Steaming ahead with variable speed
Dean Palmer reports on a novel range of condensate recovery units that use variable speed technology to reduce running costs
A new range of condensate recovery units (CRUs) saves energy by using variable speed technology. Switching pumps on and off more regularly returns hot water to the system before it can cool down – slashing the cost of re-heating.
The developer is Amarinth, a UK-based firm that specialises in the design and manufacture of centrifugal pumps and pumping systems for industrial, chemical and pharmaceutical applications.
For 18 months, Amarinth worked with one of its customers – steam heating specialist Spirax Sarco – on developing a new range of CRUs. The client wanted better performance and greater reliability than competing CRUs, but at a competitive price. Variable speed technology was also a ‘must-have’. The tanks were to be made from stainless steel, the footprint was to be smaller and the range of CRUs significantly rationalised.
The result – the ‘Ci-Nergy’ CRU, launched in January 2006 – should cut running costs of steam heating and process systems. The system’s speed control acts as a soft start system, enabling the pump to be stopped and started frequently. The accumulated condensate – and associated latent heat – are then returned to the system more rapidly than by traditional methods, using fixed speed pumps with float switches or controls. The returning condensate can be up to 10°C hotter, which could save re-heating costs of up to £10,000 per annum.
Reductions of up to 75% in electricity costs can be achieved by running the pump as slow as 10Hz. The savings are exponential (often referred to as the ‘cubed law’) and so by using variable speed technology, Ci-Nergy operates anywhere between 10 and 50Hz, to match the level of condensate arriving in the tank.
According to Oliver Brigginshaw, managing director at Amarinth: “This delivers typical electricity savings of a further £1,000 per year per unit.”
He adds that, in most cases, payback on the CRUs “is between six and nine months”.
Amarinth’s design concept was one of ‘plug-and-play’. Using just two tank and pump sizes with eight motors to handle condensate of 500-35,000kg/hour, the original range of 59 CRUs was cut to just eight, in the 18-month re-design project. Reducing the number of components also cut the price of the new unit compared to the original. Amarinth eliminated one complete standby control pump from the original CRUs. The new units have just one pump with variable speed control.
“Steam heating and process systems generally collect condensate and return it into the system to recover its sensible heat,” says Brigginshaw. “The quicker the condensate can be returned, the more sensible energy is recovered – reducing operating costs. Traditional systems rely on centrifugal pumps to return the condensate, but the volume of condensate varies with the demands placed on the system.”
With a limit of around 10 motor starts per hour, condensate has often lost much of its sensible energy by the time it is returned to the system.
Amarinth looked at ‘affinity laws’, which show that power consumption reduces with speed-cubed.
“This means that running a pump at 30Hz instead of the usual 50Hz results in a 78.5% drop in power consumption. If the pump can be varied to run at the minimum speed required to pump the condensate, there are significant benefits to be gained,” says Brigginshaw.
The key to the project was developing software algorithms to control the speed of the pumps and ensure that the pump-system matching was optimised at all times. Amarinth knew that centrifugal pumps were ideal to control via their speed because their performance is so predictable.
“We combined a standard centrifugal pump set with an inverter to replace the original motor control panel and we introduced a process control sensor with an electronic output compatible with the inverter,” says Brigginshaw.
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