Microchannel Heat Exchangers

Thermal engineering is about abstract topics: heat transfer, thermodynamics and fluid flow. One of the convergence practical products is the heat exchanger (HE), which the function is to provide a mean to transfer thermal energy between fluids flowing at different temperatures. Heat flow is always running from hot to cold. In this article we will limit ourselves to HE in which the “cold” flow is the room air pushed by ventilators and cooled off fluid is the refrigerant gas (chiller condensers) or water (drycoolers). Conventional HEs often used in air conditioner devices, are produced in serpentine shape straight copper tubes connected and disposed in lines and columns crossed by aluminum fine tin plates, fins, that are pressed in the surface. The fin, which function is to compensate air lowest capacity to transfer heat the fluid flowing inside the tubes, is delicate and can be easily damaged which makes its cleaning process a very difficult task to be performed. Also, the contact between the tube walls and fins is precarious, which limits the improvement effect of thermal exchange and can cause galvanic corrosion in ocean smell or acid rain environments, reducing shelf life of the HE.

With water price increased and environmental awareness peaking the way it is, there is a clear trend to use air condensers in chillers and air conditioner devices. One of the limiting factors is the space taken by the condenser, especially if the chiller is designed to operate in tropical climate. Microchannel technology exchangers, widely used for over 20 years in car radiators and condensers, started to be used with advantages as condensers. They are manufactured with a resistant aluminum alloy and consist of: (a) several flat bars with 1 mm diameter channels and arranged perpendicular to the air flow; (b) two collectors that connect the channels and serve as connection to the cooled off fluid; (c) fins shaped as crossed strips welded between bars. They are manufactured in automatic lines with a brazing furnace to ensure the perfect contact between the parts and eliminate leakage possibility. The advantages of microchannel HE are:

• Shelf life higher than 20 years even in facilities by the edge of the sea;

• Lower resistance to air passage reducing the electricity consumption of ventilators and noise level;

• Low speed flow in channels providing energy savings in the compressor (or pump);

• Great gain in efficiency in heat exchange, resulting in more compact HE and about 70% lighter;

• Reduction of up to 40% refrigerant load when used as condensers;

• HE is thinner and has more resistant fins reducing airlock possibility and facilitating cleaning.

This innovation is already present in special chillers and Modular DryCooler (refer to cover article). As of 2010, Mecalor chillers will be supplied regularly with microchannel condensers allowing its continuous operation in environments of up to 42ºC. “The customers win, because they get more durable equipment, easy to clean, with lower energy consumption and smaller refrigerant load. All of this in a competitive price and within highest quality standards which are Mecalor’s trademark”, comments Engineer Raul Imada, technical director of Mecalor.

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