Small adjustments to your refrigeration system can make a big difference
One common issue that we see in refrigerated warehouses across the country is excessive humidity levels on docks that lead to condensation issues on the dock and frost problems in freezers. There are many operation and design factors that can play a major role in this, such as dock doors being left open, bad seals around trailers and doors, air infiltration through the building envelope, dirty air unit coils, and undersized dock refrigeration, but we will focus more on the refrigeration system itself assuming the building and refrigeration system are adequate.
Besides being a conditioned space to stage product, the dock should act as a vestibule when attached to a freezer, and the dock air units should dehumidify the air as much possible before it makes its way to the freezer. When dock humidity levels are too high, frost will build up around doorways and on air units, and “snow” can drop onto the floor causing it to become slick. Below are two psychrometric charts showing a -10°F freezer (left) and 0°F freezer (right) with a 34°F dock. Without getting too technical, the thick black line on the graph is the saturation line, and the colored lines are the air paths from air going from the dock to the freezer. If the air path crosses to the left of the saturation line, fog and frost will form in the freezer. More frost is formed as the air path crosses further in the saturation zone. As you can see in the graphs, in a -10°F freezer, the humidity level in a 34°F dock will need to be 55% relative humidity (RH) or less to keep any frost from forming. In a 0°F freezer, the humidity level can be up to 70% with no frost, and even at 80% there should only be a small amount forming. This shows that both the dock humidity levels and freezer temperatures greatly affect the amount of frost buildup, and that dock humidity becomes more of a concern as freezer temperatures drop.
Freezer door with no frost in properly conditioned environmentThe simplest way to reduce dock humidity is to make sure the dock air units are clean and cooling as much as possible. If the units are not cooling, they will not be removing any moisture from the air. During certain conditions, it may be necessary to set up the control system to run one or more air units in heating mode in order to provide a false heat load in the dock and keep the other units cooling. This will act similar to a reheat mode on certain air units and HVAC systems. It is also important to keep freezer doors closed as much as possible since open doors will cool the dock and keep the air units satisfied.
If the air units are cooling a majority of the time and humidity levels are still high, it may also be necessary to decrease the evaporating temperature (increase the temperature difference [TD]) of the dock units. For example, depending on the conditions, an air unit running with a 10°F TD in a 34°F dock may only decrease humidity to 77% RH, but if the TD is increase to 15° F, the humidity level can drop to roughly 65%.
In more extreme cases, it may be necessary to add desiccant dehumidifiers, vestibules, heated air curtains, or other equipment, but most of the time we have been able to control the humidity using these steps. These adjustments will increase the safety and efficiency of the freezer with little or no additional capital cost.
For help with your warehouse, please email us with any questions or call us at (260) 490-3000.