Thermodynamic Analysis of Steam Cooling Process in Marine Power Plant by Using Desuperheater

Abstract

Thermodynamic (energy and exergy) analysis of steam cooling process in the marine steam propulsion plant is presented in this research. Steam cooling is performed by using Desuperheater which inject water in the superheated steam to obtain wet steam. Wet steam is used in auxiliary heaters for various heating purposes inside the marine steam propulsion system. Auxiliary heaters require wet steam due to safety reasons and for easier steam condensation after heat transfer. Analysis of steam cooling process is performed for a variety of steam system loads. Mass flow rates of cooling water and superheated steam in a properly balanced cooling process should have the same trends at different system loads - deviations from this conclusion is expected only for a notable change in any fluid temperature. Reduction in steam temperature is dependable on the superheated steam temperature (at Desuperheater inlet) because the temperature of wet steam (at Desuperheater outlet) is intended to be almost constant at all steam system loads. Energy losses of steam cooling process for all observed system loads are low and in range between 10–30 kW, while exergy losses are lower in comparison to energy losses (between 5–15 kW) for all loads except three the highest ones. At the highest system loads exergy losses strongly increase and are higher than 20 kW (up to 40 kW). The energy efficiency of a steam cooling process is very high (around 99% or higher), while exergy efficiency is slightly lower than energy efficiency (around 98% or higher) for all loads except the highest ones. At the highest steam system loads, due to a notable increase in cooling water mass flow rate and high temperature reduction, steam cooling process exergy efficiency significantly decreases, but still remains acceptably high (between 95% and 97%). Observation of both energy and exergy losses and efficiencies leads to conclusion that exergy analysis consider notable increase in mass flow rate of cooling water which thermodynamic properties (especially specific exergies) strongly differs in comparison to steam. Such element cannot be seen in the energy analysis of the same system.