Chilled Water System: The Ultimate Guide (Types & Diagrams)

Chilled water systems are considered the holy grail of air conditioning. They are big, complex and yet used in some of the tallest buildings in the world like the Burj Khalifa in Dubai, the Merdeka 118 in Malaysia and many more. Almost all large buildings of any kind, shape and application (hotel, office, shopping mall, hospital, etc.) primarily use a chilled water system for cooling and air conditioning.

To engineers, technicians, maintenance personnel and even business owners or executives, the importance of understanding chilled water systems can’t be understated. This is because a survey conducted in Malaysia’s 12 commercial sub-sectors suggests that about 42% of the total energy consumption is used for space cooling. If you understand the system well enough, you’ll have a better chance to optimize it and massively reduce the building’s energy consumption.

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In this post, I’ll start by taking a closer look at the major chilled water system components so you understand how they function as a system. Then, I’ll look into the key design decisions that lead to a high-performing chilled water system. After that, I’ll check out chilled water system diagrams and explore some of the remarkable designs. Let’s get started.

What is Chilled Water System?

Chilled water system is a type of air conditioning system that uses chilled water (low-temperature water) for cooling and dehumidification. It is a combination of multiple components that serve different purposes such as water cooling, water circulation, air cooling, dehumidifying of air and heat rejection.

Unlike conventional air conditioning systems which use refrigerant to cool the air, chilled water systems cool the air by circulating low-temperature water through cooling coils. As each air handler circulates air through their cooling coil, the room is cooled and dehumidified.

While the ultimate goal of a chilled water system is to produce chilled water, how efficiently the chilled water is produced and how consistent the performance of the system lies in the components of the chilled water system. Let’s break it down.

What are the Components of Chilled Water System?

The reason why it is crucial to understand the components of chilled water system is because for each component, there are many different types and they affect how well the system performs over one another in certain situations.

The more you learn about the components, the better you can decide which type is best for your application and how each component interacts to make the system work efficiently. Below I’ll the major components of chilled water system and their primary function.

1. Chiller

Let’s start with the most important component – the chiller. Chiller is the heart of the chilled water system. It is the one that produces chilled water or low-temperature water for air handlers or AHUs to perform the cooling and dehumidification process. Chiller is also the most “power-hunger” component.

The basic working principle of a chiller is similar to the air conditioner used in our home. It follows the 4 refrigeration processes:

1. Compression – Increases the refrigerant pressure and temperature
2. Condensation – Takes away the heat in the refrigerant
3. Expansion – Decreases the refrigerant pressure and temperature
4. Evaporation – Use the refrigerant to absorb heat

Essentially, you can view chiller as a giant air conditioner. The difference is chiller doesn’t directly cool the air but rather, it cools the water which later be used to cool the air. I know it sounds counter-intuitive but hear me out.

In large buildings, it is troublesome to have hundreds or not thousands of air conditioners that directly cool the air. The manpower needed to maintain these air conditioners is enormous. Not to mention there are thousands of compressors which is the most expensive part of an air conditioner. Having thousands of it means the failure rate is higher and so does the cost to repair or replace.

That’s why when the application (building size or total cooling demand) exceeds a certain threshold, it is beneficial to centralize the expensive part – the compressor. Once we have a giant compressor in one place, we can then use it to produce low-temperature water and supply it to thousands of air handlers that are comprised of less expensive parts such as fan, cooling coil and filter.

Now, why water? Why do we use water as a means of energy transfer? Well, simply because water is readily available and easily accessible. Plus, water is good at storing energy. It has 4 times the energy storage capacity than air (specific heat of water 4.2 kJ/kgK vs air 1.005 kJ/kgK).

However, as chiller produces chilled water to absorb the heat in the building, the heat must be rejected somewhere or else, the heat absorption or the cooling stops. The chiller can’t magically make the heat disappear. That brings us to the next component.

2. Cooling Tower

Chillers have two ways to dissipate the heat they absorb in the building. One way is to use the outdoor air just like regular air conditioners with their condensing unit or the outdoor unit. The other way is to use water which requires help from cooling towers.

Cooling towers are the big square things you see on the roofs of large buildings. A large portion of their surface area is used to spread out water so that the water cools better. They use fans to draw the ambient air through the water. As the water evaporates, its temperature drops.

Water-cooled chillers transfer the heat they absorb from the chilled water to what’s called the condenser water. The condenser water is also water but instead of low-temperature, it is slightly warmer than the water that comes out of our home water tap. The condenser water carries the heat from the chiller to the cooling tower and the heat eventually gets released to the surrounding air.

As mentioned earlier, the chiller can also directly use the surrounding air to release heat. This type of chiller is known as air-cooled chiller. They eliminate the need for cooling towers and equip themselves with fans to draw in the ambient air and cool their condenser.

3. Pump

With the chilled water and the condenser water, something must help them move around. This is where chilled water pump and condenser water pump come in.

Unlike regular air conditioners where the compressor can drive the refrigerant around the indoor and outdoor unit, chillers by themselves can’t move chilled water. We use one or more pumps to drive the chilled water around the chiller and the AHU, and so does the condenser water for circulation between the chiller and the cooling tower.

4. AHU (Air Handling Unit)

AHUs or air handling units are the air conditioning units we put in each room. They use chilled water to cool and dehumidify the room. As mentioned earlier, AHUs don’t have a compressor. They only consist of a fan, a cooling coil and a filter.

The temperature and humidity level of the rooms in a building are controlled by AHUs. Depending on the size of the room, one AHU can serve one room, or a single unit can serve multiple rooms. In some buildings, a single AHU can even serve the entire floor, saving the space needed to house the equipment.

As the name suggests, AHUs are air handling units. They handle the air. Additional and different types of AHU components such as filters, UV lamps, humidifiers and some heat recovery devices can be put inside an AHU to improve the air quality, reduce the maintenance frequency, save energy, better condition the air to the building’s requirement or meet other objectives.

Other Components of Chilled Water System

A chilled water system is made up of more than just chiller, cooling tower, pump and AHU. There are a handful of other components that are essential to the system. For example, a makeup water tank is needed to replenish the condenser water as cooling tower uses water evaporation to cool the condenser water.

Another non-major component of chilled water systems is the water filtration system and water treatment system. Both systems ensure good water quality to minimize corrosion and extend the lifespan of the system.

Expansion tank is another important component that gives space for the closed-loop chilled water to go due to thermal expansion. It also serves as a means to add chilled water and exert a certain pressure to keep the air from getting into the piping system which hinders heat transfer.

Different variants of chilled water systems have extra components. For example, in a district cooling system, thermal energy storage tanks and their associated pumps are used to store energy at night and release the energy during daytime to save operating costs. I’ll show you this system in the diagram section below.

Chilled Water System Design

Because chilled water systems are large and complex, involving many different components and each component has many different types to choose from, the design decisions made to every chilled water system ultimately determine if the system can perform well for the building.

While there are more than a handful of design decisions to make, below I listed some of the major decisions to make when designing a chilled water system. You must answer these design questions no matter what buildings or applications you face.

Chilled Water Temperature

The first and foremost design decision to make is what chilled water temperature you want. Well, the common ones are 6.7°C (44°F) supply, 12.2°C (54°F) return and 7°C (44.6°F) supply, 12°C (53.6°F) return. But, can that meet your cooling needs and energy requirements?

The difference between the supply and return temperature is known as delta T. Traditionally, a delta T of 10°F has been the standard for chilled water temperature. However, in recent times, high energy standards call for at least 15°F delta T.

According to physics, the lower the temperature, the harder it is to maintain it. What this means is if you use a lower chilled water temperature, for example 5.5°C (42°F) supply, your chiller will need to use more power to reach that low temperature compared to a 6.7°C (44°F) supply.

However, it doesn’t mean you shouldn’t use lower chilled water temperature. Many chilled water systems use low chilled water temperatures and achieve great results. Chilled water temperature is just one of the design aspects. There are others that will affect the final outcome of the system.

What Type of Chiller Should You Use?

Chillers come in different types. The first categorization of chiller is based on its heat transfer method. As I mentioned earlier, a chiller that rejects heat to the condenser water is known as a water-cooled chiller. A chiller that rejects heat directly to the surrounding air is known as an air-cooled chiller. And, there are a few more variants:

• Water-Cooled Chiller: Use water for heat rejection
• Air-Cooled Chiller: Use air for heat rejection
• Evaporative Air-Cooled Chiller (Hybrid Chiller): Use both water and air for heat rejection
• Heat Recovery Chiller: Reuse the heat for other purposes (eg: hot water production)
• Absorption Chiller: Driven by external heat (eg: waste heat) instead of a compressor

Chillers are also further categorized by their compressor type. There are 4 major types of compressors used in chiller: a) centrifugal, b) single or twin screw, c) scroll and d) reciprocating. Some of the common chiller types involving both categorizations are:

• Water-Cooled Centrifugal Chiller
• Water-Cooled Screw Chiller
• Air-Cooled Screw Chiller
• Chiller with Magnetic Bearing

Each type of chiller and compressor type makes a difference in how well the chilled water system performs for the building. For example, centrifugal compressors are prone to problems when the cooling demand is low. So, in an office building where there is a low cooling demand at night for overtime work, a chiller with screw compressors may be a better design choice.

Another example is when you have excessive heat in a factory or processing plant. You can reuse the heat by using an absorption chiller which can save you massively operating costs by eliminating the power-hunger compressor.

How Many Chillers Do You Need?

Sometimes, the type of chiller by itself may not help you solve problems like operating in low cooling demand. Even screw chillers have a minimum cooling demand requirement often known as turndown ratio. Just like a plane, you can’t fly too slow or else, the plane will stall.

Here’s where the number of chillers matters.

Assume that a building needs 1000 tons of cooling capacity. You have a few choices. You can either use two 500-ton chillers or four 250-ton chillers. The fewer chillers you have, the cheaper it is to install and operate. However, if the lowest cooling capacity needed in the building is 100 tons, you might have trouble running the 500-ton chillers.

Most chillers have a turndown ratio of about 30%. That means the most a chiller can reduce its cooling capacity before tripping is 30% of its rated cooling capacity. A 500-ton chiller with a 30% turndown ratio means the lowest cooling capacity is 150 tons. This is still higher than the building’s 100 tons requirement. So how?

In this case, more chillers with lower cooling capacity are a better choice. Alternatively, you can also use three 300-ton chillers and one 100-ton chiller. The 100-ton chiller can be used to tackle the low cooling demand period.

Pump Arrangement

While pumps are simple machines that move water, at a large scale, they become very important because pumps affect how well the chilled water is supplied to the AHUs and how the chillers react to the change in the cooling demand. Essentially, the transfer of the heat from the building to the chiller and then to the cooling tower relies on the pump.

In a chilled water system, pumps can be arranged in a few ways. The way pumps and chillers are arranged is sometimes known as system configuration. The two most commonly used arrangements are:

• Primary-Secondary Flow
• Variable-Primary Flow (VPF)

If a chilled water system uses the primary-secondary flow configuration, there will be 2 sets of chilled water pumps, one is called primary pump and the other one is called secondary pump.

Primary pumps are used to ensure the chillers always have enough water flowing through them to prevent the chillers from tripping due to low pressure. Secondary pumps are used to change the water flow supplied to the AHUs based on how much is needed for each AHU using a variable frequency drive.

On the other hand, VPF only uses one set of chilled water pumps. These chilled water pumps are similar to the secondary pumps in the primary-secondary flow configuration but they also need to ensure the chillers never trip due to low water flow. Basically, they handle two jobs at once.

Which system configuration to use depends on many factors. For example, VPF has a lower first cost because you have fewer pumps. However, it requires you to have a good understanding of the control system since you need to balance both the AHU and the chiller’s requirements.

Where Should You Use Chilled Water?

In a building where chilled water system is used for space cooling, there are some areas where it is not practical to use chilled water. For instance, in an office building, spaces used for office work typically operate during the daytime. However, server rooms that hold the data for employees to access remotely require cooling 24/7. These server rooms are typically small and it is not possible to operate a chiller to cool these spaces.

So, where should you use chilled water becomes a very important question to answer. Not only it determine how much chilled water you actually need in a building, but it also affects the size, cost, space and installation time required for the chilled water system.

Chilled Water System Diagrams

By now, you have a basic understanding of the chilled water system. You know what components make up the system, and what are involved in the design process and how they affect the outcome of the cooling system. Let’s check out some chilled water diagrams and what are they.

1) 4000 Ton Water-Cooled Centrifugal Chiller

A standard chilled water system diagram consists of the chiller, cooling tower and pump. The chilled water distribution to AHUs and FCUs is usually included unless the system is large until a separate diagram is needed. The diagram will show the quantity of chillers and cooling towers, how are they connected, what valves and sensors are used, the pipe sizes, how many sets of pumps, what type of water filtration system is used and the zones which the chilled water is supplied to the AHUs and FCUs.

2) District Cooling System with Thermal Energy Storage

A single chilled water system can be used to serve multiple buildings and it is known as a district cooling system. A district cooling system can use thermal energy storage tanks to take advantage of off-peak tariffs. In such a system, the diagram will include the thermal energy storage tank capacity, physical size and the pumps used for the charging circuit. The chilled water supply and return to and from AHUs and FCUs are typically not included as the piping to these destinations is long and complicated.

3) Chilled Water Network Piping

Further to the district cooling system diagram, the chilled water supply/return to/from AHUs and FCUs is included in what is known as a network piping diagram. This diagram will show the piping route from the plant room where the chillers are located, to each building. Often, the chilled water is not directly supplied to the AHUs and FCUs in the building. Rather, the chilled water transfers its energy to a separate chilled water circuit within the building via a heat exchanger.

4) Chilled Water Piping System

High-rise buildings with many floors typically have a chilled water piping diagram that shows how the chilled water pipe connects to each AHUs and FCUs at different floors from the chilled water plant. From the piping diagram, we can see how many AHUs and FCUs are in the building, where are they located at and where is the chiller plant room. The sequence of the piping connection can also be seen in this diagram. For example, which FCU is connected first and which FCU is connected last.

5) Air-Cooled Chiller Detail Connection

Detailed piping connections between pumps and chillers are needed to show the position of all valves, strainers, gauges and sensors. While this type of detailed diagram is not needed if the overall schematic diagram contains the information, complex systems where too much information can’t fit into a single diagram require multiple detailed connection diagrams for contractors, installers and other designers to understand how the system works.

Designing a Chilled Water System

Designing a chilled water system from scratch can be very challenging if you don’t have a lot of experience. That’s why I developed the Chilled Water System Design Course which uses a modern high-rise building as an example and takes you through the step-by-step design process. If you want to learn about chilled water systems, go to the course outline below now:

Check Out the Chilled Water System Design Course Outline

Although there are many things to learn about chilled water systems, take it as the ultimate goal as an HVAC engineer which we can work towards mastering. Not a bad progressive learning isn’t it?

PS: Whenever I learn something new from chilled water system, I’m excited and enjoy every bit of the process. I think continuous learning motivates me and keeps me happy.