Minimizing the energy consumed by buildings is of great interest because of stringent state and federal regulations, LEED green-building-certification-system requirements, and ever-increasing energy costs. One way to decrease energy consumption is to recover heat from the air exhausted from buildings. Many heat-recovery devices are available. They include:

• Air-to-air heat exchangers with metal walls.
• Air-to-air heat exchangers with permeable walls.
• Heat wheels.
• Enthalpy wheels.
• Heat pipes.
• Runaround coils.

While each of these devices has its merits, runaround coils have the unique feature of offering complete flexibility in terms of the location of supply- and exhaust-air streams; all of the other devices require the air streams to be close together. Another advantage of runaround coils is the impossibility of cross-contamination between supply air and return air; this is a risk with all of the other devices. For that reason, runaround coils are widely used for energy recovery in hospitals and laboratories.

Energy recovery with runaround coils can be greatly enhanced with evaporative cooling of exhaust air, a long-established fact that seems largely forgotten by design engineers and building owners, as all of the applications the author has come across in recent years use dry coils only. Examples are 13 hospitals in California that recently won awards for energy efficiency.1 The hospitals use runaround coils—without the evaporative-cooling enhancement—to recover energy from exhaust air. Many of the hospitals are in climates that are quite hot. As this article will show, energy recovery would be several times greater if exhaust air were evaporatively cooled.

The Runaround-Coil System
Figure 1 shows the essential features of a typical runaround-coil energy-recovery system without evaporative cooling. One coil is located in the exhaust-air stream; another is located in the supply air-handling unit (AHU) upstream of the heating and cooling coils. The two coils are interconnected with pipes. A pump circulates water (or glycol) through the coils. During summer, water cools in the exhaust stream and, in turn, cools outdoor air passing through the other coil. A three-way valve with controls is placed in the circuit to prevent freezing during winter.

Figure 2 shows a runaround-coil energy-recovery system with an evaporative-cooling section. An evaporative-cooling section can be of many types, including:

• Wetted pad.
• Rigid media.
• Sprayed coil.
• Spray chamber.
• Mist generator.

In a mist generator, water is forced through fine nozzles; any small amount of unevaporated water is drained away. A minimum water pressure of about 40 psi, which normally is available in water lines, is required. In the other types, unevaporated water drains to a sump and is recirculated. In the wetted-pad and rigid-media types, water is recirculated by a pump to wet the surface of a pad or rigid media. In the sprayed-coil type, a pump sprays water on the surface of a heat-recovery coil. The spray chamber consists of rows of spray nozzles, through which the pump forces water to form fine droplets.

Saturation Effectiveness
Saturation effectiveness, E, is defined as:

E = (T1 − T2) ÷ (T1 − Ts)

where:
T1 = Dry-bulb temperature of air entering evaporative cooler, degrees Fahrenheit
T2 = Dry-bulb temperature of air leaving evaporative cooler, degrees Fahrenheit
Ts = Wet-bulb temperature of air entering evaporative cooler, degrees Fahrenheit

All of the previously mentioned systems are capable of providing 80- to 95-percent effectiveness.