Chemical Engineering License Review, Second Edition
This text contains 146 problems with detailed step-by-step solutions, and is an ideal study guide and the first truly practical, no-nonsense review for the difficult PE Exam.
TABLE OF CONTENTS Chemical Engineering License Review, Second Edition
Chapter 9: Psychrometry and Humidification
In dealing with processes involving exchange of mass and energy between a liquid and an essentially insoluble gas, the principles of psychrometry are useful. Important operations to consider are the cooling and humidification and dehumidification of a gas. A familiarity with the use of the psychrometric chart for the air water-vapor system at atmospheric pressure is also essential.
Definitions in Psychrometry
The following is a review of the important definitions used in psychrometry.
Humidity Y Sometimes this is also called specific humidity or humidity ratio. When P is the total pressure which is low enough (near atmospheric) for the gas mixture to be considered ideal, the humidity is given by
| where M A | = molecular weight of vapor |
| M B | = molecular weight of dry gas |
| p A | = partial pressure of vapor in gas, atm |
| P | = total pressure, atm |
| Y | = lb of vapor per lb of vapor-free (dry) gas |
Saturation Humidity Y s When p A = P A, the vapor pressure of the liquid at the prevailing temperature, the gas is saturated with the vapor and the saturation humidity is given by
Molal Absolute Humidity Y ? This is defined as the number of moles of vapor per mole of the dry gas or
Thus from Eqs. (9-1) and (9-3),
Percent Relative Humidity or Relative Humidity This is a term which expresses the actual humidity of a gas at a given temperature as a percentage of its saturation humidity...
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