Handling foods at low temperature
23 November 2009
EXCLUSIVE Even at temperatures as low as -30°C drive technology in deep-freeze storages must be flawless. Lenze has put together three packages where standard products are adapted for use at such temperatures.
These products include ac geared motors, servo geared motors, frequency inverters and servo drives which cover the vast majority of requirements for transport and positioning in cold stores.
Refrigeration of food starts as early as the manufacturing process allows and continues until the goods reach the point of sale. The requirements vary depending on the type of food. Dairy products, salads and fruits are stored at temperatures between 0°C and -7°C. Beef, chicken and butter are stored a little colder at -12°C whilst ice cream, bread and ready meals are frozen to -28°C to achieve a high quality and long shelf life. High priority is given to continuously maintaining the cooling through the supply chain, and stock rotation is important to make effective use of the shelf life. Standard drives are rated down to 0°C, or at best -10°C, without protection against condensation. Therefore it can be seen that there is the potential for failures in cold stores unless special measures are taken.
The use of drive technology in the automation of cold stores is increasing. Staff working in these low temperatures need to be equipped with special clothing and tools. Working and break times must be strictly controlled to protect their health. This brings higher operating costs. Special tools such as fork lift trucks with heating add to the running costs as the extra heat has to be removed. This is why the automation of cold stores becomes increasingly attractive for economic reasons.
The primary products used for the automation of cold stores are geared motors. These are often found in large numbers on conveyor and elevator systems. Lenze offer the G-motion range from 60W to 45kW with options for encoder feedback, blowers and brakes. In addition, automated storage and retrieval systems will use servo geared motors to achieve the dynamic travel profiles and accurate positioning required. All these motors, gearboxes and accessories all have to work reliably in the cold conditions. The inverters and servo drives are generally less of a problem as they are panel mounted and can be located outside the cold area. Alternatively if that is not possible, heated and insulated control cabinets are recommended if the ambient is below -10°C.
Where the air temperature remains above zero and the humidity is below 85%, no drive modifications are needed. However condensation levels are often higher, for example in the storage of vegetables, fruit and salads, and here special measures are needed to prevent corrosion of the geared motor. The corrosion protection package introduced by Lenze includes measures such as an additional coating of the motor windings, stainless steel output shafts, condensation drain holes, stainless steel nameplates and application of a 2 component paint finish.
Servo motors are sometimes used without gearboxes and here the motor shaft is constructed in stainless steel. The use of encoder feedback is avoided, instead Lenze fit resolvers which are much more robust. Where brakes are needed, special surface treatment such as hard chroming is applied to prevent rust. Permanent magnet brakes are set to be engaged with reverse polarity at standstill so as to maintain a small amount of heating.
As the temperature falls for the storage of fish (-5°C), butter and meat (-12°C) the drive task actually becomes easier as there is no condensation. However care must be taken in the area of doors where the air temperature can fluctuate and condensation becomes possible. The Lenze drive package for temperatures down to -12°C involves no more than a change of gearbox oil. Low temperature synthetic oil is used to ensure lubrication is maintained at all times and there is no undue wear.
The third Lenze drive package suits deep-freeze areas down to -30°c suitable for the storage of ice cream, bread and convenience foods. The low air humidity prevents icing but a number of measures are needed for the drives. The viscosity of lubricants falls sharply so roller bearings are filled with low temperature grease and gearboxes with synthetic lubricants.
Experience shows NBR seals are the optimum, but life can be reduced by frequent stop/starts. Servo motors cannot use blowers below -15°C, but encoders and resolvers are suitable. For all motors and geared motors, the starting torque needs careful consideration and an extra service factor needs to be applied at these low temperatures. Depending on size of the drive, this factor will be up to 20%. Commissioning and testing of drives specified for low temperatures should not be done at normal temperatures in order to prevent damage.
Operators of cold store and deep-freeze storages expect their drive technology to be maintenance and trouble free. However the severe operating conditions mean that standard drives are not suitable in many cases. The Lenze project-specific approach to these requirements brings together field experience with drive knowledge. Three product packages – corrosion protection, extended standard and deep-freeze – bring solutions to the handling of all foods in cold storage.
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