KEVOS® MAKEOS · Engineering
Cooling Coil Load with Dehumidification (Psychrometric Balance, Condensate, Sensible Heat Ratio)
What total cooling load, condensate flow and sensible heat ratio does it take to cool and dehumidify this air flow from the inlet to the outlet state, and can the coil you have deliver it?
Steady-flow mass and energy balance on a cooling coil that cools and dehumidifies moist air: the humidity ratio, enthalpy and dew point in and out, the dry-air mass flow, the condensate removed, the total coil load, its sensible and latent parts and the sensible heat ratio, against the coil’s rated capacity.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Total (barometric) pressure | kPa | Barometric (total) pressure: 101.325 kPa at sea level, lower at altitude. |
| Air volume flow at the coil inlet | m³/s | Volume flow of air entering the coil from the fan schedule or a flow measurement. |
| Air entering the coil | ||
| Inlet dry-bulb temperature | °C | Dry-bulb temperature of the air entering the coil (after mixing outdoor and return air). |
| Inlet relative humidity | % | Relative humidity of the air entering the coil. |
| Air leaving the coil | ||
| Outlet dry-bulb temperature | °C | Dry-bulb temperature of the air leaving the coil, from the design or a measurement. |
| Outlet relative humidity | % | Relative humidity of the air leaving the coil; typically 85 to 95 % for a coil that removes moisture. |
| Coil | ||
| Rated total capacity of the coil at these conditions | kW | Total (sensible plus latent) capacity of the coil at these air and coolant conditions, from the coil maker’s selection. |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
What this means: Total (barometric) pressure
- What it is
- The total pressure of the moist air at the coil, the barometric pressure at the site, used in the humidity ratio and specific volume.
- Units and format
- Enter a number in kPa: kilopascals. The form accepts 50 kPa to 110 kPa. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 101.325 kPa as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Changes the humidity ratios, the dry-air mass flow and so the load.
- Get it right
- Use about 101.325 kPa at sea level; at high-altitude sites use the local value.
What this means: Air volume flow at the coil inlet
- What it is
- The volume of moist air entering the coil each second, which with the inlet specific volume gives the dry-air mass flow.
- Units and format
- Enter a number in m³/s: m³/s. The form accepts 0.001 m³/s to 1000 m³/s. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1 m³/s as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The load and condensate rise in proportion to the air flow.
- Get it right
- Use the flow at the coil inlet conditions; fan data at standard air may differ from the real volume.
Air entering the coil
What this means: Inlet dry-bulb temperature
- What it is
- The dry-bulb temperature of the air arriving at the coil, after any mixing of outdoor and return air.
- Units and format
- Enter a number in °C: degrees Celsius. The form accepts 1 °C to 60 °C. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 30 °C as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A warmer inlet raises the sensible load and, at the same relative humidity, the latent load.
- Get it right
- Measure after the mixing box and filter, upstream of the coil face.
What this means: Inlet relative humidity
- What it is
- The relative humidity of the air arriving at the coil, which with its temperature sets the moisture the coil must remove.
- Units and format
- Enter a number in %: percent: the number is read as a percentage, so type 8 for 8 %. The form accepts 0.1% to 100%. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 60% as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Higher inlet humidity adds latent load and condensate and lowers the sensible heat ratio.
- Get it right
- Humid outdoor air dominates the latent load; use the design outdoor and return mix.
Air leaving the coil
What this means: Outlet dry-bulb temperature
- What it is
- The dry-bulb temperature of the air leaving the coil, the supply condition the coil is asked to produce.
- Units and format
- Enter a number in °C: degrees Celsius. The form accepts 1 °C to 60 °C. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 14 °C as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A colder outlet raises both the sensible and the latent load.
- Get it right
- It must be below the inlet temperature; leaving air is often 12 to 15 °C for comfort cooling.
What this means: Outlet relative humidity
- What it is
- The relative humidity of the air leaving the coil, which with its temperature fixes how much moisture remains.
- Units and format
- Enter a number in %: percent: the number is read as a percentage, so type 8 for 8 %. The form accepts 0.1% to 100%. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 95% as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A lower outlet humidity means more water removed and a larger latent load.
- Get it right
- Air leaving a wet coil is usually close to saturated; the outlet must not hold more moisture than the inlet.
Coil
What this means: Rated total capacity of the coil at these condi…
- What it is
- The total cooling the coil can deliver at these entering air and coolant conditions, sensible plus latent, from the coil selection.
- Units and format
- Enter a number in kW: kilowatts. The form accepts 0 kW to 1000000 kW. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 40 kW as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Only moves the pass or fail line of the capacity check.
- Get it right
- Coil ratings change with chilled-water or refrigerant temperature and air velocity; use the selection at your conditions.
Method and formulas
- Each state from temperature and relative humidity: p_v = φ p_sat(T) (Hyland–Wexler), ω = 0.622 p_v ÷ (P − p_v), h = 1.005 T + ω (2500.9 + 1.82 T) kJ/kg of dry air.
- Dry-air mass flow ṁ_a = V̇₁ ÷ v₁ with v₁ = 0.287 (T₁ + 273.15) ÷ (P − p_v1); condensate ṁ_w = ṁ_a (ω₁ − ω₂).
- Energy balance: Q = ṁ_a (h₁ − h₂) − ṁ_w h_w, with the condensate leaving at the outlet air temperature, h_w = 4.18 T₂ kJ/kg.
- Sensible part Q_s = ṁ_a (1.005 + 1.82 ω₁)(T₁ − T₂); latent part = Q − Q_s; sensible heat ratio = Q_s ÷ Q.
- Example: 1 m³/s of air cooled from 30 °C and 60 % to 14 °C and 95 % at sea-level pressure needs 37.22 kW (sensible heat ratio 0.505) and removes 26.88 kg/h of water; a 40 kW coil can meet it.
Limitations
- Steady flow with no fan heat, duct gains or bypass; the outlet state must be one a real coil can reach, which the coil maker’s selection confirms.
- Temperatures from 1 to 60 °C at the coil; the condensate enthalpy uses the specific heat of liquid water, 4.18 kJ/kg·K.
- A screening balance, not a coil selection; coil performance depends on fins, rows, face velocity and coolant temperature, which only the maker’s data covers.
- Example values are illustrations. Replace them with values for your actual air-handling unit.
Typical uses
- Size or check a cooling coil in an air-handling unit or process air cooler
- Estimate the condensate a coil produces for drain and pump sizing
- Split a cooling load into sensible and latent parts
Who it is for
- Mechanical services designers
- HVAC and refrigeration technicians
- Process and facility engineers
What you get
- Total, sensible and latent coil load and the sensible heat ratio
- Condensate removed
- Dry-air mass flow
- Humidity ratio, enthalpy and dew point in and out
- Coil capacity against the load
Interpreting the result
- A sensible heat ratio well below 0.7 means much of the load is drying the air; the coil and the chilled-water or refrigerant temperature must be cold enough to do that.
- Condensate in kilograms per hour is close to litres per hour for drain sizing.
Before relying on the result
- Check the coil selection at the same entering and leaving conditions with the coil maker.
- Measure entering and leaving temperature and humidity on the running unit to confirm the load.
References
- Çengel Y. A. and Boles M. A., Thermodynamics: An Engineering Approach, 5th edition (McGraw-Hill, 2006), chapter 14 (sections 14-2 and 14-7: humidity, air-conditioning processes, cooling with dehumidification). Method reference; no tables, figures or examples are reproduced.
- ASHRAE Handbook—Fundamentals (SI), chapter 1 Psychrometrics: saturation pressure and moist-air relations.
- AHRI Standard 410, Forced-Circulation Air-Cooling and Air-Heating Coils (named by number and scope only).
Learn the Concepts Behind This Tool
These KEVOS® Library readings explain the method this tool uses, with worked examples.
Method explained here · Library collection
Psychrometrics for HVAC and process air: humidity, dew point and the loads you cannot feel
In Fluids, HVAC and Building Services
What it covers: A practical guide to moist air: dry-bulb, wet-bulb, dew point, humidity ratio and enthalpy, the main processes on the psychrometric chart, sensible and latent loads, and condensation.
Method explained here · KEVOS® original book
Cooling coils and dehumidification
In Humidity Control by the Numbers
What it covers: Calculate a cooling coil's total, sensible and latent load and condensate, find its apparatus dew point and contact factor, compare coil and room sensible heat ratios, and avoid sensible-only sizing.
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