The chiller is the main component of a cooling system. The chiller centralizes three heat exchanging cycles: the refrigeration cycle, the chilled water process, and cooling water process. The chiller creates chilled water in a centralized building location that can be distributed for water usage and air conditioning purposes. The chiller is an all-in-one system that operates under the vapor compression cycle, using refrigerants as the working fluid.
Refrigeration Cyclefrom your skin. Similarly, when the refrigerant evaporates, it draws heat from the warmer water in the evaporator, which cools the water. When the refrigerant evaporates it loses pressure but then regains it in the compressor thus repeating the refrigeration cycle.
Chilled Water Loop
The chilled water loop transports water through the distribution network to supply the air handlers with cooling for air conditioning. This water first enters a chiller evaporator, which is essentially a heat exchanger. There are many types of evaporators. One such evaporator would be a spray shell and tube heat exchanger which consists of a fluid-filled outer pressure vessel that contains bundles of tubes that hold a different fluid. The low-pressure refrigerant is sprayed evenly over the surfaces of the tubes that contain the chilled water. The heat transfer process transfers the energy from the water to the refrigerant as the water passes through the evaporator. After the water has traveled the length of the evaporator, the resulting chilled water is sent to designated destinations.
Cooling Water Loop
How Stuff Works → Four-Pipe Chiller System
A four-pipe Heating, Ventilation, and Air-Conditioning (HVAC) system consists of four lines in two sets: two supply lines and two return lines. One set is for hot water while the other is for chilled water. The hot water is kept at a temperature between 150 and 200°F, and the
There are advantages to having a four-pipe HVAC system compared to a two-pipe HVAC system. One is that because the four-pipe system has two sets of pipes, one for heating and one for cooling, hot or chilled water is always available, so the system can immediately use whichever is needed. A second advantage is that the four-pipe system can be used to cool and heat a building at the same time. This capability is especially useful in office buildings or apartments, to satisfy the needs of all occupants.
One disadvantage to the four-pipe system compared to the two-pipe system is that it is much more expensive to install and maintain. Four-pipe systems have twice as many valves, controls, and pipes, and thus proportionally higher chances to fail or break down.
Chillers are categorized according to the way the refrigerant is compressed. The most common types of compressors used for vapor compression systems are reciprocating, screw, scroll, and centrifugal compressors. Generally, reciprocating compressors are used for small applications up to 150 tons, a rotary-screw for medium applications up to 1000 tons, and a centrifugal generally for large applications up to 2000 tons . However, other considerations such as refrigerant properties, compression ratio, part-load efficiency are also important factors in determining the specific compressor that needs to be used.
Physical sites that include absorption chillers:
The College of New Jersey
The cooling cycle for absorption chillers is the same as for electric chillers. The basic difference between the two chillers is the way that the pressure of the refrigerant is raised. In an electric chiller, an electrical motor operates a compressor, raising the pressure of the refrigerant vapors. In absorption chillers, a pump raises refrigerant vapors to a high pressure.
In the condenser, the cooling water absorbs the heat from the vaporized refrigerant, transforming the refrigerant to a liquid. The liquid refrigerant travels from the condenser through an expansion valve to the evaporator where the pressure and temperature of the liquid refrigerant drop. The liquid refrigerant is discharged into a pan in the evaporator, and then pumped to the chilled water tube where it is sprayed onto the tube bundles containing the water. The liquid refrigerant extracts energy from the water, and any liquid refrigerant that is not vaporized drops down into the evaporator pan and is re-circulated.
Vaporized refrigerant will then travel to the absorber. The vaporized refrigerant enters the absorbent solution spray, such as lithium-bromide, and absorbs the vaporized refrigerant forming a liquid solution. This reaction is an exothermic one (a reaction that will release heat to its surroundings). As amount of refrigerant absorbed to the solution increases with decreasing temperature. The cooling water is also circuited through the absorber to remove the heat produced through the reaction. After the liquid absorbent solution gives up its heat to the cooling water, it takes one of two paths: either it mixes with the concentrated lithium-bromide solution and is pumped back to the spray nozzles, or it is pumped to the generator/concentrator. The function of the generator is to remove the refrigerant from the liquid solution. As this is an endothermic reaction (a reaction which will draw energy from its surroundings), it will require a heat input. This is done by either steam, hot water (indirect effect) or a fuel burner. The generator/concentrator, raises the temperature of the solution which then vaporizes the refrigerant allowing it to travel to the condenser while the absorbent concentration flows back down to the absorber.
The spray shell-and-tube evaporators spray refrigerant evenly over a distributor, where it receives energy from the warm condenser water returning from buildings that are flowing through the tube bundles. As the refrigerant gains energy, it boils and travels through an eliminator. Since compressors cannot have liquid flowing across it, the eliminator would keep out (or eliminate) the liquid before refrigerant enters the compressor. The water passes through the tube bundles in a defined number of passes and is expelled at a low temperature and used for building air conditioning. A direct expansion shell-and-tube evaporator is designed with refrigerant flowing through the tubes and chilled water flowing through the shell.
To increase overall efficiency of a chiller, many manufacturers employ a multiple stage expansion and compression process.Since part of the refrigerant from the economizer does not need to be compressed fully, the total work required by the compressor is reduced.
Considered a high-pressure refrigerant, R-134a has evaporator and low side and high side condenser pressures of approximately 50 psig and 210 psig (when it is at 80°F), respectively . HCFC-123, on the other hand, is considered a low-pressure refrigerant and has evaporator and condenser pressures of approximately -8.9 psig and 6.1 psig, respectively. Recall that a negative gauge pressure indicates that the absolute pressure is below the atmospheric pressure. There are advantages and disadvantages to operating with each refrigerant with regard to: higher/lower compressor work input, fast or slow leaks if they were to occur, and different effects the refrigerant may have on the environment.
R-134a requires a higher compressor input and has a potentially higher leak rate. As R-134a is a high-pressure refrigerant; if a leak develops in the piping, the expulsion rate is higher than other refrigerant and it will most likely leak out into the chiller room. Interestingly, this particular refrigerant possesses no ozone depletion potential because it does not contain any chlorine, but it has a higher global warming potential as compared to HFCF-123 .
Refrigerant Numbering System
Refrigerants names have two different types of prefixes. They can start with a prefix “R-“ that stands for refrigerant or the prefixes which can indicate the type of compounds they contain such as CFC-12, HCFC-141b, and HFC-134a.
To understand their meaning, it is important to understand what these prefixes actually mean. Typically, refrigerants will contain atoms of chlorine, hydrogen, fluorine, carbon and bromine. The letters in the prefixes indicate these atoms. For CFCs and HCFCs, the “C” to the left of “F” indicates chlorine while the “C” to the right of “F” indicates Carbon. H, F and B represent hydrogen, fluorine and bromine respectively. The chlorine is the ozone-depleting substance. For this reason, refrigerants containing chlorine are currently being phased out. Therefore, CFCs and HCFCs both pose threats to the ozone layer while HFCs do not.
For Non-mixture refrigerants, their numbering are based on their chemical composition, the following method is used.
Take R-22 for example, the first step is to add 90 to the number.
So, 22 + 90 = 112
This result gives you the number of carbon, hydrogen and fluorine.
However, this is not the end of the story as chlorines are not yet accounted for. To calculate the number of chlorine atoms, one more piece of information is required. As these refrigerants contain only single bond with Carbon at the center, the number of bonds available can be calculated by 2(#C) + 2. For this case, we only have 1 carbon atom. The number of bonds available is thus 2(1) + 2 = 4. Hydrogen is occupying 1 bond and Fluorine occupies the other 2, that leaves 4 – 2 -1 = 1 bond for Chlorine. Therefore, R-22’s chemical composition is CHClF2.
Blends of refrigerants are numbered by their respective refrigerant mixtures. They may or may not have ozone depletion potential depending on whether they contain chlorine. They are 400 and 500 series refrigerants followed by a capital letter. The numbers indicate the type of refrigerants it contains and the letter represents the percentage composition of each refrigerants. The 400 series are Zeotropes and the 500 series the Azeotropes. Azeotropes are mixtures of refrigerants with similar boiling points that act as a single fluid with a defining boiling point with predictable properties. Zeotropes, on the on the other hand, do not have that property and will change from liquid to vapor on a range of temperatures when pressure is constant. Organic compounds are the 600 series refrigerant and inorganic compounds are the 700 series refrigerants .
|Check out this animation showing how the refrigeration cycle works.|
|Line drawing showing a basic refrigeration cycle at MNSU.|
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|Components of an Absorption Chiller .|
|Line drawing of an absorption refrigeration system.|
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|Line drawing showing a refrigeration cycle with an economizer.|