The improvement of heat exchange processes represents an important challenge in the modern energy scenery; in particular, the industrial need to be able to have innovative solutions with limited dimensions and high efficiency is increasingly accentuated: therefore, compact heat exchangers are used. This thesis presents the study of the thermal design of a dry cooler heat exchanger, in which the currents of fluid involved are air (humid) external side and water (glycol) internal side. Consequently to an accurate bibliographic review, in which it was possible to study in deep the main issues under examination and identify the criteria commonly adopted for the resolution of this type of problems, a calculation code was implemented in the Python environment which applying the method epsilon-NTU, changing the geometric configuration and the operating conditions of the heat exchanger, is able to process its thermal power, its effectiveness and other quantities of interest. A case study was defined and a parametric analysis was conducted with the aim of achieving a validation of the numerical model by comparing the results obtained with those elaborated by a commercial program currently in use in the company (already validated). Analyzing the results, it was possible to conclude that the model is overall solid; in particular, the correlation of Wang and Chi proved to be precisely suitable for predicting the thermal performance of a staggered heat exchanger with the inlet air at medium-low temperatures (percentage error of 2%). Furthermore, it has been found that the correlation proposed by McQuiston is the most accurate as regards the calculation of the external friction factor, and then for the evaluation of the air side pressure drops (maximum percentage error of 15%, largely lower than the other correlations); the set of equations proposed by Gray and Webb was found to be the most precise for determining the thermal performance for inline heat exchangers (maximum percentage error of 3%). In conclusion, it can be observed that, while the results on the thermal part are very encouraging, the predictions regarding the pressure drops on both sides are overall less accurate, so it suggests the future need to calibrate the model on the basis of experimental data.
Il miglioramento dei processi di scambio termico rappresenta un’importante sfida nel panorama energetico moderno; in particolare, è sempre più accentuata l’esigenza industriale di poter disporre di soluzioni innovative con dimensioni contenute e ad elevato rendimento: si ricorre quindi all’adozione di scambiatori di calore compatti. Questa tesi propone lo studio del design termico di uno scambiatore di calore a pacco alettato, in cui le correnti di fluido coinvolte sono aria (umida) lato esterno e acqua (glicolata) lato interno. Conseguentemente ad un’accurata revisone bibliografica, dove si è potuto approfondire le principali tematiche in esame ed individuare i criteri comunemente adottati per la risoluzione di questo tipo di problemi, è stato implementato in ambiente Python un codice di calcolo che, al variare della configurazione geometrica e delle condizioni operative dello scambiatore di calore, applicando il metodo epsilon-NTU è in grado di elaborarne la potenza termica scambiata, l’efficacia e altre grandezze caratteristiche di interesse. È stato quindi definito un caso di riferimento relativamente al quale è stata condotta un’analisi parametrica con l’obiettivo di conseguire una validazione del modello numerico confrontando i risultati ottenuti con quelli elaborati da un programma commerciale attualmente in uso in azienda (già validato). Analizzando i risultati si è potuto concludere che il modello è complessivamente solido; in particolare la correlazione di Wang e Chi si è dimostrata precisamente adatta per prevedere le prestazioni termiche di uno scambiatore di calore a tubi sfalsati con aria in ingresso a temperature medio basse (errore percentuale del 2%). Inoltre si è potuto constatare che la correlazione proposta da McQuiston è la più accurata per quanto riguarda il calcolo del coefficiente d’attrito esterno, ovvero per la valutazione delle perdite di carico lato aria (errore percentuale massimo del 15%, di gran lunga inferiore rispetto alle altre correlazioni); il set di relazioni proposto da Gray e Webb è risultato essere il più preciso per la determinazione delle prestazioni termiche per scambiatori di calore a tubi allineati (errore percentuale massimo del 3%). In conclusione si può osservare che, mentre i risultati sulla parte termica sono molto incoraggianti, le previsioni riguardanti le cadute di pressione su entrambi i lati risultano complessivamente meno precise, ciò suggerisce la necessità futura di calibrare il modello sulla base di dati sperimentali.
Sviluppo di un codice di calcolo per scambiatori di calore a pacco alettato in ambiente Python
PIVA, ENRICO
2018/2019
Abstract
The improvement of heat exchange processes represents an important challenge in the modern energy scenery; in particular, the industrial need to be able to have innovative solutions with limited dimensions and high efficiency is increasingly accentuated: therefore, compact heat exchangers are used. This thesis presents the study of the thermal design of a dry cooler heat exchanger, in which the currents of fluid involved are air (humid) external side and water (glycol) internal side. Consequently to an accurate bibliographic review, in which it was possible to study in deep the main issues under examination and identify the criteria commonly adopted for the resolution of this type of problems, a calculation code was implemented in the Python environment which applying the method epsilon-NTU, changing the geometric configuration and the operating conditions of the heat exchanger, is able to process its thermal power, its effectiveness and other quantities of interest. A case study was defined and a parametric analysis was conducted with the aim of achieving a validation of the numerical model by comparing the results obtained with those elaborated by a commercial program currently in use in the company (already validated). Analyzing the results, it was possible to conclude that the model is overall solid; in particular, the correlation of Wang and Chi proved to be precisely suitable for predicting the thermal performance of a staggered heat exchanger with the inlet air at medium-low temperatures (percentage error of 2%). Furthermore, it has been found that the correlation proposed by McQuiston is the most accurate as regards the calculation of the external friction factor, and then for the evaluation of the air side pressure drops (maximum percentage error of 15%, largely lower than the other correlations); the set of equations proposed by Gray and Webb was found to be the most precise for determining the thermal performance for inline heat exchangers (maximum percentage error of 3%). In conclusion, it can be observed that, while the results on the thermal part are very encouraging, the predictions regarding the pressure drops on both sides are overall less accurate, so it suggests the future need to calibrate the model on the basis of experimental data.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/153438