Fact is, all plate, rotary and heat pipe air to air energy recovery devices leak – but it is a very small amount compared to the overall air movement in a building. Swegon uses all forms of energy recovery devices including plates in GOLD PX, Rotors in GOLD RX and heat pipes as secondary recovery devices. They all have advantages and disadvantages. While a plate unit can come close to matching the sensible energy recovery performance of a rotary heat exchanger, nothing can come close to the latent performance of an enthalpy rotor. In applications where humidity control (summer or winter) is important, the energy savings of an enthalpy rotary recovery device is the best.
|Rotor||Sensible Plate||Enthalpy Plate||Heatpipe||Run Around Loop|
|Energy Recovered||Sensible or Enthalpy||Sensible||Enthalpy||Sensible||Sensible|
|Efficiency||50 to 80%||50 to 75%||55 to 75%||40 to 60%||45 to 65%|
|EATR||0.5 to 10%||0 to 2%||0 to 5%||0 to 1%||0%|
|OACF||0.99 to 1.10||0.97 to 1.06||0.97 to 1.06||0.99 to 1.01||1.0|
To help understand what the leakage level might be both Eurovent and AHRI 1060 standards include a measurement for leakage. The term is known as Exhaust Air Transfer Ratio or EATR. EATR compares a tracer gas concentration difference between the supply air and the outdoor air and the difference between the return air and supply air. It is expressed as a percentage. The test is performed in a lab as part of the certification process.
Looking at the range of EATRs for various energy recovery devices, it can be seen there is a large range, particularly for rotary devices. It takes a lot of practice to achieve low rates with rotors but it can be done. Swegon RX units are certified to 0.45% by independent labs.
There many steps that can be taken to minimize leakage when rotary energy recovery components are used including seals, purge and special controls.
Leakage in any energy recovery device is proportional to the pressure differential between the supply and return air streams. In a plate heat recovery device the leakage can occur between the plates (which are not perfectly sealed) or the seals around the plate and the cabinet.
For rotary energy recovery devices, the leakage occurs at the seal of the rotor perimeter (peripheral) and the seal between the supply air return air paths. In addition, the air in the core of the rotor can be carried from one air stream to another as the rotor turns. This is managed using a purge and controls.
Keeping the pressure differential between air streams as close to neutral as possible will improve the EATR for any recovery device. In Practical Guidance for Epidemic Operation of Energy Recovery Ventilation Systems by ASHRAE TC5.5, it is pointed out the fan position will have a significant impact on the pressure differential and thus the leakage level.
The figure above shows the 4 possible fan arrangements used in energy recovery units. Arrangement 4 should not be used as it will certainly lead to return air leakage to the supply air stream (very poor EATR value). Arrangement 1 and 2 are the most common with Swegon using arrangement 1. The EATR value given in an AHRI 1060 certified printout is for the component only so fan position is not taken into consideration. It is a good tool for comparing different products but a poor predictor for a field application.
The image above can be used to show that system design will also impact the pressure differential and thus the leakage rate. The image shows a typical Dedicated Outdoor Air System (DOAS) design. The supply fan total static pressure (TSP) is 3.45 inches w.c. while the exhaust fan is 2.3 inches w.c. Knowing this will not help understand the pressure gradient in the ventilation unit. To know the gradient, the pressure drops for both air paths must be plotted out as shown. Whether the duct static pressure is on the inlet or the exhaust side of the fan will not impact the TSP but it will impact the pressure differential. Maintaining building pressurization will also impact the pressure gradient. The same unit used in a different building application would yield a different pressure gradient and thus leakage rate.
In the example above, the pressure gradient on the leaving side of the rotor (based on supply air) is -1.8 inches w.c. – (-1.5 inches w.c.) = -0.3 inches w.c. Left as is, this system will leak air from the exhaust air side to the supply air side which is not desirable.
To resolve the issue, consider adding a pressure drop (add a damper) in the return air path just before the return air filter. Adjust the pressure drop until it is 0.3 inches w.c. across the device. While this will increase the exhaust fan TSP from 2.3 inches w.c. to 2.6 inches w. c. but it will also set the pressure gradient to zero so there will be no air movement between streams into the supply side.
Swegon GOLD RX units come standard with baffle plates that can be adjusted to deliver 0 to 0.08 inches w.c. pressure differential and avoid leakage.
Dynamic systems where the airflow rates change such as Demand Control Ventilation (DCV or variable air volume (VAV) will also see the pressure gradients change. By motorizing the damper in the return air path and adding a pressure transducer between the two air streams, it is possible to automate the pressure gradient control. For the GOLD RX unit, this option is known as Air Quality Control.
The Swegon selection software AHUD takes into account fan position and duct static pressures to calculate the EATR. It automatically calculates the pressure drop in the return air path (if required) and takes this into account for fan sizing and EATR calculation.
Through good design and application, the leakage rates between different energy recovery devices can be negligible. The decision of which energy recovery device to use should be based on the energy performance goals considering the choice will impact how the building operates for the next 20 years.