Monthly Archives for January 2011

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 … Continue reading

27. January 2011 by admin
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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 … Continue reading

27. January 2011 by admin
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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On modern condensing units, low-pressure control switches are largely superseded by thermostatic-control switches.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 … Continue reading

27. January 2011 by admin
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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 … Continue reading

27. January 2011 by admin
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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 … Continue reading

27. January 2011 by admin
Categories: Control Devices | Tags: | 4 comments

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 … Continue reading

27. January 2011 by admin
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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, … Continue reading

17. January 2011 by admin
Categories: Controlling Refrigerant | Tags: | Leave a comment

HVAC Level-Master Control Oil and halocarbon systems

With halocarbon systems (Refrigerants 12, 22, 502, etc.) the oil and refrigerant are miscible (capable of being mixed) under certain conditions. Oil is quite soluble in liquid Refrigerant 12 and partially so in liquid Refrigerant 22 and 502. For example, … Continue reading

17. January 2011 by admin
Categories: Controlling Refrigerant | Tags: | Leave a comment

HVAC Level-Master Control Oil and ammonia systems

For all practical purposes, liquid ammonia and oil are immiscible (not capable of being mixed). Since the density of oil is greater than that of ammonia, it will fall to the bottom of any vessel containing such a mixture if … Continue reading

17. January 2011 by admin
Categories: Controlling Refrigerant | Tags: | Leave a comment

HVAC Level-Master Control Oil return

All reciprocating compressors will allow some oil to pass into the discharge line along with the discharge gas. Mechanical oil separators are used extensively. However, they are never completely effective. The untrapped oil passes through the condenser, liquid line, expansion … Continue reading

17. January 2011 by admin
Categories: Controlling Refrigerant | Tags: | Leave a comment

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