Thermodynamics

Thermodynamics States About Energy Conversion Thermodynamics is the branch of science that embodies the principles of energy transformation in macroscopic systems. The general restrictions which experience has shown to apply to all such transformations are known as the laws of thermodynamics. These laws are primitive; they cannot be derived from anything more basic. The first law of thermodynamics states that energy is conserved; that, although it can be altered in form and transferred from one place to another, the total quantity remains constant. Thus, the first law of thermodynamics depends on the concept of energy; but, conversely, energy is an essential thermodynamic function because it allows the first law to be formulated. This coupling is characteristic of the primitive concepts of thermodynamics. The words system and surroundings are similarly coupled. A system is taken to be any object, any quantity of matter, any region, and so on, selected for study and set apart (men...

Psychrometry

Psychrometry is concerned with determination of the properties of gas-vapor mixtures. The air-water vapor system is by far the system most commonly encountered. Principles involved in determining the properties of other systems are the same as with air-water vapor, with one major exception.

Whereas the psychrometric ratio (ratio of heat-transfer coefficient to product of mass-transfer coefficient and humid heat, terms defined in the following subsection) for the air-water system can be taken as 1, the ratio for other systems in general does not equal 1. This has the effect of making the adiabatic-saturation temperature different from the wet-bulb temperature. Thus, for systems other than air-water vapor, calculation of psychrometric and drying problems is complicated by the necessity for point-to-point calculation of the temperature of the evaporating surface. For example, for the air-water system the temperature of the evaporating surface will be constant during the constant-rate drying period even though temperature and humidity of the gas stream change. For other systems, the temperature of the evaporating surface would change.


TERMINOLOGY

Terminology and relationships pertinent to psychrometry are: Absolute humidity H equals the pounds of water vapor carried by 1 lb of dry air. If ideal-gas behavior is assumed, H = Mwp/[Ma(P - p)], where Mw = molecular weight of water; Ma = molecular weight of air; p = partial pressure of water vapor, atm; and P = total pressure, atm.

When the partial pressure p of water vapor in the air at a given temperature equals the vapor pressure of water ps at the same temperature, the air is saturated and the absolute humidity is designated the saturation humidity Hs.

Percentage absolute humidity (percentage saturation) is defined as the ratio of absolute humidity to saturation humidity and is given by 100 H/Hs = 100p(P - ps)/[ps(P - p)].

Percentage relative humidity is defined as the partial pressure of water vapor in air divided by the vapor pressure of water at the given temperature. Thus RH = 100p/ps.

Dew point, or saturation temperature, is the temperature at which a given mixture of water vapor and air is saturated, for example, the temperature at which water exerts a vapor pressure equal to the partial pressure of water vapor in the given mixture.

Humid heat cs is the heat capacity of 1 lb of dry air and the moisture it contains. For most engineering calculations, cs = 0.24 + 0.45H, where 0.24 and 0.45 are the heat capacities of dry air and water vapor, respectively, and both are assumed constant. Humid volume is the volume in cubic feet of 1 lb of dry air and the water vapor it contains. Saturated volume is the humid volume when the air is saturated.

Wet-bulb temperature is the dynamic equilibrium temperature attained by a water surface when the rate of heat transfer to the surface by convection equals the rate of mass transfer away from the surface. At equilibrium, if negligible change in the dry-bulb temperature is assumed, a heat balance on the surface is

kg l(ps - p) = hc (t - tw)

where kg = mass-transfer coefficient, lb/(h×ft2×atm); l = latent heat of vaporization, Btu/lb; ps = vapor pressure of water at wet-bulb temperature, atm; p = partial pressure of water vapor in the environment, atm; hc = heat-transfer coefficient, Btu/(h×ft2×°F); t = temperature of air-water vapor mixture (dry-bulb temperature), °F; and tw = wet-bulb

temperature, °F. Under ordinary conditions the partial pressure and vapor pressure are small relative to the total pressure, and the wet bulb equation can be written in terms of humidity differences as

Hs - H = (hc /lk¢)(t - tw)

where k¢ = lb/(h×ft2) (unit humidity difference) = (Ma /Mw)kg = 1.6kg.

Adiabatic-Saturation Temperature, or Constant-Enthalpy Lines If a stream of air is intimately mixed with a quantity of water at a temperature ts in an adiabatic system, the temperature of the air will drop and its humidity will increase. If ts is such that the air leaving the system is in equilibrium with the water, ts will be the adiabatic saturation

temperature, and the line relating the temperature and humidity of the air is the adiabatic-saturation line. The equation for the adiabatic-saturation line is

Hs - H = (cs /l)(t - ts)