January 2021 - Basic Dehumidification of Refrigerated Spaces by Jayson Noetzelman

               Well, winter is once again upon us in Minnesota. Even though shoveling the driveway and raking the roof can be bothersome tasks at home, if you own or operate a refrigeration system, this is probably a relatively easy time of year to tend to them. Obviously, the low outdoor air temperatures make easy work for our refrigeration equipment. However, another factor makes winter easier on our refrigeration equipment and our refrigerated spaces, low humidity.  Let’s take a little time to talk about how keeping humidity under control year-round can alleviate many common headaches in refrigerated workplaces.

                Whether you are working with a small walk-in cooler or freezer, a cold storage warehouse, a large ammonia cooled production facility, or anything in between, humidity can be troublesome. Humidity in chilled spaces can cause ice build-up, condensation or ice on floors that can become a trip hazard, damage to product, and many sanitation issues associated with dripping water. If you have owned or operated a refrigerated space, you have likely seen these issues before. These issues are caused by humidity in the air condensing into liquid water. Let’s take a second to discuss why condensation is so common in refrigerated spaces and then let’s discuss what we can do about it.

                Stated simply, condensation occurs when air is cooled below its dew point. A helpful analogy I’ve heard working at Carlson & Stewart Refrigeration is imagining the air like a sponge. Hot air can hold more water than cold air can, so imagine hot and humid air like a sopping wet sponge. As the air is cooled, it’s like squeezing the sponge. The more you squeeze the sponge, the more water falls out. While you are compressing the sponge, it is always holding the most water it possible can at its current size, or what we might call the sponge being completely saturated. It isn’t until you release your grip on the sponge that it springs back to its original shape and is no longer completely saturated.  The reason we so often see condensation in refrigerated spaces is because in the summer months humid air can infiltrate refrigerated spaces and interact with cool surfaces in the room. These cool surfaces squeeze the water out of the air so to speak and leave it behind in many different forms. Such as an ice ball near the crack in your walk-in freezer, water droplets dripping form sprinkler pipes, or water dripping down the walls. Condensation is normal, but what can we do about it?

                The only surface we want condensing water in our refrigeration system is our evaporator coils. Evaporator coils are almost constantly gathering condensation on the cold evaporator tubes by “wringing out” the air by lowering its temperature below the dew point and then draining excess water via the condensate drain lines. In freezers the process is the same but requires some form of defrost to remove the frost from the coils. In most situations the refrigeration system drying out the air is a good thing, but in some situations, it isn’t enough to prevent condensation from occurring within the production space. In these situations, it may be time to consider adding desiccant dehumidification or air reheat (and of course Carlson and Stewart Refrigeration is always willing to help!).

                Desiccant dehumidification can provide much lower relatively humidity levels and more consistency than reheat can. This process typically involves a desiccant wheel made of material particularly suited for absorbing moisture from the air. The air is forced through the desiccant wheel which removes water from the air. Most typically these are installed as part of an air handler in large scale operations and do eventually require replacement of the desiccant wheel.

                One easier and more commercially available treatment is installation of hot gas or electric reheat. This process involves specifically designing evaporator coils to cool air beyond the targeted air temperature thus removing even more water from the air. The air is then heated slightly before leaving the evaporator coil and ejected from the evaporator at the rooms target air temperature. This “wringing out” leaves the air only partially saturated, and therefore less likely to condense on items in the room as well as able to absorb a bit more humidity before being fully saturated again. Because the air in the room is constantly being circulated through the evaporator coil, this process continues and should keep the air dryer any the room less likely to develop condensation. This process does require extra energy for the additional cooling and heating happening in the evaporator, and therefore does add some operational cost to the process. The impact is lessoned if the coil can be configured to heat using refrigerant vapor, as often seen in ammonia refrigerant installations, instead of using electric heat.

If the cool dry winter months suddenly made your humidity problems go away, it might be time to tackle those issues before summer returns. Some problems can be resolved by simple operational changes like adding roll up doors, sealing cracks in your walk ins, or adding strip curtains to doorways if you (or your coworkers) have a hard time keeping the door closed. If these aren’t likely culprits or just aren’t doing enough, it may be worth considering adding some mechanical dehumidification to your refrigeration system. Carlson and Stewart Refrigeration is proud to serve the Midwest and would be happy to discuss the subject with you further and help determine the best solution for your facility.