The potential of ambient cooling systems for reducing refrigeration loads and saving energy

Transcription

1 The potential of ambient cooling systems for reducing refrigeration loads and saving energy Stephen J James, Christian James Food Refrigeration and Process Engineering Research Centre (FRPERC), The Grimsby Institute of Further & Higher Education (GIFHE), HSI Building, Origin Way, Europarc, Grimsby, North East Lincolnshire, DN37 9TZ, UK (jamess@grimsby.ac.uk) ABSTRACT The cooling and freezing of cooked products is one of the top 10 uses of refrigeration energy in the cold chain [1]. The surface temperature of cooked products is very high when they leave baking ovens or deep fat fryers, and consequently the temperature difference between the surfaces and the ambient is very large at this time. Pre-cooling using ambient air at this stage rather than immediate use of refrigerated air offers a number of advantages, not least a reduction in energy use and cost. Although ambient cooling is common in some particular sectors of the food industry, it is not widespread. It is therefore important to encourage the use of ambient cooling throughout the food industry. Two case studies are reported in this paper, ambient cooling of hash browns prior to freezing, and ambient cooling of meat and vegetable pies prior to blast chilling to demonstrate the advantages and energy savings that ambient pre-cooling can achieve. In the hash-brown example, an initial 5 minutes of ambient cooling removed 562,500 kj of heat energy from the 4.5 tonnes of hash browns every hour. In the pie example, by combining experimental studies with modelled data a two-stage process was designed incorporating an initial ambient cooling stage followed by a redesigned multi-rack chilling tunnel. This work showed that approximately 50% of the heat that had to be extracted in the total cooling pie cooling process could be removed in this ambient pre-chilling stage. Keywords: Freezing; Chilling; Energy; Weight loss; Refrigeration loads INTRODUCTION The surface temperature of cooked products is very high when they leave baking ovens or deep fat fryers, and consequently the difference between the surface and the ambient is very large at that time. To reduce energy usage, and costs, a number of food manufacturers have traditionally operated two-stage cooling operations using ambient air followed by refrigerated air [2]. In an example of cooling meat pies cited in [3] the use of an ambient first-stage did not significantly increase the cooling time (Table 1) but significantly reduced the heat load on the refrigeration system. Predictions [4] show that in the cooling of a 50mm thick tray of Bolognaise sauce that a 1-hour ambient cooling stage can reduce the heat load on a refrigeration system by 49% whilst only increasing the total cooling time by 16%. An initial higher temperature stage can also be used to improve the energy efficiency of freezing operations. As an example, the pre-chilling of Albacore tuna prior to freezing using a refrigerated seawater system (RSW) has been reported to remove almost one third of the total heat load and improve the quality of the fish [5]. The RSW system operated more energy efficiently when removing the initial heat than a low temperature blast freezer [6]. Table 1. Commercial cooling of cooked steak and kidney pies (185g) initially at 90 to 95 C in single and two-stage systems [3] Type Type of chiller Air conditions Temperature ( C) Velocity (ms -1 ) Cooling time (h) Final Temperature ( C) Single-stage Spiral -3 3 to to 20 Two-stage Two-stage Two-stage Ambient 20 3 to Spiral Ambient 20 3 to Spiral to Ambient Spiral to to 27

2 With many ambient stable products such as bread and confectionary filtered ambient air is a common cooling method after the product exit backing ovens. Investigations have shown that the water loss is very dependent on the cooling time: the lower the cooling time, the lower the water loss [7]. Optimum final cooling was obtained with forced convection, low ambient temperature and high humidity. Purpose built ambient cooling units are available, some using impingement technologies to achieve high cooling rates. However, the use of ambient cooling is not widespread within the food industry and in some cases it is not encouraged. This is due to the belief that ambient cooling is slow and that a slower initial cooling rate may encourage bacterial growth and that distributing ambient air over unwrapped products may increase bacterial contamination. HASH BROWN FREEZING A spiral freezer was designed to freeze 5 tonnes of hash browns per hour (Figure 1), the cooked product entering the spiral at +80ºC and departing at -18ºC. However, the system could not achieve the desired freezing rate. The main problem was believed to be the latent load on the system, although there was sequential defrosting for the cooling coils the defrost load on the plant was excessive and there was a problem removing all of the ice. A short study was therefore carried out to determine the amount of heat and moisture that could economically be removed using ambient cooling prior to freezing. MATERIALS & METHODS Figure 1. Hash browns emerging from fryer. Batches of three or four hash browns were selected, core temperature initially measured with a single point metal shrouded hypodermic T-type (copper-constantan) probes (1mm diameter, 50mm length; Electronic Temperature Instruments Ltd, UK) and then weighed. The hash browns were then heated in a deep fat fryer (5 litre Electric counter top fryer, model L484, Buffalo) for 90s. After heating the three hash browns were rapidly transferred to the adjacent cooling rig and calibrated single point metal shrouded hypodermic T-type (copper-constantan) probes (1mm diameter, 50mm length; Electronic Temperature Instruments Ltd, UK) inserted into the approximate thermal centre of each hash brown. The temperatures were monitored using a Grant Squirrel data logger. Initially, replicated (3) trials were carried out using an ambient of 22 C and air velocities of approximately 2, 2.5 and 3 ms -1. Further trials were carried out using a single stage at -30 C or a first stage at 22 C followed by -30 C, all at 3.5 ms -1. During separate ambient (22 C) cooling trials the weight of the hash browns were determined at 5 minute intervals. To determine the amount of heat removed, thermal modelling was carried out using FRPERC s islab program, part of the FoodTempX suite of models. islab is a finite difference model for heat flow to and within an infinite slab using numerical method based on that originally described by Dusinberre [8]. It is based on equations utilized for thawing pork legs [9] and beef blocks [10]. During operation, the model obtains the relevant thermal properties by interpolation in a temperature/property data file generated by the FoodProperties program. At the initial freezing point of many foods, there is a sharp peak in the specific heat/temperature curve. Interpolation in this region can lead to significant errors in prediction. The

3 temperature-enthalpy method proposed by Pham [11] has been incorporated into the model to help overcome this problem. RESULTS & DISCUSSION In the initial ambient cooling trials the core temperatures of the hash browns were 40±1 C after 10 minutes at all three air velocities (Table 2). After a further 10 minutes the product core temperatures were approaching (within 2.5 C) that of the ambient temperature. During the first 5 minutes of ambient cooling the hash browns lost approximately 1 g of weight and less than one third of a g in the following 5 minutes (Table 3). Table 2. Core temperatures in hash browns during ambient (22 C) cooling Air velocity (ms -1 ) Core temperature ( C) Time (min) Table 3. Weight loss from hash browns during ambient cooling Weight loss (g) during different time (min) periods During frying 0 to 5 5 to to to to to 30 Mean (SD) 2.25 (0.30) 0.94 (0.07) 0.31 (0.03) 0.22 (0.05) 0.13 (0.02) 0.15 (0.05) 0.12 (0.02) Figure 2 shows typical cooling curves for a single-stage freezing process and a two-stage process with a 10 minute ambient first-stage. Ambient cooled hash browns Direct frozen Hash browns Air temp Ambient Air temperature Temperature ( C) Time (min) Figure 2. Core temperatures during single- and two-stage freezing in air at 3ms -1. Using the calculated thermal properties of the hash browns, a fit to the experimental data was made, using the experimental air temperature of 22 C and average the product starting temperature and used to calculate the rate of sensible heat removal. From the experimental and theoretical studies it was concluded that 5 minutes of ambient cooling could remove 562,500 kj from 4.5 tonnes of hash browns per hour. The process would also stop 60 kg of ice formation on the refrigeration coil of the spiral per hour. Overall 5 minutes of ambient cooling would reduce the heat load by 31% and 10 minutes by 44%.

4 PIE COOLING In a gourmet pie (Figure 3) factory the finished pie cooling process was identified as a key bottleneck in production. Trials were therefore carried out to produce an efficient energy saving process that could be installed after the company moved to new premises. Figure 3. Gourmet pie MATERIALS & METHODS Laboratory trials simulated, as close as possible, the production process that was used in the factory. Batches of 6 uncooked pies were cooked, in an oven (Convection Oven; Professional DFS903) at 180 C for 25 min. The pies were placed on either a solid metal tray or a metal grid. Six hot pies were set on the tray/grid and a pre-calibrated single point metal shrouded T-type (copper-constantan) probes (1 mm diameter, 50 mm length) inserted in each of the pies before chilling. Chilling was carried out in an experimental wind tunnel [12], with the air temperature monitored by a T-type (copper-constantan) wire thermocouple suspended directly in the air stream. Temperatures were logged every minute during cooling using Comark Diligence EV data loggers (N2014, Comark Ltd, Stevenage, and Hertfordshire). Cooling was carried out at different combinations of air temperature and air speed using air at -10 C or 0 C (±0.3 C) and three air velocity settings. The temperature of -10 C was selected to provide directly comparable results to those obtained at the factory. The airflow around the pies was measured using an airflow AV-2 air velocity and volume flow meter. RESULTS & DISCUSSION Single stage cooling trials showed clearly the effect of air velocity, air temperature and the supporting pie structure on cooling time to 3 C (Figure 4). A chilling time of under 3 h (180 min) to a core temperature of 3 C was achieved under all the air temperature and air speed combinations used. A time of under 90 minutes could be achieved by placing the pies on a metal tray in an air temperature of -10 C and any air speed over 1.3 ms-1. If the pies were placed on a mesh grid the average air speed had to exceed 3.3 ms-1 to achieve a cooling time of less than 90 minutes. Initial trials using a 30 minute pre-chilling ambient (20 C) cooling stage before chilling at 0 C, showed that the core temperature of the pie could substantially be reduced by this process (Table 4). The overall cooling time to 5 C was increased but the time required in the blast chiller slightly reduced. In the new factory the ambient temperature in the de-tinning area would be approximately 10 C so further, more comprehensive trials were carried out using a pre-chilling ambient (10 C) cooling stage. These showed (Table 4) that after 30 minutes the core pie temperature could be reduced to approximately 46 C and a cooling time in the blast chiller of under 2 h would reduce the pie temperature to below 3 C. A 30 minute ambient (10 C) pre-chill therefore removed approximately 50% of the heat that had to extracted in the total cooling pie cooling process. To design a chilling system for the new factory several requirements/considerations were taken into account: The system should chill the pies (without freezing) to 3-5 C in 2.5 h in air at 0 C and air velocities between 1 ms-1 and 3 ms-1 over the product. Two racks, i.e. 672 pies, would be cooked and ambient cooled at the same time.

5 Hot pies would be pre-cooled in ambient air for 30 min before they are de-tinned. The de-tinned pies would then be transferred to standard aluminium baking trays, racked on standard steel trolleys and placed in the blast chiller. A production rate of 1250 pies per hour meant that 2 racks needed to be cooked every 30 min in a continuous process. With a chilling time of 2.5 h the chiller had therefore to be big enough to hold 10 racks at one time. A flow process was therefore developed that provided a virtually continuous chilling process in synchronicity with the oven. When production moved to the new premises an ambient cooling system for pies was fully designed, constructed and commissioned. After further calculations and discussions two 5-rack blast chillers were designed, constructed and commissioned. The main reasons for the change from one to two systems were reduced initial capital and running costs, increased flexibility and the provision of an alternative system to use if a breakdown occurred. 1.3 m/s 3.3 m/s 6.3 m/s Time to 3 C (min) Tray 0 C Tray -10 C Grid 0 C Grid -10 C Figure 4. Time to reach 3 C (min). Table 4. Pie temperatures after 30 minutes, total and time in chiller to reach 3 C. Control is the time achieved in a single stage system at 0 C, 3 ms -1 Conditions Pie temperature ( C) after 30 min Time to 5 C (min) Total In Chiller Control 30 min at 20 C then 0 C on grid, both 3.3ms min at 20 C then 0 C on tray, both 3.3ms Time to 3 C (min) 30 min at 10 C, 1.3 ms -1 then 0 C, 3.3ms -1 on tray min at 10 C, 3.3 ms -1 then 0 C, 3.3ms -1 on tray CONCLUSION The introduction of an ambient cooling operation prior to refrigerated chilling or freezing operations can significantly reduce the heat load and energy consumption of chilling and freezing systems. Two cases studies have been presented that clearly demonstrate this. Hash browns emerged from a fryer at 80 C and had to be frozen to -12 C before packaging at a process rate 4.5 tones/h. The existing spiral freezer was incapable of extracting the initial heat load and the moisture loss from the hash browns was causing ice to build up the evaporator. An initial 5 minutes of ambient cooling

6 removed 562,500 kj of heat energy from the 4.5 tonnes of hash browns every hour. It also prevented 60 kg per hour of water freezing on the evaporator In the pie case, by combining experimental studies with modelled data a two-stage process was designed incorporating an initial ambient cooling stage followed by a redesigned multi-rack chilling tunnel. This work showed that approximately 50% of the heat that had to be extracted in the total cooling pie cooling process could be removed in this ambient pre-chilling stage. This two-stage system has now been installed by the company and shown to achieve more controlled, uniform and faster cooling of the pies than previously achieved with a single chiller. In the next few decades climate change is likely to increase pressure on the food cold chain and the need to reduce energy consumption in food refrigeration systems whilst maintaining food safety and quality [13]. The wider uptake of ambient cooling has much to offer in meeting both of these aims. However, microbiologists and food technologists do not always encourage the use of ambient cooling. This is due to the belief that the slower cooling rate would encourage bacterial growth and that distributing ambient air over unwrapped products could increase bacterial contamination. It is quite common to use filtered air in the cooling of bread and other bakery items and filtering systems are readily available. Eliminating air born contamination would not therefore appear to be a problem in a well-designed ambient cooling system. As far as the authors know no studies have been published on bacterial increases due to the use of an ambient rather than a refrigerated cooling system. It is unlikely that the small changes in cooling on a fully cooked product would result in any measurable effect on the safety and shelf-life of the product after cooling. However, without peer reviewed data microbiologists may remain unconvinced and restrict the introduction of this energy saving technology. ACKNOWLEDGEMENTS We would like to thank defra for funding the project of which this work is a part, and the Knowledge Transfer Partnership that produced the data in this paper. We would also like to specifically thank Christina Goodacre for being such a stimulating project officer, and Tristan Hog (Pieminister) for his enthusiasm and support with the KTP. The complete output of the defra energy project can be found on at: REFERENCES [1] James S.J., Swain M.J., Brown T., Evans J.A., Tassou S.A., Ge Y.T., Eames I., Missenden J., Maidment G. & Baglee D Improving the Energy Efficiency of Food Refrigeration Operations. Proceedings of the Institute of Refrigeration, 5-1. [2] James S.J. & James C Meat Refrigeration, Woodhead Publishing Limited, ISBN [3] James S.J Cooling of cooked products. Progress in the Science and Technology of Refrigeration in Food Engineering, Proceedings of meetings of IIF-IIR Commissions B2, C2, D1, D2-D3, 24-28th September 1990, Dresden, Germany, International Institute of Refrigeration, Paper 30. [4] Anon Ambient cooling, http// [5] Kolbe E., Craven C., Sylvia G. & Morrissey M Chilling and freezing guidelines to maintain onboard quality and safety of Albacore tuna, Agricultural Experimental Station Oregon State University Special Report [6] Kolbe E Refrigeration energy prediction for flooded tanks on fishing vessels, Applied. Engineering. Agriculture, 6(5), [7] Monteau J.Y Sandwich bread cooling. Foodsim'2008: 5th International Conference on Simulation and Modelling in the Food and Bio-Industry, [8] Dusinberre G.M Numerical Analysis of Heat Flow, 1st ed, McGraw-Hill, New York. [9] Bailey C., James S.J,. Kitchell A.G. & Hudson W.R Air-, water- and vacuum-thawing of frozen pork legs, Journal of the Science of Food and Agriculture, 25, [10] James S.J. & Bailey C Air and vacuum thawing of unwrapped boneless meat blocks, Proceed. Institute of Refrigeration , 76, [11] Pham Q.T A fast, unconditionally stable finite-difference method for heat conduction with phase change, International Journal of Heat and Mass Transfer, 28, [12] James S.J., Creed P.G. & Bailey C The determination of freezing time of boxed meat blocks, Proceedings of the Institute of Refrigeration, 75, [13] James S.J. & James C The food cold-chain and climatic change, Food Research International, 43,