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<\/a>Condensation<\/h1>\n\n\n\n
Start by remembering that air carries water vapor and the cooler it gets, the greater the chance of that water vapor dropping out as condensation. Also imagine a glass of ice water on a warm humid day. The condensation on the outside of that glass is water vapor that has condensed out of the air when it came into contact with the cold surface of the glass. In technical terms, the surface of the glass is below the dew point of the air, so dew (condensation) is formed.<\/p>\n\n\n\n
Your cooler walls<\/a>, floors and ceilings can also end up doing the same thing as that glass of ice water. I\u2019m going to just refer to cooler walls, floors, and ceilings collectively as \u201cwalls\u201d from now on for simplicity. If the cooler walls are relatively cold and there is stored produce nearby that is respiring and generating heat and water vapor, it\u2019s the same situation as that glass of ice water on a humid day.\u00a0 You have a relatively warm and humid air source coming up against a cold surface. Condensation will happen. Congrats, you made rain!<\/p>\n\n\n\n As we move into deeper winter storage periods, cooler refrigeration systems are generally running less. The outside temperatures are closer to storage temperature, so there is less need for refrigeration. Most farms are also slowing down on their pack-out frequency.<\/p>\n\n\n\n Two things happen:<\/p>\n\n\n\n Here are some things to try if you are seeing condensation:<\/p>\n\n\n\n Some growers also have a hard time keeping humidity high enough. This is generally the result of either poor cooler sealing, running with very low refrigerant temperatures, or having relatively little produce in a large cooler.<\/p>\n\n\n\n It\u2019s important to remember that different storage crops have different optimal storage conditions (temperature and relative humidity (RH)). <\/p>\n\n\n\n This tends to lead us to wanting different storage zones for the different crops being stored. That doesn\u2019t necessarily mean different storage rooms or coolers. I\u2019ve been in an 8\u2019x10\u2019 cooler with a CoolBot that had a 10 \u00b0F temperature difference and a 20% humidity difference between the CoolBot (cold and dry) and the door (warmer and more humid) over the 10\u2019 length. <\/p>\n\n\n\n The key is <\/p>\n\n\n\n (1) knowing the best conditions for storage of specific crops and <\/p>\n\n\n\n (2) knowing your actual conditions. <\/p>\n\n\n\n Invest in a good, instant read, digital thermometer<\/a> that can be calibrated in a glass of ice water.\u00a0 You can use these or other remote monitoring options<\/a> to know how the conditions in your cooler differ by location. Electronic humidity sensors are notoriously inaccurate above 85% RH, so invest in a sling psychrometer<\/a> which will give you dry-bulb and wet-bulb temperatures. These two temperatures determine relative humidity<\/a> and you can either use a look-up table that comes with the psychrometer or use an online calculator<\/a>. You can also download an Excel add-in<\/a> if you\u2019re really serious. Or use the lookup table<\/a> below for common cold, humid produce storage conditions.<\/p>\n\n\n\n A word of caution, don\u2019t actually spin (or sling) the glass psychrometer in your cooler, use the air flow that a fan provides. The glass thermometers can break and then you’ve got bigger problems. You can also make a psychrometer out of your digital thermometer by placing some moist cotton shoelace over it and putting it in the air flow to measure wet-bulb temperature.<\/p>\n\n\n Condensation isn\u2019t always obvious. It can happen inside walls, ceilings, and below floors as well. If you\u2019re keeping the inside of the cooler cold and the air on the other side of any of those walls is relatively warm and humid, guess what? There\u2019s a chance of condensation inside the wall, ceiling or floor.<\/p>\n\n\n\n When there is a temperature difference across any constructed wall (again, \u201cwalls\u201d means ceilings and floors also), there is a temperature gradient. There is also generally going to be a dew point gradient. The dew point gradient results in a \u201cmoisture drive.\u201d Think of it as pressure; there is higher moisture pressure on one side and it drives the moisture toward the other side. This can occur by diffusion (water vapor moving from an area of high concentration to one of lower concentration) or by infiltration (air leaking into and even through a wall.)<\/p>\n\n\n\n The warm humid air can infiltrate into the wall and water vapor can diffuse into a wall if there is no barrier to this movement. That\u2019s the role of a vapor or moisture barrier. The other challenge, in the northeast anyway, is that our dew point gradients can swap over the course of the year.<\/p>\n\n\n\n The figure above shows a cooler wall in summer conditions and winter conditions. The basic wall construction from inside to outside is a layer of dairy board (FRP), \u00bd\u201d plywood, 4\u201d of rigid blue board, and a final outside layer of \u00bd\u201d plywood. The inside conditions are the same year-round at 34 \u00b0F and 95% RH (the resulting dew point temperature is 32.2 \u00b0F). In the summer, the outside conditions are 90 \u00b0F and 85% RH (dew point is 84.6 \u00b0F). So the moisture drive is from the outside (higher dew point) to the inside (lower dew point). We need a vapor barrier on the outside. In the winter, the outside conditions are 32 \u00b0F and 40% RH (10.4 \u00b0F dew point). The moisture drive is from the inside (higher dew point) to the outside (lower dew point). <\/p>\n\n\n\n So, we need a vapor barrier on the inside. Cooler walls should, therefore have vapor barriers on both inside and outside wall surfaces and be well sealed all around. Alternatively, a wall can be constructed to ensure areas where condensation will happen are ventilated. An intentional air gap can be designed into the wall which spans the condensation plane and which has an inlet on the bottom and an outlet on the top. This would allow for condensation that does occur to be stripped with passive, natural convective flow. In either case, a fair amount of construction skill is required.<\/p>\n\n\n\n Want to geek out on cooler walls and condensation, check out our wall thermal calculator<\/a>. Want to geek out more, call or email Chris.<\/p>\n\n\n\n A video presentation on this topic is also available here:<\/p>\n\n\n\nDeep Winter Storage\u2026 \u201cWhy am I getting condensation all of a sudden?\u201d<\/h1>\n\n\n\n
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Condensation Cures<\/h2>\n\n\n\n
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On the Other Hand\u2026 \u201cI can\u2019t keep my humidity high enough!\u201d<\/h1>\n\n\n\n
Humidity Helpers<\/h2>\n\n\n\n
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<\/a><\/a><\/a>Specific Storage Crop Issues<\/h1>\n\n\n\n
<\/a><\/a><\/figure>\n\n\n\n<\/a>The Condensation You Can\u2019t See, Until It\u2019s too Late.<\/h1>\n\n\n\n
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