What are Falling Film Evaporators?
What are Falling Film Evaporators, and why is their use an increasingly discussed topic in the HVAC industry?
In order to bring the market into line with the new 2021 standards, high unit performance ratios are required from the various HVAC manufacturers. New technologies, enabling to ensure higher capacities and high efficiency are thus beginning to make their way into the marketplace. These regulations also have an influence on both the type of compressors and the type of evaporators. If, from the point of view of compressors, there are increasing rumors about the magnetic levitation solution, there is a tendency, for evaporators, to move from the flooded to the so-called Falling Film Evaporators.
The Falling Film Evaporators have tubes sprayed with refrigerant, which falls as in a cascade, covering them with a freon film: hence the term “Falling Film”. Several tests have shown that starting from an evaporator with freon-flooded tubes and gradually decreasing the flooding, in favour of the spraying of the tubes, the evaporation temperature remains constant. This means that in the falling film we are keeping the evaporation temperature reached in the flooded evaporator constant, but with a lower freon charge and a smaller exchange surface!
The special tubes of the falling film evaporator are able to guarantee very high heat transfer coefficients. By making a quick comparison, if a grooved tube can reach a coefficient of 4000 W/(m2 K) and a flooded tube can reach 12,000 W/(m2 K), the Falling Film allows to reach an overall heat transfer coefficient of over 20,000 W/(m2 K).
What are the advantages of these evaporators? We can say that, with the same global heat transfer coefficient, in the case of falling films we have a smaller surface, so this means lower costs for the manufacturer in terms of the material used, and therefore a considerable saving. The falling films also require a reduced charge of freon (40% less refrigerant than the flooded) as it is not necessary to fill the entire tubes with liquid, but, as already mentioned, a thin film is now used to cover the surfaces. In markets such as heating and air conditioning, this can save manufacturers a lot of money due to high refrigerant charging costs. If the freon charge is reduced, we will consume less freon, thus drastically reducing the costs of buying it. The ideal configuration in terms of capacity and efficiency, in view of the new standards foreseen for 2021, would therefore require the use of Microchannel condensers, magnetic levitation compressors and evaporators of the Falling Film type.
The Falling Films are not, however, free from disadvantages, primarily due to the reduced Know-How of those who just started using these technologies, compared to the flooded evaporators. Moreover, the distribution of the fluid for Falling Film horizontal evaporators remains an interesting challenge, as the performance is significantly lowered in case of an uneven distribution of the film on the pipes, and it is therefore essential to distribute the freon correctly.
