Saving energy is a vital demand of modern society. The performance of systems used in food processing and other manufacturing operations cannot be compromised in this effort, however. The need has been defined by the International Energy Agency (IEA) as “an organized structural effort directed to [saving energy] without reducing the standards of living and productivity.” Manufacturers of refrigerating equipment for food storage have become increasingly sensitive to this need and have developed solutions that combine energy savings with the ability to obtain the highest-quality products.

One such solution recently was installed at a food processing plant in Rutigliano, near Bari in southeast Italy. The company wanted a complete plant based on a concept that would include a harvesting plant, a pre-refrigeration area, a conservation area, and a processing area for fruits and vegetables. The project’s main scope was to increase the plant’s processing rates drastically while keeping energy consumption the same as or only slightly higher than existing levels. The company also required the flexibility to process different types of fruits and vegetables in the plant, along with the ability to use the cold-storage rooms for both the conservation and pre-refrigeration stages.

The new plant’s centralized machine room includes three low-temperature chillers.

After evaluating various options, the company chose a plant developed through close cooperation between Ascon SpA, based in Baranzate (Milan), Italy, and Ortiz Srl, based in Bari, Italy, with additional assistance from Trane, headquartered in Piscataway, N.J.

The plant, which was designed by Ortiz Srl, includes a harvesting plant; five cold-storage rooms, three of which are dedicated to conservation while the other two can operate as both conservation storage and pre-refrigeration; a refrigerated processing area; anterooms for the cold-storage rooms; and a loading and unloading area for the refrigerated products. The plant uses both an air-forced and a water hydro-cooler pre-refrigeration system. The cooling system is an indirect expansion type located in a centralized machine room and includes three low-temperature chillers and one evaporative cooler. The cooling system’s primary circuit uses a limited quantity of R134a Freon inside the chiller, while the secondary circuit uses a glycol antifreeze fluid. This system provides precise control of the thermal and hygrometric conditions required for processing fruit and vegetable products. Additionally, the low volume of the primary refrigeration gas meets the latest environmental regulations.

The machine room includes three glycol chillers (R Series helical rotary chillers manufactured by Trane) that use 400 kW of power to produce an 18°F (-8°C) output temperature. The energy-efficient machines are equipped with falling-film evaporators that can provide a high coefficient of performance even when the load is low.

The water-based condensation system uses a closed-loop machine Ortiz calls an evaporative cooler. This machine offers the advantages of evaporative cooling while also allowing a considerable reduction in water consumption compared to most cooling towers or evaporative condensers. The evaporative cooler is fitted with an intermediate closed-loop heat exchanger that receives the cooling fluid from the chiller’s condenser. A dedicated pump sprays the fluid (which contains 20 percent ethylene glycol) along with water inside the evaporative cooler.

Optimized Control

The new plant includes five cold storage rooms.

The operation of the evaporative cooler can be adapted to environmental conditions by the counter-stream ventilation control system. This system allows the machine to work with air instead of water when ambient temperatures are low such as during autumn or winter. This option substantially reduces scaling and, as a result, increases machine life, optimizes efficiency for a longer period of time, and reduces maintenance costs and energy consumption.

The freezing energy is transported through the loop using an inverter-controlled, variable-speed pump that matches the fluid flow to the real energy absorption of the products in the cold storage rooms (figure 1). Close control of the machine room parameters is maintained by control strategies downloaded into AC Station series programmable loop controllers, which were developed by Ascon Spa in close cooperation with Ortiz Srl. The differential pressure across the pump on the secondary glycol line (shown as P2 in the figure) also is controlled by the programmable loop controller, which signals the inverter to actuate the pump motor. The P2 pump control ensures the correct secondary cooling fluid pressure on the pipe header connected to the cold storage rooms.

Enlarge this picture Figure 1. The freezing energy is transported through the loop using an inverter-controlled, variable-speed pump that precisely matches the fluid flow to the real energy absorption of the products in the cold storage rooms.

The inlet and outlet temperatures of the secondary loop in the machine room are measured by resistance temperature detectors (RTDs) with a class 1/3 DIN accuracy (higher than Class A accuracy), which are fed into the same AC Station controller. The AC Station controller also controls the other parameters of the machines, including the water defrost line pressure, the load distribution and the alarms.

The cold storage rooms are controlled by a distributed system that includes, for each room, an area circulation pump for the freezing fluid (shown as P5 in figure 1) as well as an ambient temperature and relative humidity control system based on a three-way mixing control valve. The majority of the fruits and vegetables require a conservation temperature near their freezing point and relative humidity values ranging from 85 to 95 percent. As a result, accurate, reliable equipment must be used to control the thermal and hygrometric parameters. Again, the programmable loop controller is used, which has a 16-bit resolution analog/digital converter and an accuracy of ±0.2°F (±0.1°C). The humidity sensor in the controller provides long-term stability; reliable measurements; long-life, interchangeable filters for different applications; and the ability to be replaced without recalibration. In addition to providing accurate measurements, the controller uses software that controls the main cold storage room parameters by acting on the three-way control valve. The software also controls the ventilation and smart defrosting system, and it optimizes plant operation, thereby reducing energy consumption.

Enlarge this picture Figure 2. The Autolink SCADA system provides information such as setpoints, recipes, alarms and data acquisition.

The operator interface is a high-definition graphic display that contains several preformatted pages with numerical values, bar graphs, trends, alarms and menus. While each multiloop controller can be used to control up to four loops, most facilities prefer to use one controller for each chiller and one controller for each cold storage room. This arrangement allows the operator to have one interface graphic display for each area on the 3 x 6" (72 x 144 mm) controller screen. The control and monitoring systems are interfaced with Ascon’s Autolink series supervisory control and data acquisition (SCADA) system (figure 2), which provides plant supervision and management with information such as setpoints, recipes, alarms and data acquisition. The SCADA also adds flexibility. For example, the recipe management system can be used to set each cold storage room to work independently, with its own recipe and parameters. The additional flexibility also allows the plant to process a range of fruits and vegetables based on the season.

As noted previously, two of the five cold storage rooms in the food processing plant can be set to work as either conservation rooms or pre-refrigeration areas. Before entering the controlled-temperature areas, the products need to be pre-refrigerated. A hydro-cooler machine quickly lowers the temperature of the products using a shower of icy water. The machine is controlled by an additional programmable loop controller, which detects the freezing fluid and process water temperatures and keeps the temperature of the water in contact with the products at 34°F (1°C). The working setpoint of the machine is adjusted by a remote control algorithm to avoid a buildup of ice, which would cause an unavoidable decline in the efficiency of the cooling heat exchanger.

AC Station programmable loop controllers provide tight control of the machine room and cold storage room parameters.

The cold processing chain is completed by the processing room, where the temperature and humidity are controlled to meet the conditions necessary for areas in which people have to work. In accordance with the hazard analysis and critical control point (HACCAP) regulations for food safety, the processing room is cooled to a controlled temperature of 54°F (12°C) by a cluster of air handling units fed with a freezing fluid. The treated air is distributed throughout the approximately 300,000 ft² (30,000 m²) room through ducts made of micro-hole fabric. A separate programmable loop controller monitors the refrigeration fluid mixing stage, the ventilation areas, the ambient air turnover and the room pressurization.

The new plant has a slightly higher energy consumption compared to the original plant. However, the amount of product processed by the plant has increased by about 100 percent. As research and development on energy-saving technologies continues, future solutions will provide additional opportunities to optimize productivity and quality, along with energy conservation.