How to know which evaporative cooling option is right
When considering evaporative cooling, understand the general applications and high-level engineering considerations
- Understand the thermodynamic principles of evaporative cooling.
- Review examples of evaporative cooling technologies commonly used in the HVAC industry.
- Discuss ancillary considerations when pursuing evaporative cooling technology in a design.
Evaporative cooling technology is an extremely versatile and effective air conditioning solution that also carries challenges for the designer to consider. From cooling large volumes of air in data centers to maintaining space temperature and humidity in commercial greenhouses, evaporative cooling technology can be applied across many industries. If deployed in an optimal climate and with the right design conditions, it is cost-effective, energy-efficient and environmentally conscious.
Along with these benefits come many considerations. Like any well-oiled machine, if one component is overlooked, the whole system could fall apart. Evaporative cooling technologies inherently possess nuances and a designer must be prepared to address these considerations ahead of design, or face potential issues once the building has been turned over to the owner.
Keeping in mind the considerations outlined in the following as well as having a firm understanding of the options available in the market, evaporative cooling technology can be an exciting and viable solution to implement.
Thermodynamic principles of evaporative cooling
Evaporative cooling is based on the adiabatic process of evaporation. As air passes over a wetted surface, it begins to cool. This cooling effect is a result of water from the wetted surface evaporating into the passing airstream. The process is fully adiabatic, resulting in a net-zero exchange of energy. Sensible heat removed from the passing airstream is equal to the heat of vaporization resulting from the moisture evaporating from the wetted surface. This results in a lowering of the supply airstream’s dry bulb temperature.
Because fluid from the wetted surface undergoes a phase change from liquid to gas and evaporates out into the passing airstream, mass transfer must be conserved. The moisture removed from the wetted surface is absorbed into the supply airstream. The amount of moisture added to the supply airstream is equal to the amount of water evaporated from the wetted surface.
Because no heat is added to or removed from the theoretical control volume, the process follows the inlet air enthalpy line up to the leaving air dry bulb target. As shown in the psychrometric chart (see Figure 1), the resulting leaving air condition of a basic evaporative cooling process is cooler and moister than the incoming air condition. Because the process follows a constant enthalpy line, entering air and leaving air wet bulb conditions remain approximately constant.
Like all thermodynamic processes, evaporative cooling is limited. This limitation is dictated by the process wet bulb depression. The wet bulb depression is defined as the difference between the entering air’s dry bulb and wet bulb temperatures. The greater the wet bulb depression, the greater the cooling potential.
Types of evaporative cooling: direct versus indirect
In short, there are two main types of evaporative cooling applications: direct and indirect. Direct evaporative cooling follows what was stated in the previous paragraphs. Air and the wetted surface come into direct contact, resulting in a reduced dry bulb temperature and increased moisture content (see Figure 1).
Indirect evaporative cooling is exactly as it sounds: an indirect exchange of energy. Two fluids interact in this configuration: a primary airstream and a secondary airstream. The key in an indirect arrangement is the inclusion of a heat exchanger.
As primary air passes through the heat exchanger, secondary air passes in the opposite direction. To maximize process cooling effect, the heat exchanger surface on the secondary side is usually wetted with a spray or mist, allowing for the direct evaporative process to occur. As the secondary airstream is adiabatically cooled, a sensible energy transfer occurs through the heat exchanger, lowering the primary air dry bulb temperature. This process is sensible because the airstreams are completely isolated from one another, preventing the moisture evaporated from the secondary side from influencing the primary air humidity level (see Figure 2).
Like direct evaporative processes, indirect evaporative cooling is limited by its wet bulb depression. However, unlike direct, the wet bulb depression is defined as the difference between the primary air entering dry bulb and the secondary side entering wet bulb temperature. The greater this depression, the greater the cooling potential.
There are applications that combine both direct and indirect evaporative technologies. However, for the purposes of this article, direct and indirect evaporative applications will be focused on in their isolated states.
Direct evaporative applications
Direct evaporative cooling is commonly achieved by passing air through a rigid media pad, which is wetted by a spray or mist. The media pads comprise various materials intentionally designed to absorb fluid and wick it away quickly. The evaporative media pads are typically enclosed within a steel or sheet metal frame, with piping to spray nozzles and a sump to collect any water that did not evaporate into the airstream. Figure 3 illustrates an evaporative media module used for a hyperscale data center application.
Common applications for direct evaporative cooling are:
Data centers: Data centers are prime candidates for evaporative cooling technology. According to ASHRAE Technical Committee 9.9’s 2021Thermal Guidelines for Data Processing Environments, there are several recommended operating envelopes that designers should adhere to and several tiers of acceptance. The widest, Class A4, includes supply air dry bulb recommendations ranging from 41°F to 113°F and moisture recommendations ranging from 10.4°F dewpoint/8% relative humidity to 75.2°F dewpoint/90% RH.
As server technology advances and becomes more tolerant to elevated temperatures and increased moisture content, direct evaporative systems become a very viable solution. Economization hours can be maximized if designed carefully to yield a highly efficient mechanical system (low power usage effectiveness, or PUE) that uses water sparingly (low water usage effectiveness, or WUE) to cool in the summertime and humidify in the wintertime.
Industrial facilities: Industrial facilities are great applications for evaporative cooling. Due to the shear square footage and volume of these facilities, ventilation air is generally a common strategy for cooling as the energy requirements to mechanically cool via direct expansion equipment would be too great. The inclusion of direct evaporative cooling allows for the ventilation strategy to be used, lowering the supply air dry bulb temperature and increasing RH.
Greenhouses: Direct evaporative cooling is extremely viable for greenhouse design applications because crops generally require a space well-regulated from a temperature and moisture content perspective. Because evaporative cooling allows for cooling and humidification to occur, crops can thrive in their optimal environment.
Indirect evaporative applications
Indirect evaporative technologies are especially useful for processes that necessitate the ability to sensibly cool, while ensuring the primary airstream remains isolated from the secondary airstream. Coils, heat pipes or heat exchangers are typically used as the “handshake” between the primary and secondary fluid streams and ensure the two remain isolated.
The most common application for indirect evaporative technology is in packaged air handlers. There are many configurations that can be chosen to ensure the indirect nature of the process is preserved. The most common “decoupler” is an air-to-air heat exchanger. These heat exchangers are generally constructed of a polymer-based material, making them very resilient to corrosion once wetted. As noted previously, water quality must be carefully analyzed to ensure that fouling or degradation of the heat exchanger will not occur to mitigate losses in heat exchanger effectiveness.
Face velocity, direct and indirect: To reduce or, more importantly, mitigate water carryover issues, it is crucial to consider media face velocity in design. If this is not considered, elevated airflow velocities will cause the water that trickles down the evaporative media pad face to become entrained into the airstream. Once entrained, water droplets can migrate downstream into ductwork or onto a mechanical room floor.
This presents safety and maintenance concerns for building operations teams and compromises the integrity of the mechanical design. Target velocities to mitigate water carryover in direct evaporative applications are 400 to 600 feet per minute.
Face velocity is also one of the variables that influences media effectiveness. Media saturation effectiveness is the ratio of the entering air and leaving air dry bulb differential to the process wet bulb depression. This metric indicates the media’s ability to maximize cooling effect by maximizing media saturation. When air is passed through media too quickly, evaporation of passing water is not optimized. As media face velocity increases, media effectiveness and thereby overall cooling potential decreases.
Evaporative pad thickness, direct and indirect: Fibrous media pads vary in thickness based on how much cooling is required; 12 inches is quite common and performs well under many inlet air conditions and thermal profiles. Pressure drop typically varies between 0.14 to 0.3 inches of water column and rises as pad thickness increases. As media thickness varies, so does media effectiveness. For the same face velocity and face area, a thicker media pad will possess a higher media effectiveness.
However, thicker media pads tend to dry out over longer periods of time. This is especially problematic for two reasons:
- Evaporative media pads must dry out periodically to retain their useful life.
- Evaporative media that is wet for indefinite periods of time could promote biological growth if water treatment is not carefully considered.
Outside air considerations, direct: One of the benefits of a direct evaporative system is that it can fully economize for large portions of the year. Full economization yields 100% outdoor airflow. When large quantities of outside air are brought into a building, outdoor air quality must be considered. Suspended particulate (pollan, dirt, sand, ash), serious pollution and wildfires would be detrimental in this mode of operation.
To combat this, filtration should be carefully considered and controls sequences should be reviewed to ensure that under these circumstances, outdoor air quality does not begin to adversely impact indoor air quality and ultimately occupant safety or equipment integrity.
Water quality requirements
Typically, most of the fluid sent to the evaporative media or wetted surface returns to the sump and will not evaporate. Because of this, a circulation pump is included in the sump to send any water that did not evaporate back through the media or over the heat transfer surface. This arrangement allows for a highly efficient and environmentally conscious arrangement from a water use perspective.
As recirculated water continues to evaporate, particulate and mineral concentrations increase within the recirculated water volume. Over time, the particulate and minerals precipitate out and scale begins to form on the evaporative surface. This contributes to reduced media effectiveness and increased media pressure drop.
To reduce the accumulation of scale and to maximize the effectiveness of the media or heat exchanger, it is important to adequately control water quality. It is critical to ensure that scale is mitigated through monitoring of conductivity for blowdown control and maintaining adequate cycles of concentration. For reference, the cycles of concentration value for a system represents the ratio of recirculation water conductivity (mineral content) to that of the makeup water.
Furthermore, scale can be reduced by incorporating softening, pH control and/or scale inhibitors — all under close review and advisement by the evaporative media pad manufacturer. Failing to do so could lead to premature degradation or failure of the media pads.
Another reason water quality is paramount with these types of systems is the possibility of biogrowth. Biogrowth may appear as a slimy or oatmeal-like substance (see Figure 4) on the evaporative media pad or wetted surface or present itself as a viral or bacterial agent within the process water. Not only is biogrowth detrimental to media performance and life over time, but it also presents a health and safety hazard to operations personnel.
To mitigate biogrowth in the evaporative cooling system, several precautions must be taken. First, filtration and/or ultraviolet sterilization of the process water and makeup water is required. This ensures all organic material, both living and inert, will remain free from the main process water loop. Also, biocides can be injected into the process water system to mitigate organic growth.
However, this should only be pursued under the direction and supervision of a water treatment provider. Great care must be taken to ensure the health of any occupants that may reside in the building.
As a general rule of thumb, the designer should always obtain a site water quality report during design. The site water quality report will indicate the parameters of the incoming water source. Important parameters that are obtained from this report are total dissolved solids, chemical content (i.e., chlorine), various water hardness values, conductivity, pH and various scaling potential indices. Once obtained, the designer should share the water quality report with two entities:
- Evaporative media manufacturer for general review and compliance checks with media material.
- Water treatment vendor for general review and compliance checks of all water treatment related chemicals and compounds against what is allowable by the media manufacturer.
Once these reviews are complete, the designer will have the water treatment recommendations and recommended cycles of concentration for use in the evaporative cooling system.
The process of evaporative cooling is extremely useful in the air conditioning industry. It is highly versatile and can be tailored to fit most applications. It provides an effective cooling strategy, while maintaining energy efficiency and environmental consciousness under the right circumstances. With general design and water quality considerations in mind, evaporative cooling technology can be deployed successfully.