Exergy of heat flows of the air-to-air plate heat exchanger

Abstract

The employing of recuperative heat exchangers in the utilization of the secondary ventilation air heat in stables aims at lowering the consumption of fuel for heating, and improving the stable's microclimate, along with heat comfort of the animals and protecting external structures against deterioration. Whether or not these results will be achieved depends above all on the construction of the heat exchanger and of the heat exchange surfaces.In the technical field the energy balances are derived from the 1 st thermodynamic law, which covers merely the instantaneous quantitative aspects of heat flows. In terms of energy conservation law these balances are expressed in the form of constant sums of exergy and anergy. And that is why they cannot, or can only to a very limited extent, be used to define the transformation of part of the energy (exergy) to the less valuable energy (anergy), or in other words, the so-called energy losses conditioned by the irreversibility of processes. Using the 2 nd thermodynamic law we have a chance to trace the energy transformation through entropy, a quality indicator of every heat process and an accompanying symptom of process irreversibility.The plate air-to-air heat exchanger, whose exergy analysis is in the spotlight of our interest, operates, in terms of the laws above, in irreversible changes, while the change of entropy induced by the heat flows depends on the construction and surface of the heat exchange areas.An overview on the utilization of a physical model of entropy change in various technical fields is given in the BEJAN's paper (1996). The EGM (Entropy generation minimization) model, when applied to heat exchangers, expresses the change of total entropy S gen as a sum of partial changes of entropies evoked by heat exchange S gen,ΔT and fluid friction S gen,Δp . The ratio of the change of entropy in the real design S gen to the change of entropy in an optimal design, in which the entropy change reaches its minimum S gen,min , is called the entropy change coefficient N S . This method, which is well suited for finding out how close the design comes to the ideal alternative under given conditions, was published by BEJAN (1978).The SEKULIC paper (1990) mentions an equation for entropy change as a function of temperature and pressure differences. In case of an air-to-air heat exchanger the influence of the pressure difference is later ignored, while a new coefficient, HERN (heat exchanger reversibility norm), is introduced to describe the quality of the energy transfer. This coefficient considers a reversible process as a limiting case and the real irreversible processes as less favourable alternatives. The author assumes that there exist such shapes and dimensions of heat exchanges surfaces, for which the increase of entropy will be minimal.Another article (DE JONG et al. 1997) presents a simplified method for evaluating air-to-air heat exchangers. In principle, it is based on previous papers dealing with the generation of entropy in an irreversible process, yet the solut...

Exergy of heat flows in exchanger consisting f gravity heat pipes

“…These conclusions are also confirmed by low values of the exergetic factor ε ex,i2 . Significantly lower values of η ex,c and η t were detected at the same ratio of V i ×V e -1 when carrying out measurements of a plate heat exchanger (ADAMOVSKÝ, ADAMOVSKÝ 2004).…”

“…These conclusions are also confirmed by low values of the exergetic factor ε ex,i2 . Significantly lower values of η ex,c and η t were detected at the same ratio of V i ×V e -1 when carrying out measurements of a plate heat exchanger (ADAMOVSKÝ, ADAMOVSKÝ 2004).The transformation of exergy can be improved by optimizing the shapes of heat-exchanging surfaces, which applies primarily to the evaporation section of the heat pipe, and by selecting an appropriate working medium. Another way is to decrease the hydraulic friction of working media vapour and condensate.…”

Continuous automated ventilation heat recovery efficiency performance assessment using building monitoring system

“…As a result, the NTU method gives us the possibility to continuously compare the result to the temperature efficiency declared by the producer of the unit. The negative side of the simplified NTU method is the fact that this method is not taking into account the influence of latent heat and condensation of water vapour in exhaust side of heat exchanger [20]. As the relative humidity in extract and supply airflows is not measured in practice, then it is not possible to consider this.…”