Mechanical Drives - 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.

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.

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.