The size effect of heat-transfer surfaces on boiling

Abstract

A sprinkled tube bundle is frequently used in technology processes where an increase or decrease of a liquid temperature in a very low-pressure environment is required. Phase transitions of the liquid very often occur at low temperatures at pressures ranging in the thousands of pascals, which enhances the heat transfer. This paper focuses on the issue of a heat-transfer coefficient that is experimentally examined at the surface of a tube bundle. The tube is located in a low-pressure chamber where the vacuum is generated using an exhauster via an ejector. The tube consists of smooth copper tubes of 12 mm diameter placed horizontally one above another. Heating water flows in the bundle from the bottom towards the top at an average input temperature of approximately 40°C and an average flow rate of approximately 7.2 L min -1 . A falling film liquid at an initial temperature of approximately 15°C at an initial tested pressure of approximately 97 kPa (atmospheric pressure) is sprinkled onto the tubes' surface. Afterwards, the pressure in the chamber is gradually decreased. When reaching the minimum pressure of approximately 3 kPa (abs) the water partially evaporates at the lower part of the bundle. Consequently, the influence of the falling film liquid temperature increase is tested. This gradually leads to the boiling of water in a significant part of the bundle and the residual cooling liquid that drops back to the bottom of the vessel is almost not heated anymore. In this paper we present the influences of the size of the heat-transfer surfaces. Keywords: sprinkled tube bundle, water, under pressure, heat transfer Pr{enje po snopu cevi se pogosto uporablja v tehnolo{kih procesih, kjer se zahteva povi{anje ali zmanj{anje temperature teko~ine v okolju z nizkim tlakom. Fazni prehod teko~ine se pogosto pojavi pri nizkih temperaturah in tlakih v obmo~ju nekaj tiso~paskalov, kar vpliva na prenos toplote.^lanek se nana{a na koeficient prenosa toplote, ki je eksperimentalno dolo~en na povr{ini snopa cevi. Cev je name{~ena v nizkotla~ni komori, kjer se vakuum ustvarja s pomo~jo aspiratorja preko ejektorja. Snop cevi sestavljajo gladke bakrene cevi, premera 12 mm, ki so name{~ene horizontalno ena nad drugo. Voda za ogrevanje te~e v snop od spodaj proti vrhu s povpre~no temperaturo okrog 40°C in povpre~no hitrostjo pretoka okrog 7,2 L min -1 . Padajoṽ odni film z za~etno temperaturo okrog 15°C in z za~etnim tlakom okrog 97 kPa (atmosferski tlak) pr{i po povr{ini cevi. Tlak se v komori postopno zni`uje. Ko dose`e minimalni tlak okrog 3 kPa (absolutni tlak) voda delno izhlapi na spodnjem delu snopa. Posledi~no je preizku{en vpliv nara{~anja temperature padajo~e vode. To postopno privede do vrenja vode na ve~jem delu povr{ine snopa in preostala hladilna teko~ina, ki kaplja na dno posode, se skoraj ne segreje ve~.^lanek predstavlja vpliv velikosti povr{ine kjer se prena{a toplota. Klju~ne besede: pr{enje po snopu cevi, podtlak, prenos toplote

Investigation of Cocoa Husks Densification Possibilities

“…The quality of biofuels depends on the characteristics of the raw material (feedstock), which includes its biomass type, moisture content, and particle size. Quality management of the manufacturing process, including operating conditions, technological variables and pelletizer type, as well as the binding agent, will also affect the quality of the final product Kracík et al (2016), Arce et al (2013). In order to cause problems in combustion systems, low-quality pellets may have undesirable effects such as fouling, corrosion or slugging.…”

“…Some problems arise from the raw material behavior, properties and composition, as well as from the structure of the raw material. These parameters can also influence the final biofuels made from cocoa husks Kracík et al (2016). It was essential to assess the impact of technological variables and raw materials treatment variables on final biofuels' quality as well as densification options for extrusions manufacturing.…”