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Run-around coil systems
These systems can recover energy in other ways besides recapturing the heat in the exhaust air. For example, waste heat in the process cooling water from the laboratory equipment can be recovered. Water chiller waste heat can provide domestic hot water and space heating for laboratories and offices with a run-around coil system. A high-performance, run-around energy exchanger can provide a large increase in overall HVAC system effectiveness from 50 percent to nearly 70 percent, large returns on investment, typically 33 percent, and short payback periods of three years. In new building designs and retrofits, a run-around system can reduce peak heating and cooling loads as well as total heating and cooling loads. The run-around system can have a significant impact upon the boiler and chiller capacity in new HVAC designs.

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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