Run-around systems

The run-around heat recovery system is a simple piping loop, containing a circulator; the loop connects a finned-tube coil in the exhaust plenum with a finned-tube coil in the make-up air plenum or AHU. The warm exhaust air heats the circulating fluid; this fluid then warms the cool make-up air. The heat recovery system typically operates to preheat outdoor make-up air but also to pre-cool the make-up air when the exhaust air stream is cooler than the outdoor makeup air. [Brown, 1993; Industrial Ventilation: A Manual of Recommended Practice - 22nd Edition, 1995]

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. [Besant and Johnson, 1995]

Depending upon its arrangement and features and a thorough sizing analysis by an energy engineer, a system's overall effectiveness can be much higher than in the past (50 percent or less) using run-around-coil-tube heat exchanger systems that are accurately designed for maximum cost-effective performance. Because of the relatively small temperature differences between the energy exchange coils, very accurate simulation and cost models are needed. [Besant and Johnson, 1995]

A main advantage of the run-around coil is that exhaust and supply duct systems can be separated by a significant distance. This feature reduces the potential for contamination of fresh intake air by exhaust air. [Industrial Ventilation: A Manual of Recommended Practice - 22nd Edition, 1995; Besant and Johnson, 1995]

Depending upon utility rates, situations with ventilation air-flow rates greater than 10,000 cfm (5000 L/s) are usually good opportunities to use run-around systems. [Besant and Johnson, 1995]

More:

Manifolded exhaust and energy recovery

Run-around vs. indirect evaporative pre-cooling

Condenser water heat recovery

Coupling fluid glycol concentration

Simulation model

Cooling load reduction

Run-around system commissioning

Run-around system limitations


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