1. CHAPTER 4 The First Law of Thermodynamics – Steady flow systems (steady means no change with time)

2. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. FIGURE 4-22 Under steady-flow conditions, the mass and energy contents of a control volume remain constant. 4-2

3. Steady flow systems W b = 0 (constant volume) m in = m out (rates also) E in = E out (rates also) W and Q are constant with time Properties of mass in and out are generally taken to be constant with time.

4. Steady State Systems Mass balance: m in = m out Energy balance – E in – E out = ΔE system = 0 –or q – w = h out – h in – Δke - Δpe

5. FIGURE 4-28 At very high velocities, even small changes in velocities may cause significant changes in the kinetic energy of the fluid. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4-3

6. FIGURE 4-30 Nozzles and diffusers are shaped so that they cause large changes in fluid velocities and thus kinetic energies. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4-4 Q = 0 W = 0 ΔPE = 0 so ΔKE = ΔH

7. Turbines, Compressors, Pumps and Fans Usually Q = 0 ΔPE ≈ 0 ΔKE ≈ 0 So w = Δh

8. FIGURE 4-35 Throttling valves are devices that cause large pressure drops in the fluid. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4-5 Often, temp drops. h 1 ≈ h 2 so: u 1 + P 1 v 1 ≈ u 2 + P 2 v 2 Therefore, u is usually exchanged for Pv or flow energy and T drops Isenthalpic process.

9. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4-6 FIGURE 4-37 During a throttling process, the enthalpy (flow energy + internal energy) of a fluid remains constant. But internal and flow energies may be converted to each other. How about an ideal gas? Will Temperature change?

10. Mixing Chambers Usually no work or heat transfer Usually, ke and pe are neglected m 1 + m 2 = m out m 1 h 1 + m 2 h 2 = m out h out

11. FIGURE 4-42 The heat transfer associated with a heat exchanger may be zero or nonzero depending on how the system is selected. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4-7 Kind of like a mixing chamber, but no mixing.