The Fundamentals Of Pressure Controls

November 1, 2002
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Pressure controls serve various functions, which may be divided into control and protection functions. Examples of control functions are compressor cycling, pumpdown, or defrost control. Protection functions included pressure limiting and cutout against excessive pressures, against loss of charge or for freeze protection.

Pressure controls (such as those manufactured by Alco, a division of Emerson Climate Technologies) are equipped with display scale and pointers to indicate the approximate settings. The display scale is printed in relative pressure units “bar” and “psi.” For precise setting of the controls, external gauges must be used.

PRESSURE SENSING

All pressures are understood as gauge pressures. The Alco PS1/PS2 and TS1 controls, for example, sense pressure by means of bellows that expand or contract when exposed to medium pressure. PS3 controls use a diaphragm rather than a bellows.

High-pressure limiters and pressure cutouts approved according to DIN 32733 feature a double bellows design. The inner bellows serves as the operating bellows and is enclosed by the outer bellows, which features larger surface area.

Should the inner bellows leak, the larger surface area of the outer bellows creates a larger force and causes the pressure control to a pre-empted cutout. This represents a fail-safe function.

Standard controls for refrigeration applications are equipped with bronze bellows and can be used with all common HFC, HCFC, and CFC refrigerants. For ammonia applications, controls with stainless steel bellows are available.

PRESSURE CONNECTORS

A variety of pressure connectors — including male and female flare-type connectors, capillary, and solder connectors — are available. The standard connector is a 7/-16 inch -20 UNF male flare connector, which, in its high-pressure versions, is equipped with a snubber to protect against pressure pulsations.

ELECTRICAL CONTACTS

PS1/PS2/PS3 and TS1 controls are equipped with high-rated double snap action contacts designed for chatter-free and reliable operation. All contacts throughout this range of controls are designed as single pole double throw (SPDT) contacts. One contact may be used for control and the other contact for alarm/status indication or auxiliary control. In addition, dual pressure control PS2 comes with two independently actuated SPDT contacts, providing for even further application flexibility by allowing for a variety of wiring options.

Gold-plated contacts are available on request for low electrical loads (for example, in electronic signaling DC applications).

SETPOINTS

PS1/PS2 and TS1 are adjustable controls with either internal or external adjustment spindles for range and differential. By turning the range spindle, the upper setpoint is defined and by adjusting the differential spindle, the differential, and hence the lower setpoint, is changed. (The upper setpoint does not change.)

The dependency between upper and lower setpoint is always as follows:

Lower setpoint = upper setpoint differential.

The following two rules should always be kept in mind:

  • An adjustment of the range spindle always affects both upper and lower setpoint.

  • An adjustment of the differential spindle affects the lower setpoint only.

    CONTACT FUNCTION

    All PS1/PS2 and TS1 controls have contacts labeled 1, 2, and 4 where 1 refers to the common pole, 2 refers to the lower setpoint, and 4 refers to the upper setpoint.

    AUTOMATIC RESET

    On temperature/pressure rise above the upper setpoint, contacts 1-2 open and contacts 1-4 close. On decreasing temperature/pressure, lower setpoint contacts 1-4 open and contacts 1-2 close.

    MANUAL RESET — LOW TEMPERATURE OR PRESSURE

    On decreasing temperature or pressure below the lower setpoint, contacts 1-4 open, contacts 1-2 close and latch. Only on pressure rise above upper setpoint and after pressing the manual reset button contacts 1-2 will open and contacts 1-4 will close again.

    MANUAL RESET — HIGH TEMPERATURE OR PRESSURE

    On increasing temperature or pressure above the upper setpoint, contacts 1-2 open, contacts 1-4 close and latch. Only on falling temperature or pressure below lower setpoint and after pressing the manual reset button, contacts 1-4 will open and contacts 1-2 will close again.

    For operational safety, all PS1/PS2 with manual reset are designed as trip-free controls. Pressing the manual reset button while the pressure has not reached its reset threshold will not operate the electrical contacts.

    The contact function for controls with internal and external manual reset is alike. The only difference between the two is that for internal manual reset the cover has to be undone, whereas the external reset controls can be reset without removing the cover.

    As a dual pressure control, PS2 has two complete sets of contacts. Their function is the same as on single pressure controls PS1, with the only difference that the contact labels are preceded by an additional index. One side of the control is labeled 11, 12, and 14 whereas the second side is 21, 22, and 24.

    The contact function of controls with convertible reset is as described above, but depends on the actual position of the convertible reset toggle (i.e., automatic or manual reset position).

    Maier is vice president of Engineering for Alco Controls. For more information, contact Alco at www.alcocontrols.com.

    Publication date: 11/04/2002

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