How important is it to choose the right tool for the job? Very. Just imagine trying to shave with an axe. Sure, the tool may be sufficient, but it is not very efficient and, in this case, even a bit scary.

When it comes to car washing, nozzle choice determines cleaning performance, water and energy consumption, and, thus, operating costs. This article focuses on the often-neglected nozzle.

Choosing a nozzle that is too small causes it to work against the pump, resulting in higher power consumption and a higher mechanical load on the pump and supply line. If, on the other hand, a nozzle that is too large is selected, the pump’s power cannot be fully converted into momentum, leading to lower cleaning performance.

Nozzle Capacity

There is a vast selection of nozzle types, shapes, and sizes.  Every nozzle has a capacity or caliber corresponding to its volumetric flow rate, allowing for the comparison of different nozzles independently of their sizes and shapes. The capacity can be regarded as a volumetric flow rate at a reference pressure.

For a given nozzle, the capacity is constant over the operational range. By reducing the pressure, the flow rate drops accordingly. In the U.S. gallons per minute as volumetric flow rate and 4,000 psi as reference pressure are used. In Europe, liters per minute as volumetric flow rate and 290 psi (20 bar) as reference pressure are commonly used.

However, every manufacturer has its own definition of nozzle capacity. To get smooth numbers, nozzle manufacturers usually multiply the flow rate by a factor of 10.

To determine the appropriate nozzle capacity (C), you only need to know the flow rate at a given pressure. The nozzle capacity can be calculated using the following equation. Although the formula was derived with various simplifications, it has proven effective in practice.

The diagram shows the flow rate as a function of pressure for a fixed nozzle. It is obvious that the equation has a non-linear relationship, which is due to the root function of the pressure.

Choosing Nozzle Capacity

The nozzle must be chosen according to the pump(s) used. Like nozzles, the pump has a similar dependence between pressure and volumetric flow rate. When drawn into the same diagram, the operational point is given at the cross section of both characteristics, as shown in the chart below. The components’ manufacturer can provide both characteristics.

In self-service areas with lance cleaning, often pumps that deliver 4 gallons per minute (gpm) at 1,550 psi are used. Considering two washing areas fed by one pump results in a flow rate of 2 gpm per nozzle when the flow by the pump is evenly split. Applying these values into equation 1 results in a nozzle capacity of around 32 per nozzle.

Nozzles with non-standard values are rare — they are typically rounded to the nearest available size. Rounding down to a nozzle capacity of 30 slightly increases the pressure, whereas rounding up the nozzle capacity to 35 increases the flow rate.

Especially when rounding up the nozzle capacity, it is often found that the pressure at the nozzle is surprisingly not 1,550 psi but 10 percent lower. It is often assumed that the pressure at the pump corresponds to the operating pressure at the nozzle. However, pressure losses occur due to piping losses, such as friction, fittings, valves, and pipe bends. A rule of thumb says that the pressure for calculating the nozzle capacity must be increased by around 10 percent compared to the number given.

In this example, the calculated nozzle capacity is much closer to the typical nozzle capacity of 30, so two nozzles with a capacity of 30 should be used.

Measuring Nozzle Capacity

To determine the nozzle capacity, the total pressure directly at the inlet of the nozzle instead of the pump should be used to avoid pipe losses. A precise and fast pressure sensor and a calibrated Coriolis mass flow meter with high measurement accuracy are recommended. However, when this expensive equipment is not at hand, a more practical method can be used to determine the appropriate nozzle capacity.

A pressure gauge manometer can be mounted at the inlet of the nozzle, and the assembly can then be placed in a container with a small hole in the lid. In high-pressure applications, the jet has a high impulse, and the lid prevents spray water from being lost.

The picture below shows a typical situation that can be found in a self-service area operated with a manual high-pressure lance. To determine the nozzle capacity manually, the pressure is recorded while the nozzle is operated, and the water volume is captured over a certain time. The nozzle is pointed towards a canister or a bucket with a lid and operated for 30 seconds.

Afterward, the contents of the container are poured into a measuring cup or put on a scale to determine the amount of water. In our example, 1.17 gallons of water was sprayed from the nozzle within 30 seconds, resulting in a volume flow of 2.34 gpm. During this time, the pressure gauge showed a pressure of 1,500 psi. These values are used in equation 1.

Since it is not always possible to place a pressure gauge directly before the nozzle, we also measured the pressure directly at the pump — the pressure gauge at the pump showed a value of 1,700 psi. Since the water volume remains the same, this results in a nozzle capacity of 35.9. This underlines the rule of thumb from above, stating that the pressure loss within the piping is around 10 percent.

A laboratory testing system with highly accurate equipment leads to a nozzle capacity of 40.2. What is the cause of this difference?

The two main error sources in this example are manual time measurement, measuring the water volume with a measuring cup, and using a conventional slow-pressure gauge. However, this easy setup allows for measuring nozzle capacity at most washes without any hassle within an accuracy of 5 percent compared to a reference system, which is usually sufficient.

Summary

When operating a car wash facility, the aim should be to preserve the system and minimize operating costs. Besides choosing the right pump, choosing the right nozzle is crucial and often neglected. If the nozzle is chosen too small, the pressure inside the piping increases, imposing higher forces on the overall system. Using a nozzle that is too big leads to higher water consumption and reduced cleaning effectiveness.

 Choosing the right nozzle capacity can be done easily and accurately with standard tools. We recommend changing the system’s nozzles rather than the pump. Of course, in terms of longevity and saving operating costs, the most beneficial system is one in which all components are selected by their specific characteristics.

Thomas E. Rossegger is managing director for sales and marketing (CSO) at FDX Fluid Dynamics (www.fdx.de/en/), strengthening the management team with his experience in marketing, sales, and business development. He graduated in industrial engineering and management with a focus on mechanical engineering from the Graz University of Technology and has held various managing director positions in export-oriented industries worldwide for more than two decades.