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 discuss what air cooled chillers are, their components, and their working principle.
Air cooled chillers are refrigeration systems that cool fluids and work in tandem with the air handler system of a facility. Air cooled chillers are types of chillers that rely on the use of fans to reject heat outside the building rather than relying on cooling towers.
Air cooled chillers consist of four major parts: a compressor, an evaporator, a condenser, and an expansion valve. These chillers are utilized in large buildings like indoor malls, industrial buildings, hotels, and hospitals. They are also used in locations that may consist of many buildings requiring cooling, like theme parks and outdoor malls. They are versatile workhorses capable of providing cooling power to large and small operations.
Air cooled chillers have a compact design that makes them portable, inexpensive to maintain, and easy to install. Portable air cooled chillers can be utilized in large events or emergency situations. Air cooled chillers generally have a simpler structure than water cooled chillers. Most air cooled chillers use either screw compressors or scroll compressors.
Air cooled chillers are also called modular chillers since they can be stacked horizontally to increase their cooling capacity. The cooling capacity available for an air cooled chiller can range from about 10 RT to 550RT. When scroll compressors are used, a single air cooled chiller can be designed to have a maximum cooling capacity of about 200 RT. On the other hand, with screw compressors, a single air cooled chiller can be designed with a maximum capacity of up to 550RT.
An air cooled chiller usually consists of at least two compressors. If one compressor malfunctions, the remaining one will continue to function, with the total cooling capacity of the chiller reduced to 50%. Air cooled chillers are not very efficient compared to water cooled chillers. The energy efficiency of air cooled chillers is only around 1.00 kW/ton; hence, the efficiency of water cooled chillers is twice that of air cooled chillers.
The components of an air cooled chiller include:
This part serves to provide the driving force that moves the refrigerant around the system. There are different types of compressors, which are screw, reciprocating, scroll, rotary, and centrifugal compressors. Centrifugal compressors are different from the other four types of compressors mentioned. Centrifugal compressors are dynamic, while the other four make use of positive displacement in their working principle.
In positive displacement compressors, there is a continuous reduction in a certain amount of gas in terms of volume. This is done by a screw, piston, spiral, or rotation, and this will, in turn, increase the vapor pressure. While in centrifugal compressors, the compression of the gas occurs as a result of being accelerated by an impeller’s rotation followed by the speed reduction in a diffuser. This will increase the 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.
The compressor and the motor are separate and distinct parts of the assembly. These types of compressors are available with mid-range capacity, which is more than a hermetic compressor. In an open compressor, the electric motor is separated from the enclosure of the casing but is connected to it by means of a coupling. These types of compressors are often made with high capacity.
The condenser is a heat exchanger. This condenser permits the transfer of heat from the refrigerant to the air. The condenser is cooled by air. Air cooled condensers consist of tubes made of copper, running liquid refrigerant, and aluminum fins that serve in the transfer of heat with the passing air. The condensers in air cooled chillers come in three types: air cooled, evaporative condensers, and water cooled condensers. In air cooled condensers, the function of the air is to serve as a cooling agent that distills the refrigerant.
On the other hand, water is used for the cooling of the refrigerant in a water cooled compressor. Evaporative condensers are capable of using both air and water. Although evaporative condensers increase the temperature of the passing air to some extent, the distillation of the refrigerant is done by the evaporation of water sprayed on the condenser. The air serves in the promotion of evaporation by removing the vapor that results. Air cooled chillers often make use of condensers of the first type mentioned.
These fans are what differentiate air cooled chillers and water cooled chillers. Their purpose is to move air across the condenser. As they move air across the condenser, unwanted heat is removed from the system, and the cycle starts all over again.
This component serves to expand the refrigerant from its liquid form into a gaseous form before entering the evaporator. This valve creates a pressure drop across the path of refrigerant that runs to the evaporator.
This is what causes the transformation of liquid refrigerants into hot gas. The hot gas is then transformed into cold gas. The expansion valve of the chiller can either be compression, thermostatic, or electronic.
In this component, chilled water is created when unwanted heat is drawn into the refrigerant before it’s fed into the condenser. The evaporator is a heat exchanger that aids in the transfer of heat from water to the gaseous refrigerant.
The purpose of this component is to cool down the refrigerant. It achieves this by transforming it into gas. The purpose of the cooldown of the refrigerant is for the collection of heat from the water that comes from the fan coil to the evaporator, or air handling units to the evaporator.
The evaporators are classified into three categories based on their structure:
This improves the contact between the tube running the refrigerant and the plates.
The purpose of this component is to protect the chiller by the removal of unwanted contaminants and moisture from the system. They need to be replaced regularly as they get clogged after repeated use.
The working principle of air cooled chillers is based on the absorption of heat from processed water. Once the air handler system uses up the water, it becomes warm and is returned to the chiller. The chiller’s evaporator is utilized to transfer heat away from the water. A low-pressure gas is created by the evaporation of liquid refrigerant over the evaporator tube. Next, the refrigerant that has been evaporated travels to the compressor. At the compressor, low-pressure gas is compressed into high-pressure gas.
The gas will then leave and travel to the air cooled condensers. While inside the condenser, heat is removed from the chiller by blowing out the heat to the outside through the fans located on the side of the condenser. It is ideal if the chiller is placed outdoors or in a large indoor location. This is so because the heat is blown outside the chiller, and the excess heat effect is reduced.
The refrigerant turns into a high-pressure liquid soon after the gas leaves the condenser. The high-pressure liquid then flows to the evaporator via the expansion valve. As this process takes place, the refrigerant changes from a high-pressure liquid to a low-pressure liquid. Since an industrial air cooled chiller’s working principle is also based on a continuous cycle, this low-pressure liquid will then be transferred back to the evaporator. At the evaporator, the whole cycle repeats itself.
These types of chillers have many things in common. They both come from the vapor compression chilling family, they both use an electronically driven mechanical compressor to force refrigerant around the system, and many of their components are the same. What distinguishes these two is how the unwanted heat is ejected from the system.
In water cooled chillers, water is pumped via a sealed condenser and dispersed through a cooling tower. On the other hand, air cooled chillers make use of fans that force air through the condenser. The tower of a water cooled chiller, as well as its pump, uses up more space than air cooled chillers’ fans. This main component difference makes air cooled chillers have a more compact footprint. Water cooled chillers are more expensive due to their cooling towers, which are expensive to maintain. On the other hand, air cooled chillers are cheaper since their maintenance cost is low. Generally, the upfront costs of air cooled chillers are low.
Some considerations when choosing an air cooled chiller include:
When opting for an air cooled chiller, the environmental characteristics and the device's capacity must be considered. When there is a cold environment and a large temperature difference between day and night, air cooled chillers are preferred. However, in those cases, the capacity of the device, whether minimum or not, must be considered. When the appliances are of low capacity, it is ideal to use air cooled chillers.
The height of the surroundings is an important factor that must also be considered. Always pay attention to the height factor when purchasing an air-cooled chiller because the air cooled chiller capacity decreases as the altitude increases.
When choosing an air cooled chiller, operators must consider how the chiller will be controlled. This is so because devices that can be easily controlled are more likely to promise more efficient work as well as performance.
Any types of chiller must be protected against high pressure as much as possible, whether the water is cold or the air is cool. For this purpose, the quantity of air that flows in the condenser must be high. At the same time, medium and low-pressure refrigerants should be preferred.
In air cooled devices, the device’s remaining capacity is reduced by 1% for every 1°C. This is an important factor that will largely affect the choice when opting for an air cooled chiller. This change also greatly affects the compressor and increases the amount of power that is used by the compressor by 1%.
In air cooled chillers, the pressure drop value of the evaporator must not exceed 60 kPa by a large amount. This must not occur in the air-cooled chillers that prefer the lowest pressure drop.
The size of the chiller is an important factor to consider. In the space of the building in which the chiller is going to be used, a small chiller must be chosen according to the available space. Usually, most air cooled chillers use little space.
The issue of the noise produced by the device during operation must also be considered. In places where noise is an important factor, categories of air cooled chillers with sound pressure less than 75dBA at a distance of 16.4ft (5 m) in an open space must be selected. There should be a preference for categories with low noise levels.
The number of compressors that an air cooled chiller contains is important. These are written in the specifications of the device.
The air cooled chillers can be grouped into different types of compressors, which are screw, reciprocating, scroll, rotary, and centrifugal.
The label “portable air cooled chiller” refers to the system's basic configuration, not its mobility. Most portable chiller units can never be moved once they are installed. The term portable air cooled chiller refers to any chiller system that contains all the ideal components such as the refrigeration circuit, the pump or pumps, and the reservoir.
These are all contained within a single footprint. The controls of a portable air-cooled chiller unit are made to include all motor starters. These controls require a single-point electrical connection. Chillers are made to enable them to give notifications for predictive 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 standard flow chiller system is fitted with an integrated tank. This chiller system is used in a closed-loop cooling application. In this application, all the water pumped from the chiller returns to the main unit under pressure generated by the pump.
The reverse flow portable chiller is not fitted with a tank. The reverse flow portable chiller is a component in an open-loop cooling application where an external tank or trough is put to use. The water in this tank or trough is supplied into the chiller pump by the effect of gravity. The movement is through the chiller and filter and back to the tank or trough.
The continuous feed portable chiller units work by cooling a fluid source on a single pass. The continuous feed portable chillers are typically equipped with a pump, a tank, and a secondary heat exchanger followed by a filter.
A “stationary air cooled chiller” refers to any chiller that does not have a pump or a tank. The main component in a stationary air cooled chiller is a stand-alone chiller unit connected by piping to a pump and a separate reservoir to function properly. This system actually has separate controls for the chiller and the pump or pumps.
Each of these components has an electrical connection. The electrical controls should be wired together properly to prevent the chiller system from cooling until the circulating pump starts operating. The stationary air cooled chillers can be found in sizes ranging from 2 tons to 200 tons.
The gas contained in the reciprocating chiller is compressed inside with the use of pistons, not unlike a car engine. The gas is compressed by the use of multiple pistons to heat it. The difference is that the hot gas inside the system is not exhausted from a tailpipe. The demand for this type of chiller is matched by the adjustable intake and exhaust valves that can be opened to allow the piston to simply idle.
Leaving the piston when there is a demand for chilled water helps manage capacity. This chiller system is very flexible and can cope with the specific demands of load on the system. It can also be possible to manage the capacity to match the demand with a hot gas bypass. This process is not considered to be as efficient as possible. Some chiller systems utilize both capacity control systems to unload pistons. The same chiller systems can also utilize the hot gas bypass to match demand.
The screw compressor can also be referred to as a helical compressor. Inside its stationary housing, it contains helically grooved rotors. When the helical rotors are in rotation, the direct volume is reduced.
The capacity of a rotary screw compressor can be found in the range of 20 to 450 tons. The rotary screw compressor is controlled by a sliding inlet valve or variable speed drive.
One of the main features of the centrifugal compression chiller is its ability to provide a high cooling capacity in a compact design. The centrifugal compression chiller operates via an impeller, which is much like a water pump in design. This impeller functions by compressing the refrigerant. The centrifugal compression chillers can be outfitted with both variable speed drives and inlet vanes.
The variable speed drives and inlet vanes are used to regulate the control of the chilled water capacity. These centrifugal compression chillers are high capacity and can handle 150 tons and up.
The design of frictionless centrifugal chillers is much like a regular centrifugal design. They function via the same principles but do so with magnetic bearings. Frictionless centrifugal chillers use magnetic bearings to eliminate the need for lubricants.
Frictionless centrifugal chillers also feature variable-speed DC motors. These featured motors are direct-drive and attached directly to the chillers. The frictionless centrifugal chillers have a capacity ranging from 60 to 300 tons.
Unlike other chillers that utilize a mechanical compressor, the absorption chillers use a heat source as the driving force behind the refrigeration cycle. These absorption chillers typically use two liquids, one to cool and one to absorb. The absorbent liquid is usually lithium bromide or ammonia, and the coolant used is usually water. The absorbent and coolant are separated and recombined during the absorption cycle. The water can change phase easily due to the low-pressure conditions in the chiller. Water and the absorption liquid also function efficiently in chillers because of their natural affinitive properties.
The refrigeration cycle begins with the heating of the combined liquids. This act then boils the water out of the absorption liquid at high pressure. The next stage is to send the refrigerant water vapor past a condenser coil. Here the heat is rejected, and the water vapor is phased into a high-pressure liquid. Following this stage, the high-pressure liquid is passed along to the lower-pressure evaporator, where adiabatic flash evaporation returns the water to a gas. This process leads to the heat being absorbed from the water that needs to be chilled. In the final stage, the concentrated absorption liquid is directed back to be recombined with the lower pressure water vapors coming from the evaporator.
This chapter will discuss the applications and benefits 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, these 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. Chiller systems have the accuracy and precision for the chilling process and can be central process chillers or compact process chillers.
The common types of pharmaceutical chillers are reciprocating, centrifugal, and absorption. In power generation, power plants generate a great amount of heat so as to produce electrical power. Due to this case, chillers are used to cool components and processes by absorbing the heat they generate. Medical instruments require precision temperature control. CT scanners, MRI scanners, and LINAC machines produce heat that has to be removed and controlled. Air cooled chillers are used to provide a constant source of cool temperatures. This is done in order to ensure the efficient operation of critical equipment.
Air cooled chillers can be brought quickly into installed systems,saving time and money. Installation teams require less time to install this type of chiller system, which can lead to significant upfront cost savings. Another benefit of air cooled chillers is that they have fewer assemblies to maintain as compared to water cooled systems. Without a condenser pump, a cooling tower, and other parts, these chillers require less maintenance than other systems with a more complex component structure. The air cooled chillers are ideal in an area prone to drought or that has a limited water supply as they provide a water-saving alternative to traditional water cooled systems. An air cooled chiller is able to eliminate the water requirement by cooling the process with ambient air instead of water, creating room for optimal performance. Air cooling systems do not modify the ambient humidity or temperature as compared to other chiller systems.
Air cooled chilled may encounter a shortened lifespan. This is because air cooled chillers reside outdoors, so they are exposed to environmental elements that could shorten their lifespan such as ice, rain, snow, wind, and hail. Finally, air cooled chillers have a high level of noise output as a result of the cooling fans.
Air cooled chillers are refrigeration systems that cool fluids and work in tandem with the air handler system of a facility. Air cooled chillers are types of chillers that rely on the use of fans to reject heat outside the building rather than relying on cooling towers. They consist of four major parts: a compressor, an evaporator, a condenser, and an expansion valve. Air cooled chillers can be found in various types, which include portable air cooled chillers and industrial stationary air cooled chillers. The working principle of air cooled chillers is based on the absorption of heat from processed water. Once the air handler system uses up the water, it becomes warm and is returned to the chiller. The chiller’s evaporator is utilized to transfer heat away from the water.
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