Source: The Engineer Online

Published: 27 February 2008 12:00AM

Link: http://www.theengineer.co.uk/Articles/304789/Mitsubishi+-+Potato+store.htm

Industrial variable speed drives from Mitsubishi Electric are helping to ensure that consumers have fresh potatoes throughout the year.

Today’s supermarket shoppers may be surprised to hear that potatoes are a seasonal vegetable; they appear all year-round in every store. Their parents though are likely to remember that potatoes where harvested in autumn, stored in outside clamps for perhaps 6 months, after which they were likely to be sprouting in all directions, ugly and hard work to prepare for cooking. 

These days potatoes are stored in climatically controlled buildings in a state of virtual suspended animation so that they retain the ‘just out of the ground’ appearance and taste so vital to modern consumer marketing.

Leader in the provision of the necessary control systems is Crop Systems Ltd of North Walsham in Norfolk. Its Technical Consultant is Ray Andrews, who explains: “A typical bulk potato store may hold between 500 and 3000 tonnes of potatoes with the more modern unit being 30m by 30m by 7m high. It will be totally insulated and have a large fan house at one end.”

The objective is to maintain optimum storage conditions by controlling the temperature, humidity, and CO2 content of the atmosphere. Until recently the philosophy was to have large, fixed speed fans running flat out regardless of conditions, a set up which made it impossible to truly optimise conditions.

"We have developed the Evolution controller to give many varying levels of control through different types of sensors located around the store, from this we can adjust the fan speed according to the exact requirements,” says Andrews.

In fact, when newly harvested potatoes are put into storage, their surface moisture content is high and over the first few weeks this moisture needs to be removed relatively quickly. The system is designed with an automatic load cure program, this keep the fans running at an economical speed until the stock has settled down. The potatoes also emit heat, which if left build up would compromise storage; the fans adapt their speed to compensate for different conditions.

“We have also developed a automatic fogging program so that we can better address the distribution of CIPC fog within the storage area, and we have proven that ‘stirring’ and re-circulating the air at reduced air speeds helps give far better efficacy and extends the fogging intervals dramatically, this speeds the payback period to less than 18 months in some cases.

Central to the storage processes above is the use of an F740 variable speed drive from Mitsubishi. The F740 is the fan rated version of the F700 range of drives and offers improved motor efficiency during constant speed operation and acceleration periods, and optimum torque patterns which accurately match the power delivered to that required at any instant. The speed of optimisation is effectively instant, so the drive is always giving maximum performance and efficiency to the load.

Its ease of commissioning, its removable terminal block, on-board EMC filter, predictive maintenance monitor and tripless reliability, as well as its ability to optimise energy usage, made the F740 the obvious choice for this fan drive application. 

Andrews continues: "It has been shown that the use of Mitsubishi Inverters has more than just short-term advantages to our customers, as the amount of power consumed is dramatically decreased adding to the already considerable cost benefits."