Mechanical Drives | My Business | Mississippi Power | A Southern Company

Mechanical Drives

Chiller compressors can be driven by electric motors, reciprocating engines, gas turbines, or steam turbines. The selection of alternative drive technologies rests primarily on the issues of first cost and operating cost, as well as any fuel diversity and power reliability criteria. While there are other issues involved in the selection process, including CFC phaseout and other refrigerant-related issues, the selection between the alternatives just mentioned will probably not be driven by CFCs. In other words, a refrigerant that might be applicable for a chiller driven by a reciprocating engine would also work for an electric motor drive. A discussion of these criteria can be found elsewhere in this digital reference library.

While mechanical drives other than electric motors are also discussed, the primary alternatives presented will be reciprocating engines in the 100-500 ton range and steam turbines which are typically much larger.

Gas turbine-driven chillers are seldom seriously considered for three reasons:

  • The limited number of gas turbine sizes available
  • Their economic reliance on heat recovery and
  • Their relatively poor on-peak performance during hot weather.
  • Technology types (resource)

    Electric motors

    The electric motor is far and away the most common chiller compressor drive. Most of these are fixed speed motors (typically 1,800 or 3,600 rpm). Since compressor power requirements are proportional to the difference between evaporator and condenser pressures and refrigerant flow requirements, motor loads vary accordingly. Load variations are handled by cylinder unloading or multiple compressor staging for reciprocating units, slide vane capacity control in screw compressors, and inlet guide vanes (and infrequently hot gas bypass) for centrifugal compressors.

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    In cases where the ability to change compressor speed may offer a better way to modulate compressor capacity and/or performance, a variable speed electric motor should be considered. This approach is seldom utilized in new chiller installations since chiller manufacturers can now build in excellent modulation control. Variable speed motors have been more often used in retrofit applications. One word of caution: always consult the chiller manufacturer for warranty and performance verification before accepting the claims of anyone wishing to modify an existing chiller in this way.

    Steam turbines (back pressure & condensing)

    Steam turbines, reciprocating engines, and sometimes gas turbines are used to drive chiller compressors. The most common applications are very large (over 1500 tons) steam turbine-driven centrifugal chillers used in cogeneration applications for large hospitals or industrial cooling. In situations where electrical demand charges are high (say over $25 per kW per month) or where a demand ratchet could make an electric-driven chiller too expensive to operate for a few months a year, steam turbine-driven chillers are often specified.

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    Why not reciprocating engines or gas turbines? Steam turbines use the existing boiler system so they don't have to worry about fuel supply or air emissions. Since the steam needs of the site are usually dictated by the colder months, the existing steam generating capacity is often more than adequate to support cooling. Plus, the operating and maintenance characteristics of steam turbine-driven chillers are much better than reciprocating engines or gas turbine-driven equipment. Finally, where existing boiler capacity is adequate, steam turbine-driven chillers cost less than reciprocating engines or gas turbines. There are two basic steam turbine designs: back pressure and condensing. These indicate whether steam leaving the turbine goes on into the steam distribution system to satisfy process or heating requirements (this is "back pressure"), or whether the steam leaving the turbine goes straight to a dedicated steam condenser where it is rejected via a cooling tower or river water. Logically, condensing steam turbines are more expensive and less efficient than the back pressure designs.

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    When site steam requirements are reasonably steady and in excess of the steam flows necessary to drive the chiller, the back pressure design makes the most sense. Where this is not true, and power costs would be high for an electric-driven machine, a condensing steam turbine may be the most cost effective alternative. In many cases where steam turbines are considered, rather than apply them to a chiller operating relatively lower hours a year, the turbine is typically used to drive a generator to take maximum advantage of its power generating capabilities.

    Selecting a chiller design like this requires careful consideration of site-specific conditions. Steam turbine driven chillers represent a complex design in any situation. It is wise to consult with qualified design professionals and reputable equipment manufacturers before making a final decision.

    Reciprocating engines

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    Reciprocating engines are usually selected to drive chillers in the smaller range — 100 to 500 tons. The compressor (usually a screw or centrifugal model) is usually directly coupled to the engine drive shaft. Engines are often considered where the site can use the energy in the hot water and/or hot air exhausts produced. Roughly one-third (or less) of the total fuel input is converted to compression power. Therefore, the economics of reciprocating engine drives usually depend on the cost-effectiveness of heat recovery. Engine jacket water (which can reach temperatures as high as 220°F) is easily recovered and also represents about one-third of the fuel input. The heat in the engine exhaust represents the remaining third of fuel input, but this heat is generally not fully recoverable.

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    Engine-driven chiller cost effectiveness can best be determined using a cautious, conservative assessment by a professional that considers these three factors:

    1. Heat recovery that reflects actual site-specific heating efficiencies and needs,
    2. Conservative annual heating requirements, and
    3. Realistic operating and maintenance costs (which are typically higher than any other mechanically driven chiller alternative).
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    Once realistic heat recovery estimates have been factored into the equation, the only other major issue is that of O&M expense. Here, the Gas Research Institute uses $0.01 per ton-hour more than an electric-driven chiller design. While your costs could be different, a figure of $0.01 to 0.12 per ton per operating hour represents a reasonable first cut estimate. Always rely on qualified design professionals and reputable equipment manufacturers for installed cost, operating cost, and performance estimates.

    Gas turbine designs

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    Gas turbines are seldom selected to drive chiller compressors because the efficiency of the cogeneration system using a gas turbine relies heavily on recovering the engine's waste heat. Most sites simply don't have a use for all the waste heat. In cases where the heat can be used, the gas turbine is typically used to drive a generator to take maximum advantage of its power generating capabilities. The main problems associated with using gas turbines as chiller drives include:

    1. Gas turbine power levels (and the resulting chilled water production) are significantly reduced (~ 25-35%) at high ambient temperature levels. This means that at the very time the site needs maximum power to drive a chiller compressor, the gas turbine is least capable of delivering it. One solution might be to use some of the chilled water production to cool gas turbine inlet air, but this also reduces net chilled water production.
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    2. Operating and maintenance procedures are relatively sophisticated. The engines must be protected against inlet dust, contaminants, frosting, or damage from foreign objects. When placed in the hands of qualified, experienced personnel, and run continuously, gas turbines have recorded extremely high annual availability and low maintenance costs. Unfortunately, chillers seldom run continuously.
    3. If the gas turbine is fueled with natural gas, gas pressures have to be higher than with any other mechanical driver — typically 300 - 400 psig for the gas turbine. These pressures aren't always available from suppliers, and therefore require a supplemental gas compressor. Since this gas compressor is relatively unreliable, a "spare" is usually added in the system design, making it an expensive design attribute. Coupled with the power used to compress the natural gas fuel input, this compressor becomes a significant element in the cost-effectiveness equation.
    4. Careful matching of the turbine and compressor, both available in limited size increments is essential. Starting and stopping torques are specially important. These requirements typically increase the chiller cost not economically supportable.

    This doesn't mean that the gas turbine is a necessarily bad choice for a mechanical drive application, it just highlights the primary concerns the designer and owner should consider in evaluating the alternatives. Therefore, it would be prudent to rely on qualified design professionals and reputable equipment manufacturers for gas turbine installed cost, operating characteristics, and site-specific performance estimates.