Indirect cooling technologies

Indirect evaporative cooling allows for space cooling without the addition of moisture. The cooled air never comes in direct contact with the conditioned air, which should help reduce erosion and possible leaks. Consider these codes, standards, and best practices of indirect evaporative cooling.

04/18/2014


Learning objectives:

  1. Understand the evaporative cooling process by comparing and contrasting direct and indirect evaporative cooling.
  2. Learn about specific applications of indirect evaporative cooling systems.
  3. Know which standards and guidelines should be considered.

This article has been peer-reviewed.The cooling effect of evaporating water into air is a simplistic, ancient method for building cooling. It is primarily known for having substantial energy savings potential over conventional refrigerant-based cooling for various applications. While this may be true for direct evaporative coolers, indirect evaporative coolers have a wide and growing range of applications.

Evaporative cooling has gained acceptance by the modern HVAC industry. Depending on the application, evaporative cooling can provide one of the simplest methods for cooling over other mechanical refrigeration alternatives while boasting lower energy consumption, no chlorofluorocarbon (CFC) refrigerants, and cost-effectiveness. Though water conservation is of great concern today, evaporative cooling can be appropriately applied to minimize excess water use. Common applications range from comfort cooling in residential and commercial buildings to spot cooling for industrial applications such as mills, foundries, and power plants, and even cooling for highly critical environments such as data centers.

Evaporative cooling works by using the evaporation of water across an airstream to provide the cooling effect, reducing the air’s dry-bulb temperature while increasing its humidity ratio. The air undergoes what is known as an adiabatic saturation process by which the amount of heat removed from the air is equivalent to the heat absorbed by the water as the heat of vaporization. No heat is added to or extracted from the adiabatic process, which follows the line of constant wet-bulb temperature on the psychrometric chart (see Figure 1 for direct cooling). Note that the total enthalpies before and after the adiabatic saturation are the same. The evaporative cooling process is most commonly applied to projects located in dry climates and requiring high air change rates or where operating cost savings can be demonstrated over mechanical refrigeration technologies.

Limitations of evaporative cooling are primarily based on climate conditions, specifically wet-bulb temperature, makeup water consumption, and increased maintenance required to minimize scale accumulation. Water treatment for the prevention of algae and bacteria growth is also an important factor of the design. All of these need to be taken into consideration when specifying an evaporative cooling system. It is important to note that water scarcity in climates ideal for evaporative cooling can often be the limiting factor in the application of this technology.

Figure 1: The psychrometric process is shown for direct and indirect evaporative cooling. Courtesy: jba consulting engineersDirect, indirect evaporative cooling

Direct evaporative cooling (DEC) is the process by which the primary airstream comes in direct contact with the water, either by an extended wetted surface material or with a series of spray nozzles. This direct contact reduces the primary airstream dry-bulb temperature and also increases relative humidity but is limited by the wet-bulb temperature of the entering airstream. Under higher wet-bulb temperatures in humid climates, DEC can be economical for spot cooling in a number of industrial applications or be applied as makeup air for kitchens and laundries. Under lower design wet-bulb temperatures, DEC can be practically applied for comfort cooling. Care must be taken when designing DEC systems to ensure that space relative humidity levels are not compromised.

Indirect evaporative cooling (IEC) combines the benefits of the evaporative cooling effect for sensible cooling without the addition of moisture into the primary air stream. During this process a secondary air source is used to remove heat from the primary air through the use of a heat exchanger. The indirect application has the added benefit of lowering the primary air wet-bulb temperature. The cooling process is similar to cooling with mechanical refrigeration when plotted on a psychrometric chart (see Figure 1). Although indirect evaporative coolers are limited by the effectiveness of the heat exchanger, they can allow for enhanced cooling capability by applying building exhaust or precooled air into the secondary airstream to lower its wet-bulb temperature. This produces lower dry-bulb temperatures that approach the wet-bulb temperature of the secondary airstream. The process can further reduce the leaving air temperature of the primary airstream, improving its range of cooling beyond that of the direct evaporative cooler alone. Latent cooling of the primary airstream can also occur when the secondary air wet-bulb temperature is below the primary air dew point.

IEC and associated technologies are being used on projects globally for many different applications. IEC technology can be divided into two main categories: packaged indirect evaporative air coolers and cooling tower/coil systems.

Figure 2: This is an example of a packaged IEC unit, which provides primary cooling for a large data center project. Courtesy: MuntersPackaged indirect evaporative air coolers

The basic packaged IEC unit consists of a primary and secondary air moving device, an evaporative heat exchanger, and a water collection and recirculation system that includes a wetting apparatus and pump. Packaged IEC units may be selected in various configurations of this arrangement with all components in a single-unit casing. Some packaged units are also available with supplemental direct expansion (DX) refrigeration or chilled water coil, a direct evaporative cooling section, and/or heating section. The folded metal or plastic sheet-constructed heat exchanger typically is protected from airborne debris with air filters and may be provided with a corrosive-resistant or moisture-retaining coating. The arrangement of the different sections and components listed above, and the order in which they interact with the primary and secondary airflow, affects the unit’s performance and efficiency under different operating conditions.

A typical packaged IEC unit component arrangement involves placing the IEC heat exchanger upstream of the refrigeration coil in the primary airstream to reduce the coil’s sensible cooling load. The intent of this arrangement is to increase the unit’s overall efficiency during peak load periods. The energy savings is approximately equal to the difference of the energy reduction from the decreased refrigeration load and the increase from the various components. These energy components include IEC water pump, evaporative cooler fan motor, and losses experienced by the primary fan due to the pressure drop across the IEC heat exchanger. The unit efficiency is increased by placing the DX condenser coil in the secondary airstream downstream of the IEC heat exchanger so that the condenser coil is, effectively, direct evaporative cooled. Along with the unit’s efficiency relative to electrical energy consumption, the IEC section’s water consumption rate must also be considered when determining the unit’s overall operating cost.

Packaged IEC units can also be arranged to incorporate heat recovery systems. IEC units that use energy recovery wheels, heat pipes, or run-around coils to precool raw outside air before additional cooling methods will increase the cooling effect by including a DEC section upstream of the heat recovery device to lower the dry-bulb temperature of the exhaust airstream. During any required heating operation, this unit arrangement can operate normally (use the heat recovery device to exchange heat from the hot exhaust airstream to the cold outside airstream) by disabling the DEC section. The DEC section is freeze-protected because it is separated from the outside air during freezing conditions.

Figure 3: In this graphic, the schematic layout and major componentry of a recirculation air conditioning by evaporation (RACE) type packaged IEC unit is shown. Courtesy: jba consulting engineersPackaged IEC units can be an attractive option for data center cooling. The energy efficiency of evaporative cooling combined with rapidly advancing data center server technology allows for increasingly higher dry-bulb temperature cold aisles, which contribute to the application of IEC. Figure 2 shows a packaged IEC unit specifically designed for data center cooling applications. This particular unit, also known as a recirculation air conditioning by evaporation (RACE) unit, includes air-to-air evaporative cooling heat exchangers.

Figure 3 shows a schematic layout of the unit and system components. In general, the unit operates by rejecting heat from the data center hot aisle return air to the atmosphere through the heat exchanger. With low enough ambient temperatures and certain hot and cold aisle design criteria (100 F hot and 70 F cold aisle dry-bulb temperatures are increasingly becoming common), the heat exchanger can reject all of the heat generated by the data center without the added need for evaporative cooling. The heat exchanger is referred to as being “dry” during this mode of operation, and the unit is essentially functioning as an indirect airside economizer.

Figure 4: Air-to-air heat exchanger tubing is found in a packaged IEC unit. Courtesy: MuntersFigure 4 shows a dry air-to-air heat exchanger consisting of watertight polymer tubing through which the primary or recirculated air flows via a supply air fan. The secondary, raw outside air flows around the tubing during dry operation via a scavenger fan. This heat exchanger should operate between 45% and 50% efficiency.

When ambient conditions do not allow for dry operation, the heat exchanger is wetted via the recirculation water system. The water evaporates as secondary air is drawn through the heat exchanger and the primary air is cooled. In this mode, the heat exchanger should be able to cool the hot aisle primary air dry-bulb temperature to within 70% or 80% cooling effectiveness of the secondary air wet-bulb temperature. The primary supply air temperature can be controlled by adjusting the secondary airflow via the scavenger fan variable frequency drive (VFD).

In addition to efficient operation, packaged IEC units with air-to-air heat exchangers benefit data centers by limiting the introduction of outside air pollutants to sensitive and expensive data center equipment during economizer mode periods. A data center cooling unit in economizer mode without indirect heat exchange operation may provide 100% outside air (OA) directly to the space during poor air quality conditions unless controlled otherwise. Indirect systems also do not require barometric relief and will have little effect on space humidity since they operate by sensibly cooling space recirculated air.


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