Operation and Maintenance of Industrial Refrigeration Systems

Introduction

The operation of industrial refrigeration systems with intermediate coolant is a challenge in which the quality of mode control and the accuracy of diagnostics have a significant impact on equipment life. In such systems, the stability of flow, superheat, boiling and condensing pressures determines the life cycle of compressors, heat exchangers and pumping equipment, and any deviations create a cascade of disturbances leading to emergency shutdowns and costly repairs. A practical approach to operation should take into account the actual physical processes in the system, rather than relying only on calendar maintenance intervals.

This article describes the approach to operating such plants, focusing on the bottlenecks that engineers encounter on a daily basis and describing only those technical solutions that have a direct impact on equipment life and accident prevention.

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System management, daily operating procedures

SCADA trend analysis

Despite the abundance of data, only four groups of parameters are of real value: superheat, boiling pressure, condensing pressure, coolant temperatures and compressor starts. The rest of the data play a secondary role and are only used when investigating complex deviations, not in daily operation. Any deviation with a trend (e.g. gradual increase in HP, SH fluctuations, LP fluctuations) is more important than one-off peaks, because it is the trends that confirm gradual degradation, which the automation does not yet detect as an emergency, but which personnel can detect a day or two before restrictions or shutdowns occur.

Fig. 1 – SCADA panel of evaporator monitoring on the control panel

Fig. 2 – SCADA panel for compressor monitoring on the control panel

Fig. 4 – SCADA panel on the remote operator’s post, logging of compressor power consumption

Fig. 5 – SCADA panel at the remote operator’s station, control and logging of refrigeration system parameters

Control of actual flow rate and ∆T by evaporator

The operator checks the current flow rate and temperature drop against the baseline values: if ∆T has changed by 1-2 °C without load change, this is a sign of hidden faults in the distribution network, while a drop in flow rate is the root cause of most overheating and LP problems. Daily monitoring of these two parameters ensures early detection of blockages, partial filter blockages and unstable pump operation – disturbances that, without diagnostics, lead to evaporator instability within a day.

An operating ∆T of 3-6 °C is an indicator of the condition of the entire network. If the ∆T rises above 7-8 °C or fluctuates during the shift, this is a clear signal of problems in the consumers: either one of the heat exchangers is operating out of mode, or the control valves are creating hydraulic shocks, or the bypass is not working correctly. Unstable ∆T always leads to unstable evaporation, so it is important for the operator to check not the chiller but the distribution network – in 70 % of cases the root of the problem is located there.

Control of refrigerant superheat (SH), boiling pressure (LP), condensing pressure (HP)

A normal superheat of 6-8 °C is the “healthy mode” of an evaporator with a mechanical TRV. An upward deviation may indicate underfilling with refrigerant, dirty filter-drier, TRV problems or unstable flow rate. A downward deflection indicates a risk of liquid refrigerant entering the compressor, especially dangerous for reciprocating compressors. The most important thing for the operator is the stability of the superheat. Overheating fluctuations of ±2-3 K at a stable heat load are the first and most reliable indication that the system is out of design mode. The cause of unstable overheating is most often not the TRV itself, but a clogged filter-drier, incorrect calibration of sensors, so attempts to “tweak” the TRV usually have the opposite effect.

At a stable heat load, the boiling pressure should remain within the amplitude range of ±0.1-0.2 bar from the set point. Any LP pulsations indicate evaporator fouling problems, fluctuating coolant flow rates, malfunctioning VFD pumps or delayed RTD control. Experienced engineers start the diagnosis of an unstable system by analysing the LP trend, as unstable LP appears days or weeks before low pressure alarms and is one of the most reliable early indicators of degradation.

A rise in condensing pressure, even if small (e.g. 50-100 kPa over a week), indicates degradation of the condenser heat exchanger – fouled microchannel sections, uneven blowing or reduced water flow. If the HP is unstable at a stable outdoor temperature – this is a sure signal of problems that do not yet cause accidents, but significantly increase the load on the compressor and increase energy consumption.

Visual assessment of compressor and pump condition

During a walk-around, the engineer looks for casing temperatures, heated oil odour, abnormal sounds and vibration of compressors and pumps: even a small increase in vibration or resonant noise indicates initial wear, misalignment or bearing degradation that can lead to an emergency shutdown in a matter of weeks; visual and tactile diagnostics often detect problems much earlier than instrumentation.

Checking the condition of pipework and connections

The engineer inspects pipework for leaks, oil stains, traces of refrigerant or micro-droplets in the area of flanges, fittings and welded joints; any traces of oil on the refrigeration circuit are a direct indicator of a leak, even if the automation has not yet reacted by reducing the refrigerant pressure or level, and in the coolant circuit leaks are often only apparent by a change in tank level and a drop in ∆T, so daily visual inspection prevents the development of critical defects.

Checking the condition of the condenser heat exchanger surfaces

For air chillers, the engineer assesses airflow uniformity, fan noise and visual condition of the fins: even partial fouling leads to increased HP and compressor load, while uneven flow indicates faulty individual fans or blocked sections of the battery; for water systems, daily monitoring of pressure drop and flow stability, as a decrease in ∆P of even 10 % is already an indicator of heat transfer deterioration.

Monitoring the level, transparency and condition of the cooling medium

The engineer checks the level in the expansion tank, the transparency of the solution and the absence of foam or suspended solids: turbidity, sediment or discolouration are early signs of glycol degradation, corrosion or air ingress. A level deviation with no visible leakage indicates an air lock or faulty operation of the make-up unit, and foam is often associated with cavitation of the pumps, which must be corrected immediately to prevent a drop in flow.

If the solution pH drops by 0.3-0.5, mechanical impurities appear or the solution becomes cloudy, the operator should assume that the heat exchanger degradation process has already started: corrosion rate changes, ∆P increases, heat transfer deteriorates; unlike in the refrigeration circuit, coolant degradation develops slowly, but the consequences are irreversible, so pH and suspended solids analysis is one of the most important preventive tools.

Evaluation of the operation of the consumers’ control valves

The correct operation of control valves at consumers is recorded on a daily basis: if one of the valves modulates between 0-100% at a stable load, this creates hydraulic fluctuations that are transmitted to the evaporator and cause the TRV to “hunt”; the operator must realise that instability in one branch affects the chiller more than the consumer itself, so detecting such irregularities is a key task in the daily rounds.

Checking the correctness of sensor and automation readings

Comparison of temperature and pressure sensor readings with reference values or duplicate measurements (manual thermometer, manometer) allows to detect sensor drift, which leads to false modulation of compressors, pumps and TRVs. Even an error of 1-2 °C or 0.1-0.2 bar distorts control algorithms and causes operational errors, so daily sensor accuracy verification is one of the most important procedures, although it is often underestimated.

Record all deviations in the operating logbook

Any deviation – unstable SH, rising HP, flow fluctuations, vibration changes – is recorded in the logbook with the time and magnitude of the deviation. It is the logbook that identifies trends that cannot be detected by eye or in a single shift, and the lack of recording means that degradation goes unnoticed until an accident occurs; a good logbook is the basis for correct scheduled maintenance and setpoint revision.

Scheduled maintenance: frequency and necessity criteria

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Weekly hydraulic stability check

Once a week, an engineer records evaporator pressure drop, current flow rate and ∆T stability: a 10-15% increase in ∆P, a decrease in flow rate or a drift in ∆T indicates initial filter contamination, pump deterioration or an imbalance in the consumer network; weekly analyses can detect most hidden problems 1-3 weeks before they become emergencies, so monitoring hydraulics is a key element of routine maintenance.

Monthly inspection of the condition of the heat exchangers and the quality of the coolant

Evaporator and condenser ∆P, plate fouling factor (based on temperature and ∆P change) and coolant parameters: pH, transparency, presence of mechanical inclusions are checked monthly. Even a slight deterioration of these parameters is an unambiguous signal of the need to flush or restore the inhibitor composition, since the degradation of the coolant and plate fouling are the most common causes of performance degradation in glycol systems.

Quarterly diagnostics of the refrigeration circuit and automatics

Each quarter we check overheating, subcooling, TRV stability, filter-drier condition, pressure and temperature sensors, and compressor error log analyses. If the superheat is trending upwards or the subcooling is unstable, this indicates problems with the TRV, partial filter contamination or a change in refrigerant mass. During the same period, the oil circuit of screw compressors is checked – oil ∆P, oil quality and heater operation.

Annual inspection of heat exchangers and pumping group

Once a year deep maintenance is carried out: disassembly of plate heat exchangers with mechanical or chemical cleaning, flushing of tube boards of shell and tube units, high pressure cleaning of air condensers, as well as revision of the pumping station – assessment of bearings, alignment, vibration, impellers and VFD efficiency. The annual inspection eliminates accumulated latent defects that cannot be diagnosed by operating parameters alone.

Electrical and automation diagnostics with insulation measurement

Power circuits are checked annually, the insulation resistance of electric motors is measured and contactors, terminals and connections are inspected. Superficial signs of overheating, darkened terminals or a 10-15 °C increase in temperature in control cabinets are signs of degraded contact connections and potential failure of the frequency converter or motor. Electrical components are often the cause of non-thermodynamic accidents and require systemic attention.

Criteria for mandatory unscheduled maintenance

Unscheduled maintenance is required when there is a trending increase in HP, unstable overheating, a drop in flow rate below 90% of rating, an increase in ∆P at the evaporator or condenser of more than 20%, an increase in vibration of 15-20%, sensor drift, or an increase in the number of compressor starts. These criteria are more important than calendar dates because they reflect the actual technical condition of the system and prevent the progressive development of defects.

Adjustment of maintenance intervals based on operational data

The revision of maintenance schedules is based on the analysis of long-term trends:

• If the condenser is fouled every 3-4 months instead of the nameplate 12, the maintenance period is shortened
• if pumps are stable with low vibration, some operations are postponed
• if SH or LP tend to drift, VFD settings or TRV setpoints are revised
• adapting routines to the actual condition of the equipment always has a better effect than following formal schedules.

Conclusion

Efficient operation of an industrial refrigeration system is achieved not by the number and frequency of performed maintenance work, but by the stability of key parameters, which is ensured by a properly organised cycle of monitoring, analysis and adjustment of modes. If mass flow rate, superheat, boiling and condensing pressures are kept within a narrow range, and the condition of the coolant, heat exchangers and pumping equipment is monitored by trends, the system operates predictably and demonstrates a resource significantly exceeding the nameplate values. Competent diagnostics of early deviations makes it possible to eliminate defects before they develop into mechanical damage, which is the basis for reliable operation.

With the high cost of downtime and expensive compressor equipment, operation engineering must focus on preventing cascading failures, which are formed from small instabilities that go undetected for weeks. The integration of SCADA data, operational logs and regular trend analyses creates a closed-loop control loop in which the system itself tells maintenance personnel when and what actions need to be taken. This approach strikes the right balance between reliability, energy consumption and equipment life, making the industrial refrigeration system sustainable and long-lasting.

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Author of the article:
Sergey Stafiychuk, Sales Manager

12.12.2025