Air cooled chillers are among the most commonly used refrigeration systems for commercial and industrial facilities. They work by absorbing heat from process water and then transferring this heat, meaning that they should be used in areas where heat discharge is not a serious issue. Like water cooled chillers, they have a compressor, condenser, evaporator, and expansion valve that work to chill water and absorb heat. As opposed to water cooled chillers, they use air to fuel condenser cooling. Most if not all outdoor applications for chillers will use air cooled chillers as excess heat is released into the ambient air.
This type of chiller system is generally used in applications where the additional heat it discharges is not a factor. In fact, it's often practical to use the excess heat to warm a plant during the winter, thus providing additional cost savings.
The term "portable" is not referring to its mobility, but to the basic configuration of the system. Normally, once installed, the portable chiller unit is never moved. This term describes any chiller system that contains the refrigeration circuit, the reservoir and the pump or pumps, all necessary components.
Industrial water chillers are used in a variety of applications where chilled water or liquid are circulated through process equipment. Commonly used to cool products and machinery, water chillers are used in a multitude of different applications including injection molding, tool and die cutting, food and beverage, chemicals, lasers, machine tool, semi-conductors and mor
The function of an industrial chiller is to move heat from one location (usually process equipment or product) to another place (usually the air outside the manufacturing facility). It is very common to use water or a water/glycol solution to transfer the heat to and from the chiller which may require the process chiller to have a reservoir and pumping system. Regardless of your industry and process, making sure that you have sufficient cooling is critical to productivity and cost savings.
No industrial process, machine, or motor is 100% efficient, with heat being the most common byproduct of those inefficiencies. If this heat is not removed, it will accumulate over time causing reduced production times, equipment shutdowns, and even premature equipment failure. It is necessary to incorporate cooling into industrial process system design to avoid these issues.
Using a chiller to provide cooling has multiple benefits. A chiller provides consistent temperature and pressure to your industrial process. Eliminating temperature and pressure variables simplifies the process development and optimization, ensuring the highest quality product. Instead of a wasteful, single-pass-through system, a chiller recirculates the cooling water. The recirculation minimizes the cost of water consumption which can be expensive and environmentally unfriendly.
How It Works
In most process cooling applications, a pumping system circulates cool water or a water/glycol solution from the chiller to the process. This cool fluid removes heat from the process and the warm fluid returns to the chiller. The process water is the means by which heat transfers from the process to the chiller.
Process chillers contain a chemical compound, called a refrigerant. There are many types of refrigerant and applications depending on the temperatures required but they all work on the basic principle of compression and phase-change of the refrigerant from a liquid to a gas and back to a liquid. This process of heating and cooling the refrigerant and changing it from a gas to a liquid and back again is the refrigeration cycle.
The refrigeration cycle starts with a low-pressure liquid/gas mix entering the evaporator. In the evaporator, heat from the process water or water/glycol solution boils the refrigerant, which changes it from a low-pressure liquid to a low-pressure gas. The low-pressure gas enters the compressor where it is compressed to high-pressure gas. The high-pressure gas enters the condenser where ambient air or condenser water removes heat to cool it to a high-pressure liquid. The high-pressure liquid travels to the expansion valve, which controls how much liquid refrigerant enters the evaporator, thereby beginning the refrigeration cycle again.
There are two types of condensers used in chillers; air-cooled and water-cooled. An air-cooled condenser uses ambient air to cool and condense the hot refrigerant gas back down to a liquid. It can be located inside the chiller or can be remotely located outside, but ultimately it rejects the heat from the chiller to the air. In a water-cooled condenser, water from a cooling tower cools and condenses the refrigerant.
Refrigeration is a thermodynamic cycle. Chillers use refrigeration to extract heat from the process circulation fluid and then ultimately reject it to the atmosphere. This system uses a chemical compound called a refrigerant. There are many types of refrigerants and applications depending on the temperatures required, but they all work on the basic principle of compression and phase-change of the refrigerant from a liquid to a gas and back to a liquid. This process of heating and cooling the refrigerant and changing it from a gas to a liquid and back again is the refrigeration cycle. Changing the physical state of a compound (for example, from gas to liquid) is an extremely efficient means to absorb or expel energy.
The refrigeration cycle in a chiller consists of four key components: compressor, condenser, expansion valve, and evaporator as illustrated below.
Condenser is a heat exchanger that transfers heat from the refrigerant gas to an external cooling source (water or air are typical mediums). This transfer causes a phase change from gas to liquid condensation.
Expansion valves reduce high temperature, high pressure liquid refrigerant to a low temperature, low pressure liquid/vapor mixture. This small amount of phase change cools the mixture for a low temperature refrigerant supply to the evaporator. The expansion valve controls the amount of refrigerant supplied to the evaporator by maintaining superheat at the outlet of the evaporator.
Evaporators are heat exchangers that transfer heat from the process fluid into the refrigerant causing a phase change, evaporation.
Other common refrigeration circuit components are: liquid line solenoid valve, filter dryer, hot gas bypass valve, and sight glass.
With limited resources and a heightened awareness of the need to reduce energy and water consumption, the use of fluid coolers has become a popular alternative to conventional evaporative cooling towers.
Similarly, a fluid cooler uses ambient air to cool the process water. However, this is done through a cooling coil and without exposing water to the atmosphere. This method is effective but limited by the temperature of the ambient air. In most cases, the practical limit is a process water temperature leaving the dry fluid cooler about 10°F warmer than the entering air temperature.
Brazed plate heat exchangers are a variation of the plate and frame design. The plate pack is permanently brazed together. This eliminates the need for the frame, gaskets, and end plates. This design improves the efficiency over a standard plate and frame. These are also much more compact but are not easily cleaned. Brazed plate units are often used as evaporators in chillers.
Shell and Tube Heat Exchangers use an outer shell vessel with internal tubes to isolate the two fluids. Large passageways between the tubes inside the shell avoid clogging but this reduces the efficiency of heat transfer. Shell and tube heat exchangers are commonly used in applications where one of the fluids contains high levels of contamination. Variations of these heat exchangers are used as condensers in water-cooled chillers.
Coil heat exchangers are most often air-to-water or air-to-refrigerant units. These consist of tubes with fins stacked together to form flat pieces. A common example of a coil heat exchanger is a radiator in a car. Forced air through the coil's fins conduct heat from the fluid in the tubes, through the fins and into the air. These are often condensers in air-cooled chillers.
Chillers incorporate two types of heat exchangers - evaporator and condenser.
In an evaporator, refrigerant enters as a low pressure liquid/vapor mixture and exits as a low pressure gas. The change of state from liquid to gas occurs at a constant temperature and absorbs energy. A chiller's evaporator achieves superheated refrigerant vapor. Superheat is when all the liquid refrigerant has evaporated, and the gas temperature increases above its saturation temperature. The process fluid enters as a hot liquid and exits at a lower temperature after transferring energy to the refrigerant.
In a condenser, refrigerant enters as a high temperature vapor and exits as a high temperature liquid. Condensers exhaust the heat from the chiller to the surrounding air or cooling water. The condenser design covers the "Total heat of rejection." This means the condenser rejects the heat from the evaporator as well as the compressor. The refrigerant exiting the condenser is a subcooled liquid. Subcooling is when all the vapor refrigerant is condenser and cooled below its saturation temperature.
A compressor is designed to increase pressure (and temperature) of refrigerant and circulate it though the system. By increasing the pressure of the refrigerant, the saturation temperature increases. With the elevated saturation temperature, the condenser easily subcools the refrigerant.
Scroll compressors use positive displacement to compress the refrigerant from a low pressure gas to a high pressure gas. The compressor motor is cooled by refrigerant passing over it. Oil is a critical component of a scroll compressor for lubrication. Scroll compressors are hermetically sealed and often replaced rather than repaired.
Screw compressors also use positive displacement to compress the refrigerant. The compressor uses two meshing screw-rotors. These rotate in opposite directions to increase the refrigerant gas pressure. Screw compressors require oil for lubrication. Screw compressors can be disassembled for maintenance and repair.
Centrifugal Compressors are dynamic and raise the refrigerant pressure by rotating an impeller. This creates centrifugal force which compresses the gas. Certain centrifugal compressor designs, like those on the Thermal Care TC and TCF series, use magnetic bearings and do not require any oil for lubrication. Centrifugal compressors can be disassembled for maintenance and repair.
valves reduce the high pressure, high temperature refrigerant liquid to a low
temperature, low pressure liquid/vapor mix. The valves are installed in the
refrigeration circuit after the condenser and before the evaporator. Sensing
the temperature at the evaporator's outlet (superheat), the valve controls the
amount of refrigerant supplied to the evaporator by maintaining superheat.
There are thermal and electronic expansion valves. Thermal expansion valves use a bulb and capillary tube to mechanically control the position of the valve. The bulb is mounted at the outlet of the evaporator to sense the superheat. The capillary tube connects back to the valve adjusting the valve's position. An electronic expansion valve is controlled based upon the input of a sensor mounted in the refrigeration piping at the evaporator's outlet. A controller's algorithm adjusts the position of the valve.
Thermal expansion valves are robust, simple, and inexpensive. The electronic expansion valves are more expensive but offer greater level of control in the refrigeration circuit. This affords the system designers the ability to optimize the performance for greater performance and reliability.
Compressor is a vessel used to increase the pressure and temperature of refrigerant gas and circulate it through the process cooling system.
Condenser is a heat exchanger used to transfer heat from the refrigerant gas to an external cooling source (water or air are typical mediums). This transfer causes a phase change from gas to liquid condensation.
Expansion valve reduces the high pressure, high temperature refrigerant liquid to a low temperature, low pressure liquid/vapor mix.
Evaporator is a heat exchanger that transfesr heat from the process fluid into the refrigerant causing a phase change - evaporation.
Dryer is installed in the refrigeration circuit. It filters any
particles of dirt, metal or other debris. This protects the operation of
the components especially the expansion valves. The dryer absorbs any
residual moisture remaining in the system after evacuation and charging the
refrigerant. It is important to remove moisture to prevent possible
freezing. Moisture can also form acids when in contact with the oil in
Hot gas bypass valve is used for capacity control of a chiller. It provides an artificial load on the evaporator by introducing high pressure, high temperature gas to the evaporator. This prevents the compressor from short cycling the system when the demand for cooling is low. Additionally, the hot gas keeps the evaporator from freezing and prevents low refrigerant pressure alarms.
Liquid Solenoid Valve prevents liquid refrigerant migration to the compressor when the system is shutdown.
cooling is a method of removing heat from where it is not wanted (the process)
and moving it into the air outside a manufacturing facility
Refrigerant sight glasses provide a visual indication of refrigerant as it flows through the system. The sight glass includes a moisture indicator. Visual bubbles in the system can indicate a clogged filter-dryer or low refrigerant charge.
Reservoir is the tank where water or water/glycol mix is stored.
Sensors measure pressure in the system.
Difference of Water-Cooled & Air-Cooled Chillers
Water cooled and air cooled chillers work in a rather similar manner. They both have an evaporator, compressor, condenser and an expansion valve. The main difference is that one uses air to fuel condenser cooling and the other uses water.
-All chillers require basic maintenance in order to perform at optimum levels, but air cooled chillers are easier and less expensive to maintain than water-cooled units.
-Air cooled chillers do not require a cooling tower or a condenser water pump.
-Air cooled chillers consume around 10% more power than a water-cooled unit; wet surfaces are better at transferring heat than dry surfaces.
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