Troubleshooting and Fault-Finding
First decide whether the load changed or the plant changed — that single question splits every refrigeration fault in two.
Key Principles at a Glance 8 points
- Fault-tracing is a formal eight-step deduction — detection, knowledge of the system, observation, identification, decision, action, test, record — and step 8, recording the incident, is the one most often skipped.
- Air in the system gives a high discharge pressure with a NORMAL condensing temperature; poor cooling gives a high discharge pressure with a HIGH condensing temperature. That distinction alone separates the two commonest causes of a high condenser gauge.
- Undercharge is the most common single fault: running hot, low suction and discharge, large bubbles in the sight glass. Find and repair the leak BEFORE charging.
- Overcharge only happens when the system has been charged; poor cooling, air and iced regulator occur spontaneously. That timing is the clue.
- The oil return line from the oil separator should feel just above ambient. If it is cool or cold, liquid refrigerant is vaporising in it and the separator shell is too cool.
- Short cycling is usually a frosted-coil symptom, not a control fault — check the defrost system first.
- A ruptured bursting disc shows as suction and discharge pressures being about equal with no cooling.
- A dirty air filter is by far the most frequent cause of air-conditioning malfunction.
1. The Fault-Tracing Method
The maker's manual, the SMS and the class rules override. Hasty decisions and random efforts to get the plant working again are to be shunned, since more damage may result.
System faults fall into two general classes: the sudden catastrophe of a mechanical breakdown, and the slow fall-off of performance which can be detected as a malfunction in its early stages but will also lead to a breakdown if not rectified. Identification of the first will be obvious. To track down the cause of a malfunction will be more complicated.
Fault-tracing is a multistep process of deduction, ending in normal operation again and a record of the incident to inform other operatives. The steps are as follows:
The supporting rules:
- A lot of help in fault-tracing may be had from charts for specific pieces of apparatus, prepared by the manufacturer.
- Detailed examination of a sophisticated item may be beyond the skills of the plant operators and require the assistance of a specialist, such as an electronics engineer. Where such complications are part of the system, it is an advantage to know beforehand where such specialist help can be reached.
- It should be accepted that fault analysis can be a slow process and that it usually defies prior estimates of the time it will take, regardless of the pressures of persons who are affected by the interruption of the service.
- Training courses are available in analytical methods of fault-tracing. Computers are also in use which monitor a number of parameters and draw attention to any observed abnormality.
- The control/monitoring device may then make a judgement as to the cause, or this may rely on the interpretation of the operator.
Electrical faults are responsible for a large number of refrigerator problems. Ship vibration, in turn, is the reason for many of the electrical faults. Loose connections and broken wires and earths, resulting from chafed insulation, are examples. The wiring diagram for the installation should be available, for location of all fuses etc.
2. Master Symptom Table
| Symptom | Most likely cause |
|---|---|
| High condensing pressure, normal condensing temperature | Air in the system |
| High condensing pressure, high condensing temperature, condenser ends hot | Poor cooling water / dirty condenser |
| High condenser gauge, full liquid sight glass | Overcharge |
| Low condenser gauge, large bubbles in sight glass | Undercharge / leak |
| Compressor running hot, low suction and discharge | Undercharge |
| High suction and high discharge | Overcharge |
| Icing/frost at the expansion valve | Moisture in the system |
| Compressor runs continuously, insufficient cooling, noisy, high suction, low discharge, warm cylinder head | Defective discharge valve |
| Compressor runs continuously, insufficient cooling, noisy, high suction pressure | Defective suction valve |
| Starved evaporator, high superheat, rapid condenser pressure rise | Choked expansion valve |
| Cold room temperature slowly rising, compressor runs long then cuts out on LP and restarts | Excessive frost on evaporator / defrost failure |
| Compressor starts and stops immediately | LP cut-out, oil cut-out, defrost timer, low oil, oil foaming, motor overload |
| Compressor repeatedly runs a few seconds and cuts out | Short cycling |
| Oil level in compressor dropping, poor cooling, frost on suction line | Oil in the system |
| Liquid returning to the compressor suction | Flooding |
| Liquid level in condenser too high, HP switch trips | Overcharge / air / iced regulator |
| Suction and discharge pressures about equal, no cooling | Ruptured bursting disc |
| Discharge pressure 0.6 bar higher than a week ago | Higher ambient, higher load, dirty condenser, fan stopped, non-condensables, dirty strainer |
3. The Faults in Detail
3.1 Undercharge (loss of refrigerant)
This is the most common single fault.
| Element | Detail |
|---|---|
| Indication | Compressor is running hot and performance of the compressor falls off due to high superheat temperature at the suction side of the compressor; suction and discharge pressure of the compressor is low; large vapour bubbles in the liquid sight glass; low gauge readings in the condenser |
| Cause | Leakage from the circuit — pipework, joints, shaft seal, solenoid, valve glands. With R12, the suction pressure is below atmospheric, so the leak can also admit air |
| Action | When undercharge is suspected a leak test is necessary to confirm and locate the fault. Locate and repair the leak first. Then charge |
| Confirmation | Charging is continued until the bubbles disappear from the sight glass. Then the condenser gauge should read about 7 °C above sea water inlet and the suction gauge about 7 °C below the evaporator, on equivalent saturation temperatures |
| Prevention | Regular leak test as part of the maintenance schedule, especially for halocarbons; always test the shaft seal; keep the running log so a slow loss shows up |
Symptoms of undercharge are a low condenser gauge reading and the appearance of large bubbles in the sight glass. The compressor will tend to work hot also. The result of undercharge is that the performance falls off.
Leak detection: The mechanical seal on the shaft is tested because freon from the crankcase may be lost through wear or a fault.
Charging method (McGeorge): the freon storage cylinder is connected to the filling valve on the regulator outlet, loosely. The bottle valve is cracked open to clear air from the connecting pipe and the nut is tightened on the nipple of the filling valve. The charging connection is made to the liquid stop valve or suction stop valve if there is no valve after the regulator. The bottle must be kept upright to prevent entry of liquid when the connection is made to the suction side of the system. The charging valve is opened to one turn off the back seat and, with the compressor running, the bottle valve is fully opened.
3.2 Overcharge (excess refrigerant)
| Element | Detail |
|---|---|
| Indication | The liquid level in the condenser is too high (high condenser gauge reading). This will reduce the available condensing surface, with a corresponding increase in the saturation temperature and pressure. High pressure switch of the refrigerant compressor activates and stops the compressor. The suction and the discharge pressures are high. High condenser gauge reading with a full liquid sight glass |
| Cause | Excessive refrigerant has been charged in the system; air in the system may also cause overcharge indication; formation of ice on the regulator |
| Action | Remove the refrigerant from the system — connect a cylinder to the liquid line charging valve, start the compressor, then operate the charging valve. Purge the air from the system and maintain effective cooling. Remove ice from the regulator by using any of the defrosting methods |
| Confirmation | Suction and discharge pressures return to normal; sight glass shows liquid with no excess liquid level in the condenser |
| Prevention | Weigh the cylinder before and after charging and record the amount in the log book; do not top up without investigating the reason for the loss |
The charge is pumped to the condenser and the excess refrigerant is released to atmosphere through a pipe connected up for this purpose. When pumping the charge to the condenser, the cooling water is left on so that the gas from the compressor will be liquefied. The high pressure cut-out will stop the compressor as the pressure rises. Condenser cooling becomes less effective with accumulation of liquid and at the end of the process it will be necessary to restart the compressor by hand in order to fill the condenser.
There are other faults which will result in high condenser pressures — poor cooling, air in the system and icing of the regulator. Such faults occur spontaneously, but overcharge only happens when the system has been charged.
3.3 Moisture in the system
This normally comes with the ingress of air into the system.
| Element | Detail |
|---|---|
| Indication | Water circulating with the freon tends to freeze on the regulator, causing a build-up of pressure on the condenser side and a drop in pressure on the evaporator side due to the blockage. The machine tends to be stopped by the high-pressure cut-out. Icing of the expansion valve. Moisture may freeze at the expansion valve, giving some of the indications of undercharging. It will contribute to corrosion in the system. It may cause lubrication problems and breakdown of the lubricating oil in the refrigerant compressor. In halocarbon circuits, indicated by the colour trace on the sight glass |
| Cause | Ingress of air carrying moisture; exhausted drier; moisture in new oil; inadequate evacuation after opening the circuit |
| Action | Renew silica gel in case of minor moisture. Collect refrigerant and remove all air and moisture by vacuum pump if the amount is huge. The drier should be changed and the sight glass watched for reversal of the colour to "dry". Bad cases of contamination may need a second change of drier |
| Confirmation | The chemical (either activated alumina or silica gel) is renewed and the compressor restarted after the ice has melted. Usually the ice in the expansion valve will melt due to the ambient temperature. Sight glass colour reverts to "dry" |
| Prevention | Keep oil in tightly sealed containers; always fit a drier and renew it on schedule; evacuate properly before recharging; do not leave the circuit open to atmosphere |
Immediate action is required, especially with a hermetic or semi-hermetic compressor, before damage is caused.
3.4 Air in the system
| Element | Detail |
|---|---|
| Indication | The refrigeration compressor may overheat, with a high discharge pressure and normal condensing temperature. Small air bubbles in the liquid sight glass of the condenser. Condensing pressure of the refrigerant in the condenser may be high. If there is excessive air, it may reduce the cooling capacity of the system, making the compressor run for an extended period of time. The gauge pointer of the condenser may jump indefinitely. A steady increase in the high-pressure gauge reading. Accumulation of air reduces the effective area of condenser available for condensing refrigerant. Abnormally high condenser pressure gauge reading |
| Cause | During charging, air may enter the system. If Freon-12 is used, air may leak into the suction line because the working pressure of the Freon-12 refrigerant is less than atmospheric. |
| Action | Collect the system gas in the condenser, leaving the condenser cooling water on, and vent the air out from the top of the condenser, because air will not be condensed in the condenser but remains on top of the condenser above the liquid refrigerant. Connect the collecting cylinder to the purging line of the condenser, open the valve, and collect air in the cylinder. After purging the air from the system don't forget to shut the purging valve. Check the level of the refrigerant in the system. If required, charge the system with fresh refrigerant. Restart the compressor with all safety precautions |
| Confirmation | Close the condenser outlet liquid valve and the compressor will trip on LP cut-off. Sea water is left circulating in the condenser for a few hours to achieve equilibrium. Note the HP gauge reading. If no air is present in the condenser, there will be no change in the reading. If air is present, then the reading will be higher than the previous value. Then purge and repeat — the reading should stabilise |
| Prevention | Purge the charging connection before tightening; keep the suction pressure above atmospheric where possible (this is an advantage of R22 over R12); keep all joints gas-tight; do not over-cool a leaky circuit |
Air in the system gives a high discharge pressure with a NORMAL condensing temperature (because the air occupies condenser volume without condensing). Poor cooling gives a high discharge pressure with a HIGH condensing temperature. This distinction is worth memorising.
3.5 Oil in the refrigeration system
| Element | Detail |
|---|---|
| Indication | Temperature is not dropping in the cold rooms as normal, due to the fact that oil acts as insulation in the evaporator. Excessive frost on the suction line. Refrigerant compressor runs for an extended period of time. Lubricating oil level in the compressor will drop. Refrigerant level will fall if oil has caused blockage. Compressor may take high capacity current during starting |
| Cause | Oil separator not working properly. Oil may carry over from the compressor and may not come back to the compressor due to blockage in the system. Defective piston rings or worn-out liner of the compressor may cause the oil to carry over along with the refrigerant. |
| Action | Check the oil separator for proper functioning. Check the drier for proper cleaning and if it requires cleaning, clean it. Evaporator coil should be drained to remove any trace of oil. If there is oil in the cooling coils, increase the condenser and evaporator temperature differentials and remove excess frost on the suction pipe. Heat pipes with a blow torch |
| Confirmation | Cold room pull-down returns to normal; compressor sump oil level stabilises; oil separator return line is warm to the touch, not cold |
| Prevention | Maintain the oil separator (float, needle, heater); maintain the correct oil level; check piston rings and liners at overhaul; treat long-term oil migration in a dry expansion circuit as a design fault and put it right |
Feel the oil return line from the oil separator. It should be just above ambient temperature. If it is cool or cold, liquid refrigerant is vaporising in it — the separator shell is too cool and liquid refrigerant has collected in the bottom.
3.6 Flooding of refrigerant
This is seen as liquid getting back to the suction of the refrigerant compressor.
| Element | Detail |
|---|---|
| Cause | Faulty or incorrectly adjusted expansion valve; solenoid valve leakage; overcharging of the refrigeration system |
| Effect | May lead to an iced-up evaporator; liquid knock; compressor damage |
| Action | Check and re-set the TEV superheat; test the solenoid for leakage; correct the charge |
| Prevention | Solenoid liquid stop valve shuts just before the machine cuts out so that the compressor clears the suction line before it stops — this prevents liquid knock when restarting |
3.7 Evaporator coil icing
Icing of the evaporation coils may happen due to:
| Cause | Action |
|---|---|
| Too low temperature setting | Increase the coil temperature by adjusting the TEV or its sensor |
| The coil capacity is less | Install large capacity evaporator coils |
| Defrost is not operational | Check if the defrost system is functioning at regular intervals |
Also consider: abnormal operation of the TEV, overcharge of the system, moisture in the system owing to a dirty drier, defective suction valve.
3.8 Compressor starts but stops immediately
When the compressor in the reefer circuit starts and suddenly stops, it can be because of the following reasons:
| Cause | Action |
|---|---|
| Low pressure cut-out gets activated | Ensure that all the suction line valves are in open condition, the refrigeration is properly charged, and the low pressure cut-out is not defective |
| Defective oil pressure cut-out | Check for proper functioning of the oil pressure cut-out and replace the defective cut-out |
| Defrosting timer is getting activated frequently | If the defrost timer is getting activated frequently, leading to cut-out of the compressor, check and repair the defrost timer |
| The lube oil level is below required level | This can be because of leakage of lube oil from the seal or carry-over of oil. Rectify the leakage and refill the oil level |
| Foaming of oil leading to reduced oil pressure | Ensure no foaming takes place; renew the oil if required |
| Motor overload cut-outs are activating | Ensure that electrical motor trips are working properly |
Welch's equivalent list — the motor driving an open compressor is switched on, and ten minutes later the compressor is not turning. The possible reasons:
- No mains electric supply
- Fuse blown
- One phase blown, out on single-phasing trip
- Belts broken or slipping
- Out on high-pressure cut-out (various)
- Out on low-pressure cut-out (this may have reset, but the compressor contactor is held out by the restart delay timer)
- Short of oil
- Out on thermostat
- Flow switch open, in load or condenser water
3.9 Other operational faults
| Fault | Indication | Cause | Action |
|---|---|---|---|
| Poor cooling water | Condenser pressure gauge high; condenser ends and pipes feel hot; refrigerant not efficiently liquefied | Choked sea water pump strainers; chokage of the system; pump fault | Clean strainers; check pump; check valves |
| Oil deposits in condensers/evaporators | Reduced heat exchange | Oil carry-over | Chemical cleaning; maintain the oil separators |
| Frost on evaporator coils | Cold room temperature gradually rises; compressor runs continuously at first, then cuts out on low suction pressure, then restarts as the refrigerant passing through the still-open solenoid builds up pressure | Failure of the defrost arrangement | Remove ice by washing with a hot water hose with the plant shut down, after clearing the drip tray drain; restore the defrost system |
| Blockage in the system | Low/erratic pressures | Moisture forming ice on the expansion valve; blocked strainers; closed valves; solenoids which have failed to open | Identify and clear; renew driers; clean strainers |
| Electrical faults | Various, often intermittent | Ship vibration — loose connections, broken wires and earths, chafed insulation | Use the wiring diagram to locate all fuses; check connections; renew damaged cable |
| Excessive icing up at compressor suction | Frost on the suction line | Abnormal operation of TEV; overcharge of the system; moisture in the system owing to a dirty drier; defective suction valve | Investigate each in turn |
| Defective suction valve | Continuous running of compressor; insufficient cooling effect; noisy operation; high suction pressure | Worn or broken valve | Overhaul the valve assembly |
| Defective discharge valve | Continuous running; insufficient cooling; noisy operation; high suction pressure during running; low discharge pressure during running; suction pressure rises faster after compressor shutdown; warm cylinder head | Worn, broken or held-open discharge valve | Overhaul; test by shutting the suction and discharge valves and watching the gauges |
3.10 Testing for a defective discharge valve
If the discharge pressure falls roughly by 1 bar and above in five minutes, and simultaneously the suction pressure rises, then the discharge valve is leaking.
3.11 Choked expansion valve
| Element | Detail |
|---|---|
| Cause | Dirt and freeze-up of water present in the system |
| Effect | Starved evaporator; high superheat temperature; rapid condenser pressure rise can cause stopping of the compressor |
| Remedy | Clean the expansion valve and filter; renew the dehydrator |
Related: if the liquid line leaving the drier or strainer (if separate) is colder than the inlet, there is a severe pressure drop within, indicating dirt. A new drier, or cleaning of the strainer, will cure this.
3.12 Short cycling
Short cycling is the term used to describe a compressor unit repeatedly running for a few seconds and then cutting out. This is the result of operation of the L.P. controller.
The mechanism:
The control is arranged to operate when all solenoids have closed and suction pressure drops, to stop the machine. It restarts it when the suction pressure rises from solenoids reopening. Thus any condition which varies suction pressure over this range will cause the compressor to cut in and out.
The classic shipboard cause:
If a solenoid is opened in the normal way by high cold room temperature, the refrigerant in passing through will build up suction pressure and the compressor will be started. If the supply is restricted and insufficient for compressor demand, the suction pressure will drop and the L.P. controller will stop the machine. The thermostatic valve may be the restricting device, due to low superheat of the gas leaving the evaporator (from the insulating effect of ice on the coils) it will close in to reduce the refrigerant flow.
So: short cycling is often a frosted coil symptom, not a control fault. Check the defrost system first.
3.13 Ruptured bursting disc
A safety bursting disc is fitted between the compressor discharge and the suction. This may be of nickel with a thickness of 0.05 mm. A ruptured disc is indicated by suction and discharge pressures being about equal.
| Element | Detail |
|---|---|
| Indication | Suction and discharge pressures about equal; no cooling |
| Cause | Liquid carry-over, or excessive discharge pressure |
| Action | Stop the machine; renew the disc; investigate why it ruptured (liquid carry-over from flooding, or HP control failure) |
| Prevention | Maintain the HP cut-out and the discharge valve lifting arrangement; avoid flooding |
3.14 Discharge pressure higher than it was
A discharge gauge reads 0.6 bar higher than the reading a week ago. List four possible reasons.
Answer:
- Higher ambient, dry or wet bulb
- Higher load temperature or more flow
- Dirty condenser
- Condenser fan stopped
- Non-condensible gas in system
- Pump strainer dirty (condenser water)
Note how the answer splits into "the world changed" (ambient, load) and "the plant changed" (dirty condenser, fan, non-condensables, strainer). That split is the basis of all refrigeration fault-finding: first decide whether the load changed or the plant changed.
4. Air Conditioning Specific Faults
| Fault | Cause | Action |
|---|---|---|
| Reduced air flow, higher running cost | Dirty air filter — by far the most frequent cause of malfunction of air-conditioning equipment | Clean or replace; use the manometer across the filter; never run fans without filters in place |
| Water side fouling | Scale, algae in the water circuit | Clean with sulphamic acid, brushing or high-pressure water jets; remove all traces of chemical before return to service |
| Compressor overheats on a high-discharge-temperature refrigerant | Ammonia requires water-cooled cylinder heads | Check cooling water to the heads |
| Tower water freezing on the packing in winter | No thermostatic fan control | Fit/repair thermostatic fan control |
| Legionella risk | Stagnant water or wet deposits of slime/sludge | See the air conditioning chapter for the safety obligation |
5. The Fault-Finding Flow That Earns Marks in an Oral
When asked "the plant isn't cooling — what do you do?", answer in this order: