Have you ever wondered how your freezer works? In the earliest stages of cold storage, they literally used ice. Later on technology was developed which allowed them to use Freon, a chemical which has a super cooling effect when it manifests in gas form. This was later found to be harmful to the environment, so the switch was made to tetrafluoroethane, or HFC. HFC turns to liquid at -15.9 Celsius, and in liquid form is can be moved through coils which effectively pull the heat out of confined spaces, creating the environment needed to cool or freeze goods.
What if you don’t need to freeze items in a small area though? What if you have an entire warehouse of goods that you need to keep cold? Surely the same rules don’t apply. With larger facilities, the first thing you have to take into consideration is what you’re storing, and how long you can store it for under certain temperatures before it goes off. For example, pacific salmon can keep for approximately 10 months at -25ºC, or 7 months at -15ºC. Knowing what you’ll be storing determines what kind of temperatures you need to maintain, and after that you have to think about how much space you’re going to need.
For this, you need to know what variety of products you will be storing, in what quantities, and for how long. With this taken into account, you have to start thinking about how you’re going to stock things, and the amount of space you’ll need between isles (as well as what kind of forklifts and other equipment you’ll need, bearing that in mind). This then brings into play what kind of insulation you’ll need, based on scale and outside temperature.
With all this information taken into account, it’s then a case of choosing either a central or distributed refrigeration system. In the former case, there is a central generator in a ‘machine room’, with several compressors and pressure vessels mounted nearby. The cooling units will be dotted around the facility in such a way to assure consistent temperatures, and everything is connected using insulated piping. This system also requires condensers to be erected outside to cool the air being let in. The actual coldness comes from a chemical carrier, often it’s HFC, but ammonia is also sometimes used.
With a distributed system, ‘condensing units’ are placed throughout the facility at intervals. These are effectively a combination of a compressor and a condenser built into a single cabinet, typically mounted to the roof, with an air cooling unit directly connected on the ceiling below. These tend to be thought of as a poor cousin to a central plant system, as central plants make it easier to regulate the temperature consistently, you’re free to choose between air or water cooled condensers, and it’s easier to install a ground heating system.
Central plant systems are, however, more expensive, as you have to build a machine room within the facility. You also need a far more complex piping system, which not only affects building costs, but also places certain limitations on the layout of the facility. Ammonia is considered to be a more effective chemical for mass cooling than HFC, but it’s far more hazardous. Distributed systems, comparatively, are lower cost and lower maintenance, but they have a high energy cost and once you reach beyond a certain size that cost makes them no more financially viable than a central plant system.