OpenSpecEngineering

Finds the heat an airflow gains or loses as it passes through a coil, a heater or a room. Sensible heat changes the air’s temperature, latent heat its moisture content, and total heat its enthalpy [1]. HVAC designers use it to size coils and check equipment capacity.

Equations

q_s = ρ·Q·c_p·ΔT

q_L = ρ·Q·h_fg·(W₁ − W₂)

q_t = ρ·Q·(h₁ − h₂)

Q is the volumetric airflow and ρ the air density, so ρ·Q is the mass flow of air. c_p is the specific heat of the air and ΔT its temperature change. W₁ and W₂ are the entering and leaving humidity ratios, the mass of water vapor per mass of dry air. h_fg is the latent heat of water vapor, 2501 kJ/kg by default [1]. h₁ and h₂ are the entering and leaving enthalpies. The Sensible + latent mode adds q_s and q_L to give q_t, and the sensible heat ratio SHR = q_s/q_t, which a cooling coil must match.

Pick an Air preset to fill in ρ and c_p. The dry-air presets are at 20 °C and the standard-atmosphere pressure for the site elevation [1]; the hot-air presets come from [3]. Higher sites have thinner air, so the same airflow carries less heat.

Limits and common mistakes

• Use the density of the air where Q is measured. Supply air at 13 °C is denser than return air at 24 °C.
• With latent heat, every change is entering minus leaving, so a positive result is heat removed, as in cooling. A negative latent load means the air gained moisture.
• Enter humidity ratios and enthalpies read from a psychrometric chart, not relative humidity.
• The SHR is defined only when both loads are removed from the air.

Worked example

A cooling coil at a sea-level site handles 2000 CFM. Air enters at 26.7 °C with W = 11.1 g/kg and leaves at 13.9 °C with W = 9.3 g/kg. What load must the coil carry?

1. Choose Heat to Calculate Sensible + latent — total and sensible heat ratio, and enter Airflow Rate 2000 CFM.
2. Choose Air Dry air, 20°C, sea level. The density fills in as 1.204 kg/m³ and c_p as 1006 J/(kg·K).
3. Enter Temperature Drop 12.8 °C, Entering Humidity Ratio 11.1 g/kg and Leaving Humidity Ratio 9.3 g/kg. Keep h_fg at 2501 kJ/kg and press Solve. The Solution shows, among other rows: Sensible Heat Load (qs): 14.6338 kW
   Latent Heat Load (qL): 5.11607 kW
   Total Heat Load (qt): 19.7499 kW
   Sensible Heat Ratio (SHR = qs/qt): 0.740957
   Total heat (tons): 5.61579 ton

Check with the US rules of thumb: q_s = 1.08 × CFM × ΔT(°F) = 1.08 × 2000 × 23.04 = 49,800 BTU/hr, against the page’s 49,932.7. q_L = 4840 × CFM × ΔW = 4840 × 2000 × 0.0018 = 17,400 BTU/hr, against 17,456.7.