Selecting chillers, chilled water systems

Selecting the right chiller is generally dictated by capacity, and there are many philosophies on the best way to control, operate, and calculate system operational costs.

09/16/2013


Learning objectives

  1. Understand the variety of chiller options based on load requirements.
  2. Learn to calculate a simplified cost/ton estimate for estimating chiller initial investment costs.
  3. Know the appropriate calculations for determining chiller plant operational costs. 

Figure 1: The Northwestern Mutual Van Buren Office Building in Milwaukee was an existing building retrofit that uses two modular chillers to produce chilled water for cooling. In the foreground is a 50-ton heat recovery chiller module that produces chilleChilled 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.

Chiller compressors

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.   

Figure 2: Two 500-ton water-cooled centrifugal chillers serve an office building with space for future chillers. Courtesy: Ring & DuChateauThe 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.


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