Upgrading mixed ventilation systems in industrial conditioning

Mixing ventilation systems (MVS) involve injecting cold air from the upper part of the room. The resulting turbulent mixing of the cold stream with ambient air determines a cooling of the whole volume and establishes an uniform temperature level with a very low vertical temperature gradient. While this method is commonly practiced it may not be energy efficient as the entire room volume gets cooled while often only the lower room volume, occupied by personnel at the shop floor level, needs conditioning. This may be a severe drawback especially in high-ceiling buildings and when process equipment with high thermal loads are utilized. In this paper an easily retrofitted improvement solution to existing MVS, called retrofit hybrid displacement ventilation system (RHDVS) is suggested and compared with a traditional MVS taking into account technical and economic performances. The study was carried out resorting to experimental measurements on a single-diffuser pilot installation of the RHDVS in an actual industrial facility, to characterize the existing MVS and estimate the performance improvement obtainable with the proposed RHDVS. An economic analysis was finally performed to assess the retrofit economic feasibility in case it is extended to the entire plant. Respect mixing ventilation, a reduction of the air flow rate cooling requirement from 12 to 6 kJ/kg resulted in the considered application, with an average cost saving of 50,000 (sic)/yr. A pay back time of less than 1 year followed, making the upgrade to RHDVS an interesting alternative. (C) 2009 Elsevier Ltd. All rights reserved.