Post Harvest Cooling/Storage for Cut Flowers
David S. Ross, Extension Agricultural Engineer, University of Maryland
Refrigerated storages provide growers with the capability of extending the useful life of cut flowers and therefore widening the market window for the product. Cut flowers are alive. At lower temperatures flowers have a lower respiration rate and consume their stored energy much slower. The flower stays alive longer at the cooler air temperature.
A refrigerated room should be well insulated to keep the outside heat from penetrating to add to the heat load to be removed from the inside. Doors should seal tightly to keep cold air and moisture from leaking out or warm, moist air from leaking in to condense on the cold surfaces.
Water vapor moves from high vapor pressure to low vapor pressure. Generally, the cooler the air, the lower the moisture holding capacity and the vapor pressure are. A vapor moisture barrier should be placed on the outside wall side of the insulation to stop the penetration of moisture into the insulation where it might condense and lower the benefit of the insulation.
High humidity is desirable in the refrigerated storage to prevent a water vapor differential between the flowers and the air. Keep the humidity high so that the flowers do not dry out and wilt. Flowers need to hold onto their vital moisture content to live.
Moisture inside a cold storage room moves from the flower to the air and from the air to the cooling coils of the air conditioning/refrigeration system. The cooling coil (evaporator) typically operates at a lower temperature than the storage room (necessary for heat transfer) and condenses water out of the air onto the cold surface of the coil. The continual water loss contributes to the lower humidity in the storage room air. Examples of this process are shown by condensate dripping from window air conditioners, auto AC units, and regular household units. The air becomes cooler and dryer with most systems.
After the air exits the cooling coil it is heated slightly by the storage room air. The greater the difference between the air temperature exiting the coils and the storage room air, the lower the relative humidity will be in the room. This lower relative humidity air has a lower vapor pressure than the flowers and drying will occur again. This is a continuous process.
Cooling equipment must be selected correctly to obtain a high relative humidity storage room. The refrigeration system must have a cooling coil (evaporator coil) with a large surface area for the air to pass over. The evaporator must operate at a temperature very close to the storage room air temperature. The temperature differential between the evaporator coil and storage air must be small. The smaller temperature differential, the better conditions are for high relative humidity.
Typically, the air velocity is also relatively low for cut flowers so that the air movement does not damage the flowers. The fan may be sized larger in diameter to maintain a large airflow at a low velocity. Large diameter, low velocity fans should be less noisy than smaller diameter high-speed fans.
Moisture can be added by spraying water onto the storage room floor and letting it evaporate. Also, water can be sprayed or misted into the room air with a misting nozzle.