High Efficiency Cooling Plant for Cold Stores for the Conservation of Fruits and Vegetables

Energy saving is a vital demand of the modern society, this demand has been defined by the International Energy Agency (IEA) as “an organized structural effort directed to the energy saving without reducing the standards of living and productivity”. The manufacturers of refrigerating equipment for the conservation of food are more and more sensitive to this demand and have found solutions which combine energy saving with the requirement to obtain the highest quality level of the products processed in the cold stores.

The scope of this article is to describe the application of the instrumentation and control system developed by ASCON Spa for a plant ordered by a company acting in the fruit and vegetable sector based in Rutigliano, near Bari in the Southeast of Italy. The company was looking for a complete plant based on a new conception and including: the harvesting plant, the pre-refrigeration area, the conservation area, and the processing area for fruits and vegetables.

The main scope was to drastically increase the processing rates of the plant with an energy consumption equal or slightly higher than the one used for plants with normal processing rates. Also it were required the flexibility necessary to process different types of fruits and vegetables and the possibility to use the cold rooms for both the conservation and pre-refrigeration stages. The development of the control system has been completed thanks to the close co-operation of ASCON Spa with the plant main contractor and engineering company, ORTIZ Srl which has 40 years of experience in the realization of industrial refrigeration plants which include the latest state-of-the-art technological equipment. The co-operation also included the company TRANE, a leading global manufacturer of machines for the industrial and residential cooling applications.

Plant and Control System Description

The plant includes: the harvesting plant; five cold stores, three of them are only dedicated to the conservation while, the remaining two, can operate both as conservation store and pre-refrigeration area; the refrigerated processing area; anterooms for the cold stores; loading and unloading area for the refrigerated products. The available pre-refrigeration systems are of two types, air-forced type and hydro cooler water type. The cooling system is located in a centralized machine room (figure 1) and includes three low temperature chillers and one evaporative cooler. The cooling system is of the indirect expansion type, the primary circuit uses a limited quantity of R134a freon, this is due to the fact that the gas is only present inside the chiller. The secondary circuit uses a glycol antifreeze fluid. This system well matches the requirements of accurate and extremely stringent control of the thermal and hygrometric conditions typical of the fruit and vegetable products. Also, the low volume of the primary refrigeration gas, meets the latest requirements to protect the environment.

The machine room includes three chillers for glycol with 400kW power and with a minus 8 degrees Centigrade output temperature. Machines are of the series R helical rotary chillers manufactured by TRANE with high energy efficiency and equipped with falling film type evaporators able to guarantee a high C.O.P. (Coefficient Of Performance) even when the load is extremely low. This latter condition is the sore point of the majority of these type of plants, often the efficiency of the machines, when the plant has reached the steady state, is not kept in due consideration.

The condensation system is of the water type and is realized by using a new closed loop machine called evaporative cooler which offers all the advantages of the evaporative cooling but also allows a considerable reduction of the water consumption if compared with the one of the cooling towers or of the evaporative condensers. The evaporative cooler is fitted with and intermediate closed loop heat exchanger receiving the cooling fluid of the condenser of the chiller.

The cooling fluid, a fluid containing the 20% of ethylene glycol, is sprayed with water inside the evaporative cooler using a dedicated pump. The evaporative cooler operation can be optimized and adapted to the environmental conditions by means of the counter stream ventilation control system. This system allows the machine to work by using only air, instead of water, when, during autumn or winter, the ambient temperatures are low. The advantages are: substantial reduction of the scaling and, as a consequence, increase of the life of the machine, keeping of the machine optimal efficiency for a longer period, reduction of the maintenance costs and, again, fairly good energy saving.

The transportation system of the freezing energy is entrusted to a loop using a variable speed pump controlled by an inverter suitable to exactly match the fluid flow on the basis of the real energy absorption of the loads, i.e. the cold stores (figure 2). The close control of all the parameters of the machine room is realized by the control strategies downloaded into programmable loop controllers of the ASCON’s AC Station series and resulted by the close co-operation of ASCON Spa with the plant engineering company ORTIZ Srl. The differential pressure across the P2 pump on the secondary glycol line is controlled by the AC Station controller which acts on the inverter actuating the pump motor. The P2 pump control guarantees the correct secondary cooling fluid pressure on the pipe header connected to the cold 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 (highest than Class A accuracy) and fed into the same AC Station controller which also controls all the parameters of the machines including the water defrost line pressure, the load distribution and the alarms.

The cold rooms are controlled by means of a distributed system which includes, for each cold room, an area circulation pump of the freezing fluid (P5) and an ambient temperature and relative humidity control system acting on a three way mixing control valve. The majority of the fruit and vegetables require a conservation temperature near to their freezing point and relative humidity values ranging from 85%RH and 95%RH. Due to this it is essential to use, as control equipment of the thermal and hygrometric parameters, extremely accurate and reliable equipment. Again the solution of the application has been found by using the microprocessor based AC Station controllers manufactured by ASCON Spa. These controllers offer a 16bit resolution analogue/digital converter and an accuracy of 0.1