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Flow switches are used to detect the movement of air or liquid through a duct or pipe. Air flow switches are of ten called sail switches because the sensor mechanism resembles a sail (Ill. 1). The air flow switch is constructed from a snap-action micro switch. A metal arm is attached to the micro switch. A piece of thin metal or plastic is connected to the metal arm. The thin piece of metal or plastic has a large surface area and offers resistance to the flow of air. When a large amount of air flow passes across the sail, enough force is produced to cause the metal arm to operate the contacts of the switch.
Air flow switches are often used in air conditioning and refrigeration circuits to give a positive indication that the evaporator or condenser fan is operating before the compressor is permitted to start. A circuit of this type is shown in Ill. 2. When the thermostat contact closes, control relay CR energizes and closes all CR contacts. This energizes both the condenser fan motor relay (CFM) and the evaporator fan motor relay (EFM). The compressor relay (Comp.) cannot start be cause of the two normally open air flow switches. If both the condenser fan and evaporator fan start, air movement will cause both air flow switches to close and complete a circuit to the compressor relay.
Notice in this circuit that a normally closed over load contact is shown in series with the compressor contactor only. Also notice that a dashed line has been drawn around the condenser fan motor and overload symbol, and around the evaporator fan motor and over load symbol. This indicates that the overload for these motors is located on the motor itself and isn't part of the control circuit.
Liquid flow switches are equipped with a paddle that inserts into the pipe (Ill. 3). A flow switch can be installed by placing a tee in the line as shown in Ill. 4. When liquid moves through the line, force is exerted against the paddle, causing the contacts to change position.
Regardless of the type of flow switch used, they generally contain a single-pole double-throw micro switch (Ill. 5). Flow switches are used to control low current loads such as contactor or relay coils or pilot lights. A circuit that employs both the normally open and normally closed contact of a flow switch is shown in Ill. 6. The circuit's designed to control the operation of an air compressor. A pressure switch controls the operation of the compressor. In this circuit, a normally open push button is used as a reset button. The control relay must be energized before power can be supplied to the rest of the control circuit.
When the pressure switch contact closes, power is supplied to the lube oil pump relay. The flow switch detects the flow of lubricating oil before the compressor is permitted to start. Note that a red warning light indicates when there is no flow of oil. To connect the flow switch in this circuit, power from the control relay contact must be connected to the common terminal of the flow switch so that power can be supplied to both the normally open and normally closed contacts (Ill. 7). The normally open section of the switch connects to the coil of the compressor contactor, and the normally closed section connects to the red pilot light.
Regardless of whether a flow switch is intended to detect the movement of air or liquid, the NEMA symbol is the same. Standard NEMA symbols for flow switches are shown in Ill. 8.
Flow switches are used to detect liquid flowing through a pipe or air flowing through a duct. Flow switches, however, cannot detect the amount of liquid or air flow.
To detect the amount of liquid or air flow, a transducer must be used. A transducer is a device that converts one form of energy into another. In the case of a flow sensor, the kinetic energy of a moving liquid or gas is converted into electrical energy. Many flow sensors are designed to produce an output current of 4 to 20mA. This current can be used as the input signal to a programmable controller or to a meter designed to measure the flow rate of the liquid or gas being metered (Ill. 9).
Liquid Flow Sensors: There are several methods that can be used to measure the flow rate of a liquid in a pipe. One method uses a turbine type sensor (Ill. 10).
The turbine sensor consists of a turbine blade that must be inserted inside the pipe containing the liquid.
The moving liquid causes the turbine blade to turn.
The speed at which the blade turns is proportional to the amount of flow in the pipe. The sensor's electrical output is determined by the speed of the turbine blade.
One disadvantage of the turbine type sensor is that the turbine blade offers some resistance to the flow of the liquid.
Electromagnetic Flow Sensors: Another type of flow sensor is the electromagnetic flow sensor. These sensors operate on the principle of Faraday's Law concerning conductors moving through a magnetic field. This law states that when a conductor moves through a magnetic field, a voltage will be induced into the conductor. The amount of induced volt age is proportional to the strength of the magnetic field and the speed of the moving conductor. In the case of the electromagnetic flow sensor, the moving liquid is the conductor. As a general rule, liquids should have a minimum conductivity of about 20 micro-ohms per centimeter.
Flow rate is measured by small electrodes mounted inside the pipe of the sensor. The electrodes measure the amount of voltage induced in the liquid as it flows through the magnetic field produced by the sensor (Ill. 11A). Since the strength of the magnetic field is known, the induced voltage will be proportional to the flow rate of the liquid. A cut-away view of an electromagnetic flow sensor with a ceramic liner is shown in Ill. 11B.
Orifice Plate Flow Sensors: Orifice-plate flow sensors operate by inserting a plate with an orifice of known size into the flow path (Ill. 12). The plate is installed between two special flanges (Ill. 13). The flanges are constructed to permit a differential pressure meter to be connected across the plate. When liquid flows through the orifice, a difference of pressure is produced across the plate. Since the orifice is of known size, the pressure difference is proportional to flow rate. It is the same principle as measuring the voltage drop across a known resistance to determine the amount of current flow in a circuit. The disadvantage of the orifice plate sensor is that it does add restriction to the line. A differential pressure sensor is shown in Ill. 14.
Vortex Flow Sensors: Vortex flow sensors operate on the principle that when a moving liquid strikes an object, a swirling current, called a vortex, is created. Vortex sensors insert a shedder bar in the line to produce a swirling current or vortex (Ill. 15). This swirling current causes the shedder bar to alternately flex from side to side. The shedder bar is connected to a pressure sensor that can sense the amount of movement of the shedder bar (Ill. 16). The amount of movement of the shedder bar is proportional to the flow rate.
Several different sizes of vortex flow sensors are shown in Ill. 17.
Airflow Sensors: Large volumes of air flow can be sensed by prop driven devices similar to the liquid flow sensor shown in Ill. 10. Solid-state devices similar to the one shown in Ill. 18 are commonly used to sense smaller amounts of air or gas flow. This device operates on the principle that air or gas flowing across a surface causes heat transfer. The sensor contains a thin-film thermally isolated bridge with a heater and temperature sensors. The output voltage is dependent on the temperature of the sensor surface. Increased air flow through the inlet and outlet ports will cause a greater amount of heat transfer, reducing the surface temperature of the sensor.
1. What are typical uses of flow switches?
2. Draw a line diagram to show a green light that will glow when liquid flow occurs.
3. Draw a one-line diagram showing a bell that will ring in the absence of flow. Include a switch to turn off the bell manually.
4. What is a transducer?
5. What is the most common output current for flow sensors?
6. What is Faraday's Law concerning conductors moving through a magnetic field?
7. What type of flow sensors use Faraday's Law as their principle of operation?
8. What is the operating principle of the solid-state air flow sensor described in this text?
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