Mercury Contacts

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Mercury Contacts

Some thermostats use the expanding bimetal arrangement to cause a tube of mercury to move. As the mercury moves in the tube, it comes in contact with two wires inserted into the glass tube. When the mercury comes in contact with the two wires, it completes the electrical circuit. This type of thermostat needs to […]

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HVAC Heating and cooling thermostats

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HVAC Heating and cooling thermostats

Some thermostats can be used for both heating and cooling. The thermostat shown in Fig. 14-15 is such a device. The basic thermostat element has a permanently sealed, magnetic single pole double throw (SPDT) switch. The thermostat element plugs into the subbase and contains the heat anticipation, the magnetic switching, and a room temperature thermometer. […]

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HVAC Bimetallic-type thermostat

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HVAC Bimetallic-type thermostat

Temperature changes can cause a bimetallic strip to expand or contract in step with changes in temperature. These thermostats are designed for the control of heating and cooling in air-conditioning units, refrigeration storage rooms, greenhouses, fan coils, blast coils, and similar units. This is the type used in most homes for control of the central […]

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HVAC Bellows-type thermostat

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HVAC Bellows-type thermostat

On modern condensing units, low-pressure control switches are largely superseded by thermostatic-control switches. A thermostatic control consists of three main parts: a bulb, a capillary tube, and a power element or switch. The bulb is attached to the evaporator in a manner that ensures contact with the evaporator. It may contain a volatile liquid, such […]

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HVAC Solenoids

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HVAC Solenoids

Solenoid valves are used in many heating and cooling applications. They are electrically operated. A solenoid valve, when connected as in Fig 14-10, remains open when current is supplied to it. It closes when the current is turned off. In general, solenoid valves are used to control the liquid-refrigerant flow into the expansion valve or […]

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HVAC Motor-start relays

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HVAC Motor-start relays

Relays are a necessary part of many control and pilot-light circuits. They are similar in design to contactors, but are generally lighter in construction so they carry smaller currents. Compressors used for household refrigerators, freezers, dehumidifiers, vending machines, and water coolers have the capacitor-start, induction-run type of motor. This type of compressor may have a […]

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HVAC Power Relays

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HVAC Power Relays

One of the control devices is the power relay. It is one of the most often used controls for controlling compressors in refrigeration and air-conditioning. The power relay is also referred to as the main conductor. It is used to apply the main line voltage to the motor circuit. The coil of the relay is […]

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HVAC Water valves

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HVAC Water valves

Manually operated valves are installed on water circuits associated with refrigeration systems—either on cooling towers or in secondary brine circuits. They are installed for convenience in servicing and for flexibility in operating conditions. These valves make it possible to recircuit, bypass, or shut off water flow as desired (see Fig. 11-54). These manually operated shutoff […]

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