Question

An industrial process is using river water to cool 10⁵ kg/h of saturated steam in a heat exchanger at 0.1 bar. The cooled steam exits the heat exchanger as a saturated liquid at the same pressure of 0.1 bar to be used in the process. The river water has a volumetric flow rate of 500 ft³/s and is the coolant in the heat exchanger., having properties close to pure water (i.e., density – 1000 kg/m³). It exists the heat exchanger with the same mass flow rate, and is returned to the river, but at a higher temperature. The river water on a hot summer day is 20 ⁰C at the heat exchanger inlet, and near the mixing point of the discharged water from the heat exchanger. Operation is at steady state and kinetic and potential energy can be neglected. Since a higher river temperature results in lower dissolved oxygen there may be a risk of killing fish or even “stressing” them. For example, rainbow trout stop growing at sustained river temperatures of 73°F (23° C), and die at about 80 ⁰F (27 ⁰C). Should the owner of this industrial process be concerned for the fishes and local fisherman’s livelihood? Please comment by calculating the temperature rise of the river at the heat exchanger’s exit that discharges into the river.

Hint: properties of the saturated and compressed water can be found in tables: Table A-3 and Table A-5.

Answer 1