Due to the internal leakage, the mass flows of extract air/exhaust air and outside air/supply air are changed; this has many effects:
- The exhaust air and outside air flows must be adjusted according to the leakage.
- This causes an increase of the air output of the fans.
- Caused by the leakage, the extract air can be transferred to the supply air; therefore, the quality of the supply air can be impaired.
- Due to the leakage, the configuration conditions of the heat recovery unit and thus its technical data (temperature efficiency, pressure loss) are modified.
Because of these effects, the leakage must be kept as small as possible; the planner can make a decisive contribution to this through a low pressure loss (from the outside air to the exhaust air) and the heat recovery system manufacturer through high-quality sealing systems.
Two leakage figures are defined which indicate the mass flows in relation to leakage-free operation::
L1 = m11 / (m11 - m1-2) = m11 / m1 Leakage figure exhaust air
L2 = m21 / (m21 - m2-1) = m21 / m2 Leakage figure outside air
m1 = leakage-free exhaust air flow
m2 = leakage-free outside air flow
In addition, the circulating air figure U describes the proportion of circulating air in the supply air:
U = m1-2 / (m21 - m2-1) = m1-2 /m2 Circulating air figure
With these three key figures, a heat recovery unit can be comprehensively defined in terms of leakage; the values should always be given as a specification.
If no project-specific data is available at the time of design, the leakage-free flows for exhaust air m1 and outside air m2 must be increased by 10% each.