How evaporative cooling exhaust air can greatly—and inexpensively—increase runaround-coil energy recovery
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, E, is defined as:
E = (T1 − T2) ÷ (T1 − Ts)
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.
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Formulas for Calculating Energy Recovery
Runaround coils usually are used with 100-percent-outdoor-air systems. With 100 percent outdoor air, the amount of supply air equals the amount of return air, all of which is exhausted to the outdoors.
The amount of energy recovered by runaround coils depends on:
- Temperature approach between air and water.
- Saturation effectiveness.
With no limits on coil size, the temperature of air leaving a coil could be almost the same as the temperature of water (or glycol) entering the coil. Large coils, however, are expensive and usually do not fit in available spaces. A 5°F approach usually provides a good economical design.
For these calculations, it is assumed that:
The dry-bulb temperature of air leaving a runaround coil in an outdoor-air intake is 5°F higher than the temperature of the water entering the coil.
The temperature of water leaving a runaround coil in an exhaust stream is 5°F higher than the dry-bulb temperature of air entering the coil.
The amount of energy recovery (in British thermal units per hour), Q, from a system without evaporative cooling, then, is:
Q = V × 1.08 × [(TOA,DB − TEA,DB) − 10]
where: V = Airflow through each coil, cubic feet per minute TOA,DB = Dry-bulb temperature of outside air, degrees Fahrenheit
TEA,DB = Dry-bulb temperature of exhaust/return air, degrees Fahrenheit
For a system with evaporative cooling, energy recovery is:
Q = V × 1.08 × [TOA,DB − TEA,DB + E(TEA,DB − TEA,WB) − 10]
TEA,WB = Wet-bulb temperature of exhaust/return air, degrees Fahrenheit
The above formulas assume only sensible cooling of outside air. This will be the case in most areas. Any latent cooling will increase energy recovery beyond what is calculated here.
Energy-Recovery Calculations and Results
Calculations were performed for four U.S. cities: Dallas, Los Angeles, Miami, and New York. Return air was assumed to have a dry-bulb temperature of 75°F and a relative humidity of 40 percent. That is fairly typical. If relative humidity were below 40 percent, energy recovery would be higher. Calculations were performed with saturation effectiveness at 80 percent and 95 percent.
The results of the calculations are given in numerical form in Table 1 and graphically in figures 3 and 4. The outdoor-air temperatures listed are the 1-percent conditions from ASHRAE Handbook—Fundamentals.2 With the addition of evaporative cooling of exhaust air, energy recovery increases severalfold. Thus, in Miami, energy recovery increases by almost 300 percent at 80-percent saturation effectiveness and even more at 95-percent saturation effectiveness. It is interesting to note that even in the mild climate of Los Angeles, the addition of evaporative cooling results in a significant amount of energy recovery; without evaporative cooling, runaround coils are detrimental to performance because of parasitic losses.
Thus, a substantial increase in energy recovery is attainable with the addition of evaporative cooling of exhaust air.
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Design engineers and building owners are likely to have the following concerns:
Corrosion. The possibility of corrosion of a coil—its fins in particular—could be a concern. With proper bleed-off of water and chemical treatment, however, corrosion can be minimized. Galvanic corrosion of fins could be eliminated by using copper fins on copper tubes. It should be noted that cooling and dehumidifying coils with aluminum fins usually last 15 to 20 years in air handlers. Further, no unusual corrosion problems have been reported for sprayed coils.3
Additional cost and space. The cost of incorporating evaporative cooling is quite low; it is significantly less than that of a unitary evaporative cooler. Copper fins, if used, will increase the cost of a coil, but the total increase still will be small. An evaporative section typically adds 4 ft to the length of an exhaust-air unit. This usually is not a problem, as exhaust air handlers are much shorter than supply air handlers.
Additional power consumption. Additional power is consumed for water circulation and because of pressure drop as air flows through an evaporative section.
Atomizing evaporative-cooling sections normally do not require power, as line pressure usually exceeds the minimum 40 psi required. The pumping power required for wetted-pad and rigid-media sections is small and may be considered negligible. Power consumption for a spray chamber or sprayed coil will be higher, but is unlikely to exceed 1 hp for a 10,000-cfm system. (To put the added power consumption into perspective, consider that in Miami, it is recovering an additional 130,000 Btuh.)
Sprayed-coil and atomizing nozzles add negligible resistance to airflow. Wetted pads and wetted rigid media can add a pressure drop of 0.25 in. of water or more. For a 0.25-in. pressure drop in a 10,000-cfm system, the additional power consumption is about 0.6 hp. During cooling season, this amount is quite small compared with the gain in energy recovery. During heating season, it is a parasitic loss.
Annual energy-efficiency calculations will, in most cases, show net savings in energy are substantial despite the additional power consumption.
Maintenance. Additional maintenance consists of periodic cleaning of nozzles and wetted media. If the water supply is soft and clean, not much effort will be required. If water quality is poor, softening and filtration will be needed.
Experience With a Large System
The author observed a large air-conditioning system with sprayed coils serving a hospital complex in the Middle East over a period of about 10 years. The system consisted of numerous AHUs with 100 percent outdoor air and a total capacity of about 6,000 tons. Part of the system became operational around 1970, the rest around 1980. In 2004, most of the coils installed around 1980 were in fairly good condition. Of those installed around 1970, many had deteriorated, but many still were in operation.
Evaporative Cooling With Other Heat-Recovery Systems
Evaporative cooling could be used with other sensible-heat-recovery systems, such as heat pipe and air-to-air heat exchanger, with similar success. At least one heat-pipe manufacturer offers a spray system for the exhaust-air-side portion of a coil as an option.
One-hundred-percent-outdoor-air systems are common in hospitals and laboratories. Usually, runaround coils are used to recover heat from exhaust air to ensure no cross-contamination of supply air.
Evaporatively cooling exhaust air can increase runaround-coil energy recovery significantly at little cost.
Heat recovery with runaround coils enhanced by evaporative cooling helps to achieve the high energy-efficiency goals set by American Society of Heating, Refrigerating and Air-Conditioning Engineers standards; the LEED green-building-certification system; and government regulations.
1) Switenki, P. (2010, April). 2010 ASHRAE technology awards: Health-care facilities—Greening hospitals. ASHRAE Journal, pp. 42-48.
2) ASHRAE. (2009). ASHRAE handbook—Fundamentals. Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineers.
3) Coad, W.J. (1988, November). The sprayed coil option. Heating/Piping/Air Conditioning, pp. 121-122.
Mirza M. Shah, PhD, PE, long has been active in design, analysis, and research in the areas of HVAC, refrigeration, energy systems, and heat transfer. His formulas for boiling and condensation heat transfer are widely used and included in most engineering reference books. He is a fellow of both the American Society of Heating, Refrigerating and Air-Conditioning Engineers and ASME International. For more information, visit his Website at http://www.mmshah.org/, or contact him at email@example.com.
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