Metoder för att öka temperaturverkningsgraden för en motströms plattvärmeväxlare. En undersökande studie av parametrar som inverkar på temperaturverkningsgraden.

In passive and low-energy houses the energy consumption of the heating system should be minimized in order to meet the standards of such houses. The ventilation system should therefore be optimized to recover as much heat as possible from the exhaust air. The company REC Indovent produces air to air counterflow fixed plate heat exchangers for this type of houses. They provide two heat exchangers for this master thesis, which are tested practically with focus on thermal efficiency. The first one is a standard unit and the second is custom made with a smaller distance between the lamellas creating a denser heat exchanger. Measurements show that the denser heat exchanger returned a higher thermal efficiency, likely because its larger heat transfer area. Experiments also show that the airflow was uneven distributed through the exchanger on the exhaust side since the fan is placed right before the heat exchangers inlet. Calculations showed that an even distributed airflow can improve the thermal efficiency. Air guides were used to direct the air more even and consequently the thermal efficiency was increased. The benefits of using air guides proved to be more distinct for the original heat exchanger than for the modified, since the last one showed a relatively good air flow distribution without using any type of air guidance. This is probably dependant on the fact that the pressure drop is larger for the modified unit than for the original one and then works as a diffuser. The use of air guides resulted in an increase of thermal efficiency from 73.0% to 78.4% for an airflow of 55 l/s and from 74.6 to 81.0% for an airflow of 86 l/s. The thermal efficiency of the heat exchanger with a larger surface area was measured to 82.3% for an airflow of 55 l/s and 80.8% for an airflow of 82 l/s. The power consumption was logged during the test in order to predict how the modifications affected the Specific Fan Power, SFP. The SFP was slightly increased when using air guides in both units. The increase was measured to 0.01 kWh/(m3/s) for the lower airflow and up to 0.18 kWh/(m3/s) for the higher airflow. Since an increased thermal efficiency and also a more compact product will increase the risk of attaining freezing damages in the heat exchanger, these modifications should also be tested during cold conditions when freezing is likely to occur