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Thermal & HVAC Simulator

Cooling Coil Load Simulator — Sensible, Latent & Total Heat

From inlet and outlet air states, instantly compute the sensible, latent and total heat loads, dehumidification rate and sensible heat factor SHF of a cooling coil. The cooling process is plotted on a simplified psychrometric chart so you can build engineering intuition for HVAC design.

Parameters
Inlet dry-bulb temperature Tin
°C
Inlet relative humidity RHin
%
Outlet dry-bulb temperature Tout
°C
Airflow V
m³/min

Outlet relative humidity assumes near-saturation at the coil surface and is fixed at RHout=95%; atmospheric pressure p is fixed atatm=1013 hPa.

Results
—
Inlet T / RH
—
Outlet T / RH
—
Inlet absolute humidity w
—
Outlet absolute humidity w
—
Sensible load Q_s
—
Latent load Q_L
—
Total load Q_t
—
Sensible heat factor SHF

Left: warm, humid air (red) is cooled and dehumidified by the coil, becoming cool, dry air (blue) on the right. Bottom bar: proportions of sensible heat (red) and latent heat (blue). Condensate drips from the coil below the dew point.

Simplified Psychrometric Chart & Cooling Process
—
Sensible load Q_s
—
Latent load Q_L
—
Total load Q_t
—
Dehumidification dW
—
Sensible heat factor SHF
—
Air mass flow rate m_a
Theory & Key Formulas

Horizontal axis = dry-bulb temperature T (°C); vertical axis = humidity ratio w (g/kg DA). Red dot = inlet (In), blue dot = outlet (Out). Horizontal component = sensible heat change, vertical component = latent heat change, blue curve = saturation curve.

Theory & Key Formulas

Saturation vapor pressure es(T) and relative humidity RH are used to determine the water-vapor partial pressure and humidity ratio w, then calculate enthalpy h on a dry-air basis.

Saturation vapor pressure (Magnus equation):

$$e_s(T) = 6.112\,\exp\!\left(\frac{17.62\,T}{243.12+T}\right)\ \text{[hPa]}$$

Humidity ratio (dry-air basis):

$$w = 622\,\frac{e}{p_\text{atm}-e}\ \text{[g/kg DA]}$$

Specific enthalpy of moist air:

$$h = 1.006\,T + \frac{w}{1000}\,(2501 + 1.86\,T)\ \text{[kJ/kg DA]}$$

Air mass flow rate and heat loads:

$$m_a = V \cdot \rho \approx V \cdot 1.2\ \text{[kg/s]}$$ $$Q_s = m_a\,c_p\,(T_\text{in}-T_\text{out}),\quad Q_L = m_a\,\frac{w_\text{in}-w_\text{out}}{1000}\,L_v$$ $$Q_t = m_a\,(h_\text{in}-h_\text{out}) \approx Q_s + Q_L,\quad \text{SHF} = \frac{Q_s}{Q_t}$$

Here, cp=1.006 kJ/(kg·K)、Lv≈2501 kJ/kg and ρ≈1.2 kg/m³ are used. The dehumidification rate is dW = ma(win-wout)/1000 × 3600 [kg/h].

What is the Cooling Coil Load Simulator?

🙋
When picking an air conditioner in summer, the rating says something like "X kW". Isn't that just the heat needed to drop the temperature?
🎓
Good question. The heat a cooling coil removes from the air actually splits into two parts: the "sensible" heat that lowers the temperature and the "latent" heat that condenses water vapor. Roughly speaking, cooling humid summer air requires as much energy for dehumidification as for cooling itself. With the defaults (28°C / RH 65% inlet, 16°C outlet, 100 m³/min), you can see Q_s and Q_L come out nearly equal.
🙋
Oh really! So on a humid day the coil has to work much harder?
🎓
Exactly. Try pushing the inlet RH from 65% to 85%. Q_L (latent load) and dW (dehumidification rate) jump up sharply. On the psychrometric chart the vertical distance between the red (inlet) and blue (outlet) dots gets larger — that means the coil is wringing out a lot of water. In the rainy season or in tropical climates, the air feels cool only slowly because the coil is busy dehumidifying.
🙋
What is the "sensible heat factor SHF" shown in the results?
🎓
SHF = Q_s / Q_t is the share of the coil's work that goes into temperature reduction. Typical offices run around 0.7, but humid kitchens or systems with heavy outdoor-air intake can drop below 0.5. A low SHF means you need colder chilled water or more coil rows. In real design you overlay the SHF (slope of the process line) with the coil characteristic on the psychrometric chart to select equipment.
🙋
Doubling the airflow doubles the load. That feels intuitive.
🎓
Right, Q_t = m_a (h_in - h_out) is linear in airflow. But you can't just shrink airflow to save energy because ventilation requirements and a minimum supply temperature (usually not below 14°C to avoid condensation) constrain it. The practical workflow is: required cooling -> airflow -> outlet temperature, then check SHF and the process line on the chart. This tool is great for building first-cut intuition.

Frequently Asked Questions

Physically that would be heating, so the tool clamps any negative load to zero. The cooling-coil model is out of scope in that case. The sliders are constrained to inlet 20–40°C and outlet 10–25°C, but if you push the inlet below the outlet the displayed loads simply become zero.
In real coil selection the bypass factor (BF ≈ 0.05–0.20) prevents the outlet from reaching full saturation. This tool uses an idealized RH_out = 95% model for early design exploration. For detailed selection you should use the manufacturer's BF, derive the apparatus dew point (ADP) and recompute the loads with the bypass mixing model.
The enthalpy method Q_t = m_a (h_in - h_out) is exact, including the temperature dependence of water-vapor heat capacity and latent heat. Q_s + Q_L uses the dry-air heat capacity and a constant latent heat, which is an approximation. The two typically differ by 2–3%, which is negligible for design. The tool computes Q_t from enthalpy and Q_s, Q_L separately, so a small mismatch appears.
Only as a first estimate. Final selection also needs the coil rows, fin pitch, chilled-water inlet temperature, water flow rate, bypass factor, and air- and water-side pressure drops. Typical practice is to grasp Q_t and dW with this tool, then feed those into a vendor selection program. If the SHF is very low (below 0.5) you should also consider reheating or low-temperature supply-air designs.

Real-World Applications

HVAC design for offices and commercial buildings: Sizing the cooling coil from outdoor and indoor design conditions is a fundamental task. For example, when outdoor air at 32°C / RH 70% is conditioned to maintain 26°C / RH 50% indoors, this tool gives a quick estimate of the outdoor-air coil's total load and dehumidification rate, useful for chiller sizing and annual energy estimates. In humid climates where the room SHF falls below 0.6, the need for reheat after cooling-and-dehumidification can also be evaluated.

Precision cooling for data centers and server rooms: Server heat is mostly sensible (SHF ≈ 1.0), but the outdoor-air intake brings latent load. By separating Q_s and Q_L of the outdoor-air portion with this tool, you can tune CRAC chilled-water set points and dehumidification operating modes. It also helps decide whether raising the supply-air temperature to avoid condensation is feasible.

Food factories and clean rooms: Food processing requires tight humidity control, so latent capacity dominates. Knowing dW (kg/h) directly drives drain-pipe diameter, slope and drain-pan capacity. Clean rooms may further evolve into combined chilled-water-coil + desiccant-rotor designs, but the load split here is the starting point.

Boundary conditions for HVAC CFD: The h_out and w_out from this tool can be used directly as supply-air boundary conditions in a CFD model of room airflow. Comparing the tool's Q_t and SHF with CFD-derived loads also validates wall heat transfer and occupant heat sources in the model. A 1-D enthalpy balance check before running CFD is standard engineering hygiene.

Common Pitfalls and Notes

The most common mistake is to believe that "coil capacity is set by temperature difference alone". In humid air, latent load can equal or exceed sensible load, so the true requirement can be nearly double what the temperature difference suggests. Comparing inlet humidity of 50% vs. 90% in this tool shows that the total load Q_t changes dramatically even when the temperature drop is the same. A sizing based only on "temperature × airflow × cp" will be undersized in humid weather.

Next is the mismatch between room SHF and coil characteristic. If the room load's SHF (say 0.85) does not match the coil's apparatus-dew-point SHF (say 0.70), the system over- or under-dehumidifies and the room humidity drifts away from the design value. After checking the Q_s/Q_L split here, consider adding a reheat coil or reducing outdoor-air intake when SHF is too low. In real design you overlay the slope of the process line with that of the coil characteristic on the chart.

Finally, the assumption that "outlet air is always saturated". This tool fixes RH_out = 95% for early-stage estimates, but real coils never reach full saturation because of the bypass factor (typical 0.05–0.20). With coils that have a large bypass factor (few rows or high face velocity), outlet humidity stays higher and the actual dehumidification is smaller than this tool predicts. Final selection always needs a detailed calculation that includes the manufacturer's BF. Use this tool for "ballpark numbers and intuition building".

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How to Use

  1. Set inlet air temperature (°C) using the slider; typical range 25–35°C for return air in commercial buildings.
  2. Adjust inlet relative humidity (%) with the slider; residential cooling usually operates 40–60% RH.
  3. Enter outlet air temperature (°C) via the slider; design practice maintains 10–15°C temperature drop across coil.
  4. Input volumetric airflow rate (m³/s) using the slider; office spaces typically require 0.15–0.25 m³/s per occupant.
  5. Read outputs: sensible load Q_s (W), latent load Q_L (W), total load Q_t (W), dehumidification rate dW (g/s), SHF ratio, and mass flow m_a (kg/s).

Worked Example

Office space return air enters cooling coil at 28°C, 65% RH, with 1.2 m³/s airflow. Coil outlet set to 13°C. Assuming standard atmospheric pressure (101.325 kPa) and density ≈1.15 kg/m³, air mass flow m_a = 1.38 kg/s. Inlet humidity ratio ≈12.5 g/kg, outlet ≈4.8 g/kg (at 13°C, saturated ~5.4 g/kg). Sensible load Q_s ≈ 1.38 × 1.006 × (28–13) = 20.7 kW. Latent load Q_L ≈ 1.38 × 2450 × (12.5–4.8)/1000 = 20.8 kW. Total load Q_t ≈ 41.5 kW. SHF = 20.7/41.5 ≈ 0.50. Dehumidification rate ≈ 1.38 × (12.5–4.8) × 1000/3600 ≈ 2.9 g/s.

Practical Notes

  1. SHF below 0.65 indicates high latent load—common in humid climates (Florida, Southeast USA); verify coil face velocity stays below 2.5 m/s to prevent carryover.
  2. Design coils with outlet dew point 2–3°C above apparatus dew point to avoid coil freeze-up in low ambient or part-load conditions.
  3. Latent load dominates in data centers (SHF ~0.8+); sensible load dominates in dry climates (SHF ~0.3–0.4)—adjust coil rows and fin spacing accordingly.
  4. Monitor dehumidification rate: 2–3 g/s removal is typical for 10–20 kW cooling; rates below 0.5 g/s suggest insufficient surface area or high outlet RH.