The design of affordable, temperature efficient cold storage doors has eluded engineers for decades. Only recently have significant breakthroughs been made in cold storage door technology, allowing a combination of fast cycling with high-efficiency insulation and effective perimeter sealing. To appreciate the importance of these breakthroughs, one must first understand the inherent problems cold storage presents and how engineers have dealt with them in the past.

The design of cold storage doors must address two problems: conduction and infiltration. Conduction is the property that allows heat to move through the door, even when it's closed. Panels with sufficient thermal resistance, or R-value, experience minimal loss. However, those with low R-value allow condensation, or frost buildup, to collect on the door due to a lower surface temperature on the warm side. There is also the problem of infiltration, which is the property that allows heat to seep in at the door's edges and through any seams where door sections meet. High-speed doors and tight seals do help to decrease infiltration costs.

The most common door design to combat these properties thus far has been the hard-core door. Made of rigid panels of high R-value materials and designed to seal tightly around the perimeter, these doors manage to conserve much of the heat normally lost to conduction and infiltration. But because they are typically slow operating and because their drive systems have not been designed for frequent opening and closing, they are incapable of accommodating a continuous flow of traffic. Hard-core doors also pose a problem because of their susceptibility to forklift damage.

The cold storage door systems designed for heavy traffic are strip curtains, air systems, high-speed foldable doors and roll-up doors. Each of these has traditionally had its advantages and disadvantages. The first type, strip curtains, are inexpensive, easily replaceable and allow forklifts to pass in and out of the cold storage area rather quickly. But with their low R-value, frost buildup typically occurs. This obscures the forklift driver's vision and puts pedestrians at risk. Also, because the strips remain in place and do not clear the opening, products and lift truck operators are frequently struck by the strips as they come and go. The second type, air systems, control the humidity of the air inside the cold storage area. With this system, visibility is typically clear. Conversely, air systems are costly to install and do not seal at the opening. They also take up a large amount of floor space and consume a great deal of electricity. The high-speed folding door, the third type, opens quickly and features flexible panels that absorb impacts. Unfortunately, these doors do not provide an optimal seal around the door perimeter and, because of their low R-value, require panel defrost systems. They also require a backup door during shutdown periods as a defense against conduction and infiltration. Lastly, roll-up doors occupy little floor space but, like folding doors, have very low R-value and require defrosting. Though they open rapidly, overhead operations make them slower to clear openings than doors that open to either side. Most require a sensing edge to reverse the door's movement should it come down on an object or person, but these sensors are often unreliable.

Cold storage door technology is now able to combine the best aspects of all these models: the insulating and sealing qualities of hard-core doors and the fast action of high-speed doors. Door-makers have begun using thick, yet flexible, foam core panels that are better able to withstand impact and have a sufficient enough R-value to make defrosting unnecessary. On the mechanical angle, they have also developed faster opening rolling doors and more effective perimeter seals. What's more, the latest systems require less floor space to install. In addition, automatic reversing is now possible without the use of sensing edges. Brought together, these technologies result in lower long-term energy costs, higher handling efficiency and a safer overall environment for cold storage workers.

Even with the advantages this new technology affords, it pays to do some homework before choosing a door for your cold storage facility. This means taking a close look at the intended application and estimating both the investment and long-term costs. In addition to the price of the door itself, start-up costs include the price of defrost systems and the calculated product storage loss due to the operating space the door system requires. The question to ask when calculating long-term costs is "How energy efficient will it be?" A thorough energy analysis can pinpoint which type of door system is the best economical choice. An energy analysis looks at the differences in temperature and humidity between cooler, freezer and outer room. It also takes into account factors such as the size of the door opening, the number of work hours per day, the number of door cycles per hour, the door cycle time, the cost of electricity and the defrost system energy rating. These are the variables that affect infiltration and conduction levels and which determine the cost of a defrost system. Companies shopping for cold storage door systems have some good news as far as technology is concerned. They would still do well to compare systems and calculate both the short and long term costs.

Source: Shutting in the Cold
Al Hochstein
Material Handling Management
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