Venting of and managing excess low-pressure flash steam is a common problem in many process facilities. If condensate recovery is critical, typically, design engineers either choose to vent the low-pressure steam into the atmosphere or install air-fin condensers. In some plants, process engineers have added thermocompressors to utilize the vented low-pressure steam.
Part one of this series, www.ChemicalProcessing.com/articles/2011/recover-low-level-heat-pt-1.html, highlights several innovative concepts for recovering such heat, including installing heat pipes to preheat the combustion air and blowdown heat recovery systems to recover vented flash steam. These are not the only options to improve critical process cooling areas. Plant engineers may want to consider using the flash steam in an absorption chiller to produce chilled water.
It may seem funny to say heat can create cooling, but that is perfectly true with the help of absorption chillers. The most interesting characteristic of an absorption chiller is its ability to recover very low-grade heat, such as hot water.
For example, at a Wyoming refinery, processing capacity was limited at the fluid catalytic cracking (FCC) unit due to the main air blower's performance — especially during the summer months. A condensing steam turbine supplied with cooling-tower water drove the air blower. In the same refinery, excess low-pressure steam was continuously vented to atmosphere from a 15-psig steam header.
We recommended installing an absorption chiller to recover rejected low level heat to produce the chilled water, which was then used to reduce the temperature of the cooling water supplied to the turbine condenser. This low level heat recovery application improved the process unit's performance during summer months.