Evaporative Cooling
Evaporative cooling is the use of the cooling effect of introducing water that evaporates into an airstream. Evaporation is an adiabatic process, which means that heat from the airstream itself is used to evaporate the water. The heat that goes toward this evaporation removes heat from the air and drops the temperature of the airstream. This cooled air can then be supplied to the building for space conditioning.
There are many ways to include evaporative cooling equipment in the HVAC system of a building, but they fall into two distinct categories:
- Cooling the air used in the building
- Heat rejection, such as cooling towers
This page is focused only on the cooling of air used in buildings.
The different types of evaporative cooling systems include:
- Direct evaporative coolers
- Indirect evaporative coolers
- Indirect/direct evaporative coolers
- Evaporative cooling used in conjunction with standard cooling systems
Due to the wide variety of configurations that can take advantage of evaporative cooling, component-based BEM software may be more appropriate for less common configurations. Non-component-based software often includes the most common configurations.
Maximizing the impact of the evaporation of water usually occurs when the local air is driest, so traditionally, the most common locations for the use of evaporative cooling were in arid or semi-arid climates. These are still the climates that work the best but are not necessarily the limitation of where effective cooling can be done with evaporation. The consideration of water usage and its associated cost for evaporative cooling should also be considered when choosing equipment.
How It Works
Direct Evaporative Coolers
A direct evaporative cooler is a device with a fan that blows air across a pad with water flowing through it, across rigid media that is corrugated with water flowing over it, or with water sprayed directly into the air that is supplied to the building. These different approaches may impact the direct saturation efficiency, a measure of how close the leaving air temperature is to the entering wet-bulb temperature. Due to the high humidity in the supply air and the discomfort associated with it, direct evaporative cooling is not always chosen, and often other configurations are used when the goal is to provide human comfort. A common name for direct evaporative coolers when applied to residential cooling is “swamp cooler.”
Since this configuration is simple and less expensive than others, it is not unusual to see this approach being used to provide spot cooling in industrial facilities or provide cooling to greenhouses or farm animals. In those applications, it is not uncommon for the exhaust fan to pull air over the water evaporating device that is located at the other end of the facility.
Direct evaporative cooling has a very long history in arid or semi-arid climates. It was used and continues to be used on both a small scale with jars wrapped in a wet cloth to keep food cool to larger scales to cool buildings by using wind towers to draw air through water-filled tunnels.
Indirect Evaporative Coolers
Indirect evaporative cooling systems provide sensible cooling (reduce the drybulb temperature) of a primary airstream via direct evaporative cooling of a secondary air stream. Water is not in direct contact with the air that enters the building. Instead, a secondary air stream, such as exhaust air from the building or outdoor air, is cooled by direct evaporation and that secondary air cools the primary airstream via a heat exchanger.
One disadvantage to an indirect evaporative cooling system is that it provides less reduction in drybulb temperature due to the use of a heat exchanger when compared to a direct evaporative system. However, the advantage is that the indirect evaporative cooling system does not add moisture to the primary airstream and therefore can be used in applications where the moisture added by a direct evaporative system is undesirable.
The following diagram and explanation provides more details on how evaporative cooling works:
- An air stream, often from the building exhaust, shown as point “A” in the figure below, is cooled the same way using air that is blown across a pad, over a corrugated material, or with water sprays and cools to point “B.”
- Instead of entering the building, it is used to cool another air flow from outdoors “D” that actually enters the building at condition “E” through the use of an air-to-air heat exchanger. The heat exchanger allows only the sensible heat to be removed from the air stream, actually entering the conditioned space without any additional moisture.
- After the heat exchanger, the original air stream is exhausted at point “C.”
The type of heat exchanger used can include cross-flow heat exchangers, heat pipes, rotary sensible heat exchangers, run-around loops, and coils connected to remote cooling towers. A common type of heat exchanger consists of a set of tubes through which primary air flows with water sprayed on the outside of the tubes where it evaporates into a secondary airstream. The measure of efficiency for indirect evaporative coolers is wet-bulb depression efficiency (WBDE) and is based on the supply inlet and outlet temperature as well as the wet airstream’s inlet wet bulb temperature. The higher the value, the greater the efficiency.
WBDE = 100 x (Tdry_bulb_supply_inlet - Tdry_bulb_supply_outlet) / (Tdry_bulb_supply_inlet - Twet-side_inlet_wetbulb)
The ASHRAE Handbook for HVAC Applications 2020 has a very useful table comparing these and is summarized below:
| System Type | WBDE
(%) |
Heat Recovery Efficiency
(%) |
Wet-Side Air Delta-P
(inch H2O) |
Dry-Side Air DeltaP
(inch H2O) |
Pump Power (hp per 10,000 CFM) | Parasitics (kW/ton cooling) | Winter Heat Recovery | Cross Contamination | Best For |
|---|---|---|---|---|---|---|---|---|---|
| Cooling tower to coil | 40 - 60 | NA | NA | 0.4 - 0.7 | Varies | Varies | None | None | Multiple air handling units |
| Cross flow plate | 60 - 85 | 40 - 50 | 0.7 - 1.0 | 0.4 - 0.7 | 0.1 - 0.2 | 0.12 - 0.20 | Low | Possible | Lower airflows |
| Heat pipe | 65 - 75 | 50 - 60 | 0.7 - 1.0 | 0.5 - 0.7 | 0.2 - 0.4 | 0.15 - 0.25 | Medium | Possible | Large airflows |
| Heat wheel | 60 -70 | 70- 80 | 0.6 - 0.9 | 0.4 - 0.65 | 0.1 - 0.2 | 0.20 - 0.30 | High | Possible | High airflows |
| Run around coil | 35 - 50 | 40 -60 | 0.6 - 0.8 | 0.4 - 0.6 | Varies | > 0.35 | None | Separated supply and return ducts |
The configuration of indirect evaporative cooling is very similar to the heat exchanger configuration used with heat recovery applications in dedicated outdoor air systems (DOAS). Because of this similarity, when considering DOAS, the addition of an indirect evaporative cooler may provide significant benefits with only a small additional investment.
Direct/Indirect Evaporative Coolers
These systems combine direct and indirect evaporative coolers to provide an even lower supply-air temperature. In this type of system, primary air flows first through an indirect evaporative cooling section and then through a direct evaporative cooling section. The resulting air drybulb temperature is lower than could be achieved through either direct or indirect evaporative cooling alone. The return air stream is usually used, although sometimes just an outside air stream, with an evaporative cooler to cool air prior to a heat exchanger. The other side of the heat exchanger is the airstream from outdoors to supply conditioned air to the building. This portion of the configuration is an indirect evaporative cooler. In the outdoor air stream that is intended for conditioning the building, after the heat exchanger, is another evaporative cooler which cools the air further and is the direct evaporative cooler portion of the configuration. The heat exchanger is using heat pipes, cross-flow, or a heat wheel. The figure below shows this configuration graphically and is from the ASHRAE Handbook on HVAC Applications Chapter 53.
With either indirect or direct/indirect configurations, it is common to also include either a standard chilled water coil or DX cooling coil with these configurations to provide cooling when conditions would otherwise be too humid for its operation.
Due to the presence of the extra heat exchanger and direct evaporative cooling media, the fan has an additional pressure drop to overcome, so it is important to understand the impact of the fan sizing on the overall design and energy usage.
Maisotsenko cycle
The Maisotsenko cycle, or m-cycle, uses a heat exchanger configuration that iteratively cools the process and the delivered air so that the delivered air temperature is below the wet-bulb temperature and approaches the dew-point temperature. It is a patented technology that is available from only a few vendors. See References for more information.
Information Needed for the Model
For evaporative coolers, the following information is typically required for modeling:
- How the airflows are connected to the unit
- Supply and “wet” airstream fan power, speeds, and placement
- Airflow rates for supply and “wet” airstream or a ratio between them
- Fraction of the supply air cooled
- Pad areas and depth
- Water pump power and capacity
- Rated temperatures
- Minimum and maximum dry bulb temperatures
- Maximum wet bulb temperature
- Wet-bulb depression effectiveness
- Performance curve modification for effectiveness often as a function of air flow rate
- Sump water tank volume
- Drift loss (unevaporated water that is lost to the airstream)
- Blowdown (sump water that is routinely drained to prevent build-up)
- Heat exchanger effectiveness and control options
- Pressure drop across the heat exchanger and evaporative media
- Water consumption rate
Because of the variety of models used, not all of these modeling inputs are required for all modeling tools, but a subset is often used.
Common Measures
The most common measures to use with evaporative cooling are the use of different configurations. Trying direct, indirect, or a combination of direct and indirect are probably the most common options. Another variation that may be considered is changing the type of heat exchanger in an indirect evaporative cooler which has different effectivenesses and power consumptions. These different configurations often have significant first-cost differences as well.
Within indirect evaporative cooling, the physical configuration of the heat exchanger, including the size, number and size of channels, and depth, will impact both the effectiveness and the pressure difference. An evaluation of the impact of each option on annual energy consumption is necessary to identify the optimal option.
For indirect evaporative cooling, an evaluation of the source of the secondary air stream is often performed and can be either the exhaust air from the building or the outside air.
Common Control Options
Evaporative cooling systems have a number of control options, including:
- Limits on the use of evaporative cooling based on the minimum or maximum outdoor dry-bulb temperature or wet-bulb temperature, or other humidity metric.
- A flag indicating if the evaporative cooling system can operate at the same time as a conventional DX or chilled water system.
- A flag indicating if the heat exchanger is used during heating periods for heat recovery of exhausted air. This only makes sense if exhaust air is used for an evaporative cooling system for the secondary air stream.
Common Applications
In arid and semi-arid climates, evaporative cooling systems may be a very good fit assuming that sufficient water is available at a reasonable cost. Since this is not always the case, it is important to evaluate both the water consumption cost and energy costs using building energy modeling. In these climates, some type of evaporative cooling can be a good match for many different building applications. Other climates with significant hours of low-humidity air may also make sense for evaporative cooling.
In many climates, evaporative cooling may be used in industrial settings where processes need cooling, but comfort is not as important. It is also commonly used in “spot” applications in industrial facilities to provide cooling to workers due to the high temperatures of the industrial process, such as near furnaces, casting equipment, and some chemical processes, including paper processing. This may use floor outlets or outlets just above head height in a high bay facility to cool employees. Similar approaches have been used in laundry facilities.
Evaporative cooling is also used for applications that can be served with supply air temperatures that are higher than typically required for comfort cooling applications. Data centers are a common example.
On farms where cooling animals or plants are required to aid in production, direct evaporative cooling is often employed. For farm animals, since evaporative cooling is usually 100% outdoor air, it matches well with exhaust ventilation of animal fumes. For greenhouses, it is not uncommon for the exhaust fan to pull air over the water-evaporating device that is located at the other end of the facility in order to keep the plants cooler during higher ambient temperatures.
Model Output Checks
Probably the most important model output check is to confirm the state points on a psychrometric chart for the evaporative cooling process selected. This should be repeated by using timestep outputs from the simulation for a few different conditions of outdoor dry-bulb temperature and wet-bulb temperature. To do this, find a time during the cooling season when the operating conditions, primarily the outside dry-bulb and wet-bulb temperatures, are similar to design conditions and make sure the output conditions are as expected. For indirect evaporative cooling, both airstream conditions should be confirmed. Water consumption, pumping power, and heat exchanger efficiency should be confirmed. This check should be repeated for multiple operating conditions by using the psychrometric chart.
If using an indirect evaporative cooler in heating mode as just a heat recovery device, repeat this process during the design heating conditions as well as other off-rated conditions.
Related Energy Code Requirements
ASHRAE 90.1-2019 Energy Standard for Buildings Except Low-Rise Residential Buildings includes a number of requirements specifically mentioning evaporative cooling, including:
- Table 6.5.3.1-2: this allows fan systems with evaporative cooling and heat exchangers to have additional fan power to overcome the additional pressure drop they add to the system.
- Section 6.5.3.2.1: this requires variable speed supply fans with exceptions for fan motors less than ¼ hp for evaporative cooling or under one hp for evaporative cooling when the fan is not used to provide ventilation and cycles with the load.
Ratings of direct and indirect evaporative coolers using one of the following ASHRAE methods of test:
- ASHRAE 133-2015 Method of Testing Direct Evaporative Air Coolers
- ASHRAE 143-2015 Method of Test for Rating Indirect Evaporative Coolers
Similar or Related Systems
Evaporation of water to aid in heat rejection is used as part of other cooling systems for buildings, including:
- Evaporative condensers
- Cooling towers
Additional Resources
2023 ASHRAE Handbook - HVAC Applications Chapter 53 - “Evaporative Cooling”
2020 ASHRAE Handbook - Heating, Ventilating, and Air Conditioning Systems and Equipment. Chapter 41 - “Evaporative Air-Cooling Equipment”
Wikipedia article on evaporative cooler
Evaporative Air Conditioning Handbook (3rd Edition) - John R. Watt, Will K. Brown
Evaporative Cooling Design Guidelines Manual for New Mexico Schools and Commercial Buildings - J. D. Palmer
Building America - Evaporative Cooling Systems
Indirect Evaporative Cooling - Zero Net Energy Technology Application Guide
DA29 Evaporative Air Cooling Systems - Vincent Aherne
ASHRAE TC 5.7 Evaporative Cooling
References
ASHRAE 133-2015 Method of Testing Direct Evaporative Air Coolers
ASHRAE 143-2015 Method of Test for Rating Indirect Evaporative Coolers
Rogdakis E, Tertipis D. Maisotsenko cycle: technology overview and energy-saving potential in cooling systems. Energy and Emission Control Technologies. 2015;3:15-22
Taler, J.; Jagieła, B.; Jaremkiewicz, M. Overview of the M-Cycle Technology for Air Conditioning and Cooling Applications. Energies 2022, 15, 1814.
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