How boiler generates energy?

What is steam?

Steam is an energy fluid.?It is one of the most widely used commodities for conveying heat energy. Steam is an energy carrier. Steam has five or six times the potential energy of an equivalent mass of water. Steam is one of the most commonly used mediums for transporting heat over long distances. Because steam flows in response to pressure drops along the line, expensive circulating pumps are not required. When water is heated in a boiler, it begins to absorb energy. Depending on the pressure in the boiler, water will evaporate at a certain temperature to form steam. The steam contains a large amount of stored energy, which will eventually be transferred to the process or the space to be heated.

?Steam production

Fundamentals

In a closed box, steam is not an energy. It is thermally balanced with itself. The criteria for equilibrium are [1] constant temperature and pressure. [2] constant chemical potential and [3] lowest free energy. All of these conditions are met when natural steam is boxed up in a closed vessel. When steam is consumed, it becomes energy. The chemical potential has shifted. When there is a consumption or drop in pressure, it indicates that energy has been depleted. This indicates that the steam has performed thermodynamic work and expended its internal energy. If a substance lacks free energy, it cannot perform work.

If the free energy change is zero, dG = dH -TdS = 0, 0r, dH =TdS. H stands for enthalpy or total energy, and S stands for entropy. The difference between total energy and TdS is the net loss in free energy dG. The second law of thermodynamics, which states that [1] entropy always increases and [2] energy always moves to a lower level from a higher level, is applicable here. Entropy propels a substance to perform work by causing a loss of energy, requiring a new energy supply to keep the work going. When the change in Gibbs free energy is negative, this occurs in any spontaneous thermodynamic process. This is a fundamental concept that should never be forgotten. This is a natural universal law.

Steam production in a boiler

Steam table: Refer to steam properties at 100 degc

There are twelve columns. The meaning of each column has been explained by calculation.

The?role?of?a?boiler?in?supplying?energy?to?water

A?boiler?is?a?closed?vessel?that?heats?a?fluid?(usually?water). The?fluid?does?not?always?boil. The?heated?or?vaporized?fluid?is?discharged?from?the?boiler?for?use?in?various?processes?or?heating?applications.

The?boiler?frees?the?intermolecular?bonds,?which?are?the?custodians?of?enormous?potential?energy,?and?transfers?this?energy?to?steam as kinetic energy, which?can?perform?work?that?water?cannot?due?to?its?incompressibility.

The energy of a gas is predominantly kinetic energy since there are fewer intermolecular bonds in the gas

The?potential?energy?of?water?is?transferred?to?vapor?in?two?stages.

When?water?expands?due?to?the?release?of?intermolecular?bonds,?it?adds?internal?and?work?energy. The total energy of vaporization at 1 atm pressure and 100 degc is 2260 kj/kg.

Steam making

Please follow the steam table [100 degc row above.]

Energy at start

1st and 2nd columns are temperature and pressure.

They do not need an explanation.

3rd column

The density of water at 100 degc= 958.05 kg/m3

Specific volume of water, vf = 1/958.05 = 0.001043 m3/kg

Energy – Work in progress

4th column

Water to steam volume expansion

Density of steam at 100 degc =0.6 kg m^-3

Specific volume of steam, Vg = 1/0.6 = 1.670 m3/kg

vfg is the saturated water-vapor specific volume = the difference between the specific volume of the saturated vapor and saturated liquid

Saturated specific volume of vapor-water, vfg = (vg - vf) = [1.670-0.001043] = 1.6689 m3/kg

5th column

The internal energy of saturated water, uf at 100 degc

The internal energy of saturated water at 100 degc is its stored energy which is mass x specific heat x delta t. The specific heat of water at 0 degc, = 4.191 kJ/kg/k, ug = 1x 4.19x100 = 419.1kJ/kg. This is the sensible heat that water contributes at 100 deg.

6th column

ug, the internal energy of saturated steam

P= 0.1014 mPa, vg = 1.670 m3/kg]

1 mPa = 1000000 Newton/m2

0.1014 mPa = 1000000x 0.1014

PV= 1000000x 0.1014 x 1.670 = 169338 Newton-meter [PV = Work]

1kJ = 1000 Newton-meters

Therefore, PV = 169.338 KJ,

ug = hg - PV

hg = 2675.6 kJ/kg [ Important point: Here there is consumption of energy [endothermic] as H bonds break and water expands

ug = 2675.6-169.338 = 2506.3 KJ/kg?

7th column

hf, enthalpy of water,

Calculation

Since hf = uf + w and w [work] is zero for water, hf = uf, therefore,

hf = 419.1 kJ/kg

Column 8

hfg, the enthalpy of saturated water-steam vapor inside the bell curve

Calculation

hfg = hg-hf [ difference of enthalpy of saturated steam on the saturated steam line of TS diagram and enthalpy of saturated water on the saturated water line of TS diagram.

hfg = [2675.6-419.1] = 2256.5 kJ/kg

The energy of finished product

Column 9

hg, the enthalpy of saturated steam. This is a property of water, 2675.6 KJ/kg at 100 degc and 0.1014 mPa pressure.?

hg = 2675.6 kJ/kg : this is the latent heat of steam

Column 10

?sf, the entropy of water

Calculation

Along with this sensible heat water also brings entropy Sf = Cp x ln T2/T1 [ T1 = 0+273.15= 273.15k] and [T2 = 100+273.15, T2/T1 = 373.15/273.15 =1.366]

Sf = Cp x ln 1.366 = 4.191x 0.312 = 1.30 kJ/kg/k

Sf = 1.3 kJ/kg/k

Column11

sfg, the entropy of saturated water-steam in equilibrium

Calculation

Sfg = vfg/T = 2256.4/373.15 = 6.0495 kJ/kg/k

Column 12

sg, the entropy of saturated steam on the saturated steam .

Calculation

It is the sum of sf + sfg [entropy of saturated water +entropy of water-steam in equilibrium]

sg = 1.3+6.0495 = 7.3495 Kj/kg/k

This is the final loss of energy as entropy

Energy distribution

Finished product energy

Enthalpy of water = 419 kj/kg

Enthalpy of saturated steam= 2256.5 kJ/kg

Latent heat of water = 419+2256.5 = 2675.5 kj