Lesson 7FIRST LAW OF THERMODYNAMICS

• STATE the First Law of Thermodynamics.

• Using the First Law of Thermodynamics, ANALYZE an open system including all energy transfer processes crossing the boundaries.

• Using the First Law of Thermodynamics, ANALYZE cyclic processes for a thermodynamic system.

• Given a defined system, PERFORM energy balances on all major components in the system.

• Given a heat exchanger, PERFORM an energy balance across the two sides of the heat exchanger.

• IDENTIFY the path(s) on a T-s diagram that represents the thermodynamic processes occurring in a fluid system.

First Law of Thermodynamics

Energy can neither be created nor destroyed, only altered in form.

First Law of Thermodynamics• Energy Transfer

– Mass and energy crossing the control boundary– External work and/or heat crossing the boundary– Change of stored energy within the control volume.

• Mass Flow of Fluid - Associated with the kinetic, potential, internal, and "flow" energies that affect the overall energy balance of the system

• Energy balanced by the exchange of external work and/or heat

Conservation of Energy

• Open System Σ(all energies in) = Σ (all energies out) + Δ(energy stored in system)

(Σ E in = Σ Eout + ΔE storage)

• Closed System– No mass crosses the boundary, but work and/or heat do

• Isolated System – Mass, work and heat do not cross the boundary - the

only energy exchanges taking place are within the system

First Law of Thermodynamics

First Law of Thermodynamics

First Law of Thermodynamics

Energy Balance – Control Volume

• A fixed region in space is established with specified control boundaries

• The energies that cross the boundary of this control volume, including those with the mass crossing the boundary, are then studied and the balance performed.

Control Volume Concepts

Open System Control Volumes

Open System Control Volumes (Cont.)

Multiple Control Volumes in Same System

Energy Across the Boundary• Forms of energy crossing the control boundary with the mass

are given as m (u + Pv + ke + pe).

• Enthalpy, has been defined as h = u + Pv. This results in the above expression being written as m (h + ke + pe).

• Externally applied work (W) (or “Shaft Work”) also may cross the system boundary.

• In order to complete and satisfy the conservation of energy relationship, energy that is caused by neither mass nor shaft work is classified as heat energy (Q).

Energy Relationship in Symbolic Form

Processes• In some processes, the relationships between pressure,

temperature, and volume are specified as the fluid goes from one thermodynamic state to another.

• The most common processes– Isothermal - Constant Temperature– Isobaric - Constant Pressure– Isovolumetric - Constant Volume

• Cyclic Process - Fluid passes through various processes and returns to the same state it began with

T-s Diagram with Rankine Cycle

Rankine Cycle

ab: Liquid is compressed with no change in entropy (by ideal pump).

bc: Constant pressure transfer of heat in the boiler. Heat is added to the compressed liquid, two-phase, and superheat states.

cd: Constant entropy expansion with shaft work output (in ideal turbine).

da: Constant pressure transfer of heat in the sink. Unavailable heat is rejected to the heat sink (condenser).

Typical Steam Plant Cycle

Steam Plant System Components• Heat source to produce the thermal energy (e.g. nuclear or fossil fuel)

• A steam generator to change the thermal energy into steam energy

• Pumps to transfer the fluid back to the heat source (reactor coolant pumps in a nuclear reactor)

• Pressurizer to ensure that the primary system maintains its desired pressure

• Piping to ensure the fluid passes through each stage of its cyclic process.