Question: In the design of an energy-efficient house, a difficulty arises when the house is sealed up so well that various types of air pollution become both unpleasant and dangerous. The obvious solution is to provide ventilation, but this destroys the reason for sealing, i.e., to prevent the leak of thermal energy. A way to address this problem is to provide forced draft ventilation through a heat exchanger in such a way that the exhaust air (warm) transfers heat to the fresh air (cool). The objective of this problem is to design a heat exchanger to accomplish this. We will specify that the heat exchanger have a single-pass cross-flow arrangement. It will be of plate-fin construction and must use We would like to make the effectiveness as high as "practicable," although we do not really know what that term implies without heatexchanger cost information. Lacking this, let us use 0.85.
We cannot afford very large pressure drops on the two sides because the electric power to blow the air through might otherwise be used more profitably to provide house heating. But let us assume that a further constraint is that air velocity in the ducts leading to and from the heat exchanger must be held down to no more than 4 ft/s (1.2 m/s) to keep sound levels low. Pumping power should be checked, but this constraint in itself may keep the pumping power level low. Let us further specify that the frontal areas of the two fluid faces of the heat exchanger are to be the same as the duct cross-sectional areas, so that if rectangular ducts are used, no transition sections will be necessary. The system specifications are that the ventilation rate must be one house volume per hour. The house has 2000 ff (186 m") of floor area with an 8 ft (2.4 m) ceiling. The inside temperature of the house will be 70°F (21°C). The outside air temperature will be 35°F (2°C).