EVALUATION OF THE THERMAL PERFORMANCE OF FIVE REFRIGERATED VANS

Transcription

1 EVALUATION OF THE THERMAL PERFORMANCE OF FIVE REFRIGERATED VANS S. ESTRADA-FLORES, A. EDDY and N. SMALE Supply Chain Innovation, Food Science Australia PO Box 52, North Ryde, NSW 7, Australia. Fax: ; S. Estrada-Flores@csiro.au ABSTRACT An experimental evaluation of insulation effectiveness, pull-down times and effectiveness of the mechanical refrigeration unit was performed on five refrigerated panel vans. Additionally, a mapping of temperature variability in the cargo space was carried out during a simulated logistic scenario encompassing several door openings. The temperature variability measured in the vans was neither correlated to the effectiveness of the insulation nor the time required to achieve complete cooling of the cargo space. These criteria are common thermal performance indicators used to evaluate refrigerated vehicles in international standards. The effectiveness of insulation was correlated to the pull-down times, presenting the possibility of calculating the former as a function of the latter. The temperature variability was correlated with the time required for the unit to recover temperature control after a door opening interval and the difference between the maximum and minimum temperatures reached during a door opening cycle. Keywords: thermal performance; vehicles; insulation; temperature; regulation; cooling; refrigerated; 1. INTRODUCTION Thermal performance standards serve as benchmarking instruments to compare refrigerated transport systems and to ensure that these systems provide a minimum level of operating effectiveness. Insulated and refrigerated sea, rail and road freight containers must comply with the international standard ISO (International Organization for Standardization, 1996). In Europe, the specifications for refrigerated vehicles are covered by the International Agreement for the Transport of Perishables (United Nations Economic and Social Council, 23), which has the status of a standard. The Australian Standard for refrigerated vehicles (Standards Australia, 23) defines testing specifications and procedures to evaluate the thermal performance of refrigerated equipment, used to transport perishable goods by road. The equipment covered by the standard includes insulated trailer, truck and van bodies (single or multi-compartment) fitted with any form of refrigeration. Key thermal tests described in AS and relevant to refrigeration plants already fitted to the unit are: (a) Insulation effectiveness (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 2 o C between the cargo space and the external environment. The method includes measurement of at least twelve temperatures covering the corners, [all] walls, ceiling and floor of the vehicle. (b) Performance test of the refrigeration system: This test evaluates the ability of the truck to maintain the set-point temperature within the cargo space during hours of uninterrupted operation. The vehicle is tested for a further hours with an additional heat load. The AS specifies an ambient external temperature of 3 o C for this test. Innovative Equipment and Systems for Comfort and Food Preservation, Auckland, NZ, 26

2 (c) In-service (pull-down) test: This assessment measures the time required to cool down the empty truck to the classification temperature. The evaluation of vehicles following the AS and similar standards rely on temperature measurements in selected locations throughout the tested body; however, these locations usually reflect the thermal behaviour of the layers adjacent to the walls of the van, rather than the thermal behaviour of the cargo space. Furthermore, most regulations do not test vehicles under realistic logistics scenarios. For example, vehicles carrying out home delivery operations will have at least three deliveries (i.e. three door openings) per hour (Clancy, 2). Frequent door openings can lead to increased evaporator frosting, thus increasing the need for defrosts, particularly in humid weather conditions. This paper presents the results of thermal performance tests carried out on five refrigerated vans, following the methodology described in the Australian Standard Additionally, a detailed mapping of temperature variability in the cargo space of each van was carried out during an -hour simulated delivery schedule. 2. METHODOLOGY Tests were carried out in Food Science Australia s controlled-environment shipping-container test facility Instrumentation of the vans The characteristics of the vehicles tested are summarised in Table 1. Table 1. Mechanical characteristics of the vehicles tested. VAN MODEL PAYLOAD CAPACITY (m 3 ) 1 Mercedes Sprinter 3CDI SWB 2 Ford Transit/Therma Truck (mid-roof, long wheelbased) 3 Mercedes Sprinter 3CDI SWB Mercedes Sprinter 3CDI SWB 5 Mercedes Sprinter 3CDI LWB High Roof MEAN SURFACE AREA (m 2 ) COOLING PLANT AND CAPACITY AT 2,5 RPM Transfridge chiller model Transfridge chiller model Transfridge chiller model Transfridge chiller model Transfridge chiller model All vehicles were insulated post-production with polyurethane foam.type-t thermocouples (diameter =.5 mm; nominal accuracy ±.2 ºC) were placed in each van, as described below: (a) K value test: Sixteen measurement points in the inside and outside of the vans were used to determine their thermal behaviour. Care was taken to avoid placement of thermocouples on thermal bridges (e.g. metallic fittings, pockets in the insulation). (b) Temperature mapping in the cargo space: The cargo space was divided into five imaginary slices. Each slice had nine thermocouples distributed throughout the van s height and width, in a 3x3 grid. Innovative Equipment and Systems for Comfort and Food Preservation, Auckland, NZ, 26

3 All thermocouples were connected to two 6-channel, battery-powered data loggers Eltek SQ 1 (Eltek Ltd, Cambridge). The measuring systems were previously calibrated with ice slurry, by means of a standard measuring system, encompassing a -wire platinum resistance thermometer sensor connected to a Hart Scientific Instrument Model 152 (system resolution =.1 o C) Heat leakage ( K value ) test Testing was carried out inside the test facility at 1 o C. Once all thermocouples were attached to the van s body, the van was brought to thermal equilibrium with the external air. Two fans and a heater were placed inside the van and connected to an uninterruptible power supply (Powertech, 5kVA). The power consumption of fans and heater were measured during the test, by means of a PM39 multifunction electricity meter (accuracy =.5% of reading; Northern Design Electronics Ltd, UK). Additionally, twenty-five fans were distributed to ensure air velocities between 1 and 2 m s -1 over the external surface of the van. Floor drains, defrost drains and relief valves were functioning as normal. The doors of the van were closed as normal. The heater power was adjusted through a Variac autotransformer (model W2HMT3A -2V 2 VAC) to obtain a temperature difference greater than 2 o C. Temperature data was recorded every minute during the warm-up and the final steady-state -hr period. The data was analysed to obtain the mean air and skin temperatures inside and outside the truck during the steady-state period. The K value was calculated as follows: K value Q = (1) S m ( T T ) ms, i ms, o Therefore, the K value is a reflection of an overall heat transfer coefficient, rather than a formal expression of the thermal conductivity of the van s walls. The K value obtained experimentally reflects: (a) the properties of the van s insulation; (b) the van s design and (c) the van s manufacturing process. An insulated body may be insulated with a highquality, low conductivity material, but due to its design and/or manufacturing process, the body may have significant thermal losses in the form of structural pockets, air gaps incorporated in the foaming process, metallic latches, structural ribs, hinges, rivets or screws. These losses cannot be predicted theoretically without an in-depth knowledge of the dimension and location of the thermal bridges in each body tested Pull-down test Immediately after the K value test, the temperature of the test facility was set to 3 o C and the van s doors were opened to reach thermal equilibrium with the external temperature. Once the external and internal temperatures were within a ±3 o C tolerance band around 3 o C, the doors were closed and the refrigeration unit of the van was turned on. The temperature set-point of the unit was set to 2 C. The unit was left to operate, until the internal temperature was 2 o C, as read by the sensor signalling the start of the normal on/off cycling of the refrigeration unit. The time to achieve this cooling was considered as the pull-down period (t pull-down, hrs). 2.. Effectiveness of the mechanical refrigeration unit in maintaining a specified temperature The pull-down phase was immediately followed by an -hr period of uninterrupted, normal operation at 2 o C. After this first period, the heater inside the cargo space was activated and adjusted to increase the internal heat load. The AS specifies that the extra heat load should be 35% of the infiltration heat load, using the K value calculated with Eq. (1). After the K value test was performed, it was apparent that some of the vans would not be able to maintain temperature control with an extra 35% heat load. Therefore, for the purposes of this paper, it was decided to add only an extra 3% heat load. The extra heat load Q h was calculated as: Innovative Equipment and Systems for Comfort and Food Preservation, Auckland, NZ, 26

4 ( T T ) Q h.3 K value S m ms, o ms, i = (2) The van was left to operate with the additional heat load for a further -hr period. A pass or fail status was given to each van, depending on its ability to sustain the 2 o C temperature set-point during the -hr steady state period with no extra heat load and the -hr steady state period with extra heat load Temperature mapping during door opening tests This test was conducted in the test facility at a temperature of 3 o C and a relative humidity in the range of 5 to 9%. The relative humidity was kept high, to assess the performance of the vans under extreme ambient conditions. The trial simulated a logistic schedule for an hr working day, encompassing a pull-down of one hour followed by fourteen 1-minute door openings at approximately 3-minute intervals. Only one rear door was opened. Cargo space temperatures were measured every seconds during the whole testing period. The analysis of the temperature variability was performed by means of histograms, showing the frequency temperature distribution of the data collected for each van. The following measures were calculated: (a) Mean temperature T m, represented by the average of all the temperatures measured in the cargo space in each van. (b)total variation, represented by the standard deviation σ. (c) Fraction of temperatures fulfilling the 2 to o C temperature guideline for pharmaceutical products (P x ), calculated as the number of temperature values falling within the aforementioned range divided by the total number of temperature measurements. (d) Recovery time (t recovery ) or the time required for the unit to recover temperature control in the cargo space after a door opening, as measured by the grid of sensors installed inside the van. (e) Amplitude of the mean temperature rise during the door opening cycle (A do ), calculated as the average difference between the peak mean cargo space temperature reached at the end of the door opening, and the minimum temperature reached when temperature control was regained. Once T m, σ, P x, t recovery and A do were calculated, the relationship between these measures and the two parameters obtained from the thermal performance tests (K value and t pull-down ) was investigated by means of a correlation analysis. 3. RESULTS AND DISCUSSION Table 2 summarises the results obtained after testing of the K value, refrigeration performance and pull-down tests. Even though Table 1 shows that Models 1, 3 and have similar mechanical characteristics, their different K values suggest variations in the effect of thermal bridges in these three models, possibly due to different methods used during the foaming of the insulation. Differences in the age of the insulation could have also led to different K values. The K value measured in the five vans ranged from.2 to 1.2 W m -2 o C -1. Models 2 and 3 showed similar values, which were approximately 1.5 times higher than the value found for Model 1. Innovative Equipment and Systems for Comfort and Food Preservation, Auckland, NZ, 26

5 Model Table 2. Summary of results of the K value test, pull-down test and effectiveness of the mechanical refrigeration unit. K value (W m -2 o C -1 ) t pulldown (h) Refrigeration capacity during -hrs, no extra heat load Heat load entering the cargo area (W) Pass/fail criteria Refrigeration capacity during -hrs, extra heat load added Extra heat load added (W) Pass/fail criteria Pass 253 Pass Pass 369 Fail Pass 375 Fail Pass 26 Pass a 1 Fail 75 Fail a The unit failed to achieve a full pull-down (cooling) to +2 o C. Although Models 1 to achieved normal cycling conditions, only Models 1 and continued to operate normally during the period of hours with an additional heat load. These two models also showed the shortest pull-down times and the lowest K value. Model 5 failed to achieve a pull-down, even though its K value was comparable with the values found for Models 2 and 3. Insufficient refrigeration capacity in Model 5 may have produced this result. Figure 1 shows the average temperatures measured in the cargo space during the last 7 hours of the door opening test. Only Model 1 was able to return to a normal cycling temperature operation after each door opening. Although Model performed well during the first three door openings, temperatures below o C were observed after this point, particularly after each door opening. This was possibly due to the control system of the cooling unit attempting to regain temperature control after each opening by excessively lowering the air delivery temperature. Other models showed an initial loss of temperature control due to a slow pull-down period (e.g. Model 2), a gradual loss of control during the test (e.g. Model 3) or a combination of both (e.g. Model 5). Figure 2 shows the frequency distribution charts of the temperature data collected during the 7-hour period for each model, representing around 2, values measured in 5 locations, distributed as explained in Section 2.3. The initial one-hour pull-down period was not included in the analysis. Innovative Equipment and Systems for Comfort and Food Preservation, Auckland, NZ, 26

6 2 Model 1 2 Model Model Model Model Figure 1. Mean cargo space temperatures measured in Models 1 to 5, during the final 7 hours of the door opening test. The dotted lines indicate the temperature set-point (2 o C). Table 3 shows the results obtained for the five temperature variability indicators calculated. The proportion of measured temperatures falling within the required temperature range of 2 to o C, for the logistics operations commonly performed in these vans, ranged from 93% (for Model 1) to 6% (for Model ). Although the latter van achieved a full pull-down and passed the refrigeration performance tests, the van was not capable of complying with the 2 to o C specification. Table 3. Parameters calculated during the door opening test for all models. MODEL T m ( o C) σ P x t recovery A do ( o C) (hrs) Innovative Equipment and Systems for Comfort and Food Preservation, Auckland, NZ, 26

7 25 Model 1 25 Model Model 3 25 Model Model Figure 2. distribution charts encompassing temperature data measured in Models 1 to 5, during the final 7 hours of the door opening test. The dotted lines indicate the recommended temperature range for pharmaceutical products (2 to o C). Table shows the correlation coefficients calculated for the seven performance indicators. As expected, vans that presented T m values close to the targeted temperature mean (5 o C) presented the highest P x values. The strong correlation between K value and t pull-down was not expected, though there is an intuitive relationship between these two parameters (i.e. lower K value should lead to shorter pull-down times). This presents the possibility of predicting a K value as a function of t pull-down, thus decreasing the time and cost of testing procedures. However, further experimental work is required to assess the robustness of this relationship for a wider range of K value and sizes of bodies. Innovative Equipment and Systems for Comfort and Food Preservation, Auckland, NZ, 26

8 Table. Correlation coefficients between thermal performance indicators and capability indices T m σ P x K value t pull-down t recovery A do T m 1 σ.1 1 P x K value t pull-down t recovery A do K value correlated poorly with the five parameters describing the temperature variability in the cargo space. However, the correlation of σ (which represents the total temperature variation within the cargo space) with respect to t recovery and A do was strong. This suggests that (a) the most significant factor affecting temperature variability during in-transit conditions is the management of door openings; (b) K value influences the initial cooling rate of the van, but not the cargo space temperature during typical working conditions.. CONCLUSIONS The temperature variability of the cargo space measured in five refrigerated panel vans, when subjected to realistic logistics scenarios, was neither correlated to the effectiveness of the insulation (measured as an overall heat transfer coefficient) nor the time required to achieve complete cooling of the cargo space (measured as a pull-down time, which is in turn an indication of the available refrigeration power). Effectiveness of insulation and pull-down time are common thermal performance indicators used to evaluate refrigerated vehicles in international standards. The effectiveness of insulation was correlated to the pull-down time, presenting the possibility of calculating the former as a function of the latter, thus decreasing the time and cost of the testing procedures. The temperature variability during a simulated delivery schedule, encompassing several door openings, was correlated with the time required for the unit to recover temperature control after a door opening and the difference between the maximum and minimum temperatures reached during a door opening cycle. ACKNOWLEDGEMENTS The authors gratefully acknowledge the support of Mr Robert Itaoui and Mr Micheal Gammon during the development of this project. NOMENCLATURE A do amplitude of mean temperature during the door opening cycle ( o C) K value overall heat transfer coefficient (W m -2 o C -1 ) P x fraction of temperatures fulfilling the 2 to o C temperature guideline for pharmaceutical products Q electrical power dissipated inside the body by the heaters and fans (W) Q h total heat load during the final -hr period in the refrigeration capacity test (W) S m mean surface area of the body (m 2 ) T m mean temperature ( o C) Innovative Equipment and Systems for Comfort and Food Preservation, Auckland, NZ, 26

9 T ms arithmetic mean of skin temperature ( o C) t pull-down pull-down time (h) recovery time (h) t recovery Greek letters σ standard deviation Subscripts o outside, external i inside, internal REFERENCES 1. Clancy, S. 2. Small vehicles, great expectations. E-logistics magazine. Issue 2, May. From website: 2. International Organization for Standardization ISO E. Series 1 Freight Containers Specification and testing Part 2: Thermal containers. 51 pp. 3. Standards Australia. 23. AS 92-23: Thermal performance of refrigerated transport equipment Specification and testing. 5 pp.. United Nations Economic and Social Council. Economic Commission for Europe. 23. Agreement on the International Carriage of Perishable Foodstuffs and on the Special Equipment to be used for such Carriage.United Nations. Geneva, including amendments thereto up to Sept pp. Innovative Equipment and Systems for Comfort and Food Preservation, Auckland, NZ, 26