Tougher standards, soaring energy prices, and more intense competition are all factors pushing the makers of air-conditioning and refrigeration equipment to design more energy efficient products. The heat exchangers employed as evaporators and condensers in such products play a critical role in overall system efficiency. While the core principles have underlying finned tubing heat exchangers have remained the same for years, a great deal of research and development has been focused on improving their heat transfer performance. New configurations have also increased design options for makers of cooling products, given heat exchangers have a significant impact on the size and shape of the end product.
For example, the size of both indoor and outdoor coils was a big consideration for makers of residential air conditioners and heat pumps as they prepared to meet the 13 SEER standard that went into effect in January. York International, Unitary Products Group, York, Pa., was one of the OEMs that looked to redesigned heat exchangers as a path to 13 SEER. The company achieved the goal through use of microchannel heat exchangers, using coils supplied by Delphi Thermal & Interior, Lockport, N.Y.
Microchannel heat exchangers have flat, streamlined tubes. One larger tube is split into multiple smaller, parallel ports.
The metallurgical tube-to-fin bond eliminates contact resistance, allowing fins to transfer heat more efficiently. Where higher grade aluminum materials are used, the metallurgical bond allows the heat exchanger to be formed in unlimited configurations.
There are three basic components in the overall heat transfer equation: air-side heat transfer (between the fins and the ambient air), heat conduction (between the fins and tubes) and refrigerant-side heat transfer (between the tubes and the refrigerant).
Because air is essentially an insulator, overall heat transfer is limited by the air-side portion of the equation. This limitation is common to both fin-tube and microchannel heat exchangers. Consequently, a great deal of research has been dedicated to improving air-side heat transfer through modification of fin geometry. Some of these modifications include louvers, lances and rippled edges.
The heat conduction portion of the equation depends heavily on the bond between the fin and tube. In fin-tube heat exchangers, that bond is created when hairpins are expanded. If the fins are loose on the hairpins, (either from under-expanded hairpins or splits in the fin extrusions due to over-expanded hairpins) air gaps between the two materials reduce heat transfer. With microchannel heat exchangers, a mechanical bond is created by brazing the fins directly to the tubes, eliminating the possibility for air gaps to reduce heat transfer.
The refrigerant-side heat transfer occurs at the boundary between the refrigerant and the internal surface area of the hairpin — typically known as the “wetted perimeter.” In general, as the internal surface area of a tube increases, the refrigerant side heat transfer increases. For fin-tube heat exchangers, internal surface area of the hairpins can be increased by adding groves to the inside of the tubing.
For microchannel heat exchangers, internal surface area is increased by two methods: one, increasing the number of microports in each tube’s cross section, and two, increasing the number of tubes in each heat exchanger (decreasing the tube spacing.)
Microchannel technology should be suitable for any HVAC/R application that uses refrigerant and air for heat transfer. This would include residential and commercial condensing units, heat pumps and evaporators as well as refrigeration systems. In addition to the heat exchanger heights and lengths that are normally modified to create a variety tonnages and efficiencies, the manifold baffles can be relocated to change the refrigerant-side pressure drop and the split between the “de-superheat” and subcooling portions of the heat exchanger. Contingencies must also be made for condensation removal when the heat exchangers are used in evaporator or heat pump applications.
“Now that the commercial feasibility of microchannel technology has been proven for HVAC applications, additional research should probably be dedicated to optimizing the tube geometry for the various refrigerants used in our industry,” says Darren Warnecker, senior project engineer, Cooling for York International-Unitary Products Group. “However, even though the microchannel tubing has dramatically improved the overall heat transfer by increasing the refrigerant-side portion of the equation, HVACR heat exchangers are still limited by the air-side portion. Consequently, additional studies will be necessary to determine the optimum fin geometry to compliment the microchannel tubing geometry.”
The fin is what differentiates PF2 heat exchangers from conventional micro channel heat exchangers. It allows tubes to be oriented vertically or horizontally to function as a condenser or evaporator in refrigeration and air-conditioning applications.
With a few notable exceptions, most current HVAC heat exchangers are constructed from aluminum fins and copper hairpins, both of which may have enhancements to improve heat transfer. The aluminum fins are long and thin and have round extruded holes through which the hairpins are laced and expanded to provide an interference fit for heat conduction. Circuiting is performed with various return bend combinations that are brazed to the open ends of the hairpins. Parallel flow plate fin, or PF2 technology from Modine Manufacturing, Racine, Wis., is a heat exchanger innovation designed specifically for the HVAC/R market. The fin is what differentiates PF2 from conventional micro channel heat exchangers. It allows tubes to be oriented vertically or horizontally to function as a condenser or evaporator in refrigeration and air-conditioning applications. PF2 also solves water drainage issues.
Modines’s patented PF2 coil technology provides a means to meet or exceed mandated R-22 refrigerant phase out, performs well as an evaporator in any orientation and meets large capacities with its multiple row design. It also has multiple pass circuitry and enhanced mounting designs.
Benefits of the PF2 technology include:
- Reduced coil depth that further lowers air side static pressures.
- Reduced face area to cut cabinet costs.
- Reduced internal volume to reduce refrigerant charge.
- Increased design versatility.
Potential applications include:
- Refrigeration equipment such as ice machines, beverage dispensers, refrigerated display cases and food service refrigeration.
- Air conditioning applications such as residential air conditioners, rooftop air conditioners, chillers, geothermal heat pumps, PTACs and electronic cooling.
The PF2 concept mainly remains the same from application to application. Only coil size or depth will vary to meet performance needs.
With PF2 engineers can design cooling solutions around the application, not the heat exchanger technology. Consequently, PF2 provides for lower system costs, higher system efficiencies, lower operating costs and quieter operation.
Parallel flow plate fin, or PF2 technology from Modine, is a heat exchanger innovation designed specifically for the HVAC/R market.
A plate-on-tube condenser from Bundy improves airflow over the condenser surface by using the Coanda effect. The Coanda effect is named for a Hungarian researcher who, during World War II, discovered that air that flows over a curved surface, tends to follow that surface. The technology was first used to improve aerodynamics in warplane designs. Traditional tube-on-plate condensers have louvers that are angled in the same direction to allow air to pass through the condenser. Nevertheless, because there is no significant pressure drop between the two sides of the condenser, a very limited amount of air actually passes through the louvers.
Launched in mid-2005, Bundy’s Coanda-effect louver design allows air to flow from one side of the plate to the other, and back again, flowing much like a wave up through the middle of the condenser.
The technology was designed mainly with upright refrigeration appliances in mind, but can replace any plate-on-tube, and potentially wire-on-tube, condenser.
The company currently has OEM customers in Europe, who purchase the Coanda solution for household refrigerators and upright freezers.
The curved design of the heat exchanger fins disrupts the formation of boundary layer on the condenser and creates turbulence, which improves the efficiency of the fins, and in turn, the condenser. Tests have shown a 5 percent to 7 percent improved heat transfer compared to standard plate-on-tube condensers.
Peter Espersen, technical director for Bundy notes that the future of heat exchanger technology lies in not just tinkering with the basic heat exchanger design, but the development of entirely new products.
“I think there will be increasing interest to develop new heat exchanger products that will not only improve efficiency, but also reduce costs. This business has been quite static for a number of years,” he says. “There has not really been any development because the main benefits that we have seen over the last couple of years have been due to innovations in gas compression and insulation, not because of systematic development of a newly developed product, and this is where we believe there is going to be interest in the years to come.”
Modine's PF2 concept mainly remains the same from application to application. Only the coil size or depth will vary to meet performance needs.
While metal heat exchangers are constantly being improved, they still have some inherent drawbacks due to their metal construction. These heat exchangers are subject to corrosion, oxidation and microbiological attack. Metal heat exchangers additionally experience slow degradation and loss in heat transfer capacity. And commonly used chemical water treatment can adversely affect metal heat exchangers. Another difficulty with metal heat exchangers is friction or contact between coils during operation. Vibrations during operation often result in rubbing of the heat exchanger coils. This friction can cause the coils to wear on each other. Over time, this wear and tear can cause serious damage and even fluid leaks.
New plastic heat exchanger technology developed by Powercold, La Vernia, Texas, offers an interesting alternative to metal heat exchangers. Powercold designs, develops and markets energy efficient HVAC systems for new and retrofit applications. Powercold uses Caltrel plastic from Dupont for its heat exchanger tubing to provide for effective energy transfer in most applications. In fact, Powercold says that the heat transfer capacity of this plastic tubing is comparable to that of copper heat exchangers, and that transfer rate remains consistent for both heating and cooling.
When used in HVAC applications, fluid coolers fitted with 3/8-in.Caltrel tubing with a 0.20-in. wall thickness, have comparable heat transfer rates to 5/8-in. copper with a 0.034-in. wall thickness, which is the industry standard for fluid coolers. When comparing the heat transfer of the 1/8-in. plastic tubing with a 0.007-in. wall thickness and aluminum finned 3/8-in. copper coil, the plastic exhibits the same heat transfer performance up to 400 cfm airflow. For airflow higher than 400 cfm, however, the plastic exhibits higher heat transfer rates than copper coil, according to Powercold.
The Coanda effect is named for a Hungarian researcher who, during World War II, discovered that air that flows over a curved surface, tends to follow that surface. The technology was first applied to improve aerodynamics in warplane designs.
Caltrel also offers improved capacity, strength, weight, fluid flow and lifespan. Unlike conventional copper heat exchangers, the plastic heat exchangers are not subject to corrosion, oxidation, microbiological attack or galvanic action. (With galvanic action, dissimilar material can set up an electrical current, causing the metal to decompose.) This allows the plastic heat exchangers to function effectively under conditions in which conventional systems would not survive. For example, the plastic tubing is unaffected by exposure to salt water, aggressive vapors or corrosive fluids. The plastic heat exchangers therefore will not experience the slow degradation and loss in heat transfer capacity other materials experience over time. Instead, Caltrel maintains heat transfer rates closer to that of its original design capacity for the duration of its lifespan. As a result, these plastic heat exchangers have a longer life than conventional heat exchangers, regardless of environment.
The plastic is not adversely affected by chemical treatment. Systems utilizing plastic heat exchange tubing can be treated with these chemicals much like copper coil fitted systems.
The plastic material is far more resistant to impact damage during handling because unlike copper and aluminum, it has elasticity and “memory.” If pinched or dropped during handling, this plastic will return to its original position/configuration almost immediately.
A plate-on-tube condenser from Bundy improves airflow over the condenser surface by using a Coanda effect.
Friction or contact between coils during operation is not an issue for the plastic for two key reasons — one, the coils naturally dampen the vibrations in the system, and two, the Caltrel surface is smoother, permitting little friction between tubing rows even if they do come in contact. Plastic is additionally lightweight. The plastic heat exchangers typically weigh 75 percent less than similarly sized metal heat exchangers, providing for greater mobility as well as ease of installation and maintenance. And unlike most metal heat exchangers, the plastic-fitted heat exchangers have no sharp edges, greatly reducing the risk of injury during handling.
The surface of plastic tubing is smoother than copper and resists the buildup of material deposits, which can restrict both fluid and airflow through and around the coils. In the event that an internal buildup does occur, it can be removed from the plastic by flushing it at moderate pressure.
External mineral deposits can significantly decrease the heat transfer capacity of heat exchange coils. Evaporative condensers and fluid coolers are constantly battling scale buildup and often require water treatment or softeners. Because of its higher coefficient of friction, copper bonds more easily to scale. Scale does not bond easily to the smooth surface of plastic coils, and is less of a problem for evaporative systems utilizing plastic tubing.
The heat transfer capacity of plastic tubing from Powercold is comparable to that of copper heat exchangers, and that transfer rate remains consistent for both heating and cooling.
Another important benefit of a smoother surface is that pressure drops, internally and externally, are lower than that of copper. Fluids flowing through the coils encounter lower internal friction and suffer lower pressure drops at similar flow rates. This allows engineers to lower pumping horsepower for a system while maintaining required fluid flows. The same holds true for airflow across the outside of the plastic tubing. Lower pressure drops across the heat exchanger results in lower fan horsepower requirements for the system. A unique benefit of plastic material is its natural dampening characteristics, which minimize vibrations and reduce noise levels. While in operation, plastic-fitted systems create less noise than traditional heat exchangers. This characteristic is especially appealing in instances where fan coil heat exchangers are located in hotel rooms less than 10 feet from customers’ beds.
Unlike metal coils, plastic tubing is easy to contour and shape. This makes plastic heat exchangers entirely customizable for almost any application, having the ability to fit any space or configuration.
It is not as easy to customize a metal heat exchanger for unique applications, and metal heat exchangers are limited by manufacturing costs. Production costs of plastic heat exchanges are competitive or lower than conventional finned metal tube heat exchangers.
Despite their significant advantages, these plastic heat exchangers are not suitable for all applications. They cannot be used with refrigerants nor high-pressure or high-temperature systems.
Plastic is not a suitable heat transfer media for systems with operating temperatures higher than 220 DegF or pressures higher than 150 psi. Plastic additionally cannot be used with any gaseous systems because it does not serve as a sufficient vapor barrier.
Although the primary market for plastic heat exchangers is the HVACR industry, it is not limited to the industry. Plastic is versatile enough to be utilized by almost all industries requiring some form of energy transfer.