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What are chiller systems?
Every building needs an air conditioning system to produce cooling effects as well as dehumidify air. This is essential because the buildings generate heat as a result of internal and external factors. Some of the external factors include outside temperature, humidity and wind. Occupants of the building, lighting and equipment are some of the internal factors. Chillers are an essential component of a wide variety of facilities, including but not limited to, hotels, restaurants, hospitals, sporting arenas, industrial and manufacturing plants
Chiller systems are the single largest consumers of electrical usage in most facilities and consume over 50% of the total usage. They also use approximately 20% of the total electric power generated in North America, according to the US Department of Energy. US DOE also estimates that they expend up to 30% of the energy inefficiency. This inefficiency costs companies and building facilities billions of dollars annually.
A chiller uses a heat-transfer device that uses refrigeration to remove heat from an internal environment and transfer it externally. Chillers are the heart of any central HVAC system and they are also used to dehumidify air in buildings.
How does a chiller work?
A chiller works on the principle of vapor compression or vapor absorption. Chillers are designed to provide a continuous flow of coolant to the cold side of a process water system at a desired temperature of about 50°F (10°C). The coolant is then pumped through the process, extracting heat out of molds and machinery as it flows back to the return side of the process water system.
A chiller uses a vapor compression mechanical refrigeration system that connects to the process water system through a device called an evaporator. See Image below. Refrigerant circulates in evaporator, compressor, condenser and expansion valve of a chiller. A thermodynamic process occurs in each of above components of a chiller. The evaporator is a heat exchanger in which heat captured by the process coolant flow is transferred to the refrigerant. As the heat-transfer takes place, the refrigerant evaporates, changing from a low-pressure liquid into vapor, while the temperature of the process coolant is reduced.
The refrigerant then flows to a compressor, which performs multiple functions. First, it removes refrigerant from the evaporator and ensures that the pressure in the evaporator remains low enough to absorb heat at the correct rate. Second, it raises the pressure in outgoing refrigerant vapor to ensure that its temperature is high enough to release its heat when it reaches the condenser, where the refrigerant returns to a liquid state.
The latent heat given up as the refrigerant changes from vapor to liquid is carried away to the environment by a cooling medium (air or water).
Types of Chillers:
Chillers use a refrigeration system that cools a fluid using either a vapor compression or absorption to cool or dehumidify air in facilities.
The latent heat given up as the refrigerant changes from vapor to liquid is carried away to the environment by a cooling medium (air or water). Thus, there are two types of condensers: air-cooled and water-cooled.
- Air-cooled condensers resemble the “radiators” that cool automobile engines. They use a motorized blower to force air across a grid of refrigerant lines. Unless they are specially design for high-ambient conditions, air-cooled condensers require ambient temperatures of 95°F (35°C) or below to operate effectively.
- Water-cooled condensers perform the same function as air-cooled condensers, but require two steps, rather than one, to complete heat transfer. First, heat moves from refrigerant vapor into the condenser water. Then the warm condenser water is pumped to a cooling tower where the process heat is ultimately discharged to the atmosphere.
There are two types of chillers – Water Cooled and Air Cooled
Water Cooled Chillers: There are many types of chillers that are used to lower the temperature of water and to cool it but the mainly used ones are water-cooled chillers. It is mainly used in industries to cool down a large amount of water. Water-cooled chillers range in size from small 20-ton capacity models to several thousand-ton models that cool the world’s largest facilities such as airports, shopping malls and other facilities.
A typical water-cooled chiller uses recirculating condenser water from a cooling tower to condense the refrigerant. A water-cooled chiller contains a refrigerant that is dependent on the entering condenser water temperature (and flow rate), which is a function of the ambient wet-bulb temperature. Since the wet-bulb temperature is always lower than the dry-bulb temperature, the refrigerant condensing temperature (and pressure) in a water-cooled chiller is often significantly lower than in an air-cooled chiller. This is why water-cooled chillers are more efficient.
Water-cooled chillers are typically installed indoors and hence last longer than air-cooled chillers.
Although water-cooled chillers are more efficient and last longer than air-cooled chillers, they are often expensive to install due to the large number of condenser pumps required as well as need more maintenance than air-cooled chillers.
Air Cooled Chillers: A typical air-cooled chiller uses propeller fans or mechanical refrigeration cycles to draw ambient air over a finned coil to condense the refrigerant. It may contain multiple or single compressors. The refrigeration cycle of an air-cooled chiller includes two important processes:
- The evaporation of the liquid refrigerant in the evaporator, which absorbs heat and lowers the temperature of the chilled-water system
- The condensation of the refrigerant vapor in the air-cooled condenser and rejection of heat to the atmosphere
Air-cooled chillers are the most commonly used type of chillers due to their all-inclusive nature and reduction in costs due to the absence of cooling towers.
Air cooled chillers offer the advantages of lower installation costs and lesser maintenance apart from the fact that they occupy less space since they are mostly located outside a facility. They also have the ability to operate in below-freezing temperatures and hence are attractive to building owners in order to reduce overall maintenance costs.
They also eliminate the maintenance requirements associated with a water-cooled chiller due to the fact that they do not have condenser water treatments, condenser cleaning or cooling tower services.
Actions to increase efficiency of Chiller Systems:
Chiller costs are a substantial part of your building’s utility bills. A number of energy savings measures can be undertaken to obtain energy savings as well as the long-term goal of reducing building energy consumption.
Below is a list of actions that can be implemented to ensure that the existing chiller systems operate efficiently in your building facilities:
- Inspect and clean condenser coils: Heat transfer has a large effect on chiller systems. The larger the chillers, the longer are the condenser ducts. Hence, high heat transfer is a fundamental point to keep in mind when considering efficiency of chiller systems. It is important to inspect the coils for clogging and free air passage. Replacing, cleaning and vacuuming the coils regularly will help with reducing the energy consumption.
- Maintain refrigerant charge: A chiller’s cooling quotient is dependent on the how much refrigerant moves through the compressor. Hence it is important to properly maintain the refrigerant charge. Maintaining this can reduce cooling costs by nearly 5-10%.
It is important to determine if the refrigerant charge is at an acceptable level or above or below the level. Use the manufacturer’s manuals to adjust and maintain adequate refrigerant charge.
- Maintain condenser water: Condenser water loops are often used in the cooling towers of the chillers. Any debris like sand, erosive solids and contamination materials can affect the condenser water. Further more, condenser water may reach warmer temperatures. Resetting the condenser water to the lowest possible temperature and inspecting the chilled water loops help with increasing chiller operating efficiency.
Additional measures like cleaning fan blades, adjusting belts and inspecting valves will certainly help in improving your cooling operations.
In conclusion, chillers form a large percentage of your facilities’ HVAC (Heating, Ventilation and Cooling) systems. Chillers are the equipment that cools and dehumidifies your buildings.
Making your chiller systems as efficient as possible is an important component of reducing building operating costs, increasing energy efficiency and reducing energy consumption.