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Lesson 38 · Sensible heat ratio
The share, not the split
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The share, not the split

A cooling coil does two jobs at once, and the balance between them decides whether a room ends up comfortable or merely cold and damp. That balance has a name: the sensible heat ratio.

SHR=Q˙sQ˙s+Q˙l\mathrm{SHR} = \dfrac{\dot{Q}_s}{\dot{Q}_s + \dot{Q}_l} — read aloud S-H-R equals Q-sensible over Q-sensible plus Q-latent. Q˙s\dot{Q}_s is the sensible load, the temperature-changing part, in kilowatts; Q˙l\dot{Q}_l is the latent load, the moisture-removing part, in the same kilowatts; and their sum is the total load. SHR is a power over a power, so every unit cancels and it travels naked, always between 0 and 1.

The denominator is where the marks are lost. Sensible over LATENT is a different number, it has no name, and it is usually greater than 1 — which is the tell. If your SHR ever lands above 1 you have divided by the wrong thing, because the sensible part can never be more than the whole.

What the number means, in a sentence: a dry office in a cold climate might run SHR 0.9 — nearly all the work is temperature. A packed restaurant in July runs 0.65, and a coil selected for 0.9 will hit setpoint long before it has taken the moisture out, then switch off and leave the room at 24 °C and clammy. Comfort is not one number.

QsQlSHR

SHR=Q˙sQ˙s+Q˙l\mathrm{SHR} = \frac{\dot{Q}_s}{\dot{Q}_s + \dot{Q}_l}

  • SHR\mathrm{SHR}= Sensible heat ratio
  • Q˙s\dot{Q}_s= Sensible load (power)
  • Q˙l\dot{Q}_l= Latent load (power)
Sensible Heat Ratio (SHR) solver →
V̇ΔTQs

Q˙s=ρacaV˙ ΔT\dot{Q}_s = \rho_a c_a \dot{V} \, \Delta T

  • Q˙s\dot{Q}_s= Sensible heat rate (power)
  • V˙\dot{V}= Airflow (volumetric flow rate)
  • ΔT\Delta T= Dry-bulb ΔT (temperature difference)
Air Sensible Heat (1.08 Rule) solver →
V̇ΔhQt

Q˙t=ρaV˙ Δh\dot{Q}_t = \rho_a \dot{V} \, \Delta h

  • Q˙t\dot{Q}_t= Total heat rate (power)
  • V˙\dot{V}= Airflow (volumetric flow rate)
  • Δh\Delta h= Enthalpy change (specific latent heat)
Air Total Heat (4.5 Rule) solver →