The carry-over can be minimized by use of a purging sector; which is a plate that blocks off a sector of the rotor for the extract air and lets a small amount of outdoor air purge the rotor of the extract air. This eliminates the carry-over of extract air to supply at the air velocity for which the size of the purging sector is designed for. At lower velocity, there will be a small amount of carry-over.
The GOLD control system IQlogic features a function that controls the speed of the rotor according to the air velocity in order to ensure correct purging. By matching the rotor speed to the air flow, slowing down the rotor if necessary, transfer of extract air to the clean supply air is minimized.
For the purging function to work, it is necessary to have a pressure difference to drive the airflow through the sector. This requires the pressure before the rotor on the extract side to be lower than on the supply side after the rotor.
In GOLD units, the pressure balance can be measured using the tappings provided, and adjusted using the perforated pressure plates included in the unit.
Air Quality Control
The GOLD control function includes an option that ensures correct pressure balance even when the external pressures are changing. A pressure sensor is used to control a damper on the extract inlet so that the correct pressure is maintained continuously. This is ideal for DCV systems.
OACF and EATR
The most established factors for characterising internal leakages in rotary heat exchangers are EATR and OACF, which are both defined in EN 16798-3.
EATR defines how much extract air leaks to the supply air. OACF defines the ratio of outdoor air to supply air and is a measure of the leakage between outdoor and exhaust air.
EN 308 provides methods for measuring the performance of heat exchangers, including OACF and EATR. All Eurovent certified air handling units are measured in accordance with EN 308.
SFP is all about pressure and efficiency. To reduce the SFP there is a need to reduce pressure drop and increase the total fan efficiency.
A correct calculation of the SFP for the unit must include the leakages and the additional pressure needed to get a correct pressure balance.
The definition of SFPv demands that the leakages are included, and the operating points of the fans are calculated with the effects of leakage. This means that SFPv will be higher than SFP calculated without the leakage.