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Chilled water systems are cooling systems that circulate chilled water throughout a building for cooling and dehumidifying a building’s air. They come in all shapes, sizes, and configurations. Chilled water systems are closed-loop systems, meaning that the system water is continually recirculated and not exposed to atmospheric pressure, similar to domestic water systems. While selecting the type of chiller to use is generally dictated by capacity, there are still many philosophies on the best way to control, operate, and calculate system operational costs.
The first step in chiller selection is understanding the options available. A building’s block load will determine the overall capacity, whereas part load will determine the number and quantity of chillers required, with multiple chillers providing the ability to stage chillers in response to load. A block load will take into account building diversity and load changes based on exposure, internal and external loads, and building schedules because all portions of the building will not be peaking simultaneously. The function of a space may also dictate sizing and plant reliability. Essential services, such as data centers or hospitals, require reliability and redundancy with the use of a backup chiller or chillers for N+1 or 2N redundancy based on an owner’s requirements. Furthermore, the hourly building profile run time may require equal or unequally sized chillers.
The first category of chillers is defined by the method used to compress the refrigerant. Positive displacement compressors operate with two mechanical devices, such as scroll- or screw-shaped rotors. These devices trap refrigerant vapor and compress it by gradually reducing the volume to increase the pressure.
Most small chilled water plants—up to approximately 200 tons in capacity—use scroll compressors for production of chilled water. Scroll chillers start as low as 20 tons and increase in size to approximately 200 tons. As the capacity increases, the chillers increase the quantity of scroll compressors, typically of equal sizes to provide the total chiller capacity required. The disadvantage is that chiller capacity control is provided as stepped control instead of modulating control. Although the multiple compressors may be a disadvantage for capacity control, generally they are piped with multiple refrigerant circuits which provide some system redundancy. For example, an 80-ton chiller may have four 20-ton compressors, with two compressors on each refrigerant circuit. Failure of one compressor will cause a loss of capacity but will still allow the chiller to remain in service and provide partial cooling output.
Once the capacity exceeds the size of multiple scroll compressors, typically four to six 30-ton scroll compressors, chillers use screw compressors. Screw compressors are available in sizes up to about 500 tons. Screw compressors have the ability to vary the cooling output capacity from 100% to 20% via the use of a slide vane to limit refrigerant delivery to the compressor and provide a smooth, modulating transition between capacities. It is important to note that screw chillers have only one compressor, so a loss in the compressor would cause a complete loss in chiller capacity. Screw chillers typically have very good full-load and part-load kW/ton efficiencies. Screw compressors also are generally louder than scroll compressors, with higher noise levels in the lower frequencies octave bands.
The third and final type of compressor, a centrifugal compressor, operates on a different compressor philosophy that relies on dynamic compression to compress and raise the refrigerant pressure. A rotating impeller is used to accelerate the refrigerant and allow the conversion of velocity energy into pressure energy. Centrifugal chillers start at approximately 200 tons and go up to thousands of tons depending on the number of compressors. Centrifugal compressors are typically used for compressing large volumes of refrigerant to relatively low pressures and can be configured specifically to the application by changing the number of stages, compressor speed and size, impeller diameter, refrigerant type, and condenser and evaporator shell sizes. Capacity control of centrifugal chillers is accomplished through inlet vanes at the inlet of the compressor that varies the refrigerant flow in stages in response to the building load. A variable frequency drive (VFD) also could be used for capacity control to vary the speed of the impeller rotation in conjunction with inlet vanes. Inlet vanes and VFDs accomplish different objectives: inlet vanes are used for buildings that may have a large load variation, while VFDs should be used for buildings that have large variations in lift, which equates to changes in condenser relief. VFDs are not always an appropriate option for chillers and their use greatly depends their ability to vary temperatures. Regardless, close attention should be paid to low load conditions near 20% capacity, in which the efficiency degrades rapidly and causes the chiller to operate in a condition known as surging.
Centrifugal chillers also operate at high speeds, which can result in more vibrations and noise transmission into the building structure, but are extremely reliable and robust devices. Centrifugal compressors have great efficiencies throughout their operating range and are relatively compact for the amount of tonnage that can be provided per sq ft of mechanical room space. A centrifugal chiller can vary capacity continuously in lieu of stepped control, which can provide capacity output based on the building load profile. This enables accurate temperature control while using only the energy required. Figure 2 shows two centrifugal chillers serving an office building.
The next category of chillers is defined based on the type of heat exchanger used in the chiller. Heat exchanger selection can be either air- or water-cooled and will have a large impact on the efficiency and cost of the chiller. Air-cooled chillers are limited in size to 500 tons of capacity, whereas water-cooled chillers range to almost 9,000 tons.
Air-cooled chillers operate on the concept of using air to reject the building’s heat, which approaches the outside ambient dry bulb temperature. Consequently, air-cooled chillers must raise the refrigerant temperature and pressure to a higher condition and will require more energy to do the same amount of cooling compared to a water-cooled chiller. Despite this, air-cooled chillers offer the advantage of a packaged system with a single source of responsibility. Design and installation time are also reduced due to less equipment being involved: there is no requirement for cooling towers and associated freezing issues, consumption of makeup water and chemical treatment, or condenser water pumps. Air-cooled chillers use scroll compressors up to 200 tons and use screw compressors above 200 tons of capacity.
The alternative is water-cooled chillers. They operate by using water to reject the building’s heat, which approaches the outside ambient wet bulb temperature—which is typically lower than the dry bulb temperature. Therefore, water-cooled chillers are more energy efficient as the condensing temperature is lower and they require less work by the compressor to raise the refrigerant temperature and pressure. Although the chiller energy may be less than that of a comparable air-cooled chiller, one must still evaluate all the costs of the chiller system, including the cooling tower and condenser water pumps. The smallest water-cooled chillers (up to 200 tons) begin with multiple scroll compressors, those from 200 to 500 tons use screw compressors, and those above 500 tons mainly use centrifugal compressors. Water-cooled chillers typically last much longer than air-cooled chillers due to the location of the chiller inside of the building and lower operating pressures using water as a condensing fluid.
Another item to be aware of is the method of cooling the compressor and motor, which can be an open drive or hermetically sealed design. With an open drive chiller, the heat is rejected directly to the mechanical room and must be cooled or ventilated by ambient air, possibly requiring high volumes of air to cool the space. Also, contaminants can potentially get to the motor and refrigerant seals might leak if the seals are not properly maintained. Hermetic or semi-hermetic chillers are refrigerant cooled, and the heat is rejected to the refrigerant, reducing the heat rejection load in the mechanical room. These machines require no seals as the motor is completely contained within an enclosure, so there is no potential for refrigerant to leak out of the system.
Modular chillers are the newest option in the chiller market and have been increasing in popularity. They are similar to building blocks in that you can add modules of varying capacities to reach the overall desired capacity. Typical modules are available in sizes of 30, 50, 70, and 85 tons consisting of scroll or screw compressors. The chillers are specifically designed to fit through doorways in new or retrofit applications and are extremely compact. Chiller modules up to 85 tons require only 10 to 12 sq ft of building footprint without the large clearance spaces required for evaporator or condenser tube pull. These modules can then be combined to approximately 1,000 tons of overall chiller capacity. When building loads increase or additions are built, an additional module of the necessary capacity can be installed, provided the distribution systems are set up for the additional load. Similarly, instead of purchasing a completely redundant chiller for N+1 capacity, an additional module can be added, providing redundancy for one module at fraction of the cost of purchasing an entire redundant packaged chiller.
Despite the advantages of modular chillers, one major disadvantage is the significant initial cost. Most modular chillers are about 150% to 200% the cost of a similar tonnage water-cooled chiller. Second, modular chiller efficiencies are typically not as attractive as standard water-cooled chiller efficiencies due to the compressors used in the chillers. Third, water treatment is extremely important as modular chillers use a plate and frame heat exchanger instead of a shell and tube heat exchanger, which must be cleaned to prevent fouling and plugging the heat exchangers. Duplex, external strainers should always be used to provide service of one strainer while maintaining operation through the other strainer. Similarly, the heat exchangers shall be fully serviceable with individual isolation valves to isolate the heat exchanger without shutting down the chiller. Figure 3 provides an example of a modular chiller with the heat exchanger in a fully serviceable position.
Chiller categories tend to have a lot of overlap and options within each capacity range and category described above. Figures 5 and 6 provide a simplified method to observe the ranges and compressor types for air-cooled and water-cooled chillers from the major chiller manufacturers.
Upfront costs are always a critical discussion in any project. In some projects, initial costs can outweigh lifecycle and operating costs. Since chiller costs vary so much, it is difficult to provide an overall initial cost for a chiller based on its size as multiple factors are associated with each chiller size based on manufacturer.
In general, scroll chillers are the most expensive of the three types of chillers discussed. Centrifugal chillers are the most cost-effective option but are not available in all tonnages. When screw and centrifugal compressors are both available options based on the capacity, screw compressors are typically 10% to 15% more expensive. Similarly, air-cooled chillers are typically a more expensive initial investment than a similarly sized water-cooled chiller when comparing strictly the initial cost of an air-cooled chiller versus that of a water-cooled chiller.
However, when evaluating the total installed cost of an air-cooled versus a water-cooled system, including all the necessary components for the system to be fully operational, the investment for the required components in a water-cooled system are always more expensive. The reason: An air-cooled system only requires chillers and chilled water pumps, whereas a water-cooled system requires the chillers, chilled water pumps, condenser water pumps, and cooling towers for fully functional system and to reap the energy efficiency benefits.
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