Free cooling is a type of process cooling system design that takes advantage of ambient temperatures to reduce or even eliminate chiller operation. Chillers consume large amounts of energy; so, reducing a chiller’s operating hours per year can result in significant bottom line savings for your company. In this article, we will review a typical free cooling system design, some of the considerations for your system, and finally, how these considerations impact your system’s ability to capitalize on the free cooling operation.
The practice of free cooling uses low ambient temperatures to achieve a setpoint that would normally require a chiller with mechanical refrigeration. Access to these low ambient temperatures is considered “free” because it is simply based on your geographic location and climate. Using this environmental benefit to your advantage saves the cost of operating a chiller.
First, before we can dive right in with free cooling, we need to step back and review the typical components of a large process cooling system. These are the central chiller, fluid cooler, and pumping system. Understanding what each of these components do and how they interact with each other will be the basis for our free cooling design discussion.
What is a Central Chiller?
Central chillers are typically used to support the total process cooling needs of a facility.
A central chiller is a large capacity cooling unit that typically supports an entire facility’s processing needs or at least a large portion of it. These units are available in three main configurations: air-cooled, water-cooled, and remote air-cooled. An air-cooled chiller is a packaged unit where the refrigeration circuit, including the compressor, is housed within the same framework as the condenser and fans. It represents a self-contained, packaged solution. The fans are used to force air over the condenser and reject the heat removed by the chiller to the atmosphere. A water-cooled chiller uses a water-to-refrigerant heat exchanger as the condenser instead of an air-to-refrigerant heat exchanger. This requires a cooling tower system to provide the water to cool the condenser. A remote air-cooled chiller, also known as a split system, takes a packaged air-cooled chiller and separates the chiller’s compressor and most of the other refrigeration components, from the condenser and fans. The piece of equipment with the condenser and fans is known as the remote condenser. The indoor chiller and outdoor remote condenser are then connected with refrigerant piping in the field.
The packaged air-cooled chiller is typically installed outdoors because it rejects the heat removed from the process into its surrounding area. A water-cooled unit is installed indoors, and the split system chiller is installed indoors with the remote condenser installed outdoors where it can expel the system’s heat.
What is a Fluid Cooler?
Fluid coolers are installed outdoors and use ambient air to provide cooling.
A fluid cooler is very similar to a radiator in your car. It uses forced air over a coil to remove heat from a fluid. These units are installed outdoors and use ambient air to provide cooling. As the air temperature rises, the ability for the unit to reject heat to the atmosphere is reduced. This means during warm months, the fluid cooler is unable to achieve lower fluid temperatures. Conversely, during cooler periods of the year, the fluid cooler can achieve lower temperatures similar to a chiller. This is a key distinction to understand because it is a primary concern when designing a free cooling system.
What is a Pumping System?
Pump tanks work in conjunction with a central chiller and fluid cooler to provide a central water distribution system.
The last major component of a process cooling system is the pump/tank skid. These units have a tank and pumps mounted to a common skid. The tank provides additional mass to the system which acts as a thermal flywheel. This buffers any process heat load spikes and helps maintain steady temperature control. The pumps are used to circulate the fluid through the system. This fluid, typically water or a water/glycol mixture, acts as the medium to transfer heat from the process back to the cooling system’s equipment. The cooling equipment is used to extract the heat and reject it from the system.
The most common pumping configuration uses separate process pump(s) and recirculation pump(s) coupled with a dual well tank (hot and cold wells). The process pump circulates fluid from the tank’s cold well, pumps it to the process to remove heat, and then returns to the other side of the tank – the hot well. A recirculation pump pulls from the hot well and pumps the fluid to the system’s cooling equipment, i.e. – a chiller or fluid cooler, where the heat is rejected from the system. The now cooled fluid returns to the tank’s cold well to start the circulation loop over again.
Pump/tank skids like these use a partial divider to not only allow the two wells to equalize if the flow rates from each side are different, but also provides enough separation to help maintain the hot and cold sides. This contributes to better temperature stability. The other variation of this pumping design uses a full divider. This design completely isolates each side of the tank and requires an external heat exchanger to transfer the heat from the process loop to the recirculation loop. There is some efficiency loss through the heat exchanger but it allows the two fluid loops to be completely separate. The key benefit of a fully divided tank, when considering free cooling designs, is that it allows the use of glycol. The glycol mixture loop is recirculated through the chiller and/or fluid cooler located outdoors, while the process loop can still use pure water to support the equipment in the facility that generates the heat.
What is Free-Cooling?
A free cooling system reduces energy use by using a fluid cooler or cooling tower to cool the process fluid in place of a chiller.
Now that system components have been defined, understanding a free cooling system is much more straightforward. The most effective design for free cooling installs a fluid cooler in series before the chiller in the recirculation pump loop. This means that the fluid in the loop passes through the fluid cooler prior to reaching the chiller. Another common configuration has the fluid cooler installed in parallel to the chiller. In this design, the recirculation loop supplies either the chiller or fluid cooler. This is accomplished with a diverting valve or with a dedicated pump loop for each piece of equipment.
The parallel setup was most common when free cooling designs were first implemented for process cooling. The control setup was simple and easy to use. However, the energy saving benefits were reduced. There was no ability to use partial capacity to take advantage of a chiller’s ability to unload, lowering its energy consumption.
Installing the fluid cooler in series is now widely accepted as the best solution for free cooling. The key advantage of a series design is known as load shedding. This is in addition to the 100% load coverage available with a parallel system design. In the series configuration, the hot fluid passes through the fluid cooler first before reaching the chiller. Even in instances when the fluid cooler cannot meet the full cooling requirement, such as when the ambient temperature is not low enough, it can still remove part of the heat load. The now partially cooled process fluid leaves the fluid cooler and enters the chiller at a lower temperature. This signals the chiller that only part of its available cooling capacity is required allowing the chiller to unload.
As unloading technology has improved, energy savings have followed. For example, VFD controlled centrifugal compressor chillers with magnetic bearings can decrease their speed much lower than a traditional style compressor. A magnetic bearing compressor has a much lower minimum pressure differential because there is no oil. As speed is reduced, the energy consumption is reduced by a cubed relationship. Even for the same amount of load shedding, the energy savings between a traditional style compressor and a magnetic bearing compressor is drastically improved.
There are also chiller designs that incorporate multiple refrigeration circuits that each use multiple compressors. This creates many discrete stages of unloading by turning compressors off. Each stage reduces the energy consumption. Coupling this design with advanced refrigeration controls such as floating head pressure controls can improve energy efficiency even more.
A few of the key items to consider when determining if a free cooling system is right for you:
- Does your cooling system’s setpoint allow you to take advantage of ambient conditions?
- By truly understanding your system’s setpoint requirement, you may be able to gain large periods of time during the year for free cooling. In some instances, simply adjusting your setpoint up by 5°F can add 500+ hours per year when your chiller can be completely shut off.
- Is the climate in your area conducive to a free cooling system?
- Many Northern climates have enough time per year at lower temperatures that a free cooling system can replace a chiller for over 50% of the year.
- Even some Southern climates can shut off a chiller for over 25% of the year, especially if the system’s setpoint is optimized for the facility and the climate.
- Do you have the space available outside of your facility for the additional equipment?
- Depending on the capacity of the cooling system, the fluid cooler can require a fair amount of real estate.
- Are your facility and process equipment capable of supporting a glycol/water process fluid?
- Some processing equipment cannot tolerate glycol. Also, glycol management adds more handling requirements for your facility staff.
- This is when a full divider in your pump/tank may be useful.
You can review all of these items and more with a professional application engineer. They will guide you through the process to understand your particular needs and tailor a system to maximize your energy savings while minimizing your upfront costs.
About the Author
Tom Stone is the National Sales Manager of Industrial Markets for Thermal Care. He has been in the process cooling field for 14 years after graduating from Purdue University’s School of Mechanical Engineering.
For more information visit https://www.thermalcare.com.
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