Steam Recompression - Example

A manufacturing plant's operation resulted in a 50,000 lbs/hr excess of 40 psig steam. At the same time, this plant needed additional 300 psig steam. A steam compressor open-cycle heat pump was examined.

A two-stage rotary screw compressor or a three-stage centrifugal compressor unit was proposed. Each bidder offered a compressor, 5,000-hp electric motor, gear box, couplings, controls, and instrumentation on a factory mounted single skid. With 50,000 lbs/hr of 40 psig inlet steam, the heat pump would deliver 58,500 lbs/hr of 300 psig steam when supplied with 3,525 kW of electric power and desuperheating water.

With power at $0.05/kWh, the repressured steam cost is:

50.05 x 3,525 kW x 1,000 divided by 58,500 lbs/hr= $3.00 per 1,000 pounds.

At an installed cost of $1,250,000 and operating 8,000 hrs/yr, a payback of 2 years or better on energy savings would be achieved when the plant's boiler steam cost amounted to:

$1.25 million divided by 16,000 hours, and 58,500 lbs/hr steam Or, $1.34 per 1000 lbs

Therefore, steam recompression would meet the required economic guideline where steam was worth more than $4.34 per 1,000 pounds.