Choosing a cold room starts with the temperature at which the product will be stored. Fruit and vegetables, dairy products and delicatessen goods are kept in a chilled storage room at 0 to +10 °C; meat, fish and frozen food in a frozen storage room at −18 to −25 °C. To freeze a product quickly, a blast freezer working at −35 to −45 °C is used. Once the temperature is set, the panel thickness, floor insulation, door type, humidity and the cooling capacity that follows from the daily product intake fall into place one after another.
Temperature first: three types of room
- Chilled storage room (0 to +10 °C): Keeps the product fresh without freezing it; in most applications the set point lies between +2 and +8 °C. Supermarkets, restaurant kitchens, fruit and vegetable stores, dairy, delicatessen and pharmaceutical stores belong to this class. Volumes range from a 5 m³ kitchen room to stores of more than 1,000 m³.
- Frozen storage room (−18 to −25 °C): Keeps already frozen products for long periods: meat and poultry, fish, ice cream, frozen ready meals. It is built from a 10 m³ buffer store up to cold stores of more than 2,000 m³.
- Blast freezer (−35 to −45 °C): Not a store but a process room. It brings down the product's core temperature in a short time; in a product that freezes quickly the ice crystals stay small and the product keeps its texture when thawed. It is sized from 400 kg to 10 tonnes per cycle, and a typical cycle takes 8–12 hours depending on the product. The blast-frozen product then moves to a frozen storage room.
Products that need different temperatures and humidity are not put in the same room. Some fruit gives off ethylene, a gas that speeds up ripening and shortens the shelf life of sensitive vegetables stored next to it; in such cases the room is divided or two separate rooms are built. The spec table for each type of room is on its own page: chilled storage room, frozen storage room and blast freezer.
Panel thickness and floor insulation
The heat entering through the walls and ceiling is proportional to the temperature difference between the inside and the outside of the room. That is why the panel gets thicker as the room gets colder:
- Chilled storage room: 80–100 mm sandwich panel with PUR / PIR core
- Frozen storage room: 120–150 mm panel
- Blast freezer: 180–200 mm panel, floor included
Thickness is not only a question of energy. When insulation falls short, the outer surface of the panel drops below the dew point and condensation starts; in frozen rooms this turns into ice at the panel joints. For the same reason it matters that panels are joined with cam locks and without thermal bridges. In food rooms the corners are closed with hygienic radiused profiles, and the sheet surface is chosen in painted galvanised steel, stainless steel or a food-safe coating.
As the temperature drops, the floor becomes critical too. In frozen storage rooms and blast freezers the floor is insulated with XPS boards or panels; under an uninsulated floor the ground freezes over time, heaves and cracks the slab. If pallet trucks or forklifts will use the room, a non-slip finish rated for the load is laid over the insulation. In chilled storage rooms, floor insulation is assessed according to the product, the size of the room and the ground beneath it.
The door: where most heat gets in
Every time the door opens, warm, humid air enters; in a room that is opened often, this load can exceed the heat coming through the walls. The door type is therefore chosen for how heavily it will be used: hinged doors for small rooms that are rarely opened, sliding doors for stores with pallet truck and forklift traffic. A strip curtain or air curtain reduces the amount of air that enters while the door is open.
Double EPDM gaskets and an inside emergency release handle are standard on chilled storage room doors. In frozen rooms, a heating cable in the door frame warms the gasket so the door does not ice up and stick to the frame. These rooms also need a pressure relief valve: after the door closes, the warm air that came in cools and contracts, the room is pulled into a vacuum and without the valve the door is hard to open.
Humidity, air distribution and defrost
Most fresh fruit and vegetables are stored at high relative humidity; when the humidity drops the product loses water and weight and wrinkles. The humidity in the room is largely decided by the choice of evaporator: an evaporator with a large surface and a small temperature difference between the air and the refrigerant draws less moisture out of the air, so the room stays more humid. Air velocity is also kept at a level that does not dry out the product's surface.
In a blast freezer it is the other way round; the goal here is speed. Evaporators with high air velocity and wide fin spacing are used, and air at about 5 m/s flows between the products on the racks. Racks and trolleys are arranged so as not to block this flow.
The defrost method also depends on the temperature. Chilled storage rooms use automatic electric or hot gas defrost. In frozen rooms, electric defrost runs with smart interval control and starts only when ice has built up on the evaporator.
Product load and capacity calculation
The capacity of the refrigeration unit is the sum of several loads: heat entering through the panels, air entering with door openings, cooling of the product brought into the room, the respiration heat of fresh produce, lighting, fans, people working inside and defrost. Of these, the one that varies most from project to project is the daily product intake.
Example: let us assume that 2,000 kg of fruit enters a chilled storage room every day at +25 °C and has to be cooled to +4 °C within 24 hours. Taking the specific heat of the fruit as about 3.6 kJ/kg·K, Q = m · c · ΔT = 2,000 × 3.6 × 21 ≈ 151,200 kJ, or about 42 kWh. That means an average load of 1.75 kW through the day just for cooling the product. If the product is to be cooled in 12 hours instead of 24, this share doubles. For frozen products the calculation has three stages: cooling down to the freezing point, the latent heat that has to be removed during freezing, and cooling below the freezing point; of these, latent heat is usually the largest item.
Once the total load is known, the unit is usually selected for 16–20 hours of running per day, leaving room for defrost and off-cycles. Monoblock units are used for small and medium volumes, split systems for large rooms, multi-compressor central systems for large frozen stores, and units with two-stage reciprocating or screw compressors for blast freezers. R449A and R452A are used in all three types of room; R134a and R290 in chilled storage rooms, CO₂ in large frozen stores and blast freezing plants, and ammonia in large blast freezing plants are also among the options. Digital control, temperature logging and SMS or e-mail alarms also provide the records needed for HACCP inspections.
Hot water from the condenser's waste heat
The refrigeration unit of a cold room rejects through its condenser all the heat it draws from the room plus the energy the compressor uses. Plants that process meat, fish or milk, meanwhile, need plenty of hot water for washing and cleaning at the same time. The two needs can be combined.
There are two ways to do it. A heat recovery exchanger added to the condenser takes heat from the superheated gas leaving the compressor and preheats water directly. When a higher temperature is needed, lukewarm water from the condenser side is fed to a heat pump as its source, and the heat pump raises this heat to domestic hot water at 55–65 °C. Because the source is warmer than the outdoor air, the heat pump usually runs at high efficiency in this arrangement; we explain the logic of the cycle in our article what is a heat pump. Whether recovery makes sense depends on how well the hot water demand overlaps with the hours the refrigeration unit runs; in a project the two are assessed together.
What we need for a quote
To size your cold room, this is enough: the product to be stored and the required temperature, the internal dimensions of the room, the daily product intake and the temperature at which the product enters the room, how many times a day the door will be used and how (by hand, with a pallet truck, with a forklift), whether the room will be inside a building or outdoors, and the power supply. We calculate the panels and the refrigeration unit together from this information and send a quote. You can see every type of room on the industrial refrigeration page and send your request through our quote form.
If what you need to cool is not a product but the water that feeds a machine or a production line, the right equipment is a chiller; see our article on chiller selection. For maintenance and service after installation, we work with our sister company Aktek Elektromekanik.