Actual Process Control and Operational Conditions of Commercial Vacuum Cooling Plants and its Improvement Instructions

Abstract

A comparative investigation of commercial vacuum cooling plants has been carried out to obtain the improvement instructions in both process control and cooling performance for a continuous operation.
Three models were selected from among the most typical systems operated in Japan, and their system's components, actual operating conditions and controlling methods and cooling performance of vegetables were evaluated by applying a standardized measure of vacuum cooling processes, which was defined from the viewpoints of most rational control procedures based on the cooling mechanism of vegetables.
The results and recommendations obtained from this study have been summarized as follows;
1. Although commercial vacuum plants are usually consist of two vacuum chambers, one or two cold traps and two or three oil rotary vacuum pumps as shown in Fig. 1, there are many variations in their design and combination as well as controlling methods of the components in a evacuation system, depending on the manufacturers and the steps of cooling process.
2. To compare and evaluate the controlling and operational methods of each plant, a standardized vacuum cooling process was defined by dividing it into four steps such as air-evacuation, flash-cooling, finishing and pressure recovering as shown in Fig. 2. The evacuation and controlling methods each model are classified as indicated in Table 2 by comparing these with the standardized process.
3. The features of controlling methods for each model of the plants as well as step of the process are summarized in Table 3 and the corresponding cooling characteristics of lettuce are presented in Fig. 3-5.
4. As the improvement instructions of the plant for cooling perfommance, controlling and operated methods, the following items have been indicated; namely a) the develapment of controlling method to set the timing from the air-evacutation stage to flash-cooling, predicting the flash point from the infomation of initial product temperature and the capacity a evacuation system, b) the rationalization of both assessment method for the refigeration load of the cold trap and c) the arrangement of evacuation system considering a required capacity of each cooling step. d) the finding of operational procedures to disappear the temperature gradients within the cooled vegetables which were found to be affected by the variation of the cooling load.
5. Although at present the process is essentially operated and controlled based on the chamber pressure, it is not necessarily considered to be an appropriate factor for monitoring the cooling conditions of vegetables. As an alternative, we have appointed the possiblity of the wet-bulb temperature in the vacuum chamber as a monitoring factor product temperature especially after the flash point of the cooling process.