How to improve process cooling and chilled water efficiency
Why chilled-water temperature is the master variable, plus free cooling, sequencing, pumping and load reduction for efficient process cooling.
How a chilled-water system uses energy
A typical process-cooling system has a chiller that produces cold water, pumps that circulate it to loads, and a heat-rejection system — cooling towers or air-cooled condensers — that dumps the absorbed heat to atmosphere. The chiller compressor is usually the largest single energy user, but the pumps and fans around it add a significant share, and they are often left running flat out regardless of demand.
The efficiency story is about three things: making the chiller work less hard for each unit of cooling, moving water and air no faster than needed, and not generating cooling load that did not have to exist. The biggest single lever sits inside the first of these.
Chilled-water temperature: the master variable
A chiller works harder the bigger the gap between the temperature it must produce and the temperature it rejects heat to. So the colder the chilled water it is asked to make, the lower its efficiency. Many systems are set to produce water far colder than the loads actually need, simply because the set point has never been revisited.
Raising the chilled-water set point, where the process allows, is often the single largest efficiency improvement available — it lifts the chiller's efficiency directly. Likewise, anything that lowers the temperature heat is rejected to (cooler condenser water, cooler ambient) reduces the lift the compressor must overcome. Narrowing this temperature gap is the heart of efficient cooling.
Free cooling
When the outside air is cold enough, the cooling towers or dry coolers can meet some or all of the cooling load without running the compressor at all — this is free cooling. For sites with cooling demand through cooler months, the savings can be large, because the most energy-hungry component is switched off or unloaded.
Realising it needs the controls to detect the opportunity and switch over automatically, and a chilled-water set point high enough that ambient conditions can meet it for a useful part of the year. Free cooling and a sensible set point reinforce each other.
Pumps, fans and variable speed
Chilled-water pumps and condenser/tower fans are classic centrifugal loads, where flow and power fall steeply as speed reduces. Yet many run at fixed speed against throttling valves, wasting energy. Converting to variable flow — varying pump speed to hold a differential pressure, and fan speed to hold a condenser temperature — captures large savings at part load, which is where these systems spend most of their life.
The same applies to staging: rather than running every pump and fan continuously, control them to demand. Part-load operation is the normal condition, so getting it right matters more than the full-load rating.
Sequencing multiple chillers
Where a site has several chillers, how they are sequenced sets system efficiency. Running too many machines lightly loaded, or the wrong machine for the conditions, wastes energy. Good sequencing loads the most efficient machines first, matches the number of running chillers to the load, and uses each chiller in its efficient range.
This is increasingly automated by plant-optimisation software that accounts for each machine's part-load curve, the current load and ambient conditions, and dispatches the combination with the lowest total power. The gains from sequencing are real and require no new mechanical plant.
Reducing the load and monitoring
The cheapest cooling is the cooling you never have to provide. Heat gains into chilled spaces and loops — through poor insulation, infiltration, oversized flows and process inefficiency — all add load that the chiller must then remove. Reducing unwanted heat gain at source cuts the cooling demand directly and lets every other measure work on a smaller load.
None of this is manageable without measurement. Metering chiller power against cooling delivered gives an efficiency figure that can be trended; monitoring chilled-water and condenser temperatures, flows and pump and fan power reveals drift and confirms savings. Continuous monitoring turns a cooling plant from a fixed cost into a controllable one.
Frequently asked questions
What is the single biggest way to make a chiller more efficient?
Raise the chilled-water set point as far as the process allows. A chiller works harder the colder the water it must produce, so many systems waste energy making water colder than the loads need. Lifting the set point improves chiller efficiency directly.
What is free cooling?
When ambient air is cold enough, the cooling towers or dry coolers can meet some or all of the cooling load without running the chiller compressor. The savings can be large for sites with cooling demand in cooler months, but it requires suitable controls and a high enough chilled-water set point.
Why convert chilled-water pumps and fans to variable speed?
They are centrifugal loads whose power falls steeply as speed reduces, and cooling systems spend most of their time at part load. Fixed-speed operation against throttling valves wastes energy, while varying speed to hold pressure or temperature captures large part-load savings.
Does chiller sequencing really matter?
Yes. With multiple chillers, running too many lightly loaded or the wrong machine for conditions wastes energy. Sequencing the most efficient machines first and matching the number running to the load improves system efficiency with no new mechanical plant.
Related guides
How to improve industrial refrigeration efficiency
The big refrigeration energy levers — suction and condensing pressure, defrost, compressor control, heat recovery and load reduction — and how to manage them.
Cooling tower efficiency
Cooling towers reject process heat to the air, and small improvements in approach, fan control and water treatment cut both energy and water use. The levers that matter and the faults that waste them.
Pump efficiency
Pumps are among the largest electricity users in industry, and many run far from their best efficiency point. Where pump energy is wasted — oversizing, throttling, wear — and how to recover it.
Fan and VFD optimization
Fans move air for ventilation, combustion, drying and cooling — and like pumps, they are often controlled by wasteful damping. How variable-speed drives and better system design cut fan energy.
Software that helps
Schneider EcoStruxure
IoT platform for energy and plant resource management.
AVEVA Predictive Analytics
Early-warning analytics for critical process and power assets.
Seeq
Advanced analytics for time-series process data.