Psychrometry and Thermodynamics Vocabulary

Vocabulary flashcards covering key psychrometric and thermodynamic properties, definitions, indices, and thermodynamic processes.

Dry-Bulb Temperature

The temperature indicated by a conventional thermometer, which serves as a direct measure of sensible internal energy ($t = \,^{\circ}\text{C}$).

Wet-Bulb Temperature

The temperature ($t_{wb} = \,^{\circ}\text{C}$) indicated by a thermometer covered with a wetted cotton wick and exposed to an airflow of $3\,\text{m/s}$; used together with dry-bulb temperature to measure the evaporative capacity of air.

Dew-Point Temperature

The temperature ($t_{dp} = \,^{\circ}\text{C}$) to which a sample of moist air ($v_{air}$) must be cooled at constant pressure ($P$) for the condensation of water vapour to begin, at which point the air reaches 100% saturation.

Partial Pressure

The pressure ($P_a$ for dry air or $P_v$ for water vapour, in $\text{kPa}$) that an individual gas component in a sample of air would exert on the container walls if it alone occupied the container at constant temperature.

Total Pressure

The sum of the partial pressure of dry air and the partial pressure of water vapour in a quantity of moist air, expressed as $TP = P_a + P_v$ in $\text{kPa}$.

Vapour Pressure

The partial pressure ($P_v$ in $\text{kPa}$) exerted exclusively by the water vapour present in a sample of moist air.

Saturated Vapour Pressure

The partial pressure ($SVP$ or $P_s$ in $\text{kPa}$) exerted by water vapour when the moist air sample is fully saturated.

Moist Air ($v_{air}$)

An atmospheric mixture composed of dry air and water vapour.

Dry Air ($d.a.$)

The gaseous component remaining when all water content has been completely removed from moist air.

Absolute Humidity

The mass of water vapour present per unit volume of moist air, expressed in $\text{g/m}^3$.

Specific Humidity

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

Apparent Specific Humidity (ASH)

The ratio defined as the mass of water vapour present divided by the mass of water.

Relative Humidity

The ratio of the partial pressure of water vapour present ($P_v$) to the saturated vapour pressure ($SVP$) at the same temperature, expressed as a percentage ($\% = \frac{P_v}{SVP}$).

Superheat

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

Enthalpy of Moist Air

The sum of sensible internal energy and pressure energy of dry air and water vapour components (in $\text{kJ/kg\,d.a.}$), calculated by adding total heat relative to a datum temperature of $0\,^{\circ}\text{C}$.

Sigma Energy (Sigma Heat)

The total enthalpy of dry air and water vapour minus the enthalpy component of water vapour in the liquid phase at the wet-bulb temperature, which remains nearly constant at a constant wet-bulb temperature (in $\text{kJ/kg}$).

Globe Temperature

The temperature ($t = \,^{\circ}\text{C}$) measured by a thermometer centered inside a hollow copper sphere of $150\,\text{mm}$ diameter painted matt black on the outside, used to measure radiant heat from surroundings.

Wet-Bulb Globe Temp Index (WBGT)

An index derived from wet-bulb temperature and globe temperature to assess heat stress on a person, calculated as $WBGT = 0.7 t_{wb} + 0.3 t_g$.

Effective Temperature

The temperature ($ET$ in $\,^{\circ}\text{C}$) of motionless saturated air that induces the same thermal sensation as the ambient air being evaluated.

Gas Constant of Air ($R$)

The ideal gas constant for air, equal to $R = 0.287\,\text{kJ/kg\,K}$, derived from Boyle's law and Charles's law ($\frac{PV}{T} = R$).

Adiabatic Process

A thermodynamic process in which a change in temperature and pressure occurs without loss or gain of heat (such as auto-compression), measured as sensible heat.

Isothermal Process

A thermodynamic process in which a change in state occurs without any change in temperature (such as ice at $0\,^{\circ}\text{C}$ changing to water at $0\,^{\circ}\text{C}$), measured as latent heat.