What is the difference between rotary and counterflow heat exchangers?

As well as heat (sensible heat), rotary heat exchangers are also able to transmit moisture (latent heat). For use in singlefamily houses or dwellings with sensible efficiencies of up to more than 85 per cent.

The lamellar structure of the rotor (similar to corrugated paper) and the continuous rotary motion between the warm and cold air currents result in the rotor heating up in the extract air. In winter this heat is transferred to the cold supply air. This effect can be used in reverse in summer, because heat from the outdoor air is transferred to the cooler extract air. This enables a cooling effect to be achieved in summer, or also it can be used to recover cooling energy in air-conditioned buildings.

The heat exchange matrix is made from pure aluminium which transfers moisture precisely at the moment when condensation forms on the extract air side and is then reabsorbed by the outdoor air. For large temperature differences, the degree of recovered moisture can rise above 60 per cent. With our rotors, moisture is primarily transferred when it is needed, i.e. in winter. This enables problems with overly-dry air to be reduced or even avoided altogether. The moisture transfer can be controlled by adjusting the rotor speed.

The rotor rotation can result in small amounts of extract air entering the supply air (entrained rotation). A combination of air leakage in the housing and the rotor speed, in conjunction with the pressure differential of the unit (caused by filters, for example), can result in a certain amount of extract air being transferred to the supply air. In general, our equipment has an internal leakage rate of less than 5 per cent. This even falls to below 3 per cent for equipment certified for use in passive houses. This means that nowadays, units with a rotor are certainly comparable with plate heat exchangers, at least concerning internal leakage.

Thanks to the transfer of moisture, it is not necessary to drain away condensation in residential buildings. This means the ventilation unit does not need to be connected to a waste water pipe. Also, down to around -20 °C, the rotor does not freeze while transferring condensation, making frost protection for the heat exchanger unnecessary. Therefore, the entire capability of the heat exchanger can be exploited when outside temperatures are low. The colder the better.

Aluminium and plastic counter flow heat exchangers transfer heat (sensible heat) and are able to achieve efficiencies of over 90 per cent. Heat transfer In plate heat exchangers, the air flows through a series of parallel plates. Here supply air and extract air always. flow alternately on opposite sides. The heat energy is transferred through the plate from the warm air current to the cold air current.

If conventional aluminium or plastic plate heat exchangers are used, the resulting condensate must be removed from the unit via a waste water pipe. The siphon must always be filled with water to prevent air from being sucked through the duct system.

Supply air and extract air are completely separate from each other so, in theory, there is no portion of extract air in the supply air. In practice however, there can be internal leakages due to a combination of manufacturing tolerances of approx. 0.8 per cent for plate heat exchangers (manufacturer's specifications) and leakages in the housing, depending on the pressure difference in the unit. Here too, the limit for internal leakage is 5 per cent and 3 per cent for a passive house.

The higher the efficiency of the heat exchanger, the more condensate can form when temperature differences are large. The efficiency also affects the temperature below which the heat exchanger must be protected from frost, i.e. when the condensate starts to freeze. For high-performance counter flow heat exchangers with efficiencies of around 90 per cent, this may already be the case for outside temperatures below -3 °C. The lower the efficiency, the lower the freezing point. This also means the energy expenditure for frost protection varies correspondingly. For reliable frost protection, and to prevent too much heat being extracted from the heat exchanger, the outdoor air can either be preheated, or guided past the heat exchanger via a bypass and then over a heater. Another option is often used: the supply air fan is throttled, creating an imbalance between the supply air and the extract air. However this no longer works for modern, airtight houses. Frost protection can be achieved without using too much energy by investing in a brine-geothermal heat exchanger. No matter how it is done, frost protection costs money and energy! Special types of plate heat exchangers, such as themembrane counter flow heat exchanger, can also transfer moisture as well as heat. However the sensible heat transmission efficiency is reduced to around 75 per cent, i.e. below that of the rotors.

Membrane exchangers can transfer moisture and do not need to be connected to a waste water pipe. Depending on the humidity difference, moisture can also be transferred from the outdoor air to the extract air in summer. However, membranes are subject to wear; the pores start to close up and eventually the heat exchanger must be replaced. This means that the original moisture transfer performance reduces continually over time.

At around -8 °C, the frost point of membrane heat exchangers is somewhat lower than that of aluminium or plastic heat exchangers. Just like for aluminium or plastic heat exchangers, energy must be added to protect the heat exchanger from freezing when outside temperatures are low.

Every type of heat exchanger has advantages and disadvantages, which are more or less significant depending on the requirements. A plate heat exchanger can make the best use of its high efficiency if the outside temperature does not fall (or falls only briefly) below its specific frost point. Then frost protection is unnecessary and, due to the mild climate (not colder than around -3°C), the indoor climate does not become too dry, even without moisture transfer. A membrane heat exchanger plays to its strengths wherever as much moisture as possible is to be transferred continuously. Aluminium or plastic heat exchangers are often replaced by membrane heat exchangers after around two years, once the moisture from construction has gone. However, the moisture transfer cannot be controlled here and the membrane gradually closes. A rotor performs most strongly in climates down to around -20 °C. Here the rotor is able to generate maximum heat recovery without any frost protection at all, as well as transferring moisture as needed. Nowadays internal leakages are no longer a problem, since they are comparable with other heat exchangers. Within a dwelling a slight transfer of odour is unimportant, since the proportion transferred via the ventilation system is considerably smaller than, for example, via doors, clothes or the body of air able to move between rooms. With the open plan (living-eating area) designs common today, this is no longer of any significance.