THE USE OF THERMOGRAPHY TO AID DESIGN OF REFRIGERATED ROAD VEHICLES S. ESTRADA-FLORES and A. EDDY Supply Chain Innovation, Fax: +61 02 94908530; Email: S. Estrada-Flores@csiro.au; Food Science Australia PO Box 52, North Ryde, NSW 1670, Australia. ABSTRACT A thermographic survey was performed on two types of insulated bodies: (a) truck bodies, holding an internal temperature of 28 o C at an ambient temperature of 38 o C; and (b) insulated panel vans, holding an internal temperature of 2 o C at an external ambient temperature of 30 o C. Relevant results related to the design of the vehicles include evidence of heat leakage through the seals of the back and side doors. Hardening of the seals due to aging and sudden internal temperature changes are likely explanations for the former, whereas thermal expansion is the reason for the gaps in the side door seals. Heat leakages through the pockets of the side lights, where the insulation is usually at least 12 mm thinner than in the rest of the body, were evident. Thermal bridges due to the presence of structural ribs in the roof and sides of trucks were also observed. 1. INTRODUCTION During the past three years, the authors have assessed a variety of insulated bodies used for cold chain logistics, following the methodology described in the Australian Standard 4982-2003 (Standards Australia, 2003). A key test in this standard is the insulation efficiency (heat leakage) test by the inner heating method. This test determines the effectiveness of the insulated body, measured as a K value, by means of creating a temperature differential of no less than 20 o C between the cargo space and the external environment. The K value obtained reflects the properties of the insulation and the manufacturing process of the body. For example, if a particular truck body encompasses a layer of high-quality insulation, but the manufacturing process and/or design of the body allows for significant thermal bridging, both factors will affect the resulting K value. The heat leakage test does not provide information on the location of the defects which contribute to a high K value. Therefore, the authors have used thermographic surveys to determine the likely reasons for failure of some designs to achieve adequate insulation effectiveness. Thermography is a two-dimensional, non-contact technique that allows fast surface temperature mapping of objects subjected to a large temperature difference from their surroundings (Meola et al., 2004). Given certain assumptions in terms of the radiative heat transfer, the radiation captured by a thermographic camera can be interpreted as surface temperatures. The temperature values can then be displayed as either greyscale or pseudocoloured images. The radiation detected is in the infrared spectrum, usually in the 2 to 5.6 µm and 8 to 14 µm regions (Avdelidis and Moropoulou, 2003). By taking snapshots of several parts of the insulated truck body, the exact location of problematic features in truck bodies that present high K values can be ascertained and solutions for these can be implemented. This paper summarizes the findings of qualitative surveys undertaken in two types of insulated bodies: (a) 45-ft trailer bodies, holding an internal temperature of 28 o C at an ambient temperature of 38 o C; and (b) insulated panel vans, holding an internal temperature of 2 o C at an external ambient temperature of 30 o C. The most common heat leakages found in these surveys relate to general design features of refrigerated road vehicles, which are also discussed in this paper.
2. METHODOLOGY The effectiveness of the insulated body of the two trucks and three of the panel vans, measured as a K value, was determined previous to the thermographic survey. The methodology and procedure followed is outlined in the Australian Standard 4982-2003 (Standards Australia, 2003) and also described by Estrada-Flores and Tanner (2005). The thermographic survey was carried out by means of a thermographic camera (ISI 525 Snapshot Thermal Camera, range 0 to 350 o C, accuracy 2% reading). The spectral band covered by the camera used was 8 µm to 12 µm, with a thermal resolution of 0.1 o C at 30 o C. In all the surveys undertaken, the camera was mounted on a tripod, about 1.5 m from the floor and between 1.5 and 2.5 m away from the surface photographed. Visible light photographs were taken with a digital camera at the same angle and at a similar field of view, for later analysis. 2.1. Truck bodies tested K value tests and thermographic surveys were carried out in Food Science Australia s controlledenvironment shipping-container test facility. The ambient temperature was 38 o C and the refrigeration set-point of both trucks was set to 28 o C. No solar radiation (either direct or indirect) was incident on the external surface of the trucks during the thermographic survey. The total survey time was about 1 hour. The truck bodies were both manufactured with composite (sandwich panel type) walls. The external and internal layers were made of fibreglass and foamed polyurethane was used for the middle layer. The K values and other characteristics of the trucks tested are presented in Table 1. Table 1. Description of trucks tested. TRUCK INTERNAL DIMENSIONS 1 Length = 13.21 m Width = 2.42 m Height = 2.68 m 2 Length = 13.28 m Width = 2.41 m Height = 2.76 m INSULATION THICKNESS (m) Roof and ceiling: 0.13 Sidewalls: 0.08 Front wall: 0.10 Rear doors: 0.07 Roof and ceiling: 0.13 Sidewalls: 0.04 Front wall: 0.10 Rear doors: 0.07 K value (W m -2 K -1 ) REFRIGERATION PLANT AGE 0.35 Carrier Ultima XTC 2 months 0.54 Carrier Ultima XTC 1 month 2.2. Panel vans tested K value tests were carried out in Food Science Australia s controlled-environment shipping-container test facility. Thermographic surveys were carried out inside a loading facility in Granville, NSW. The facility was insulated but the temperature of the loading dock was not controlled. The ambient temperature around the vans tested was about 30 o C and the refrigeration set-points were set to 2 o C. there was no direct solar radiation on the external surface of the vans during the thermographic survey. The surveys were completed in about 3 hours. The K values and other characteristics of the vans tested are presented in Table 2. The values observed were higher than those calculated for the trucks; the panel vans had been in use for more than one year when they were tested. The exact age of each insulated body was unknown, as these vans were converted to refrigerated vehicles before the present owner purchased them.
Table 2. Description of panel vans tested. VAN INTERNAL DIMENSIONS (Length x width x height, m) K value (W m -2 K -1 ) REFRIGERATION PLANT 1 3.27 x 1.74 x 1.63 0.82 Transicold OED6 (3.5 kw 2 2.93 x 1.55 x 1.34 --- Transicold OED6 (3.5 kw 3 3.27 x 1.74 x 1.63 1.24 Transicold OED6 (3.5 kw 4 3.27 x 1.74 x 1.63 --- Transicold OED6 (3.5 kw 5 4.22 x 1.74 x 1.86 m 1.11 Transicold OED6 (3.5 kw AGE 3. RESULTS AND DISCUSSION During the thermographic inspections, some practical guidelines were established: (a) The difference in emissivity of surface coatings (e.g. paint, rust or highly reflective plastic light casings) may cause difficulties during the interpretation of the thermograms. A change in emissivity from one surface to another is difficult to distinguish from a true temperature change. In some cases, a highly polished surface may be interpreted as a thermal bridge, creating a false positive. An understanding of the design features and a temperature check with a second measuring device (e.g. a thermocouple or thermometer) can help to distinguish these false positives from actual temperature differences. (b) In view of (a), the thermographic survey gives more accurate results in dark conditions (e.g. the lights in the testing facility are off). (c) The thermographic image captured with the camera changed considerably, depending on the viewing angle relative to the vehicle s surface. When the surface is viewed at right angles, the changes in surface temperature appeared to be scaled differently to the changes detected at glancing angles However, sources of heat leakages could still be observed qualitatively and identified at different angles. (d) The refrigeration units diesel engines and condensers produce waste heat, some of which warms nearby surfaces being measured. Care needs to be taken to ensure that these sources are not mistaken for a thermal bridge, particularly when the joint between the refrigeration plant and the insulated body is being assessed. Tables 3 and 4 present the most significant heat leakages observed during the thermographic surveys of truck 1 and truck 2, respectively. The tables also present an explanation of the photographs and the relevant design features of the insulated body that influence the particular thermal behaviour described in the thermograms. The temperature scale is shown in the right hand side of each thermographic image. In general, red and yellow areas represent the warmest zones of the body, whereas blue and purple areas represent cold zones (i.e. zones that present heat leakages).
Table 3. Representative results obtained during the thermographic survey of Truck 1 Structural component Thermographic image The plates behind the latches, detected as blue (cold) zones, secure the latch against the door. Steel creates a significant thermal bridge. Elements such as hinges, corners and brackets can introduce a significant heat conduction load towards the internal Emergency door release load space. Bottom front corner of side door Underneath plant refrigerating Left hand side (upper corner) rear door Heat leakages through the seals of the side door. Rippling the external skin of the door indicated that thermal expansion of the external layer of fibreglass had occurred, thus distorting the door. This was a likely explanation for the gaps observed between the door seals and the body. A cold zone across the width of the back of the vehicle was detected at floor level, suggesting that the edge of the T-section metallic floor, or the bonded joint between the floor and the wall, was a source of heat leakage. Heat leakages through the seals of the rear door were observed. This effect is often observed in used trucks, due to deterioration and hardening of the seal. However, this source of heat leakage can also occur in trucks with new seals, due to the pressure difference between the internal (cargo) space and the external environment. Structural pockets to fit lights and handles usually decrease the thickness of the insulation by 12 to 15 mm. Side light
Table 4. Representative results obtained during the thermographic survey of Truck 2 Structural component Joint between left hand side wall (as seen from the refrigeration unit side) and roof Left hand side wall Thermographic image The manufacturing of refrigerated vehicles usually encompasses three types of joints: mechanical fastened, welded and adhesively bonded. The first category involves drilling the two parts and then coupling with bolts or rivets. This seems to be the technique used in this joint, leading to heat leakages due to drilling, use of highly conductive materials, and/or poor adhesion between the insulation layers. Good manufacturing practices for joints include sealing the gaps left between insulation layers with foamed polyurethane. Insulated bodies require a rigid structure with posts, beams or internal reinforcement. These structural features can act as a source of heat leakage, even if the posts and beams are foamed with insulation. The vertical blue lines indicate the existence of internal webs of fibreglass reinforcement, confirmed with a carpenter s electronic stud-finder. In other designs, this could be due to vertical discontinuities in the foaming process, due to the use of sectional foamed panels that have not been fitted correctly. Detail of the back door latch and seal Similarly to truck 1, heat leakages through the seals of the rear door were observed. The same explanation applies in this case. Figure 1 presents two examples of the thermographic surveys undertaken for the five vans. The heat leakages detected in the body of the van seem to be related to the foaming-in-place insulation technique used to convert the vans to refrigerated vehicles. In this technique, the insulation (usually polyurethane) is foamed to fill the space between a wooden frame and the metallic structure of the cargo space. Wet-laid insulation is not very efficient at preventing heat entering the cargo space, because the metal ribs in the side walls act as thermal bridges. The effect of these heat leakages can be observed in Fig. 1(a), where the average temperature difference between the warmest and coldest areas of the external surface of the van was 16 o C. In Fig. 1(b), the heat leakage through the seals of the rear doors was evident; the average temperature difference between the warmest and coldest areas in the rear doors was 15 o C.
(a) (b) Figure 1. (a) Side view of Model 2; and (b) rear doors of Model 5. The thermographic images show 12 sections of each visible light image. In summary, examples of design and manufacturing aspects that decrease the insulation effectiveness of refrigerated road vehicles observed in these surveys include: (a) Structural pockets to fit lights and handles usually decrease the thickness of the insulation by 12 to 15 mm in those areas. (b) Metallic latches, structural ribs, hinges, rivets or screws often impinge on the insulation layer. (c) Pockets of air may be incorporated in the insulation a s it is foamed in place.
(d) Poor bonding of materials and sub-optimal distribution of insulation thickness between the horizontal walls (roof and floor) and sidewalls can create air gaps, which also decrease the insulative efficiency of the body. (e) Door gaskets may be poor insulators when manufactured and heat infiltration may increase as the gaskets deteriorate with time. Pressure differences between external and internal environments can also lead to an increase in air infiltration through door seals. (f) In the long term, accelerated aging of the insulation may occur if moisture migrates to the insulation layer; this can occur if there are gaps in the seams or rivet holes. A strategy frequently recommended for reducing the sources of heat leakages observed in this study is to increase the overall insulation thickness. However, this strategy needs to be analyzed in the context of current demands in the logistics industry: A strong market-driven influence on the design of insulated trailer bodies is the maximizing of revenue per shipment by means of maximizing the cargo space available per unit. A stow using Australian standard pallets (1.17 m x 1.17 m) requires 2.34 m clear internal width, which is suitable for dry goods (i.e. uninsulated trucks). A stow using ISO standard pallets (1.00 m x 1.2 m) requires 2.2 m clear internal width, thus allowing a design with thicker insulation; however, few logistics companies in Australia use ISO pallets because the manufacturing industry sizes packaging for the Australian standard pallet. Government regulations establish fixed external dimensions for trailer bodies, therefore, the insulation thickness is compromised. These drivers result in a push towards thinner wall insulation in refrigerated road vehicles. Thin, inefficient insulation has a negative impact in the profitability of logistics operations (i.e. more cooling power is required to avoid unsafe temperatures during transport of perishables) and in the environment (i.e. the increase in cooling power leads to the use of more diesel, thus increasing harmful exhaust emissions and noise pollution). New insulating materials, such as vacuum panels, could fulfill the requirements of decreasing the thickness of the wall and simultaneously decreasing the energy usage of refrigerated vehicles. The reduction of thermal bridges by improving the current design of refrigerated vehicles is also an alternative to reduce heat leakages. For example, materials which are poor thermal conductors can be used to break the thermal bridges caused by aluminum or steel structural reinforcement, by inhibiting direct contact between the external (ambient) environment and the cargo space. 4. CONCLUSIONS Current thermal performance standards for refrigerated road vehicles provide a means to assess the quality of the insulated body against a benchmark value, thus allowing comparisons between insulated bodies of various sizes and manufacturing processes. However, they do not provide information on the location of defects that negatively affect the effectiveness of insulation. Thermographic imaging is a practical tool that enables the qualitative assessment of insulation deficiencies in insulated bodies and may assist in improving current designs. Some aspects detected as factors decreasing the effectiveness of insulation in the vehicles surveyed during this study include: the thinning of insulation in structural pockets to fit lights and handles, thermal bridges in the form of metallic structures (e.g. latches, structural ribs, hinges, rivets or screws) and poor bonding of materials between the horizontal walls (roof and floor) and sidewalls, amongst others.
The interpretation of the images requires background knowledge of the characteristics of the insulated bodies surveyed and the effect of the surveying conditions (e.g. solar radiations, shadows, emissivity of surfaces) on the results. Future research focused on thermographic imaging, as a tool to perform quantitative surface temperature mapping of truck bodies, could enable its use as an accurate measure of local thermal resistivity. Thermography can also be a valuable tool during design optimisation of insulated bodies, enabling the calculation of relative contributions of design features on the overall heat leakage value. It would also allow comparison of materials, interventions, dimensioning tolerances and other features, in terms of their effect in improving the efficiency of insulation. ACKNOWLEDGEMENTS The authors gratefully acknowledge the support of Mr Robert Itaoui and Mr Micheal Gammon during the development of this project. REFERENCES 1. Meola, C., Carlomagno, G. and Giorleo, L. 2004. The use of infrared thermography for materials characterization. Journal of Materials Processing Technology 155 156: 1132 1137 2. Avdelidis, N.P. and Moropoulou, A. 2003. Emissivity considerations in building thermography. Energy and Buildings 35: 663 667. 3. Meola, C., Carlomagno, G., Squillace, A. and Giorleo, L. 2004. The use of infrared thermography for nondestructive evaluation of joints Infrared Physics & Technology 46: 93 99 4. Birk, A.M. and Cunningham, M.H. 2000. Thermographic Inspection of Rail-Car Thermal Insulation. Transactions of the ASME, 122: 494-501. 5. Standards Australia. 2003. AS 4982-2003: Thermal performance of refrigerated transport equipment Specification and testing. 54 pp. 6. Estrada-Flores S and Tanner, D. 2005. Linking the Cold Chain. Loaded: Trailer and Body Technology. (Aug-Sept): p. 33-36 NOMENCLATURE K value overall heat transfer coefficient (W m -2 K -1 )