Chilled Water Coil Performance Simulator

A chilled-water coil is a heat exchanger inside an air-handling unit (AHU) that cools air by passing it over a bundle of finned tubes carrying chilled water. Chilled water near 7 °C, produced by the chiller, flows through the coil; air touching the fins gives up heat and its temperature drops. This is sensible cooling. If the coil surface falls below the air's dew point, water vapor condenses on the fins and drains away. The heat removed during condensation is the latent (dehumidification) load, which is what lets a coil both cool and dry humid summer air.

This simulator draws, in real time, air flowing left to right through the coil rows and cooling down, while chilled water warms up in counterflow. Condensate drips off the lower fins once the air falls below its dew point, and the leaving temperature and relative humidity, total capacity (sensible + latent), water-side ΔT, LMTD, and bypass factor all update together.

The energy balance equates the air side and water side. With air capacity rate $C_a=\dot m_a c_{pa}$ and water capacity rate $C_w=\dot m_w c_{pw}$, let $C_{min}=\min(C_a,C_w)$ and $C_r=C_{min}/C_{max}$. The counterflow effectiveness is $\varepsilon=\dfrac{1-e^{-NTU(1-C_r)}}{1-C_r e^{-NTU(1-C_r)}}$ with $NTU=UA/C_{min}$.

Sensible capacity is $Q_{sens}=1.21\,\dot V\,(T_{in}-T_{out})$ ($\dot V$ in L/s; the 1.21 factor is the volumetric heat capacity of standard air). Latent capacity follows the drop in humidity ratio, $Q_{lat}=\dot m_a\,h_{fg}\,(w_{in}-w_{out})$ ($h_{fg}\approx2501\,\mathrm{kJ/kg}$). Whether dehumidification occurs depends on whether the apparatus dew point $T_{ADP}$ (≈ coil surface temperature) is below the inlet air dew point. The bypass factor $BF=(T_{out}-T_{ADP})/(T_{in}-T_{ADP})$ measures the fraction of air that slips past the coil; the smaller it is, the closer the outlet approaches the apparatus dew point. The water temperature rise is $\Delta T_w=Q/C_w$, and the driving force is evaluated with the log-mean temperature difference (LMTD).

In the animation, air shifts from orange toward blue as it passes the coil rows (temperature drop), and condensate appears on the lower fins once it falls below the dew point. The counterflow chilled water (arrow at the bottom) is warmest near the outlet.

In the capacity bars, compare the heights of the sensible (blue) and latent (cyan) bars, i.e. the SHR. SHR = 1 means sensible only; the lower it is, the more dehumidification dominates.

On the psychrometric chart, the direction from the inlet point to the outlet point reveals "cooling only" (horizontal left) versus "cooling plus dehumidification" (sloping down-left).

First-pass selection of AHU chilled-water coils for offices and commercial buildings (sizing required rows, UA, and water temperature).

Checking dehumidification capacity in humid summers (whether sub-dew-point cooling handles the latent load).

Diagnosing capacity loss in existing coils (separating fin fouling / UA loss from outlet-temperature rise and lost dehumidification).

Evaluating shoulder-season energy savings (the effect of raising water temperature to switch to sensible-only operation).

"Outlet temperature = water temperature" is wrong. The bypass factor and effectiveness limits always keep the outlet above the apparatus dew point. With 7 °C water, an outlet near 10 °C is normal.

Ignoring latent heat badly underestimates capacity. In humid air, latent can be nearly half the total; sizing on sensible alone leaves both the coil and the chiller short.

This is a simplified model. The apparatus dew point is a surface-temperature approximation, and air is assumed at standard density 1.2 kg/m³ and specific heat 1.006 kJ/(kg·K). Correct with measured or vendor data at high altitude, high humidity, low water velocity, or special fins.