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

Adamovský, R.; Adamovský, D.; Herák, D.

doi:10.17221/4939-rae

Cited by 3 publications

(3 citation statements)

References 7 publications

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Section: Resultsmentioning

confidence: 88%

“…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.…”

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confidence: 99%

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Exergy of heat flows in exchanger consisting f gravity heat pipes

Adamovský¹,

Neuberger²,

Herák³

et al. 2005

Res. Agric. Eng.

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ABSTRACT:The paper deals with the analysis of the impact of inlet air temperature on the exergy efficiency and exergy of the losing heat flow and determination of the relation between the exergy and thermal efficiency in an exchanger consisting of gravity heat pipes. The assessment of heat processes quality and transformation of energy in the exchanger are also dealt with. a theoretical study (ABTAHI et al. 1988) on the basis of the standard balance of energy flows between the cooled and heated air currents, the coat and working medium of the tube. KeywordsThe purpose of this paper is to analyze the impact of the inlet air temperatures on exergy efficiency values, exergy values of heat flows and to determine the relation between the exergy efficiency and thermal efficiency in a heat pipe heat exchanger. The paper also deals with evaluating the quality of thermal processes and exergy transformation in the heat exchanger. METHOD Theoretical analysisThe exergy of the mass flow of 1 kg of gas during its passage through an open thermodynamic system can be determined from the following adiabatic (isoentropic) and isothermal expansion illustrated in Fig. 1.Gas expands from the initial state determined by the pressure p 1 and temperature T 1 to the state of the surrounding environment determined by the pressure p 0 and temperature T 0 . In the case of an adiabatic expansion, the temperature drops to T 2 = T 0 and the pressure p 2 > p 0 is reached. During the subsequent reversible isothermal expansion with a simultaneous heating, the gas pressure drops to p 3 = p 0 .In this case, the maximally utilizable energy (exergy) is equal to the technical work a t1.3 carried out between the initial and final states. The first law of thermodynamics implies that:where: da t -change in specific technical work (J/kg), dq -specific transferred heat (J/kg), dh -change in specific enthalpy (J/kg).Integrating equation (1), we obtain the following result:As 1-2 is dq = 0 in adiabatic expansion, the following applies to transferred heat:Substituting the figure in equation (2), we obtain the following result:This maximum technical work a t1,3 corresponding to the reversible process between the gas state determined by h 1 = h and s 1 = s and the state of the environment determined by h 3 = h 0 and s 3 = s 0 represents the maximum work content of the substance, i.e. exergy e x :The non-utilized specific energy, i.e. anergy a n , is h 0 at T 0 and shared heat at T 0 :According to equation (5), the change in the specific exergy of a heat flow during any process is as follows:In the event of an isobaric heat transfer, the specific enthalpy change dh is equal to the transferred heat dq. The specific entropy change is ds = dq/T in case of a reversible process. The above-mentioned facts indicate that equation (7) can be put as follows:T T Equation (6) can be put as follows:When heat is transferred between cooled air i and heated air e, heat flow dQ transmits exergy flow dE i (10) according to equation (8). Heated air e, however, receives only the ex...

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Section: Resultsmentioning

confidence: 88%

mentioning

confidence: 99%

Exergy of heat flows in exchanger consisting f gravity heat pipes

Adamovský¹,

Neuberger²,

Herák³

et al. 2005

Res. Agric. Eng.

Self Cite

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show abstract

“…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.…”

Section: Introductionmentioning

confidence: 99%

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

et al. 2021

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The performance of ventilation heat recovery has high impact to the total energy consumption of modern buildings and its sub-optimal performance results in a remarkable energy penalty. There are several issues, which can significantly affect the heat recovery efficiency such as the inaccuracy of sensors, errors in control systems, mechanical defects and incorrect setting of the system. In addition, the direct comparison of the designed and measured heat recovery efficiency is not necessarily meaningful due to varying boundary conditions e.g. mass flow rates. The main focus of this paper is to develop and demonstrate a simple automated method for monitoring the heat recovery efficiency of ventilation units using building monitoring system (BMS). As the supply and extract air mass flows and temperatures may differ from the calculated initial design parameters, the proposed solution is to analyse the heat recovery efficiency using the number of transfer unit (NTU) method. With this method the efficiency is always calculated by the limiting mass flow, meaning that the warm exhaust air can not transfer more energy to the cold supply air than it is able to contain. 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 developed method demonstrated that the application of NTU method enables identifying sub-optimal performance of ventilation heat recovery, which would not have been revealed by direct comparison of temperature efficiencies. In some cases, low measured temperature efficiency was associated with problems not connected to the heat recovery heat exchanger. The method also enabled to estimate the additional heating costs due to the decreased heat recovery efficiency.

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Uncertainty estimation of the mean specific heat capacity for the major gases contained in biogas

Trávníček¹,

Vitázek²

2020

Res. Agric. Eng.

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The paper is focused on the uncertainty estimation of the mean isobaric and isochoric specific heat capacity calculation. The differences in the data among the individual sources for the technical calculation are presented in the first part of the paper. These differences are discussed in this paper. Research of scientific work with listed values of measurement uncertainties has been carried out in the second part of the paper. Furthermore, mathematical models were calculated which describe the dependence of the specific heat capacities and temperature. The maximal error models were carried out. Two approaches were used for the calculation of the mean specific heat capacity. The first approach is the calculation with help of integration of the function which describes the dependence of the specific heat capacity and temperature. The second approach is the calculation of a simple arithmetic mean of the specific heat capacity related to the maximal and minimal value of the temperature interval. The conclusion of the work shows that the time-effective second way is applicable in the case of a narrow temperature range. A value of 5.5% (Δ<sub>t</sub> = 200 K) was reached for the relative uncertainty. This is a similar value to that in the case of using the first way.

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

Cited by 3 publications

References 7 publications

Exergy of heat flows in exchanger consisting f gravity heat pipes

Exergy of heat flows in exchanger consisting f gravity heat pipes

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

Uncertainty estimation of the mean specific heat capacity for the major gases contained in biogas

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