A chiller is a heat transfer device that uses refrigeration to remove heat from a process load and transfers the heat to the environment. So, chillers are the cooling machines used for various 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 moulding 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. This can even be used simply to cool potable water to desirable levels. In all the applications and processes, water temperature control plays an important role in many of the activities that affect our everyday lives.
Types of Chillers
The different types of HVAC (Heating Ventilation and Air conditioning) chillers are classified as shown in figure 1. The two primary types of chillers are:
Figure 1: Classification of chillers
- Absorption chillers.
- Vapour Compression Chillers.
Absorption chillers use a heat source such as natural gas or steam to create a refrigeration 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. The refrigerant compression chillers are mainly classified into air cooled chillers and water cooled chillers. 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 almost the same as those cooled by water regarding the refrigerant cycle except that the cooling medium on the condenser is air instead of water and are intended for outdoor installation and operation. They 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 unlike water cooled chillers, since the air rejects to the atmosphere. The refrigerant chillers can be further classified based on the type of compressors used like reciprocating compressors, centrifugal compressors, rotary screw compressors and rotary scroll compressors.
Absorption Cycle
Absorption chiller is a machine which operates based on absorption cycle. The absorption 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 take place inside generator and condenser, while inside evaporator and absorber there will be low pressure. The cycle starts when we input waste heat into generator and as a result of this heat input, the solution inside will be separated into refrigerant and weak solution. The refrigerant part will enter into condenser where it will be cooled and changed into liquid and the solution part will enter absorber. The refrigerant will flow inside evaporator and will absorb heat from cooled water that is in circulation inside evaporator. As a result of this process, temperature of circulated water will decrease and is used for air-conditioning. 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. Schematic diagram of a vapour absorption chiller has been shown in figure 2.
Figure 2: Vapour absorption cycle for chiller
The absorption chillers as compared to compression chillers provide certain disadvantages like lower response time, higher costs and a smaller Coefficient of Performance (COP) which makes absorption chillers less attractive. The advantage of absorption systems is that they can be used in an integrated way in energy cogeneration systems. Also, the thermal waste from these systems can be used to decrease the direct consumption of electricity for cooling, which is not the case when using compression refrigeration. Absorption chillers could also operate indirectly by providing hot water in the temperature range of 70 – 95°C by using solar energy to heat water.
Vapour Compression Chiller
Vapour-compression water chillers have been widely used to cool water or secondary coolant for air-conditioning and refrigerating applications in both commercial and industrial fields. Fig. 3 shows that the main components of a vapour-compression water chiller include compressor, condenser, throttling device, and evaporator (liquid cooler). The coefficient of performance (COP) for water chillers is defined as the ratio of the evaporator cooling capacity to the compressor input power.
Figure 3: Schematic representation of vapour compression chiller
Heat Addition to Refrigerant in Evaporator
Refrigerant gets vaporized by taking heat from chilled water in evaporator. It leaves the evaporator as vapours 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 in shell and tube design. Refrigerant vapours come out of evaporator and then get compressed by chiller compressor to high pressure and temperature.
Compression of the refrigerant (2-3):
The refrigerant vapours from the evaporator get compressed to higher pressure in the compressor and then sent into the condenser to get converted into liquid phase by rejecting heat.
Heat rejection by refrigerant in condenser (3 – 4):
Refrigerant rejects its heat to outside cooling liquid or air. In this way, refrigerant gets condensed and outside media is heated. Outside media e.g. cooling water may be cooled by cooling tower and recycled again into condenser.
Expansion of refrigerant in expansion valve (4 -1):
Refrigerant in condensed form coming out of condenser is expanded in expansion valve and its pressure and temperature is reduced to level of evaporator so that above cycle is repeated again.
Environmental Effect of Refrigerants
Vapour compression refrigeration systems (VCR) uses different refrigerants as working fluids to provide cooling effect. These refrigerants are a great threat to the environment. The two important parameters which are used to measure the impact of the working fluids on the environment are the ozone depletion potential (ODP) and the global warming potential (GWP). Various protocols and international agreements were signed as the part of steps taken to decrease the harmful effect of refrigerants on the environment. The protocol of Montreal in 1987 limited the use of the CFC and HCFC refrigerants while Kyoto protocol in 1998 was signed to check the CO2 and the greenhouse gases emissions. The intension behind these agreements is to make countries reduce their greenhouse gas emissions by at least 5% compared with the level of 1990, during the period of commitment from 2008 to 2012. In 2015, the United Nations Climate Change Conference (COP21) was held in Paris, France. The objective of this conference was to achieve, a universal agreement on climate, from all the nations of the world, with the aim of keeping global warming below 2 ºC. On April 2016, 174 countries signed the agreement and began adopting it. The current researches in VCR systems deal with the modelling and the exergy analysis of systems. The refrigerants with low GDP and zero ODP are the need of the hour.
Low GWP Refrigerants for Chillers
Midgley first discovered the excellent thermodynamic properties of halogenated CFC refrigerants. But in 1974 Molina and Rowland stated the adverse effects of these refrigerants category on environment which leads to banning of those CFC and HCFC refrigerants. Refrigerants such as CFCs and HCFCs including R11, R12, R22 and R502 have depleted the ozone layer for years and consequently lead to the greenhouse effect on the climate. Hence, efforts are being made in order to find out alternative refrigerants to replace the present high GWP refrigerants. Kabeel et al. suggested R1234yf and R1234ze as alternative refrigerants for R134a, which has a GWP of 1430. The alternative refrigerants R1234ze and R1234yf have zero ODP and the GWP values of 6 and 4 respectively with mild flammability and low toxicity. They observed greater value of COP and lower values of compressor power, evaporator capacity, mass flow rate, discharge pressure and suction pressure using R1234ze as shown in table 1. There are many other researches which proposed alternative refrigerants. Leighton et al. found through steady state analysis of domestic refrigerator–freezer that, for R1234ze, the evaporator capacity was decreased by about 21.5% and the COP was increased by about 7.9%. Francisco et al. introduced a theoretical energy performance evaluation of different single stage vapor compression refrigeration configurations using R1234yf and R1234ze(E) as working fluids. They noticed that, there was an increment increase of 9% to 15% for R1234yf and 11% to 20% for R1234ze on COP with respect to basic cycle. However, the main disadvantage of these refrigerants on the system was the increase in complexity of the system and increase of cost. Though the alternative refrigerants like R1234yf and R1234ze which belong to the hydro-olefin family of refrigerants offer the above said disadvantages, necessary steps should be taken to overcome them because these refrigerants offer very less harmful environmental effect in terms of ODP and GWP as compared to the conventional refrigerants with high GWP values.
Radhouane Ben Jemaa et al considered an air-cooled vapour compression chilled water (VCCW) system and carried out energy and exergy analysis using working fluid R1234ze as alternative to R134a. They developed a thermodynamic model using the Engineering Equation Solver (EES). They investigated the effect of evaporator temperature and ambient temperature on the energy and exergy efficiencies, the total exergy destruction and the exergy losses in different components of the system. The exergy destruction for different components has been shown in figure 4.
Figure 4: Exergy destruction for different components
It was observed that the exergy destruction was less in all the components for R1234ze as compared to that for R134a. Also, the maximum exergy destruction was observed in the compressor.The effect of evaporator temperature and ambient temperature on COP has been shown in figures 5(a) and 5(b) respectively.
In both the cases there was only slight difference in COP for R1234ze and R134a. For both refrigerants, no important differences are observed between the energy and the exergy efficiencies. The relative irreversibilities for different components using R134a and R1234ze have been shown infigures 6(a) and 6(b) respectively. The irreversibility obtained in the unit using R1234ze is lower than the R134a. R1234ze is a good alternative to R134a in the VCCW systems.
Figure 6(a): Relative irreversibility of different components (R134a)
Figure 6(b): Relative irreversibility of different components (R1234ze)
Variable Speed Control Chillers
The operation of chillers commonly used in central air-conditioning systems consumes the large amount of electricity in commercial buildings. The reduction in power consumption can be achieved by using variable speed control applied to system components as building cooling demand varies with weather conditions. Al-Bassam and Alasseri identified energy savings of 5.8% when dual speed control was replaced by variable speed drives for evaporative cooling tower fans in a chiller system operating for summer seasons in Kuwait. Hartman proposed an equal marginal performance principle to optimize the energy performance of chiller systems with variable speed control for chillers, condenser water pumps and cooling tower fans. Bahnfleth and Peyer analyzed how variable speed control for chilled water pumps helped save pumping energy while maintaining proper temperature difference across the supply and return headers of a chilled water circuit. Oil-free magnetic bearings are used in the chiller compressors to give friction-free rotation which brings improved compressor efficiency at speed regulation. Heat transfer effectiveness at the evaporators can be maximized as no lubricating oil is brought in the refrigerant. Yu et al. examined the energy improvement of using oil-free chillers in a system retrofit. They found that by using oil-free chillers with variable speed control brought an energy saving of 9.6% in the total electricity consumption of a shopping arcade when they operated for a wide range of system cooling demands. The monthly total electricity consumption saving by using oil-free chillers has been shown in figure 7.
Figure 7: Monthly total electricity consumption saving by using oil-free chillers
Conclusions
- Hydrocarbons as refrigerants offer the possibility of good efficiencies. HFOs will become the new mainstream refrigerants of choice for chillers if necessary changes in the system are made.
- 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.
- The advantage of absorption refrigerants that they can be used efficiently in integrated co-generation systems should be considered.
AUTHORS CREDIT & PHOTOGRAPH
Bijan Kumar Mandal
Department of Mechanical Engineering
Indian Institute of Engineering Science and Technology, Shibpur, Howrah
Madhu Sruthi Emani
Post graduate student, Indian Institute of Engineering Science and Technology, Shibpur, Howrah
Ranendra Roy
Research Scholar, Indian Institute of Engineering Science and Technology, Shibpur, Howrah
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