Psychrometry

PSYCHRMETRY:

Moisture, air, and heat interact with some consequences that are threats to, and other consequences
that are opportunities for, building performance. In winter, condensation within insulation due to falling
air temperatures can be disastrous. In summer, adding moisture to hot, dry air can lower its drybulb
(DB) temperature while raising its humidity to more comfortable levels.

These moisture, air, and heat interactions are complex. As air temperature rises, its capacity to hold
moisture rises also, and the warmer air becomes less dense. These combined interactions are described
by psychrometry, the study of moist air. Fortunately, these interactions can be combined within a single
chart. Dry‐bulb (DB), wet‐bulb (WB) and relative humidity (RH) elements are combined in the schematic chart, where the term saturation line, at 100% RH, is introduced. This is also called the dew point (DP) because dew forms (water vapor condenses) when saturated air touches any surface at or below the air’s dew point temperature. This saturation is sometimes undesirable, as within walls or roofs, or on ceiling, air duct, or glass surfaces. However, it is often desirable, as on air‐conditioner coils, where the resulting reduction of the moisture content in the air is deliberate.

The psychrometric chart may be used to graph a wide variety of processes. The first addition
is the humidity ratio, which indicates the amount of moisture by weight within a given weight of dry air.
Air treatment processes that travel along these horizontal lines of constant humidity ratio are
the familiar processes of simple heating (air passing through the heating coil of a furnace or through
a solar collector) and simple (sensible) cooling (air passing through the cooling coil of an air conditioner
before saturation). The humidity ratio is used in calculating latent heat gains from outdoor air.

The next addition shows how the density of air varies as its temperature and moisture content vary.
These lines are those of specific volume, the reciprocal of density, a useful quantity in air‐conditioning
calculations and helpful in understanding the stack effect in passive design. The specific volume is given
in ft3/lb (m3/kg) of dry air.

The next addition  involves enthalpy, the sum of the sensible and latent heat content of an air–moisture mixture relative to the sum of the sensible and latent heat in air at 0°F (0°C in SI units) at standard atmospheric pressure. Enthalpy units are Btu/lb (kJ/kg) of dry air. Enthalpy lines are almost parallel to those of WB temperature. Perhaps the most familiar process to travel along the lines of constant enthalpy is evaporative cooling, whereby increased moisture and lower air DB temperature are obtained without changing the enthalpy (total heat content) of the air. There is indeed a drop in sensible heat as the temperature drops, but this is matched by an increase in latent heat as the moisture content increases. The opposite process is chemical (desiccant) dehumidifying, whereby decreased moisture content is obtained at
the price of increased air DB temperature; again, no change in enthalpy (total heat) occurs.