Guiding principles: Full cone spray nozzle design In the field of spray nozzle development, several interrelated design levers can be adjusted to reach desired performance characteristics. The shape of internal and external spray nozzle geometries plays a major role in droplet sizes, drop- let trajectories, spray angle, and clog resistance. However, certain guiding principles in full cone spray nozzle develop- ment cannot be circumvented. For instance, under identical operating conditions (same flow rate and differential pres - sure), two nozzles with different exit orifices will generate different droplet sizes. A nozzle with a larger exit orifice will generate larger droplet sizes and vice versa. Moreover, in the case of two full-cone nozzles delivering 10 gallons per minute (GPM) at 10 pounds per square inch gauge (psig) differential, the free passage of a nozzle with a 90-degree spray angle will be larger than that of a nozzle with a 120-degree spray angle. Also, it is generally observed that the spray angle of a nozzle is inversely related to its free passage, assuming all other factors remain constant. Regarding droplet size, when two spray nozzles oper- ating at the same differential pressure are observed, the nozzle with a greater flow capacity produces larger drop - lets than a smaller capacity nozzle. For example, a nozzle designed to spray 10 GPM at 10 psig will produce larger droplets than a mechanically identical nozzle designed for 5 GPM at 10 psig. It is important to consider all these guiding principles in chorus during the conceptual design phase of a new spray nozzle in an effort to satisfy both process and operational expectations. Maximising free passage Conventional spray nozzles that are widely used today in the wash sections of vacuum distillation columns were primarily designed to be fouling resistant. The design of these nozzles centres on maximising the ‘free passage’ of the spray nozzle in the hope of reducing nozzle clogging with little consideration for their atomisation characteris- tics. Traditional maximum free passage-style nozzles tend to create a significant percentage of fine droplets, espe - cially when operating at higher differential pressures. In an effort to overcome limitations tied to conventional wash bed spray nozzles, Lechler embarked on a product devel- opment journey to design a spray nozzle specifically for use in counter-current wetting applications. During the con- ceptual design phase, a wish list of design parameters was gathered from key distillation stakeholders and ordered in terms of importance to both process and operational con- siderations: entrainment reduction, clog resistance, good distribution, and self-draining axial design. Spray nozzle internals are critical to both droplet forma- tion and clog resistance. Differential pressure is converted to kinetic energy within a spray nozzle to promote the rota- tional motion of the liquid being sprayed. A spray nozzle does not produce a uniform droplet size at any given oper- ating condition; instead, the spray plume is comprised of a spectrum of droplet sizes. The characteristics of this drop- let size spectrum are largely dependent on the mechanism of atomisation within the nozzle or, more specifically, the
Figure 1 Image of traverse PDA spray plume analysis
of small droplets, which have a higher tendency of becom- ing entrained. Moreover, as vapour side velocity increases, it exerts a greater drag force on spray droplets, essentially increasing the threshold of entrained droplets to a larger droplet diameter. Spray nozzle hydraulics also impact entrainment. As dif- ferential pressure across the spray nozzle increases, more energy for atomisation is introduced, creating finer droplets that are more easily entrainable. Entrainment thus becomes more pronounced when both the liquid feed rates and vapour feed rates are relatively high. High levels of entrain- ment in a vacuum distillation column can pose undesirable consequences, such as a higher propensity for coking and reduced yield, which are detrimental to the overall distilla- tion efficiency of a tower.
Figure 2 SMDmax model 4HR.208 during testing
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