Evaporative Cooling: The Inside Story
Using evaporative cooling for space cooling allows industrial facilities to improve labor productivity.
Evaporative cooling is not a modern concept — in fact, its origins can be traced to ancient times when Egyptians hung wet blankets across the doors of their homes to cool the space as fresh air blew in. During the 1800s, New England textile mills used a similar approach. Today, more sophisticated evaporative cooling systems are used for applications from industrial processes to dry cleaning facilities to schools to modern data centers.
Evaporative cooling is an efficient and relatively inexpensive method for cooling buildings and products, yet it remains underused. While the basic technology of the process of evaporative cooling is well understood, its potential — and its limitations — often are not. An understanding of evaporative cooling starts with the three different categories of systems: direct, indirect and hybrid.
Direct Evaporative Cooling. In direct evaporative cooling systems for space cooling, outside air is drawn over a damp surface and then into the space that is to be cooled. As the warm air moves across the wet surface, water evaporates, shedding molecules into the air and changing from a liquid state to a vapor state. The evaporation absorbs some of the heat from the hotter air and cools it as the water and air reach equilibrium.
In simple terms, heat is absorbed from the air and the remaining liquid water, lowering the air temperature and increasing its relative humidity. It is a bit of a controlled balancing act: if the air gets too cool, the water will not evaporate. If the humidity gets too high, the saturated air cannot absorb any more water.
Optimally, the relative humidity of the air leaving the system will be in the 80 to 90 percent range. Also, because direct systems use 100 percent fresh air, they can be used to offset process exhaust air losses while tempering the air.
Direct evaporative cooling tends to be the most effective because — other than the evaporative media itself — there is no other heat transfer device required. Every additional step of heat transfer results in lower effectiveness. (No heat transfer device is 100 percent effective.)
Indirect Evaporative Cooling. The indirect evaporative system utilizes a physical heat transfer component in which a secondary airstream is used to produce cold water by a direct evaporative cooling system. That colder water then goes through a heat exchanger, where it cools the primary airstream. The cooled primary stream then is circulated via blowers.
Using the heat transfer device results in no humidity increase, but it is not as effective as direct cooling because of the inherent losses in any heat transfer device. In other words, a direct evaporative cooling unit will always produce cooler air than will an indirect evaporative cooling unit.
An indirect evaporative cooling solution can use recirculated air from the space if desired. Therefore, it sometimes can be used effectively with variable process exhaust air losses by modulating the amount of outside air and return air to the unit.
Hybrid (Indirect/Direct) Method. This combination system offers the best of both methods and can supply air at the lowest temperature. In this method, some moisture is added to the airstream though less than in the direct system. The first stage of this process typically involves an indirect evaporative solution that lowers the temperature of the air that will enter the direct stage. The direct system then lowers the air temperature further — actually below the ambient wet bulb temperature — while adding some moisture.
Selecting an Evaporative Cooling System for Comfort Cooling in Industrial Spaces
Factors to consider when comparing evaporative cooling systems for space cooling include cost, function, setting, water usage and effectiveness.
Capital vs. Operational Costs. From a capital cost perspective, the direct system is the least expensive to install, and the hybrid indirect-direct is the most expensive. However, when you look at operational expense, the hybrid system often can have the lowest cost of ownership while the indirect system might have the highest cost over time.
Comfort Cooling and Air Volume. Most industrial facilities are quite large and have high bays, so the volume of air required for comfort cooling can be significant.
As an example, consider a facility with approximately 80,000 ft2 of production space. The company installed a comfort cooling system providing nearly 280,000 cfm of direct evaporative air to space. After the company installed the system, the direct labor-productivity rate increased by 20 percent.
Spot Cooling. It is much more common to use direct evaporative cooling for spot cooling rather than large-volume cooling. A drawback with large-volume comfort cooling is that much more electrical energy is required to run the fans and provide sufficient air movement.
Spot evaporative cooling can be used to offset process exhaust in a specific location and, at the same time, provide some operator cooling in the area. As with any makeup air application, it is important to be aware of air velocities near the process to avoid disrupting the performance of the exhaust system.
System Size Requirements. Because it is not possible to direct exactly where the evaporatively cooled air goes, there tends to be spillover in the areas around the process area. This means that the amount of air needed to supply the area will be three or four times more than the actual exhaust requirement. Indirect evaporative cooling will have many of the same considerations with the exception of the issues regarding moisture and powders.
Impact of Relative Humidity. Another consideration is that the air is moist, so if the manufacturing process involves powders of any sort, the powder’s affinity for water must be taken into consideration.
The increase in humidity that comes with evaporative cooling has consequences. Over the long term, it can lead to rust on unprotected metal surfaces. The plant maintenance procedure should always include regular visual inspections of columns, beams, machine tools and any other metal structures or production elements in the facility. Generally, it means the facility should plan regular painting as part of the maintenance to protect the metals in the building.
Water Usage and Quality. Water consumption is sometimes a concern in evaporative cooling applications, but a number of techniques can reduce water use. Much of the water that is consumed comes from draining (blowing down) the evaporative cooling system sump. A sump can contain hundreds of gallons of water, so implementing a system that reduces the number of times draining occurs can reap significant benefits.
A traditional practice is to simply schedule draining the sump on an arbitrary frequency. With the advent of more advanced computerized controls and the increase in available data, it is now possible to schedule the sump draining based on operating hours of the equipment. An even more sophisticated approach is available on some systems that use a conductivity sensor in the sump to determine the concentration of solids in the water and then trigger the draining of the sump based upon those results.
Using water that is not suitable for drinking but is not severely contaminated, or gray water, also can lower operating costs by reducing purchased, potable water consumption. This technique opens the door to rainwater-harvesting systems or recycling of condensate water from some processes. This method typically will increase capital costs because it normally will require the installation of a storage tank, pump and filter system. When combined with a conductivity sensor system, however, it can produce the lowest purchased water costs.
See the related feature article, "Evaporative Cooling: The Inside Story," to learn how using evaporative cooling for space cooling allows industrial facilities to improve labor productivity.