Fri. Oct 22nd, 2021

Vary Your speed

Hong Kong's leading energy consultant, energyLAB, John Herbert

HVAC equipment and industrial machinery are designed to operate at the peak capacity, to handle the worst case scenario.  However, during normal operation periods of partial load occur when the load is less than the design load.

For example, comfort cooling air conditioning systems operate for 99% of the year inside the design condition, particularly the autumn and spring months demand less capacity and injection molding machines hydraulics cycle through full load to no load, and back to full load.

Control by Obstruction

Motors are dynamic, driving equipment that moves fluids, hydraulic fluid, water, or air, from one place to where it is needed.  Without variable speed motor control we need an additional device to either bypass or create an artificial resistance to flow.  Briefly, there are two primary means of operation:


In this situation a valve or other device might be used to divert the excess flow under part load conditions.  Commonly used applications include:

  1. the differential pressure relief for chilled water systems and injection molding machines;
  2. the hot gas bypass used in refrigeration systems and chillers;


In this situation, a device is installed to create an artificial pressure loss, and therefore reducing the flow rate.  Common applications include:

  1. Inlet vanes used on centrifugal fans;
  1. Regulating valves used on hydraulic systems;

However, both modes waste energy. To capitalize on the potential energy saving, the speed and hence the power consumption of the motor needs to be lowered during part load conditions.


One solution is the tried and tested VFD (Variable Frequency Drive) it is an electronic motor speed controller that varies the motor speed to match the required load.  During periods of part load operation,  lower speeds translate into reduced power consumption, and operating cost savings.

Even a relatively small speed reduction provides attractive energy savings.  for example reducing the speed of 1500 rpm motor by 150 RPM (approximately 10%) equates to a power reduction of  27%.

Because a VFD can vary the motor speed from zero RPM thorough to full speed, it reduces the mechanical and electrical shock and stresses during normal operation extending the operating life of your plant and equipment.

— John Herbert, Consultant

Learn more about Ambrose Air HVAC service.