Heat exchanger improves air conditioning efficiency

Racine, WI--Boosting air conditioning efficiency is simple: Just enlarge the heat exchangers. Unfortunately, most residential condensers and evaporators are already too big.

By using a new heat exchanger design, however, engineers say they can achieve greater efficiency without adding more bulk. Developed by engineers at Modine Manufacturing Co., the new heat exchanger employs a novel tube-and-fin design.

The new tube-and-fin arrangement uses flat aluminum tubes metallurgically bonded to aluminum fins. Each tube contains a series of microchannel passages for refrigerant. Because of the shape, small size, and reduced internal volume of the passages, the tubes provide greater heat-exchange efficiency than conventional tubes.

  • Counterflow furnaces

  • Horizontal air handlers

  • Freezers

Additional heat-exchange efficiency is provided by the metallurgical bonding of the tubes to the fins. Modine engineers designed the unit's serpentine fins with a silicon-aluminum cladding on them. The cladding, which has a lower melting point than the aluminum fins, actually flows into the joints between the fins and the tubes during manufacturing. After it cools, the cladding provides a fillet-weld-type joint. In contrast, conventional round tube plate fin (RTPF) heat exchangers use a mechanical process to press tubes and fins together. Employing the metallurgical bond, rather than a mechanical fit, offers greater contact between tube and fin, and thus a more efficient path for heat removal.

The smaller internal volume of the new heat exchanger also enables Modine engineers to use far less refrigerant than needed for a conventional A-frame-type unit. In one application, they used 84 ounces of refrigerant in a 2.5-ton air conditioning system, compared to 130 ounces of refrigerant in a conventional 2.5-ton system. The effective surface area of the tubes, combined with the fabrication method described above and the reduced height of the fins, also results in better dehumidification characteristics. Because the system condenses more water, Modine engineers say that they achieve 25 to 30% more dehumidification than conventional systems.

Called the PFV Evaporator, the system employs a heat exchanger bent and formed in a V-shape. By using the V-configuration, rather than a conventional A-frame, the system reportedly drains more readily and consequently uses a smaller condensate tray.

When serving as an evaporator, the PFV offers greater thermal efficiency and better dehumidification than traditional units, while fitting within the confines of a conventional furnace. And as a condenser in a heat pump application, it offers higher efficiency without adding bulk to the indoor heat exchanger.

"There are other ways of enhancing heat transfer that are very costly and cumbersome," notes Terry L. Chapp, applications engineering manager in Modine's Commercial Products Division. "But this is a much cleaner, much simpler, approach."

Additional details...Contact John B. Welker, Modine Heat Transfer, Inc., 1500 DeKoven Ave., Racine, WI 53403, (414) 636-1808.

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