REFRIGERANTS VENTILATION
Preventing ice & mist with dehumidifi ers
Dave Marshall-George, sales director at Condair, explains how dehumidifi ers can be used in cold stores and freezers to prevent ice and misting.
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ommercial cold stores and freezers are tightly sealed environments, but excess water will always fi nd a way to get in. Cleaning routines, evaporation from
produce and people, or simply carried in by the air through open doorways, moisture ingress is inevitable. It can lead to ice on fl oors, walls and ceilings, frost damage to produce and mechanisms, or air misting and reduced visibility. All of these things will impact an operation’s productivity, profi tability, and health and safety. Dehumidifi ers specially designed for use in this
environment will extract humid air from the freezer, remove the excess moisture and return dry air back to the storage environment. This dry air delivery is normally directed to where a problem is being experienced, for instance above an entrance if the issue is doorway icing. With an ante-room or airlock area just outside the entrance, to minimise the volume of moisture initially entering the freezer. Beyond improving the internal environment of a cold
store, removing moisture with a dehumidifi er will also reduce the amount of ice build-up on the refrigeration system’s evaporators. By lowering the ambient humidity level, the need to defrost can be signifi cantly reduced and the operating effi ciency of the system improved. In some blast freezer projects, we’ve seen the need to defrost be extended from once per week, to once every three months. The dehumidifi ers were blowing directly on to the evaporators, leaving them running during the defrost cycle reduced the defrost time from a whole day to just a few hours. Removing moisture from a cold environment requires desiccant dehumidifi er. This is a type of dehumidifi er that passes wet air through a slowly rotating desiccant rotor. It literally absorbs water from the air like a sponge. As this rotor revolves, a section of it continually passes through a hot air fl ow, which removes the water and allows the rotor to remain absorbent. This is called ‘regeneration’. This type of dehumidifi er therefore needs two ducted airfl ows to operate. A process airfl ow, which takes air either from the cold store or ante-room area, passes it through the dehumidifi er’s drying rotor, then re-introduces the now dry air back to the cold store environment. The other airfl ow is the
34 September 2023 •
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regeneration airfl ow. This draws in fresh ambient air, heats it, then passes the hot air through the rotor to regenerate it, before fi nally being exhausted externally, along with the moisture it collected from the rotor. This type of dehumidifi er isn’t unique to cold store
projects, as it’s used in many close control, low humidity or low temperature drying applications. But there are some features that are specifi cally needed for successfully drying a cold store or freezer. These can include exceptionally thick insulation, a thicker than normal rotor, diff erent rotor speeds, special heaters, and pressure confi gurations that avoid the potential for air leakage and undesirable thermal transfer that will lead to internal freezing or condensation. Freezer dehumidifi cation systems can be tailored to include a very fl exible range of options to suit any cold store requirement. Models are available that can be located outside the cold store, in an ante-room or airlock area, inside the cold store or even outside the building. Each strategy has pros and cons, but a particularly useful feature is the energy savings that are possible only when placing the dehumidifi er outside the cold store. As these systems are operating 24/7, minimising energy consumption is vital. The majority of the energy used by a desiccant dehumidifi er is for heating the regeneration airfl ow. In cold store applications, it’s possible to use the fridge system’s condenser coil to heat this airfl ow. Using this alone can be suffi cient to get the air to 65-70°C and obtain enough drying capacity to achieve a project’s humidity control objectives. This strategy is not possible when installing the dehumidifi er inside the cold store. Another energy saving feature involves using a heat
recovery system on the exhaust side of the regeneration airfl ow. Before the hot wet air is vented, running it through a heat recovery unit can transfer around 75% of the energy back to pre-heat the incoming regen airfl ow. Incorporating both these strategies can see a dehumidifi er’s energy consumption reduced signifi cantly and could result in a net reduction in overall energy use, when considering the improved effi ciency and energy savings on the chiller plant. But whatever location or energy saving options are selected for a project, being able to proactively reduce the humidity of the air inside a freezer environment has obvious benefi ts for commercial operators and can produce rapid return on investment.
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