L3 · HVAC Loads and Psychrometrics

Foundation lesson · 25 minute study session

Describe a moist-air state without confusing its properties

Relative humidity, humidity ratio and dew point describe different features of the same air. State pressure first, then use two independent properties.

By the end of this lesson

Identify a state from two independent properties and pressure.
Use humidity ratio on a dry-air mass basis.
Interpret sensible heating on a psychrometric chart.

Name each property

Dry-bulb temperature describes ordinary air temperature. Humidity ratio is the mass of water vapor per mass of dry air. Relative humidity compares vapor pressure with saturation vapor pressure at the same temperature. Dew point is the temperature at which the same vapor pressure reaches saturation. They are related, but cannot be substituted for one another. $W$ is a mass ratio; $\phi$ is a vapor-pressure ratio. They are different properties.

Locate and read the state

Choose the chart appropriate to the air pressure and temperature range. Locate two independent properties, then read humidity ratio, enthalpy and specific volume from that state. Wet-bulb and enthalpy lines are close in many ordinary HVAC conditions, but their meanings and exact relationships differ.

Follow a process

Sensible heating at fixed pressure with no moisture addition keeps humidity ratio constant. Dry bulb increases, relative humidity usually falls, and dew point stays constant. Cooling first follows a constant-humidity-ratio path; moisture removal begins when the process actually causes condensation.

A decision map for this lesson

Four properties, four different meanings

At the stated pressure, distinguish a temperature, a vapor-pressure ratio and a dry-air mass ratio.

Choose the governing relationship and catch the wrong route
Condition or question	Governing decision	Error to avoid
Dry-bulb temperature	Use it to describe the air temperature; pair it with an independent moist-air property.	Treating another temperature-unit conversion as a second independent property.
Relative humidity	Compare vapor pressure with saturation pressure at the same dry bulb: $\phi=p_w/p_{ws}$ .	Using 50% RH as a humidity ratio of 0.50.
Humidity ratio	Use $W=0.622p_w/(p-p_w)$ , with absolute pressures.	Replacing the dry-air mass basis with a volume fraction.
Dew point	Find the temperature whose saturation pressure equals the current vapor partial pressure.	Using the current dry bulb as dew point for unsaturated air.

On a narrow screen, scroll across the table to compare the decision and the error.

Relationships and their limits

Humidity ratio

W=\frac{m_w}{m_{da}}=0.622\frac{p_w}{p-p_w}

Use absolute pressures in matching units. W is lb water/lb dry air.

Relative humidity

\phi=p_w/p_{ws}(T)

Use saturation pressure at the dry-bulb temperature. Multiply the ratio by 100 for percent.

Symbols and unit basis
Symbol	Meaning	Units or basis
$W$	Humidity ratio	lb water/lb dry air
$p$	Total absolute pressure	psia
$p_w$	Water-vapor partial pressure	psia
$p_{ws}$	Saturation vapor pressure at dry bulb	psia
$\phi$	Relative humidity as a ratio	dimensionless
$m_w$	Water-vapor mass	lb water
$m_{da}$	Dry-air mass	lb dry air
$T$	Dry-bulb temperature for saturation property	°F

One decision, fully worked

Apply the method.

Moist air is at 14.7 psia. Water-vapor partial pressure is 0.18 psia and saturation pressure at the air dry bulb is 0.36 psia. Find humidity ratio and relative humidity.

Use vapor partial pressure in the humidity-ratio relationship.
Subtract vapor pressure from total pressure for the denominator.
Use the given saturation pressure to compute relative humidity.
Keep humidity ratio in lb/lb rather than percent.

W=0.622(0.18)/(14.7-0.18)=0.00771\ \mathrm{lb_w/lb_{da}}

\phi=0.18/0.36=0.50=50\%

Decision

The air contains about 0.00771 lb water per lb dry air and has 50% relative humidity.

Independent check

The same humidity ratio can occur at many relative humidities when dry-bulb temperature changes.

Specialist extension

Check saturation before trusting a computed state

Humidity ratio describes water vapor mass per dry-air mass. Relative humidity compares vapor partial pressure with saturation pressure at the same temperature. A humidity-ratio calculation above the saturation value cannot represent an equilibrium vapor-only state at that temperature and pressure. Check pressure, units, temperature and whether condensation or frost is occurring.

A new applied case

At a supplied temperature, saturation vapor pressure is 0.50 psia and total pressure is 14.7 psia. Find the maximum equilibrium vapor humidity ratio.

W_s=0.622\frac{p_{ws}}{p-p_{ws}}=0.622\frac{0.50}{14.7-0.50}=0.02190

Decision. The saturation limit is about 0.02190 lb water/lb dry air.

Cross-check. A proposed W = 0.030 exceeds this supplied-temperature saturation limit; investigate the state before calculating a coil.

Unit basis. W is lb water/lb dry air; p and vapor partial pressures are absolute psia.

NCEES PE Mechanical Reference Handbook v2.1; see the module reference path.

Open targeted practice in the study studio →

Transfer the method

HVAC Loads and Psychrometrics · HV-LC-L3

Try a new decision.

Select one

At a dry-bulb temperature with supplied saturation pressure 0.50 psia, moist air has water-vapor partial pressure 0.20 psia and total pressure 14.7 psia. The humidity ratio and relative humidity, respectively are most nearly:

A0.0138 lb/lb; 40%B0.00858 lb/lb; 40%C0.00858 lb/lb; 1.36%D0.0219 lb/lb; 100%

A lesson check supports practice, not a readiness score. Your response stays in this page session and clears on reload.

Catch the common errors

Treating 50% RH as 0.50 lb water/lb dry air.
Using gauge pressure in a vapor-pressure ratio.
Reading a sea-level chart for a different pressure without correction.

Handbook navigation drill

Find it, then explain it.

Search: Psychrometric Properties

Locate humidity ratio, relative humidity and the sea-level chart. Pick an ordinary state and read h and v.

Check your navigation

You can name the units and identify the fixed-pressure condition for every property read.

Reference path: §7.1–7.4, pp. 385–397. These are printed handbook page numbers; section names and search terms remain the primary navigation method. See technical review status.

Theory sources

NCEES handbook v2.1, §§7.1–7.4.
Grondzik and Kwok, Mechanical and Electrical Equipment for Buildings, 12th ed., §4.2, Psychrometry.
Lechner, Heating, Cooling, Lighting, 4th ed., §§4.5–4.7.

Worked examples and lesson prose are original ReasonSpan^™ material. Obtain authorized publications separately; this preview does not reproduce the supplied books or handbook.