon the exceptional durability of copper tubes for ACR applications.
Familiarity. Tube suppliers, OEMs, mechanical-systems engineers and HVAC contractors all enjoy a high level of familiarity with copper tubes and aluminum fin (CTAF) technology. Up and down the value chain, the materials and processes are well understood. The fabrication, assembly, installation, service, repair and recyclability are not substantially changed in the migration from conventional copper tubes to economical, eco-friendly copper tubes.
- Why are OEMs suddenly interested in economical, eco-friendly tubes for ACR coils?
While energy efficiency has always been a high priority of HVAC manufacturers, the continual drive for higher efficiencies combined with environmental changes to refrigerants used calls for a new look at system designs. Energy efficiency can be achieved at a lower material cost with smaller diameter tubes. Overall system size can be reduced by using small diameter tubes. Smaller tube diameters result in reduced usage of tube materials, fin materials and refrigerants, contributing to overall reduction in system cost. Also, smaller diameter tubes can operate at higher pressures.
- Why do small-diameter tubes dissipate heat more efficiently?
The human body provides an example of the benefits of small diameter and even microscopic tubes. Heat flows into extremely small tubes close to the skin surface and is distributed over a larger cooling surface area. Although copper tubes will never be as small as the blood vessels, a modest reduction in diameter from the conventional tube diameter markedly increases cooling rates.
- Does this have to do with the surface-to-volume ratio?
Yes. It is well known from experience that small bodies are easier to cool than large bodies. Elephants need large ears to keep cool. The surface-to-volume ratio increases as tube radius decreases.
- Is there another way to increase the surface-to-volume ratio?
Yes. The surface area can be increased by rifling or grooving the inside surface of the tube. Furthermore, the enhanced surface helps to mix the refrigerant and homogenize the refrigerant temperature across any tube section, resulting in more efficient convective heat transfer. Typically, such surface enhancement can significantly increase performance.
- What tradeoffs are involved in reducing tube diameter?
The refrigerant "pressure drop" increases for smaller diameter tubes. More work is required to circulate the refrigerant through a given length of tube when the pressure drop is high. To offset the pressure drop, coils can be designed with shorter tube lengths and the tube circuitry can be configured with fewer tubes per branch and more branches per coil.
- What effect does refrigerant type have on tube diameter selection?
Small diameter tubes are more desirable as operating pressures increase. Higher pressures typically are required to condense new refrigerants (e.g., R410a or R744) compared to refrigerants that are being phased out (e.g., R22). Working pressure is directly proportional to wall thickness and inversely proportional to diameter. In other words, for tubes with the same thickness, smaller diameter tubes can withstand higher pressures than larger diameter tubes.
- What is the typical weight savings for tube materials when using economical, eco-friendly copper tubes?
In one design study for functionally equivalent 5-kW HVAC heat exchangers, the weight of the tube materials in the coils was 3.09 kg, 2.12 kg and 1.67 kg for tube diameters of 9.52 mm, 7 mm and 5 mm, respectively. The tube weight was reduced by 31 percent and 46 percent when copper tube diameters were downsized from 3/8 inch to 7 mm and from 3/8 inch to 5 mm, respectively.
- What fin designs are suitable for economical, eco-friendly copper tubes?
A variety of fin designs have been developed for use with small-diameter copper tubes. A white paper titled "Development of a Small-Diameter Tube Heat Exchanger: Fin Design and Performance Research" compares the performance of slotted fin and louvred fin designs as various fin dimensions are varied. Simulations were used to the optimize performance of fin designs. Contact ICA for more information.
- What is the status of the development of air conditioners with small diameter copper tubes among OEMs?
The migration to small-diameter copper tubes is inevitable because of their inherent advantages. The small-diameter tube project in China involves the major manufacturers who together account for more than 80 percent of HVAC production. Several OEMs in North America are already marketing residential air-conditioner products with economical, eco-friendly, smaller-diameter copper tubes.