Experimental and numerical study of air distribution and thermal environment control for chilled food manufacturing facilities

Abstract

Thermal environment control of chilled food manufacturing facilities plays a critical role in maintaining the required food product temperature during processing to ensure food quality and maximise shelf life. The provision of cooling to maintain the required temperatures in the processing halls, which should be in the range between 4 °C and 12°C also impacts on energy consumption and CO2 emissions. Chilled food manufacturing facilities normally have high ceilings to provide flexibility in their use and accommodate different height equipment. In these facilities cooling is commonly provided by fan coil units installed at ceiling level that recirculate air in the space through the cooling coils with high velocities. Small amounts of fresh air can also be provided to the space through a separate fresh air system. The low temperatures and high velocity of air circulating in the space lead to uncomfortable environments for the workers and high energy consumption. Refrigeration systems in chilled food manufacturing facilities account for more than 60% of the energy consumption in the plant so identifying ways of improving the thermal environment in these facilities and reducing energy consumption can lead to increased productivity and profitability of chilled food operations. This thesis makes a contribution to this challenge by investigating alternative air distribution approaches for both existing and new facilities. A primary consideration was to identify solutions that could be easily retrofitted to existing cooling systems in the space at low cost and minimum disruption to the production. The research involved the investigation of two chilled food manufacturing spaces with different cooling system arrangements to establish their performance characteristics and ability to provide the required conditions of temperature and velocities at low level in the space to minimise thermal discomfort. Learnings from these investigations were used to develop in the laboratory a test facility that could reproduce chilled food manufacturing environments at a smaller scale and enable the investigation of different cooling systems and air distribution arrangements. CFD models were also developed and validated against temperature and air velocity data from the chilled food spaces in the factories and the test facility. The models were then used to evaluate different chilled air distribution designs prior to them being manufactured and installed for evaluation in the test facility. The main objective was to achieve temperature stratification and low air velocities at low levels in the space. Key findings and contribution to knowledge for science and technology of cold processing areas are the follow: i) The monitoring of the two case studies provided evidence of the air-temperature distribution issues in existing chilled food facilities such as high velocities, poor temperature distribution, cooling of the whole space and increased energy consumption. ii) Numerical and experimental results of this research provided guidelines of how air distribution solutions in existing chilled food facilities can be improved regarding their air temperature efficacy and energy efficiency. For example, supplying air from evaporator coils at medium level with circular or semi-circular fabric ducts as air distribution solutions, could achieve temperature stratification in the space with lower temperatures at low level covering the manufacturing area and higher temperatures towards the ceiling; In addition, medium level air supply with fabric duct was shown to provide in the region of 9% reduction in energy consumption compared to high level supply with the same duct; Furthermore, medium level air supply with a fabric duct provided 23% energy savings compared to air supply with an un-ducted fan-coil system which is the most common air distribution method in chilled food factories; iii) Numerical and experimental results derived guidelines of which air distribution systems should be avoided in new chilled food facilities. Tests and CFD modelling comparing air distribution with circular fabric duct and metal duct with linear diffusers showed that the circular fabric duct provided a better thermal environment in terms of temperature uniformity and low air velocities; Furthermore, comparing the air flow velocities obtained from the air distribution system via non-ducted coil and fabric ducts as air distribution solutions, it can be highlighted that the fabric duct provided much lower air flow velocities. This is beneficial to achieve some temperature stratification in the space and reduce the discomfort of the workers produced by high velocities as seen in the case of the non-ducted coil. iv) A simulation tool developed that couples refrigeration system and CFD modelling has been shown to be able to simulate the dynamics of air distribution and refrigeration system energy consumption in chilled food spaces. The tool can be used to optimise the design of air distribution systems from both thermal environment and energy consumption perspectives.