Benefits of brazed heat exchangers – better heat transfer

One of the largest potentials to increase efficiency of heat exchangers lies within the heat-transfers: reducing condensing temperatures by 3 °K will improve overall system efficiency by approx. 10 % for a standard R 410 A air conditioning cycle. A minimization of the temperature difference between the air flows and the phase change temperatures of the refrigerants can be achieved by improving the heat transfer efficiency of the heat exchangers. Brazed microchannel heat exchangers have already proven that they are a cost effective solution for the utilization of this optimization potential – as well as boasting a number of other benefits (see below). Brazed microchannel heat exchangers have been the technology of choice in the automotive industry for the past 10 to 15 years, and are already making inroads into the stationary HVAC&R industry for the following convincing reasons.

Poor contacts between fins and tubes account for approximately 5 – 10 % of heat transfer resistance in a standard heat exchanger manufactured by mechanical or hydraulic expansion of the round tubes because this always leaves imperfect connections between the parts. The microscopic image shows the small gaps between fins and tubes responsible for contact resistance that reduces heat transfer performance.

Heat transfer

Fig. 1: Small gaps between fins and tubes reduce heat transfer performance in mechanically or hydraulically manufactured heat exchangers.

Brazed connections are much better because they metallurgically bond the fins and tubes in a single conductive material, eliminating all potential sources of contact resistance.

Heat transfer 2

Fig. 2 Excellant heat transfer performance because no gaps in brazed connections.

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Active surface finishing with a brazing agent

NOCOLOK® Flux is the world’s most widely used flux for aluminium brazing in a controlled atmosphere. Well-proven in the automotive industry, NOCOLOK® Flux is also increasingly used for brazing aluminium coolers for air conditioning and refrigeration systems. In the well-known standard applications NOCOLOK®Flux is not corrosive. To improve the positive properties under extreme conditions even further, Solvay Fluor, has developed a new brazing agent for the markets: NOCOLOK® Li Flux.

This new flux builds a very smooth surface residue. The new physical-chemical properties present an optimization of the compatibility in hydrous environments. NOCOLOK® Li Flux has passed several test series with good results and is meanwhile in the testing phase in many companies.

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Cs flux for brazing Al with a higher Mg content

NOCOLOK® Cs is a special mixture that is intended for brazing Al alloys with a higher Mg content. By adding Mg to the alloy (types 6000 or 3000) higher material- and pressure resistance as well as better machine processing is now given, compared with NOCOLOK® standard Flux which could tolerate only low Mg contents.

During brazing, Mg diffuses to the Al surface and joins the flux. These chemical compositions have a higher melting point than the filler metal, so removal of the oxides by the flux is inhibited.

In this situation Cs can function as a buffer while the original potassium-based flux remains unchanged in its composition. So alloys with a Mg content of up to 0.8% can be brazed in a CAB furnace.

While flame brazing, the brazeability rate is even higher (1.5 % Mg) due to quicker heating.

The synthetic mixture of only 2% Cs represents an optimized ratio with respect to efficiency and profitability whereas the physical characteristics of NOCOLOK® standard grade such as melting point and granulometry remain nearly unchanged. The following benefits are also very interesting:

  • Lower leakage rates and less porosity in production
  • Better finish of the brazed work piece after brazing
  • No change with respect to standard flux loading
  • No change in brazing cycle.

The market trends are clearly obvious. Enormous global cost pressure requires reduction in costs for basic Al material (decrease in wall thickness) and lowering of production costs. NOCOLOK® Cs can help in this and make headers of heat exchangers or fittings more easily brazeable.

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The six fundamental rules for successful brazing

1. Have a clean surface

2. Heat the joint evenly to brazing Temperature

3. Choose the right brazing alloy for the job

4. Select the appropriate means of removing the oxide skin from the faying surfaces of the joint

5. Use a capillary gap of the appropriate size

6. Apply the brazing alloy to the last part of the joint to reach brazing temperature.

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