CHILLER COMPRESSORS

In HVAC industry, the refrigeration machine that produces chilled water is referred to as a “Chiller”. A chiller package operates either on the principles of vapor compression or vapor absorption. The vapor compression system uses mechanical energy in the form of electric motor to drive the cooling cycle whereas absorption chillers use heat to drive the process. The vapor compression chiller system, which is far more prominent in commercial buildings, consists of four major components: the compressor, evaporator, condenser and expansion device all packaged as a single unit. The classification of vapor compression chiller packages is generally by the type of compressor: centrifugal, reciprocating, and screw being the major ones.

                                            Chillers are the largest consumer of energy in a commercial building and it is therefore important to understand the relative benefits and limitations of various types in order to make the right economic decisions in chiller installation and operation.

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

Positive-displacement:- 

                                Positive-displacement compressors physically compress the vaporized refrigerant into a smaller volume and higher pressure, and include reciprocating, rotary, and scroll types. These deliver a constant volume of gas under a constant speed.

Dynamic:-                

                Dynamic compressors increase vaporized refrigerant pressure by the kinetic energy imparted on refrigerant by a rotating impeller. A centrifugal compressor is a dynamic compressor and is not a constant displacement type.

Reciprocating Compressor:-

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 pressure exerted 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.

1. Hermetic     
   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.
2. Semi-hermetic 
   In the semi-hermetic unit the motor is also part of the unit, however it is not sealed. Semi-Hermetic compressors have the advantage over hermetic compressors in that they can be re-built a number of times if necessary giving a much longer service life.       
3. Direct driven 
   In a direct drive unit the motor and compressor are separated by a flexible coupling. These types of units utilize older technology and are not commonly used today.              

     Advantages of Reciprocating Compressor :-

• low cost and efficiency when applied in low capacities.
• Multiple reciprocating machines can be installed for higher building loads.
• Simple controls and the ability to control the speed
• Available in both air-cooled and water cooled heat rejection configurations
• These chillers are available from 0.5 to 150 tons of refrigeration (TR).

   Drawback of Reciprocating Compressor :-

• High level of maintenance requirements in comparison with other chiller types.
• Reciprocating chillers have more moving parts than centrifugal or rotary chillers.
• Reciprocating chillers also generate high levels of noise and vibration.
• Reciprocating chillers are not well suited for applications with cooling loads in excess of 200 tons.

Screw Compressor:-

Screw compressors are positive displacement machines that use helical rotors to compress 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.

Single-screw :- 

                       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.

Twin-screw :-

                   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 counter-rotating motion. The twin-screw compressor allows better control and variations in suction pressure without much affecting the operation efficiency.

 Advantages of Screw Compressor :-

• High compression ratio and few moving parts.
• screw chillers are compact, smaller and lighter than reciprocating and centrifugal chillers of the same cooling capacity.
• They are designed for long periods of continuous operation, needing very little maintenance.

 Drawback of Screw Compressor:-

• The major drawback of screw chillers is their high first cost.

Centrifugal Compressor:-                    
                                                           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.

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.

                                              

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.

Advantages of Centrifugal Compressor :-

• Centrifugal machine is their high operational efficiency at full load.
• They are well suited to compressing large volumes of refrigerant to relatively low pressures.
• 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.

Drawback of Screw Compressor:-

• 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.
• Centrifugal chillers also can be difficult to operate at low cooling loads.

Centrifugal Compressor:-  

                                                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 center 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 center of the upper disk.

Advantages of Scroll Compressor :-

Scroll compressors are a relatively recent development that is rapidly overtaking the niche of reciprocating chillers in comfort cooling. They provide small size, low noise and vibration and good efficiency. Available in air-cooled and water cooled configurations, scroll chiller capacity can reach approximately 30 tons or less, which makes them good candidates for spot cooling or make-up cooling applications.

Drawback of Scroll Compressor:-

The biggest drawback is that these cannot be repaired and there have been issues of scroll compressors losing oil at low temperatures. On relatively small sizes, these do not affect the life cycle economics drastically and therefore not discussed further in this course.