Water-Cooled Chiller Efficiency: How It Degrades and How to Measure It

Key takeaways

• Chiller energy rises with lift; FEMP estimates 2.5% to 3.5% more energy per degree of higher condenser temperature.
• Trend evaporator and condenser approach temperatures against the chiller's own clean baseline.
• Condenser tubes on open tower water foul far more than evaporator tubes on closed loops.
• Tube brushing cleans the water side; an oil-film treatment targets the refrigerant side.
• Oil-free magnetic-bearing and absorption chillers are not candidates for an oil-film treatment.

A water-cooled chiller is usually the single largest electrical load in the building it serves. That makes it the place where small efficiency changes become large dollar figures. This page explains how these machines work, the specific ways their efficiency slips, how to measure that slippage with numbers your operators already collect, and where an internal oil-film treatment fits into a chiller plant program.

How a water-cooled chiller works

A chiller is a refrigeration machine that cools water instead of air. In a vapor-compression chiller, liquid refrigerant boils in the evaporator, a shell-and-tube heat exchanger, absorbing heat from the chilled water flowing through the tubes. The compressor raises the refrigerant vapor's pressure and temperature and sends it to the condenser, where it gives up its heat to condenser water. That water carries the heat to a cooling tower, which rejects it to the atmosphere by evaporation. An expansion device drops the refrigerant back to evaporator pressure and the cycle repeats (FEMP O&M Best Practices Guide).

The FEMP guide groups mechanical chillers by compressor type: reciprocating, screw (two meshing helical rotors) and centrifugal (a high-speed impeller, typically 100 to 10,000 tons). Absorption chillers are a separate family that uses heat, usually steam or hot water, and a lithium bromide and water solution instead of an oil-lubricated compressor.

The idea that matters most for efficiency is lift: the difference between the refrigerant's evaporating temperature and its condensing temperature. The compressor's job is to supply that lift, and the energy it needs rises with it. Anything that pushes the evaporating temperature down or the condensing temperature up costs energy.

How efficiency degrades over time

Water-cooled chillers lose efficiency through a predictable set of mechanisms. Most of them show up as a larger temperature difference between the refrigerant and the water, which forces more lift.

Waterside scale and biofilm

FEMP notes that chiller heat transfer surfaces "tend to collect various mineral and sludge deposits from the water," and that "any buildup insulates the tubes," requiring a larger temperature difference between water and refrigerant. The condenser is the usual problem because its water passes through an open cooling tower that picks up dust, scale-forming minerals and biological growth. Evaporator tubes in a closed chilled water loop typically foul much less. When AHRI revised Standard 550/590, it reduced the evaporator fouling allowance based on ASHRAE-sponsored research showing that evaporators in closed-circuit water systems see very little performance degradation over time (ACHR News). AHRI publishes separate guidance on how fouling factors affect evaporators and water-cooled condensers (AHRI Guideline E).

Air and non-condensables in the condenser

Low-pressure chillers can draw air in through small leaks. FEMP explains that trapped air raises discharge pressure and compressor horsepower, with "the same effect as scale buildup in the condenser." A purge unit that runs excessively is an early warning.

Condenser water problems

A clogged strainer, a tower with poorly distributed water, or failing tower fans all raise condenser water temperature. FEMP estimates the chiller consumes 2.5% to 3.5% more energy for each degree of higher condenser temperature, and gives the example of a 100-ton chiller with a $20,000 annual energy bill paying roughly $500 to $700 more per year for every degree.

Refrigerant charge and oil management

Low refrigerant charge starves the evaporator and lowers the evaporating temperature. On the refrigerant side, a small fraction of compressor oil always travels with the refrigerant. Researchers have studied how lubricant in the refrigerant affects boiling, condensation and pressure drop for decades; Shen and Groll's two-part review in HVAC&R Research summarizes that literature (IIR record). FEMP's own description of oil-free chillers makes the practical point: their heat transfer efficiency improves "because no oil enters the evaporator or the condenser" (FEMP). For the general mechanism, see what oil fouling is and how compressor oil migrates.

Controls drift

Setpoints get lowered after a hot-call complaint and never raised again, reset schedules get disabled, and sequencing logic runs two chillers where one would do. FEMP estimates that raising chilled water temperature by 2°F to 3°F can improve centrifugal chiller efficiency by up to 3% to 5%, and that each 1°F increase cuts chiller energy by about 1.7% for centrifugal and 1.2% for reciprocating machines.

How to measure a water-cooled chiller's efficiency

kW/ton

Measured kW/ton is compressor electrical input divided by the cooling delivered. Cooling in tons can be calculated from the chilled water side as flow (gpm) × temperature difference (°F) ÷ 24. A chiller drawing 300 kW while cooling 1,200 gpm from 54°F to 44°F is producing 1,200 × 10 ÷ 24 = 500 tons, or 0.60 kW/ton. Compare that figure against the same chiller at a similar load and condenser water temperature, not against the nameplate.

For reference, the efficiency levels FEMP requires federal buyers to meet for a new full-load-optimized water-cooled centrifugal chiller of 300 to 399 tons are 0.544 kW/ton at full load and 0.520 kW/ton IPLV (FEMP). IPLV (integrated part load value) is a weighted average of efficiency measured at 100%, 75%, 50% and 25% load under AHRI Standard 550/590 (Trane); the standard weightings are 1%, 42%, 45% and 12% respectively (MEP Academy).

Approach temperatures

Approach temperatures are the single most useful trend for spotting heat exchanger problems:

• Evaporator approach = leaving chilled water temperature minus saturated evaporator refrigerant temperature.
• Condenser approach = saturated condensing refrigerant temperature minus leaving condenser water temperature.

A clean chiller at a given load has a characteristic approach, recorded at startup or after the last tube cleaning. When the approach widens over time at similar load and flow, heat is having a harder time crossing the tube wall. Modern chiller controllers display both values; if yours does not, they can be calculated from refrigerant pressures and water temperatures.

Daily log and trend data

FEMP's sample chiller log records leaving chilled water setpoint, run hours, volts and amps on each phase, and operating pressures. Its maintenance list also calls for using superheat and subcooling readings and discharge line temperatures to keep the machine at maximum efficiency. Logged daily, these numbers show slow drift long before comfort complaints arrive.

Maintenance that matters

FEMP's chiller checklist is a solid baseline. Highlights include:

• Clean condenser tubes at least annually as part of shutdown, and clean evaporator tubes at least annually.
• Eddy current test condenser and evaporator tubes as required to assess tube wall thickness.
• Conduct oil and filter analysis, check the oil pump, seals, heater and strainers.
• Test water quality for proper chemical balance, and assess evaporator and condenser water flows.
• Leak test compressor fittings, oil pump joints and relief valves; record refrigerant added and fix the leak.
• Verify chilled water reset, motor load limit and vane control settings against the manufacturer's specifications.

Cooling tower care belongs in the same program. FEMP recommends adjusting tower blowdown to hold two to four cycles of concentration of dissolved solids, balancing water distribution across the fill, and keeping biological growth under control. For broader context on preventive maintenance built around efficiency, see building a PM program for efficiency.

Where an internal oil-film treatment fits

Tube brushing and water treatment clean the water side of the tubes. They do nothing to the refrigerant side. An internal treatment such as CryogenX4 is aimed at the refrigerant side. According to the company, oil that migrates past compressor seals circulates with the refrigerant and coats internal heat exchanger surfaces, and CryogenX4's polarized molecules lift that oil and return it to the sump, condition the metal surfaces for better heat transfer, and improve the lubricity of the existing oil.

A reasonable candidate chiller usually looks like this:

• It uses an oil-lubricated compressor (screw, scroll, reciprocating or oil-lubricated centrifugal).
• Waterside tubes have been cleaned recently, the purge works, charge is correct and flows are at design, yet approaches or kW/ton remain worse than the machine's own startup baseline.
• It is mechanically sound and has years of service life left, so a treatment intended to last the remaining life of the equipment (a company statement) has time to pay back.

CryogenX4 reports treating water-cooled chillers up to 1,600 tons, installing while the system runs with no downtime and no modifications, typically in one day. The company states energy savings of up to 30% and a typical payback of 12 to 36 months; results vary by equipment condition, and savings are not guaranteed.

Questions to ask before treating a chiller

1. What are this chiller's evaporator and condenser approaches today, and what were they at startup or after the last tube cleaning?
2. When were the tubes last cleaned and eddy-current tested? Is the purge running excessively?
3. Is the refrigerant charge verified, and is there a leak history?
4. What does the latest oil analysis show?
5. Has the manufacturer or your service contractor given a written position on compatibility with this refrigerant and oil? (CryogenX4 states that Intertek tested and certified its compatibility with all refrigerants and refrigerant oils.)
6. How will savings be measured? Agree on a baseline period, metering points and a method aligned with IPMVP, the protocol maintained by the Efficiency Valuation Organization (EVO). See how HVAC energy savings are measured.
7. What controls measures (chilled water reset, condenser water reset, sequencing) are still on the table? These are often the cheapest savings in the plant.

Next step

Pull twelve months of chiller logs and calculate monthly average approaches and kW/ton at comparable loads. If the trend is rising after waterside maintenance, a measured pilot on one chiller is a low-risk way to test an internal treatment; see how a pilot program runs or request an assessment.

Frequently asked questions

How often should water-cooled chiller tubes be cleaned?

FEMP's checklist calls for cleaning condenser and evaporator tubes at least annually during shutdown. Plants with poor tower water quality may need more frequent condenser cleaning; let the condenser approach trend tell you when performance is slipping.

What is a good kW/ton for a water-cooled chiller?

It depends on size, compressor type, load and condenser water temperature. As a reference, FEMP's purchasing requirement for a new full-load-optimized 300 to 399 ton centrifugal chiller is 0.544 kW/ton at full load. Older machines run higher. Compare your chiller with its own history at similar conditions.

Can raising the chilled water setpoint really save energy?

Yes, when the building can still meet its loads and humidity targets. FEMP estimates about 1.7% chiller energy reduction per 1 degree F increase for centrifugal machines. Watch indoor humidity when you reset upward.

Why would approach temperatures stay high after the tubes were cleaned?

Possible reasons include air in the condenser, low water flow, incorrect refrigerant charge, sensor error, and resistance on the refrigerant side of the tube, such as accumulated oil. Work through them in that order before considering a refrigerant-side treatment.

Sources

1. Operations & Maintenance Best Practices: A Guide to Achieving Operational Efficiency, Release 3.0 — U.S. DOE Federal Energy Management Program / Pacific Northwest National Laboratory
2. Magnetic-Bearing Chiller Compressors — U.S. DOE Federal Energy Management Program
3. Purchasing Energy-Efficient Electric Chillers — U.S. DOE Federal Energy Management Program
4. ARI Posts Addendum to Standard 550/590 — ACHR News
5. Guideline E: Fouling Factors: A Survey of Their Application in Today's Air-Conditioning and Refrigeration Industry — AHRI
6. Chiller Performance Testing Program (AHRI Standard 550/590 test points) — Trane Technologies
7. How to Calculate Chiller IPLV — MEP Academy
8. Critical review of the influence of lubricants on the heat transfer and pressure drop of refrigerants (Shen and Groll, HVAC&R Research, 2005) — International Institute of Refrigeration (FRIDOC record)
9. International Performance Measurement and Verification Protocol (IPMVP) — Efficiency Valuation Organization