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.
Closed system balance:
combining the first two equations,
Open System Energy Balance Equation:
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.
ΔEk = 0 if the system doesn't change velocity
ΔEp = 0 if the system doesn't move vertically
Q = 0 if the system doesn't exchange heat with the surroundings, that is, if the system is adiabatic or insulated
Ws = 0 if there are no moving parts, no electrical currents, or radiation exchanged between the system and the surroundings.
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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