Process Cooling

Chillers

Chiller Performance Starts With a Clear Measurement Boundary

A chiller transfers heat from a chilled-water circuit to another medium. In a water-cooled installation, condenser-water pumps and cooling towers also affect the performance of the cooling system.

Distinguish chiller-only electrical consumption from whole-plant consumption. An improvement at the chiller can be offset by additional pumping or fan power. Record what your calculation includes before comparing results.

Reference for system-level thinking: Trane — Chilled-Water System Design Trends.

Worked Example: Cooling Duty and Efficiency

Illustrative example: these assumed values demonstrate arithmetic, not actual plant results or recommended operating targets.

For water, a useful approximate heat balance is:

Cooling duty (kW) = mass flow (kg/s) × specific heat (kJ/kg·K) × temperature difference (K).

Assume 100 m³/h of water, density 1,000 kg/m³, specific heat 4.186 kJ/kg·K, return temperature 12°C, and supply temperature 7°C.

Mass flow = 100 × 1,000 ÷ 3,600 = 27.78 kg/s.
Cooling duty = 27.78 × 4.186 × 5 ≈ 581 kW.
Cooling capacity = 581 ÷ 3.517 ≈ 165 refrigeration tons.

If the measured chiller electrical input is 110 kW:

COP = 581 ÷ 110 ≈ 5.29.
Chiller efficiency = 110 ÷ 165 ≈ 0.67 kW/TR.

These are illustrative values, not a pass/fail target. Use actual fluid properties for glycol mixtures, and verify flow and temperature measurements. Small sensor errors become significant when the temperature difference is small.

When Results Change

Record load, chilled-water temperatures, condenser conditions, electrical input, and control status. Compare similar operating periods before concluding that efficiency has deteriorated.

A low water-temperature difference alone does not identify a refrigerant fault. The load, water distribution, bypassing, controls, and measurement quality all deserve review. Refrigerant diagnosis and work belong to qualified personnel using the equipment procedure.

Next step: Calculate cooling duty and clearly label whether your electrical input covers the chiller or the entire plant.

Record observations only where it is safe. Inspection inside equipment, electrical testing, chemical work and changes to protective controls require qualified personnel, approved isolation and the installed equipment manual. Never bypass a trip or repeatedly restart unsafe equipment. Engineering disclaimer

Measurements To Record

Evaporator entering and leaving water temperatures
Condenser entering and leaving water temperatures
Water flow and chiller electrical input
Refrigerant pressures and alarm history

Maintenance Checklist

Record chilled-water and condenser-water temperatures.
Trend flow, differential pressure and kW per cooling output.
Review refrigerant alarms and compressor start counts.
Plan water treatment and heat-exchanger inspection.

Evidence-Based Diagnosis

Insufficient cooling

Possible Causes: Higher load; reduced water flow; fouling; condenser problems.
Evidence To Collect: Verify water temperatures and measured flow; compare loading and condenser conditions with the design baseline.
Qualified Follow-Up: Address confirmed water-side problems first. Use a licensed refrigeration technician for refrigerant-side assessment.

High condenser pressure

Possible Causes: Warm condenser water; restricted airflow on air-cooled units; fouled heat-transfer surfaces.
Evidence To Collect: Compare condenser conditions with ambient temperature, cooling-tower performance and previous clean-system readings.
Qualified Follow-Up: Arrange inspection of airflow or water circuits and cleaning. Do not bypass high-pressure protection.

Frequent cycling

Possible Causes: Low load; narrow control band; inadequate system water volume; sensor error.
Evidence To Collect: Review starts per hour, demand trend and temperature-sensor agreement.
Qualified Follow-Up: Have a controls specialist assess staging, system volume and permitted start intervals against the manufacturer instructions.

Record The Confirmed Result

Keep the asset model, time, load, ambient conditions, alarm code and relevant readings together. Compare with the last healthy baseline. Record the confirmed cause, approved corrective action and readings after restoration; check that the improvement persists before closing the investigation.

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How It Works

1
Refrigerant absorbs heat in the evaporator, cooling the chilled-water loop.
Low-pressure liquid refrigerant boils inside the evaporator (flooded shell-and-tube or DX plate type) at 2–4 °C, absorbing the latent heat of the returning chilled water and cooling it from 12 °C to 7 °C. Superheat of 3–6 K at the suction confirms full evaporation before the gas reaches the compressor.
2
Compressor raises refrigerant pressure and temperature.
The compressor — screw, centrifugal, scroll or reciprocating — lifts the refrigerant from evaporator to condenser pressure. This 'lift' is the whole energy cost of the machine: reduce condenser water temperature or raise chilled-water setpoint and lift falls, so kW/TR falls with it. Capacity is trimmed by slide valve (screw), inlet guide vanes plus VFD (centrifugal) or cylinder unloading.
3
Condenser rejects heat to cooling water or ambient air.
Hot discharge gas de-superheats, condenses and sub-cools in the condenser, rejecting the evaporator load plus the compressor work to cooling-tower water (32/37 °C) or ambient air. Sub-cooling of 3–6 K increases the refrigerating effect per kg and protects against flash gas.
4
Expansion device drops pressure, restarting the cycle.
The expansion device — electronic expansion valve, float valve or orifice plate — throttles liquid to evaporator pressure, flashing part of it and dropping the temperature. It modulates on superheat so the evaporator stays fully wetted at all loads, then the cycle repeats.

Key Components

Compressor

Screw / centrifugal / scroll

Screw (100–1500 TR, robust, part-load friendly with VFD), centrifugal (>300 TR, best full-load efficiency, risk of surge at low load/high lift), scroll (small, multiple circuits, good redundancy). Oil temperature, differential pressure and motor winding temperature are the key protections.

Evaporator

Refrigerant absorbs process heat

Shell-and-tube barrel with enhanced tubes, or brazed-plate for small units. Chilled water must maintain minimum flow — low flow causes freeze-up. Watch the approach (LCHWT minus saturated suction temp); a rise above 2 K means fouled or oil-logged tubes.

Condenser

Rejects heat to cooling medium

Water-cooled shell-and-tube (best efficiency) or air-cooled coil. Condenser approach above 2–3 K signals scaling, biofilm or non-condensable gases; each 1 K rise in condensing temperature costs roughly 2–3 % on power.

Expansion Valve

Meters refrigerant flow

Electronic (EEV) with stepper motor and superheat control, thermostatic (TXV) with bulb, or float valve on flooded machines. A hunting valve shows up as swinging suction pressure and unstable chilled-water temperature.

Chilled Water Pump

Circulates CHW to load

Primary/secondary or variable-primary arrangement. Primary loop keeps constant evaporator flow; the secondary VFD pump matches building/process demand. Poor ΔT syndrome (return water too cold) is usually 3-way valves, dirty coils or bypass, not a chiller fault.

Typical Operating Parameters

These reference ranges are illustrative context, not approved operating limits. Verify the installed model, duty, fuel or water conditions and applicable site requirements before use.

CHW Supply/Return

7 / 12 °C

Condenser Water

32 / 37 °C

Approach

1 – 2 K

COP

4.5 – 6.5

Safety Notes

•Refrigerant leak detection — HFC/HFO/ammonia hazards.
•Never bypass HP/LP cut-outs.
•PPE for cold surfaces and refrigerant handling.

Maintenance Schedule

•Weekly log: pressures, temps, motor current, approach.
•Quarterly: tube brushing / eddy current inspection.
•Annual: oil analysis, vibration analysis, refrigerant purity.

Problem Solving Matrix

Listed causes are hypotheses, not confirmed findings. Any corrective work requires qualified assessment, approved procedures and follow-up measurements.

Issue	Likely cause	Fix
High condenser pressure	Fouled tubes / low CW flow	Clean tubes, verify CT
Low evaporator pressure	Low charge / dirty filter drier	Check subcooling & charge
Short cycling	Oversized / low load	Sequence multiple units, VFD

Equipment Running-Cost Estimate

Use measured electrical input, operating hours and your actual electricity tariff to estimate energy costs for chillers. Compare replacement options before setting a project budget.

Calculate running costs

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