Main Factors Affecting Spray

All the data relating to our nozzles contained in our catalog are based on spraying water at standard temperatures. When spraying liquids other than water, performance is likely to be different. The following presentation describes how various liquid properties and operating conditions affect performance.

FLOW RATE

Any nozzle will produce a certain flow rate at a given pressure differential. The differential pressure is the difference between the pressure of fluid in the pipe just before exit minus the pressure of the vessel it is being sprayed into so it is important to compensate for friction losses and if the fluid is being sprayed into a pressurised vessel.

The flow rate from a given nozzle can be calculated by the following formula:

The K factor is a unique constant for that particular nozzle which should be listed in the nozzle data table.

Nozzle Discharge Coefficient K-factor

Spray nozzles are designed to produce certain spray characteristics, most notable of which is the relationship between fluid flow rate and inlet pressure. In attempting to gain some commonality between various manufacturers, styles and capacities, it became readily accepted by the fire protection industry to use the nozzle discharge coefficient (or K-factor) for system design. ...

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For many nozzles n = 0.5 which means the quotation simply become:

This is commonly used to apply to all nozzles but it is in fact erroneous to do so for some nozzles. In particular non-spiral design full cone nozzles and wide angle full cone nozzles will have an n exponent of 0.46 or 0.44. 

A further tip is to ensure that the K factor is in the correct units. Whilst it is technically a unit-less constant it will depend upon whether metric or imperial units are being used for P and Q. There is a metric K factor and an imperial K factor for each nozzle. So caution needs to be exercised when when reading K factors from data sheets i.e. one needs to check whether the K  factor is for metric or imperial units.

Viscosity also plays a role in capacity. Increasing viscosity reduces the turbulence of the rotational flow inside full-cone and hollow cone nozzles at a given pressure since the internal flow velocity decreases. The net effect is an increase in capacity, although usually, at the expense spray pattern integrity. Other types of nozzles, such as flat fan nozzles, which do not rely on rotational flow, generally show a decrease in capacity with increasing viscosity, simply because of a lower exit velocity.

The effects of viscosity on capacity were discussed in the previous paragraph. Viscosity is a property of liquids that measures the intermolecular attraction between its molecules. The greater this attraction (higher viscosity), the greater will be the resistance for the molecules to move over one another, which is what occurs during liquid flow. High viscosity liquids have a profound effect on the spray pattern. In general the pattern deteriorates and the spray angle narrows considerably compared to the equivalent value for water.

Viscosity is highly temperature dependent. Small increases in a liquid’s temperature can dramatically reduce its viscosity.

VISCOSITY

The effects of viscosity on capacity were discussed in the previous paragraph. Viscosity is a property of liquids that measures the intermolecular attraction between its molecules. The greater this attraction (higher viscosity), the greater will be the resistance for the molecules to move over one another, which is what occurs during liquid flow. High viscosity liquids have a profound effect on the spray pattern. In general the pattern deteriorates and the spray angle narrows considerably compared to the equivalent value for water.

TEMPERATURE

The liquid temperature is an important factor in nozzle performance since it has a direct bearing on the other elements that affect performance; namely specific gravity, viscosity, and surface tension. The chart below summarizes these effects.

SURFACE TENSION

This property of liquids refers to the behavior of those molecules that lie at or near its surface and are in contact with a different medium (either a solid surface or air.) The surface tension is essentially a force at this interface that minimizes the potential energy of all the molecules involved. Water has a very high surface tension. Most all other liquids exhibit lower values. The “beading up” of water on a glass surface is indicative of a high surface tension. Adding soap or some other surfactant to water dramatically lowers the surface tension as evidenced by the water now “spreading out” over the glass surface.