Coil Loop Recovery Systems 2


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

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.

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.

Best Applications

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.

Any building where reducing the sensible load on the cooling equipment is advantageous.

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.

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.