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A typical coil energy recovery (runaround) loop system places extended surface, finned tube water coils in the supply and exhaust air streams of a building or process. The coils are connected in a closed loop via counterflow piping through which an intermediate heat transfer fluid (typically water or a freeze-preventive solution) is pumped.
This system operates for sensible heat recovery only. In comfort-to-comfort applications, energy transfer is seasonally reversible—the supply air is preheated when the outdoor air is cooler than the exhaust air and precooled when the outdoor air is warmer.
Advantages
- does not require that the two air streams be adjacent to each other,
- several air streams can be used,
- has relatively few moving parts -- a small pump and control valve,
- relatively space efficient,
- the cooling or heating equipment size can be reduced in some cases,
- the moisture removal capacity of existing cooling equipment can be improved,
- no cross-contamination between air streams.
Disadvantages
- adds to the first cost, to the fan power to overcome add coil pressure drop, and for the glycol circulating pump,
- requires added glycol pump and piping, expansion tank, and a three-way freeze protection control valve,
- requires that the air streams must be relatively clean and may require filtration.
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Increased ventilation air requirements and rising energy prices have stimulated interest in heat recovery systems. With heat recovery, existing system capacity can be increased without adding chiller or boiler capacity. This system is best applied in buildings where most of the supply ventilation air and the exhaust air is in one or two ducts that are not too far apart.
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As this system is for sensible heat recovery only, it is best applied in locations where there is a sizable heating season. Projects that require a large percentage of outdoor air can increase system efficiency by transferring heat in the exhaust to either precool or preheat the incoming air.
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Any building where reducing the sensible load on the cooling equipment is advantageous.
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Where there are a number of intake or exhaust air ducts that must be piped, the benefits are likely not to offset the higher fan and pump energy and first cost.
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Coil energy recovery loop systems are highly flexible and well suited to renovation and industrial applications. The system accommodates remote supply and exhaust ducts and allows the simultaneous transfer of energy between multiple sources and uses. An expansion tank must be included to allow fluid expansion and contraction. A closed expansion tank minimizes oxidation when ethylene glycol is used.
Standard finned tube water coils may be used; however, these need to be designed using an accurate simulation model if high effectiveness values and low costs are to be realized. Integrating runaround loops in buildings with variable loads to achieve maximum benefits may require combining the runaround simulation with building energy simulation.
Moisture must not freeze in the exhaust coil air passage. A three-way temperature control valve should be used that prevents the exhaust coil from freezing. The valve is controlled to maintain the temperature of the solution entering the exhaust coil at 30°F or above. This condition is maintained by bypassing some of the warmer solution around the supply air coil. If a dual- purpose valve is used, it can also ensure that a prescribed air temperature from the supply air coil is not exceeded.
Manufacturer's design curves and performance data should be used when selecting coils, face velocities, and pressure drops, but only when the design data are for the same temperature and operating conditions as in the runaround system.
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The coil energy recovery loop cannot transfer moisture from one airstream to another; however, indirect evaporative cooling can reduce the exhaust air temperature, which significantly reduces cooling loads. For the most cost-effective operation, with equal airflow rates and no condensation, typical effectiveness values range from 45 to 65%. Highest effectiveness does not necessarily give the greatest net cost savings.
The following example illustrates the capacity of a typical system:
A waste heat recovery system is desired to heat 10,000 cfm of air from a 0°F design outdoor temperature using an exhaust airstream at dry-bulb temperature of 75°F and a wet-bulb temperature of 60°F (at 100% effectiveness a maximum heating load of 810,000 Btuh) Air flows through identical eight-row coils at a 400 fpm face velocity. A 30% ethylene glycol solution flows through the coils at 26 gpm.
Freeze control typically maintains the heat recovery capacity constant for outside air temperatures below about 20°F. This constant output occurs because the valve has to control the temperature of the fluid entering the exhaust coil to prevent frosting. Above about 20°F the heating capacity gradually declines to 0 Btuh at 60°F outdoor air temperature (OAT). As the exhaust coil is the source of heat and has a constant airflow rate, entering air temperature, liquid flow rate, entering fluid temperature (as set by the valve), and fixed coil parameters, energy recovered must be controlled to prevent frosting in the exhaust coil. If the coils are selected for a 50% sensible heat effectiveness at 0°F OAT, the actual heat recovered is .5 x 810,000 = 405,000 Btuh.
When the three-way control valve operates at outside air temperatures of 20°F or lower, a maximum of 405,000 Btu/h is recovered. At the 0°F design temperature and a sensible effectiveness is 50%, the leaving air dry-bulb temperature is 35.5°F (= 405,000/{10,000 x 1.08)) and the 75°F exhaust air is cooled to 37.5°F.
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Thermosiphon Heat Exchangers
Heat Pipes
Enthalpy Wheels
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