Compressor of Centrifugal Water Chiller
Compressor of Centrifugal Water Chiller
The vapour-compression refrigeration cycle in water chillers vary by the type of compressor used in it. As per size of chillers following type of compressors typically use:
For small size chillers: Reciprocating and scroll type compressors.
For Medium size chillers: Helical-rotary or screw type compressors.
For large size chillers: Centrifugal type compressors.
Centrifugal water chillers can also be divided into two types based on the method used to reject heat to the atmosphere:
Most centrifugal chillers are water-cooled. Water-cooled centrifugal chillers are generally available from 100 to 3,000 tons [350 to 10,500 kW] as prefabricated machines, and up to 8,500 tons [30,000 kW] as built-up machines.
This particular centrifugal water chiller makes use of a shell-and-tube evaporator where refrigerant absorbs heat from the water flowing through the tubes. The compressor is made up of 1 or more centrifugal impellers. A second shell-and-tube heat exchanger serves as the water-cooled condenser, where refrigerant is condensed inside the shell and water flows inside tubes. Refrigerant is metered through the system using an expansion device such as a fixed orifice plate. An economizer can be used to enhance the efficiency of a chiller with multiple compressor impellers. A control panel is also provided on the chiller and a starter is either mounted on the chiller or located remotely.
The centrifugal compressor uses the principle of dynamic compression, which involves converting energy from one form to another, to increase the pressure and temperature of the refrigerant. It converts kinetic energy to static energy.
The core component of a centrifugal compressor is the rotating impeller. The centre, or eye, of the impeller is fitted with blades that draw refrigerant vapour into radial passages that are internal to the impeller body.
The rotation of the impeller causes the refrigerant vapour to accelerate within the impeller passages, increasing its velocity and kinetic energy.
The accelerated refrigerant vapour leaves the impeller and enters the diffuser passages. These passages start out small and become larger as the refrigerant travels through them. As the size of the diffuser passages increases, the velocity, and therefore the kinetic energy of the refrigerant decreases.
The first law of thermodynamics states that energy is not destroyed—only converted from one form to another. Thus, the refrigerant’s kinetic energy is converted to static energy or static pressure. Refrigerant, now at a higher pressure, collects in a larger space around the perimeter of the compressor called the volute. The volute also becomes larger as the refrigerant travels through it. Again, as the size of the volute increases, the kinetic energy is converted to static pressure.
Due to its pressure and temperature, the refrigerant leaving the compressor is in a condition that allows its heat to be rejected from the chiller.
Again, in the passages of the rotating impeller, the refrigerant vapour accelerates, increasing its velocity and kinetic energy. As the area increases in the diffuser passages, the velocity, and therefore the kinetic energy, of the refrigerant decreases. This reduction in kinetic energy is offset by an increase in the refrigerant’s static energy or static pressure. Finally, the high pressure refrigerant collects in the volute around the perimeter of the compressor, where further energy conversion takes place.
The resulting pressure and temperature of the refrigerant is now high enough that its heat can be rejected from the chiller.
Centrifugal compressors use 1 or more impellers to compress the refrigerant. A multi-stage compressor uses 2 or 3 impellers to increase the pressure of the refrigerant in steps instead of performing the task within a single impeller. Compressed refrigerant vapour travels from the outlet of the first-stage compressor impeller to the inlet of the second-stage compressor impeller. After the accelerated refrigerant vapour leaves the last impeller, it collects in the compressor volute and travels on to the condenser.