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Electric Controls for Heating Systems
Learning Outcome
When you complete this chapter you will be able to… Describe and explain the function of the various components of an electric control circuit.
Learning Objectives
Here is what you will be able to do when you complete each objective.
- Describe the basic construction and operation of electrical thermostats, humidity controllers and pressure controllers.
- Describe the function and operation of the controlled devices in an electrical control systems.
- Explain the operating sequence of a basic electric control circuit.
Introduction
Electrical control systems provide control by starting and stopping a flow of electric current or by varying the voltage and current between controller and controlled device. They either employ line voltage, normally 120 or 240 volts ac, or they use low voltage 12 or 24 volts that is usually provided by a step down transformer. Using electricity for control purposes has the following advantages:
- It is readily available.
- The wiring is usually simple and easy to install.
- The signals received from sensing elements can be used to produce more than one electro-mechanical output.
- Single controller/actuator combinations are possible, without the need for a main air source as required by pneumatic controls.
Objective One
When you complete this objective you will be able to… Describe the basic construction and operation of electrical thermostats, humidity controllers and pressure controllers.
Learning Material
Electric Control System Components
Thermostats
The thermostat is one of the most important controls on a heating system since its function is to maintain the temperature of a heated space or substance at the desired level. It accomplishes this by regulating the heat supply. Various designs of thermostats are available. They may be classified according to: their application, the type of temperature sensing element used, the type of control action provided, the type of switch used, the voltage applied, and the temperature range. A description of some of the common types of thermostats follows.
1. Bimetal Thermostat This thermostat has found widespread use in residences, commercial, and institutional buildings. It has a bimetal sensing element that reacts to the changing room temperature and operates a two-position electric switch. This switch opens and closes the circuit to the controlled device that regulates the heat supply. The bimetal sensing element consists of a strip made up of two thin layers of different metals which expand at widely different rates when heated. These layers are bonded together, and one end of the strip is held in a fixed position while the other end is free to move. The movement of this free end is used to operate an electric switch. The principle of operation of the bimetal thermostat is illustrated in Fig. 1. When the room temperature is below the setting of the thermostat, the strip is straight and the contacts of the switch are closed (a), thus energizing the controlled device on the heat supply. When the room air warms up, the metals of the bimetal strip expand at different rates and, as a result, the strip starts to bend. When the temperature of the room air reaches the setting of the thermostat, the strip will have bent so far that the contacts of the switch open (b) and de-energize the controlled device so that the heat supply is stopped. When the air temperature drops again, the bimetal strip cools down, straightens out, and the contacts close again. Figure 1. Operating Principle of Bimetal Thermostats
prevents the temperature of the room air from overshooting the set temperature of the thermostat. On-off control thermostats are equipped with either an open-contact switch, a mercury bulb switch, or a snap switch. Open-contact switches are only used in light-duty thermostats controlling low-voltage current circuits. Heavy-duty thermostats, controlling high-voltage and/or heavy currents, require switches that open and close instantly to prevent burning of the contact points. Mercury and snap switches are more suitable for these applications. Fig. 3 shows an internal view of a thermostat equipped with a coil-shaped bimetal element operating a mercury bulb switch. This type of thermostat is quite sensitive, having a temperature differential of less than 1°C (1.8°F). Its operating range is between 10° and 32°C (50° and 90°F). The required temperature is set by adjusting the lever on top of the thermostat. The heat anticipator mounted below the bimetal element is adjustable, providing a means of regulating the "on-off" operation of the controlled device for greatest comfort. Figure 3. Internal View of Thermostat Showing Bimetal Element
2. Remote Bulb Thermostat This thermostat, illustrated in Fig. 4, is used to control the temperature of air in ducts, or of liquids in tanks or boilers. It is designed for use with line voltage or low voltage. The sensing bulb/capillary tube/bellows combination contains a volatile liquid. Part of the liquid in the sensing bulb is turned into vapor when the temperature of the controlled medium rises. The resulting increase in pressure expands the bellows which, in turn, operates the control switch via a lever system at a set temperature. When the temperature of the medium falls, the opposite happens. Figure 4. Remote Bulb Thermostat (Courtesy of Honeywell)
3. Immersion Thermostat (Aquastat) This type of thermostat is used to sense and control the temperature of water or other liquid in boilers, piping, tanks, etc. The sensing element of this thermostat is liquid filled and the switch is actuated by the expansion and contraction of the liquid when there is a temperature change of the controlled medium. 4. Duct Thermostat The duct thermostat is used to control the air temperatures in ducts and plenums. It contains a bimetal sensing element protected by an open metal guard tube which is inserted through the wall of the duct or plenum. 5. Modulating Thermostat The operating principle of the electric modulating thermostat is basically similar to that of the thermostats already described. It employs a bimetal strip, bellows, or remote bulb sensing unit, but it differs in the way it controls the operation of the controlled device. Since a modulating thermostat adjusts the position of the controlled device in direct proportion to the heat demand, a simple on-off switch cannot be used. Instead, this thermostat is equipped with a special regulatory mechanism. One type of regulatory mechanism commonly used is the variable potentiometer. 6. Multiposition Thermostat The operation of a multiposition thermostat is similar to that of an on-off thermostat. However, this thermostat is equipped with two or more switches which are operated in sequence, each switch controlling part of the heat supply. Fig. 5 shows a diagram of the control circuit of a two-stage heating system. Figure 5. Two-Stage Control Circuit
When used with heating systems, the humidistat is designed to open the contacts of the switch, thus stopping the operation of the humidifier when the required relative humidity has been reached. This cutout point can be changed by adjusting the dial to the desired relative humidity setting. If the humidity drops below this setting, the element becomes shorter, closes the contacts, and puts the humidifier back into operation. Humidistats should be kept dust free, and covers must permit free air circulation over the element.
Pressure Controllers
Pressure controllers can be divided into two classes according to the pressure range of the measured variable. High pressure controllers measure and control high pressures or vacuums measured in kPa or in mm of mercury (e.g. steam or water pressures in an air conditioning system). Low pressure controllers measure and control low pressures and vacuums measured in mm of water (e.g. pressure in an air duct). High and low pressure controllers have different size diaphragms. In both types, one side of the diaphragm is exposed to the pressure to be controlled and the other side is in contact with a reference pressure. Pressure can be measured in respect to atmospheric pressure or another pressure source. One available low pressure controller is a bleed-type design illustrated in Fig. 6. Figure 6. Bleed-Type Static Pressure Controller (Courtesy of Honeywell)
Objective Two
When you complete this objective you will be able to…
Describe the function and operation of the controlled devices in an electrical control systems.
Learning Material
Controlled Devices
Many different types of controlled devices can be used to perform a certain function in response to the signal of the controller. For example, an electric thermostat may control:
- The fuel valve in the supply line to the burner of a furnace or boiler.
- An electric relay that switches the circulator in a hydronic system on or off.
- An electric radiator valve in the supply line to one or more steam or hot water convectors.
- A three-way mixing valve controlling the temperature of the water supply to a heating system.
- A three-way diverter valve controlling the water flow to a heating system.
- A damper motor operating air louvres. A short description of various controlled devices will follow.
Electric Radiator Valve
An electrically operated, single-seated valve is used for two position (on-off) control of steam or hot water to radiators, convectors, etc. Valve and actuator form one assembly. The actuator includes a small electric motor that opens the valve when the controller energizes the circuit, and a spring that returns the valve to closed position when the control circuit is de-energized. The actuator can be manually opened during power-off periods, but will automatically come under the control of the controller when power is restored.
Motor-Operated Valve
Motor-operated valves are used in heating systems for modulating or two-position control of steam or hot water supply lines where tight shutoff is required. The motor operator consists of a reversible driving motor and reduction gears mounted in a sealed casing and completely submerged in oil. The operator places the valve in an open or partially open position by means of a linkage system that works against the force of a return spring. The valve is equipped with a position indicator.
Three-Way Valve
Fig. 9 shows the valve used in diverting service. In this application the flow of water, supplied at constant temperature, can be diverted to bypass the heat exchanger, thus reducing the heat supply to match a reduced heat demand. Figure 9. Three-Way Valve Installed for Diverting Service
Control Motors
Electric control motors are used for many purposes; in general, they can be divided into two classifications:
- Motors for valve operation.
- Motors for damper operation. There are several types of control motors. One of these, the capacitor-type control motor, is illustrated in Fig. 10. The internal construction of this motor may be varied to provide two-position, floating, or modulating control. This type of motor is usually built with the rotor and gear train sealed in a housing which is partly filled with oil, eliminating any
need for further lubrication during service. An example of such a control motor is shown in Fig. 11. Figure 10. Capacitor-Type Control Motor (Courtesy of Honeywell) Figure 11. Louvre-Type Damper with Motor Control
Objective Three
When you complete this objective you will be able to… Explain the operating sequence of an electric control circuit
Learning Material
Basic Electric Control System To illustrate a basic electric control system we will use a typical warm air heating example. This system consists of a thermostat which controls the operation of the burner of a heating unit in the following sequence:
- The thermostat, which is the controller, senses a drop in air temperature (the controlled variable) below the set point temperature.
- The thermostat switch, which is the controller mechanism, closes to send an electric signal.
- The electric current is transmitted through an electric wire, which is called the connecting link, to the burner, also called the controlled device.
- The burner starts up, raising the temperature of the heat transfer medium, which in this case is air, and in turn transferring the heat to the air in the building.
