Open Systems

- How to write and solve an energy balance for an open system.

Explanation: Energy balances on open systems are actually very similar to energy balances for closed systems. However, since there is the work from flow in all cases, we like to rewrite the equation so that this is taken into account. Derivation: Closed system balance: _{k} + ΔE_{p} = Q + W_{s (moving parts)} + W_{flow}_{flow} combining the first two equations, _{k} + ΔE_{p} = Q - W_{s} - W_{flow}ΔH + ΔE_{k} + ΔE_{p} = Q - W_{s}Open System Energy Balance Equation: _{k} + ΔE_{p} = Q - W_{s}ΔH = 0 if there are no temperature changes, phase changes, or chemical reactions. Also, ΔU = 0 for small pressure changes pf a few atm for most systems. ΔE ΔE Q = 0 if the system doesn't exchange heat with the surroundings, that is, if the system is adiabatic or insulated W
Two streams of water are mixed to form the feed to a boiler. Note: Often, even for the simplest energy balances on open systems, we will need to calculate ΔH for changes in temperature and pressure(unlike in the previous example, where the enthalpys were simply given). In this book, obtaining ΔH from changes in temperature and pressure are given in chapter 8. So, even though the equations have already been given here, we will continue these problems in that section. |

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