Energy recovery in air handling units (AHU) can be carried out in several ways. One way is the use of cross-flow or hexagonal heat exchangers, which utilize formed hexagonal heat exchanger plates built into a housing that causes two air streams to pass in opposite directions and transfer energy from one air stream to the other. Hexagonal heat exchangers allow for more efficient energy recovery compared to cross-flow heat exchangers due to the increased heat transfer surface resulting from the elongation of one dimension. Hexagonal heat exchangers are countercurrent heat exchangers realizing energy recovery in a passive system (without supplying additional electricity as is the case in regenerative rotary heat exchangers). One of the leading producers of highly energy-efficient hexagonal heat exchangers (Fig. 1) is the Swiss Rotors company (http://swissrotors.com).
2. Energy balance of the hexagonal heat exchanger
The operation of the hexagonal heat exchanger in a ventilation unit is subject to the laws of thermodynamics, with the basis for calculating its operating parameters as shown below.Figures 2a and 2b present the basic thermodynamic transformations for heat recovery applications as illustrated on the Mollier or Psychrometric chart. Figure 2a presents heat recovery without condensation of moisture, and Figure 2b shows the thermodynamic transformations taking place in the hexagonal heat exchanger with the transformation of moisture.
Fig 2. Interpretation of thermodynamic changes occurring in the hexagonal heat exchanger
a) without condensation of moisture;
b) with condensation of moisture from the exhaust air. Below are the basic energy and mass balance equations for hexagonal heat exchangers.
Fig 3. Key thermodynamic quantities required to balance a hexagonal heat exchanger
- Mass balances for outside (fresh or supply air)
- Mass balances for exhaust air
- The total energy balance for a hexagonal heat exchanger can be calculated as
To additionally obtain parameters related to the selection of the heat exchanger surface in the case of a hexagonal heat exchanger, it is necessary to write the equations related to the logarithmic temperature for the counter-current exchanger.
Fig 4. Logarithmic temperature difference for the hexagonal heat exchanger
3. Characteristics of exchangers manufactured by Swiss Rotors
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).
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 (http://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).
AHUs often use additional control automation systems to prevent excessive condensation and freezing of the heat exchanger. These systems monitor the dew point temperature at the air intake before the hexagonal heat exchanger. Depending on its value, they adjust the fan speed accordingly, thus affecting the mass flow rate of the air stream. If additional heating is possible at the outside air inlet to the hexagonal heat exchanger, the automation system can activate the heat source to prevent frosting of the hexagonal heat exchanger (at the expense of the energy efficiency of the entire ventilation unit).
4. Production of hexagonal heat exchangers For the hexagonal heat exchangers to operate with high efficiency both initially and over the design life of the device, the manufacturing technology used in their production is also important. Figure 6 shows the Swiss Rotors hexagonal heat exchanger production line.
The technology of producing Swiss Rotors hexagonal heat exchangers (http://swissrotors.com) ensures perfect repeatability due to the utilization of fully automated production lines that are maintained per standard operating procedures. For more information, please visit the Swiss Rotors website: http://swissrotors.com