# Steady Flow Processes

**In this notes sheet:**

The basic form of the first law, **ΔQ – ΔW = ΔE, only applies to closed systems **- no mass can transfer across the system boundary, only energy in the form of heat and work.

In reality, perfectly closed systems are quite rare (take a turbine, for example: air flows in as well as heat, shaft work and hot air flow out), and as such a different model is required:

The energy transfers across the control volume surface are:

Shaft work

Heat transfer

Energy in the working fluid (kinetic, potential, and internal energies)

To simplify the process, we only look at the energy inputs and outputs: what goes on inside the control volume is irrelevant.

We call where the working fluid enters and exits * ports*.

The control volume above has two ports: one inflow and one outflow port.

## The Steady Flow Energy Equation (SFEE)

In order to solve problems involving steady flow through a control volume, we use the **Steady Flow Energy Equation (SFEE)** instead of the simple first law equation:

In a less mathematically intimidating form:

**Left hand side:**rate of energy transfer – rate of shaft work**Right hand side:**sum of output mass energy flow rates – sum of input mass energy flow rates

**Note: **The dot above the letters means it is a rate of change with respect to time: