Mechanical Drives - Reciprocating Engines

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.

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).

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.