Hexagonal heat exchangers in air handling units can be arranged in series to increase the amount of energy recovered. The installation of two hexagonal heat exchangers in series means that the outside air and the exhaust air inlets are geometrically at the same level as the outside air and exhaust air outlets. Mounting only one hexagonal heat exchanger in a ventilation unit results in a geometrical exchange of the air inlet with the air outlet for both air streams (Fig. 5).
a)
b)
Fig 5. Air flow directions in a ventilation unit in the case of using a) two hexagonal heat exchangers and b) one hexagonal heat exchanger
Hexagonal heat exchangers manufactured in a fully automated Swiss Rotors factory are characterized by:
- spacing between plates of 2-3mm which produces effective heat transfer while also achieving low airside pressure drop,
- a specially shaped heat exchange surface enhancing turbulence of the air flow, thus improving the overall energy efficiency of the heat recovery unit,
- reversible energy recovery (building cooling or heating) with an efficiency of 90% while maintaining separation of supply and exhaust air streams,
- suitability for chemically aggressive environments and ventilation systems used in health care, chemical and process engineering, food engineering and food processing,
- system applications that require dry supply air in corrosive environments (e.g. swimming pools, sewage treatment plants, etc.)
- high corrosion resistance associated with the polymer or aluminum materials used in the hexagonal heat exchanger,
- standalone applications (systems without air handling or ventilation units) mounted directly to the ventilation ductwork
One of the significant differences between hexagonal heat exchangers and rotary heat exchangers manufactured by Swiss Rotors (http://swissrotors.com) is the ability to handle frosting. To a large extent, the frosting process of these heat exchangers should be eliminated through the use of appropriate controls automation in the air handling unit system.
Hexagonal heat exchangers can be used in suspended air handling units with horizontal configuration and in classic standing air handling units with vertical installation. Because the heat exchangers maintain separation of the outside (supply) air from the exhaust air, it is possible to use them in ventilation systems where the basic requirement is zero mass exchange between these streams.
Because mass exchange does not occur in hexagonal heat exchangers, moisture regeneration in these exchangers is not possible (which is the case in rotary exchangers also manufactured by Swiss Rotors (https://swissrotors.com)). Due to the lack of mass exchange in these exchangers, in specific conditions (below the dew point temperature) it is possible to condense the moisture. In this case, condensate drainage systems are necessary in these exchangers. This is especially true in winter when it is also possible to freeze the exchanger, thereby causing significant deterioration of the heat exchange.
Alternative methods have been used to prevent freezing of the exchanger. A typical technique to prevent exchanger freezing is the use of by-pass dampers. The heat exchanger is bypassed by the cold outside air stream. The bypass separates the outside air into two streams, one being directed to the regeneration exchanger and the other to the bypass. After the hexagonal heat exchanger, both outside or supply air streams re-combine. However, when the bypass is used, if the outside air is not properly heated, additional air heating systems (water heater, electric heater) can also be found in units with a bypass system. If it is permissible to mix the outside or supply air and exhaust air in ventilation units, there are recirculation systems (in this case no bypass is used) where some of the exhaust air returns to the building and is mixed with the outside air. Such systems can be found in exchangers installed on buildings with water pools (e.g., hotels, water parks).
