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New generation microchannel heat exchangers - Part I: Background

October 20, 2016

 

There are numerous heat exchangers designs based on microchannel technology with folded fins and flat tubes which are connected to the manifolds. These designs have been predominantly used for automotive radiators, condensers, and more recently, automotive air conditioning evaporators.


Attempts to apply the microchannel technology in HVAC applications have achieved limited success based on the product features, design objectives, and operating issues of HVAC applications are significantly different and more diverse than automotive applications.


Previous conventional microchannel heat exchangers, such as those configured for automotive applications use flat microchannel tubes and a brazed manifolds exhibit deficiencies when used in most HVAC applications, especially as evaporators.


Typical single- and multi-pass microchannel heat exchanger designs exhibit high refrigerant pressure drops during operation. These pressure drops are required to compensate for pressure drop losses in the manifolds. While not an issue in automotive designs, where manifold pressure drop can be low, ignored or factored into the single operating design, this pressure drop is not acceptable in most HVAC applications and can cause other system operating issues.


Conventional construction of the manifold is using the smallest round material stock size possible to match the microchannel tube width, for reasons relating to lower material costs and manufacturing associated with integral brazing of the microchannel tubes to the manifold.


Such a construction is generally not suitable for most HVAC applications. That is, for broad-based use in HVAC applications, this or similarly sized manifold diameters impose significant operational limitations regarding the capacity range of the microchannel heat exchanger, and also causes major performance issues and losses due to pressure drop in the manifolds, as well as refrigerant entrapment in the manifold area. In microchannel condensers, this tube-to-manifold size combination corresponds to about 5% to 20% operating capacity loss at various refrigerant flow conditions. In microchannel evaporators, this size combination results in a loss of operating capacity that can easily exceed 30%.

 

The pressure drop of refrigerant and fluids in the conventional manifolds is one of several phenomena that can induce mal-distribution of refrigerant vapour entering the microchannel tubes. Mal-distribution can arise in microchannel heat exchangers functioning as condensers or evaporators. In microchannel condensers, an increase in the manifold pressure results in less refrigerant being provided to microchannel tubes positioned further from the inlet of the manifold. The effect can be worse for multi-pass arrangements, depending on the number of microchannel tubes, refrigerant flow rate, as well as other reasons. Imposing additional increases in pressure through the use of multi-passes can help compensate the incorrect distribution in microchannel condensers, however, results in a significant additional refrigerant pressure drop and loss of heat transfer capacity of the heat exchanger. In microchannel evaporators, multi-pass arrangements can cause incorrect distribution, that increasingly occurs in each fluid flow pass through the tubes. In single pass microchannel evaporators, incorrect distribution of a refrigerant can be caused both in the entrance and exiting manifolds.

 

One way to avoid mal-distribution in microchannel condensers and evaporators has been to provide extremely low manifold header pressure losses as a ratio of tube pressure drop losses. In microchannel evaporators, the ratio of exit pressure drop due to the exiting manifold versus the pressure drop due to the microchannel tubes can be an important consideration. That is, the tubes near the connection may be subjected to a reduced pressure drop when compared to the pressure drop of the tubes positioned further away from the connection. If the manifold has lower pressure drop over its length than the pressure drop in the microchannel tubes, the tubes closest to the exit connection will have an increased refrigerant flow compared to tubes positioned further from the connection. Since the mass fluid flow rate is exponentially related to the induced pressure drop, the pressure drop over the length of the manifold can cause an imbalance of the amount of fluid being evaporated in each microchannel tube.

 

Conventional microchannel heat exchangers have unpredictable performance due to internal manifold baffling. Tube pressure drop losses combined with manifold pressure drop losses in multi-pass designs require an extremely complex analysis in order to predict both full load and part load performance of the heat exchanger. The refrigerant charge level can significantly affect the available condenser heat transfer surface and thus, refrigeration system capacity and energy efficiency.

 

Because of the relatively low ratio of manifold cross-sectional area to the cross-sectional area of microchannel tube and manifold to overall system capacity, there is typically insufficient refrigerant holding charge in a conventional microchannel condensers. Without the use of a refrigerant receiver, the refrigeration system is thus said to be critically charged. That is, a very small addition of refrigerant to the system may cause the microchannel condenser to back up with refrigerant inside the tubes, thus reducing the amount of heat transfer surface, thereby increasing the condensing pressure. In contrast, refrigerant undercharge in a critically charged system can cause the microchannel evaporator to have insufficient refrigerant, resulting in reduced evaporator temperatures, which in turn results in loss of refrigeration capacity and lower energy efficiency. In some cases, the low evaporator temperatures may result in system safety shutdown or evaporator failure.

 

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90571 Schwaig
Germany


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