In order to reduce energy losses as much as possible, the cold store and ante-room need to be properly separated. The most common techniques for sealing such passages are the strip curtain, high speed door and the air curtain.
Cost-effective passageways for cold stores / freezer rooms
In moments when a passageway is not in use, there needs to be a fixed freezer door directly before or behind it with which to seal it. The question is: which type of door is the best choice in a given situation?
1. Strip curtain
Of the three types of barriers, the strip curtain is the most common. Under normal circumstances, it creates a good barrier between the ante-room and the freezer room and requires little investment. However, there are disadvantages. A strip curtain compromises hygiene because products carried on forklifts come into contact with the strips, which could then leave behind microorganisms on the products subsequently brought through the curtain. Forklifts need to navigate the passage carefully and slowly, which leads to loss of time, and the driver’s visibility is limited during passage. This can create dangerous situations, for instance for employees working behind the passage. Strip curtains require regular maintenance, which primarily takes the form of mending and replacing the strips.
With a new strip curtain, the strips are still aligned to form a flat surface and do a good job of separating the air zones, but this almost always changes after some time. The strips become warped and cold air leaks away. They become clouded and workers may cut strips short or push them aside, leading to an even greater loss of cold. The poor visibility can cause collisions with the freezer door, necessitating repairs that can cost a few thousand euros every time they are needed. A strip curtain is only a cost-effective solution if it is used infrequently. It is a cheaper investment compared to the other two solutions.
2. High speed door
One benefit of a high speed door is the fact that a sealed freezer door is not always required, particularly when the passageway sees limited use. However, it is not an ideal solution, because a high speed door never seals the passage off completely. Air from outside leaks in, the moisture in the air freezes and ice forms on the floor, making it slippery and leading to a loss of energy. Every time a forklift passes through, the door is fully opened and there is no barrier between the cold store and the ante-room whatsoever, leading to an even greater loss of cold.
There are special models that offer improved separation of temperature-controlled zones, but these are significantly more costly. There are manually-operated and automatic doors. With a manually-operated high speed door, time is lost because the forklift needs to come to a complete stop. An automatic system is better, because a sensor senses when a forklift approaches the passageway and the door is opened immediately or after a slight delay, depending on the settings. If the door is opened immediately, cold is lost, and if there is a delay, the forklift needs to slow down, which costs time. If transport in and out is frequent, the high speed door barely has time to close and will often remain open, causing much cold air to be lost. Due to wear and damage, maintenance costs are often high.
3. Air curtain
Air curtains consist of a portal in which air flows downwards. The flow of air is generated by a high-pressure radial fan and is so powerful that it creates an effective barrier between different zones. Normally, the outlet nozzles are slits of 5 to 15 cm, but in order to minimalise air consumption, those in AFIM air doors are 0.6 to 2 cm wide. Measures are in place that distribute the supplied air equally over all nozzles, and the air is guided so that there is a laminar outflow across the full length of the passageway. The result is a tight air curtain that provides a clean separation of air across the full width and height of the portal. There is good visibility and there is no risk of collision. There are no waiting times and proper hygiene is ensured, especially if the curtain is fitted with an air filter.
The fan turns on immediately when the door opens, and turns off again as soon as the door is closed. A common phenomenon is a reduction of air pressure inside the freezer room compared to the ante-room as a result of the decrease in air volume, and air curtains help equalise the pressure. If the air curtain is equipped with a dryer, there is no ice formation. The blowers work quietly, with a maximum noise level of 63 dB(A). Of course, besides their advantages air curtains also have disadvantages. One of these is the fact that an air curtain can never completely seal off a passage, meaning some mixing of air cannot be prevented; 20% is common. Air doors are most effective when transport frequency is high, simply because the other two solutions entail a greater air loss. Air curtains are a greater investment than strip curtains and cost around the same as a high-speed door. Of the three types of barriers, air curtains can most easily be adapted to changes in the workplace. A small company could start off with a basic model, for example, and easily switch to a more advanced solution at a later date which can hold the passage open throughout the day. In cold stores where transport is frequent and the passageway often remains open, an air curtain is the most cost-effective solution.
Air doors are not yet a common sight, but their benefits are so great that this is slowly changing. There are models fitted with an absorption dryer that extracts moisture from the air to bring it to a dew point of around -40 °C, thereby preventing ice from forming on the floor. Well-sealing air curtains are also known as air doors. The largest air doors have a clear opening of 7 by 7 m. Air doors are used in various countries across Europe, America and Africa for cold storage rooms larger than 2000 m2. They do a better job of separating air than air curtains and their evaporators are more energy-efficient.
“Recently, an air door user observed a net reduction of 13.5% of the total energy costs of their freezer room compared to using a traditional air curtain. Thanks to the air door’s high efficiency, it it possible for it to pay back for itself in less than 2 years.”
Air door maintenance
Air doors require minimal maintenance. For those that use a filter, the filter element needs to be changed regularly. The fan works without metallic contact and therefore does not wear, apart from the ball bearings, which nonetheless can stand 35,000 hours of operation even in the most unfavourable conditions, which corresponds to 10 to 15 years of normal use. Lubrication is not necessary, not even for the fan.