Centrifugal and Screw Chillers

In a typical commercial building, chillers consume more electricity than any other single energy-consuming device, except for an occasional extremely large fan. Thus, inefficient chillers can waste significant amounts of electricity, and even modest improvements in efficiency may yield substantial energy savings and attractive paybacks.

However, it’s important to select a chiller (and its associated efficiency) carefully. Choosing a chiller that’s most efficient at full or part load, according to standard ratings, might be counterproductive because the ratings don’t measure the efficiency of the overall cooling system. To maximize cost-effectiveness, we recommend analyzing the entire chilled-water system as well as exercising care in specifying the efficiency of the chiller itself—it may be wiser to invest in a less-efficient chiller and instead spend more on efficient auxiliary equipment and improved operating strategies. (See the Chiller Terminology sidebar for more information.)

Chiller Terminology

Tons: One ton of cooling is the amount of heat absorbed by one ton of ice melting in one day, which is equivalent to 12,000 Btu per hour (h), or 3.516 kilowatts (thermal).

Chiller performance is certified by the Air-Conditioning, Heating, and Refrigeration Institute (AHRI), a manufacturer trade organization, according to its Standard 550/590-2003: Performance Rating of Water-Chilling Packages Using the Vapor Compression Cycle. Various efficiency metrics are commonly used for chillers.

Full-load efficiency: This metric is the efficiency of the chiller at peak load and at AHRI standard conditions, measured in kilowatts (kW) per ton. This metric is a specific case of the broader metric, power input per capacity—sometimes called the “kW/ton rating”—which can be measured at any given set of rating conditions. A lower kW/ton rating indicates higher efficiency.

Part-load efficiency: This is the efficiency of the chiller at part load, measured in kW/ton by either integrated-part load value (IPLV) or nonstandard part-load value (NPLV), depending on the particular AHRI part-load test conditions. Both give the efficiency of the chiller averaged over four operating points according to this formula:

Where

  • A = kW/ton at 100 percent load
  • B = kW/ton at 75 percent load
  • C = kW/ton at 50 percent load
  • D = kW/ton at 25 percent load

Coefficient of performance (COP): The ratio of the cooling capacity output power to the total power input at any given set of rating conditions, expressed as watts of output per watts of input.

Energy-efficiency ratio (EER): This metric is frequently used for smaller chillers; it is the ratio of the cooling capacity to the total power input at any given set of rating conditions, expressed as Btu per watt-hour. Use these conversion factors to relate COP, EER, and kW/ton:

  • COP = 0.293 EER; EER = 3.413 COP
  • kW/ton = 12/EER; EER = 12/(kW/ton)
  • kW/ton = 3.516/COP; COP = 3.516/(kW/ton)

AHRI standard conditions: Standard reference conditions at which chiller full-load performance is measured. For water-cooled chillers, this means a constant flow rate of 2.4 gallons per minute (gpm) for water leaving the evaporator at 44° Fahrenheit (F) and a constant flow rate of 3.0 gpm for water entering the condenser at 85°F.

What Are the Options?

Electric chillers cool water via a vapor-compression refrigeration cycle and circulate it to provide air conditioning in large commercial and industrial buildings. In a typical large commercial building (Figure 1), a central chiller plant consists of one or more chillers and their auxiliary systems. The chillers produce cold water, which is pumped to one or more air handlers throughout the building, where the water absorbs heat from warm indoor air. The cool air is then distributed around the building through a network of ducts. Chillers can be water-cooled or air-cooled: In the former, water absorbs heat from the condenser and disperses it through a cooling tower; in the latter, a flow of outside air across the condenser removes heat. Water-cooled chillers are typically more efficient than air-cooled chillers. The auxiliary, or ancillary, devices in a chiller plant include chilled-water pumps, condenser-water pumps, and cooling towers.

Figure 1: Components of a typical chilled-water system
The chilled-water system—the chiller, cooling tower, and pumps—provides cooled water as a heat sink throughout the building. The ventilation system—the outside air intake, air handler, ducts, and diffusers—provides fresh air. The terminal unit combines the two systems to exchange heat, and the boiler provides heat through separate hot water pipes.

Of the four types of compressors used by chillers—reciprocating, scroll, screw, or centrifugal—the latter two are the most efficient. We cover the options for those two types here.

Centrifugal Compressors

Centrifugal compressors spin the refrigerant vapor from the center of an impeller wheel outward so that centrifugal forces compress it (Figure 2). Some machines use multiple impellers to compress the refrigerant in stages. They have the highest efficiency of any compressor type, particularly at full load. They are amenable to operation with a variable-speed drive (VSD), thus enabling even higher efficiencies. Because they have few moving parts, they are reliable and have low maintenance requirements. Surge (the disruption of refrigerant vapor flow at low partial loads) can be a problem for centrifugal compressors, causing noise, vibration, and sometimes serious damage to the compressor. A technique called hot-gas bypass prevents surge by artificially loading the chiller, but this incurs a large efficiency penalty. Centrifugal compressors dominate the market for capacities of 300 tons or larger, with factory-assembled units available up to several thousand tons. Larger field-assembled units are also available.

Figure 2: Centrifugal chiller cutaway
The wheel-shaped compressor uses centrifugal force to compress the refrigerant vapor.

Although centrifugal compressors have traditionally had a much smaller presence in the under-300-ton market, that’s changing thanks to the Turbocor centrifugal compressor, which was designed for this smaller size range. Available from the HVAC compressor manufacturer Danfoss Turbocor Compressors Inc., the Turbocor’s biggest benefit is that it has enabled water-cooled chillers that use it to achieve one of the best part-load and full-load efficiencies in the under-300-ton chiller market. These results are due to its unique compressor design: It uses magnetic fields to levitate the compressor shaft in midair, eliminating the need for traditional oil-lubricated bearings (Figure 3). This design makes variable-speed operation possible—not a readily available option in this size range in the past. It also eliminates mechanical friction. Other benefits include dramatically lower maintenance costs (because no lubricating-oil system is needed), small size, modularity, low noise levels, and external digital communications.

Figure 3: Floating on a magnetic field
The Turbocor’s shaft floats on front and rear radial magnetic bearings, eliminating all mechanical friction. The axial magnetic bearing maintains shaft alignment.
Screw Compressors

Screw compressors are used primarily in chillers with capacities under 300 tons, though they are available up to about 1,000 tons. They are positive displacement devices, which means the refrigerant is compressed by reducing the volume of the refrigerant chamber. A screw compressor does this by squeezing refrigerant between two rotating helical rotors that mesh. Screw chillers are rugged and fairly quiet, owing to the small number of moving parts and the rotary motion. They are also up to 40 percent smaller and lighter than centrifugal machines, so they’re popular as replacements. A single-screw compressor consists of a cylindrical main rotor positioned between identical gaterotors (Figure 4). Twin-screw compressors that use two mating twin-grooved rotors are also available.

Figure 4: Compression process in a single-screw compressor
During rotation of the main rotor, a groove open to the suction chamber gradually fills with suction gas. As the main rotor turns, the groove engages a tooth on the gaterotor and is covered simultaneously by the cylindrical main rotor casing. The gas is trapped in the space formed by the sides of the groove, casing, and gaterotor tooth. As rotation continues, the groove volume decreases and compression occurs. At the geometrically fixed point where the leading edge of the groove and the edge of the discharge port coincide, compression ceases, and the gas discharges into the delivery line until the groove volume has been reduced to zero.
Chiller Efficiencies

Technological advancements have significantly boosted the best available part-load chiller efficiency, which is good news for chiller operators because chillers spend most of their operating time at only 40 to 70 percent load. From 1999 to 2009, the best available IPLV increased by about 27 percent, largely due to VSD technology. The best available full-load efficiency only increased by about 4 percent over the same period.

Though higher IPLVs are now available, it’s important to remember that, for several reasons, focusing on just the efficiency of the chiller will not necessarily lead to the most cost-effective savings. One reason is that IPLV data is provided at too few operating points to give an accurate indication of performance. Also, neither IPLV nor full-load efficiencies account for pumps and fans in the cooling system. And finally, both IPLV and full-load efficiency metrics apply only to single chillers (80 percent of all chillers are part of multiple-chiller installations).

To provide a sense of current chiller efficiencies, Table 1 shows the latest minimum efficiency specifications from the ASHRAE (the American Society of Heating, Refrigerating, and Air-Conditioning Engineers) 90.1 standard, “Energy Standard for Buildings Except Low-Rise Residential Buildings,” which is used as the basis for many local building codes. There are no federal minimum efficiency standards for chillers.

Table 1: VSDs prompt ASHRAE to tighten chiller specifications
Citing significant advancements in integrated part-load value (IPLV) due to the use of variable-speed drives (VSDs), ASHRAE has tightened efficiency requirements in its Standard 90.1 by creating addendum M. Paths A and B, shown here, are for applications where significant time is expected at full load and part load, respectively. Addendum M also added a 600-ton-or-greater category for centrifugal chillers and split the under-150-ton category into two categories for positive displacement chillers. The best available efficiencies from spring 2009 are also listed for comparison.
How to Make the Best Choice

Several factors influence the type of chiller compressor that you will select for an application. A major factor will be load size: For over 300 tons, centrifugal compressors will offer the most options; below 300 tons, screw compressors and the Turbocor will do so. Also, centrifugal compressors have better full-load efficiency than screw compressors, but screw compressors can achieve comparable part-load efficiencies. In addition, screw compressors are stable down to about 10 percent capacity, whereas centrifugal compressors can begin to surge at higher capacities (typically around 20 to 40 percent). Screw compressors also work well in high-lift applications (where there is a large difference between the evaporator and condenser temperatures, such as with ice-making for thermal storage) and provide an excellent match to evaporative condensers. Lastly, some screw compressors require large amounts of lubricant oil and an oil/gas separator that reduces efficiency.

Buying a more-efficient chiller can be cost-effective, though it may not be the most cost-effective option for the chiller plant. Annual energy costs for a chiller may amount to as much as a third of the purchase price, so even a modest improvement in efficiency can yield substantial energy savings and attractive paybacks. For example, paying an extra $6 per ton for each 0.01 kW/ton improvement to raise the full-load efficiency of a 500-ton chiller from 0.60 kW/ton to 0.56 kW/ton would increase that machine’s first cost by $12,000. But that change might reduce operating costs by as much as $3,000 per year—assuming 1,500 equivalent full-load hours and electricity at an average price of $0.10 per kilowatt-hour (kWh), including demand charges, simple payback would take just four years. However, only an analysis of the entire chiller plant, preferably using simulations, will tell whether a more-efficient chiller is a better option than upgrading the auxiliary equipment instead.

Some basic advice for navigating the major considerations when selecting a chiller includes:

  • Plan ahead. Start planning early to allow sufficient time to evaluate various chiller or cooling-system scenarios and to identify a comprehensive system approach that best meets your budget and facility needs.
  • Buy only as much chiller as you need. First reduce building loads and improve air-side distribution (see the Energy Star Building Upgrade Manual for more details), then size the chiller. Buying more cooling than you need not only increases equipment costs, it also increases monthly utility bills.
  • Know what equipment is available. Chillers are available in a range of technologies and efficiencies. Compare what manufacturers are offering to guidelines established by various organizations, such as ASHRAE.
  • Select a chiller that is most efficient over the entire year. Picking a chiller that’s most efficient at peak load—a condition rarely encountered—doesn’t provide the most efficient (or least-cost) chiller operation over the year. Instead, consider chillers that will operate most efficiently under the part-load conditions that they will likely experience.
  • Select different-size machines for multiple-chiller installations. Select one machine small enough to meet light loads efficiently and others to meet larger loads efficiently. Start additional chillers only when the chillers that are already running are near full capacity. This strategy ensures that chillers operate near their most efficient loadings and avoids excess pumping power. It also allows for more stages of chiller sequencing. For example, two equal-size chillers offer two stages (50 and 100 percent load), whereas two chillers sized for 33 percent and 67 percent offer three stages (33, 67, and 100 percent load).
  • Consider a chiller with a VSD to maximize part-load efficiency. Variable-speed centrifugal chillers are now offered by all of the major manufacturers, and Carrier Corp. offers a VSD screw chiller. All of those available offer energy performance that is superior to traditional constant-speed chillers under most conditions, but particularly part-load operation. Chillers equipped with VSDs typically have IPLV values between 0.35 and 0.45 kW/ton, which is considerably better than their constant-speed brethren. Some manufacturers are fairly new to the VSD chiller market, however, so it pays to research the track record of specific products before you make a purchase.
  • Use simulations to maximize system efficiency. The interplay of the chiller with cooling-tower fans, condenser- and chilled-water pumps, climate, and internal cooling loads will significantly affect overall chiller-plant efficiency—but these system interactions are difficult to analyze without computer simulations. Using simulations not only helps guide the selection of efficient components, it also helps you determine efficient operating strategies. For example, chiller efficiency increases with higher-temperature chilled water and lower-temperature condenser water (called lower “lift”). However, these operational changes will require pumping more chilled water and blowing more air, which increases pump and fan power. Simulations can help identify optimal temperatures for balancing these opposing trends and minimizing system energy consumption.
  • Use zero-tolerance bids. Chillers can be manufactured to much tighter tolerances than the AHRI allows for in the test conditions used for certification. Some manufacturers take advantage of this to make the performance of their chillers look better. For example, the AHRI tolerances allow a manufacturer to claim that a chiller has a capacity of 550 tons at 40 percent load when its actual capacity (with the tolerances removed) is only 500 tons. The Chilled Water Plant Design and Specification Guide (PDF), an in-depth resource for the technical design audience from Pacific Gas and Electric Co., points out that using a zero-tolerance bid would require the manufacturer to submit performance data with AHRI tolerances removed, which will give a much more accurate measure of performance.
  • Include a liquidated damages clause with bids. This clause ensures that the manufacturer will be held financially responsible should a purchased chiller not measure up to the bid specification. The Chilled Water Plant Design and Specification Guide includes a sample chiller bid specification with the appropriate language.
What’s on the Horizon?

Most chillers using chlorofluorocarbons (CFCs) have been converted or replaced since the U.S. moratorium on CFC production took effect in 1996, and the chiller industry’s focus has turned to the tradeoffs among next-generation refrigerants. These tradeoffs are embodied by the popular refrigerants R-134a (a hydrofluorocarbon, or HFC) and R-123 (a hydrochlorofluorocarbon, or HCFC).

R-134a has zero ozone-depleting potential (ODP), but its global-warming potential (GWP) is about 11 times greater than that of R-123. R-134a also has a greater indirect effect on global warming than R-123 because the chillers that use it aren’t as efficient at full load (there is some debate as to what the relative effect is at part load). The higher energy use of R-134a chillers at full load results in more carbon dioxide being emitted from the generation of extra electricity. Because R-123 does have a slight ODP, and given U.S. acceptance of the Montreal Protocol—an international treaty to phase out any refrigerant with an ODP—the U.S. Environmental Protection Agency is scheduled to halt the manufacture of R-123 chillers in 2020 and end production of the refrigerant in 2030.

However, some contend the phaseout of R-123 should be rethought because of this refrigerant’s very low GWP and ODP. In effect, this has set up a conflict between the ozone-focused Montreal Protocol and the global warming–focused Kyoto Protocol. It remains to be seen whether the political will can be mustered to change the Montreal Protocol. Regardless of the outcome, obtaining refrigerant for an R-123 chiller should not be a problem for many years. At worst, buyers would have until 2030 to stockpile the small amount of refrigerant that they will likely need to last the lifetime of an R-123 chiller purchased before 2020.

Meanwhile, the search goes on for refrigerants of the future. Mike Thompson, director of environmental affairs at Trane, says that the automotive industry is leading the search because of Europe’s 2011 phaseout of R-134a in automotive applications. Leading contenders include hydrofluoro olefins (such as HFO-1234yf) and carbon dioxide. According to Thompson, the HVAC industry is evaluating the emerging options to identify alternatives that best balance ODP and GWP while being efficient, cost-competitive, and safe.

The phaseout of R-22, an HCFC, on January 1, 2010, will likely result in increased overall chiller efficiencies for one segment of the market because it means that most packaged reciprocating chillers under 80 tons capacity will no longer be available. Small reciprocating chillers use R-22, and Richard Lord, fellow for Carrier Corp., says that most manufacturers are finding it easier to replace them with scroll chillers than to redesign them for use with other refrigerants. Because reciprocating chillers are the least-efficient type available, their disappearance will boost the average efficiency of what’s available on the market.

On the chiller-efficiency front, Lord also says that stricter ASHRAE 90.1 efficiency standards are coming in 2015, which will likely require significant redesign of chiller technology.

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