Dehumidification Using Cooling Coils (Calculation Guide)
- nexoradesign.net
- Mar 14
- 7 min read
Updated: 2 days ago
Introduction
Dehumidification using cooling coils is one of the most important psychrometric processes in HVAC design, especially in hot and humid climates, hospitals, laboratories, commercial buildings, and ventilation systems with high outdoor air fractions. Engineers often face a practical challenge: a cooling coil may reduce air temperature, but unless the coil surface temperature is below the entering air dew point, it will not remove enough moisture.
In real projects, this issue directly affects indoor comfort, mold risk, condensation control, occupant health, and equipment reliability. A system may appear to have enough total cooling capacity, yet still fail to maintain indoor relative humidity because the latent load was underestimated or the cooling coil was selected only for sensible cooling.
This guide explains how dehumidification using cooling coils works, how to calculate moisture removal, and how engineers use psychrometrics, apparatus dew point, and bypass factor to evaluate coil performance. (Dehumidification Using Cooling Coils)
Definition :
Dehumidification using cooling coils is the HVAC process in which moist air is cooled below its dew-point temperature so that water vapor condenses on the coil surface. The air loses both sensible heat and latent heat, resulting in lower dry-bulb temperature and lower humidity ratio.
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What is Dehumidification Using Cooling Coils
A cooling coil in an air handling unit, rooftop unit, or fan coil unit is designed to transfer heat from air to chilled water or refrigerant. When the coil surface is cold enough, two processes occur at the same time:
Sensible cooling: air temperature decreases
Latent cooling: moisture condenses and is removed from the air stream
System purpose (Dehumidification Using Cooling Coils)
The main purpose is to control both temperature and humidity in the conditioned space or in the supply air stream.
Where it is used
Cooling coil dehumidification is commonly applied in:
comfort air conditioning systems
dedicated outdoor air systems
hospitals and clean spaces
museums and archives
food processing facilities
basements and underground spaces
schools and commercial offices in humid regions
Why engineers apply it
Engineers use this process because indoor humidity that is too high can lead to:
microbial growth
discomfort at otherwise acceptable temperatures
condensation on diffusers, ducts, and glazing
reduced indoor air quality
instability in process-controlled spaces
Engineering Principles
Dehumidification using cooling coils is governed by psychrometrics, heat transfer, and mass transfer.
1. Dew point and condensation
Condensation begins when the air contacting the coil surface is cooled below its dew point. If the coil surface temperature remains above the dew point, no moisture removal occurs.
2. Sensible and latent heat transfer
The coil removes total heat from the air:
Total Heat = Sensible Heat + Latent Heat
Sensible heat changes dry-bulb temperature
Latent heat removes moisture by phase change
3. Humidity ratio
The humidity ratio, usually expressed in kg water/kg dry air, is the main property used to quantify moisture removal. The difference between entering and leaving humidity ratio determines condensate rate.
4. Apparatus Dew Point (ADP)
The ADP is the effective coil surface temperature at which the process line on the psychrometric chart would intersect the saturation curve. It represents the coil condition required to produce the leaving air state.
5. Bypass Factor (BF)
Not all air contacts the coil surface equally. Some portion effectively bypasses full treatment. The coil bypass factor is:
BF = (t_leave - t_ADP) / (t_enter - t_ADP)
A lower bypass factor means better contact and stronger dehumidification.
6. Sensible Heat Ratio (SHR)
SHR indicates how much of the total coil load is sensible:
SHR = Sensible Heat / Total Heat
A low SHR means stronger latent performance, which is important in humid applications.
Step-by-Step Engineering Process
Step 1 – Determine entering air conditions
Establish the entering dry-bulb temperature and relative humidity, or use dry-bulb and wet-bulb values. From the psychrometric chart or software, determine:
humidity ratio
enthalpy
dew point
Example entering air condition:30°C DB, 60% RH
Approximate properties:
humidity ratio, W1 ≈ 0.016 kg/kg
enthalpy, h1 ≈ 71 kJ/kg
dew point, approximately 21.4°C
Step 2 – Define leaving air condition
Assume or calculate the leaving air state based on required supply conditions or selected coil performance.
Example leaving air condition:14°C saturated air
Approximate properties:
humidity ratio, W2 ≈ 0.010 kg/kg
enthalpy, h2 ≈ 39 kJ/kg
Because the leaving air is saturated, the coil is clearly performing both cooling and dehumidification.
Step 3 – Calculate moisture removal
Moisture removed from the air is based on the reduction in humidity ratio:
ṁwater = ṁair × (W1 - W2)Where:
ṁwater = condensate removal rate, kg/s
ṁair = dry air mass flow rate, kg/s
W1, W2 = entering and leaving humidity ratio, kg/kg dry air
If air flow rate is 2.5 kg/s:
ṁwater = 2.5 × (0.016 - 0.010) = 0.015 kg/s
That equals:
0.9 kg/min
54 kg/h
This is the actual dehumidification rate of the coil.
Step 4 – Calculate coil cooling load
The total cooling load across the coil is:
Q_total = ṁair × (h1 - h2)Using the example:
Q_total = 2.5 × (71 - 39) = 80 kW
Then estimate sensible load:
Q_sensible = 1.02 × ṁair × (T1 - T2)
Using SI air-side approximation:
Q_sensible = 1.02 × 2.5 × (30 - 14) = 40.8 kW
Then latent load is:
Q_latent = Q_total - Q_sensible = 80 - 40.8 = 39.2 kW
This shows nearly half the coil load is latent, which is common in humid outside air applications.
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Practical Engineering Example
Consider a fresh air handling unit serving a lobby in a coastal climate.
Design data:
Outdoor air: 32°C DB, 24°C WB
Supply air after coil: 13°C saturated
Dry air flow rate: 3.0 kg/s
Approximate psychrometric properties:
Entering humidity ratio, W1 ≈ 0.0185 kg/kg
Entering enthalpy, h1 ≈ 79 kJ/kg
Leaving humidity ratio, W2 ≈ 0.0093 kg/kg
Leaving enthalpy, h2 ≈ 36 kJ/kg
Moisture removal
ṁwater = 3.0 × (0.0185 - 0.0093) = 0.0276 kg/s
That is:
1.66 kg/min
99.4 kg/h
Total coil load
Q_total = 3.0 × (79 - 36) = 129 kW
Sensible load
Using 32°C to 13°C:
Q_sensible = 1.02 × 3.0 × (32 - 13) = 58.1 kW
Latent load
Q_latent = 129 - 58.1 = 70.9 kW
Engineering interpretation
This coil is performing significant dehumidification because:
leaving air is near saturation
coil surface temperature is below entering dew point
latent load is greater than half the total load
In practice, this would require proper condensate drain design, corrosion-resistant materials in coastal regions, and careful control valve sequencing to avoid humidity drift at part load.
Technical Comparison Table
Parameter | Sensible Cooling Only | Cooling with Dehumidification |
Coil surface temperature | Above air dew point | Below air dew point |
Dry-bulb temperature | Decreases | Decreases |
Humidity ratio | No significant change | Decreases |
Condensate formation | No | Yes |
Latent heat removal | Negligible | Significant |
Supply air RH | May remain high | Usually near saturation at coil leaving |
Best application | Dry climates or low latent loads | Humid climates or high ventilation loads |
Drain pan required | Not critical | Essential |
Psychrometric path | Horizontal left | Downward left |
Risk if misapplied | High indoor humidity | Possible overcooling if not controlled |
Advantages
Cooling coil dehumidification offers several engineering benefits:
provides combined temperature and humidity control
supports ventilation air pretreatment
reduces indoor condensation risk
improves comfort in humid climates
helps maintain IAQ and building durability
works with chilled water or DX systems
integrates well with AHUs, DOAS, and packaged systems
For many commercial buildings, it is the most practical and economical first-stage humidity control method.
Read related topic :
Humidity Control in HVAC: Calculations & Design Guide
Hospital Humidity Control Calculations in HVAC Design
Common Engineering Mistakes
Engineers often run into performance problems because of avoidable design assumptions.
1. Ignoring latent load
Sizing a coil only from room sensible load can lead to under-dehumidification.
2. Using dry-bulb temperature alone
Air leaving temperature does not tell the full story. Humidity ratio and enthalpy must also be checked.
3. Assuming the coil leaving air is always saturated
Real coils may have a nonzero bypass factor. Leaving air may not be exactly on the saturation curve.
4. Not checking apparatus dew point
Without ADP evaluation, the selected coil may not achieve the required moisture removal.
5. Poor condensate management
Improper drain slope, trap design, or pan insulation can create water carryover and hygiene issues.
6. Part-load control problems
At reduced chilled water flow or high supply air temperature reset, the coil may lose latent capacity.
Future Trends
Cooling coil dehumidification is evolving with better controls and digital system integration.
Smart latent control
Modern BAS platforms increasingly monitor dew point, humidity ratio, and coil valve position instead of relying only on dry-bulb control.
Dedicated outdoor air systems
DOAS units are becoming more common because they separate ventilation latent treatment from zone sensible conditioning.
AI-assisted optimization
Advanced analytics can predict when a coil is drifting from expected latent performance due to fouling, valve issues, or sensor error.
Digital twin integration
Digital twins allow facility teams to compare expected and actual coil behavior, including moisture removal and condensate trends.
High-performance coils
Improved fin geometry, lower bypass factor designs, and better hydrophilic coatings can increase latent capacity while reducing pressure drop.
FAQ Section
1. When does a cooling coil start dehumidifying air?
A cooling coil starts dehumidifying when its effective surface temperature drops below the dew-point temperature of the entering air.
2. How do you calculate moisture removed by a cooling coil?
Use the dry air mass flow rate multiplied by the difference between entering and leaving humidity ratio:ṁwater = ṁair × (W1 - W2)
3. Why is leaving air often close to saturation?
Because air in direct contact with the cold coil surface approaches the apparatus dew point, which lies on or near the saturation line.
4. What is the role of bypass factor in coil calculation?
Bypass factor shows how much air is not fully treated by the coil. Lower bypass factor means colder leaving air and better dehumidification.
5. Can a coil provide humidity control at part load?
Yes, but only if the control strategy maintains sufficiently low coil temperature. Otherwise the system may satisfy sensible load while failing to remove latent load.
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Conclusion
Dehumidification using cooling coils is more than a temperature reduction process. It is a combined sensible-latent treatment that depends on dew point, humidity ratio, enthalpy difference, apparatus dew point, and coil bypass factor. For accurate HVAC design, engineers must evaluate both total cooling and moisture removal instead of relying on dry-bulb temperature alone.
In humid buildings and outdoor air systems, a well-selected cooling coil can remove large amounts of water vapor while maintaining stable supply air conditions. The key calculation steps are straightforward: determine entering and leaving psychrometric properties, calculate humidity ratio difference, estimate condensate rate, and verify total, sensible, and latent load performance.
Author Note :
Nexora Design Lab publishes engineering insights on HVAC design, MEP systems, and sustainable building technologies used in modern construction projects.
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