This article will take an in-depth look at air cooled chillers.
The article will bring more detail on topics such as:
This chapter will cover the basics of air-cooled chillers, including their components and how they operate.
Air-cooled chillers are refrigeration systems that cool fluids and work in conjunction with a facility's air handling system. Unlike traditional industrial chillers that use cooling towers, air-cooled industrial chillers rely on fans to dissipate heat outside the building. These chillers use air to cool water and other fluids circulating through the system, helping to lower the temperature of equipment, processes, and areas by absorbing and removing heat.
Air-cooled industrial chillers comprise a compressor, an evaporator, a condenser, and an expansion valve. They are utilized in large industrial operations as well as various structures such as indoor malls, industrial complexes, hotels, and hospitals. These chillers are also suitable for locations with multiple buildings, such as theme parks and outdoor malls, providing reliable cooling power for both large and small operations.
The compact design of air cooled industrial chillers makes them portable, inexpensive to maintain, and easy to install. Portable air cooled industrial chillers can be used for large events or emergency situations and have a simpler structure to that of water cooled industrial chillers. Most air cooled industrial chillers use screw compressors or scroll compressors.
Air-cooled industrial chillers are often referred to as modular chillers because they can be stacked horizontally to increase their cooling capacity, which ranges from around 10 refrigeration tons (RT) to 550 RT. A single air-cooled industrial chiller with scroll compressors can achieve a maximum cooling capacity of about 200 RT, while those with screw compressors can reach up to 550 RT.
These chillers generally feature at least two compressors. If one compressor fails, the other continues to operate, though the overall cooling capacity will be reduced to 50%. Air-cooled industrial chillers are less efficient than water-cooled chillers, with an energy efficiency of approximately 1.00 kW/ton, compared to the double efficiency of water-cooled models.
The components of an air cooled chiller are:
The compressor provides the driving force that moves the refrigerant through the system. The different types of compressors are screw, reciprocating, scroll, rotary, and centrifugal. Centrifugal compressors are the most dynamic and operate differently from the other types, which use positive displacement as their working principle.
Positive displacement compressors continuously reduce the volume of gas using screws, pistons, spirals, or rotations to increase vapor pressure. In contrast, centrifugal compressors compress gas by accelerating it with an impeller's rotation and then reducing its speed in a diffuser to increase pressure.
Centrifugal compressors are well suited for large-capacity applications. Compressors can be categorized into three categories: hermetic compressors, semi-hermetic compressors, and open compressors. In hermetic compressors, the compressor and the electric motor are placed in an enclosed casing that cannot be opened when maintenance needs to be done. This type of compressor is utilized in low-capacity applications as a result. In semi-hermetic compressors, there is a two-piece casing in which the electric motor and the motor are enclosed.
Unlike hermetic compressors, the compressor and motor are separate units in semi-hermetic compressors, allowing for mid-range capacity applications. Open compressors have the electric motor separated from the casing enclosure but connected via a coupling. These compressors are designed for high-capacity applications.
The condenser is a heat exchanger that transfers heat from the refrigerant to the air and is cooled by air. Air-cooled condensers consist of copper tubes carrying liquid refrigerant and aluminum fins that facilitate heat transfer to the passing air. Condensers in industrial air-cooled chillers can be categorized as air-cooled, evaporative condensers, or water-cooled condensers. In air-cooled condensers, air functions as the cooling agent that removes heat from the refrigerant.
Water is used for cooling the refrigerant in a water-cooled compressor, while evaporative condensers can utilize both air and water. Evaporative condensers increase the temperature of the passing air slightly to facilitate refrigerant condensation through the evaporation of water sprayed onto the condenser. Air assists in this process by promoting evaporation and removing resulting vapor. Air-cooled chillers often employ the first type of condenser mentioned.
These fans are what distinguish air-cooled industrial chillers from water-cooled industrial chillers. Their primary function is to circulate air across the condenser, thereby removing unwanted heat from the system. This process allows the cycle to restart effectively.
The expansion valve controls the flow of refrigerant from the condenser and adjusts it based on changes in cooling load. It receives filtered liquid refrigerant and reduces its pressure before spraying it into the evaporator. The restriction created by the expansion valve causes the refrigerant to back up and expand into a mixture of low-pressure liquid and vapor. As pressure increases, the liquid refrigerant transitions into a hot gas and then a cold gas.
There are seven types of expansion valves: thermal, low-pressure, high-pressure, electronic, and automatic valves among others. Each type is designed to regulate the flow of refrigerant to the evaporator and maintain the pressure difference between the condenser and evaporator. These valves depressurize the liquid refrigerant to facilitate its transformation from liquid to vapor inside the evaporator.
In the evaporator, heat is extracted from the refrigerant before it enters the condenser. The evaporator is a heat exchanger that transfers heat from refrigerant to water or coolant . The various forms of evaporators are coil, shell and tube, and plate. The efficiency of the heat transfer is an essential function of an air cooled industrial chiller, with high efficiency being measured by an evaporators ability to transfer more energy using less heat exchanger area.
The function of this component is to lower the temperature of the refrigerant by converting it into a gas. This cooling process allows the refrigerant to absorb heat from the water flowing from the fan coil to the evaporator, or from the air handling units to the evaporator.
Evaporators can be categorized into three types depending on their structural design:
This enhances the contact between the refrigerant-carrying tubes and the plates.
This component's purpose is to safeguard the chiller by eliminating unwanted contaminants and moisture from the system. It requires periodic replacement due to clogging from extended usage.
The operational concept of air-cooled industrial chillers revolves around extracting heat from processed water. Once the air handler system utilizes the water, it warms up and returns it to the chiller. The chiller's evaporator facilitates heat transfer away from the water. Liquid refrigerant evaporates over the evaporator tube, forming low-pressure gas. This evaporated refrigerant then moves to the compressor, where low-pressure gas is compressed into high-pressure gas.
The gas then exits and travels to the air-cooled condensers. Inside the condenser, heat is extracted from the chiller and expelled outside through fans located on the condenser's sides. Placing the chiller outdoors or in a large indoor area is optimal because it mitigates the excess heat effect by dispersing heat outside the chiller.
After leaving the condenser, the refrigerant transitions into a high-pressure liquid. This high-pressure liquid then flows to the evaporator through the expansion valve. During this process, the refrigerant changes from a high-pressure liquid to a low-pressure liquid. Since the operational principle of an industrial air-cooled chiller is based on a continuous cycle, this low-pressure liquid returns to the evaporator to repeat the entire cycle.
Air-cooled and water-cooled industrial chillers share several common factors. Both belong to the vapor compression chilling family and utilize electronically driven mechanical compressors to circulate refrigerant through the system. The key distinction between air-cooled and water-cooled chillers lies in their methods of heat removal from the system.
In water-cooled industrial chillers, water is pumped through a sealed condenser and then circulated through a cooling tower. In contrast, air-cooled industrial chillers use fans to direct air over the condenser. The cooling tower and pump of a water-cooled industrial chiller occupy more space compared to the fans of air-cooled chillers. This significant component difference results in air-cooled industrial chillers having a more compact footprint. Water-cooled industrial chillers tend to be more expensive due to the cost of maintaining their cooling towers. Conversely, air-cooled industrial chillers are less expensive both in terms of maintenance and upfront costs.
When selecting an air-cooled chiller, several considerations should be taken into account:
When choosing an air-cooled industrial chiller, it's important to consider environmental conditions and the device's capacity. Air-cooled chillers are preferred in cold environments with significant day-night temperature differences. However, ensure the chiller's capacity meets the minimum requirements for optimal performance. For appliances with lower capacities, air-cooled industrial chillers are ideal.
Height is a critical factor to consider when opting for an air-cooled chiller. As altitude increases, the cooling capacity of air-cooled chillers decreases.
When selecting an air-cooled chiller, the control system is crucial. Well-controlled devices tend to offer more efficient performance.
Industrial chillers must withstand high pressures effectively, regardless of whether they cool water or air. Adequate airflow through the condenser is crucial, and lower-pressure refrigerants are preferable.
In air-cooled systems, capacity reduces by 1% for every 1°C increase in ambient temperature. This factor significantly influences the choice of air-cooled industrial chillers and impacts compressor power consumption.
In air-cooled industrial chillers, the evaporator's pressure drop should not exceed 60 kPa significantly. Opt for chillers with minimal pressure drops.
Chiller size is critical for fitting within available building space. Air-cooled chillers typically have smaller footprints.
Noise levels during operation are important considerations. Choose air-cooled chillers with sound pressure levels below 75 dBA at a distance of 16.4ft (5m) for noise-sensitive environments.
The number of compressors in an air-cooled chiller is specified in the device's technical details.
Air-cooled industrial chillers are categorized based on the types of compressors they use, which include screw, reciprocating, scroll, rotary, and centrifugal.
The term "portable air cooled industrial chiller" describes the system's standard configuration rather than its mobility. Once installed, most portable industrial chiller units cannot be relocated. This designation refers to any chiller system that includes essential components such as the refrigeration circuit, pump(s), and reservoir.
These are all contained within a single footprint. The controls of an air cooled industrial chiller unit include motor starters with a single-point electrical connection. Industrial chillers provide notifications for maintenance milestones, fault codes, technical alarms, and alerts to nearly eliminate downtime. Portable chillers can be found ranging from 2 to 100 tons in capacity.
The conventional flow chiller system comes equipped with an integrated tank. It operates within a closed-loop cooling setup where all water pumped from the chiller circulates back to the main unit under pressure from the pump.
The reverse flow air-cooled industrial chiller does not include a tank but utilizes an open-loop cooling system, relying on an external tank or trough. Water in the tank or trough flows to the chiller pump via gravity, passes through the chiller and filter, and returns to the tank or trough.
Portable industrial chiller units with continuous feed operate by cooling a fluid source in a single pass. They feature a pump, a tank, and a secondary heat exchanger, followed by a filter.
A "stationary air cooled industrial chiller" denotes a chiller without an integrated pump or tank. The primary component of a stationary air cooled industrial chiller is a standalone chiller unit connected via piping to a pump and a separate reservoir for proper operation. This system includes distinct controls for the chiller and the pump(s).
Each of these components requires an electrical connection. Proper wiring of the electrical controls is essential to ensure the chiller system does not begin cooling until the circulating pump starts operating. Stationary air cooled chillers are available in sizes ranging from 2 tons to 200 tons.
In reciprocating industrial chillers, gas is compressed by pistons, with the pressure applied by the pistons heating the gas. Adjustable intake and exhaust valves open to control piston movement. The piston disengages when there is a demand for chilled water to manage capacity. Reciprocating industrial chiller systems are adaptable and can meet specific load demands efficiently. They use hot gas bypass to match capacity with demand, although this process may not be maximally efficient. Some chiller systems employ both capacity control systems and hot gas bypass to unload pistons and match demand.
Each component requires an electrical connection. Properly wiring the electrical controls is crucial to ensure the chiller system does not initiate cooling until the circulating pump starts operating. Stationary air cooled industrial chillers are available in sizes ranging from 2 tons to 200 tons.
The screw compressor, also known as a helical compressor, features helically grooved rotors inside a stationary housing. Rotation of the helical rotors reduces the volume directly.
The capacity of a rotary screw compressor ranges from 20 tons up to 450 tons. Rotary screw compressors are controlled either by a sliding inlet valve or a variable speed drive.
The centrifugal compression industrial chiller is prized for its compact design and high cooling capacity. It utilizes an impeller, similar to a water pump, to compress the refrigerant. These chillers often incorporate variable speed drives and inlet vanes for enhanced operational efficiency.
Variable speed drives and inlet vanes control the chilled water capacity in centrifugal compression industrial chillers. These chillers are high-capacity units capable of handling 150 tons and more.
Frictionless centrifugal industrial chillers are designed similarly to regular centrifugal chillers, operating on the same principles but equipped with magnetic bearings that eliminate the need for lubricants.
Frictionless centrifugal industrial chillers also include variable-speed DC motors that are direct-drive and integrated into the chiller. They have a capacity ranging from 60 tons to 300 tons.
Unlike other industrial chillers that use a mechanical compressor, absorption industrial chillers operate using a heat source to drive the refrigeration cycle. They employ two liquids: one for cooling and one for absorbing heat. Typically, the absorbent liquid is lithium bromide or ammonia, while water serves as the coolant. These liquids are separated and then combined again during the absorption cycle. Water easily changes phase due to the low-pressure conditions in the chiller. The affinity between water and the absorption liquid contributes to the efficiency of absorption industrial chillers.
The refrigeration cycle starts by heating the combined liquids, causing the absorption liquid to boil and release water vapor at high pressure. Next, the refrigerant water vapor moves to a condenser coil, where heat is expelled and the vapor condenses into a high-pressure liquid. This high-pressure liquid then flows to the lower-pressure evaporator, where adiabatic flash evaporation converts it back into gas, absorbing heat from the chilled water in the process. Finally, the concentrated absorption liquid returns to recombine with the lower-pressure water vapors from the evaporator.
This chapter will explore the applications and advantages of air-cooled chillers.
For industrial operations, air cooled chillers are used to create heat through the use of friction, high-powered equipment, and furnaces or ovens. They are able to increase the lifespan of heavy-duty equipment by circulating cooled liquid through equipment to maintain their efficiency and productivity. Due to the fact that the food industry has strict regulations regarding the storage temperatures of ingredients and products, air cooled industrial chillers are used as a traditional cooling system for food. In the pharmaceutical field, medicines require using chilled water in the manufacturing process and precise temperature control. Industrial chiller systems have the accuracy and precision for the chilling process and can be central process chillers or compact process chillers.
Common types of pharmaceutical chillers include reciprocating, centrifugal, and absorption units. In power generation, power plants generate significant heat during electrical power production. Chillers are employed to cool components and processes by absorbing the generated heat. Medical instruments, such as CT scanners, MRI scanners, and LINAC machines, require precise temperature control to manage the heat they produce. Air-cooled chillers are utilized to maintain consistently cool temperatures, ensuring the efficient operation of critical equipment.
Air cooled chillers can be quickly integrated into existing systems, saving both time and money during installation. Their simpler setup compared to water cooled systems means installation teams require less time, resulting in significant upfront cost savings. Another advantage of air cooled chillers is their reduced number of components to maintain compared to water cooled systems. They do not require a condenser pump, cooling tower, or other complex parts, thereby lowering maintenance needs. Air cooled chillers are particularly beneficial in areas prone to drought or with limited water supply, offering a water-saving alternative to traditional systems. By using ambient air instead of water for cooling, air cooled chillers eliminate the need for water, optimizing performance. Additionally, air cooling systems do not alter ambient humidity or temperature as significantly as other chiller systems.
Air cooled chillers may have a shortened lifespan due to their outdoor location, exposing them to environmental elements such as ice, rain, snow, wind, and hail. Additionally, air cooled chillers tend to produce higher levels of noise because of the cooling fans.
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