Complete Chiller Systems: Chapter 2
Cold Water Cycle
In this module, we will learn about how chiller systems produce cold water.
We will also look at cold water cycle components and learn about them in detail. Skip to quiz!
Introduction
Recall that chiller systems usually contain a shell and tube type of evaporator. Shell and tube evaporator consist of a bundle of copper tubes inside a large metal shell.
In the chiller systems, the evaporator coils filled with refrigerant have water flowing over them instead of air. So the refrigerant cools the water. Chiller systems use this cold water to provide cooling to the building.
Cold water that is circulated throughout the building cools the air inside the building. So, the refrigerant cools the water, and the cold water cools the air in the building. For this reason, water is also called secondary refrigerant in the chiller systems.
Water does not take part in the basic refrigeration cycle. It does not undergo any phase change. But it cools the air in the building.
To cool large buildings, we would need a large amount of refrigerant. However, we can not run refrigerant lines for long distances and each room inside the building. Recall that some refrigerants are toxic and flammable. It can be hazardous if they leak.
We provide cooling to a large building through cold water because:
Water is cheap and readily available in nature.
It is cost-effective to use water.
It is not toxic. So leakage of water inside the building not hazardous.
It is much safer to use water in place of refrigerants.
Chiller systems are also used in applications like ice factories and cold storage, where we need to maintain the temperature below 0ºC. Recall that the water freezes at 0ºC. So we can not use water. For such applications, we use brines.
Brine is a solution formed by mixing a soluble substance in the water. The soluble substance could be salts like table salt, or calcium chloride, or glycol. By mixing these substances in water, its freezing point decreases. This means the mixing solution does not freeze at 0ºC.
Cold Water Cycle
The main components of the cold water cycle are:
Evaporator in the chiller,
Cold water pumps,
Cold water piping, and
AHUs or FCUs.
We will go over each of these components and learn how they work. Recall that chillers consist of shell and tube evaporators. These evaporators produce cold water.
Recall that water pumps circulate cold water into the building. Centrifugal pumps are the most popular and commonly used for circulating water in chiller systems.
Centrifugal pumps are very similar to centrifugal compressors. Compressors are used for gases, and pumps are used for liquids. Let us see a video to understand the working of centrifugal pumps.
In the chiller systems, pipes are used to carry water. The piping used to circulate cold water is made up of steel. Water pumps circulate water through these pipes.
AHU stands for ‘Air Handling Unit.’ As the name suggests, it handles the air inside the building. It is usually a large rectangular metal box that is connected to the ductwork. Recall that ductwork is a closed path for circulating air.
Recall that an air handling unit contains an air filter, blower, and heating and cooling components. It is used to cool or heat and circulate a large amount of air throughout the room via ductwork. Let us see a video to understand the AHU working.
FCU stands for ‘Fan Coil Unit.’ It is similar to AHU but small in size. It provides cooling for a single room only. It does not require much ductwork as it is placed within the room. It cools and recirculates room air only. There is no adding of outdoor fresh air.
AHUs found in commercial applications are much larger than residential applications. FCUs are generally found in commercial applications. We will look at AHUs and FCUs in commercial applications in detail in later courses.
Recall that the chiller system generates cold water. This cold water is circulated to AHUs or FCUs within the building through water piping. Water pumps are used to circulate cold water through water piping. Let us see a video to understand how the whole cold water cycle works.
Chiller systems use this cold water to provide cooling to the building. Cold water is called secondary refrigerant. Brine has a lower freezing point than water. Brines are used in applications like ice factories and cold storage to maintain the temperature below 0ºC.
The main components of cold water cycle are:
Chiller,
Water pumps,
Water piping, and
AHUs or FCUs.
Water pumps circulate cold water to AHUs or FCUs within the building through water piping.
Cold Water Distribution
In this module, we will learn about how cold water is circulated to the building. We will also look at pipe fittings and piping insulation. Skip to quiz!
Cold Water Pumping
Recall that the chiller system generates cold water.This cold water is circulated to AHUs or FCUs within the building through water piping. Water pumps are used to circulate cold water through water piping.
The cold water pumping includes two sets of pumps. They are named as
primary pumps, and
secondary pumps.
The set of pumps that return water from the building to the chiller are called primary pumps. The other set of pumps that supply cold water from the chiller to the building are called secondary pumps. Let us see a video to understand cold water pumping.
The set of pumps that return water from the building to the chiller are called primary pumps. We need to maintain minimum water flow in the chiller. If the water flow falls below the specified value, chiller can freeze which may result in chiller failure. So we generally use primary pumps as constant speed pumps.
The set of pumps that supply cold water from the chiller to the building are called secondary pumps. Sometimes we need not maintain cooling for some rooms. We stop supplying cold water to those rooms. So it varies the flow to building. So we generally use secondary pumps as variable speed pumps.
The primary and secondary circuits are connected by a pipe known as a decoupler. Water in the decoupler can flow either way or no flow depending on the operation of the chiller.
Pipe Fittings
Pipe fittings are piping components that help in directional changes, size changes, and branch connections. Some examples of pipe fittings are:
Elbows,
Tees,
Reducers,
Couplings, and
End caps.
Let us see a video to recall the usage of different pipe fittings.
Elbows and tees change the direction of the water flow. Reducers change the size of the pipe. Couplings connect different components. End caps cover the end of the pipe and stop the water flow.
Valves
Recall that a valve is a device that is used to decrease or stop the flow of a fluid. It is a type of fitting that controls the flow inside the piping. There are different types of valves used in chiller systems. Let us learn each of them in detail.
Recall that ball valves are 2-way, 2-position valves that are used for shut-off or isolation of a system. The 2 ways are the inlet and outlet of the pipe. Open and close are the two positions for which ball valves are used.
The butterfly valve is very similar to the ball valve. The ‘butterfly’ disk is connected to the rod. It closes when the rod rotates the disc by a quarter turn to a position perpendicular to the piping. It opens when the rod is rotated back to its original position.
Recall that the gate valve is also used for starting or stopping the flow. Gate valves work by inserting a rectangular gate or wedge into the path of a flowing fluid.
Recall that the globe valve is used to stop, start and regulate flow. Unlike the gate valve, the globe valve can control the flow of fluid. 2-way globe valves that are used for single inlet flow and outlet flow.
Recall that check valves are generally installed in pipelines to prevent backflow. Backflow is when water flows backward in the supply line due to pressure changes. Such backflow can be prevented by safety devices such as a check valve.
Check valve permits fluid to move in one direction. It is also called a non-returning valve. It opens with the forward flow and closes against the reverse flow. It acts as a safety device.
Strainer
A strainer is a pipe fitting used to filter the water flowing through the pipeline. Strainers have wire mesh inside them that traps dirt in the water. These protect piping equipment from potential damage due to dirt by water.
Strainers are mainly two types:
T strainer
Y strainer
Let us learn more about these strainers.
As its name implies, it is in T shape. It is also called pot strainer and basket strainer. It can only be installed in horizontal piping. It has a large size mesh. So it can hold more dirt.
As its name implies, it is in ‘Y’ shape. It can be installed horizontal or vertical piping. It can handle more pressure than the T strainer. It has a small mesh, and it can hold less dirt than the T strainer. So this strainer requires frequent cleaning.
Other fittings
As the name implies, flexible bellows are flexible elements that absorb movements in the piping. These are used to create a fitting that will cope with movement, caused either by
thermal expansion and contraction, or by
vibration.
Air and dirt separator removes both air bubbles and dirt particles from the piping. It is usually placed before the water pump. Lets us see a video to understand the working of it.
Piping Insulation
Piping insulation is materials wrapped around the pipe, which decreases the heat transfer. It creates a barrier between cold water piping and outside air.
Recall that cold water is circulated through water piping. When outside air passes over the cold surface of water piping, moisture in the in turns into water droplets. This creates wet surfaces in the buildings.
Recall that wet surfaces encourage the growth of mold. Mold creates health problems for the occupants of the building. Wet surfaces also cause piping to corrode and damage quickly. To overcome these problems, we have to insulate the cold water piping.
The most common insulating materials in chiller systems are:
cellular glass,
glass fiber,
polyurethane foam, and
phenolic foam.
Let us learn more about these insulating materials.
Cellular glass is made by heating crushed glass and a material such as a limestone or carbon. It is lightweight, non-flammable, and does not absorb moisture. Fabricators transform the melted glass into sheets used as piping insulation.
Glass fiber is a material made from a large number of the extremely fine thread of glass. The glass fiber is usually flattened into a sheet, randomly arranged, or woven into a fabric. It is also fire and moisture resistant.
Polyurethane foam is commonly known as spray foam. It is found in mattresses, home furniture, and car seats. It has a comparably higher thermal resistance. It stops the heat transfer from the outdoor air to cold water piping.
Phenolic insulation is manufactured by a process in which a plastic foam forms an insulating core between two flexible tissue-faced layers. These are more stable in the case of fire and release almost no emissions. We can get optimal insulation results by using less material. Phenolic foam is efficient among all four.
The two sets of the pumps
primary pumps, and
secondary pumps.
Pipe fittings are piping components that help in directional changes, size changes, and branch connections.
Air and dirt separator removes both air bubbles and dirt particles from the piping. The most common insulating materials in chiller systems are:
cellular glass,
glass fiber,
polyurethane foam, and
phenolic foam.
Cooling Tower
In this module, we will learn about the cooling tower and its working principle. We will also learn about water treatment for the cooling tower. Skip to quiz!
Cooling Tower
Recall that the water-cooled chiller’s condenser is cooled with water. Water absorbs heat while cooling the condenser. We have to cool the hot water and reuse it for the effective cooling of the condenser.
The device used to cool hot water from the condenser is a cooling tower. A cooling tower is a large box designed to remove heat from water by spraying it down through the tower.
The main components of the cooling tower are:
Basin,
Fills,
Nozzles,
Drift eliminator, and
Fan.
Let us learn more about these components.
Basin is located at the bottom of the cooling tower. It collects the cooled water that flows down through the tower. Fills slow down the falling of water and spread it over a large area to increase water and air contact.
Nozzles are attached to the hot water pipe. These spray water on the fills for cooling. These work similar to the garden sprinklers. Drift eliminator installed above the hot water pipe. It traps the large water droplets from escaping.
Fans are installed at the top or bottom of the cooling tower to circulate the air through the tower. Let us see a video to understand the working of the cooling tower.
Warm water is sprayed from the top using nozzles, and outside air enters from the bottom. As the water flows down the tower, fills spread it over a large area to increase water and air contact. This process reduces the temperature of water by evaporation process.
The cooled water is collected at the bottom of the cooling tower. This cooled water is used again to cool the condenser.
Types of Cooling Towers
The two most common cooling towers used in the HVAC industry are:
Induced draft, and
Forced draft.
These are divided on how the air is moved by the fan through the cooling tower. Let us see a video to understand the above types of cooling towers.
In an induced draft cooling tower, a fan is placed on top of the cooling tower. Axial fans are always used for this type of draft. The fan pulls air upwards against the downward flow of water.
In a forced draft cooling tower, a fan is placed at the base of the cooling tower. Both axial fans and centrifugal fans can be used. The fan pushes air upwards against the downward flow of water.
Two different types of cooling towers based on the airflow pattern:
Crossflow, and
Counterflow.
Induced draft and forced draft cooling towers are built in these configurations. Let us see a video to understand these configurations.
Fills are placed on two sides of the tower. Air comes from two sides of the tower. It uses gravitational force to distribute water in the tower.
Fills are placed horizontally in the tower. Inlet air comes from all the sides below fills. It uses a water pump to distribute water in the tower.
Condenser Water Circuit
The condenser water system consists of:
Shell and tube condenser,
Condenser water pump,
Condenser water piping, and
Cooling tower.
Let us look a video to understand how these work together.
Water Treatment
Recall that water in the cooling tower evaporates to get cooled. As water evaporates, impurities in the water remain in the basin. These impurities can create problems like scaling, corrosion, and fouling potential.
Impurities in the basin make water acidic or basic in nature. Lime water is an example of an acidic nature liquid. Acidic nature liquids generally taste sour. These react with metals and make them corrode.
Basic nature water is slippery feel to touch. When water passes over rocks and picks up impurities, it becomes basic in nature. Basic nature water increases the scale-forming tendency.
Pure water is neither acidic nor basic. It is neutral in nature. Because of the impurities added to water, it becomes acidic or basic. The pH scale is used to measure degree of acidity or basicity of liquid. The pH of water is a very important measurement concerning water quality in the cooling tower.
pH scale ranges from 0 to 14. pH value 7 indicates neutral. pH value less than 7 indicates acidity, and greater than 7 indicates basicity. pH is measured with an electronic pH meter. Color indicators are also available.
Water with a pH value below 7 is acidic in nature and corrode metal parts. Water with a pH value above 7 is basic in nature and forms scale on condenser tubes.
We have to maintain cooling tower water with a pH of about 7. To maintain water quality, we add chemicals to the water in the cooling tower.
The hardness of water is the amount of dissolved calcium and magnesium in the water. Let us see a video to understand the hardness of the water.
Hard water is high in dissolved minerals, largely calcium and magnesium. Hard is unfit for drinking, bathing, washing. It also forms scale in the chiller systems.
Scale
Recall that water contains a lot of dissolved impurities, which contribute to the hardness of the water. The solubility of dissolved impurities, like calcium carbonate, decreases as temperature
rises. That is when the impurities separate out from the water and deposit on the condenser tubes as scale.
Scale is a hard deposit that can be removed only by using chemicals. It offers resistance to the heat transfer between refrigerant and water in the condenser.
Cooling Tower Blowdown
Recall that as the water evaporates in the cooling tower, impurities in the water remain in the basin. Makeup water from softener or other water supply added to the cooling tower to replace the evaporated water. A softener is a machine that removes the hardness of the water. It works similar water purifier in the home.
Over a period of operation, the number of impurities that accumulate in the tower basin in increases. It will accelerate the scale formation on condenser tubes. A small amount of cooling tower is drained out to overcome this problem. This is called blowdown of the cooling tower.
The condenser water system consists of:
Shell and tube condenser,
Condenser water pump,
Condenser water piping, and
Cooling tower.
Cooling tower is a large box designed to remove heat from water by spraying it down through the tower.
Question #1: Water generated in the chiller system goes through a phase change.
True
False
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Answer: False
Water generated in the chiller system does not undergo any phase change. Only the refrigerant used in the chiller system undergoes a phase change.
Question #2: Brine has a lower freezing point than water.
True
False
Scroll down for the answer...
Answer: True
Brine is a solution formed by mixing a soluble substance like table salt or calcium chloride, or glycol in the water. It has a lower freezing point than water. It does not freeze at 0ºC.
Question #3: In the chiller systems, cold water piping is made up of _________.
Plastic
Steel
Copper
None of the above
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Answer: Steel
In the chiller systems, cold water piping is made up of steel.
Question #4: Which of the following does not require much ductwork?
Air Handling Unit
Fan Coil Unit
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Answer: Fan Coil Unit
FCU does not require much ductwork. It cools and recirculates room air only.
Question #5: _______ circulate cold water from the chiller to the building.
Primary pumps
Secondary pumps
Tertiary pumps
All the above.
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Answer: Secondary pumps
Secondary pumps circulate cold water from the chiller to the building.
Question #6: Which of the following pipe fittings change the direction of water flow? (Select all that apply)
Tee
Reducer
End cap
Elbow
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Answer: Tee
Elbow
Elbows and tees change the direction of the water flow.
Question #7: Controls systems are important because they allow for efficiency.
True
False
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Answer: True
True, controls allow for automation, making the system both efficient and intelligent.
Question #8: Which of the following valve does not allow back flow of water? (Select all that apply)
Ball valve
Check valve
Gate valve
Globe valve
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Answer: Check valve
Check valve does not allow backflow of the water. It is also called a non-returning valve.
Question #9: What is another name for T strainer? (Select all that apply)
Pot strainer
Y strainer
Basket strainer
Butterfly strainer
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Answer: Pot strainer
Basket strainer
T strainer is also called pot strainer and basket strainer.
Question #10: Which of the following strainer can handle more pressure?
T strainer
Y strainer
Pot strainer
Basket strainer
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Answer: Y strainer
Y strainer can handle more pressure than T strainer.
Question #11: Which of the following absorbs the movement in the piping?
Flexible bellow
Reducer
Y strainer
Butterfly valve
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Answer: Flexible bellow
Flexible bellows are flexible elements that absorb movements in the piping
Question #12: Which of the following insulation material is the more efficient among all?
Glass fiber
Cellular glass
Phenolic foam
Polyurethane foam
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Answer: Phenolic foam
Phenolic foam is the more efficient insulation material among all.
Question #13: Which of the following component traps the large water droplets from escaping?
Fills
Basin
Nozzles
Drift eliminator
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Answer: Drift eliminator
The drift eliminator is the component of cooling tower. It is above the hot water pipe. It traps the large water droplets from escaping.
Question #14: What device cools the hot water from the water-cooled chiller’s condenser?
Evaporator
Compressor
Cooling tower
Metering device
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Answer: Cooling tower
The cooling tower cools the hot water from the water-cooled chiller’s condenser.
Question #15: In a crossflow cooling tower, fills are placed on two sides of the tower.
True
False
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Answer: True
In a crossflow cooling tower, fills are placed on two sides of the tower.
Question #16: Which of the following component that is not the part of condenser water circuit?
Condenser
Evaporator
Cooling tower
Condenser water pump
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Answer: Evaporator
The condenser water system consists of:
Shell and tube condenser,
Condenser water pump,
Condenser water piping, and
Cooling tower.
So evaporator is not the component of the condenser water circuit.
Question #17: pH value equal to 4 indicates _______.
acidity
basicity
neutral
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Answer: acidity
pH value less than 7 indicates acidity. Since pH value 4 is less than 7, it indicates acidity.