Rotary Screw Chillers
What are the options?
How to make the best choice
If your commercial building has a chiller, you should know that chillers typically consume more electricity than any other single energy-consuming device, except for an occasional extremely large fan. Inefficient chillers can waste significant amounts of electricity, so 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—buying a chiller that is highly efficient may not be cost-effective in all cases. It is also important to remember that chillers are actually part of a chilled-water system, and the efficiency and control of pumps and cooling towers can have a significant impact on overall efficiency. Maximizing the efficiency of the chiller alone does not ensure that the system will operate efficiently. To maximize cost-effectiveness, we recommend that you analyze the entire chilled-water system in addition to exercising care in specifying the efficiency of the chiller itself.
What are the options?
Rotary screw chillers, available in sizes ranging from 100 to 1,100 tons, dominate the market for small to midsize chillers. They are most commonly used in applications of 300 tons or less. The screw compressor is a relative newcomer to comfort conditioning, although it has long been used for air compressors and low-temperature refrigeration.
Screw compressors are positive displacement devices. The refrigerant chamber is actively compressed to a smaller volume by the twisting motion of two interlocking, rotating screws. Refrigerant trapped in the space enclosed between the two rotating screws is compressed as it makes its way from the inlet to the outlet of the compressor. A slide valve is used to adjust the compression effect by varying the amount of compression that occurs before the refrigerant is discharged. A single-screw compressor consists of a cylindrical main rotor positioned between identical gaterotors, as shown in Figure 1. A twin-screw compressor consists of two mating twin-grooved rotors, as shown in Figure 2.
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Figure 1: Compression process in a single-screw compressor
Suction: During rotation of the main rotor, a typical groove in open communication with the suction chamber gradually fills with suction gas. The tooth of the gaterotor in mesh with the groove acts as an aspiring piston. Compression: 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 three sides of the groove, the casing and the gaterotor tooth. As rotation continues, the groove volume decreases and compression occurs. Discharge: 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. Source: Text from and illustrations adapted from 1992 ASHRAE Systems and Equipment Handbook; photo courtesy of McQuay |
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Figure 2: Compression process in a twin-screw compressor
Suction: As the rotors begin to unmesh, a void is created on both the male side (male thread) and the female side (female thread), and gas is drawn in through the intake port. As the rotors continue to turn, the interlobe space increases in size and gas flows continuously into the compressor. Just prior to the point at which the interlobe space leaves the intake port, the entire length of the interlobe space is completely filled with gas.
Compression: Further rotation starts the meshing of another male lobe with another female interlobe space on the suction end and progressively compresses the gas in the direction of the discharge port. Thus, the occupied volume of the trapped gas within the interlobe space is decreased and the gas pressure consequently is increased.
Discharge: At a point determined by the designed built-in volume ratio, the discharge port is uncovered and the compressed gas is discharged by further meshing of the lobe and interlobe space.
Source: Text from and illustrations adapted from 1992 ASHRAE Systems and Equipment Handbook; photo courtesy of McQuay |
Key points about rotary screw chillers:
- These compressors are very rugged and fairly quiet due to the small number of moving parts and the rotary motion. The typical acoustic signature of a screw compressor contains higher frequency noise, which is easier to attenuate.
- They are up to 40% smaller and lighter than centrifugal chillers and are becoming popular as replacement chillers.
- Rotary screw chillers use HCFC-22 or HFC-134a as refrigerant.
- The most efficient water-cooled rotary screw chiller has a full-load efficiency of 0.58 kW/ton and that of the most efficient air-cooled unit is 0.94 kW/ton. (see Chiller Terminology.)
- Screw compressors operate well at partial loads and are stable down to about 10% capacity. Table 1 presents the most recent version of the standard governing chiller efficiency from ASHRAE (the American Society of Heating Refrigerating and Air-Conditioning Engineers).
| Table 1: Screw chiller efficiency standards from ASHRAE 90.1-2001 | ||
|---|---|---|
This table presents the most recent standard governing the efficiency of screw chillers. Note that many building codes require compliance with earlier, less demanding, versions of the code. | ||
Chiller size (tons) | Full-load efficiency (kW/ton) | IPLV efficiency (kW/ton) |
| <150 | 0.79 | 0.68 |
| 150-300 | 0.72 | 0.63 |
| >300 | 0.64 | 0.57 |
| Note: kW = kilowatts; IPLV = integrated part-load value. Source: E Source; data from ASHRAE 90.1-2001 | ||
How to make the best choice
The bottom line is that you can install the highest-efficiency chiller available in your building, but if it is part of an inefficient system, you won't be capturing all the benefits of the chiller. You need to optimize the entire chiller system to reap the best savings. (See Figure 3).
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Figure 3: Components of a typical chilled water system Source: E Source |
Before selecting a chiller
Before you select a chiller, you'll be better off if you:
Reduce cooling loads. About half of the cooling load in an inefficient building comes from solar gain and lighting, so careful treatment of these two sources of heat gain can yield impressive savings. A lighting retrofit offers direct energy savings in addition to reducing the cooling load for the building. Figure 4 illustrates several ways to reduce solar heat gain.
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Figure 4: Building-shell cooling load reduction measures Source: E Source |
Optimize HVAC auxiliary systems. Cooling tower fans, condenser and chilled-water pumps, and air and water distribution systems should all be analyzed for potential efficiency improvements.
Optimize control of the system and its components. Evaluate the energy use of HVAC auxiliary systems when determining how to stage chillers. Through a combination of analysis and observation, you are likely to find that there is an optimum combination of chillers for any given cooling load. Also, do not forget to include the energy use of the air distribution systems in the building because operating strategies that reduce chiller energy use may be more than offset by increased fan energy use. For example, raising the chilled water supply temperature will likely make the chillers operate more efficiently, but will increase fan energy.
When deciding which chiller to buy
Compare chillers at a variety of efficiency levels to determine the best buy. 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 $7 per ton for each 0.01 kW/ton improvement to raise the efficiency of a 250-ton rotary screw chiller from 0.66 kW/ton to 0.62 kW/ton would increase that machine's first cost by $7,000. But that change might reduce operating costs by $1,500 per year—assuming 1,500 equivalent full-load hours and electricity at 10 cents per kilowatt-hour average price, including demand charges—yielding a 4.6-year simple payback. For a chiller that operates fewer hours and would therefore produce less savings, some buyers might choose to forgo both the efficiency improvement and the additional cost. Most manufacturers have software that will help you make such evaluations.
Compare chillers under the conditions they are most likely to experience. Even though chiller performance can vary dramatically, depending on loading and other conditions, designers frequently select chillers based on full-load, standard-condition efficiency. But chillers run for most hours at 40% to 70% load, under conditions that are often considerably different from standard test conditions. To select the chiller that will have the lowest operating costs, you need to evaluate the efficiency of various chillers under the actual operating conditions the equipment is likely to be subjected to.
Account for system effects when comparing chillers. Although it is tempting to improve the efficiency of chilled-water systems by minimizing the energy consumption of each individual component, that approach does not necessarily lead to the most efficient system. The pieces of a chilled water system interact in complex ways that make such general prescriptions difficult. For example, although the efficiency of a chiller can be improved by increasing chilled-water flow, that will require more pumping power, which may exceed the saved chiller power and result in a net loss of system efficiency.
Select unequally sized machines for multiple chiller installations. Chillers operate more efficiently when they are loaded close to their full rating (about 75% for most chillers). If one chiller in a two-machine installation is smaller than the other, under most operating conditions, one of the two chillers should be able to handle the job while running close to its full load. Having the option of switching between plants with different capacities will result in more efficient operation than if one or two same-sized chillers were operating at a lighter load. For systems with two chillers, it is common to size the chillers for one-third and two-thirds of the total peak cooling load.
What's on the horizon?
The classic vapour-compression cycle has provided cooling for large air-conditioning applications for nearly a century, but in the future, other higher efficiency cooling methods may come to the fore. One interesting system now under development takes advantage of the magnetocaloric effect—the property of some metals to heat up when they are magnetized and to cool down when they are demagnetized. The prototypes are using cylinders of the rare-earth metal gadolinium, along with a superconducting magnet. Magnetocaloric chillers have the potential to be highly efficient, but currently they are much more expensive than vapour-compression chillers. Research is underway to reduce the cost of chillers based on this novel technology.
Copyright © 2008 E Source Companies LLC
Last Modified: May 9, 2009
