Low-pressure steam exhaust from industrial operations such as evaporators or cookers is usually vented to the atmosphere or condensed in a cooling tower. Simultaneously, other plant operations may require intermediate-pressure steam at 20 to 50 pounds per square inch gauge (psig). Instead of letting down high-pressure steam across a throttling valve to meet these needs, low-pressure waste steam can be mechanically compressed or boosted to a higher pressure so that it can be reused.
Vapor recompression relies upon a mechanical compressor or steam jet ejector to increase the temperature of the latent heat in steam to render it usable for process duties. Recompression typically requires only 5% to 10% of the energy required to raise an equivalent amount of steam in a boiler.
Consider a petrochemical plant that vents 15-psig steam to the atmosphere. At the same time, a process imposes a continuous requirement on the boiler for 5,000 pounds per hour (lb/hr) of 40-pounds-per-square-inch-gauge (psig) steam. If 15-psig waste steam is recompressed to 40 psig by an electrically driven compressor, the compression ratio is:
- Compression Ration = [(40 + 14.7)/(15 + 14.7)] = 1.84
Interpolating from the table above, the compressor requires 63.5 Btu/lb of delivered steam. Assuming that electricity is priced at $0.06/kWh, the annual cost of driving the compressor is:
- Compression Operating Cost = [63.5 Btu/lb x 5,000 lb/hr x 8,760 hr/yr x $0.06/kWh]/ 3,413 Btu/kWh = $48,895
If an equivalent quantity of 40-psig steam (enthalpy for saturated steam is 1,176 Btu/lb) were to be supplied by an 80% efficient natural-gas-fired boiler, the steam production costs with fuel priced at $8.00 per million Btu ($8.00/MMBtu) and 70°F feedwater (enthalpy is 38 Btu/lb) are:
- Steam Production Costs = [5,000 lb/yr x (1,176 – 38) Btu/lb x 8,760 hr/yr x $8.00/MMBtu]/(0.80 x 106 Btu/MMBtu) = $498,444
- Annual Vapor Recompression Cost Savings: $498,444 - $48,895 = $449,549
Conduct a Pinch Analysis
Based on the actual application, there may be other options to vapor recompression. The industry best practice is to conduct a pinch analysis on the steam system to reveal cost-effective alternatives and optimize steam use by eliminating inefficiencies.
For more information on how Vapor Recompression and other ways to recover steam loss, contact the team at Campbell-Sevey.
This tip is provided by the U.S. Department of Energy - Energy Efficiency and Renewable Energy and originally published by the Industrial Energy Extension Service of Georgia Tech. Click to download the complete tip sheet along with suggested actions and resources.
Posted on Tue, April 11, 2017
by Campbell-Sevey filed under