34 The Cold Chain Keeping cool with low moisture
The potential problems that can be caused by high humidity in cold rooms are typically tackled by dehumidifying the air. However, it is important to select the most appropriate solution for each application, as John Barker of Humidity Solutions explains.
understood, as are the benefits of using dehumidification to prevent this from happening. However, with several options available it’s important to select the dehumidification solution that best meets the end client’s requirements. The key issue is that moisture in the air will condense onto the evaporator coil and freeze, resulting in reduced efficiency of the refrigeration plant. There can also be problems of ice formation in other areas, potentially creating hazards and almost certainly resulting in downtime for defrosting.
T
Using a dehumidifier to remove the moisture from the air will therefore prevent or minimise condensation and help to maintain the efficiency of the system.
The most effective form of dehumidification for cold room applications is generally accepted as being the use of desiccants to adsorb the moisture from the air. In these systems there is a constant and simultaneous flow of two airflows through a desiccant rotor in opposite directions. One airflow is for drying (process air) and the other is for rotor reactivation (wet air).
After filtering, the process air enters the desiccant rotor, and relinquishes most of its water vapour content. It is then extracted from the dehumidifier as dry air, ideally using an EC fan for optimum efficiency, and returned to the cold room. Desiccant rotor systems can reduce moisture levels at temperatures as low as -70°C and are typically housed in antechambers with air locks to reduce the infiltration of humidity.
HE PROBLEMS THAT can be caused by icing up of evaporator coils in cold rooms are well
Correct sizing involves quantifying the level of humidity infiltration and determining the required dew point.
Heat recovery
One concern that some cold store operators have expressed is that the returning air is at a higher temperature than the air in the cold store, so that the refrigeration plant needs to consume more energy to reduce that temperature. One option that has recently been introduced to the market is to combine the process described above with a heat recovery process which effectively works
in reverse to the heat recovery used in other applications.
With this arrangement, the dry air passes through a recovery plate heat exchanger where it gives up the sensible heat gained in the desiccant rotor before passing back to the cold room. In this way the recovery plates enable an isenthalpic dehumidification process where there is no change in enthalpy, so there is no net transfer of heat to or from the surroundings.
As a result, the dried process air returns to the cold room at the same temperature as it left.
ACR News May 2015
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