Summary:

This paper deals with the determination of several efficiency types of a cross-current recovery exchanger which is a part of the air venting system in the swimming pool hall on the premises of the Czech University of Life Sciences (CULS) in Prague. The product is a cross-current plate exchanger with a heat-exchanging surface of antirust aluminium. According to the manufacturer, the exchanger is fit for temperatures common in air ventilation systems. The air is forced in by fans at a flow quantity of 16,000 m³, maximum speed 2000 rpm, electric motor output 7.5 kW, filters for the air coming from the outside – grade G4, pressure loss from 42 to 200 Pa, filters for the air coming from the inside – grade G3, pressure loss from 46 to 200 Pa. The results presented are derived from in-process measurements taken on 31 January 2007 and 7 February 2007, from 9.15 to 11.30 on both days. Air temperature and air humidity were measured with 9636-51B-type sensors by Ahlborn, connected to the AHLBORN ALMEMO 5990-2 centre. These sensors were placed into each of the four input/output channels, very close to the exchanger itself (Fig. 2). The data measured were saved in the centre memory every minute. Figures 3, 4, 5 and 6 show the temperature and humidity curves at the exchanger inlets and outlets on measurement days as well as outside air temperature (te) and outside air relative humidity (φe) captured by the met station on the CULS premises. Table 2 shows efficiency ranges calculated according to relations (1), (3), (4), (5), and (6) for air parameters ascertained at exchanger inlets and outlets on 31 January and 7 February 2007 and the calculated flow rates (Table 1). The difference between the outside temperature te and outside humidity φe values taken by the met station and the temperature te1 and humidity φe1 values measured at the recuperator inlet can be explained as resulting from the air being drawn in from the premises affected by the building and 8-m air piping situated in the building’s interior. Heat transmission to the surrounding air occurs despite the mineral wool heat insulation applied to the air piping. The slight increase of thermal efficiency observed on 7 February 2007 resulted from throttling down the recuperator feed air inlet flaps. Reducing the heated air discharge volume (see Table 1) resulted in a greater temperature difference t_e2 – t_e1. Energetic efficiency η is lower than thermal efficiency ηt because equation 3 takes into account the effect of condensed vapours in the cooled waste air. According to the manufacturers, the efficiency of top-class exchangers exceeds 70 %. This value might suggest that almost all the air energy available in the given space is utilised. Closer examination reveals that what is presented is thermal efficiency, which is always higher than other kinds of efficiency (see Table 2). Low exergetic efficiency is a sign that there still is a potential in terms of transmission of recovered air utilised energy (exergy).