Inverters: payback in 12 months in Heating, Ventilation, and Air-Conditioning (HVAC)?
No matter what area of plant control you choose to scrutinise, there are few optional equipment alternatives that offer the operational benefits and cost savings of frequency inverters (VVVF Drives).
Being the designer of a centrifugal fan or pump driven system inevitably means that once all the relevant information has been taken into account, a sizing decision must be made.
It goes without saying that the fan or pump selected has to be capable of meeting the greatest demands to which it will be subjected.
Anything less and the equipment simply isn't up to the job.
When maximum demand is the controlling criterion for equipment sizing, running costs can be unnecessarily high if, as is usually the case, actual demand upon the system is considerably lower.
David Randall of Lenze in Bedford, argues that it is now possible to control energy consumption by the use of frequency inverters to overcome the high running cost implications when large fans or pumps are deployed, and in many cases, payback times of twelve months or less are typical.
His reasoning is very persuasive.
Let's look at the issues.
Although system components may be designed for a particular (maximum demand) eventuality, in practice that eventuality may never or only rarely be experienced.
We can therefore safely conclude that most system equipment will normally be running at much less than its design capacity.
Compelling evidence exists to prove that considerable cost savings are possible when a fan or pump is run at less than 100% of its design duty.
How is this so? Torque loads for centrifugal fans and pumps correlate to their running speed.
As speed increases, the load increases in proportion to the square of the speed.
Since power consumption is proportional to torque x speed, power is proportional to the cube of the speed and it is within these basic formulae that the cost saving potential exists for fans running at less than 100%.
Motors which are connected directly to the mains absorb reactive power by induction.
Anything more than a few motors can (and does) increase the cost of electricity provided by the local supply authority.
A way round this is to install power factor correction (PFC) equipment to the satisfaction of the supply authority which involves considerable capital investment.
Since frequency inverters absorb almost no reactive power, installations controlled by these units are efficient to the degree that the need for power factor correction (PFC) equipment is obviated.
Moreover, frequency inverter (AC Drives, frequency converters) controlled motors do not require starters because the frequency inverter eliminates power surges much better than any soft starter or star delta starter.
The removal of power surges (or momentary peaks in demand) can result in cabling of a smaller cross sectional area than would otherwise be the case.
These are all factors which can result in a lower electricity supply tariff.
But what are the other advantages? For air conditioning applications, lower running speeds means lower air movement levels and thus lower noise and draughts.
By running the system constantly to match demand, cut-in and cut-out frequencies are minimised so both the system and the building occupants benefit considerably.
By using frequency inverters (VFDs, ASDs), it is possible to switch and control the system manually or automatically via a PC.
When running a system via automated PC commands, it is possible to select from a variety of fixed speeds, or use control functions such as an integrated PID process controller to monitor and maintain a variety of operating criteria, ie: flow level; temperature; humidity or pressure.
Feedback signal scales can be adjusted to suit whatever measuring device is deployed and can be set to 0-10V, 4-20mA, 0-20mA, 0-5V.
A problem with any ducted system is that if the fan vibration frequency just happened to match the resonance frequency of the ductwork, then one would set up a vibration in harmonic sympathy with the other.
The result is discomfort for building occupiers and accelerated plant wear.
Frequency inverters allow this unwanted noise and vibration to be removed by setting 'skip' frequencies with adjustable 'bandwidths' which can be omitted from the inverter output.
A flying 'restart' circuit is built in which synchronises the inverter to the fan speed in the event that the fan is already cycling when the inverter is switched on and obviates the energy wastage normally experienced when having to halt the rotation and restart from rest.
Moreover, the restart circuit prevents the fan from running backwards in the event that it is freewheeling in the wrong direction when started.
David Randall added a precautionary note.
"It is appropriate to mention that inverters do cause some radio frequency interference, making it necessary to fit a filter and a choke between the mains supply and the inverter for larger powers.
This is a minor cost modification but by doing so, one eliminates interference, ensures compliance with legislation and standards, increases the operational life of the inverter and protects it from power surges." He concluded.
"No matter what area of plant control you choose to scrutinise, there are few optional equipment alternatives that offer the operational benefits and cost savings of frequency inverters (AC Drives, frequency converters).".
