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Laminar-flow Liquid-to-air Heat Exchangers - Energy-efficient Display Cabinet Applications

Haglund Stignor, Caroline LU (2009)
Abstract (Swedish)
Popular Abstract in Swedish

Resultaten som presenteras i denna doktorsavhandling visar att det går att spara stora mängder energi genom att använda samma typ av värmeväxlare, som normalt används i bilar och lastbilar, i livsmedelsbutikernas kyldiskar.



För att hålla maten kall när den ligger ute i butiken cirkuleras en kyld luftström runt i kyldiskarna. Denna kyls i en så kallad värmeväxlare som ofta finns i botten på kyldiskarna under den nedersta hyllan. Bakgrunden till forskningsprojektet är att det i Sverige under 90-talet infördes en anvisning som inte tillät att luften i kyldiskarna kyldes direkt med hjälp av konstgjorda köldmedier. Anvisningen gällde vid nybyggen av butikernas kylsystem. Istället... (More)
Popular Abstract in Swedish

Resultaten som presenteras i denna doktorsavhandling visar att det går att spara stora mängder energi genom att använda samma typ av värmeväxlare, som normalt används i bilar och lastbilar, i livsmedelsbutikernas kyldiskar.



För att hålla maten kall när den ligger ute i butiken cirkuleras en kyld luftström runt i kyldiskarna. Denna kyls i en så kallad värmeväxlare som ofta finns i botten på kyldiskarna under den nedersta hyllan. Bakgrunden till forskningsprojektet är att det i Sverige under 90-talet infördes en anvisning som inte tillät att luften i kyldiskarna kyldes direkt med hjälp av konstgjorda köldmedier. Anvisningen gällde vid nybyggen av butikernas kylsystem. Istället skulle en vätska kylas i kylmaskinerna och denna vätska, den så kallade köldbäraren, skulle sedan pumpas ut till kyldiskarna. Dessa system kallas indirekta kylsystem. Anledningen var att de konstgjorda köldmedierna bidrar till växthuseffekten om de släpps ut i luften. Genom att använda indirekta system behöver systemen inte fyllas med lika mycket köldmedium och det blir då lättare att hålla läckagen låga.



Köldbärarvätskorna är ofta ganska trögflytande vid låga temperaturer vilket gör att värme inte överförs så bra från luften till vätskan när luften kyls. Detta leder i sin tur till att det måste vara stora temperaturskillnader mellan luft och vätska när luften ska kylas. Ju större temperaturskillnad, desto mer elenergi måste användas. De värmeväxlare, så kallade kylbatterier, som normalt finns i kyldiskar har runda rör och var från början konstruerade för att ett köldmedium skulle koka vid låg temperatur inne i rören. När luften istället ska kylas med en trögflytande köldbärarvätska skulle kylbatterierna behöva byggas på ett annat sätt för att vara mer energieffektiva.



I bilar och lastbilar används ofta värmeväxlare med platta tuber istället för runda rör, eftersom värmeväxlarna då kan göras mindre och lättare. I detta forskningsprojekt har vi undersökt hur en värmeväxlare med platta tuber ska se ut för att fungera optimalt i en kyldisk. I bilar är luften torr och dess hastighet genom värmeväxlaren hög. I en kyldisk är hastigheten låg och dessutom bildas kondens av vattenånga när luften kyls. Dessa annorlunda förutsättningar gör att värmeväxlarna behöver se lite annorlunda ut, t.ex. vara glesare, för att fungera bra. För att komma fram till förslag på hur värmeväxlare med platta tuber bör se ut när de ska användas i kyldiskar har experiment varvats med beräkningar i många omgångar.



Resultaten visar att åtminstone 15 % elenergi kan sparas med hjälp av dessa ”nya” värmeväxlare och det kan bli stora mängder eftersom 5 meter kyldisk i en affär använder ungefär lika mycket energi per år som en eluppvärmd villa. Dessa ”nya” värmeväxlare kan eventuellt bli dyrare att producera. Hur mycket dyrare är svårt att säga eftersom det till stor del beror på tillverkningsvolym.



Den anvisning som inte tillät direkt kylning med ett konstgjort köldmedium som infördes i Sverige på 90-talet har nu tagits bort och ersatts med EU-lagstiftning. Enligt denna finns det inte längre någon motsvarande anvisning, men ju mer konstgjort köldmedium som en kylanläggning fylls med, desto oftare måste man kontrollera att den inte läcker. Svenska butiker har många positiva erfarenheter av de indirekta kylsystemen – de är något dyrare att bygga, men billigare att sköta och de underlättar en jämn temperaturhållning av matvarorna. Dessutom har intresset för indirekta kylsystem ökat i resten av världen, eftersom de är miljövänliga alternativ till system med konstgjorda köldmedium. Därför är sannolikheten stor att de indirekta kylsystemen kommer att fortsätta att användas i stor utsträckning och att användningen till och med kommer att öka i framtiden.



Detta forskningsprojekt utfördes på SP Sveriges Tekniska Forskningsinstitut, i samarbete med Lunds Tekniska Högskola, LTH, där avhandlingen publiceras. Chalmers Tekniska Högskola och ett antal företag som arbetar med livsmedelskyla eller värmeväxlare har också deltagit och bidragit till projektet. Projektet har huvudsakligen finansierats av Energimyndigheten, Svensk Fjärrvärme, SP och LTH. (Less)
Abstract
Provisions are stored and displayed in supermarkets and grocery stores, at a temperature lower than the ambient, in display cabinets, which are responsible for a significant amount of the energy use in this sector. During the 1990s, major changes in the regulations governing the use of synthetic refrigerants took place in Sweden. This resulted in many refrigeration systems being converted to systems with indirect cooling by means of a liquid secondary refrigerant. The cooling coil is an important component in a display cabinet and traditionally, different kinds of tube-coils, with aluminium fins on expanded circular copper tubes, have been used. Many secondary heat transfer media have relatively high viscosities at low tem¬peratures, and... (More)
Provisions are stored and displayed in supermarkets and grocery stores, at a temperature lower than the ambient, in display cabinets, which are responsible for a significant amount of the energy use in this sector. During the 1990s, major changes in the regulations governing the use of synthetic refrigerants took place in Sweden. This resulted in many refrigeration systems being converted to systems with indirect cooling by means of a liquid secondary refrigerant. The cooling coil is an important component in a display cabinet and traditionally, different kinds of tube-coils, with aluminium fins on expanded circular copper tubes, have been used. Many secondary heat transfer media have relatively high viscosities at low tem¬peratures, and so the flow regime is often laminar, which may lead to poor heat transfer. However, it is possible to achieve high heat transfer coefficients even for laminar flows if an ap¬propriate design of the heat exchanger is applied. Flat-tube heat exchangers have been used for a long time in automotive applications, where compactness is important. The display cabinet application involves low air velocities in combination with condensation of water vapour, and sometimes even frosting, and is therefore different from other applications where flat-tube heat exchangers have been used earlier.



The objective of this research work has been to find a suitable, more energy-efficient, heat exchanger design for indirectly cooled display cabinets. An efficiency that would mean that temperature differences would be so small that frosting could be avoided was aimed for. This research work has sandwiched experimental investigations with theoretical modelling and parameter studies. Initially, the heat transfer and pressure drop performance of conventional cooling coils operating with liquid secondary refrigerants was studied in full-scale experiments. Thereafter, the liquid-side (or tube-side) heat transfer was studied experimentally in small-scale experiments on three single multiport extruded flat tubes with different cross-sections. In both studies, the best agreement was found with predictions using the Gnielinski correlation for thermally developing laminar flows with the constant temperature boundary condition. After this, two different heat exchangers having flat tubes and plain fins on the air side were evaluated experimentally - one with continuous plate fins and one with serpentine fins. The results show that, for conditions similar to those of display cabinets, the heat transfer and pressure drop performance is affected to little or no extent by the occurrence of condensate water. Models for different types of flat-tube heat exchangers were thereafter created using the results from the experimental studies.



In order to find appropriate optimising criteria for the display cabinet heat exchangers, the energy efficiency of a complete cooling system of an imagined supermarket has been studied. Finally, different designs of flat-tube heat exchangers with plain fins have been evaluated theoretically in a parameter study. The results show that considerable savings in the required electric drive power can be obtained in comparison with the traditional cooling coil. The savings can be up to 15 %. In addition, the flat-tube heat exchangers could operate with a minimum temperature difference of around 1 K. This makes frost-free operation possible for many display cabinet applications and thereby the savings potential even larger. (Less)
Please use this url to cite or link to this publication:
author
supervisor
opponent
  • Professor Emeritus Granryd, Eric, Division of Applied Thermodynamics and Refrigeration, Department of Energy Technology, Royal Institute of Technology, Sweden
organization
publishing date
type
Thesis
publication status
published
subject
keywords
energy-efficiency, indirect cooling, display cabinet, pressure drop, heat transfer, flat-tube heat exchanger, cooling coil, laminar
pages
98 pages
publisher
Faculty of Engineering, Department of Energy Sciences, Division of Heat Transfer
defense location
Room M:D, M-building, Ole Römers väg 1, Lund University, Faculty of Engineering
defense date
2009-03-06 10:15
ISSN
0282-1990
ISBN
978-91-628-7700-2
language
English
LU publication?
yes
id
5833ab0f-83f9-405a-97ce-63fe37610344 (old id 1292491)
date added to LUP
2009-02-11 08:33:00
date last changed
2016-09-19 08:44:46
@phdthesis{5833ab0f-83f9-405a-97ce-63fe37610344,
  abstract     = {Provisions are stored and displayed in supermarkets and grocery stores, at a temperature lower than the ambient, in display cabinets, which are responsible for a significant amount of the energy use in this sector. During the 1990s, major changes in the regulations governing the use of synthetic refrigerants took place in Sweden. This resulted in many refrigeration systems being converted to systems with indirect cooling by means of a liquid secondary refrigerant. The cooling coil is an important component in a display cabinet and traditionally, different kinds of tube-coils, with aluminium fins on expanded circular copper tubes, have been used. Many secondary heat transfer media have relatively high viscosities at low tem¬peratures, and so the flow regime is often laminar, which may lead to poor heat transfer. However, it is possible to achieve high heat transfer coefficients even for laminar flows if an ap¬propriate design of the heat exchanger is applied. Flat-tube heat exchangers have been used for a long time in automotive applications, where compactness is important. The display cabinet application involves low air velocities in combination with condensation of water vapour, and sometimes even frosting, and is therefore different from other applications where flat-tube heat exchangers have been used earlier.<br/><br>
<br/><br>
The objective of this research work has been to find a suitable, more energy-efficient, heat exchanger design for indirectly cooled display cabinets. An efficiency that would mean that temperature differences would be so small that frosting could be avoided was aimed for. This research work has sandwiched experimental investigations with theoretical modelling and parameter studies. Initially, the heat transfer and pressure drop performance of conventional cooling coils operating with liquid secondary refrigerants was studied in full-scale experiments. Thereafter, the liquid-side (or tube-side) heat transfer was studied experimentally in small-scale experiments on three single multiport extruded flat tubes with different cross-sections. In both studies, the best agreement was found with predictions using the Gnielinski correlation for thermally developing laminar flows with the constant temperature boundary condition. After this, two different heat exchangers having flat tubes and plain fins on the air side were evaluated experimentally - one with continuous plate fins and one with serpentine fins. The results show that, for conditions similar to those of display cabinets, the heat transfer and pressure drop performance is affected to little or no extent by the occurrence of condensate water. Models for different types of flat-tube heat exchangers were thereafter created using the results from the experimental studies. <br/><br>
<br/><br>
In order to find appropriate optimising criteria for the display cabinet heat exchangers, the energy efficiency of a complete cooling system of an imagined supermarket has been studied. Finally, different designs of flat-tube heat exchangers with plain fins have been evaluated theoretically in a parameter study. The results show that considerable savings in the required electric drive power can be obtained in comparison with the traditional cooling coil. The savings can be up to 15 %. In addition, the flat-tube heat exchangers could operate with a minimum temperature difference of around 1 K. This makes frost-free operation possible for many display cabinet applications and thereby the savings potential even larger.},
  author       = {Haglund Stignor, Caroline},
  isbn         = {978-91-628-7700-2},
  issn         = {0282-1990},
  keyword      = {energy-efficiency,indirect cooling,display cabinet,pressure drop,heat transfer,flat-tube heat exchanger,cooling coil,laminar},
  language     = {eng},
  pages        = {98},
  publisher    = {Faculty of Engineering, Department of Energy Sciences, Division of Heat Transfer},
  school       = {Lund University},
  title        = {Laminar-flow Liquid-to-air Heat Exchangers - Energy-efficient Display Cabinet Applications},
  year         = {2009},
}