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HVACR317 – Core for Refrigeration

Operating Controls 2 Pressure SwitchesOil Pressure Safety

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Pressure Switches

• Pressure switches are used to start and stop electrical loads such as motors or coils.

• Pressure controls contain a bellows, a diaphragm, or a Bourdon tube to create movement when the pressure is changed.

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Pressure Switches

• When used as a switch the bellows or tube is connected to linkage that operates the electrical contacts.

• When used as a valve they operate the valve directly.

• The electrical contacts are the components that actually open and close the circuit.

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Pressure Switch

• The electrical contacts either open or close with a snap action on a rise in pressure.

• The pressure switch can also sense a pressure differential and be designed to open or close a set of electrical contacts based on this differential.

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Pressure Switch

• The pressure control can either be an operating type control or a safety type control.

• It can operate either at low or high pressures depending on the type of design.

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Pressure Switch

• Pressure controls can sometimes be recognized by the small line running to them for measuring fluid pressures.

• Pressure switches are manufactured to handle control voltages or line currents to start a compressor up to 3 hp maximum.

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Pressure Switch

• The high pressure and low pressure controls in the refrigeration and air conditioning industry are the most two widely used pressure controls in the industry.

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Pressure Switch

• Some pressure switches are adjustable and some are not.

• Some controls are automatic reset and some are manual reset.

• In some pressure controls the high-pressure and low-pressure controls are built into one housing.

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Pressure Switch

• The point or pressure setting at which the control interrupts the electrical circuit is known as the cut-out.

• The point or pressure setting at which the electrical circuit is made is known as the cut-in.

• The difference in the two settings is known as the differential.

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Pressure Switch Differential

Controls pressure

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Pressure Switch Differential

Controls temperature through pressure.

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Pressure Switch

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Pressure Switch

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Pressure Switch

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Pressure Controls - High

• High Pressure controls open on a rise in pressure.

• If you hold a high pressure control in your hand at room pressure it is closed.

• Primarily used as a safety device to shut off compressor if pressure rise to high.

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Pressure Controls - High

• Most high pressure controls have a manual reset.

• This is to prevent cycling in an unsafe condition.

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Pressure Switch - High Type

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Pressure Controls - High

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Pressure Switch - Ranco

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Pressure Switch

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Pressure Switch - Low

• Low pressure switches are used to sense a drop in pressure.

• Most often the low switch is used to sense a loss of charge or refrigerant.

• The low pressure switch is also used to control the compressor in a pump-down system.

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Pressure Switch - Low

• It is not desirable to run a system with a low charge because:– The compressor motors use refrigerant to

cool the windings.– If the system goes into a vacuum because

of a leak it may pull air and noncondensable gases into the system and cause more damage.

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Pressure Switch - Low

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Pressure Switch - Combo

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Oil Pressure Safety

• The oil pressure safety switch is a low and high pressure switch that is a pressure differential control.

• The differential control takes the high and low pressure and compares the two numbers coming up with a net pressure.

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Oil Pressure Safety

• The net oil pressure is the higher number minus the lower number.

• The net oil pressure is the useable oil pressure.

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Oil Pressure Safety

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Oil Pressure Safety

• The oil pressure is caused by a pump attached to the crankshaft of the compressor.

• When the compressor is not running there is no oil pressure.

• Because of this the oil pressure safety contains a timer that delays the action of the switch for roughly 90 seconds.

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Oil Pressure Safety

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Oil Pressure Safety

• The oil pressure is measured between the suction side (compressor crankcase) and the oil pump outlet.

• Most compressors must have a minimum of 30psi of oil pressure.

• Always follow manufactures directions.• Oil pressure safety controls have a

manual reset.

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Oil Pressure Safety

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Pressure Switch Symbols

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PRESSURE STATE with CUT-IN/OUT ADJUSTMENTS

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VARIOUS PRESSURE STATS(2-Position | ON/OFF)

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LOW PRESSURE HIGH PRESSURE SWITCHES

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DEFROST TIMER SWITCH

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DEFROST TIMER SWITCH

TIMING MOTOR

TIMED CONTACTS

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COMPRESSOR ANTI-SHORT CYCLE SWITCH