Adiabatic cooling is an eons-old method of cooling spaces using the absorption of heat by water as it changes phase from a liquid to a vapor. Ancient Egyptians took advantage of this method by hanging wet cloths in the openings of their living quarters and letting the dry desert air pass through to provide a cooling effect.

While the basic principles remain the same, since the time of the Egyptians, evaporative cooling has evolved to include a fan (to replace the desert wind) and thick pads of wet corrugated material (to replace the single layer of cloth in a window).

To some, however, evaporative cooling is still an unsophisticated method, suitable primarily for spot cooling in some factories and facilities as well as residential cooling in the Southwest. Basic, single-speed, single-stage, all-or-nothing systems are still in widespread use and can be quite inexpensive to purchase and operate. Users simply switch on the system in the morning and switch it off at night. The evaporative system provides whatever cooling it can given the ambient dry-bulb and wet-bulb temperatures.

While the basic principles remain the same, since the time of the Egyptians, evaporative cooling has evolved to include a fan (to replace the desert wind) and thick pads of wet corrugated material (to replace the single layer of cloth in a window).

To some, however, evaporative cooling is still an unsophisticated method, suitable primarily for spot cooling in some factories and facilities as well as residential cooling in the Southwest. Basic, single-speed, single-stage, all-or-nothing systems are still in widespread use and can be quite inexpensive to purchase and operate. Users simply switch on the system in the morning and switch it off at night. The evaporative system provides whatever cooling it can given the ambient dry-bulb and wet-bulb temperatures.

  • The surface area of the water-absorption pads.
  • Absorption pad thickness.
  • Absorption pad material type (cellulose, fiberglass, metal).
  • The size of perforations and flute angles manufactured into the absorption pads.
  • The airflow rate of the fan section.
  • The water flow rate over the absorption pads.
  • The face velocity of the air passing over the absorption pads.

In a typical, simple industrial cooler, manufacturers usually provide a standard cellulose pad with the following characteristics:

  • A thickness of 10 to 12".
  • A standard face velocity of 500 ft/min.
  • A standard (but manually adjustable) water flow rate of 1.5 gal/min per square foot of pad surface.

These parameters essentially are fixed in a simplified system and provide cooling effectiveness of around 90 percent. This means that the leaving air dry-bulb temperature will be reduced by 90 percent of the temperature spread between the entering dry-bulb temperature and the entering wet-bulb temperature.

For example, if the entering air dry-bulb temperature is 90°F (32.2°C) and the simultaneous wet-bulb temperature is 80°F (26.6°C), then the leaving dry-bulb temperature will be 81°F (27°C) degrees. (The calculation is (90-80) x 0.90, which produces 9°F dry-bulb temperature reduction).

The nature of the direct evaporative cooling process also means that the relative humidity of the air entering the space will be approaching 85 to 90 percent. In a basic spot cooling application in a large space, those leaving air conditions usually are acceptable because the temperature of the entering air is reduced, and the high relative humidity is diluted into the surrounding space.

In addition, because direct evaporative cooling systems use 100 percent outside air, they frequently are used to replace the room air exhausted by the manufacturing process. This further reduces the impact of humidity because the humid air is removed quickly. Typically, the only goal is to provide a lower dry-bulb temperature near the machine operator.

In evaporative cooling systems intended for more precise temperature and humidity control, manufacturers will design a system specifically to meet the target conditions by evaluating all of those critical factors previously mentioned. Engineers will use combinations of those choices to optimize the unit design. For instance, rather than provide a single 12" pad, the system might be designed with multiple layers of pads with slight spaces between the pad faces so that staging of the leaving air temperature and relative humidity can be controlled.

PC 1021 Mestek Fiberglass. Banks of fiberglass absorption pads with separate water control valves are shown.

FIGURE 1. Reducing pad thickness also reduces cooling effectiveness, research shows. Banks of fiberglass absorption pads with separate water control valves are shown. Image provided by Mestek

Research over the past eight years at the University of Texas at Arlington has verified that reducing pad thickness also lowers the cooling effectiveness. For example, a 4" cellulose pad is only 53 percent effective at the same face velocity (and water flow rate) as a comparable 90 percent effective 12" pad (figure 1). This example might mean only a 5°F temperature change and a more manageable 50 percent relative humidity. By using solenoid valves to activate water flow to the different stages of pad surface, it is possible to incrementally control the leaving air temperature for greater accuracy (figure 2).

PC 1021 Mestek Fiberglass. Using solenoid valves to activate water flow to the different stages of pad surface.

FIGURE 2. By using solenoid valves to activate water flow to the different stages of pad surface, it is possible to incrementally control the leaving air temperature. A water flow management section with multiple valve configurations is shown. Image provided by Mestek

System effectiveness can be further refined by controlling the volume of air passing over the cooling pads. Research has shown that as face velocity over the pads is increased, cooling effectiveness is decreased. This has led to the use of variable-speed fans in evaporative cooling systems. In a further effort to reduce energy consumption in these systems, systems with energy-efficient electronically electrically commutated motors (ECM) can be used (figure 3). Combined with multiple stages of pads and variable volumes of airflow, these direct evaporative cooling systems can achieve levels of temperature control similar to those of mechanical cooling systems.

PC 1021 Mestek Fiberglass: Face velocity over pads increased.

FIGURE 3. As face velocity over the pads is increased, cooling effectiveness is decreased, according to research by the University of Texas at Arlington. Image provided by Mestek

Controlling the relative humidity of the air entering the space has always been a greater challenge for direct evaporative cooling systems. This is especially true when the space demands the greatest change in dry-bulb temperature. Psychrometric charts show that the greater the reduction in dry-bulb temperature, the greater the increase in relative humidity of the air. This is an inevitable result of the evaporative cooling process.

In order to mitigate this result and provide better humidity control, a direct evaporative cooling system might include damper systems that bleed some of the drier entering air into the leaving air, thus reducing the relative humidity of the air entering the space (figure 4). An alternative approach is to split the face area of the absorption pads into strips that have alternating wet and dry sections. While this second approach might not provide as precise a level of humidity control, it can be somewhat less expensive. Both of these solutions raise the leaving air dry-bulb temperature, so a delicate balancing act is required to meet both of the required leaving air criteria.

PC 1021 Mestek Fiberglass: Evaporative Cooling System

FIGURE 4. To provide better humidity control, an evaporative cooling system might include damper systems. Image provided by Mestek

Managing this balancing act as the ambient outside air conditions change throughout the day is beyond what could be expected from a human operator. This constantly changing task in an evaporative cooling system is handled by microprocessors or onboard computer systems with control software and sensor inputs (figure 5).

PC 1021 Mestek Fiberglass: microprocessors or on-board computer systems.

FIGURE 5. Control of the evaporative cooling system is handled by microprocessors or on-board computer systems with control software and sensor inputs. Image provided by Mestek

Temperature sensors measuring outdoor air, temperatures leaving each stage of the evaporative pad and the final leaving air temperature are basic requirements. Relative humidity sensors in similar locations also are required as well as sensors to measure static pressure across filters, pads and fans.

Water quality is an important factor in maintaining the performance of an evaporative cooling system. Failing to provide relatively clean water to the system can result in the cooling pads becoming clogged with debris or calcium deposits. In a simple industrial system, water quality often is maintained by routinely draining and refilling the water sump based on a calendar schedule. This could be done via automation or handled manually. Similarly, evaporative cooling systems can include particulate and pH sensors that will allow the control software to drain and refill the system at an optimum point.

Evaporative cooling systems usually are specified or purchased for their lower operating cost when compared to mechanical systems. Eliminating the need for compressors or chiller systems obviously reduces the electrical operating cost. In continuous operations, these savings can be significant.

If the facility uses an air-cooled mechanical system, some of the electrical savings will be offset by the increased water cost, however. In a water-cooled mechanical system, this would not be the case.

Other than cost reductions, the need for such evaporative cooling systems is process based. Often, it is easier to eliminate certain applications by considering some key factors.

Probably the easiest applications to eliminate are those that involve powders or materials that can be damaged by the absorption of moisture. Although some evaporative cooling systems can control relative humidity better than a simple system, they do still add moisture to the air. Applications such as food processing, printing and some applications involving dip vats might need to be eliminated as well.

Processes that produce high levels of heat, by contrast, will create local zones of low relative humidity. The combination of high local temperatures and low local relative humidity become obvious targets. If those processes also benefit from tighter control, then the adiabatic solution becomes a good choice. Examples of those processes might include:

  • Robotic work centers.
  • Operations near autoclaves, ovens and kilns.
  • Manufacturing of composite structures for aircraft or automotive use.

Each of these potential applications shares one common characteristic: the process is impacted by variations in temperature or humidity that cause dimensional changes to the materials or equipment involved. Controlling those conditions with evaporative cooling can lower the operating cost.

Robotics typically include an assortment of electric motors, hydraulic pumps and electronic sensors and controls. Many of those elements produce locally high temperatures and humidity levels.

Evaporative cooling systems can provide both tighter temperature control for predictable robot appendage expansion and humidity control that can mitigate the risk of electrostatic discharges caused by low humidity levels.

Using direct evaporative cooling for spot cooling near ovens and kilns is a classic application of evaporative cooling systems. While a simple system might seem to be sufficient in many of these cases, the temperature range of the material as it enters the device must be controlled in order to achieve greater accuracy in the heating process result. These applications also are difficult on the operator due to the high local temperatures, so any degree of cooling has a positive impact.

The manufacturing of composite materials is known to be sensitive to conditions in the work centers. Resins and composite materials themselves are susceptible to temperature and humidity deviations from their design targets. For example, FAA AC 21-26 specifies that the operational environment in the manufacturing of composites for aircraft must be within the boundaries of 65 to 75°F (18 to 23.9°C) and relative humidity between 46 and 63 percent. Interestingly, these conditions are similar to those used for data center design where direct evaporative cooling is widely used.

These are just a few of the possible applications of direct evaporative cooling systems.