In microchannel evaporators, overall thermal performance is strongly governed by the uniformity of refrigerant distribution among parallel microchannels.
Uniform refrigerant distribution and effective condensate drainage have long been among the main engineering challenges in microchannel evaporator design. While Kaltra’s previous coil generations successfully solved these aspects individually, certain demanding conditions — such as low-load operation, frosting, or use in reversible heat-pump systems — continued to push design limits.
Previous studies have examined refrigerant distribution across tubes in microchannel evaporators. Various methods have been explored to minimize refrigerant maldistribution between tubes, including specialized inlet manifold designs with DX distributors, flash gas bypassing, coil geometry optimization, and refrigerant charge control.
Continuing its research on reducing quality-induced maldistribution in microchannel evaporators, Kaltra introduces the vapor/liquid separation method combining flash gas bypassing (FGB, discussed in the previous article) and vented inlet manifolds.
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 for ensuring the efficient function of heat exchangers and is critical for the correct operation of evaporators.
Hydrophilic topcoats, or water-resistant surfaces, are effective in environments with excessive condensation, as they enable water to flow easily off the heat exchanger surface based on the 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 provide excellent 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.
The company’s research center completes work on the streamlined microchannel evaporator design that displays amplified performance under partial load conditions. Evaporator’s outlet manifold was subjected to optimization to achieve optimum refrigerant distribution between multiport tubes.
Microchannel heat exchangers have seen increased application in air-to-water heat pumps due to offering significant charge reduction, compactness, lower air pressure drop, and consequentially, lower fan power consumption compared to traditional round tube-plate fin coils.
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.
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.
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.
