First law of thermodynamics

This session is a course reminder, and a brief discussion of energy forms put into play and the first law of thermodynamics..

It shows the different terms that appear in the expression of the first principle in steady state, in closed or open system.

The three links below provide access to long excerpts of the textbook, to which you should refer. The first document outlines the basic concepts and various definitions used, and the two others are written materials on this session.

Course reference:

“Fundamentals of Thermodynamics / Energy put into play”
“Fundamentals of Thermodynamics / First law”

To follow the presentation, go to next step

(Session realized on 06/15/11 by Renaud Gicquel)

THERMODYNAMICS FUNDAMENTALS

Energy exchange with the surroundings: 2 forms
heat δQ (on the system boundaries)
work δW:

pressure forces δW_A (on the boundaries )
gravity δW_v (in the system volume, negligible)

THERMODYNAMICS FUNDAMENTALS

Work δW, by action of pressure forces on the boundaries

$m δ W_{A} = - P A d x = - P d V$
$δ W_{A} = - P d v$ (2.2.1)
$W_{A} = - \int_{1}^{2} P d v$ (2.2.2)
$δ W_{v} = - g d z$ (2.2.3)

THERMODYNAMICS FUNDAMENTALS

Shaft work τ (open system)

$τ = W + P_{2} v_{2} - P_{1} v_{1} = W + Δ (P_{v})$ (2.2.5)

THERMODYNAMICS FUNDAMENTALS

Shaft work τ

$τ = W + P_{2} v_{2} - P_{1} v_{1} = W + Δ (P v)$ (2.2.5)
if reversible : $τ = \int_{1}^{2} v d P$ (2.2.6)
$δ τ = v d P$

THERMODYNAMICS FUNDAMENTALS

Heat transfer

δQ differential form of the fluid mass state
$δ Q = c_{p} d T + μ d P - δ π$ (2.2.7)
δQ heat exchanged with the surroundings
δπ heat dissipated by internal friction

Examples of calorimetric equations

For example, the state of a pure substance in liquid-vapor equilibrium at constant pressure exchanging heat dQ with the outside is governed by the equation:

dQ = L dx

L being the heat of vaporization of the fluid, and x is its vapor quality

Heating at constant pressure of a fluid is determined by the equation:

dQ = Cp dT

Cp being the heat capacity of the fluid, and T its temperature

THERMODYNAMICS FUNDAMENTALS

First law: energy conservation

closed system: internal energy
$d u + d K = δ W + δ Q$ (2.3.1)
open system: enthalpy h = u + Pv
$d h + d K = δ τ + δ Q$ (2.3.4)

THERMODYNAMICS FUNDAMENTALS

Application to industrial processes

$τ + Q = h_{2} - h_{1} + Δ K + g Δ z$
$τ + Q = h_{2} - h_{1} + \frac{C_{2}^{2} - C_{1}^{2}}{2}$ (2.3.7)

THERMODYNAMICS FUNDAMENTALS

Application to industrial processes

heat exchangers: Q = Δh
adiabatic machines: τ = Δh
throttling: Δh = 0

First law of thermodynamics

This session showed that two forms of energy are involved in energy systems components: mechanical work of pressure forces and heat exchanged with the surroundings.

The different terms appearing in the expression of the first law in steady state, in closed or open system, were presented.

The sessions on components show how to evaluate these terms in practice.