Selecting chillers, chilled water systems
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.
A sample project is also presented to illustrate the variation in initial chiller costs. This project involves the replacement of two 180-ton centrifugal, water-cooled chillers from 1980.To determine the best chiller for the project, a procurement specification was issued designating the chiller capacity, maximum evaporator and condenser pressure drops, efficiency criteria, maximum sound levels, and the maximum size constraints. Manufacturers were then encouraged to bid any type of chiller that met the requirements of the performance specification. The results included a mixture of efficiency levels, scroll chillers, screw chillers, and modular chillers. For this tonnage there is a 44% variation in costs.