Everything You Need To Know About Flow Switches

Air flow switches play a crucial role in air filtration and supply systems, duct heating, exhaust venting, and many other applications. Similar to their water counterparts, they come in both mechanical and non-intrusive forms, allowing them to sense the rate of passage through various means.

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In the following sections, we'll explore several common uses of air and gas flow switches.

Air Flow Switches in Ducting and HVAC Systems

Air flow switches are extensively used in ducting and HVAC systems. Often, they employ a mechanical paddle-type mechanism to activate a microswitch when the flow rate surpasses or drops below a predetermined level. Within HVAC and ducting setups, these switches also assist in air handling, fan monitoring, and filter oversight.

Paddle Air Flow Switches

Similar to water paddle flow switches, paddle air flow switches, also known as vane-type switches, are mechanical sensors that partially extend into the duct being observed. The movement of gases or air through the duct shifts the paddle within the airflow channel. This triggers an action when the flow rate exceeds or falls below the set threshold.

Air Flow Switches for Boilers

In boiler systems, air flow switches ensure proper venting of exhaust gases via fans through the flue. These switches often enable ignition only when they detect that the fans are operating at the required speed. If there's a flue blockage or a malfunctioning fan, the switch detects an inadequate pressure drop and prevents the boiler from igniting until the issue is resolved.

Inline Air Flow Switches

Inline air flow switches are integrated directly into the gas or air duct, making initial installation and maintenance somewhat complex. However, they are usually more multifunctional and provide better continuous flow readings over extended periods, requiring less upkeep than insertion or paddle flow switches.

Paddle Type Flow Switches

Paddle flow switches are mechanically activated by the medium flowing through a duct or channel. The paddle, made of metal or plastic, moves under the medium's pressure, completing a circuit and initiating an action or alarm response when the flow rate reaches the designated level.

Thermal Dispersion Flow Switches

Thermal dispersion flow switches, consisting of no moving parts, use a sensor probe inserted into the flow path of a duct or pipeline. The probe is consistently heated, and the rate of heat loss to the passing gas or liquid determines the flow rate. These switches are known for their accuracy, versatility in measuring various flow rates, durability, and ease of installation.

Oil Flow Switches

Fuel and oil flow switches function similarly to other liquid flow switches, often available in paddle or thermal variants. Due to the demanding nature of fuels and oils, these switches are typically more robust, suited to higher temperatures, and feature sturdy seals and housings.

Ultrasonic and Non-Intrusive Flow Switches

Ultrasonic flow switches are a popular type of non-intrusive switch that do not penetrate the pipe or duct walls. These switches use sensors to transmit and receive signals, calculating flow rate using the Doppler effect. Because they rely on particulates or aeration in the liquid, they're not suitable for drinking water and are more often used in wastewater monitoring.

Mechanical Flow Switches

Mechanical flow switches typically involve moving parts that come into direct contact with the monitored medium. Paddle-type switches are a common example that needs periodic testing and replacement due to wear and tear from continuous physical motion.

PVC Flow Switches

PVC flow switches are generally easy to install and maintain, offering good chemical compatibility and anti-corrosive properties. These switches provide a quick visual indicator of flow status and are typically used in less complex applications.

Industrial Flow Switches

Industrial flow switches are more robust versions of standard switches, capable of handling high volumes, pressures, and flow rates. They are designed for harsh conditions, often avoiding magnetic parts to function better with metal-containing fluids.

Calorimetric Flow Switches

Calorimetric flow switches, much like thermal dispersion switches, use two temperature sensors to measure flow rate by observing the temperature difference. These switches are valued for their high accuracy and reliability in different flow conditions.

Pneumatic Flow Switches

Pneumatic flow switches use an air valve instead of a mechanical paddle to control air flow through relays or valves. These switches are handy in applications needing pressure differential control between different ducting segments.

Adjustable Flow Switches

Adjustable flow switches allow easy recalibration of the desired flow settings without disassembling the unit, typically using a simple screwdriver to adjust the sensitivity of an internal vane or paddle.

Rotary Flow Switches

Rotary flow switches operate like a mini-turbine, using a multi-bladed wheel within the duct to measure flow rate. Available in various designs, these switches are commonly used for liquid media measurement.

A positive displacement (PD) flow meter is well-regarded for its high accuracy, often around ±0.1% of the actual flow rate, and excellent repeatability. They require no power supply and don't need long, straight pipe runs for accurate readings, making them ideal for measuring the flow of viscous fluids like oils, fuels, and solvents. As a result, they are widely used in household water meters and various industrial and petrochemical applications.

How Does a Positive Displacement Flow Meter Work?

PD flow meters measure flow by passing isolated, known volumes of fluid through gears or chambers in the meter. By counting these volumes, a flow measurement is obtained. Each design has a unique method of isolating and counting these volumes. The pulse train frequency measures the flow rate, while the total pulse count calculates the batch size. The kinetic energy of the fluid powers PD meters, and metering pumps add kinetic energy to the fluid while determining the flow rate.

These meters come in sizes from 1/4" to 12" with turndowns as high as 100:1, though more common ranges are 15:1 or lower. They offer increased accuracy with higher fluid viscosity but require clean, contaminant-free process fluids.

Testing, Calibration, and Provers

Meters with moving parts need periodic testing, recalibration, and repair due to wear. Recalibration can be done in a lab or online with a prover. Gas systems are recalibrated using a bell-jar prover, while liquid systems use secondary standards or gravimetric flow loops. Field-mounted and in-line provers are also available for on-site recalibration.

Figure 1: Field-Mounted, In-Line Flow Prover

Provers offer repeatability of about 0.02% and can handle operating flows from 0.001 gpm to 20,000 gpm, with bench-top, truck-bed, trailer, or inline mounting options.

PD Meter Accessories

PD meter accessories include strainers, filters, air/vapor release assemblies, pulsation dampeners, temperature compensation systems, and various valves for dribble cut-off in batching systems. Mechanical registers can also feature ticket printers for inventory control, and there are options for batching flow computers, transmitters, and AMR devices for remote reading.

Benefits of Positive Displacement Flow Meters

Positive displacement flow meters offer numerous benefits:
Accuracy: High precision in measurements due to minimal clearance between internal components.
Range and Repeatability: Capable of handling a wide range of viscosities with impressive accuracy and repeatability.
Reliability: Long-lasting performance with minimal error when selected correctly for the application.
Low Maintenance: Maintenance levels depend on the fluid's lubricating properties, often requiring minimal upkeep compared to other system components.

Limitations of PD Flow Meters

Despite their robustness, PD flow meters have limitations:
Large particles in fluids must be filtered out prior to measurement.
They are unsuitable for fluids with substantial air pockets unless air eliminators are used.
Consideration of pressure drops caused by the meter is essential in system calculations.

Additional reading:
What is a fully automatic water meter test bench?

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Types of Positive Displacement Flow Meters

Positive displacement flow meters operate by separating fluids into measured increments and counting them through a register. This makes them suitable for measuring viscous liquids and for applications requiring precise batching and accounting.

Liquid Positive Displacement Flow Meters or Nutating Disk Meters

Nutating disc meters are commonly used as residential water meters. As water flows through the metering chamber, it causes a disc to wobble (nutate), turning a spindle and rotating a magnet coupled to a register or pulse transmitter. The meter housing is usually bronze or plastic for corrosion resistance. These meters are available in sizes from 5/8 inch to 2 inches and can tolerate 150-psig operating pressures.

Figure 2: Positive Displacement Flow Meter Designs

Rotating vane meters use spring-loaded vanes to trap and meter fluid increments. They are used in the petroleum industry, capable of handling high flow rates and achieving high accuracy. Rotary displacement meters operate similarly, using a fluted rotor and wipers to measure flow.

Oscillating Piston Flow Meters

Commonly used for viscous fluids like oil, oscillating piston meters feature a cylindrical measurement chamber and piston with numerous openings for fluid flow. They are available in sizes from 1/2 inch to 3 inches, handling flow rates up to 65 gpm continuously and up to 100 gpm intermittently.

Figure 3: Piston Meter Designs

Oval Gear and Lobe Meters

Oval gear meters use two fine-toothed gears to measure flow accurately. These meters can achieve high accuracy for fluids with viscosities over 10 centipoise. Variations like rotating lobe and impeller meters handle different flow conditions and temperature limits.

Figure 4: Rotating Positive Displacement Meters

Helix Meters

Helix meters use radially pitched helical gears to measure flow, with low pressure drops compared to other PD meters. They are ideal for highly viscous fluids like glues and polymers.

Figure 5: Effect of Viscosity on Low-Flow Accuracy

Metering Pumps

Metering pumps are PD meters that also impart kinetic energy to fluids. Types include peristaltic, piston, and diaphragm pumps. These pumps are used in laboratories, medical applications, and various industrial processes.

Figure 6: Metering Pump Designs

Gas Positive Displacement Flow Meters

PD gas meters, similar to liquid PD meters, measure flow by counting entrapped gas volumes. Diaphragm gas meters are commonly used for natural gas metering, while lobe gear and rotating vane meters are also employed in various gas services.

High-Precision Positive Displacement Systems

High-precision gas meters combine PD meter technology with motor drives to eliminate pressure drops and slip flows. These systems offer exceptional accuracy for both gas and liquid measurement applications.

Figure 7: High-Pressure PD Meters Equalize Inlet and Outlet Pressures

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