80 Journal on Processing and Energy in Agriculture 19 (2015) 2 Biblid: 1821-4487 (2015) 19; 2; p 80-82 Expert Paper UDK: 66.047.55:641.13 Stručni rad AUTOMATIC CONTROL OF THE AIR TEMPERATURE FOR THE FRUIT DRIER CHAMBER AUTOMATSKA REGULACIJA TEMPERATURE VAZDUHA U SUŠARI ZA VOĆE Dragan MATIĆ, Vladimir BUGARSKI, Ilija KAMENKO, Perica NIKOLIĆ University of Novi Sad, Faculty of technical sciences, 21000 Novi Sad, Trg Dositeja Obradovića 6, Serbia e-mail: dmatic@uns.ac.rs ABSTRACT In this paper, it is given a proposal for air temperature control in the process of fruit drying for one real system. The process of temperature control for convective drier can be presented as a single input single output system. The convective drier consists of the inlet branch equipped with the fan for supply of atmospheric air to the drying chamber. Atmospheric air passes through the heat exchanger and heats up. Heated air passes on to the fruit drier chamber where the fruit material is placed. For the regulation of the air temperature a PID digital controller is used. The controller has one standard current analog input and output 4-20 mA. It is equipped with display for temperature setpoint and process value presentation. For air temperature measurement Pt100 temperature probe with the transmitter is used. As the actuator electrically driven three way valve is used which regulates amount of hot water going into the heat exchanger. Key words: control, temperature, convective drier. REZIME U ovom radu je dat predlog regulacije temperature vazduha u procesu sušenja voća za jedan realan sistem. Sušara se sastoji od ulazne grane sa ventilatorom kojom se doprema spoljašnji vazduh do komore za sušenje. Pre ulaska u komoru za sušenje spoljašnji vazduh prolazi kroz izmenjivač toplote gde se vrši njegovo zagrevanje. Izmenjivač toplote je povezan na sistem toplovoda. Vazduh zasićen vlagom se sistemom cevovoda izbacuje u okolinu dok se jedan deo vraća nazad u sistem radi uštede energije. Za regulaciju temperature odabran je digitalni PID regulator sa analognim ulazom i izlazom opsega 4-20 mA. Regulator je sa displejem na kom je moguć prikaz zadate i trenutne temperature vazduha sušenja. Regulator sa pratećom niskonaponskom sklopnom opremom (osigurači, sklopke, kleme...) je ugrađen u nazidni ormar izrađen u IP66 stepenu zaštite. Displej regulatora se postavlja na vrata ormana i lako je dostupan operateru. Za merenje temperature predložena je Pt100 sonda sa transmiterom. Kao aktuator odabran je trokraki elektromotorni ventil. Trokraki ventil poseduje pozicioner sa ulazom 4 do 20 mA koji je direktno proporcionalan otvorenosti ventila od 0 do 100%. Sva specificirana oprema je renomiranih svetskih proizvođača i pripada standardnom prodajnom asortimanu što u mnogome olakšava primenu i održavanje. Primenom ovako savremenog rešenja omogućeno je jednostavno rukovanje i precizna regulacija temperature vazduha sušenja. Na ovaj način obezbeđen je bolji kvalitet finalnog proizvoda, a ostvarena je i izvesna ušteda energije. Analizom troškova nadogradnje sistema utvrđeno je da je jedna ovakva investicija tehnički i finansijski opravdana. Ključne reči: regulacija, temperatura, konvektivna sušara. INTRODUCTION One of the oldest techniques for food preservation is dehydration. Dehydration means reduction of moisture content and water activity to safe levels. The main goal of drying process is to preserve food for longer period of time not reducing quality. Solar and hot air drying (convective drying) have been the most common food drying techniques from historical perspective. The solar drying is the cheapest technique but its application depends on weather conditions and requires large processing areas which are significant drawbacks. In the practice it is necessary to protect the fruits from insects and small animals and from contact with airborne dirt. In the convective drying, it is possible to control these adverse factors and increase the average drying rate (Zotarelli et al., 2014). There are three stages in material drying. The first one is heating of material while water evaporates from the surface. The second part indicates falling of the rate period. Dry patch occurs on the surface and drying rate begins to decrease. Further drying leads to a dry surface layer and the zone of water evaporation retreats towards the center of the body and this period of drying is treated as the third part which is the fully falling rate period (Jaturonglumlert and Kiatsiriroat, 2010). In this paper, it is given a proposal for air temperature control in the process of fruit drying for one real system. The main goals are to present a technical solution for automatic control of the air temperature for convective drier, to make analysis of the cost for system upgrade and to verify investment viability. Controls of the moister and air speed for convective drier are not covered by this work and are done manually. MATERIAL The convective drier is of traditional design. It is a batch dryer with racks made of stainless steel. The capacity of the drier is 250 kg per charge. It has 25 racks with capacity of 10 kg per rack. Dimensions of the rack are 800x1000 mm. This dryer is suitable for drying up to 1000 kg of fresh fruit per day (Babić et al., 2011). Outer dimension of the chamber are 1630x2080x1120 mm. The commonly dried fruits are: apple, peach, apricot and plum. In practice air temperature should be held from 50 to 60 °C depending on the fruit and quantity. The drier consists of the inlet branch equipped with the fan for supply of atmospheric air to the drying chamber. On the way to the chamber, atmospheric air passes through the heat exchanger and heats up. The ribbed heat water - air exchanger is connected to the external hot water supply system. Maximal inlet temperature of the water reaches 90 °C. The outlet water temperature is around 70 °C. Heated air passes to the drier