Spray technology ranges from rudimentary (a hose-end nozzle for watering lawns) to rocket science (liquid injectors in rocket engines). Hundreds of different spray nozzles are available for process plant applications. So picking the optimum one can be challenging. However, the right choice can provide important benefits such as reducing the amount of material sprayed, providing a more-consistent product or cutting energy cost.
Before getting into nozzle selection, let’s first review some functions that sprays can perform:
• generating additional surface area to enhance evaporation, heat transfer or mass transfer;
• distributing material over a surface;
• dispersing material through a volume of gas; and
• producing surface impact force to better clean equipment.
Many applications require a combination of these functions to yield the desired results.
One useful way to classify spray nozzles is on the basis of their energy input, because all require energy to overcome surface tension to produce a dispersion of drops. Some nozzles rely on kinetic energy — in the form of high-velocity liquid in a single-fluid (hydraulic) nozzle or high-velocity gas in a two-fluid (atomizing gas or air atomized) nozzle. Other nozzles use mechanical energy in the form of vibration in an ultrasonic nozzle or rotation in a rotary atomizer wheel. These four types of nozzles account for the vast majority of atomizing technology used at process plants.
Single-fluid nozzle. This is the most common nozzle. A precision device, it comes in a wide variety of materials, including metals, plastics and ceramics. Fluid pressure results in a high-velocity stream of liquid at the nozzle outlet. The nozzle’s design provides a specific spray angle and spray pattern; these are the first two characteristics to use to narrow the choice of nozzles for an application. The effective spray angle is the included angle of the visible boundary of the spray at the nozzle tip. The nozzle’s internal components and liquid passages are used to produce a variety of spray shapes, which are known as patterns or footprints, and liquid distributions. Figure 1 depicts some common spray patterns.
You can find a number of videos showing the spray patterns and qualitative characteristics of sprays on YouTube and spray-nozzle-vendor websites. These videos clearly illustrate that sprays are three-dimensional and vary with time.
Two-fluid nozzle. This is the second most common type of nozzle. Compressed air usually is the atomizing gas but many other gases, including steam, oxygen and methane, can be used. The nozzle provides two advantages: it can spray more-viscous material, and enables the average drop size to be adjusted independent of liquid flow. The downside is that the energy input is much higher per mass of material sprayed. The hidden cost of supplying the compressed air is easily overlooked.
Ultrasonic nozzle. This nozzle is used in specialty applications where a very narrow range of drop diameters and low velocity sprays are valuable. It has significant limitations, including a low flowrate (under 20 kg/h), a maximum temperature limit on the nozzle, and a limited maximum effective viscosity of the material sprayed. The cost per nozzle is much higher than that of a single- or two-fluid nozzle with equal liquid capacity.
Rotary atomizer. This device, widely used in spray dryers, discharges liquid from the perimeter of a wheel rotating at high speed. One key advantage is that the wheel’s rotational speed can be adjusted to fine-tune the average drop size. The disadvantages are that the required high-speed drive mechanism (shaft, bearings and wheel) adds cost and can pose reliability problems.
The quantitative spray characteristics your system requires depend upon the application. The most common spray parameter is the average drop size. As Table 1 indicates, different drop sizes suit different services — so, use the application as the starting point for selecting a nozzle. A smaller average drop size will produce a larger total surface area, i.e., drop surface area per volume of liquid. Because a high specific surface area is best for enhancing mass transfer or heat transfer, a fine spray is used in such cases. On the other hand, drop size is irrelevant in cleaning applications. Spray impact force is critical there. The distribution of the impact force is a key metric of performance. Surface impact force depends upon the distance between the nozzle and the surface as well as the attack angle between the spray and the surface.