Variable speed pumps help cut environmental impact of sweet plant

1st April 2013

A new, centralised, hot water circulation system has been installed at the largest Mars confectionery plant for Western Europe. The operating speeds of system's pumps are controlled by pump-mounted, frequency control units, which have provided great benefits in the form of flexibility of use and significant energy savings. By Steve Minett and Keld Fenwick.

In common with most major companies in the world, the Mars BV factory at Veghel in The Netherlands, pursues an active policy to reduce the impact of its operations on the environment. Reduction of energy use is a major plank of this policy and significant savings have been achieved by investing in a microprocessor-controlled, variable speed central hot water circulating system.

The newly-installed system needs to be pressurised to a constant 5.5bar and to maintain this pressure despite varying usage from a number of points around the ring. Having already had experience of ITT Vogel's Hydrovar for a fire extinguishing circuit at the plant, utilities manager Harry Klerkx had no hesitation in specifying the latest Hydrovar package for the critical hot water system.

Harry Klerkx explains that: "We decided to install a central hot water system when the municipal water authority changed its source supply: they started drawing water from aquifers with harder water; 13 degree Deutz instead of the 8 degree Deutz which we'd been used to. The increased calcium and magnesium in the supply caused deposits in our heat exchangers, which had to be de-scaled every four weeks. With a central, circulating hot water system, we could install a single, ionisation water softener at a single inflow to the plant.“ (This system replaces the calcium and magnesium ions with sodium ions.) "In addition to sugar dissolving for production, this softened water is now also available throughout the plant for equipment cleaning purposes.“ A new central buffer tank was also installed to guarantee constancy of supply.

Constant duty

Like many process plants which need to maintain precise temperatures for operational efficiency, the Mars factory operates 24 hours a day for seven days a week. It produces 170000 tonnes of finished products per year ­ the well-known Mars range of confectionery which must have unvarying top quality ­ and 35000 tonnes of semi-finished products which go for further processing. This operation obviously requires scrupulous cleanliness and, therefore, the intermittent use of hot water throughout the plant. This cleaning water demand is superimposed on the pattern of process water demand.

At the heart of the new system is a four-pump booster station under Hydrovar microprocessor control which not only maintains the necessary pressure but is also capable of a maximum flow of 30m3 per hour. These requirements are met with a technology which can result in electricity savings of some 70 per cent compared with other methods of water supply. A significant advantage is that pump monitoring and control can be included in the plant's main display panel which means that the whole plant can be overseen from a single point. The Hydrovar is also renowned for its reliability and ease of operation.

The heart of the energy-saving principle of variable-speed pumps is the basic hydrodynamic law that the power consumed by centrifugal pumps varies as the cube of impeller speed. So if pump speed is reduced by one unit, energy consumption is reduced by four units. ITT Vogel's Hydrovar also has intelligent controllers which improve on this already significant saving by minimising friction losses associated with the fluid flow.

Manfred Sacher, who is the product manager for the Hydrovar speed control system at ITT Vogel just outside Vienna, likes to use a car analogy. He suggests that constant speed pumps operate very much as if a person was driving a car with one foot constantly on the accelerator and using the brakes to control the car's speed and to stop. "The Hydrovar system,“ he says, "is like a car with an automatic gearbox; and, extending the analogy, even like an automatic car with cruise control.“

Frequency conversion

The speed of a simple induction motor depends on the frequency of the ac power supplying it. In most of Europe mains supply is at 50 Hz (cycles per second) and in the United States 60Hz, so motors connected directly to the mains turn at multiples of these figures depending on how the motor is wound. To alter the frequency of the motor supply and thus regulate pump speed, the Hydrovar system rectifies the mains supply to dc and then inverts it under command from the controller to provide the frequency required to match pump demand.

The key to this level of automation and flexibility in the Hydrovar, Manfred explains, is the inclusion of a micro-processor actually in the Hydrovar unit. Pressure and flow sensors are also attached to the actual pump. The Hydrovar concept therefore is to have all the equipment necessary for a variable speed control system mounted on the pump itself. One of the many advantages to this apump-mounted' solution is that air from the motor cooling fan can be used to cool the electronics. Exceptionally, for pumps operating in hostile environments, the control gear is available as a wall mounted unit. Since the Hydrovar unit is so self-contained, it can be moved from one pump motor to another and can also be retrofitted to existing pumps.

Manfred comments that several competitors offer frequency inverters but without micro-processors which must be supplied separately. This means that they have to be connected to remote control units, thus introducing additional installation costs. They also have to be programmed before the variable speed system can be used. Many inverters are limited to a maximum rating of 7.5kW whereas the Hydrovar is rated up to 22kW.

A feature of the Hydrovar is its application in multi pump systems, as at the Veghel Mars plant. With the plumb in and pump principle, the only extra installation work is for each pump in the system to be connected with an interface cable to its neighbour and for the pumps to be named so that the micro processors can identify each pump. Hydrovar can be retrofitted to existing multi-pump systems and it can include a friction loss compensation system.

The multi-pump system is usually available for four units and it ensures a step-less transition between each pump coming to maximum speed and the next one starting. Uniquely to Hydrovar, built-in redundancy ensures that if any component on any one pump fails, for example sensors, inverter or micro processor, the other three can maintain system pressure and avoid breakdown.

Constant speed pumps are cheaper and it would have been possible to use one or more to maintain adequate circulation in the hot water circuit. However, to ensure that each station could have water at the required pressure, an additional pump, delivering 5.5bar, would have been necessary. Harry Klerkx says: "The total cost of the installation would have been far higher than the four-pump booster station, with Hydrovar control, which we actually installed.“

In addition, the circuit pump would have been driving water at a speed necessary to satisfy the highest forecast demand whether in fact that demand was there or not. At low fluid flow speeds the head lost by friction is proportional to the velocity but at high fluid speeds the lost head is proportional to the square of the velocity. Constant running would therefore have used large amounts of energy in circulation as well as the energy used by each delivery pumps.

Other options could have included controlling the flow of the circulating pump either by throttling the discharge or by returning the excess flow to the suction side of the pump.

A by-pass could have reduced the flow to the pump but this would have induced cavitation in the impeller causing additional wear. Hydraulic accumulators could have been used to absorb excess flow and store it under pressure; these are expensive, take up space, have limited capacity and seldom produce a smooth and constant flow. All these solutions would have been energy-hungry.

Future planning

Harry Klerkx was considering the future when he specified Hydrovar control; he has plans for additions to the system. Firstly, the circuit could be used to provide water for a new system to hydrate milk powder. Secondly additional heat exchangers, taking heat from the central hot water system, are being considered for various locations around the plant.

The Veghel Mars plant, which employs 1150 people, has had a constant falling energy consumption over the last five years despite increased automation which usually raises energy demand. The new hot water circulation plant was one phase of an ongoing programme of plant upgrading. Cooling systems have been fitted with variable speed compressors, generating savings of about 10 per cent. Air conditioning units are centrally controlled and monitored thus allowing direct access to the control function and an increase of efficiency. Variable speed fans have been fitted in the air conditioning systems supplying the office accommodation; these have an automatic low speed night setting which have generated savings of at least 10 per cent. p

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