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Control techniques helps crack cashew capacity constraint.

An unusual combination of Control Techniques' process and speed control equipment has helped Playtime Foods to almost double production of roasted salted cashews. In addition to increasing production rate, product quality has been enhanced to almost eliminate waste, resulting in an attractice project pay-back period of a few months.

Playtime Foods, well known for its popcorn, has expanded rapidly in recent years into salted peanut, cashew nut and speciality nut markets at the larger of its two Leeds sites. Their nut frying facility comprises a new 2000kg per hour peanut fryer and an older 250kg per hour cashew nut fryer.

In the process used at Leeds, cashew kernesl are unpacked from 11kg tins, inspected for unwanted debris and pieces of metal then fed into 500kg mobile containers. These bulk containers are taken to the frying room where they are positioned over the feed inlet of the fryer.

The fryer itself is a stainless steel bath containing hot oil that is continuously recirculated and filtered. Gas ribbon burners underneath the fryer provide the heat and a basket transport mechanism runs through it. Raw cashews are loaded into special baskets that are pulled through the fryer to cook the cashews. Optimum temperature in the fryer is 140[degrees]C and the residence time is around 8 minutes.

On exit from the oil bath, product is transferred to a salting and cooling plant prior to final packing.

In any continuous cooking process, the three variables which need to be controlled are the cooking time, process temperature and quality of product being fed into the fryer. On this machine, the cooking time was controlled by a graduated knob, the temperature by an on/off thermostat and product feed rate (amount per basket) by the operator. With no indication of actual temperature or cooking time and with varying product weights per basket, great skill and experience was required from the operator to obtain the maximum output of acceptable product.

As the cashew nut business expanded, the equipment was struggling to meet demand. Playtime Foods' engineering and production staff, working closely with Instrument Installations, Control Techniques' main area distributor, discovered that the frying machine was short of heating capacity. As a result, the gas burners were operating at full capacity all the time but the oil never reached the desired temperature, unless there was no product in the baskets, such as at break times. When production was restarted, the first basket or two would be overcooked until the oil returned to its usual lower temperature.

Having identified the problems, Instrument Installations presented complete design proposals to the nut roasters company, based on a brief to improve quality, increase production and improve process control with minimum possible downtime. The 10,000 pounds project, which included process instrumentation and drive control equipment, valued at only 1400 pounds, was given the go-ahead and Instrument Installations provided a project engineer and all the mechanical and electrical drawings.

Due to the limitations of the gas burner system, it was decided to retain the existing gas burners and install fifteen 2kW electrical heating elements in the bath. The depth of the oil bath was doubled to allow the elements to be incorporated inside the bath. The track was also lowered to increase the depth of oil over the nuts and the capacity of the baskets.

Control Techniques 510 thyristors were chosen to switch the heating elements and these were fitted with the partial load failure facility, which immediately pin-points a breakdown in any of the elements. Thyristors were selected instead of electro-mechanical contactors because of their longer life and lower maintenance costs. Their ability to switch more quickly and frequently also provides better control performance.

Temperature control was provided by a Control Techniques Type 453 controller, which features a large digital display, giving measured value and setpoint readings, and accurate three term control. The main control output switched the thyristors, with the gas burners controlled from one of the auxiliary alarm outputs. The other alarm output inhibited the automatic oil top-up system until the set-point was realised.

Other features of the 453 controller include an autotune facility which allows easy commissioning. The three term controller also ensures that the fryer is brought back online quickly and easily after a shutdown.

Speed control was furnished by a Control Techniques' Commander VC55D AC inverter drive (sold as the Jaguar Cub) fitted with a simple graduated knob to adjust the speed of the transport mechanism. The advantages of the AC motor control system are lower maintenance and improved control, and the Commander is a well proven design already used by the company.

Indication of the speed is displayed on a Control Techniques 252 digital indicator which is a bright display for good readability, an accuracy of +/-0.1 percent and is user comfortable.

Although the process instruments are manufactured to a high standard of environmental protection, it was decided to provide further protection from the rigorous hygiene clean-downs in the factory. This was achieved by using the type 911 facia cover which gives protection to IP655 standards.

The modification to the bath and the installation of the heating elements, drives and process instruments were carried out in ten days. The capacity of the line was almost doubled from 250kg to 450kg per hour. In turn, this resulted in more efficient utilisation of the downstream packaging plants. Waste burnt production was virtually eliminated with considerable savings in this area.

Quality and consistency of the product has also been improved. In addition, the plant can be utilised to process other nuts, such as almonds and hazels, giving Playtime Foods much more flexibility. Reliability of the equipment has been excellent and no problems have been encountered after six months in operation.

This project illustrates the benefit of being able to obtain drive and instrument technology from a single supplier. Compatible design, interface and communication protocol have allowed easy integration of both technologies, leading to improved control and considerable savings.
COPYRIGHT 1991 Food Trade Press Ltd.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1991 Gale, Cengage Learning. All rights reserved.

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Publication:Food Trade Review
Date:Jun 1, 1991
Words:991
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