The Professor: Diagnosing Air Conditioning Systems

June 2, 2008
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This is the second of three columns on systematic air conditioning troubleshooting and diagnostics. The first appeared in the May 5 issue of The NEWS. The third part will appear in the July 7 issue. This article will deal with modern evaporator and condenser designs to meet the new minimum 13 SEER requirements.

CONDENSERS, EVAPORATORS

Other than having different pressures and temperatures, the condenser and evaporator are very similar. Condensers are usually a bit larger than the evaporator in order to handle not only the evaporator’s latent and sensible heat loads, but also the suction line superheat heat gain, the heat of compression, and finally the motor heat loads of the compressor.

Like the evaporator, the surface area of the condenser also affects the design temperature difference between the condensing temperature and the ambient or surrounding temperature. The larger the condenser, the more expensive it is to manufacture, but the EER will be much higher.

So, what should the condensing temperature and pressure be? Believe it or not, condensing temperatures and pressures will vary with coil surface area size and SEER, just as evaporator temperatures and pressures do.



COIL TECHNOLOGY

The newer condenser and evaporator coils are becoming more efficient to meet new federal mandates. Refrigerant flow patterns; fin geometry, louvers, and lances; and rippled edges all increase the coil’s heat transfer ability.

Every central split cooling system manufactured in the United States today must have a Seasonal Energy Efficiency Ratio (SEER) of at least 13. Federal law as of Jan. 23, 2006, mandated this energy requirement. Also, with the phaseout of HCFC-22 just around the corner, manufacturers of HVACR equipment are looking for energy-efficient methods to apply to their equipment to meet these new energy requirements.

The timeline for R-22 is:

2010: R-22 use is banned at the original equipment manufacturer (OEM) level with a production reduction of 75 percent from the baseline production year of 1989.

2015: 90 percent cap on R-22 production.

2020: Total ban on R-22 production.

Equipment covered in this federal mandate includes unitary equipment from 1.5 to 5 tons and split/packaged air conditioners and heat pumps. Equipment not covered includes commercial equipment greater than 6 tons, space-constrained units smaller than 3 tons (room air conditioners), and water-source units.

SEER is calculated based on the total amount of cooling (in Btu) the system will provide over the entire season, divided by the total number watt-hours it will consume. Higher SEER reflects a more-efficient cooling system. This federal mandate will impact 95 percent of the unitary market in the United States, or about 8 million units at the time of this writing. Because of this new federal mandate of 13 SEER, most air conditioning and heat pump manufacturers are looking for more-efficient evaporator and condenser designs, more-efficient compressors and fan motors, and more-sophisticated control systems.



NEW TECHNOLOGY

One such evaporator and condenser heat exchanger design incorporates an aluminum, parallel-flow, flat plate-and-fin configuration with small, parallel channels inside the flat plate. These plates are flattened, streamline tubes with each one split into smaller, parallel ports.

Refrigerant phase changes inside the channels in the plate, while strategically shaped fins (extended surfaces) enhance heat transfer from the air into the heat exchanger. The plates and fins are bonded or soldered to increase heat transfer and to eliminate any contact resistance (air gaps) that reduce heat transfer. Headers at the inlet and outlet of the heat exchanger are also bonded to the plates through soldering.

Heat is transferred from the air to the phase changing refrigerant in three steps. The heat transfer steps are:

Airside: Between the fins and the air to be cooled.

Heat conduction: Between the fins and the tubes.

Refrigerant side: Between the tubes and the phase-changing refrigerant.

The air side of the heat exchange can be enhanced through fin geometry. The addition of louvers, lances and rippled edges all increase heat transfer. The conduction between the fins and the tubes are enhanced through a metallurgical bond (soldering) to eliminate any air gaps. The refrigerant side of the heat transfer deals with how much surface area of the inside of the tubes will come into contact with the phase-changing refrigerant. This internal surface area is often referred to as a wetted perimeter.

As the internal surface area of the tubes increase, the heat transfer increases. Internal surface area can be increased by increasing the number of parallel channels inside the flat plates, and/or increasing the number of flat plates (decreasing the spacing between them).

The capacity (tonnage) of the heat exchanger can vary with its height and length. The plates can be oriented vertically for an evaporator application or horizontally for condenser applications. The vertical orientation of the flat plates allows condensate removal to occur naturally, alleviating any water drainage issues from the evaporator. This technology is being used with condensers as well as evaporators.

Applications in the HVACR field include residential air conditioning, rooftop air conditioning, chillers, geothermal heat pumps, electronic cooling, packaged terminal air conditioners (PTACs), ice machines, beverage dispensers, refrigerated display cases, and food service refrigeration. Some of the benefits of this parallel flow, plate-and-fin heat exchanger technology are listed below:

• Reduces static pressure through the coil meaning less fan watts and horsepower.

• Reduces coil depth for evaporator and condenser that lead to easier cleaning and less airside static pressure.

• Reduced internal volume reduces refrigerant charge.

• Reduced face area of condenser and evaporator.

• Smaller footprint for the condensing unit.

• 30 percent coil weight and size reduction.

• Packing costs, size and weight reduction.

• All aluminum coil, header and fins enhance corrosion resistance.

• Lower system costs.

• Higher system efficiencies than a round copper tube heat exchanger with aluminum fins.

• Lower operating costs.

• Quieter operations.

Field repair of leaking heat exchangers can be accomplished by:

• Recovering the refrigerant.

• Cleaning the leaking area with a solution.

• Brushing the area with a wire brush.

• Using a utility knife blade, remove any fins that may be in the local area.

• Pull a vacuum with a vacuum pump.

• Apply a two-part epoxy, which will be sucked into the flat plate where the leak exists.

• Apply heat with an electric blow drier until the epoxy is cured.

• Evacuate to a 500-micron vacuum.

• Charge with the appropriate refrigerant.

Field cleaning the heat exchangers can be accomplished in the same method used for a standard round copper tube heat exchanger with aluminum fins. These methods are:

• Elevate temperature of mixed cleaner to 120°F.

• Use a power washer with a broad spray pattern.

• Use nonacidic cleaners (pH < 10.5).

• When clean, rinse coil with clean water.

Publication Date: 06/02/2008

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