Back to resources 6 June 2021 Industry information

Fogging - Technical Questions Answered

Coverage, flow rate, droplet size, and contact time. These are some technical aspects you may want to familiarise yourself with, to help you understand the application of fogging, especially when using our Hurricane range of foggers

What is Ultra-low Volume (ULV) application?

The terms used to describe the amount of liquid applied include high volume (HV), very low volume (VLV), and ultra-low volume (ULV). The quantity applied depends on the desired surface coverage by droplets and the size of the droplets. The concentration of some disinfectants can be high, and they can be applied as a concentrate (undiluted) at ultra-low volume to surfaces. Droplets of around 8 to 10 microns are usually considered to be ULV, but this definition is variable.

What is electrostatic fogging?

Electrostatic fogging is the process of adding an electrical charge to liquid droplets released from a fogging machine. It is one of the fastest and most effective ways to sanitise any area.

What is contact time and the evaporation of droplets?

Contact time or dwell time is the amount of time a disinfecting agent needs to be on a surface to kill the exposed microorganisms. Contact times are between 30 seconds and 15 minutes. Modern disinfectants dry on a surface in less than 5 minutes and reach their peak in efficacy 60 seconds after deposition. The lifetime of droplets on surfaces must match the required contact time on the product label. Research has demonstrated significant microbial reduction with contact times of 30 to 60 seconds. Factors that influence droplet lifetime are size, surface contact, and environmental conditions surrounding the droplet.

Why does droplet size matter?

The large surface to volume ratio of small droplets results in faster evaporation than large droplets. Evaporation is a function of vaporisation and small droplets lose volume quickly due to their size. The lifetime of a 100-micron droplet is about 10 times that of a 30-micron droplet. After contact with the treatment surface, droplet size decreases with the onset of evaporation. On upward-facing surfaces droplets become flat and the surface absorbs heat and evaporation increases.

What are microns?

A micron is a unit of measurement equivalent to one-millionth of a metre or one-thousandth of a millimetre. Microns are also known as micrometres. The droplets released from the Hurricane ES range from around 40 to 70 microns.

What is flow rate?

In terms of flow rate and the Hurricane ES, it is the volume output of the formulation divided by the amount of time. The Hurricane ES flow rate is 0-120 millimetres per minute, depending on which valve setting you choose to operate from. If it was operating at 120 ml/min, that would roughly equate to a litre in 8.3 minutes.

Why is coverage important?

The objective of treating surfaces is maximum coverage without runoff of large droplets. When surfaces are uniformly covered with droplets and there is no runoff, evaporation time is reduced. When droplets are separated by a distance comparable to their radius the evaporation is significantly reduced due to the saturation of the surrounding air.

What affects the rate of evaporation?

The rate of evaporation decreases when ambient temperature and humidity are high because this can result in water condensation on droplets. Exposed to 16 °C, a 56-micron droplet from a cold fogger has a lifetime of about 7 seconds in the air. A 150-micron droplet delivered from a spray nozzle has a lifetime of about 1 minute. Small droplets may not meet the contact time requirement of disinfectants if they have limited coverage area and evaporate quickly.

What is droplet lifetime?

The short lifetime of a disinfectant droplet is due to liquid vaporising from the droplet surface. It diffuses into the surrounding air until the droplet disappears. Vaporisation depends on the active ingredient and surfactants in the formulation. Standardising the lifetime of a droplet based on size is difficult. A disinfectant may be available in several formulations, and each can a different contact time because the droplets from each may have a different lifetime.