The freezing limit, which is the minimum permissible outdoor air temperature at which freezing does not yet occur, depends on the following:
- The extract air conditions (temperature t11 and humidity rF11 or x11)
- The (dry) temperature efficiency of the heat recovery unit RWZ
- The mass flow ratio (m2 : m1 = cold air : warm air)
- The exchanger design
Using the finite element method, the freezing limit can be (approximately) calculated. Practical applications reveal that the actual freezing behaviour also depends on other factors in addition to the physical boundary conditions.
Characteristic of the extract air fan
When freezing begins, the ice in the cold corner narrows the flow cross-section of the extract air towards the outlet, thereby increasing the pressure loss. In fans with a flat characteristic this reduces the warm volume flow, which accelerates ice formation and the heat exchanger finally freezes up completely. If a fan with a steep characteristic even with a smaller flow cross-section, approximately the nominal extract air volume is still conveyed. Due to the higher flow velocity, the power is reduced and a state of equilibrium is established depending on the outside temperature; therefore the heat exchanger does not usually freeze completely. The ventilation function is guaranteed – although with some limitations.
Mounting position and the air ducting
The theoretical calculation does not take into account the fact that the condensate in the heat exchanger is moved not only by gravity, but also by the flow forces. In principle, this can have two effects:
- The arising warm condensate is rich in energy. This means that colder parts of the plate heat exchanger can be kept warm if there is a sufficient amount.
- If the amount of condensate is too small, the plate heat exchanger freezes faster due to the supply of condensate.
Structure in the plate heat exchanger
The internal structure of the exchanger has a large influence on the possible freezing of a plate heat exchanger. What matters is that exhaust air and condensate can flow into all directions (open conductor) rather than being carried monodimensionally within pipes (duct conductor). The latter generally freeze faster than open conductors.
Possibilities for avoiding ice
If it must be ensured that the plate heat exchanger is fully functional at any time, the following measures are possible to prevent freezing:
- Preheating the outside air
- Change in mass flow ratio
- Thawing circuits
It is characteristic of all these measures that the overall efficiency of heat recovery is reduced. From an economic point of view, however, this is of little importance, as the risk of freezing normally occurs only during a few operating hours per year.
Attention: If the extract air humidity (at standard conditions) is less than 4 g/kg, the dew point is < 0°C, i.e. condensation does not occur. The water vapour immediately changes from a gaseous to a solid state and sublimates (it "snows"). In order to freeze, the humidity of the extract air must therefore be more than 4 g/kg air.