Compressor

Compressor is a word, not an acronym. Its Latin root refers to compression. In machinery, compression raises gas pressure and usually temperature; density also depends on the gas and temperature. The same word describes an audio signal processor, but this guide concerns gas-compression machinery.

Air, refrigerant vapour and process gas are different working media. Approval for one does not establish compatibility with another. A liquid pump, a ventilation fan and a gas compressor perform different duties. A compressor in an air conditioner does not compress room air.

Positive-displacement compressors capture gas and reduce its chamber volume. Pistons, rolling pistons, vanes, scrolls and screws create that changing space differently. Suction means gas entering the compressor; discharge means its delivery to the higher-pressure side.

Dynamic compressors transfer energy to a continuous flow through rotating blades. The gas accelerates and part of its kinetic energy becomes a pressure rise in the flow passages. Centrifugal and axial compressors belong to this group. Both principles consume drive energy, but their flow and control characteristics differ.

Refrigeration, air conditioning and heat pumps use refrigerant vapour. Pneumatic systems use compressed air for tools, actuators and processes. Gas installations compress specified process gases for transport or treatment. Gas composition, cleanliness and material compatibility are application requirements, not interchangeable labels.

This guide develops air-conditioning and refrigeration topics in detail. The wider family also includes diaphragm compressors, with a flexible membrane separating gas from the drive; liquid-ring machines, with changing gas spaces formed by a rotating liquid ring; and axial machines, with flow mainly along the shaft. A linear drive describes piston motion, rather than a separate compression principle.

In a basic vapour-compression circuit the compressor draws vapour from the low-pressure side and raises its pressure. It adds energy to the refrigerant; useful cooling occurs at the evaporator. In heating mode, useful heat is taken from the hot side. A reversing heat pump changes the roles of its heat exchangers through the circuit arrangement, without reversing the compression process.

• Compressor for air conditioning and heat pumps

A piston changes the volume of a cylinder. Suction and discharge valves admit gas and release it as the cylinder pressure changes. This mechanism can be combined with different enclosures, drives and capacity controls.

• Reciprocating compressor

In rolling-piston and vane constructions, an eccentric rotating mechanism forms gas spaces that shrink during a revolution. In air-conditioning catalogues, rotary often denotes these constructions; the broader mechanical term may also include other rotary mechanisms. The exact construction should therefore be named.

• Rotary compressor: rolling piston and vane

Two interleaved spirals form gas pockets. One scroll typically remains fixed while the other orbits without spinning about its own centre. The pockets move inward and shrink before the gas leaves through the central discharge.

• Scroll compressor

Helical rotors and the housing enclose gas spaces whose volume decreases toward the outlet. Twin-screw and single-screw arrangements differ mechanically. Oil handling, internal volume ratio and capacity control are important model-specific characteristics.

• Screw compressor

An impeller transfers energy to a flowing gas; the diffuser slows the flow and contributes to its pressure rise. The operating map connects flow, pressure lift and speed. This dynamic mechanism is used in refrigeration chillers as well as other gas-compression duties.

• Centrifugal compressor

A compressor can simultaneously be scroll, hermetic, variable-speed and designed for a heat pump. These describe mechanism, enclosure, control and application. Hermetic and semi-hermetic designs place the motor within the refrigerant enclosure; internal repair access differs. An open-drive machine uses an external drive and a sealed shaft penetration.

• Compressor: hermetic, semi-hermetic and open-drive

Capacity can be changed by switching compressors on and off, varying speed or using a supported unloading mechanism. An inverter is an electronic drive, not a compression mechanism. Single-stage and multistage describe successive pressure increases; compressors connected in parallel do not become multistage merely because several machines are present.

• Compressor capacity control and inverter drive

A compressor contains a compression mechanism, drive, housing, inlet and outlet. Bearings support moving parts. Lubricated designs need the specified oil and an engineered return path. Oil-free constructions use another bearing or lubrication arrangement; a family name alone does not identify it.

System protection may include pressure and temperature controls, motor protection, oil separation and a suction accumulator. An oil separator removes oil from discharge gas for return to the compressor. A suction accumulator can retain incoming liquid. These devices are selected for the circuit, not added universally to every compressor.

Cooling capacity is the rate of heat removal, usually in kilowatts. Electrical input is the power consumed by the drive: a different value. Displacement is geometric volume per revolution or unit time; it does not directly state cooling capacity or actual mass flow.

A valid comparison states refrigerant, suction and discharge conditions, inlet vapour temperature or superheat, liquid conditions used for the rating, and speed. Superheat is the temperature rise above saturation at the same pressure; subcooling is the drop below liquid saturation. Refrigerant blends need a stated dew/bubble convention. Refrigerant temperatures are not simply room and outdoor air temperatures.

The operating envelope is the permitted combination of conditions in the model documentation. Capacity, input and efficiency vary within it. A cooling or heating coefficient of performance compares useful thermal output with specified power input. Compressor and whole-system values can include different equipment, so compare equal boundaries and units.

Refrigerant

The working substance carrying heat through a refrigeration circuit.

Pressure ratio

Discharge absolute pressure divided by suction absolute pressure. Gauge readings cannot be used directly in this ratio.

Mass flow rate

Mass of gas passing per unit time, such as kilograms per second; depends on density and volume flow.

Oil return

Return of lubricant carried out with refrigerant through the circuit to the compressor.

Surge

Unstable operation of a dynamic compressor and connected system, with large flow and pressure oscillations that can include reverse flow.

Chiller

A machine cooling a liquid that carries cooling to equipment or a process.

Start with the cooling or heating duty and operating range. Record refrigerant, electrical supply, control, installation limits and exact equipment identity. Replacement additionally requires matched connections, mounting, oil, motor cooling and protection. Similar dimensions or nominal power do not prove interchangeability.

A stopped or noisy compressor is a symptom, not a diagnosis. Supply faults, protective trips, refrigerant flow, heat exchangers and oil return can affect operation. Establish the cause before replacement and verify the new unit in the complete circuit. A requirements brief, documented technical selection and quotation are separate stages; reference information alone confirms neither stock nor suitability.

• Compressor selection, replacement and service