Archive for January, 2011

HVAC Designing a Perimeter System

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HVAC Designing a Perimeter System

After the heat loss or heat gain has been calculated, the sum of these losses or gains will determine the size of the duct systems and the heating and cooling unit. The three factors that ensure proper delivery and distribution of air within a room are location of outlet, type of outlet, and size of […]

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HVAC Water Tower Controls

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HVAC Water Tower Controls

Temperature controls for refrigerating service are designed to maintain adequate head pressure with evaporative condensers and cooling towers. Low refrigerant head pressure, caused by abnormally low cooling water temperature, reduces the capacity of the refrigeration system. Two systems of control for mechanical and atmospheric draft towers and evaporative condensers are shown in Figs. 14-23 and […]

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HVAC Pressure Control Switches

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HVAC Pressure Control Switches

One safety feature for air-conditioning units with a compressor and condenser is a pressure-controlled switch. This switch is wired into the circuit to protect the system in case the system develops a leak. If a leak develops, it is possible to draw in moisture and air and damage the whole system. If the pressure builds […]

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HVAC Cold Anticipator

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The cold anticipator is a fixed resistor and is not adjustable. It heats the bimetallic coil that operates the points whenever the air-conditioner compressor is not on. When the compressor is on, the resistor is shorted out by the thermostat points being closed. The heating of the coil while the points are open causes it […]

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HVAC Heat Anticipator

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The reason for the heat anticipator is to limit the degree of swing between turning on the furnace and the temperature of the room. It is a resistance heater element that is inserted in series with the thermostat line that runs to the heat-contactor coil. Once the thermostat contacts are closed current flows through the […]

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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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