Chilled water systems are made up of several components. The system doesn't work even when one of the components fails. Since chilled water systems have high efficiency, they are used in many large buildings. I thought chilled water systems are cool and thus, I did some research.
In this post, I'll be explaining the major components of the chilled water system. To help you understand better, I included plenty of diagrams to show how things move and work. Also, I'll be touching on some of the applications of the chilled water system.
How Chilled Water Systems Work?
A chilled water system can be separated into water-cooled and air-cooled. Water-cooled chilled water systems are larger and more efficient than air-cooled chilled water systems.
A typical water-cooled chilled water system consists of 4 major components as below:
- Cooling Tower
- Chilled Water Pump
- Condenser Water Pump
Each component has a lot more to talk about but, in this post, I'll give you an overview of them and explain through the chilled water system as a whole.
Below are two chilled water system diagrams that I've made to explain how it works:
Instead of just blocks with texts, here is the same diagram but with actual photos:
A chilled water system can be separated into two loops; a) chilled water loop and b) condenser water loop.
The chilled water loop starts with the chiller followed by the AHUs and the chilled water pump (CHWP) before returning back to the chiller.
On the other hand, the condenser water loop starts also with the chiller followed by the condenser water pump (CWP) and the cooling tower before returning back to the chiller.
Chilled Water Loop
The chilled water loop is a closed loop piping system. The amount of water inside the chilled water loop does not increase or decrease.
Conversely, the condenser water loop is an open piping system. Hence, the make-up water tank is used to refill water losses due to evaporation at the cooling tower.
Relevant post (technical): Closed Loop Chilled Water System Pressure.
In chilled water systems, water is used to transfer the heat energy from the AHUs to the chiller thereby cooling the space. Then, a separate loop of water is used to transfer the heat energy from the chiller to the cooling tower where the heat energy is dissipated to the ambient air.
Inside the chiller is where the basic refrigeration cycle happens. A chiller is made up of the 4 major components required for the refrigeration process which are: a) compressor, b) condenser, c) evaporator and d) expansion valve.
To produce chilled water, the refrigerant absorbs heat from the water and thus, chilling the water to about 6.7°C (44°F). Afterward, the refrigerant rejects the heat to the condenser water, making the condenser water rise to about 35°C (95°F).
Below is the refrigeration cycle that happens inside a water-cooled chiller:
The chiller itself is a giant air conditioner. The chilled water pump pushes the water through the evaporator of the chiller thereby cooling the water.
At the same time, the condenser water pump pushes another loop of water through the condenser of the chiller to carry away the heat energy.
Below is how the chilled water and the condenser water flow at the chiller:
Inside either the evaporator or the condenser, the refrigerant does not meet with the water. The evaporator and the condenser of the chiller are heat exchangers, designed to transfer heat from the refrigerant to the water efficiently.
The temperature different between the refrigerant and water is called approach. For more details, see my post What is Chiller Approach?.
Below is how the chiller shell and tube heat exchanger works:
Inside a shell and tube heat exchanger, there are multiple tubes that refer to the “tube” of the shell and tube, and the “shell” is the container of the heat exchanger.
Whether it is chilled water or condenser water, the water passes through the tube while the refrigerant passes through the shell. If we dive deeper, there are actually two types of water-cooled chillers; a) flooded type and b) dry type.
When the refrigerant is passing through the shell and flooding the “tube”, it is called a flooded type chiller. Conversely, if the refrigerant is passing through the tube and hence the shell is “dry”, it is called a dry type chiller.
On the other side, air handling units (AHUs) and fan coil units (FCUs) are basic air conditioning units that are made up of mainly 4 components; a) cooling coil, b) fan blower, c) air filters and d) condensate drain line.
Below is how an air handling unit provides cooling using chilled water:
The chilled water from the chiller enters the cooling coil of the air handling unit (AHU) usually at about 6.7°C (44°F) and leave at about 12.2°C (55°F). The AHU blows air through the cooling coil and provides cooling to the room.
Since the cooling coil is very cold, water droplets will form on the surface of the coil. Thus, a condensate drain line is needed to discharge the condensate water. At the same time, air filters are used to protect the cooling coil from dust just like any other air conditioner.
Air handling units (AHUs) are mostly custom-made, suit the required airflow and cooling capacity as well as the physical dimension. Meanwhile, fan coil units (FCUs) are usually smaller with several standardized sizes.
Every AHU and FCU is fitted with a motorized water control valve. They usually use a separate thermostat to detect the room temperature and control the chilled water flow rate (by controlling the motorized valve) to control the amount of cooling given to the room.
After absorbing heat from AHUs and FCUs, the chilled water returns back to the chiller to release the heat to the condenser water.
Condenser Water Loop
The condenser water pump pushes the condenser water from the chiller to the cooling tower which is usually located on the roof. The cooling tower uses the principle of evaporative cooling to reject the heat from the condenser water to the surrounding ambient air.
Below is the basic working principle of a cooling tower:
At the cooling tower, the condenser water is sprayed onto the infill of the cooling tower to increase the surface area in order to better reject the heat. The cooling tower fan draws in the ambient air from all directions, allowing the ambient air to meet the condenser water at the infill for the ambient air to absorb the heat from the condenser water.
The condenser water is then falling from the infill onto the basin and goes back to the chiller via gravity flow. Since the condenser water loop breaks at the cooling tower, it is an open loop piping system.
Because the cooling tower uses evaporative cooling, it relies on the wet bulb temperature rather than the dry bulb temperature. Hence, dry places with low wet bulb temperature can have a very efficient chilled water system.
Through evaporative cooling, a small amount of water disappears from the condenser water loop. Hence, the make-up water tank that holds a great amount of water will replenish the same amount of water to the cooling tower.
Chilled Water System (eBook)
To truly understand the chilled water system, engineers must learn the fundamental approach. In my ebook Chilled Water System, I'll show you how we can use a fundamental approach to explain the behaviour of chilled water systems. Learn more about it here.
The working principle of air-cooled systems is pretty much the same as water-cooled systems. However, instead of using cooling towers to reject heat, air-cooled chillers simply have a fan to reject heat. So, the internal components of air-cooled chillers are slightly different from water-cooled chillers.
Below is the refrigeration cycle that happens inside an air-cooled chiller:
Air-cooled chillers are able to increase the cooling capacity usually by simply attaching more air-cooled chiller modules like legos. Often, they are known as air-cooled modular chillers.
Below is the basic working principle of an air-cooled chiller, showing how the chilled water moves in/out and how it rejects heat to the ambient air:
So, as you can see, both water-cooled and air-cooled chillers are essentially operated by the same principle. At the chiller, there is the refrigeration cycle. Then, water is used to move around the energy.
However, most air-cooled chillers don't use shell-and-tube heat exchangers. Instead, they use a brazed-plate heat exchanger and a finned-tube heat exchanger.
A finned-tube heat exchanger is just like any home air conditioner where there are many layers of alunimium fins that help to dissipate heat faster.
A brazed plate heat exchanger has multiple plates that are packed together. Inside the brazed plate heat exchanger, the refrigerant and the chilled water do not contact and they transfer heat via the high thermal conductive copper plates.
Instead of water-cooled or air-cooled chillers, the chilled water system can be run by hybrid chillers. A hybrid chiller is the combination of an air-cooled chiller and a cooling tower.
Hybrid chillers have a similar setup to air-cooled chillers but, they reject heat using the principle of evaporative cooling just like cooling towers. Hence, they are more efficient than air-cooled chillers.
Hybrid chillers have a condenser coil and infill where the condenser water is sprayed onto the coil and infill by a built-in water pump. Then, the fan of hybrid chillers will draw in ambient air to cool the condenser water.
If you are interested in hybrid chillers, check out my post on what is a hybrid chiller to understand how it works and what is the pros and cons of using hybrid chillers.
Chilled Water System Delta T
One of the elements that determine the efficiency of chilled water systems is delta T or the difference between the chilled water supply and return temperature.
Many chilled water systems operate at a delta T of about 5.5°C (10°F). Nowadays, more and more chilled water systems are getting into high delta T setup which the delta T is about 8°C (15°F).
Generally, the higher the delta T, the higher the efficiency of the chilled water system. I encourage you to read my post on high delta T chilled water systems to know more about the efficiency of chilled water systems.
One of the common problems in chilled water systems is low delta T syndrome. The problem basically is low chilled water return temperature that can be caused by multiple different issues such as dirty filters/coils, imbalance water flow and oversized air handling units.
Chilled Water Piping System
As mentioned earlier, a chilled water system can be separated into the chilled water loop and the condenser loop.
For the chilled water loop, pre-insulated carbon steel pipes are usually used to transfer the chilled water. As for the condenser water loop, non-insulated galvanized carbon steel pipes are typically used to move around the condenser water.
Relevant post: 4 Types of Pipes Used for Chilled Water.
Carbon steel pipes are made of steel and thus, they do rust over time. Hence, the flushing process is very important to remove any contaminant before the initial startup of the chilled water system. Furthermore, water treatment is essential to maintain the quality of the chilled water and the condenser water.
The condenser water loop usually uses the more corrosive-resistant galvanized carbon steel pipes because part of the condenser water loop is exposed to the weather (at the cooling tower area).
Chilled water pipes are insulated but condenser water pipes are not insulated because the condenser water temperature is often higher than the surrounding air temperature thereby not encouraging condensation.
Common Applications of the Chilled Water System
A chilled water system is commonly found in commercial buildings such as hotels, offices, skyscrapers, shopping malls, airports, train interchange stations, universities and factories.
Air-cooled chilled water systems are more common in medium-sized buildings where there is not enough space for a dedicated chiller plant room. Instead, air-cooled chillers are placed on the roof.
When it comes to large buildings, the chilled water system is almost always run by water-cooled chillers.
District Cooling Plant
Water-cooled chillers can be found in district cooling plants. A district cooling plant is usually located at a not far distance away from several commercial buildings. The plant has five to ten large water-cooled chillers that supply chilled water to several four to seven commercial buildings depending on the scale.
Because district cooling plants have very large chillers, they are considered the most efficient way of cooling buildings because the chiller plant room efficiency is very high.
The chilled water from a district cooling plant is sent via underground chilled water pipes to a heat exchanger in a commercial building. Then, separate chilled water pumps in that particular commercial building will circulate their internal chilled water loop between AHUs and the heat exchanger for cooling.
After absorbing the heat from the heat exchanger, the chilled water returns back to the district cooling plant. Most district cooling plants charge each commercial building by how much chilled water is supplied.
Sometimes, commercial buildings get penalized by the district cooling plant operating company if the cooling load is low.
Chilled Water System with Thermal Energy Storage
It is not uncommon for a chilled water system to work with a thermal energy storage system. Such a chilled water system perhaps is the most challenging and complex cooling system.
However, thermal energy storage systems can't be applied everywhere because their sole purpose is to reduce electricity cost by taking advantage of the off-peak electricity rate.
Most of the time, a thermal energy storage system is found in a district cooling plant. Both are large cooling systems and thus, it makes sense to integrate both systems.
During the day, large water-cooled chillers supply chilled water to several commercial buildings as usual. At night, cooling demand drops and some of the chillers will shut off.
However, some of the chillers remain in operation to supply chilled water into a large thermal energy storage tank. Inside the tank, there are hundreds, if not thousands of glycol balls that use the principle of latent heat capacity to store a large amount of cooling energy for later use.
Large thermal energy storage tanks are made of a concrete structure. The tanks are filled with thousands of plastic balls that are filled with formulated liquid glycol.
These glycol balls have incredible heat capacity. When the tank is filled with chilled water, each liquid glycol ball turns into ice glycol balls. The phase change makes them storage a huge amount of cooling energy.
While transferring the cooling energy from the chillers to the glycol balls, the electricity cost is at the lowest because of the off-peak rate.
On the next day, the cooling energy stored inside all of the glycol balls is released as the chilled water pump circulates water through the thermal energy storage tank and supplies the chilled water to the associated commercial buildings.
After the glycol ball cooling energy is depleted, the system switches back to the conventional chiller-based cooling system.
A high efficient district cooling plant with thermal energy storage can save a huge amount of electricity cost. However, the energy usage is actually more than the conventional pure chiller-based cooling because of energy losses.
Relevant post: District Cooling Thermal Energy Storage Explained
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