Experimental investigation of frost formation on air to air counter flow heat exchanger in air handling unit and climatic influence on dry, wet, frost operation condition

The work presented in this paper investigates frosting problem on high efficient air to air counter flow heat exchanger. The presented investigation consists of two main activities.  
Firstly, experimental tests on a counter flow plate heat exchanger in a climatic chamber have been performed to better understand under which exhaust condition frost starts forming in the exit region of the exhaust port. A special experimental set-up to investigate frost formation on the air to air heat exchanger was developed. Set up consists of a cold chamber that could maintain freezing temperatures and a warm chamber imitating indoor condition. Both temperature and relative humidity could be controlled. Experimental tests have indicated that the temperature gradient on the exhaust port is very significant. While in one part of the exhaust port occurs frosting the other part has still positive temperature. Observed temperature difference across exhaust port was up to 6.8 ᴼC. From the experiments, it is observed that the frost starts to form when mean temperature of the exhaust air is at 0°C. Furthermore, it can be stated that even though frost formation depends greatly on the outdoor temperature, the level of indoor humidity also plays some role in location of frost formation. 
Secondly, the percentage of operation condition of the exchanger – dry, wet and frost - has been analyzed for four geographical locations – Southern Scandinavia, Central Europe, Southern Germany and Austria, and Scotland. Regarding indoor conditions, data has been created using dynamic simulation software BSim with loads representative for classrooms and offices. This investigation has shown that for the respected regions risk of frosting can occur but would not be higher than 5.8 % of operational time in the most severe investigated location. Finally, for the investigated high heat recovery efficient exchanger, it is discovered that heat recovery efficiency drops below required by Ecodesign 73% when outdoor temperature would drop below -14 °C.