Internally grooved copper tubes, also known as "microfin tubes", are a small diameter coil technology for modern air conditioning and refrigeration systems. Grooved coils facilitate more efficient heat transfer than smooth coils. Small diameter coils have better rates of heat transfer than conventionally-sized condenser and evaporator coils with round copper tubes and aluminum or copper fin that have been the standard in the HVAC industry for many years. Small diameter coils can withstand the higher pressures required by the new generation of environmentally friendlier refrigerants. They have lower material costs because they require less refrigerant, fin, and coil materials. They enable the design of smaller and lighter high-efficiency air conditioners and refrigerators because the evaporator and condenser coils are smaller and lighter.
With MicroGroove technology, heat transfer is enhanced by grooving the inside surface of the tube. This increases the surface to volume ratio, mixes the refrigerant, and homogenizes refrigerant temperatures across the tube.
Tubes with MicroGroove technology can be made with copper or aluminium. Copper fins are an attractive alternative to aluminium due to the better corrosion resistance of copper and its antimicrobial benefits.
Design
To use smaller tubes instead of conventional-sized tubes in air conditioners, heat exchangers must be redesigned including the fin and tube circuits. Design optimization requires the use of computational fluid dynamics to analyze airflow around the tubes and fins, as well as computer simulations of refrigerant flow and temperatures inside the tubes. This is important because the overall heat transfer coefficient of a coil is a function of the convection of the refrigerant inside the tube to the tube wall, conduction through the tube wall, and dissipation through the fins.
Engineering considerations for using Microgroove include: Determining the best ratio of transverse tube pitch to longitudinal tube pitch by fin efficiency analysis. Optimizing transverse and longitudinal tube pitch by performance analysis and material cost. Optimizing fin pattern by comparing performances of fins with different patterns through computational fluid dynamics-based simulations. Testing the performance of heat exchangers with smaller diameter tubes. Developing empirical equations for predicting performance of heat exchangers with smaller diameter tubes.
Published experiments on MicroGroove coil performance and energy efficiency take into account the effects of fin spacing and fin design, tube diameter, and tube circuitry. Tube circuitry is substantially different than for conventional coils. Coils should be optimized with respect to the number of paths between the inlet and outlet manifolds. Typically, smaller diameter tubes require more paths of shorter lengths. Published research on tube circuitry and fin design for heat exchangers made with 4 mm tubes are available.
Research on a heat exchanger redesign with 5-mm diameter tubes demonstrated a 5% greater heat exchange capacity than that of the same size heat exchanger with 7 mm diameter tubes. Also, the refrigerant charge of the 5 mm diameter tubes was less than the 7 mm diameter tubes. In China, Chigo, Gree, and Kelon are producing air conditioners with coils that have 5 mm diameter tubes.
A variety of fin designs have been developed for use with small-diameter copper tubes. The performance of slotted and louvered fin designs have been evaluated and compared as a function of various fin dimensions. Simulations have been used to optimize fin design performance.
Refrigerants
The phasing out of CFC and HCFC refrigerants (e.g., HCFC-22, also known as R22) due to global warming concerns has helped to spur innovations in cooling technologies. Natural refrigerants such as carbon dioxide (R744) and propane (R290), as well as R-410A, have become attractive replacements for air conditioning and refrigeration applications.
Higher pressures are typically required to condense these new environmentally friendly refrigerants compared to those that are being phased out. Small diameter copper tubes are more desirable in applications with higher pressures. For tubes of the same thickness, smaller diameter tubes can withstand higher pressures than larger diameter tubes. Hence, as tube diameters decrease, burst pressures increase. This is because working pressure is directly proportional to wall thickness and inversely proportional to diameter. By designing coils with shorter tube lengths, less work is required to circulate the refrigerant. Therefore, refrigerant pressure drop factors due to small diameter tubes can be offset.