L6 · HVAC Loads and Psychrometrics

Foundation lesson · 20 minute study session

Correct the air basis before using a familiar constant

At elevation or low temperature, a familiar sea-level coefficient can describe the wrong mass flow. Identify pressure, density and actual versus standard cfm first.

By the end of this lesson

Use the handbook 5,000-ft sensible coefficient.
Distinguish ACFM from SCFM.
Select a pressure- and temperature-appropriate psychrometric chart.

Actual versus standard flow

Actual cfm describes volume at the operating state. Standard cfm expresses the volume that the same mass flow would occupy at a defined reference state. The reference temperature, pressure and moisture convention must be known. Correcting SCFM a second time for the actual density can count the same adjustment twice. $C_s$ changes with the stated density and flow basis; confirm it before solving for $\dot V$ .

Elevation changes mass per volume

For a given actual volumetric flow, lower pressure generally gives lower dry-air mass flow. The supplied handbook gives $C_s$ = 0.92 at 5,000 ft, compared with 1.10 at sea level, for its stated simplified load calculation. General mass-flow equations remain the better starting point when specific volume is supplied.

Low-temperature processes

A cold-room or freezing process may require a low-temperature chart and appropriate ice/water property basis. Do not extend an ordinary comfort-conditioning chart beyond its range. For stored products, distinguish cooling above freezing, latent freezing duty and cooling below freezing.

A decision map for this lesson

Match the coefficient to the measured air

Keep actual volume, dry-air mass, pressure, temperature and phase on the problem’s stated basis.

Choose the governing relationship and catch the wrong route
Condition or question	Governing decision	Error to avoid
Altitude changes while actual cfm is specified	Use the supplied elevation coefficient or actual-state mass conversion.	Using the sea-level coefficient unchanged.
A chart reports specific volume per pound of dry air	Convert volume to dry-air mass using the matching pressure and state.	Inverting moist-air density without correcting the mass basis.
Air is below freezing	Use the supplied low-temperature properties and the appropriate ice/frost model.	Combining liquid-water saturation data with an ice-phase energy balance.
Standard flow is compared with actual flow	Align their reference pressure and temperature before comparing volumes.	Assuming identical units mean identical flow-reference conditions.

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

Relationships and their limits

Airflow for sensible duty

\dot V=\dot Q_s/(C_s\Delta T)

Use the coefficient and actual-flow convention stated in the problem or handbook.

Mass-flow equivalence

\rho_{actual}\dot V_{actual}=\rho_{standard}\dot V_{standard}

Both densities must use the same mass basis; moist-air and dry-air bases cannot be mixed.

Symbols and unit basis
Symbol	Meaning	Units or basis
$\dot V$	Airflow on stated basis	cfm
$C_s$	Altitude-appropriate sensible coefficient	Btu/(h·cfm·°F)
$\rho$	Density on stated mass basis	lb/ft³
$\Delta T$	Air temperature difference	°F difference
$\dot Q_s$	Sensible heat rate	Btu/h

One decision, fully worked

Apply the method.

At 5,000 ft, a sensible duty of 46,000 Btu/h must be served with a 20°F temperature difference. The problem specifies actual cfm and the handbook coefficient $C_s$ = 0.92. Find the airflow and compare the sea-level shortcut.

Select 0.92 because the problem gives 5,000 ft and the matching flow basis.
Divide sensible duty by the product of coefficient and temperature difference.
Calculate the sea-level result only as an error check.

\dot V=46{,}000/[0.92(20)]=2{,}500\ \mathrm{cfm}

\dot V_{incorrect}=46{,}000/[1.10(20)]\approx2{,}091\ \mathrm{cfm}

Decision

The required actual airflow is 2,500 cfm.

Independent check

The sea-level shortcut is about 16% below the required flow, consistent with lower mass per actual cubic foot at elevation.

Specialist extension

Recognize what an altitude correction holds fixed

At fixed actual volume, a lower air density delivers less mass. Sensible capacity therefore changes with density when specific heat and temperature difference are held fixed. At fixed mass flow, actual volume changes instead. Do not apply a density correction to a standard-flow factor and then correct the same mass basis a second time.

A new applied case

A sensible process has 30,000 Btu/h capacity at one air density. Density falls to 0.80 of the original while actual cfm, specific heat and temperature change remain fixed.

\dot Q_2/\dot Q_1=\rho_2/\rho_1=0.80

\dot Q_2=0.80(30{,}000)=24{,}000\ \mathrm{Btu/h}

Decision. Available sensible capacity is 24,000 Btu/h for these fixed conditions.

Cross-check. If the task instead requires the original duty at the same ΔT, volume would need to increase by 1/0.80, assuming the equipment can deliver it.

Unit basis. ρ is mass density on a compatible air basis; actual volume is cfm; cp and ΔT stay fixed.

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-L6

Try a new decision.

Select one

A ventilation process must deliver 9,000 lb dry air/h. Supplied specific volumes are 15 ft³/lb dry air at the actual inlet condition and 13.5 ft³/lb dry air at the stated standard condition. The actual and standard volumetric flows, respectively are most nearly:

A2,250 actual cfm; 2,025 standard cfmB2,025 actual cfm; 2,250 standard cfmC135,000 actual cfm; 121,500 standard cfmD600 actual cfm; 667 standard cfm

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

Catch the common errors

Using 1.10 at 5,000 ft without a stated standard-flow justification.
Applying a density factor to an already mass-equivalent SCFM value.
Reading properties outside a chart temperature or pressure range.

Handbook navigation drill

Find it, then explain it.

Search: Elevation Corrections

Locate the sea-level and 5,000-ft coefficients and compare with the air density correction table.

Check your navigation

You can explain why the higher-elevation actual airflow is greater for the same sensible duty.

Reference path: §7.2–7.3, pp. 388–391; §9.1.8, p. 456. These are printed handbook page numbers; section names and search terms remain the primary navigation method. See technical review status.

Deeper application · Decision Lab

Cold air: separate mass basis from frost-point lookup

A familiar sea-level shortcut is not a substitute for the stated cold-air properties. A below-freezing vapor lookup also needs the correct saturation reference.

Choose the method, predict the changed-condition result and inspect the complete cross-check. Sign in before starting to keep your first response and later reflection in the separate reasoning notebook.

Open the original workshop →

Theory sources

NCEES handbook v2.1, §§7.2–7.3 and 9.1.7–9.1.8.

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