Frost formation in air to air counterflow heat exchangers... and how to prevent it?
Translated title
Rimdannelse i luft til luft modstrømsvarmevekslere... og hvordan det kan undgås?
Authors
Osman, Dzhanan Metin ; Rasmussen, Nils Kristian Kure ; State, Diana
Term
4. Term
Education
Publication year
2019
Submitted on
2019-01-10
Pages
154
Abstract
Vi undersøgte tre metoder til at forhindre frost i pladevarmevekslere: forvarmning (opvarmning af tilluften), bypass (omgåelse af varmeveksleren) og luftmængdeubalance (ulige tilluft og udsugning). Målet var at vurdere energiforbrug og indeklima og at finde de mest kritiske betingelser for frostdannelse. Med dynamiske simuleringer i BSim identificerede vi kombinationer af ude- og indendørsforhold, hvor frost kan opstå. Udeklimaerne omfattede Sydskandinavien, Centraleuropa, Sydtyskland og Østrig samt Skotland. Som indeklima blev typiske forhold for klasseværelser og kontorer anvendt. Resultaterne viser, at en relativ indendørs luftfugtighed på 20–40 % sammen med 22 °C udsugningstemperatur svarer til udetemperaturer, hvor der kan dannes frost. For at bestemme energibehovet ved frostsikring fastlagde vi frostgrænsetemperaturen for disse forhold via laboratorietests af en modstrøms pladevarmeveksler i klimakamre. Her så vi, at frost opstår på afkastporten ved 0 °C afkasttemperatur. Beregningerne viste ingen forskel i den energi, der kræves for at holde temperaturen over denne grænse, mellem de tre metoder. Indeklimapåvirkningen blev vurderet ud fra luftkvalitet og termisk komfort. For de analyserede klimaer blev den indendørs luftkvalitet ikke forringet af nogen metode. Ubalance kan dog forringe den termiske komfort på grund af øget infiltration og risiko for træk. Desuden påvirker den indendørs relative luftfugtighed, hvor på afkastporten frosten først dannes, og ved lav luftfugtighed, hvor dugpunktet er under 0 °C, kan frost opstå uden forudgående kondensation.
We studied three methods to prevent frost in plate heat exchangers: preheating (warming the supply air), bypass (routing air around the heat exchanger), and airflow imbalance (unequal supply and extract flows). Our goal was to assess energy use and indoor environment and to identify the conditions under which frost forms. Using dynamic simulations in BSim, we found critical combinations of outdoor and indoor conditions. The outdoor climates analyzed were Southern Scandinavia, Central Europe, Southern Germany and Austria, and Scotland. For indoor conditions, typical classroom and office use were considered. We found that an indoor relative humidity of 20–40% with an extract air temperature of 22 °C corresponds to outdoor temperatures that can cause frost. To determine energy demand for frost prevention, we identified the frost limit temperature for these conditions through laboratory tests on a counterflow plate heat exchanger in climatic chambers. Frost appeared on the exhaust port at an exhaust air temperature of 0 °C. Calculations showed no difference in the energy required to keep temperatures above this limit among the three methods. Indoor environmental impact was evaluated using indoor air quality and thermal comfort. For the climates studied, indoor air quality was maintained with all methods. However, airflow imbalance can harm thermal comfort due to increased infiltration and draught risk. We also observed that indoor relative humidity affects where frost first forms on the exhaust port, and that at low humidity, when the dew point is below 0 °C, frost can form without prior condensation.
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