Many factors are contributing to a climate of innovation in the ACR industry today, including:
- Phase out of high-ODP and high-GWP refrigerants & use of eco-friendly refrigerants
- Energy efficiency standards and sustainable development
- Computer simulation of components and system performance
- Responsiveness to needs and wants in the marketplace.
The phasing out of popular CFC and HCFC refrigerants has been a major factor in spurring innovations in cooling technology in the past 20 years. Likewise, energy efficiency and sustainability have necessitated invention. End-users have their eyes on energy efficiency ratings and OEMs are highly motivated to use less material in their products.
Natural refrigerants such as carbon dioxide and propane are fast becoming more attractive to OEMs and end-users for ACR and heating applications as technology advances. Also known as R744, carbon dioxide is used as a refrigerant in a growing number of applications ranging from vending machines and refrigerated supermarket display cases to ice-skating rinks.
Another natural refrigerant under consideration for use in air conditioning systems is propane. Known as R290, propane is an eco-friendly hydrocarbon (chemical formula C3H8) with outstanding thermodynamic properties that make it well suited as a refrigerant for residential air conditioners. The advantages of copper tubes in these applications include high thermal conductivity, corrosion resistance and strength. Smaller diameter copper tubes have even higher burst strengths and they allow for lower overall refrigerant volumes.
Copper is a proven technology with a well-established supply chain, including a network of trained installers with the know-how to ensure safety and reliability.
The most important factor
Perhaps the most important factor driving the development of new products is a better understanding of attitudes toward comfort and refrigeration in different climates and cultures. ACR product developers are responding better to the real needs and wants of end users in the built environment. They are right-sizing air conditioning and refrigeration products to allow for precise temperature and humidity control in specific zones without waste. The result is that end-users will enjoy healthy, eco-friendly products that deliver cooling capacity with high energy efficiency when as well as where it is most desirable. Computer modelling is now commonly used to simulate total system design. Decisions about refrigerants, coils and components are now made with the assistance of increasingly accurate performance simulations.
Advances in coil design
Redesign of the coil has seen the use of smaller diameter copper tubes with inner-grooves increasing the internal heat transfer coefficient and raising COPs. Such improvements in coil performance may also be favourable for the use of new refrigerants, less materials, higher operating pressures (due to the smaller diameter tubes) and variable refrigerant flow (due to the increased number of branches). System design is dramatically changed for the better by using smaller diameter, inner grooved copper tubes in the coil designs.
Smaller diameter, inner grooved copper tubes have been proven to be well suited for the high-volume production of residential air conditioners. Now it is time to show that the same benefits that make Micro Groove so attractive for room air conditioners also apply to commercial and industrial systems. We would like manufacturers and mechanical systems engineers to come to realise the same benefits in commercial applications that OEMs of residential products already enjoy. The Micro Groove concept is applicable across the boards: residential, commercial and industrial refrigeration and air conditioning; and for evaporators and condensers. There are benefits in every case.
Traditional copper tube/ aluminium fin coil manufacturing technology when modified for smaller diameter copper tubes of 7mm to 5 mm, can achieve significant improvements in heat transfer. When coupled with internal enhancements to the copper tubes such as higher strength, thinner walls and internal micro-grooves, newer optimised heat exchanger designs can be smaller, more efficient, and lower cost compared.
A major innovation of small diameter copper tube technology enhances heat transfer by rifling or grooving the inside surface of the tube. This increases the surface-to-volume ratio, mixes the refrigerant, moves the refrigerant into contact with the interior surface of the tube, and homogenizes refrigerant temperature across the tube, resulting in more efficient conductive and convective heat transfer. The high efficiency of the inner grooved tube stimulates and promotes the development of energy-saving, high efficiency and miniaturisation for air conditioning systems. Typically, such surface enhancement can significantly increase overall heat transfer performance, with different inner groove geometries available for optimisation under various refrigerants and conditions.
Making coils with microgroove tubes
Manufacturers are using familiar equipment to make coils with smaller-diameter round copper tubes.
Performance simulations and prototype designs of heat exchangers with smaller diameter copper tubes are indeed impressive. The savings in materials and reduction of refrigerant volume cannot be denied.
Fortunately, microgroove copper tube technology is compatible with production methods and equipment already familiar to the HVAC industry. Equipment makers have made the necessary adjustments for producing smaller-diameter tubes and assembling them into coils. Such manufacturing equipment has proven production-worthy at major companies in China such as Haier, Midea, Kelon, Chigo and Goodman who have mastered the manufacturing and now are marketing products globally.
The principles of tube insertion and tube expansion have been utilised in the industry for decades. The equipment used today expands the tubes circumferentially, i.e., the circumference of the tube is increased without changing the length. This ‘non-shrinkage’ expansion allows for better control of tube lengths in preparation for subsequent assembly operations. Tubes are inserted, or laced, into the holes in a stack of precisely spaced fins. Specially designed expanders are inserted into the tubes and the tube diameters are increased slightly until mechanical contact is achieved between the tubes and fins. The high ductility of copper allows for this process to be performed accurately and precisely. Heat exchanger coils made in this manner have excellent durability and heat transfer properties.
Modern designs of the tube expansion equipment allow for tight tolerances and exact specifications using smaller diameter copper tubes. Otherwise the equipment and production lines closely resemble the existing equipment lines that have a long and successful history.
Manufacturing in general has become more precise and accurate and the equipment for working with smaller diameter tubes is no exception and manufacturers can quickly recoup the costs of equipment upgrades because the use of smaller diameter coils allows them to make higher value products with less material.
Another factor influencing the design of air conditioning and refrigeration systems is new published research on copper’s efficacy against the spread of fungi in air conditioning systems. OEM companies such as the Chinese air-conditioning giant Chigo and Hydronic in France have already developed all-copper products expressly for their antimicrobial properties. The use of all copper coils is not new, but their use expressly to inhibit the growth of fungi and bacteria is a recent development that is expected to be an important factor in the development of innovative air conditioning and refrigeration products. Bio build up on the coil may be reduced by using all copper coils, helping in maintaining high levels of energy efficiency for longer times and avoiding energy efficiency drop off over time.