8113 Runaround Systems

Whenever it is necessary to ensure isolation of heating and heated systems, or when it becomes advantageous to use an intermediate transfer medium because of the long distances between the two systems, a run-around heat recovery system is used. Figure 8.5 shows the schematic of a runaround system which recovers heat from the exhaust stream from the heating and ventilating system of a building. The circulating medium is a water-glycol mixture selected for its low freezing point. In winter the exhaust air gives up some energy to the glycol in a heat exchanger located in the exhaust air duct. The glycol is circulated by way of a small pump to a second heat exchanger located in the inlet air duct. The outside air is preheated with recovered waste-heat that substitutes for heat that would otherwise be added in the main heating coils of the building's air handler. During the cooling season the heat exchanger in the exhaust duct heats the exhaust air, and the one in the inlet duct precools the outdoor air prior to its passing through the cooling coils of the air handler. The principal reason for using a runaround system in this application is the long separation distance between the inlet air and the exhaust air ducts. Had these been close together, one air-to-air

Preheat coil

Figure 8.5 Runaround heat-recovery system.

Preheat coil

Figure 8.5 Runaround heat-recovery system.

heat exchanger (with appropriate ducting) could have been more economical.

Figure 8.6 is the schematic diagram of a runaround system used to recover the heat of condensation from a chemical bath steam heater. In this case the bath is a highly corrosive liquid. A leak in the heater coils would cause the condensate to become contaminated and thus do damage to the boiler. The intermediate transport fluid isolates the boiler from a potential source of contamination and corrosion. It should be noted that the presence of corrosive chemicals in the bath, which dictated the choice of the runaround system, are also in contact with one side of the condensate heat exchanger. The materials of construction for that heat exchanger should be carefully selected to withstand the corrosion from that chemical.

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