Heat Transfer Coefficient and Concept of Relaxation Time in Forced Air Direct Evaporative Cooling System

Abstract

Evaporative cooling system with palm fruit fiber as cooling pad was studied at different ambient air temperature and relative humidity. Experiment was conducted with a prototyped direct evaporative cooling system for preservation of fruits and vegetable of moderate respiratory rates at a low flow rate of 0.6 m3∙s-1. Three different models were proposed and used to obtain the heat transfer coefficient at different evaporative effectiveness. For the three models, the heat transfer coefficient varied from 173 to 857 W∙m-2K-1. The relaxation time was predicted as a finite time process and the value calculated from the model can be regarded as a hypothetical value since other heat transfer methods like conduction or even radiation loss were neglected. Therefore the value calculated might be much higher than the real relaxation time. For ambient air temperature range of 26.1 to 34.8°C which was cooled to 23.2 to 25.8°C, the average relaxation time was calculated as 0.71 to 1.68 s.

Sistem evaporativnog hlađenja sa rashladnim uloškom od palminih vlakana je bio ispitivan pri različitim temperaturama i relativnim vlažnostima vazduha. Ogled je izveden sa prototipom direktnog sistema evaporativnog hlađenja za zaštitu voća i povrća od umerene respiracije sa niskim protokom od 0.6 m3•s-1. Ponuđena su tri različita modela za postizanje koeficijenta prenosa toplote pri različitim efikasnostima evaporacije. Za ova tri modela, koeficijent prenosa toplote varirao je od 173 do 857 W•m-2K-1. Period relaksacije bilo je predviđeno kao konačan vremenski proces i vrednost izračunata iz modela može se prihvatiti kao hipotetička, obzirom da su drugi postupci prenosa toplote, kao provođenje ili čak i radijacioni gubici, bili zanemareni. Zato izračunata vrednost može da bude mnogo veća nego stvarni period relaksacije. Za opseg ambijentalnih temperature od 26.1 do 34.8°C, koje su hlađene na 23.2 do 25.8°C, za srednji period relaksacije bile su izračunate vrednosti od 0.71 do 1.68 s.