What is the difference between sensible and latent load?

Sensible load Q_s is the heat needed to lower the air temperature: Q_s = m_a * 1.006 * (T_in - T_out). Latent load Q_L is the heat needed to condense water vapor from the air: Q_L = m_a * (w_in - w_out)/1000 * 2501. Total load Q_t is the sum and represents the total heat the coil removes from the air.

How is the sensible heat factor SHF used?

SHF = Q_s / Q_t is the fraction of the total load that is sensible. Typical office spaces run around 0.7, while humid kitchens with heavy outdoor-air loads can drop below 0.5. A lower SHF means dehumidification dominates, requiring lower chilled-water temperatures or more coil rows. On a psychrometric chart, the SHF is the slope of the process line between the room and supply states.

Why is the outlet relative humidity fixed at 95%?

Cooling-and-dehumidifying coils cool the air down to the surface (apparatus dew point) temperature, so the outlet air is nearly saturated. Real equipment does not reach full saturation due to a bypass factor, but RH_out = 90-95% is a standard design value. This tool uses RH_out = 95% for simplicity. For detailed selection you should apply the manufacturer's bypass factor (typically 0.05-0.20).

How do I size the drain pipe from the dehumidification rate?

The computed dehumidification rate dW (kg/h) is the continuous condensate the coil produces. To handle it safely, the drain pan capacity, drain pipe diameter (typically 25 mm or larger nominal size) and trap height must be sized accordingly. For dW = 30 kg/h, 30 liters per hour of condensate is generated, so a slope of at least 1/100 and cleanout access for clog prevention are required.

Cooling Coil Load Simulator — Sensible, Latent & Total Heat

Parameters

Inlet dry-bulb T_in

°C

Inlet relative humidity RH_in

Outlet dry-bulb T_out

°C

Airflow V

m³/min

Outlet relative humidity is fixed at RH_out=95% (saturation near the coil surface) and atmospheric pressure at p_atm=1013 hPa.

Results

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Sensible load Q_s

—

Latent load Q_L

—

Total load Q_t

—

Dehumidification dW

—

Sensible heat factor SHF

—

Air mass flow rate m_a

Simplified Psychrometric Chart & Cooling Process

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

Theory & Key Formulas

From the saturation vapor pressure e_s(T) and relative humidity RH we obtain the vapor pressure and humidity ratio w, then the specific enthalpy h of moist air per kg of dry air.

Saturation vapor pressure (Magnus formula):

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

Humidity ratio (per kg of dry air):

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

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:

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

Constants: c_p=1.006 kJ/(kg·K), L_v≈2501 kJ/kg, ρ≈1.2 kg/m³. The dehumidification rate is dW = m_a(w_in-w_out)/1000 × 3600 [kg/h].

What is the Cooling Coil Load Simulator?

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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?

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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.

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Oh really! So on a humid day the coil has to work much harder?

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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.

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What is the "sensible heat factor SHF" shown in the results?

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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.

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Doubling the airflow doubles the load. That feels intuitive.

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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".

Cooling Coil Load Simulator — Sensible, Latent & Total Heat

What is the Cooling Coil Load Simulator?

Frequently Asked Questions

Real-World Applications

Common Pitfalls and Notes

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