Psychrometrics and Moist Air Properties

Flashcards covering key definitions, units, equations, and thermodynamic concepts for psychrometrics and moist air properties from the lecture.

Absolute Humidity

The mass of $H_2O$ vapour present per unit volume of $v_{air}$, expressed in $g/m^3$ of $v_{air}$.

Specific Humidity

The quantity of $v_{air}$ defined as the mass of $H_2O$ vapour per unit mass of $v_{air}$, expressed in $g/kg$ of $v_{air}$.

Apparent Specific Humidity (ASH)

The mass of $H_2O$ vapour present per unit mass of dry air, expressed in $g/kg$ of dry air ($g/kg\text{ d.a.}$).

Relative Humidity ($\phi$)

The ratio of the partial vapour pressure of $H_2O$ vapour actually present to the saturated vapour pressure at constant temperature ($\phi = \frac{p_v}{SVP}$), usually expressed as a percentage.

True Density

The mass of $v_{air}$ (dry air plus $H_2O$ vapour) per unit volume of $v_{air}$, expressed in $kg/m^3$.

Apparent Specific Volume ($V$)

The volume of $v_{air}$ per unit mass of dry air, expressed in $m^3/kg\text{ dry air}$.

Superheat

The quantity of heat required to raise the temperature of a quantity of $v_{air}$ from its wet-bulb temperature ($T_{wb}$) to its dry-bulb temperature ($T_{db}$), expressed in $kJ/kg\text{ d.a.}$.

Enthalpy ($H$)

The sum of sensible internal energy and pressure energy of the dry air and $H_2O$ vapour components in a sample of $v_{air}$ ($kJ/kg\text{ d.a.}$), determined relative to a datum temperature taken to be $0^\times C$.

Enthalpy Component A

The heat required to raise dry air from $0^\times C$ to the dry-bulb temperature ($T_{db}$).

Enthalpy Component B

The heat required to raise the temperature of water liquid from $0^\times C$ to the wet-bulb temperature ($T_{wb}$).

Enthalpy Component C

The latent heat of vaporisation of $H_2O$ vapour at the wet-bulb temperature ($T_{wb}$).

Enthalpy Component D

The superheat of water vapour required to raise its temperature from the wet-bulb temperature ($T_{wb}$) to the dry-bulb temperature ($T_{db}$).

Sigma Energy ($\Sigma$)

The total enthalpy of a sample of $v_{air}$ minus the enthalpy of the liquid phase at the wet-bulb temperature (Enthalpy minus Component B), which remains very nearly constant at constant wet-bulb temperature ($kJ/kg$).

Wet-Bulb Globe Temp Index (WBGT)

A figure derived from a combination of wet-bulb temperature and globe temperature of moist ambient air exposed to that air ($WBGT = 0.7\,t_{wb} + 0.3\,t_g$), used as a measure of heat stress.

Globe Temperature ($t_g$)

A measure of the radiant heat of surroundings ($^\times C$), measured using a $150\,mm$ copper sphere.

Effective Temperature (ET)

The temperature of motionless, unventilated, saturated air that produces the same thermal sensation as the ambient air being considered.

Gas Constant for Dry Air ($R_{da}$)

The specific gas constant value defined as $R_{da} = 0.2871\,kJ/kg\cdot K\text{ d.a.}$ in the gas relationship $\frac{PV}{T}$.

Isothermal Process

A process in which a substance changes its state without a change in temperature ($\Delta T = 0$).

Adiabatic Process

A process in which a substance changes in temperature without a change in state.

Sensible Heat

Heat associated with dry-bulb temperature ($T_{db}$) change.

Latent Heat

Heat associated with a change in state or phase without a change in temperature.