Microchannel evaporators are being increasingly used in air conditioning and refrigeration systems, since they aid in reducing refrigerant charge, enhancing heat transfer performance, lowering tube-side pressure drop, and reducing system size. However, as with other types of evaporators, microchannel evaporators face the issue of refrigerant maldistribution. This phenomenon leads to unequal superheated vapor at tube exits and prevents optimum utilization of the heat exchanger surface.
Microchannel evaporators provide several advantages over traditional coils, the most important of which are high heat transfer rates and low internal volume, and have been widely studied with a focus on refrigerant distribution and condensate behavior. The present study reviews the impacts of refrigerant charge quantity and arrangement of refrigerant inlets and outlets on evaporator performance.
Condensation management is essential to ensuring the efficient function of heat exchangers and critical for the correct operation of evaporators.
Hydrophilic topcoats, surfaces tending not to adsorb water or be wetted by water, are effective in environments of excessive condensation, as they enable water to flow easily off the heat exchanger surface by virtue of lowered surface tension, thereby enhancing heat transfer performance in wet and frost conditions. At the same time, hydrophilic coatings protect heat exchangers from the corrosive effect of water and exhibit fine performance in providing protection in salty environments.
In carrying forward work on microchannel evaporator designs, Kaltra presented its successive generation of coils intended for both indoor and outdoor use in air conditioners and air-source heat pumps. Thanks to the ingenuity of its dual-circuit arrangement, new evaporators offer maximum efficiency while at the same time dealing with challenges of operation in frosting conditions.
Microchannel heat exchangers have been broadly used in automotive applications due to their advantages of compactness, low weight, and high efficiency. There are numerous heat exchangers designs based on microchannel technology with folded fins and flat tubes which are connected to manifolds. These designs are predominantly used for automotive radiators, condensers, and, more recently, automotive air conditioning evaporators.
Corrosion, the deterioration of metals and alloys through a physical and/or chemical reaction with the environment, is expensive. Heat exchangers used in HVAC equipment, specifically condenser and cooling/heating coils are exposed in the environment, and this can lead to corrosion, failures, and performance degradation of the equipment in the cases of improper heat exchanger protection in corrosive locations.
Compact heat exchangers, especially those of serpentine design, are widely used for cooling in many industries, products and systems, from consumer products to IT cooling to military applications. In recent times, with the advances in microfluidics and progress in manufacturing, compact microchannel heat exchangers are becoming widespread. The principal idea behind using microchannel heat exchangers is their potential for enhanced heat transfer capabilities. The use of such heat exchangers enables the transfer of large amounts of heat while benefiting from high heat transfer coefficients.
For over three decades now, air-to-refrigerant microchannel heat exchangers have been successfully used in a number of industries, including automotive and chemical processing. More than a decade ago, microchannel heat exchangers were introduced in the HVAC industry; but it must be noted that requirements concerning heat exchangers and their operating conditions in refrigeration and air conditioning systems are different from those applicable in the automotive and other industries. Because of this, the first adaptations of microchannel heat exchangers to HVAC applications, principally as condensers, were not entirely successful.
In recent times, microchannel heat exchangers (MCHE) have started to attract attention from the HVAC industry as an alternative to fin/tube evaporators. The enhancement of heat transfer and compactness are major advantages of microchannel heat exchangers, as well as reduced refrigerant charge, lower airside pressure drop, and a reduction in materials used for manufacturing. Under current tight regulations of energy efficiency, microchannel heat exchangers are appreciated by many leading manufacturers.
Corrosion is the deterioration of metals arising from the chemical reactions between a metal and the surrounding environment. In HVAC equipment, such as air-cooled chillers, condensers, and dry coolers, corrosion of the heat exchangers may lead to performance and efficiency losses, as well as to equipment failures.
In order to avoid unwanted corrosion, microchannel heat exchangers and chilled water system, in general, must be properly protected based on environmental conditions.
Alcoil announces the issuance of US Patent No. 9,459,057 and European Patent EP 2 948 725, which relates to an improved and advanced refrigerant evaporator design. The patented technology is used as an air-to-refrigerant microchannel heat exchanger to cool air or reject heat in residential, commercial or industrial evaporators and heat pump systems.
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