What does it take to refrigerate or preserve fresh flowers? Test studies reveal the combination of both temperature and humidity plays a critical role in the preservation of fresh flowers.
The optimum temperature in a floral case is 36°F with a relative humidity of between 90% and 95%. This level of relative humidity helps flowers retain their own moisture, resulting in a fresher product with a longer vase life — up to three times longer when stored at the right temperature and humidity. Storing flowers at lower humidity levels will lead to dehydration and a shorter vase life.
To achieve this level of humidity, the evaporators used in these cases will be different than those used in general refrigeration equipment. Although not even perfect conditions can improve on initial product quality, advanced refrigeration techniques mean that a flower can have the same freshness coming out of the cooler as it did going in.
Case humidity is maintained by achieving the correct evaporator temperature difference. An evaporator’s temperature difference (TD) is the difference between the saturation temperature of the refrigerant inside the coil and its entering air temperature. The greater the temperature difference, the more moisture will be removed from the air and the lower maintained relative humidity in the case. The smaller the temperature difference, the less moisture is removed from the air and the higher the maintained relative humidity in the case. Evaporators used in floral cases are designed with a TD of about 5°F, which will maintain the case humidity between 90% and 95%.
The design of the evaporator will lead to a slightly different operating suction pressure when compared to a general cooler at a higher TD. For example, in comparing a general reach-in cooler to a floral cooler, where both systems are using R-134a with an entering air temperature of 35°F and assuming no suction line pressure drop, the suction pressure of the floral cooler would be approximately 26 psig. This is based on a coil temperature of 30°F and an evaporator temperature difference of 5°F. The reach-in cooler would have a suction pressure of approximately 15 psig, based on a coil temperature of 15°F and an evaporator temperature difference of 20°F. Both cases will maintain the same case temperature, but will have different case humidity values and slightly different operating suction pressures.
The evaporators in floral coolers will utilize a low air velocity design that leads to the lower TD, which helps eliminate hot spots within the cooler and ensures a constant overall temperature. Since the air coming off the evaporator coil (fan) is at its coldest temperature, flowers should never be placed where they might be in direct contact with this initial airflow.
Ultimately, the life of flowers depends on the quality of the environment in which they are placed. Any refrigeration will not do — it must be the proper refrigeration. Even if flowers are harvested at the right stage and then receive the right pre-treatments and preservative solutions, improper refrigeration can cause early flower death.