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.
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 effectively and efficiently.
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 different, 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.
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 effectively.
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 the evaporator, increasing cycle times and energy cost, as well as uneven cooling.
From a technical point of view, the space itself is the single most important factor. Left to its own devices, air will follow the path of least resistance, and so it needs to be actively managed to ensure the correct path. It’s good practice to take velocity readings throughout the room to identify any dead air spots or return air paths. Without this detailed insight, it becomes a question of guesswork. It’s common to see equipment not working efficiently due to poor room design. A simple baffle may be all that’s needed to improve the airflow path and make it work properly.
Critically, it’s important to manage superheats and control suction return temperatures and pressures into the compressor. This calls for a maximum opening pressure device on the evaporator, which can be a suction pressure regulator, a MOP valve or an electronic expansion valve, all of which will efficiently control superheat gain on the evaporator and therefore reduce strain on the condensing unit. This is the key to safe and efficient operation.
The dimensions and materials of the packaging, and even the nature of the product itself, also play a part in the specification process. So, for example, products or packaging which is especially dense will make it harder to get cold air through it to extract heat. Similarly, certain foodstuffs, including cheese, fish and offal, give off ammonia which will quickly degrade aluminium rendering it useless. (This is especially the case in maggot farms incidentally, a surprisingly big sector dedicated to UK anglers). The choice here is to accept the damage and regularly replace the evaporator or invest in a more expensive – but durable – solution. This might include an electro-tinned copper coil or high grade (316) stainless steel evaporator with 40 per cent copper mix to accommodate heat transfer rates. The upfront costs may be higher, but the long-term return on investment balances this — a conversation worth having when specifying solutions.
Another key consideration relates to F-Gas legislation and the use of refrigerants which is forcing users to seek alternatives, including CO2. Because of the higher associated costs with maintaining the CO2 plant, we find that most users prefer switching from high to lower GWP gases such as R448/449 and are starting to look at A2L options. These gases are readily available and offer good efficiency, so they represent the best value solution. But as with anything, it’s vital to undertake any conversions in line with the manufacturer’s instructions, not just to validate warranties and prolong service life but also to ensure food safety. If the system runs outside its envelope, the result could be failed equipment and failure to meet stringent food regulations.
Finally, there’s the question of regular and ongoing maintenance. This should include coil cleaning, leak monitoring and an annual check on the evaporator temperature difference to ensure superheats are correctly set. For maximum efficiency, pro-activity is strongly recommended here.
While there are understandably budget pressures on food manufacturers which influence equipment and maintenance, the consequences of failure are also significant. With so little margin for error, it’s important to get things exactly right.
A2L refrigerant pack