Chapter 9 Thermodynamics

THE FIRST LAW OF THERMODYNAMICS 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 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 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.

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

Sorption: 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

ADSORPTION The accumulation of molecules of a gas to form a thin film on the surface of a solid Solid surfaces in contact with water vapor have the tendency to capture and localize water molecules on them . This phenomenon is called adsorption. Factors Moisture adsorbed by a given amount of solid depends on : Temperature Partial Pressure of water vapour Surface area

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 a = adsorption d = desorption R H = relative humidity M C = moisture concentration 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.

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 Participating State Potential (Difference) Gas Diffusion Vapor Vapor Pressure Liquid Diffusion Liquid Concentration Surface Diffusion adsorbate Thermal Diffusion Vapor and Liquid Temperature Capillary flow Suction Pressure Convective Flow Air pressure Gravitational Flow Height

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