When crops, particularly salad vegetables, are harvested in the summer months, the crops are at whatever temperature pertains on that day. For the crop to reach the customer it has to be kept fresh and, in order to do this, the crop is cooled very rapidly to between 2 and 4°C. If the crop has been harvested at >20°C, the degree of cooling required to bring the crop to storage temperature will damage the crop.

To counter this, moisture is added during the chill down period. However it is not possible to humidify during the whole chilling period as the duty requirement would be so high that it is not commercially viable. The humidification is therefore timed to operate when the cooling is off, to give a fog in the store so that the crop can reabsorb moisture from the air and also to provide residual moisture to prevent moisture loss from the crop during the next chill cycle.

The normal cycle is 15 minutes of cooling followed by 5 minutes of fogging, this cycle being repeated until the crop is down to temperature. There is no humidity control for this process and control is achieved by using a timing circuit in the plant control panel that runs the humidifier. It is critical that the fans from the chiller plant run all the time during this process so that the air distribution paths are maintained.

Equipment Selection
The selection of equipment is dependent on the type of chiller unit being used. Commonly, high humidity coolers are used in this application as it is felt that these will cause less damage to the crop. However, this is not the case due to the temperature difference between the chilled air and the crop. Whilst the humidity at the discharge of the cooler is at a nominal 98%rh, the humidity around the crop is often as low as 75%rh.

The best way to distribute the spray is to mount a bank of nozzles, such as those in a JetSpray compressed air and water humidifier, across the face of the evaporator in the cold store and to use the fans from the evaporator to distribute the moisture.

The nozzles should not be larger than 3.5litres/hr as the spray from larger nozzles is too heavy to distribute across the store. The nozzles should be mounted on a stainless steel line spraying into the air flow. It would be very unusual for a duty to be larger than about 50litres/hour, typically using a JetSpray 60 control panel.

The panel should be mounted outside the area or mounted in an IP65 housing to ensure that it does not get wet. Control should be via timer circuits and not via humidistats which are not typically designed to suit the application.

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