Planning advice

If humid extract air is cooled down, the relative humidity rises until condensate is formed. If the fresh air is so cold that the extract air is cooled below the freezing point of the condensate, it can freeze and block the heat exchanger on the extract air side partially – or in extreme cases even completely. 

The freezing limit, which is the minimum permissible fresh air temperature at which freezing does not yet occur, depends on:

• the extract air conditions (temperature t11 and humidity rh11 or x11)
• the (dry) temperature efficiency of the heat recovery unit
• the mass flow ratio (m2 : m1 = cold air : warm air)
• the exchanger design

The freezing limit can be calculated (approximately); however, practical applications reveal that the actual freezing behaviour also depends on other factors in addition to the physical boundary conditions.

Characteristic curve of the extract air fan 

Freezing narrows the flow cross-section of the extract air and the pressure drop increases. In fans with a flat characteristic curve, this reduces the warm mass flow and the heat exchanger ultimately freezes up completely. In a fan with a steep characteristic curve, approximately the nominal extract air volume is still conveyed even if the fan has a smaller flow cross-section, so the heat exchanger does not generally freeze up completely. The ventilation function is guaranteed – although with some limitations.

Mounting position and air flow

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 condensate produced is warm and rich in energy. This means that colder parts of the heat exchanger can be kept free of ice if there is sufficient condensate.
• If the amount of condensate is too small, the heat exchanger freezes faster due to the supply of condensate.

Possibilities for avoiding ice

If it must be ensured that the heat exchanger is always fully functional, the following measures are possible to prevent freezing:

• Preheating the fresh air
• Changing the mass flow ratio
• Thawing circuits

All measures reduce the overall heat recovery efficiency. 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.

• In wintry conditions (low absolute humidity in the fresh air), it is usually an advantage if the humidity in the extract air is transferred to the dry fresh air. Even if this humidity transfer is rarely considered in the economic efficiency calculation due to the lack of specifications, it makes a considerable contribution to increasing comfort.
• In all systems with controlled room air humidity (minimum, maximum), humidity transfer significantly reduces energy consumption and investment costs. The heat recovery unit works in the same way for both heat and humidity in that it acts as a rectifier between the extract air and fresh air; during the winter, the fresh air is humidified, while in summer conditions, it is dried.
• In systems with mechanical cooling, the necessary cooling capacity is calculated based on the summer conditions. So if humidity transfer (in this case by means of drying the fresh air) enables the required cooling capacity to be reduced, this results directly in a reduction in the investment, as this is generally greater than the additional expenditure required for humidity transfer in the heat recovery system.

• The condensate has to be collected and discharged so as not to damage the building fabric (floors, ceilings, roofs, etc.).
• The condensate contains pollutants; therefore, it has to be disposed of/treated correctly.
• If condensate reaches the supply air side due to tiny leaks, the supply air will be contaminated. (The impact of condensate leakage is generally much greater than air leakage.)
• In terms of material resistance, condensates are often much more aggressive than gaseous media. It is vital to check whether the materials used (seals!) are resistant to the condensate.

• Arrange condensate drip trays on the exhaust air and supply air sides.
• Keep the pressure difference as small as possible.
• Measure the pressure difference in the direction of the exhaust air stream.
• Use particularly tight/tested heat recovery units. 
• During cooling, it is possible for condensate to also develop on the fresh air side.

Based on decades of operational experience, it is generally to be noted that all heat recovery units demonstrate better results in terms of soiling than was initially to be expected. 

For “normal” applications – such as offices, hotels, banks, but also assembly halls, swimming pools, industrial warehouses – there are no problems whatsoever with regard to soiling. Nowadays, heat recovery units are even being used successfully in kitchen extraction systems, paint shops, welding fume extraction systems, and similar applications, provided appropriate precautions such as double filtering, automatic cleaning, etc. are taken.

Until now, the following applications have proved problematic in terms of soiling:

• Fibres, such as those in textile manufacturing
• Viscous condensate from solvents and greases
• Compounds of condensates and dust
• Paint particles that have not dried sufficiently

However, solutions are available for these applications too, although the cost-effectiveness must be reviewed in these cases.

Depending on the soiling, heat recovery units can be cleaned with a brush and vacuum cleaner, with warm water, air pressure, water and steam jets, or even with solvents. It is important that the heat exchanger is not damaged by cleaning. This applies with respect to both material resistance and strength. Particular care is required when using air pressure, water jet and steam jet equipment.

Heat recovery units requiring regular cleaning must be installed in such a way that:
they are easy to access when installed and the soiling is easy to remove (provide drain) and/or
they allow the exchangers to be removed easily for cleaning

It is necessary to control the heat recovery output if the supply air temperature is heated above the required room temperature due to internal heat loads. In other words, output control can only be omitted if:

• no/few heat loads are present
• cooling is taking place

The output can always be controlled by a bypass and, in some systems, also by means of unit-specific measures – such as speed control on the rotary heat exchanger. In practice, a bypass is used in many cases. The reasons for this are as follows:

• If a bypass for exhaust air and fresh air is used, the system can also be operated without a heat recovery unit (operational safety).
• When operating with a bypass, the heat recovery unit does not become dirty.
• The bypass can also avoid the risk of freezing.