INTRODUCTION TO ENERGY BALANCE

introduction to energy balance

Example

1 kg of steam goes through a turbine at 1 MPa at a temperature of 200 °C and a linear velocity of 20 m/s. It leaves a point 5 m below, at a pressure of 0.1 MPa and a velocity of 100m/s. The shaft work delivered by the turbine is 30 kJ and the heat loss is 0.75 kJ. Calculate the change in entalphy.

Let's first draw a flowchart

Energy Balance eq in open system :

Rearranged :

∆Ḣ+ΔĖₖ+ ΔĖₚ = Q - Wₛ

∆Ḣ = Q - Wₛ - ΔĖₖ - ΔĖₚ

ΔĖₖ = 1/2mv² = 1/2( 1 kg )( 100² - 20² ) m²/s²

= 4.8 kJ

ΔĖₚ = mgz = 1 kg ( 9.81 m/s²) (0 - 2m)

= -0.01962 kJ

Q = 0.75 kJ

Wₛ = 30 kJ

∆Ḣ = 0.75kJ - 30kJ - 4.8 kJ - (-0.01962 kJ)

= -34.03kJ

Exercise

1. Draw a flowchart for the energy balance of:

i) an open pot

ii) an electric pressure cooker (Noxxa)

along with the related equations for the energy balance

2. Aiman drops his pressure cooker from 1 m high. What is the change in entalphy of the contents inside the pressure cooker?

3. The pressure cooker is then hit by a car and it travels 60m in 2s. What was the change in entalphy when it was moving?