Field study of an underground counterflow heat exchanger for ventilation air

An underground counterflow heat exchanger, 21 m long with diameter 0.3/0.2 m, placed at a depth of 3.5 m below the ground and connected to a pig house was built and tested. In the test during 1994, it proved possible to maintain the temperature of incoming ventilation air in the range of +8 degrees C and +19 degrees C when the ambient temperature varied between -10 degrees C and +32 degrees C. In the winter the temperature effectiveness of the air/air heat exchange was high, around 1, while the aggregate heat flow from the ground to the tube was negligible over the cold season. At periods with extreme temperatures the ground, particularly the vertical intake manhole, contributed momentarily to the heat transfer. The average power in the cold season was 1.3 kW with a maximum of 3.4 kW. The average cooling power, entirely from the ground, during summer conditions was 0.8 kW with a maximum of 2.3 kW. The heat exchanger functioned well without operational problems over a period of 5 years.