VFD for HVAC & Temperature Control

A chilled water system spends most of its life at part load. Design day sets the capacity; every other hour of the year the plant is oversized, and the question is only how the surplus is thrown away — by a control valve, by a damper, or by running fewer machines. On a chiller plant where the cooling load follows occupancy and weather, a drive on the condenser and chilled-water pumps and on the cooling tower fans converts that surplus into a speed reduction, and the fan and pump cube laws do the rest.

The complication is that HVAC is a system, not a set of machines. Slowing a pump changes the differential pressure available to the far end of the loop, and the terminal units at the end of the pipe are the ones that suffer. That is why modern chilled water plants are controlled on differential pressure across the most hydraulically remote terminal rather than on a fixed speed, and why commissioning the control loop properly matters more here than in most industries.

A chiller's efficiency depends heavily on its condensing temperature, and the condensing temperature depends on how much cooling water is flowing and how cool it is. Running the condenser pumps and the tower fans harder than necessary pushes condensing temperature down and improves the chiller's coefficient of performance, but the pumps and fans then cost more than the chiller saves. There is an optimum, and it moves with the ambient.

That optimum is the reason chiller plants benefit from drives on both sides rather than on one. Controlling the condensing loop on approach temperature — keeping the leaving water a fixed margin above the wet-bulb — turns a fixed-speed pump and fan into a system that tracks the weather. In most buildings the annual saving from the condensing side alone exceeds the cost of converting it.

Evaporative condensers and integrated evaporative units add one more element: a fan, a pump and a water spray that all have to be sequenced. Starting the spray before the fan, and stopping it after, is a sequence rather than a set of interlocks, and a drive that can hold it internally removes a control panel from the roof.

A supply fan encounters three different resistances: the ductwork, the dampers and the filter. Only one of them is stable. A filter loads progressively, so the fan has to deliver a constant volume against a rising resistance — which is exactly what a fixed-speed fan cannot do and what a drive running in constant-volume mode can. The saving is real, but the more valuable effect is that the air change rate stays where it was designed to be as the filter loads.

Where the application is return or exhaust air, the fan may already be spinning when the drive is asked to take it over. Flying restart — the ability to synchronise with a coasting motor rather than trip on overcurrent — is what makes a drive usable in a duct system where several fans work in parallel and one may be stopped by a pressure switch at any moment.

Filters and fan coils also raise the pressure-loss question on the water side. When a coil fouls, the flow falls and the terminal is starved; a constant differential-pressure control hides this, and only a flow or temperature measurement at the terminal exposes it. That is a control instrument choice rather than a drive choice, but it is usually the reason a drive retrofit does not deliver the predicted saving.

In a building, the drive has two neighbours that a factory does not have: people and other electronic systems. Acoustic noise at low speed is the first concern. A drive changes its output switching frequency to keep the motor quiet, and on a fan mounted above an office ceiling the difference between a tuned and an untuned carrier frequency is the difference between a complaint and none. Motor noise can also be reduced with an output reactor, at the cost of a slightly larger panel.

Harmonics are the second interface. A building's electrical installation has more sensitive loads per kVA than an industrial one — lifts, IT equipment, laboratory instruments — and a number of harmonics on the distribution board is often written into the electrical specification. Asking for the limit before selecting the rectifier is cheaper than fitting a filter afterwards.

The third interface is the building management system, and it is the one that most often delays a project. The drive has to expose temperature, pressure, flow, power and fault status over the protocol the BMS already speaks, and it has to be possible for the BMS to command a speed as well as read one. Settling that interface — and who writes the point list — before the drives are ordered saves more commissioning time than any parameter setting.