Compressor Service And Maintenance

TYPES OF CHILLER COMPRESSORS

Most cooling systems, from residential air conditioners to large commercial and
industrial chillers, employ the refrigeration process known as the vapor compression cycle. At the heart of the vapor compression cycle is the mechanical compressor. Its function is: 1) to pump refrigerant through the cooling system and 2) to compress gaseous refrigerant in the system so that it can be condensed to liquid and absorb heat from the air or water that is being cooled or chilled.

Not all air-conditioning applications have the same capacity requirements, and for this reason the chillers are grouped by the type of compressor
1) Positive- displacement
2) Dynamic

Reciprocating Chillers

Reciprocating compressors are positive displacement machines that use the reciprocating action of a piston inside a cylinder to compress refrigerant. As the piston moves downward, a vacuum is created inside the cylinder. Because the pressure above the intake valve is greater than the pressure below it, the intake valve is forced open and refrigerant is sucked into the cylinder. After the piston reaches its bottom position it begins to move upward. The intake valve closes, trapping the refrigerant inside the cylinder. As the piston continues to move upward it compresses the refrigerant, increasing its pressure. At a certain point the pressureexerted by the refrigerant forces the exhaust valve to open and the compressed refrigerant flows out of the cylinder. Once the piston reaches it top-most position, it starts moving downward again and the cycle is repeated.

These compressors are available in 3 configurations namely
1) Hermetic
2) Semi- hermetic and
3) Direct driven versions.
In a hermetic unit, the motor and compressor are enclosed in a common housing,
which is sealed. Because the components are not accessible for repair, the entire compressor unit must be replaced if it fails. The hermetic sealed units are most common in small capacities.

Screw Chillers

Screw compressors are positive displacement machines that use helical rotors tocompress the refrigerant gas. As the rotors rotate they intermesh, alternately exposing and closing off interlobe spaces at the ends of the rotors. When an interlobe space at the intake end opens up, refrigerant is sucked into it. As the rotors continue to rotate the refrigerant becomes trapped inside the interlobe space and is forced along the length of the rotors. The volume of the interlobe space decreases and the refrigerant is compressed. The compressed refrigerant exists when the interlobe space reaches the other end. There are two types:

1) Single and
2) Twin screw configuration.

• A single-screw compressor uses a single main screw rotor meshing with two
gate rotors with matching teeth. The main screw is driven by the prime mover,
typically an electric motor.

• A twin-screw compressor consists of accurately matched rotors (one male
and one female) that mesh closely when rotating within a close tolerance
common housing. One rotor is driven while the other turns in a counterrotating motion. The twin-screw compressor allows better control and variations in suction pressure without much affecting the operation efficiency.

Centrifugal Chillers

Centrifugal compressor is a dynamic machine that uses the rotating action of an impeller wheel to exert centrifugal force on refrigerant inside a round chamber (volute). Refrigerant is sucked into the impeller wheel through a large circular intake and flows between the impellers. The impellers force the refrigerant outward, exerting centrifugal force on the refrigerant. The refrigerant is pressurized as it is forced against the sides of the volute. Centrifugal compressors are well suited to compressing large volumes of refrigerant to relatively low pressures. The compressive force generated by an impeller wheel is small, so chillers that use centrifugal compressors usually employ more than one impeller wheel, arranged in series. Centrifugal compressors are desirable for their simple design and few moving parts

Centrifugal chillers are categorized either as positive pressure or negative pressure machines depending on the evaporator pressure condition and the type of refrigerant used. A chiller using refrigerant R-22 and R-134A is a positive-pressure machine, while the one using R-123 is a negative-pressure machine. Key facts are noted
below:

Like reciprocating chillers, centrifugal units are available in both hermetically sealed and open construction. Despite its lower operating efficiency, the hermetically sealed unit is more widely used.

Favorable Factors

The main factor favoring centrifugal machine is their high operational efficiency at full load. Unlike reciprocating and rotary screw, centrifugal compressor is not a constant displacement machine, thus it offers a wide range of capacities continuously modulated over a limited range of pressure ratios. They are well suited to compressing large volumes of refrigerant to relatively
low pressures. By altering built- in design items as number of stages, compressor speed, impeller diameter, and choice of refrigerant, it can be used in chillers having a wide range of design chilled liquid temperatures and design cooling fluid temperatures.

Due to their very high vapor-flow capacity characteristics, centrifugal compressors dominate the 200 ton and larger chiller market, where they are the least costly and most efficient cooling compressor design. Centrifugals are most commonly driven by electric motors, but can also be driven by steam turbines and gas engines.

Drawbacks

A serious drawback to centrifugal chillers has been their part load performance. When the building load decreases, the chiller responds by partially closing its inlet vanes to restrict refrigerant flow. While this control method is effective down to about 20 percent of the chiller’s rated output, it results in decreased operating efficiency.

For example, a chiller rated at 0.60 kW per ton at full load might require as much as

0.90 kW per ton when lightly loaded. Since chillers typically operate at or near full load less than 10 percent of the time, part load operating characteristics significantly impact annual energy requirements.

Centrifugal chillers also can be difficult to operate at low cooling loads. When the cooling load falls below 25 percent of the chiller’s rated output, the chiller is prone to a condition known as surging. Unrestricted, surging can lead to serious chiller damage. To reduce the chances of damage from surging, manufacturers add special controls, but most of these controls further reduce the part load efficiency of the units

Scroll Chillers

The scroll compressor is a positive displacement machine where refrigerant is compressed by two offset spiral disks that are nested together. The upper disk is stationary while the lower disk moves in orbital fashion.

The orbiting action of the lower disk inside the stationary disk creates sealed spaces of varying volume. Refrigerant is sucked in through inlet ports at the perimeter of the scroll. A quantity of refrigerant becomes trapped in one of the sealed spaces. As the disk orbits the enclosed space containing the refrigerant is transferred toward the centre of the disk and its volume decreases. As the volume decreases, the refrigerant is compressed. The compressed refrigerant is discharged through a port at the centre of the upper disk.

Compressor Capacities

The size of refrigeration compressors is given in either motor input horse power (HP), motor input kilowatts (kW input), refrigeration cooling capacity (kW cooling), British Thermal Units per hour (Btu/h) or tons of refrigeration (TR). A refrigeration ton is equal to heat extraction rate of 12,000 BTU’s/hr; therefore a 3 TR chiller can remove 36,000 BTU’s/hr.

The volume and pressure that the refrigerant can be pumped at determines the
cooling capacity, specific performance and application areas where the compressor
can be used for. Note the key facts: