ASHRAE F06-2009 (I-P)

Mass transfer by either molecular diffusion or convection is the transport of one component of a mixture relative to the motion of the mixture and is the result of a concentration gradient. Mass transfer can occur in liquids and solids as well as gases. For example, water on the wetted slats of a cooling tower evaporates into air in a cooling tower (liquid to gas mass transfer), and water vapor from a food product transfers to the dry air as it dries. A piece of solid CO₂ (dry ice) also gets smaller and smaller over time as the CO₂ molecules diffuse into air (solid to gas mass transfer). A piece of sugar added to a cup of coffee eventually dissolves and diffuses into the solution, sweetening the coffee, although the sugar molecules are much heavier than the water molecules (solid to liquid mass transfer). Air freshener does not just smell where sprayed, but rather the smell spreads throughout the room. The air freshener(matter) moves from an area of high concentration where sprayed to an area of low concentration far away. In an absorption chiller, low-pressure, low-temperature refrigerant vapor from the evaporator enters the thermal compressor in the absorber section, where the refrigerant vapor is absorbed by the strong absorbent (concentrated solution) and dilutes the solution.

Chapter Table of Contents

Molecular Diffusion
Convection of Mass
Simultaneous Heat and Mass Transfer Between Water-Wetted Surfaces and Air
Symbols

ISBN: 978-1-933742-54-0
ISSN: 1523-7222

Units: I-P

Citation: 2009 ASHRAE Handbook -- Fundamentals: Chapter 6, Mass Transfer



Product Code(s): D-F062009IP

Chapter Table of Contents

Molecular Diffusion

Convection of Mass

Simultaneous Heat and Mass Transfer Between Water-Wetted Surfaces and Air

Symbols