An Evaluation of Industrial Heat Pumps for Effective Low-Temperature Heat Utilization

The implementation of industrial heat pumps utilizing waste water from various industrial processes for the production of process steam is presented as a viable economic alternative to a conventional fossil-fired boiler and as an effective fuel conservation measure. With advanced, high-performance industrial turbocom pressors capable of delivering 45,000 feet of head at 60,000 acfm, waste stream temperatures as low as 140 F can be used economically. Three heat pump systems are described and presented schematically, differing primarily in the heat exchange strategy from the effluent to the heat pump power fluid. Detailed studies were made of two open-type Rankine cycle systems using water vapor as both the heat pump and process fluid. The heat pump capital cost, maintenance and electric power costs are calculated through the year 2000, to determine the amortized cost of the process steam and are compared with conventional fossil-fuel-generated steam costs. Heat pump capacities range from 10,000 lb/hr to 250,000 lb/hr of process steam. System variables such as the waste stream temperature, process temperature, heat exchanger and approach ?T are evaluated parametrically. The salient features of the vapor turbocompressor, the heat pump's major component, are described. The high-performance turbocompressor is capable of head capacities of 45,000 feet at 80% isentropic efficiency, using a centrifugal impeller driven through a geared speed increaser. Tip speeds and rotational speeds up to 1500 ft/sec and 52,000 rpm, respectively, are characteristic of present-day, high-performance compressors ,for industrial process applications