Plant & Works Engineering
Building efficiency, safety and comfort
Published:  19 November, 2019

Regardless of the season or external conditions, variable speed drives help make HVAC applications efficient, safe and informative while providing high levels of comfort, control and safety. ABB’s Carl Turbitt takes a tour around a typical plant and highlights the opportunities.

1 Cooling tower

A cooling tower extracts waste heat into the atmosphere. Using a variable speed drive, instead of conventional direct-on-line or Star-Delta control, means the tower’s fan and pump speed can be adjusted. The pump, for instance, would only circulate the required water for the process while the fan can better regulate the output temperature. Using the drive this way achieves reduction in energy use between 20 to 60%.

2 Chiller

A chiller reduces the temperature of water or other liquid to cool down and dehumidify the indoor air. A variable speed drive used with chillers is a proven way to optimise energy utilisation at full and part loads. Unlike a constant speed chiller, a variable speed drive inherently uses up to 30% less energy and reduces inrush currents on start-up. By soft starting the chiller controlling its speed, noise is significantly reduced, maintenance costs are lowered and overall plant life is extended.

3 Condenser water pump

This pump circulates water between the cooling tower and the chiller. By adjusting the pump speed to the cooling load using a variable speed drive, energy savings can be achieved between 20 to 60%.

4 Chilled and hot water circulator pumps

These pumps circulate water (or other liquid) between the heating coil and the boiler or cooling coil and chiller. The cooling and heating loads vary a lot over time. Variable speed-controlled circulation pumps make sure that an adequate amount of water or other liquid is distributed in the building. Soft start and stop of the pump reduces hydraulic stress, such as water hammer, on pipelines and valves. By installing drives energy savings of between 20 to 60% can be achieved effortlessly.

5 Boiler

A boiler is deployed to heat up the water for building heating. The drive controls the burner fan to adjust the amount of combustion air to the heating load.

6 Air handling unit (AHU)

An AHU circulates, mixes, cleans, humidifies/dehumidifies and heats/cools air. If Variable speed drives are used to control the speed of supply and return fans, then this eliminate mechanical stress of air duct system, avoids fan resonance speeds, controls the speed and efficiency of the rotary heat exchangers, controls the dampers and monitors AHU condition including filter clogging, fan belts and heating coil freeze. Variable speed drives should also be capable of operating in the event of a fire and have the option of an override mode, or fire mode, which secures drive operation in emergency situations. Savings of between 20 to 60% can be achieved by the installation of variable speed drives on these applications.

Selecting the right motor

Variable speed drives can be integrated with several types of AC motors, including induction, permanent magnet (PM) and even synchronous reluctance (SynRM) motors. The most cost-effective option can vary from application to application considering factors such as running speed, efficiency requirements, space and integration with other equipment and systems.

Induction motors (IM) - Drives are most commonly paired with an IM for simple and reliable operation in many HVAC applications and in a wide range of environments. This means that spare motors and parts are often readily available. Further simplifying setup, the HVAC drives can be integrated with virtually any type of IM by entering the nameplate motor data only. While energy efficiency of IM is good, at lower power ranges they can struggle to match the extreme efficiency of newer motor technologies. Their asynchronous speed also generates more heat, which can lead to shorter bearing life compared to other motor types.

Permanent magnet motors (PM) - PM technology offers users high efficiency across the speed range and customised housing for applications such as fan walls and cooling towers, as well as eliminating the need for mechanical speed reduction equipment. PM achieve very high energy efficiency levels in most applications as their permanent magnets reduce rotor losses. This also brings a compact design, particularly compared to induction motors, with high torque density in a relatively small and light package. However, PM can be more costly to purchase, although this can be mitigated by efficiency savings. They are also more difficult to maintain and repair due to the magnets used. PM require a VSD which adds further to the initial cost, however this can also substantially accelerate the payback time.

Synchronous reluctance motors (SynRM) - Combining drive control technology with SynRM gives a motor and a drive package that ensures high energy efficiency, reduces motor temperatures and provides a significant reduction in motor noise. Compared to IM and PM, SynRMs deliver superior efficiency levels up to IE5, using a special cageless rotor which removes traditional rotor losses. It also produces less heat and increases operational lifecycle. As with PM, the SynRM must be used with a drive. There are relatively few downsides to SynRM, and they are particularly well suited to quadratic torque applications like pumps and fans.

Electronically commutated motors (ECM) - Rather than traditional configurations which comprise a separate motor and drive, ECMs are an integrated standalone package, which can make them very quick to install, and are often used on HVAC applications. However, ECMs typically have a life span of half of that of a variable speed drive and motor application. This leads to higher total cost of ownership as they are not repairable and will need to be replaced. ECMs also have some other technical drawbacks: they cannot ride through power dip situations, cannot catch a spinning load, have limited speed/power duties, and can generate large amounts of harmonic distortion, particularly at higher frequencies.