Remaining competitive today requires achieving higher levels of process performance. At many plants, this demands optimal spray performance over a wide range of operating rates. However, most spray applications receive insufficient attention and design consideration because they are subsystems of unit operations.
Hundreds of applications in the chemical industry rely on sprays — for example, quenching or conditioning gas, injecting catalyst and washing filter cake. The sprays primarily serve to enhance heat and mass transfer or to distribute liquid material over an area or through a volume of gas. Some applications require more than one function to achieve the best process performance. For example, rapid cooling of hot gas not only depends upon a fine average drop size so drops evaporate rapidly but also on drops contacting all the gas. Designs of these systems use multiple nozzles to achieve the contacting and average drops requirements.
Selecting the most-appropriate and cost-effective technology requires thorough process knowledge.
Plants most commonly turn to single-fluid and two-fluid nozzles to achieve a wide range of control. However, spill-return, poppet and pulsing nozzles are gaining wider usage because they offer significant advantages in some applications. So, let’s look at each of these options.
Processes involve a range of operation from startup to full production rate. The maximum design rate is critical but high levels of performance over the entire operating envelope is valuable and, in a growing number of applications, essential. This range of operating conditions requires a range of rates of sprayed material. Most commonly, plants opt for hydraulic or single-fluid spray nozzles because of their low cost, simplicity and good performance at design rate.
Often the performance of a unit operation with a spray is optimized for full rate but the system must operate acceptably over the entire operating range of a facility. In a multi-product plant, the maximum operating rate may vary from one product to another. Consequently, the operating rate of the sprays may change. In addition, the range of actual normal operations may differ from product to product. The result is a process demand for a higher level of spray performance over a wide range of operation.
Many processes require sprays to operate effectively over a 1:10 range of rates. Figures 1 and 2 show the impact of operating rate on spray characteristics of a commercially available single-fluid nozzle. Running a single-fluid nozzle over a 1:10 range results in a nozzle pressure drop that varies by a factor of 100 (Figure 1). The pressure drop can be approximated as a squared function of flow rate. Often, more important to process performance is the average drop size, DV50, which varies from 2,700 microns to 500 microns. The process performance of these sprays differs significantly. The drop evaporation time will be approximately 29 times longer for the larger spray (lower rate operation with 2,700-micron drops). The drop trajectory in a process vessel also changes. The high rate spray has a substantially higher velocity that, when combined with the flow rate, results in a much larger energy input as shown in Figure 2. (In a nozzle, the liquid pressure is converted into the kinetic energy of the spray drops.) The change in the energy input may impact the process dramatically. The high velocity sprays induce bulk gas or vapor motion. The additional gas or vapor motion may cause back-mixing and entrainment of sprays in unintended ways. An approximation of this effect can be calculated from the hydraulic power of the spray.
In contrast, selecting a nozzle with a 40-psi pressure drop for the high flow case would result in only a 0.4-psi pressure drop at the low-flow conditions, causing the spray nozzle to drizzle liquid instead of forming a spray.
For any application, always answer the following questions: