All cool room refrigeration systems have five main components:
- The compressor, which compresses the refrigerant gas
- The condenser, in which the hot gas is cooled to a liquid
- An expansion valve, which controls flow of the liquefied gas and where liquid gas expands to vapour
- Evaporator coils, where the liquid gas expands and boils. This process absorbs energy, cooling the coils
- Fan or fans, to circulate air over the cold evaporator coils, thereby cooling the cool room. Air may also be circulated over pipes containing some type of liquid antifreeze, which have themselves been cooled using the evaporator. Fans also circulate air around the cool room to ensure even distribution of the cold air and reduce temperature variations within the room.
The temperature of air leaving the coils must necessarily be lower than the cool room setpoint, in order to remove heat generated inside the room. The RH inside the room is affected by:
- The magnitude of the difference between the room set point and the coils.
- The amount of temperature variation allowed before the system switches on or off (eg. 2°C ± 2°C means the room can increase to 4°C before the fan unit turns on).
Smaller temperature differentials maximise RH.
Cool room panelling relies on trapping air, usually in a foam or polystyrene matrix, to prevent transfer of heat from the outside environment into the cool room inside. However, it is vitally important to keep this material dry. If the inside of the panelling becomes wet due to condensation and/or entry of humid air from the room then it will become ineffective. Seals around all cool room panels must be intact and waterproof enough to repel water used for cleaning (e.g. jet washing).
Concrete floors should include layers of insulating materials, and be thoroughly sealed against water from floor puddles or washing. Many commercial cool rooms do not have well insulated floors, even though good floor insulation can greatly reduce temperature leakage.
Cool room design and construction
A cool store is essentially a large, insulated box with a refrigeration system and a door. Temperature will vary in different areas inside the room depending on airflow, the way produce has been loaded inside, and the amount of heat contained within that produce.
The design of cool rooms needs to take into account a number of criteria:
- Temperature range.
- Systems which need to achieve temperatures below zero are usually more expensive than those which have a minimum temperature of 2°C or greater.
- Accuracy of control.
- A room which seeks to control temperature within ±0.2°C, for example, will be more costly than one which allows larger fluctuations.
- Degree of spatial variation within the room.
- All rooms have warmer areas, often by the door, or in the back corners. The coldest zone is almost always in front of the delivery air.
- Minimising spatial variation requires increased air circulation and volume, adding cost to equipment and materials.
- Cooling capacity.
- For example, a room might need to have sufficient cooling capacity to reduce the temperature of 20 harvested bins of broccoli (equivalent to 20% of the total room capacity) from 25°C (harvest temperature) to 5°C within 12 hours.
- Heat load from the outside environment.
- This will affect the thickness and quality of insulation materials chosen for the walls and floor.
- Using heat reflective paints and materials as well as roof shading can significantly reduce heat load on the room.
- How the room will be loaded and produce stacked.
- Air circulation should ideally be consistent with the normal orientation of pallet skids.
- How often the door needs to be open for incoming and outgoing product.
- Frequent door opening, particularly if ambient temperatures are high, greatly increases the load on the refrigeration system.
- Having a small door for foot access, in addition to a roller door for forklifts, can reduce loss of cold air.
- These can be further enhanced with flexible curtaining materials, fast automatic roller doors, or double door systems.
- An air curtain (as shown at right) can also be used to reduce entry of warm air into the cold room.
- Running costs.
- Electricity is a major cost for packing and storage facilities. Spending more on better insulation and door seals may be highly cost effective if it reduces power costs.
- If temperature control is not critical and rooms are left closed during the day, it can be possible to use mainly off peak electricity. Product is cooled overnight and allowed to slowly increase during the day.
- With water-cooled systems, off-peak electricity can be used to cool a chilled water tank (‘heat sink’), which can then be used during the day.
- Rooms need to be kept clean, so using materials that are easily washed and including provision for drains will make this easier.
Cool room loading
For the cool room to operate correctly, air must be able to circulate around the produce inside, whether it is already cool or not.
Produce should therefore never be stacked against the cool room wall. It is recommended to leave a gap of at least 10cm for air to circulate. A larger gap (10–15cm) should be left if the wall is exposed to the sun. These gaps will allow any heat transferring from the outside environment to be carried away in the room air before it can warm the product.
Likewise, a clear air space of 25cm or more should be left between the fan unit and the top of stacked pallets or bins. This will allow the cold air to move over the top of the store contents, rather than being blocked by products nearest to the refrigeration unit.
Stacking products on pallets allows air circulation between the floor and the packed products. Aligning the pallet skids to run parallel to the direction of the cooling air (i.e. towards the refrigeration system) will create a more efficient air circulation.