Economic comparison of two energy-efficient design strategies for mechanical ventilation systems.

Two systems, one with air re-circulation alone and the other a combination of air re-circulation and energy recovery using plate heat exchangers have been analysed. For these systems, a set of equations was derived for partial or full re-circulation of air with and without plate heat exchangers. Also, as part of the analysis, the reciprocal ratio of investment and running costs was considered. In this analysis, the re-circulation factor provided by ventilation units, a bypass factor and the efficiency of a plate heat exchanger were considered to be variables. For the sake of simplicity, the outdoor temperature was assumed to change uniformly throughout the year from a winter time minimum to a summer time maximum. All the final mathematical expressions derived are non-dimensional. The technical and economic estimations proposed were those which gave the most pessimistic outcome, that is, those for which the outcome gave the lowest guaranteed common profit. The numerical calculations showed that quite different results were obtained depending on specific applications. Several economic indexes were estimated including payback times and capital and running costs for typical examples. The expected payback time for a system with plate heat exchanger is something like 2.0 years for an installation in the climate zone of Saint Petersburg, installed and run in accordance with Russian legislation.