The paper analyses the leakage of air-to-air recovery heat exchanger consisting of 100 capillary thermal tubes 1.5 m long and aligned in ten rows. The operation of the recovery exchanger prototype was tested in a pigsty. The analysis focuses on how the exchanger leakage affects the heat utilisation efficiency of the air ducted from the pigsty, changes in supply/exhaust air flow rate ratio, supply air temperature, thermal output obtained, and exchanger’s total thermal and mass balances. The results obtained in in-process measurements and the mass and thermal balances show that the exchange leakage causes the outside supply air to contain 12.1–37.3% of the pigsty exhaust air. The percentage of the pigsty air increases with the increase of mass flow rates of supply and exhaust air. This leakage resulted particularly in a major change in supply and exhaust air mass flow rates, higher moisture content, temperature and thermal flow of the air conducted in the pigsty. The change in air flow rates resulted in ventilation change so that the overpressure ventilation took place instead of balanced ventilation. This change led to increased thermal losses by air penetrating through leaks in the peripheral building structures. Increased supply air moisture content made it necessary to increase ventilation intensity in order to meet the required pigsty air parameters with respect of animal wellbeing and protection of peripheral building structures against dampness deterioration. The thermal flow increase of the air conducted in the pigsty due to the high enthalpy of the pigsty air mixed resulted in the higher secondary heat utilisation efficiency. The actual efficiency, however, was considerably lower. The differences of 2.7 – 14.5% between the exchanger’s measured and actual thermal efficiency according to relation (16) and those of 18.7 – 55.8% between the exchanger’s measured and actual running efficiency (13) increase as the ratio of supply and exhaust air mass flow rates increase.
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