Thermodynamics

Chapter 9

The internal energy of a system changes from an initial value Ui to a final value of Uf due to heat Q and work W:

ΔU = Uf -Ui = Q - W

THE FIRST LAW OF THERMODYNAMICS

Q is positive when the system gains heat and Q is negative when it loses heat. W is positive when work is done by the system and Q is negative when work is done on the system.

Fundamentals of transport and storage of moisture in building materials and components

INTRODUCTION: water is essential But it can cause

deterioration and dissipation in many natural and man-made materials.

Thus role of MOISTURE is of great importance for building designers, researchers and practitioners

Important Questions

How can the transport of moisture through building material be predicted?

How can the harmful accumulation of moisture in building materials be prevented

The effect of moisture transport on energy efficiency of buildings?

The Thermodynamic State of Moisture

Water exists in three states like any other substance

All three states can exist in a building The various building materials can

capture water molecules from the surrounding air and localize them on their surfaces.

Moisture so localized is said to be in an adsorbed state

Evaporation is the stage where water travels from the Earth’s surface to the atmosphere. Water molecules are heated up by the sun, and water is changed from a liquid form to a gas form.

Evaporation

Triple Point There is only one

temperature and saturation vapor pressure at which all three states can coexist

The triple point temperature is 273.16K and the corresponding saturation vapor pressure is 611 Pa

The process in which one substance takes up or holds another (by either absorption or adsorption)

Sorption is a physical and chemical process by which one substance becomes attached to another

Absorption - the incorporation of a substance in one state into another of a different state (e.g., liquids being absorbed by a solid or gases being absorbed by a liquid)

Adsorption - the physical adherence or bonding of ions and molecules onto the surface of another phase

Sorption:

ADSORPTION

Solid surfaces in contact with water vapor have the tendency to capture and localize water molecules on them . This phenomenon is called adsorption.

FactorsMoisture adsorbed by a given amount of solid

depends on :Temperature Partial Pressure of water vapourSurface area

The accumulation of molecules of a gas to form a thin film on the surface of a solid

Sorption a mechanism for storage of moisture

If surrounding air dry -----zero adsorption IF vapor pressure increase------first

monomolecular layer form then multi-molecular

This continues until the surface layers grows large enough to form droplets of water

From absolute dry to this point of droplets formation , the material is said to be in its hygroscopic range.

Adsorption in form of mono and multi molecular layer

Once the vapor pressure is above this hygroscopic range, larger amounts of moisture begins to deposit in the pores until the structure is filled with condensed moisture. The maximum amount of moisture that can be accommodated by a material is limited only by its porosity.

Adsorption Isotherm

Building material has its own individual response to water vapor.

The relation between amount of moisture adsorbed and the vapor pressure of moisture at a given temperature is called the adsorption isotherm

Hysteresis

If a porous building material is saturated with water and allowed to dry in air at different relative humidity, it does not retrace the sorption isotherm. Usually it retains more moisture during desorption than it can adsorb at any given humidity. This phenomenon is referred to as hysteresis.

a = adsorptiond = desorptionR H = relative humidityM C = moisture concentration

Transport of moisture and energy change

Moisture can be transported from one location to another through a porous body in all four states

Vapor transport Liquid transport Solid transport Adsorbate transport Vapor and liquid transport are much

faster than solid and adsorbate transport

MOISTURE TRANSPORT EQUATION Any transport process is brought about by a

driving force or a potential JB =-K. grad ΦB

Where JB denotes a rate at which the entity B is transported ΦB is the driving potential (temperature, pressure, concentration, hight, etc.) and k is a quantitiy called a transport coefficient characteristic of the medium through transport occurs.

JB is the flux density of B

(Entity B transport across area in unit time)

Storage of Moisture and Energy Changes

Changes in vapor pressure and temperature in pores changes the amount of moisture in the vapor phase needed to fill the pores

This change the energy content of the moisture retained

A moisture is transported through a finite volume in a medium , the amount of moisture retained by the volume is altered during any transient stage of the transport process .

Due to change in local temperature or local vapor pressure

Moisture Transport process in building material and processes

Transport Process

Participating State

Potential (Difference)

Gas Diffusion Vapor Vapor Pressure

Liquid Diffusion Liquid Concentration

Surface Diffusion adsorbate Concentration

Thermal Diffusion Vapor and Liquid Temperature

Capillary flow Liquid Suction Pressure

Convective Flow Vapor Air pressure

Gravitational Flow Liquid Height

The End