Lower speed gives fast payback

maintenance, refurbishment, IMO Precision Controls, VSD, variable speed drive
Inverter drives that control the rate of air to match a building’s changing ventilation requirement are extremely cost effective.

Moving air costs money, and so does heating and cooling it — so adding inverter drives to ventilation systems can be very cost effective, as Garry Lewis of IMO Precision controls explains.

It is a long established fact that the use of inverter drives on fan and pump applications can offer substantial energy saving benefits. With around 50% of the energy consumed in most buildings being used for heating ventilation and air conditioning, it stands to reason that this area requires closer investigation.

I remember attending Government-funded industry seminars in the early 1990s where the message was both compelling and simple. Because the majority of fans and pumps used in the building-services industry operate on a square-law curve when comparing speed and torque, a relatively small reduction in fan or pump speed will yield a large reduction in energy use. Generally speaking a 20% reduction in speed can reduce the energy used by up to 50%.

At that time most of the air-handling units installed used fixed-speed fans with dampers to reduce the airflow. Opening up the mechanical damper and reducing the speed with an inverter drive was a simple yet effective way of reducing energy usage.

In the intervening years technology has moved forward, as has our understanding of the applications. New inverter products have been developed that can optimise these energy savings even further, including our own Jaguar VXH for motors from 750 W to 710 kW and with specific building-service features for AHUs, chiller pumps, heating pumps, cooling tower pumps and fans, and hot- and cold-water supply pumps.

Standard features of these drives include a real-time clock, calendar functions, PLC, kWh data logger and four PID controllers, three of which can be used to control external equipment.

Let us look at a practical application for this technology. For this example I will use simplistic approximations to get an overview of the type of savings that can be made.

Let us consider a room used by a varying number of people, such as a lecture theatre at a university or a cinema. Such a space like would have an 11 kW supply fan and an 11 kW extract fan. Such fans running for 10 hours per day, five days per week and 48 weeks a year at 12p per kWh will consuming around £6400 in electricity.

If we adopt a 1990s approach to the application we might open up the dampers and trim the speed down by 15 to 20%. By the time you take into account the equipment cost, installation and commissioning costs etc., you will be looking at a scheme which will most likely offer a payback period in the region of 18 months.

Now let’s go a stage further and apply modern techniques. We would install CO2 monitors into the extract ducting and sample the air within the room by turning the extract fan at a very low standby speed. The on-off times can be set by the inverter’s real-time clock and calendar functions. The output from the CO2 monitor will be fed through the PID controller on one of the drives and that drive will then automatically control the fan speeds to maintain a target of 1000 ppm of CO2.

This type of strategy achieves a fully automated system that will adjust itself to the requirements of the people using the room. The more people that are in the room breathing out CO2, the faster the fans will circulate the air. Obviously the real energy savings will depend on many different factors, but typically we have seen these applications offering payback periods of around nine months.

The energy savings don’t just end there, because all we have considered so far is the energy used to run the fans. If we are circulating less air then we are also venting out less heated or chilled air into the atmosphere and therefore saving more energy for heating or chilling process.

The final consideration we need to make is the total cost of ownership. Our Jaguar inverter drives are designed to operate for 10 years and are the only drives available on the market with a 5-year warranty.

Garry Lewis is marketing manager with IMO Precision Controls.

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