Compressor

The most common compressors used in chillers are reciprocating, rotary screw, centrifugal, and scroll compressors. Each application prefers one or another due to size, noise, efficiency and pressure issues. Compressors are often described as being either open, hermetic, or semi-hermetic, to describe how the compressor and/or motor is situated in relation to the refrigerant being compressed. Variations of motor/compressor types can lead to the following configuations:

• Hermetic motor, hermetic compressor

• Hermetic motor, semi-hermetic compressor

• Open motor (belt driven or close coupled), hermetic compressor

• Open motor (belt driven or close coupled), semi-hermetic compressor

Typically in hermetic, and most semi-hermetic compressors (sometimes known as accessible hermetic compressors), the compressor and motor driving the compressor are integrated, and operate within the refrigerant system. The motor is hermetic and is designed to operate, and be cooled by, the refrigerant being compressed. The obvious disadvantage of hermetic motor compressors is that the motor drive cannot be maintained in situ, and the entire compressor must be removed if a motor fails. A further disadvantage is that burnt out windings can contaminate whole refrigeration systems requiring the system to be entirely pumped down and the refrigerant replaced.

An open compressor has a motor drive which is outside of the refrigeration system, and provides drive to the compressor by means of an input shaft with suitable gland seals. Open compressor motors are typically air cooled and can be fairly easily exchanged or repaired without degassing of the refrigeration system. The disadvantage of this type of compressor is a failure of the shaft seals, leading to loss of refrigerant.

Open motor compressors are generally easier to cool (using ambient air) and therefore tend to be simpler in design and more reliable, especially in high pressure applications where compressed gas temperatures can be very high. However the use of liquid injection for additional cooling can generally overcome this issue in most hermetic motor compressors.

Reciprocating compressors

Reciprocating compressors are piston-style, positive displacement compressors.

Rotary screw compressors

Rotary screw compressors are also positive displacement compressors. Two meshing screw-rotors rotate in opposite directions, trapping refrigerant vapor, and reducing the volume of the refrigerant along the rotors to the discharge point.

Centrifugal compressors

Centrifugal compressors are dynamic compressors. These compressors raise the pressure of the refrigerant by imparting velocity or dynamic energy, using a rotating impeller, and converting it to pressure energy.

Scroll compressors

Scroll compressors are also positive displacement compressors. The refrigerant is compressed when one spiral orbits around a second stationary spiral, creating smaller and smaller pockets and higher pressures. By the time the refrigerant is discharged, it is fully pressurized

Scroll compressors have essentially displaced the older reciprocating compressor designs for small-tonnage air conditioning systems. Which is much to the operator's benefit, because scrolls are inherently more reliable and require none of the maintenance that the piston-type reciprocating compressors required. But this advantage comes with a price: comprehension. Reciprocating compressors were so much easier to understand--since the compression stroke in a piston is easy to grasp and most people are familiar with this process from the similar function of pistons in gas engines.

The secret to a scroll compressor is two high-precision spiral "scrolls" that are designed to mesh with each other to extremely close tolerances:

Compressor Lubrication

In order to lubricate the moving parts of the compressor, an oil is added to the refrigerant during installation or commissioning. The type of oil may be mineral or synthetic to suit the compressor type, and also chosen so as not to react with the refrigerant type and other components in the system. In small refrigeration systems the oil is allowed to circulate throughout the whole circuit, but care must be taken to design the pipework and components such that oil can drain back under gravity to the compressor. In larger more distributed systems, especially in retail refrigeration, then oil is normally captured at an oil separator immediately after the compressor, and is in turn re-delivered, by an oil level management system, back to the compressor(s). Oil separators are not 100% efficient so system pipework must still be designed so that oil can drain back by gravity to the oil separator or compressor.

Some newer compressor technologies use magnetic bearings and require no lubrication, for example the Danfoss Turbocor range of centrifugal compressors. Avoiding the need for oil lubrication and the design requirements and ancillaries associated with it, simplifies the design of the refrigerant system and reduces maintenance requirements.

Control

In simple commercial refrigeration systems the compressor is normally controlled by a simple pressure switch, with the expansion performed by a capillary tube or simple thermostatic expansion valve. In more complex systems, including multiple compressor installations, the use of electronic controls is typical, with adjustable set points to control the pressure at which compressors cut in and cut out, and temperature control by the use of electronic expansion valves.

In addition to the operational controls, separate high pressure and low pressure switches are normally utilised to provide secondary protection to the compressors and other components of the system from operating outside of safe parameters.

In more advanced electronic control systems the use of floating head pressure, and proactive suction pressure, control routines allow the compressor operation to be adjusted to accurately meet differing cooling demands whilst reducing energy consumption.

Other features

• A horizontal or vertical pressure vessel, equipped internally with a demister, between the evaporator and the compressor inlet to capture and remove any residual, entrained liquid in the refrigerant vapor because liquid may damage the compressor. Such vapor-liquid separators are most often referred to as "suction line accumulators". (In other industrial processes, they are called "compressor suction drums" or "knockout drums".)

• Large commercial or industrial refrigeration systems may have multiple expansion valves and multiple evaporators in order to refrigerate multiple enclosed spaces or rooms. In such systems, the condensed liquid refrigerant may be routed into a pressure vessel, called a receiver, from which liquid refrigerant is withdrawn and routed through multiple pipelines to the multiple expansion valves and evaporators.

• Filter Dryers, installed before the compressors to catch any moisture or contaminents in the system and thus protect the compressors from internal damage

• Some refrigeration units may have multiple stages which requires the use of multiple compressors in various arrangements.