ACR Journal

June | July 2021 REFRIGERATION 30 Peter Woods, technical sales director for Wolseley Climate’s refrigeration division, looks at the challenges of process cooling and suggests ways contractors can get the most out of equipment. Preserving the taste Body copy Subhead contacts Volume 7 No.4 The UK loves convenience foods, and the statistics show it. In 2019, chilled products had a sales value of c£1.6 billion, with frozen food at around £700 million. That’s a lot of food. It also represents a significant investment in the equipment needed to make sure food reaches us on time and, more importantly, safely. The food industry is one of the most heavily regulated in this respect, and so it makes sense to review equipment and make sure it’s primed to work eƒectively and e„ciently. The physics of industrial blast freezing and chilling is complex. For food to be chilled or frozen, heat needs to be drawn away from the surface area and the core of the product at the right speed and over a specific period to preserve the quality and keep the product within its cold chain parameters. Getting it wrong can have serious consequences impacting the product quality or potentially resulting in product loss. From chilled abattoirs to production lines, every application is diƒerent, and there’s no ‘one size fits all’ solution. For Wolseley Climate’s technical engineers, most customers’ projects generally begin with a specific objective: for example, to reduce temperatures from ‘A’ to ‘B’ over ‘X’ number of hours in batches of ‘Y’. Very often, it’s the product that dictates the cooling cycle time. In products like beef, fast freezing prevents the build-up of large ice crystals that spoil the meat. In other cases, it’s the production capacity cycle that needs to be considered. Calculating load The first objective of any project is to determine the extraction load, and this is calculated using Wolseley Climate’s in-house selection programs. We use the specific heat of the product above and below freezing and the latent heat of fusion, the entering temperature, leaving temperature and cycle time to calculate the kW duty required. When a cooling process starts, the product will rapidly give up heat from its surface. The amount of heat extraction will reduce as the surface temperature becomes cool, while the core of the product will always give up its heat more slowly depending on its depth or density. This means the air temperature in batch cooling or freezing needs to be between 5 and 6K below the desired temperature of the product to allow it to give up its core heat eƒectively. The initial duty surge in batch cooling must be considered in the system’s design to prevent compressors from operating outside of their envelope. Features such as suction pressure regulators, maximum operating pressure valves or electronic expansion valves, suction accumulators and compressor unloading can be incorporated in the design to ensure reliable operation and long service life. Lack of flow Another key consideration that is often overlooked is to define the correct airflow path through the product and back to the evaporator. High air velocities are required to push the air through the product, and the product needs to be stacked to allow the air to pass through. Too close and the resistance will prevent the air from reaching all areas, and uneven cooling will occur. The air path should pass through the product and then circle back from the floor into the cooler. Obstructions and a poor air path will lead to short cycling of the fan discharge air directly back into Peter Woods, technical sales director, Wolseley Climate’s refrigeration division