KEVOS® MAKEOS · Engineering
Direct Evaporative Cooler Check (Wet-Bulb, Supply Temperature, Water Use, Cooling Capacity)
How cool will a direct evaporative cooler make this air, how much water will it evaporate and how much sensible cooling will it give, and will it reach the supply temperature you need?
Direct evaporative (swamp) cooler: the wet-bulb temperature of the entering air from the adiabatic saturation balance, the leaving temperature from the pad’s saturation effectiveness, the leaving humidity, the water evaporated, the sensible cooling of the air, and whether it reaches the supply temperature you need.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Total (barometric) pressure | kPa | Barometric pressure at the site: 101.325 kPa at sea level. |
| Entering air dry-bulb temperature | °C | Dry-bulb temperature of the air entering the cooler, usually the outdoor design temperature. |
| Entering air relative humidity | % | Relative humidity of the air entering the cooler at that time. |
| Air volume flow entering the cooler | m³/s | Volume flow of air entering the cooler, from the fan rating. |
| Saturation effectiveness of the cooler pads | % | Saturation effectiveness of the pads from the maker’s data: the share of the dry-bulb to wet-bulb difference the cooler achieves. |
| Supply temperature needed | °C | Supply air temperature the space or process needs. |
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, the barometric pressure at the site, used in the humidity ratio and wet-bulb balance.
- 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 ratio and wet-bulb slightly.
- Get it right
- Use about 101.325 kPa at sea level and the local value at altitude.
What this means: Entering air dry-bulb temperature
- What it is
- The dry-bulb temperature of the air drawn into the cooler, usually the outdoor temperature on the design day.
- 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 38 °C as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Hotter entering air at the same humidity raises the wet-bulb and the leaving temperature.
- Get it right
- Pair it with the humidity at the same hour; afternoon temperature with morning humidity is a common error.
What this means: Entering air relative humidity
- What it is
- The relative humidity of the air drawn into the cooler, which sets how much the air can still evaporate and so its wet-bulb.
- 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 20% as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Higher humidity raises the wet-bulb and the leaving temperature, and reduces water use.
- Get it right
- Dry air cools most; check the humidity at the hottest time of day.
What this means: Air volume flow entering the cooler
- What it is
- The volume of air the cooler’s fan pushes through the pads each second, measured at the entering condition.
- 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 3 m³/s as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Water use and cooling capacity rise in proportion; the leaving temperature does not change.
- Get it right
- Use the fan’s delivered flow against the real duct pressure, not its free-air rating.
What this means: Saturation effectiveness of the cooler pads
- What it is
- How far the pads take the air from its dry-bulb towards its wet-bulb temperature, as a share of that difference.
- Units and format
- Enter a number in %: percent: the number is read as a percentage, so type 8 for 8 %. The form accepts 1% to 100%. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 80% as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Higher effectiveness lowers the leaving temperature and raises water use and cooling.
- Get it right
- Thicker pads and lower air velocity give higher effectiveness; dirty or dry pads give much less.
What this means: Supply temperature needed
- What it is
- The supply air temperature the space or process needs from the cooler.
- Units and format
- Enter a number in °C: degrees Celsius. The form accepts -10 °C to 60 °C. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 26 °C 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 supply temperature check.
- Get it right
- It cannot be below the wet-bulb; the tool reports when it is.
Method and formulas
- Wet-bulb T_wb from the adiabatic saturation balance (Çengel 14-14): ω₁ = [1.005 (T_wb − T₁) + ω_s(T_wb) h_fg(T_wb)] ÷ [h_g(T₁) − h_f(T_wb)], solved by bisection, with h_g = 2,500.9 + 1.82 T and h_f = 4.18 T.
- Leaving temperature T₂ = T₁ − ε (T₁ − T_wb).
- Leaving humidity from the same balance with make-up water at the wet-bulb temperature: ω₂ = (h₁ − ω₁ h_f − 1.005 T₂) ÷ (h_g(T₂) − h_f); water evaporated = ṁ_a (ω₂ − ω₁).
- Sensible cooling = ṁ_a (1.005 + 1.82 ω₁)(T₁ − T₂).
- Example: 3 m³/s of air at 38 °C and 20 % has a wet-bulb of 20.76 °C; 80 % effective pads cool it to 24.21 °C at 73.6 %, evaporating 69.2 kg/h of water for 47.26 kW of sensible cooling, below a 26 °C target.
Limitations
- Direct evaporative cooling adds moisture: it suits dry climates and spaces that tolerate humid supply air, not humidity-controlled rooms.
- The water evaporated excludes bleed-off, which is needed to control dissolved salts and can be a large share of the total water use.
- Pads and sumps need maintenance to manage Legionella risk; check your state’s public health requirements. This tool does not cover water treatment.
- Example values are illustrations. Replace them with values for your actual site and cooler.
Typical uses
- Size or check an evaporative cooler for a factory, warehouse or plant room
- Estimate water use for an evaporative cooling system
- See on which days direct evaporative cooling can reach a target temperature
Who it is for
- Mechanical services designers
- Facility and maintenance managers
- HVAC contractors
What you get
- Wet-bulb temperature and leaving air temperature
- Leaving relative humidity and humidity ratio
- Water evaporated
- Sensible cooling of the air
- Leaving temperature against the target
Interpreting the result
- No direct evaporative cooler can cool air below its wet-bulb temperature; humid days narrow the gap.
- The cooling the space receives is less than the sensible cooling shown if the supply air is warmer than the space it enters needs.
Before relying on the result
- Check the design wet-bulb for your location from local climate data, not a single day’s reading.
References
- Çengel Y. A. and Boles M. A., Thermodynamics: An Engineering Approach, 5th edition (McGraw-Hill, 2006), chapter 14 (section 14-4: adiabatic saturation and wet-bulb temperatures; section 14-7: evaporative cooling). Method reference; no tables, figures or examples are reproduced.
- ASHRAE Handbook—Fundamentals (SI), chapter 1 Psychrometrics: Hyland–Wexler saturation pressure.
Learn the Concepts Behind This Tool
These KEVOS® Library readings explain the method this tool uses, with worked examples.
Explains the concept · KEVOS® original book
Moist air and the wet-bulb temperature
In Cooling Towers by the Numbers
What it covers: Work out the air a tower uses: humidity ratio, enthalpy, wet-bulb and dew point from temperature and humidity, why enthalpy drives a tower, and catch a 3 pm temperature paired with 9 am humidity.
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.
Also in Passive and low-energy cooling for Australian workplaces: reduce the heat before you buy the cooling, HVAC for commercial and industrial buildings: loads, system choices, efficiency, controls and maintenance
Method explained here · KEVOS® original book
Evaporative cooling for the warehouse
In Humidity Control by the Numbers
What it covers: Size direct evaporative coolers from the wet-bulb limit and pad effectiveness, find their water use and real cooling to the room, see why humid days defeat them, and keep them hygienic.
KEVOS® tools are screening and decision-support utilities. Verify important financial, engineering, safety, legal or operational decisions using appropriate professional standards and your organisation’s approved data.