Chiller

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.

Condenser on a Centrifugal Trane Chiller at MNSU
Condenser on a Trane centrifugal chiller at MNSU [129].

 

Refrigeration Cycle

Centrifugal Chiller at MNSU
Centrifugal chiller at MNSU [129].
The vapor compression refrigeration cycle involves four main components in the chiller: a compressor, a condenser, an expansion valve, and an evaporator. Refrigerant enters the compressor, which raises the pressure and temperature of the refrigerant. After the refrigerant exits the compressor, it is superheated within the pipes. The refrigerant is then run through the condenser, which is essentially a heat exchanger, where heat is rejected from the refrigerant to heat a separate flow of water. The heated water is then sent to the cooling tower to be cooled. The refrigerant remains at the same pressure through the heat exchanger but exits at a lower temperature and then flows through an expansion valve that lowers the pressure of the refrigerant causing it to evaporate. When liquids evaporate, they draw in heat from the surrounding atmosphere. Consider a drop of water on the surface of your skin; it feels cold because as it evaporates it is drawing heat from 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

Return water to chillers at MNSU
Return water to chillers at MNSU [129].
The cooling water loop, separate from the chilled water loop, transfers energy from the refrigerant to the atmosphere. The temperature of the cooling water rises in the condenser as a result of the heat transfer from the condensing refrigerant. Once the heated water exits the condenser, it is pumped to the cooling towers. In the cooling tower, the cooling water is sent down the wet deck. Then, air is blown through the wet deck to evaporate a portion of the cooling water. The water that is not evaporated continues to flow down into the cold water basin. Makeup water is added to replace the evaporated water, and all of the fluid is then sent back to a centralized utility plant for reprocessing.

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

Supply and Return water lines
Supply and return water lines [129].
chilled water is kept at a temperature of 40 to 60°F. The pipes are connected to air handlers, which use the hot/chilled water to change temperature of the air.

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.

Compressors

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 [80]. 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.

Centrifugal Chiller Compressor at MNSU
Centrifugal chiller compressor at MNSU [129].
Centrifugal or dynamic compressors function in a similar way to turbines, but in reverse; they add energy to the fluid instead of extracting it. Centrifugal compressors use rotating disks or impellers to force fluid to the rim of the impeller in order to increase the fluid velocity. The blades do work on the fluid, and accelerate the flow. The fluid then flows through a diffuser where the fluid slows down and the kinetic energy of the fluid is converted into static energy, which would result in an increase in pressure of the fluid.

Absorption Chillers

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.

Evaporators

Condenser on the McQuay Centrifugal Chiller at MNSU
Condenser on the McQuay centrifugal chiller at MNSU [129].
To elaborate on the basic refrigeration cycle, there are many possible types of heat exchangers used for evaporators and condensers. Two of such examples of evaporators are spray shell-and-tube, and direct expansion (DX) shell-and-tube. Each type is defined according to the substance traveling through the tubes.

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.

Chiller Economizer

To increase overall efficiency of a chiller, many manufacturers employ a multiple stage expansion and compression process.

Economizer on Trane Centrifugal Chiller at MNSU
Economizer on Trane centrifugal chiller at MNSU [129].
Rather than throttling the refrigerant completely to the evaporator pressure, it is only expanded partially and sent through the economizer. The economizer acts as a flash chamber between the condenser and the evaporator at an intermediate pressure. This results in a two phase mixture, which is then separated into saturated vapor and saturated liquid. The saturated vapor is directed into the second stage compressor (still at the intermediate pressure), while the saturated liquid is directed into the evaporator. It is converted to saturated vapor and then sent to the first stage compressor. Since part of the refrigerant from the economizer does not need to be compressed fully, the total work required by the compressor is reduced.

Refrigerant Types

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 [81]. 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 [83].

Trane chiller plate
Trane chiller plate [129].
HCFC-123 requires less compressor power input and has a slower leak rate (if a leak occurs). Since part of the cycle with HCFC-123 drops below atmospheric pressure, if the tank developed a leak, air would be drawn in instead of refrigerant flowing out. While the plant workers would not risk breathing refrigerant if this happened, they would be required to perform additional maintenance to remove the air from the system. The chlorine based HCFC-123 depletes ozone layer and contributes to global warming; although not to the degree that some other refrigerants (like R-22) do.

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.

#C #H #F
1 1 2

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 [84].

Check out this animation showing how the refrigeration cycle works.
Line drawing showing a basic refrigeration cycle at MNSU.
What are types of chilled water HVAC systems and chilled water system problems? Brinco Mechanical Services describes them.
Want to learn about the difference between two pipe and four pipe systems? Click Here.
Centrifugal chiller walkthrough video.
Components of an Absorption ChillerComponents of an Absorption Chiller [19].
Line drawing of an absorption refrigeration system.
Want to learn about Driving Absorbtion Chillers Using Heat Recovery? Read this ASHRAE article.
Line drawing showing a refrigeration cycle with an economizer.