A heat engine is driven by $2$ kg$/$s hot air at $1200$ K that exits the heat exchanger

at $400$ K$.$ The heat engine cycle has heat rejection at $325$ K and a first law efficiency

of $0\mathrm{.}30.$ The heat rejection is done in a heat exchanger that uses ambient air at $20\backslash$deg C

to cool it$.$ Use the exergy equation to find the exergy destruction in a$)$ the low T heat

exchanger and b$)$ the heat engine plus the high T heat exchanger.