By Jon Schumacher and Kyle Justice
Door openings are among the biggest sources of energy loss inside a building. Within large warehouses, manufacturing plants, or distribution centres, this problem is only amplified. When it comes to choosing the right door to control proper temperatures in different areas, the decision typically comes down to two factors: speed and insulation.
For years, the best solution was thought to be heavy, slow-moving doors with high R-values. Recently, however, high-speed doors have stepped to the forefront, as their quick action roll-up capabilities can actually increase worker productivity and safety, while also decreasing energy loss.
Refrigerated and air-conditioned spaces are designed to trap cold inside and keep heat and humidity out. The traditional way to approach this was to install heavy, insulated, rigid doors with a high thermal insulating performance. These side-acting doors are suitable for low-traffic openings, but are typically slow-moving—this means longer door cycle times and thus, higher rates of air infiltration.
While it is true higher R-values mean less energy loss through door panels (i.e. conduction), there is also a downside. Having a slow-moving door makes maintaining temperature control difficult; it also hampers productivity, as workers must either wait for the door to open or leave it open for extended periods. Further, traditional hard-core doors are susceptible to forklift damage. Unless the damaged door is quickly fixed, there can be substantial energy losses as a result of poor sealing. Thus, for high-traffic openings, energy losses due to the door being open or sealing poorly (infiltration) can represent a significantly larger cost component than energy losses due to conduction. The highest R-value imaginable is not worth much when the door is not closed.
As an alternative to these traditional doors, high-speed roll-up doors and folding doors are sometimes chosen. These doors move quickly and help to minimize air infiltration via their short open/close cycle time. Unfortunately, some have poor sealing characteristics and many have an R-value too low to prevent frost from building on the door panel surface itself. To alleviate this problem, it is common for defrost systems to be added to these doors at an annual operating cost of $10,000.
Other traditional approaches include strip curtains and air systems. Like all options, these designs have their own pros and cons. Strip curtains, which are comparatively inexpensive, consist of clear plastic strips suspended in the opening. Although the low upfront cost is appealing, they are not particularly good at sealing most freezer applications, which ultimately makes them an expensive choice in the long run.
Additionally, their R-value is low, making frost buildup a potential problem. This can lead to hazardous working conditions as scratches in the plastic and frost on the strips can obscure a forklift driver’s vision, and the poor seal allows for frost and ice to build up on the floor.
Air systems can be either standalone or housed in multiple units integrated into a vestibule assembly. Often these systems do not seal the opening well, and proper alignment is critical for best performance. Vestibules can also require a large footprint, eating up valuable floor space.
Air units generally consist of heaters that reduce the relative humidity (RH) of the infiltrating air. Unfortunately, it usually takes a tremendous amount of energy for them to operate at a level that keeps the opening ice- and frost-free.
Traditional doors invite high energy costs. Significant air leakage at the opening makes it difficult to maintain proper temperatures. Additionally, frost and ice buildup in freezer applications can often lead to serious issues unless an expensive door-panel defrost system is included. Overall, these high costs have driven demand for better options.