Indirect evaporative cooling (IEC) cools a product air stream through a heat exchanger while water evaporates on a separate, secondary side. In normal non-condensing sensible operation, water does not contact the product air and its humidity ratio stays approximately constant. IEC is therefore a possible cooling stage, not a rated controlled-dehumidification solution, air purifier or guarantee of a particular room temperature. Its useful output depends on the entering air states, airflow and heat-exchanger design.
Follow the two air streams
- Product or primary air enters dry passages of a heat exchanger and gives up sensible heat through the separating wall. Its dry-bulb temperature falls if the secondary side provides a lower effective temperature.
- Secondary air passes a wetted surface or another evaporatively cooled component. Evaporating water cools this side and carries away the heat received from the product air. The secondary stream is then exhausted or otherwise handled by the system design.
- Fans move both streams; a water circuit wets the secondary side. Distribution, fouling, pressure drop and the ratio of the two airflows affect the delivered cooling and parasitic electricity.
The air streams should remain separated in a sound IEC heat exchanger, so the product stream is not deliberately humidified by the evaporation process. That separation does not make incoming outdoor air clean, guarantee moisture removal or certify that a particular unit has zero leakage. If a dry-side surface falls below the product-air dew point, condensation and some latent cooling may occur; this requires an explicit rating and appropriate drainage and hygienic design. Filtration and leakage checks remain separate requirements where air quality matters.
ASHRAE Handbook, Chapter 53: indirect cooling and the separate primary and secondary streams.
What sets the cooling limit?
Start with the product-air inlet dry-bulb temperature and the secondary-air inlet wet-bulb temperature. For a conventional IEC stage, the latter is the reference for the approach temperature; it is not an unconditional outlet guarantee. A higher secondary wet-bulb temperature leaves less driving difference for a given product-air inlet temperature. Using building return air instead of outdoor air as the secondary stream can change that reference state and the result.
A common conventional IEC effectiveness definition is (product inlet dry-bulb minus product outlet dry-bulb) divided by (product inlet dry-bulb minus secondary inlet wet-bulb). Quote an effectiveness only with the stream arrangement, entering states, airflows and test boundary. If the denominator is small, a percentage by itself is especially poor selection evidence. Do not transfer a result from one climate, duty or heat exchanger to another project.
Regenerative or dew-point configurations redirect or pre-cool part of the air to create a different secondary state. Their product outlet may be described against different reference temperatures. Such a rating is not interchangeable with conventional IEC wet-bulb effectiveness. Ask which cycle and denominator were tested before comparing published values.
ASHRAE Handbook, Chapter 53: wet-bulb boundary and conventional IEC effectiveness.
Choose a system from the required air state
| Design question | What to establish | Why it matters |
|---|---|---|
| Air state | Product and secondary inlet dry-bulb, wet-bulb and humidity ratio at relevant design hours | The secondary wet-bulb state constrains sensible cooling. |
| Load and supply | Cooling load, required airflow, outlet-temperature range and operating hours | A temperature drop alone does not prove the required duty is met. |
| Moisture | Outdoor-air and process latent loads plus the required humidity range | Non-condensing sensible IEC does not remove vapor; possible dry-side condensation is not a controlled dehumidification rating. |
| Equipment | Heat-exchanger rating, both fans, pump, pressure loss and controls | Whole-system energy differs from a core-only result. |
| Water | Makeup quality, evaporation, bleed-off, drainage and maintenance | Water use and scale depend on the circuit and site water. |
For a process requiring both lower temperature and lower humidity, determine the moisture-removal duty separately. A rated cooling or dehumidification stage may be required upstream or downstream of IEC, with the combined air states checked on a psychrometric chart. A dry-product-stream claim means this IEC evaporation stage does not intentionally add water vapor; it does not mean the system controls room RH. Do not count incidental condensation as dependable latent capacity without tested duty and condensate management.
For a data center, warehouse, greenhouse or workshop, use that facility’s actual heat, ventilation and moisture loads. Do not reuse a generic outlet temperature, crop setpoint, operating-hours fraction or cost saving. An illustrative calculation can explain the method, but it must be labeled as an assumption rather than a site result.
Yakeclimate engineering: define the project air-state and load brief.
Compare energy and water on the same boundary
IEC can reduce compressor operation in suitable conditions, but a compressor-free heat exchanger is not the same as a zero-energy cooling system. Count both air movers, pumps, controls, any downstream refrigeration or reheat, and their annual duty. Compare that complete boundary with an alternative designed to meet the same supply-air and humidity requirement. A single COP or percentage saving without test conditions and boundary is not a transferable project result.
Likewise, separating product air from wetted air does not by itself conserve water. Evaporation consumes makeup water; bleed-off, leakage and water treatment add to the water balance. The US Department of Energy describes scale and mineral build-up and the need to manage bleed-off and maintenance in evaporative equipment. Confirm the chosen IEC design’s actual water circuit and quality requirements instead of treating direct and indirect layouts as a universal water-use ranking.
US Department of Energy: water management and maintenance of evaporative coolers.
Check the result before committing to a plant design
- Obtain a rating at stated primary and secondary entering conditions, both airflows, pressure losses and water quality. Identify whether the device is conventional IEC, regenerative, or a multistage combination.
- Model representative hot and humid hours as well as favorable dry hours. Check the resulting product-air dry-bulb temperature, humidity ratio and required secondary cooling or dehumidification stage.
- During commissioning, record inlet and outlet air states, fan and pump power, water use and the control mode. Keep those site measurements separate from vendor ratings and hypothetical examples.
The existing page explains the IEC process and selection boundary. It does not claim a Yakeclimate IEC product, a completed customer installation or a Yakeclimate laboratory measurement. For a project with a humidity limit, the next engineering step is to specify the latent load and test whether a separate dehumidifier is needed.
Sources
ASHRAE Handbook—HVAC Applications, Chapter 53: Evaporative Cooling.
US Department of Energy, Best Management Practice #13: Other Water-Intensive Processes.
FAQ
Frequently Asked Questions
Does indirect evaporative cooling add moisture to the product air?
In normal non-condensing sensible IEC operation, the product stream stays on the dry side and receives no intentional evaporative moisture; its humidity ratio stays approximately constant. If the dry-side surface falls below the product-air dew point, condensation and some latent cooling may occur. That requires rated data plus drainage and hygienic design, and must not be assumed to provide controlled dehumidification.
Can IEC alone maintain a target room humidity?
No. A conventional IEC stage mainly changes sensible temperature. Check outdoor-air and process moisture loads and add a separately designed moisture-control stage where needed.
Can a conventional IEC outlet beat the entering secondary wet-bulb temperature?
Do not assume so from a generic IEC rating. Conventional effectiveness uses that wet-bulb state as a reference. Regenerative or dew-point cycles use another process arrangement and must be evaluated on their stated test boundary.
Is IEC always more energy- and water-efficient than air conditioning?
No universal ranking follows from the process name. Compare the same cooling and humidity duty, design weather, fans, pumps, water balance and any backup or downstream conditioning.
What measurements should an IEC supplier provide?
Request both inlet air states, product outlet state, both airflows, fan and pump power, pressure losses, water use, control mode and the reference definition for any effectiveness rating.
