In the previous chapter we have derived energy balance equation considering a control mass system. The analysis was made in a closed system. However most of the practical devices deal with open system. The mass, work and heat can enter and exit the system. We introduce concept of control volume which is a fixed region in space. The size and shape of the control volume are completely arbitrary and are so defined to best suit our analysis. The surface of this control volume is called control surface. Control surface is a closed surface. The surface may be fixed, or it may move to expand or contract. However, the surface must be defined relative to some coordinate system. The coordinate system may be fixed, rotating or moving-depending upon the situation under analysis.
Fig. 5.1 shows a control volume and control surface. Mass, heat and work transfers are also shown in Fig. 5.1. We have shown here fixed control surface and fixed datum.
Fig. 5.1 also shows that it is an open system.