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Article-Wessel.jpg

©2015 This excerpt taken from the article of the same name which appeared in ASHRAE Journal, vol. 57, no. 9, September 2015

Dennis Wessel, P.E., Fellow/Life Member ASHRAE

About the Author
Dennis Wessel, P.E., has retired as a senior vice president and director of marketing with Karpinski Engineering, Cleveland. He is a past chair of both TC 9.1, Large Building Air-Conditioning Systems and TC 9.12, Tall Buildings

If instead of using pressure reducing valves to provide steam at the necessary pressure, what happens when you use a turbine to drive an electrical generator that not only reduces pressure but also produces power? A Cleveland developer is banking on this proposition with three historic buildings transformed into a mixed-use space that uses just such a system.

 

Pressure Reducing Valves and Lost Exergy

Steam is typically delivered at elevated pressures because the pipe sizes are reduced to save costs. This is similar to power transmission over long distances that is delivered at a high voltage to reduce transmission losses (while minimizing the size of the transmission medium). But the end user needs steam to be delivered at reduced pressures, normally between 5 psig (34 kPa) and 15 psig (103 kPa). Reduced pressure has a higher latent heat value than the higher distribution pressure, and the latent heat value provides most of the heating effect as the steam is condensed.

A steam pressure reducing valve (PRV) provides the interface between the higher pressure distribution and the lower pressure end use and reduces the pressure of steam by expanding the flow through a variable orifice. The effective diameter of the orifice is adjusted by the pilot, which senses the downstream pressure and changes the orifice to maintain downstream pressure at the setpoint.

This process of steam pressure reduction causes the lower pressure steam to contain superheat. As steam passes through the PRV, the temperature remains relatively constant while the pressure is reduced. Table 1 shows the thermodynamic condition of the entering and leaving steam compared to saturated steam at the reduced pressures of 23 psig (159 kPa) and 20 psig (138 kPa).

Table 1 shows that steam pressure reduction through a PRV from 140 psig (965 kPa) to 20 psig (138 kPa) is an isenthalpic process that maintains a constant enthalpy through the pressure reduction. The temperature also remains nearly the same as the higher pressure steam, with minimal temperature loss through the process, and 51°F (28°C) of superheat (315°F to 264°F [157°C to 129°C]) in the reduced pressure steam. As the reduced-pressure steam is further distributed, it will quickly lose temperature to the point where the steam is saturated and only the latent heat of the steam is effective at producing the desired heating effect, essentially wasting the superheat.

 

Switching to a Steam Turbine Driven Generator

The buildings that make up the 500,000 ft2 (46 452 m2) “The 9” receive steam from a distribution system owned by a private district energy corporation that serves the City of Cleveland. The underground piping system distributes high pressure steam between 125 psig (862 kPa) and 140 psig (965 kPa). To deliver steam to customers, the district heating provider’s distribution system contains pressure reducing valves in manholes throughout their system (including one immediately adjacent to “The 9,” Photo 1) that reduce the delivery pressure to medium pressure at 20 psig to 25 psig (138 kPa to 172 kPa).

Early in the design phase, the suggestion was made to the owner/developer to consider using a back-pressure turbine/generator unit instead of the steam PRV in the adjacent manhole to reduce the pressure of the high pressure steam (125 psig to 140 psig [138 kPa to 965 kPa]) to medium pressure steam (20 psig to 23 psig [138 kPa to 159 kPa]) while generating electrical power.

 

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Table-1-Wessel.jpg
Table 1

 

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