2.2 Air-Side Sensible, Latent, and Total Heat Transfer

Air-Side Heat Transfer

Choose the heat quantity before choosing the equation

A TAB heat-transfer calculation is an energy-balance estimate built from measured airflow and measured air-state change. First identify whether the question asks for sensible heat, total heat, or latent heat. Then confirm that the entering and leaving readings represent the same airstream and stable operating period.

Sensible heat changes dry-bulb temperature. Latent heat is associated with moisture change. Total heat is the sum of sensible and latent effects and is most directly calculated from an enthalpy difference. Cooling and dehumidification can involve all three; an electric heater with no moisture addition is essentially sensible.

State the sign convention. For capacity magnitude, use the temperature or enthalpy change in the heat-transfer direction. For diagnostic work, retaining positive and negative signs can make an unexpected process obvious.

Standard-air sensible heat

For standard air, the familiar shortcut is:

Qs = 1.08 × CFM × ΔT

where Qs is sensible heat in Btu/h, CFM is volumetric airflow, and ΔT is the dry-bulb temperature difference in degrees Fahrenheit. The factor 1.08 comes from approximately 60 min/h × 0.075 lb/ft³ × 0.24 Btu/(lb·°F).

Example: 5,000 CFM is heated from 55°F to 85°F. The temperature rise is 30°F:

Qs = 1.08 × 5,000 × 30 = 162,000 Btu/h

The result is only as reliable as the representative airflow and temperatures. A single leaving-air point immediately downstream of a heater or cooling coil may be affected by stratification. Use the project procedure and enough measurement locations to characterize the plane.

Total and latent heat

The official NEBB formula chart gives the standard-air total-heat relationship:

Qt = 4.5 × CFM × Δh

Here Δh is the enthalpy difference in Btu/lb of dry air. The 4.5 factor is approximately 60 min/h × 0.075 lb/ft³. Enthalpy must come from valid psychrometric measurements or a chart/calculation at the applicable barometric pressure.

For latent heat, NEBB lists two equivalent forms depending on the humidity-ratio units:

Ql = 0.69 × CFM × ΔW, when ΔW is grains of moisture per pound of dry air

Ql = 4,840 × CFM × ΔW, when ΔW is pounds of moisture per pound of dry air

Do not mix the two humidity-ratio units. There are 7,000 grains per pound; that unit conversion explains why the numerical factors differ. Small rounding differences between total heat and sensible plus latent heat can occur because the standard constants are rounded, but a material mismatch calls for a unit and measurement review.

Correcting for nonstandard density

The 1.08 and 4.5 constants assume standard density. Air density changes with barometric pressure, dry-bulb temperature, and moisture content. When the required accuracy or project procedure calls for correction, calculate mass flow rather than applying a memorized altitude threshold:

Qs = 60 × ρ × Cp × CFM × ΔT

Qt = 60 × ρ × CFM × Δh

Use density ρ and specific heat Cp on a consistent basis and document the state used. A density correction affects both the heat calculation and any velocity-to-standard-flow conversion. Avoid correcting the same value twice.

Measurement boundary and timing

A valid coil calculation requires a defined boundary. Confirm which airflow crosses the coil and whether leakage, bypass, outdoor-air mixing, fan heat, or condensate changes occur between measurement planes. Entering wet-bulb or humidity readings must be paired with entering dry-bulb readings from the same condition; the same applies at the leaving plane.

Record fan state, outdoor-air condition, valve or heater command, airflow method, barometric basis, and time. Allow the system to stabilize, then collect simultaneous or closely coordinated observations. Averaging an unstable temperature without recording the variation can hide cycling controls.

On a cooling coil, leaving-air temperatures may be highly nonuniform. Traverse the applicable plane or use a procedure that accounts for stratification. Protect humidity sensors from liquid water and radiation effects, and allow them to stabilize.

Cross-checks and diagnostics

Compare sensible heat with total heat: the magnitude of total cooling should not be less than the sensible component when both are calculated on the same stable boundary. Compare Qt with Qs + Ql using consistent units. If the values do not reconcile, check humidity-ratio units, chart reading, density basis, airflow boundary, sensor placement, and timing before diagnosing equipment.

An air-side result may also be compared with a hydronic-side estimate. The two will rarely match perfectly because both have measurement uncertainty and the boundaries may include fan heat, casing loss, leakage, storage, or condensate effects. Use the comparison as evidence, not as proof that one particular defect exists.

A defensible worksheet preserves raw dry-bulb, humidity or wet-bulb, pressure, airflow, and time data; shows the selected formula and units; states whether standard or actual air was used; and records uncertainty or limitations. That record lets a reviewer reproduce the calculation instead of trusting only a final capacity number.