Energy Balances on Open Systems at Steady State

4 ENERGY BALANCES ON OPEN SYSTEMS AT STEADY STATE 

An open process system by definition has mass crossing its boundaries as the process occurs. Work must be done on such a system to push mass in, and work is done on the surroundings by mass that emerges. Both work terms must be included in the energy balance. 

In Section 7.4a we outline the calculation of the work (or more precisely, the rate of energy transferred as work) required to move fluid through a continuous process system, and in Section 7.4b we review the concepts of intensive and extensive variables introduced in Chapter 6 and introduce the concept of specific properties of a substance. Section 7.4c uses the results of the two preceding sections to derive the energy balance for an open system at steady state. 

The net rate of work done by an open system on its surroundings may be written as 

Ws — shaft work, or rate of work done by the process fluid on a moving part within the system (e.g., a pump rotor) 

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