Different Type Of Chillers & Their Application
There are basically two types of chillers which may be classified according on their refrigerent cycles and applications. Examined here are different types of chillers and their various applications.
A chiller is a heat transfer device that uses refrigeration system to remove heat from a process load and transfers the heat to the environment. Chillers may also be seen as cooling machines of choice to condition industrial, commercial, and institutional facilities.
They are used to lower the temperatures of all kinds of equipment and processes such as: robotic machinery; semiconductors; injection and blow molding machines; welding equipment; die-casting and machine tooling; paper and cement processing; power supplies; power generation stations; compressed air and gas cooling systems; medical imaging machines; chemical, drug, food and beverage production; even simply to cool potable water to desirable levels. Whether for office comfort, keeping data server centres from overheating, or specialized industrial processes, water temperature control plays a vital role in many of the behind-the-scenes activities that affect our everyday lives.
A chiller consists of a reservoir that is filled with a fluid such as water or ethylene glycol/water mixture which is continually circulated. In a typical building application, chilled water is circulated to air-handlers or now the increasingly used chilled beams in order to transfer heat from air to water, or stated the other way, transfer cooling from the water to the building air. The schematic diagram of a chiller plant is shown in figure 1.
Figure 1: Schematic diagram of a chiller Plant
Types Of Chillers
Chillers can primarily be classified as absorption chillers and refrigerant compression chillers, based on the refrigerant cycle on which they work.
Cooling processes are significantly different in the two types of chillers. Absorption chillers use a heat source such as natural gas or steam to create refrigeration or cooling effect. Refrigerant chillers use mechanical compression and are the most common. Refrigerant compression chillers consist of four main components - a compressor, an evaporator, a condenser and a valve metering system. Basically, a refrigerant gathers heat, and then uses an evaporator heat exchanger to remove that heat.
There are two main types of refrigerant compression chillers, air and water. Air condensers are cooled by utilizing the air, whereas water condensers are cooled by using water sources. Water cooled chillers are generally located within the building and use cooling towers, a pond, or river located near the building to reject water’s heat from the condenser.
Chillers with condensers cooled by air operate essentially the same as those cooled by water regarding the refrigerant cycle and the steps along the way. The cooling medium on the condenser is of course air instead of water. Air cooled chillers are intended for outdoor installation and operation.
These reject heat to the atmosphere by mechanical means such as circulation of outdoor air by a fan directly through the machine’s condenser. These types of condenser cooled units do not require a cooling tower as is common with water cooled chillers since the air rejects heat to the atmosphere. Based on compression method of the refrigerant in its vapour phase, chillers can also be classified into four categories. The compressors may be reciprocating, centrifugal, rotary screw and rotary scroll type.
Reciprocating compressors possess a crankshaft and pistons. The pistons compress the gas and the gas is heated. The hot gas is discharged to the condenser. The pistons have intake and exhaust valves that can be opened on demand to allow the pistons to idle. A few examples of this would be in an office or school, but not necessarily in a hotel or an apartment building.
Common capacities range from 20 to 125 tons but can even get up to 450 tons. Centrifugal compressors operate much like a centrifugal water pump. They contain an impeller that compresses the refrigerant. Centrifugal chillers can provide a very high cooling capacity in a compact design. They have the ability to vary capacity continuously to match a wide range of load fluctuations with near proportional changes in power consumption. This provides tight temperature control and energy conservation.
The capacity can range from 150 tons up to 2400 tons. Rotary screw or helical DNA type compressors have two mating helically grooved rotors. As the rotors rotate, the gas is compressed by volume reduction between the two rotors. These helixes require high tolerances to fit perfectly, thus driving up the initial cost.
Capacity is controlled by a sliding inlet valve or variable-speed drive (VSD) on the motor. Capacities range from 25 to 450 tons with the largest capable of 800 tons. Rotary scroll compressors use two spirals to pump and compress the refrigerant. Commonly, one of the scrolls is fixed while the other orbits eccentrically without rotating within the other fixed scroll.
This motion traps and compresses pockets of fluid between the scrolls. This design and operation makes them the most efficient of the four compressor types. The capacity of single refrigerant loop scroll compressor ranges from 2 to 25 tons. Typical chilled water cooling temperature ranges between 39-45 °F. The classification of chillers from various aspects has been shown in Fig.2.
For proper heat transfer between the circulated water to be cooled and the refrigerant, it is important to maintain sufficient chiller water flow. The commonly recommended range of chilled water flow velocity is between 3 and 12 feet per second. Therefore, it is very important for a chiller to maintain this flow for proper efficiency and corresponding energy usage as well as maintaining long-term performance.
Absorption chiller is a machine which operates based on vapour absorption refrigeration cycle. This cycle consists of four major heat exchangers, (generator, condenser, evaporator and absorber) with two kinds of solution, (refrigerant and absorbent). During this cycle high pressure will prevail inside generator and condenser, while inside evaporator and absorber there will be low pressure. The cycle starts with input waste heat in the generator. As a result of this heat input, the solution in the generator will be separated into refrigerant and weak solution. The refrigerant in the vapour form will enter into condenser and will change into liquid. The solution part will enter absorber, since there is a pressure difference between condenser and evaporator, the refrigerant will flow inside evaporator and will absorb heat from cooled water that is in circulation inside evaporator. Consequently, the temperature of circulated water decreases and then it is used for air-conditioning purpose. The evaporated refrigerant will then enter absorber where it will be mixed with weak solution, the mixture will then get the liquid state and finally it will enter generator and the cycle is repeated. The schematic diagram of the vapour absorption refrigeration cycle has been shown in Fig. 3.
Vapour Compression Chillers
The schematic diagram of chiller based on vapour compression refrigeration cycle has been shown in Fig.4. Refrigerant gets vaporized by taking heat from chilled water in evaporator thus serving its prime purpose. Refrigerant comes out of evaporator as vapour but on other side chilled water is produced. Thus, heat is added to refrigerant at constant pressure but is extracted from chilled water. Both refrigerant and chilled water don’t get mixed and are separated by some solid wall in between them in evaporator like shell and tube design. Refrigerant vapours come out of evaporator and then compressed by chiller compressor to high pressure and temperature. Compressor requires energy input for its working and hence electric energy is supplied to it. Refrigerant vapour rejects heat to outside cooling liquid or air. Refrigerant in condensed or liquid form coming out of condenser is expanded in expansion valve and its pressure and temperature are reduced to level of evaporator so that above cycle is repeated again.
A chiller can be realized as a refrigeration system that cools water. Air conditioners and dehumidifiers condition the air while a chiller, using the same refrigerating operations, cools water, oil, or some other fluid. This chilled solution can then be used for cooling in a wide range of operations. Some of the most common applications are as follow:
1. Plastics Industry: Cooling the hot plastic that is injected, blown, extruded or stamped.
2. Printing Industry-Cooling warm rollers due to friction and ovens curing the ink, along with ultraviolet lamps also for curing purposes.
3. Medical Industry: MRI Systems-The hospital MRI units need to be cooled to operate properly.
4. HVAC Industry: Large scale air-conditioning systems pump this chilled water to coils in specific areas of high rise buildings. The air handling systems for each area open and close the water flow through specific area keeping the air of the rooms at a desired temperature.
5. Laser Cutting Industry: Technology has created machines that can cut out very specific steel products with the precise use of a laser cutting machine. These lasers run at very high temperatures and must be cooled to run properly.
6. Brewery Industry- The cooling of the kettles in fermentation has become an upcoming industry where chiller have been used to keep the kettles and storage area of beer at cold temperatures.
New formulations of lubricants and refrigerants blended with nano particles could yield increased energy efficiency for chillers.
Carbon dioxide has been used in some supermarket refrigeration equipment. However, the high operating pressures with CO2 are a concern.
Hydrocarbons as refrigerants offer the possibility of good efficiencies. HFOs will become the new mainstream refrigerants of choice for chillers.
The development of oil-free centrifugal compressors, where magnetic bearings replace the use of oil for lubrication has seen even greater increases in efficiency and lower operating costs.
AUTHOR CREDIT & PHOTOGRAPH
Madhu Sruthi Emani
Indian Institute of Engineering
Science and Technology
Bijan Kumar Mandal
Indian Institute of Engineering
Science and Technology,
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