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Docking, doors and curtains in the cold chain

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By Benjamin Brits

With all of the impressive installations of cold stores, distribution centres and warehouses going up around the country, one of the aspects you generally see first – or if you are like me, land up trying to count as you drive past, are the docking bays. But what is involved in this element of a facility as well as the components that enable the moving of goods within, and how does technology play a role?

Docking bays and the related guides, shelters, seals and levelers, doors, curtains, supports, and rails all impact the function of the cold chain, from the packhouses on the farm all the way to the retail outlets (and transportation inbetween). There are several designs and technologies to choose from today as well. In this feature we aim to consider the available solutions and their functions, how these various units add value, mitigate risks, and aid in better efficiency in holistic operations – while also contributing or transferring benefits to other facility components such as their impact or integration on refrigeration and storage systems respectively.

A strip curtain example.
A strip curtain example. Image credit: Maxiflex

A facility’s loading zone is the entry or exit point for goods that are being loaded or unloaded from various types of trucks and vans aided by materials handling equipment. It is said to be one of the most important areas to be kept efficient as it manages the flow of goods, which in turn is then directly linked to the profitability of a business, supplier, client or third party such as a specialist transportation company. In the cold chain, these “zones” have an even greater importantance in maintaining temperatures (or minimising temperature fluctuation) and so the use of doors or entry/exit points needs to take several factors into consideration.

For example, this could include that when a door is left open for an extended period, air flow according to the scientific principles we all know well, will naturally attempt to reach an energy equilibrium by mixing with hot or ambient air and this is of course a problem when a controlled environment is in use – the more heat that enters a space, the “harder” a refrigeration system needs to work. This is also potentially a bigger problem when high levels of humidity are present outside a space – that when mixing with cold air can produce condensation, water vapour or ice that can affect insulation over time, affect products, as well as expose a facility to dangerous mold growth or conditions where some bacteria can thrive.

An example of a high-speed freezer door.
An example of a high-speed freezer door. Image credit: Maxiflex

So, different techniques and systems have been developed over the years to, for one, reduce the amount of temperature loss at these entry/exit points – using different materials, combined products, air curtains, strip curtains, solid plastic curtains and naturally speed of operation, remote control (or system intergration), motion sensing and even as part of airlock-type systems. Secondly the type of system selected allows for different scenarios in traffic flows and of course caters to several health and safety aspects.

Now given these couple of factors of many, finding the appropriate solution for any environment can be critical in the improvement of a facility and will be (primarily) based on objectives and parameters. It would be obvious that speed of door operation would not be a primary concern in ambient zones even though speed of throughput may well be very relevant versus other associated operational costs.

As one may well know from experience, facilities that process, manufacture and store temperature sensitive products are subject to a host of challenges for workers and managers alike. Inside such facilities, docking and doors must fulfil the role of maintaining the separation of differing temperatures that ultimately ensure a product’s integrity is upheld. However, under certain applications it can be difficult to source solutions that meet specifications – particularly the correct R-value (resistance or sustainability against temperature variation), minimal air infiltration and adequate cycle times required in order that that those held products are not compromised or that they enjoy the best possible range of conditions in extending their lifespan.

An example of a roll-up door. These can be made from several different material types from plastic to fabric.
An example of a roll-up door. These can be made from several different material types from plastic to fabric. Image credit: Assa Abloy

Suppliers therefore spend large amounts of funds on research and development of their products that help meet the comprehensive list of facility needs and so it would be common to see such benefits as fast cycle times, new materials, new methods and even improved strength to withstand being hit by equipment on many product spec sheets.

The traditional way to approach refrigeration in the commercial and industrial spaces is to install heavy, insulated, rigid doors with high R-values, and such doors still perform effectively in low traffic areas. In high traffic facilities, that we see far more commonly today, these slow moving or acting doors most often result in higher rates of air infiltration and therefore can make maintaining temperature control inside the space more difficult. In addition, hard core doors are susceptible to damage from equipment which leads to energy losses by compromised sealing/leakage.

Docking and door selection impacts for facilities

In today’s high paced environment and rising energy costs across the globe, the importance to ensure appropriate door selection and solutions has become a game changer. Critical factors such as this are often overlooked when designing storage facilities in general yet studies have shown that improper door selection can result in very high energy costs, higher maintenance costs, lower productivity, wasted man-hours and lower product lifespan. Efficient operations depend on controlling all of these types of factors.

Several other published works are available in this complex subject for further reading. In the content that follows, we aim to highlight some of the options available locally and internationally that will aid new facilities and those who aim to revamp or upgrade their existing facilities to achieve a level of energy conservation, as well as accommodating their handling equipment/system efficiency.

As a starting point, when considering a facility design and specifications, energy conservation in a cold storage facility begins with the intrusion of unwanted heat into certain spaces. This heat can be transferred in three different ways, namely: convection, conduction and infiltration. If these “forces” are not adequately controlled they can cost facilities possibly millions of rands annually in additional energy and wasted labour.

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The impact of convection

In a refrigerated warehouse, product could be moved in and out of storage thousands of times a day. And with each opening of a door, two air streams are set in motion at the doorway. Cold, dry air escapes from the storage area along the floor into the dock/holding area (depending on facility setup) and warm(er), moist air from the dock/holding area enters the cold storage area through the upper part of the door opening.

This effect is referred to as convection (mass air flooding), where heat transfer is accompanied by a transfer of air molecules from one area to another. Convection costs have been claimed that it can amount to 85% of the energy loss associated with door openings.

The warm air stream enters and mixes with the cold air just inside and above the doorway to create ice crystals on the walls, ceiling and the door itself. Conversely, cold air streams out of the door along the floor, mixing with the warm, moist dock air, to form fog and even ice and snow on the floor in front of the door.

How fast this frost build-up occurs depends largely upon the frequency and duration of door openings. With each opening of the door, more moisture enters the cold storage room and the frost/ice layer grows, acting as an insulator on the walls and evaporator coils.

As a result, system efficiency drops, ultimately causing the refrigeration units to become ineffective or inoperative. With decreased system efficiency through frost build up, the warm air entering the cold storage room requires the refrigeration system to run longer and use more energy to compensate.

As the frost build-up becomes significant, the refrigeration process must be suspended and the system must be defrosted by applying heat or manually “chipping” off ice. This process consumes energy for melting the ice, cooling the additional air, and any subsequent labour tasks.

Fortunately, the full effect of this air exchange does not occur immediately when the door is opened. The rush of the door opening, traffic going through the door, and differential pressures, along with other conditions, will initially disrupt the course of the air flow causing the warm and cold air streams to mix only a little at the doorway. The air streams typically will begin and sustain a steady intermixing flow pattern referred to as laminar flow between 10 to 15 seconds of the door being opened.

Preventing or minimising the ability of the air streams to reach laminar flow stage requires reducing open door time. Basic powered doors give you a better chance of minimising infiltration by reducing open door time, but high speed doors offer the greatest reduction in open time.

Conduction factors

In addition to the costs associated with open doorways, the transfer of heat or cold through a door itself can also be a substantial operational cost to a facility. Conduction losses occur when heat is transmitted through one medium to another of different density, such as heat transmission through a closed door.

Solid panel insulated doors have been the traditional choice to handle conduction problems in coolers and freezers where there is a large temperature differential. These applications tend to be door openings which are cycled less and spend substantial periods of time in the “closed position”. In those applications insulation value becomes a paramount issue to stop the transfer of heat and cold.

Inadequate insulation values can contribute significantly to not only the cost of operations but to the amount of frost, ice and moisture build up on the door and floor surfaces. Low insulation values can create slippery floor conditions due to condensation formation and thus contribute to safety hazards.

Conduction related problems can also necessitate the use of peripheral heated blowers or infrared heat lamps which can add up to substantial energy costs themselves.


Air passage created by an inability to provide tight seals around door surfaces can be the cause of substantial refrigeration loss and heat gain. The air creeping in through damaged or missing gaskets, through cracks in strip curtains, or under door panels is a constant daily operational cost. This is a common problem on doors in which the seals are not in good working condition or the sealing surface for the door is not level.

Aside from energy loss, the infiltrated moisture will also turn the doorway into a hazard area. The frost becomes ice on the floor and fog around the threshold, making unsafe, slippery conditions and poor visibility for personnel. Material handling speed suffers as the operators slow down in response to the hazardous conditions. Scraping away frost from floors, walls, product, racks and other areas of the cold storage room also means higher maintenance related costs and functional time losses.

An example of a rigid sliding door.
An example of a rigid sliding door. Image credit: Cold Link Africa | Benjamin Brits


Another major hidden cost of heat intrusion is removing the ice buildup already noted. Here, in addition to the energy cost connected with the ice buildup on the walls and evaporator coils, there is a cost of defrosting. Theoretically, defrosting using heat would require about 50% to 100% of the energy used to make that frost/ice in the first place. If manual ice chipping is used, then the cost of labour should also be factored into the equation.

A typical cold storage facility docking zone – some facilities can have anything up to 50 docks. Here you can see dock seals are in use.
A typical cold storage facility docking zone – some facilities can have anything up to 50 docks. Here you can see dock seals are in use. Image credit: Cold Link Africa

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Open doors

As any facility manager would put on his top 10 list of challenges, doors being left open would definitely feature. This can happen for several reasons and having insight to such can produce solutions or appropriate remedies.

Unsurprisingly, most of the problems resulting from condition losses are linked to users where doors are left open for long periods of time, even though the impact onto the facility is well understood.

Although this is often put down to unattentiveness, or even an intentional dislike of the doors, the reasons why doors are so often not closed may be more complex than anticipated. Some reasons could include:

  • Rush hours – The pace throughout a facility may become intense, as soon as one order is complete another one is due. This then results in a choice that comes down to workers falling behind schedule/penalties or what they may perceive as loss of a “little refrigeration effect” through any open doors.
  • False activations – Equipment operators moving past door openings may set off numerous false activations adding substantially to the amount of time doors stay open over a shift.
  • A lack of automation – Doors within a facility may only be a manually controlled function or operated by pull cords or push buttons thus staying open frequently owing to a monotonous task for the worker.
  • Door downtime – The facility door may not operate or seal correctly, possibly because of collision damage or commonly in South Afirca poor maintenance. For the operation to therefore be able to continue no choice is left but for the doors to be left open.
  • After hours – In cases where facility management or supervisory staff are off site, workers generally become less mindful about following procedures such as closing doors. Many times too, incidents also occur during periods of lower supervision.

Rules, regulations or operational procedures generally have a temporary effect for facility workers and land up requiring intense oversight to control bad habbits and the exceptions becoming the rule. For many of these reasons the advancement of automation in facilities has grown exponentially and the reality is that the best long term and sustainable solution is to select the right setup for the particular facility given parameters such as activity, the number of cycles per hour or day, capacity and overall facility production flow. One would also need to anticipate the effects of expectations versus reality in for example unintentionally forcing forklift operators to “always be in a hurry” to meet quotas.

This then leads onto the consideration of what is best for a facility and understanding the variety of options and solutions available.

Types of doors and selection motivation

If we had to consider a typical cold storage facility, a wide variety of door types are used depending on the temperature of storage spaces, function, product, and quantity and types of handling equipment used in the facility.

Dock doors

Starting from the external points, the typical types of doors installed here require minimal side room and can therefore be closely positioned along a “dock wall”. Some of the main types to chose from include:

  • Vertical rise sectional – This type of door is the most commonly applied and is ideal for areas with limited side and headroom. The costs for these types of doors can vary substantially based on the quality of materials, types and thicknesses of insulation, track designs, sealing systems, spring life and skin material.
    Since these doors are either manual or run at slow speeds, their application is primarily for low volume operations. Care should be taken to utilise products with appropriate insulation values for temperature spreads. Track designs which release the door panels from the gaskets during door movement would be preferential. Door models utilising weighted counterbalance vs springs can also add up to substantial future maintenance cost savings.
  • Single-piece vertical rise – In applications requiring higher levels of insulation value and seal efficiency a single piece vertical rise door moving straight up the wall is a great option. These types of doors are constructed in a single panel to avoid the seal breaks and lack of insulation associated with sectional types of doors.
    They also utilise down and in-track designs which greatly extend seal life. Most models utilise heavy duty commercial drives and counterbalancing weights rather than torsion springs, requiring little adjustment or replacement, eliminating maintenance down the road.
    Higher speed versions are available for interior applications as well. These models do require more headroom than the sectional door, but very little side clearance. A multitude of skin types (outer materials) and colors are available – such as plastic, steel, aluminium or stainless steel.
  • Vertical rise bypassIn applications requiring high levels of insulation value with more limited head clearance, bypass doors are available. These types of doors are a compromise between sectional and single piece doors. Models utilise two vertical rise panels on two independent track systems. Typical overhead space saving is about 25% of a single panel door. The bypass door has high insulation values to combat conduction, and effective seals to prevent infiltration. This type is also weight counterbalanced. This door’s design, however, incurs additional costs.
Dock levellers bridge the gap between a docked vehicle and the loading bay to ensure smooth and safe loading and unloading of goods.
Dock levellers bridge the gap between a docked vehicle and the loading bay to ensure smooth and safe loading and unloading of goods. Image credit: Stab-A-Load

Cold room and freezer door options

Proper door application is dependent upon a number of influencing factors. These include but are not limited to, the type of traffic, temperature differential, anticipated cycles, number of shifts and environmental conditions. Listed in this portion of the article are a number of different available door types and some high-level guidelines on correct application.

  • Horizontal single panel sliding Single panel sliding doors are the standard for low to moderate cycle cold storage applications at a reasonable price point. Their sliding design enables powered, high speed operation and permits traffic to go through the doorway without stopping.
    This type of door stays close to the wall as it moves and presents minimum traffic hazards. Single sliding panel doors are highly insulated and seal well to stop convection, conduction and infiltration.
    Single slide options are cost efficient because of a combination of speed, insulation, simplicity of design and relatively low maintenance.
  • Horizontal bi-part sliding – Bi-part sliding doors offer faster operation for greater material handling efficiency, reduced energy loss and a reduced possibility of door collisions. Bipart doors are also an excellent application with limited clearance on either side of their opening.
  • Strip curtains – Known as the cheapest solution to provide some degree of mass air flow and convection protection is the installation of a strip curtain system, in conjunction with the hard panel door. While this strategy cuts some of the convection loss, the presence of a strip curtain in the doorway encourages personnel to leave the solid cold storage doors open and is a detriment to material handling efficiency. A typical application for the addition of strip curtains is in areas of significant temperature spread or in areas where doors are left open for significant periods of time.
    If one includes strip curtains in a facility, make sure they are kept in good condition and use 100% overlapping strips wherever possible. Even the smallest separations between the floor and a strip curtain can result in energy losses which equals additional energy costs.
  • Infitting/personnel – Standard hinged walkthrough style doors can be constructed of fiberglass, metal or several insulation materials. The majority of these doors are manually operated. Automatic closing devices are available – particularly in facilities with high safety/contamination concerns. They are commonly used for personnel traffic and emergency exits and are mostly highly insulated and sealed to stop convection, conduction and infiltration.

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 High speed doors

High speed doors are the access solutions that have seen the most research and development over the last few years to include several benefits from manual operation right through to fully autonomous facilities.

  • Bi-folding doors – Once the standard in high speed, high cycle door technology, the bi-folding door provides impressive opening speeds.These types of doors were popular in the past because they provided high speed operation, impactability and clear vision through the door opening. Bi-folding doors provide a mini entrance at the cold storage door opening. They are typically made up of several panels of clear PVC. Clear panels are connected by means of ropes, straps or Velcro to prevent separation from pressure differentials or air currents. Freezer versions of the bifolding doors typically use heated blowers or infrared heat lamps to prevent the buildup of frost or ice on panels. Bi-folding technology has become less desirable as the energy costs associated with its operation can be high.
  • Fabric roll-up doors – Fabric roll up doors have continued to become more popular in refrigerated warehousing and are available in a variety of styles. They combine the benefits of high speed operation, and impactability with a very low profile which allows for increased dock space. These models can have a dramatic impact on reducing refrigeration loss and heat gain. Fabric roll up doors are available in non-insulated and insulated styles. Typical applications for fabric doors are moderate to high cycle applications with little to no down time. Since this category of door provides low levels of insulation value, they are not typically applied on low cycle applications. Depending on temperature differentials some models may require peripheral heat to prevent frost and ice formation. These energy usage considerations should be taken into account when applying these types of doors.
  • Impactable panel sliding doors – Recognising the disadvantages of losing insulation value and high seal compression associated with fabric doors a newer category of cold storage doors was developed. These types of doors were designed to provide higher levels of insulation and seal efficiency with the same or greater opening speeds of fabric roll-up doors. The problem with hard panel doors of the past was they moved slow and if you impacted them substantial damage occurred. Impactable panel doors utilise a number of different styles to allow the panels to flex or breakaway in the event of a fork truck collision. This combined with their high speed operation, insulation values and seal capabilities have made them a popular selection.
  • Air doors – Applications in the mid to high cycle range with little to no down time, can be well serviced by the use of air doors. This type of product provides a high velocity laminar flow of air horizontally across a cooler or freezer door opening providing high levels of seal efficiency with no physical door in the opening. These products are available in non-heated and heated versions. Heat is typically applied to suspend any moisture which attempts to form at the door openings. These types of doors can be very effective in terms of reducing and eliminating the formation of fog, frost and ice around freezer openings. This combined with unlimited material handling efficiency provides for substantial payback in today’s busiest openings.
  • Combined door systems (air systems with secondary doors) – Although most refrigerated warehouses experience high door cycles, in most applications there is still considerable downtime. Whether this is due to meal breaks, shift changes, non-peak demand times, one or two shifts or weekend shut down, most applications see a substantial period of downtime. When applying combined door systems the goal is to achieve the highest rates of material handling without sacrificing seal efficiencies during peak demand while also providing high levels of insulation and energy cost savings during low usage times.
    This desire can be accomplished through the use of combined door systems. Recirculatory air doors are used to maintain high levels of seal efficiency during peak traffic periods. When door openings go into slow traffic time spans, secondary doors are allowed to close, creating a 100% seal during non-usage. These doors are usually a high speed fabric roll up door or an insulated sliding door depending on the expected length of time associated with downtime. A further energy cost savings option is to use a timing sequence to automatically turn off the air systems after a preset period of time when the secondary door system closes.

Door actuation

Door actuation is as important to proper door selection as the door itself. To achieve the goal of energy conservation, it is the door that provides the speed, and it is the actuator devices that enables operators to limit the time the door is open. However, specifying the right actuator for the door means knowing the limitations of the facility. Also, actuator placement is as important in achieving the best operational efficiency.

Actuators, both manual, electronic and remote controlled are available and include the following devices:

  • Push buttons
  • Pull cord switches
  • Radio transmitter
  • Pedestrian openings
  • Photo eyes
  • Motion detectors
  • Presence sensors
  • Loop detectors
  • Time delay

Dock levellers

Quite simply, a dock leveller bridges the gap between a docked vehicle and the loading bay to ensure smooth and safe loading and unloading of goods.

A leveler thus keeps stock and staff protected from damage and injury. They are designed to withstand the common daily conditions of loading processes so owners and operators can be rest assured that they have a long lifespan. From manual dock plates to automatic dock levelers, one can tailor a solution to fit any facility requirement.

There are a number of different types of levellers designed to meet different needs. The most common types include one designed with swing-lip support to ensure safe and efficient loading and unloading, and second type that enables optimal load utilisation by increasing the contact area between a vehicle bed and the dock leveler with a telescopic-lip.

There are also models designed to cater to specific industry nuances, and of course the accommodation of small and large vehicles.

Dock levellers are normally built using various steel materials – making them both durable and hygienic. They are not tied to standard dimensions and may have various capacity options: ranging from 6 tons to 15 tons.

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Dock seals and shelters

An example of an installation of dock shelters.
An example of an installation of dock shelters. Image credit: Maxiflex

The seal between a truck trailer and building is paramount in preserving the cold chain at the dock face. Dock seals and shelters serve similar purposes: they both help maintain the controlled atmosphere and help to protect product as it moves in and out of a facility. In other words, they both perform the same functions, just in slightly different ways.

Seals are generally made from compressed foam materials and covered with a durable fabric. They are mounted around the perimeter of a dock door, and they then form a tight seal when a truck backs up to the door.

Shelters, on the other hand, consist of a frame that surrounds a dock door with curtains mounted to that frame. When a truck backs in, the curtains surround the trailer entry and essentially form a seal.

Both seals and shelters each come in several specific designs and material types. They can be more rigid and also come in flexible options depending on facility requirement.

When considering whether to use shelters or seals, some of these factors may be relevant:

  • Truck and trailer variability: are most vehicles going to be the same size and configuration? If so, seals are probably a better approach. If you’ve got a lot of variability in truck size, go with shelters.
  • Door sizes: how large are the dock doors? Seals are great for smaller openings and shelters are more appropriate for larger openings.
  • Trailer access: is there a need for full access to the rear of the trailer? Seals can restrict access in some cases.
  • Structure: can the dock door wall withstand the high compression forces of conventional (non-inflatable) seals or shelters?
  • Mounting surface: is the area suitable for a shelter frame the projects several inches, or do you need something that can be flush mounted?

Another subject that has a lot of intricacies and a range of products to cater to the needs of each facility. If you have any interesting technologies related to docking, doors or curtains for the cold chain, please do get in touch with the Cold Link Africa team.

Article Sources:

  1. ASHRAE Fundamentals
  2. Assa Abloy
  3. CB Engineering Company
  4. Dock Design Layout
  5. Maxiflex
  6. Raymond West Intralogistics
  7. Stab-A-Load

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