Air flow switches can be used for air filtration and supply systems, duct heating, exhaust venting and much more. Like their water equivalents, they can be designed as either mechanical flow switches or non-intrusive varieties that sense rate of passage by other means.
In the following sections we’ll cover various examples of common air and gas flow switch uses.
Air Flow Switches for ducting and HVAC
Air flow switches are widely found in all manner of ducting and HVAC systems, often using a (mechanical) paddle-type operation to trigger a microswitch when flow increases or decreases beyond a set rate or volume.
In HVAC and ducting systems, many air flow switches can also be used to perform actions such as air handling, fan monitoring and filter monitoring.
Paddle air Flow Switches
Much like their water paddle flow switch equivalents, a paddle air flow switch - sometimes known as a vane-type switch - is a mechanical sensor that partially protrudes into the duct being monitored.
Gases and air being sent through the duct will physically move a paddle sitting directly in the air flow channel, and this, in turn, will trigger an action when flow increases or decreases beyond the desired rate.
Air Flow Switches for boilers
An air flow switch in a boiler will generally monitor the proper venting of waste gases from the system by fans via the flue. These gases must be safely expelled before the boiler fires up, and the boiler air flow switch is usually rigged to only allow ignition once it recognises the fans are already spinning at the required speed.
In the case of a blocked flue or fan not working properly, the air flow switch will detect that the pressure hasn’t dropped sufficiently, and won’t allow the boiler to fire on until the problem is addressed.
Inline Air Flow Switches
Again, much like an inline water flow switch, inline air flow switches are installed as an integral part of the duct through which gas or air is being fed. This makes them trickier to install and maintain in the first instance, but they also tend to be more multi-functional, better at reading continuous flow over longer periods of time, and generally require less upkeep than insertion or paddle flow switches.
Paddle type Flow Switches
Paddle flow switches are a type of mechanical switch that’s activated directly by pressure from the medium - usually liquid - passing through the duct or channel into which the switch is inserted. The paddle is often a small strip of metal or plastic that hangs inside the pipe and is attached to either a tensioned spring or a series of magnets.
In its resting position, it is considered open or closed, depending on the function of the switch and the larger system it’s part of. Any medium passing through the duct with sufficient (or insufficient) force to move the flow switch paddle to a second position will complete a circuit, and trigger an action or alarm response.
Thermal dispersion Flow Switches
A thermal dispersion flow switch is a ‘solid state’ piece of equipment, meaning it contains no moving parts (as opposed to a mechanical paddle - or vane-type product). A thermal flow switch involves a sensor probe being inserted into the centre of the flow in a duct or pipeline.
This probe is heated continuously through an input of electrical wattage, and the principle of thermal dispersion allows the rate of gas or liquid flow to be calculated according to how quickly that heat is ‘carried away’ by the molecules flowing past.
The amount of wattage having to be sent to the probe to maintain a consistent temperature gives the necessary numbers for flow rate to be calculated very accurately. As well as accuracy, thermal dispersion flow switches are prized for their flexibility (measuring both very low and very high flow rates), their ruggedness and their ease of insertion.
Oil Flow Switches
Fuel or oil flow switches work exactly like other liquid flow switches - and as such, they come in various configurations, with the most common being paddle or thermal variants.
Switches designed for use with fuels and oils tend to be somewhat more hard-wearing than versions intended for less aggressive media, and can usually operate at higher temperatures with more robust seals and housing.
Ultrasonic and non-intrusive Flow Switches
An ultrasonic flow switch is a popular variety of non-intrusive flow switch, meaning they don’t actually have to penetrate the wall of the pipe, duct or channel you’re attempting to monitor flow through.
Instead, an ultrasonic version can be clamped to the outside of the duct, where it reads and reacts to flow rate by pinging a signal back and forth from sensors. This signal is refracted and reflected by particulates or bubbles in the stream, before returning to the sensor, and the flow rate can then be calculated using the Doppler effect. (The Doppler effect is the reason an ambulance siren seems to change pitch as it passes you in the street!)
Because ultrasonic flow switches generally rely on particulates or aeration in the liquid, they tend not to be suitable for drinking or distilled water, and are much more commonly found monitoring wastewater and other ‘dirty’ media. Clamp-on varieties will only work with certain duct types due to risk of interference with the signal, and are seldom seen on lined pipes for that reason.
Mechanical Flow Switches
Mechanical flow switches are any type of switch that has one or more moving parts triggered by direct physical contact with the medium being monitored. They usually work by the movement of components causing a circuit to be completed, which in turn triggers the action or response needed.
Paddle-type flow switches are a common example of a mechanical flow switch. Mechanical switches need to be tested and periodically replaced, as a continual physical movement of parts eventually leads to wear and tear that can affect operation.
PVC Flow Switches
PVC flow switches are often among the most basic type of product for this application and usually favoured where a quick visual confirmation of flow/no flow status is required. A PVC flow switch is usually mechanical and can be either vane-type or magnetic with one or more moving parts.
They tend to be fairly economical to install, and are generally quite rugged in construction. They’re usually designed for easy cleaning and maintenance as well as offering good chemical compatibility and anti-corrosive properties.
Industrial Flow Switches
Industrial flow switches tend to be larger, more robust versions of many of the flow switch types outlined above. They’re usually able to withstand significantly higher volumes, pressures and flow rates, and are often designed to handle far more aggressive, heavily contaminated or hazardous materials.
Industrial flow switches often eschew magnetic operating parts, in order to function well with water containing rust and other metal particulates.
Calorimetric Flow Switches
Understanding how calorimetric flow switches work is fairly simple if you’ve already got to grips with the function of their closely-related thermal dispersion equivalents. A calorimetric switch uses two temperature sensors, one of which is heated and one of which monitors the ambient temperature of the medium in the channel.
When the difference between these two readings is compared, the flow rate can again be calculated - faster flow will result in a smaller difference, because heat is carried away more quickly from the warmed sensor under higher flow rates.
Pneumatic Flow Switches
A pneumatic flow switch generally substitutes an air valve for the mechanical or paddle-type switch and can be used to control the passage of air through relays or valves when the flow starts or stops.
An air valve will often be set up to bleed pressure from a system if the flow rate falls to too low a level. Pneumatic flow switches are also useful in applications where there’s a necessary pressure differential between different areas of ducting.
Adjustable Flow Switches
An adjustable flow switch is any type of device that allows the user to quickly and easily recalibrate the desired flow settings without disassembly of the unit. This is usually done with a flat-head screwdriver, opening or closing to adjust the sensitivity of a vane or paddle located inside the housing of the switch.
Rotary Flow Switches
A rotary flow switch functions as a basic turbine, generally in the form of a multi-bladed or paddled wheel mounted entirely within the duct’s stream on a free-spinning bearing. It registers flow rate as liquid (the most common medium for rotary flow switches) passes over it. They’re available in impeller, piston (shunt) and paddlewheel designs.