High compression pressure problem is the condition where refrigerant pressure at the compressor discharge exceeds the design operating threshold, tripping the HP switch to protect the compressor and motor from burnout. Among industrial refrigeration faults, this is the most common discharge pressure abnormality — many times more frequent than low discharge pressure.
This article moves from the definition and operating thresholds for each refrigerant type, through four groups of early warning signs and six groups of root causes, to how to distinguish the two most easily confused cases: overcharge and non-condensable gases. Next comes a quantified account of the consequences of leaving high pressure unaddressed, a five-step procedure that protects the compressor first, and the scheduled maintenance that prevents recurrence. The final section collects frequently asked questions from technicians.
Across 20 years designing and installing industrial refrigeration systems at seafood and food plants in Tay Ninh (formerly Long An), Ca Mau, Soc Trang and Nghe An, the technical team at Tan Long has found that high discharge pressure accounts for the largest share of all compressor troubleshooting requests from customers.
What is high discharge pressure in a refrigeration system?
High discharge pressure is the condition where refrigerant pressure at the compressor discharge exceeds the design threshold for the refrigerant in use, causing the HP switch to trip the machine. The term is equivalent to high head pressure or high discharge pressure in English-language literature.
Discharge pressure is closely tied to condensing pressure but is not identical to it. Condensing pressure is the refrigerant pressure inside the condenser as it changes phase from vapour to liquid, while discharge pressure comprises the condensing pressure plus the pressure losses along the discharge line and through auxiliary equipment.
Measurement point and how discharge pressure builds
Discharge pressure is measured right at the compressor discharge port, upstream of the high-pressure stop valve. The compressor draws refrigerant vapour from the evaporator at low pressure, compresses it to high pressure and discharges it into the condenser. There the refrigerant rejects heat to the environment (through air, water or water spray) and changes phase into liquid.
Condensing pressure governs discharge pressure. When the condenser performs poorly, the refrigerant cannot reject heat, so condensing temperature and pressure rise and drag discharge pressure up with them. This is why the cause of high discharge pressure usually originates on the condenser side rather than in the compressor itself.
Normal discharge pressure ranges by refrigerant type
Normal discharge pressure ranges differ by refrigerant; exceeding the HP alarm threshold means the switch will trip the machine.
| Refrigerant type | Normal discharge pressure | HP alarm threshold |
|---|---|---|
| NH3 (R717) | 6.5 to 8.6 bar (95 to 125 psig) | above 12.4 bar (180 psig) |
| R404A | 14 to 18 bar (200 to 260 psig) | above 25 bar (360 psig) |
| R507A | 14 to 19 bar (200 to 275 psig) | above 26 bar (375 psig) |
| R22 | 12 to 17 bar (175 to 245 psig) | above 24 bar (350 psig) |
These values depend on the design condensing temperature, typically 35 to 40°C for water-cooled systems and 45 to 50°C for air-cooled systems under Vietnamese climate conditions.
Signs of a high discharge pressure fault
A high discharge pressure fault shows up through four groups of signs: gauge readings, protection switch behaviour, physical symptoms at the compressor, and a drop in cooling performance. Catching it while the pressure is still climbing and has not yet reached the HP threshold is many times cheaper than waiting until the switch trips the machine and forces the production line to stop.
Signs on the gauges and protection switches
The high-pressure gauge needle exceeding the design operating range is the most direct sign. If the needle vibrates and fluctuates continuously, that points to non-condensable gases having entered the system. An HP switch that trips repeatedly after being reset indicates the root cause has not been addressed. A high-pressure fault lamp on the control panel lit continuously or flashing according to
On projects where Tan Long installed PLC control panels with analogue pressure sensors, the operating logs typically show pressure rising gradually over 7 to 14 days before it reaches the HP threshold. This is the golden window for early intervention.
Signs on the compressor and in cooling performance
Four physical signs, easy to observe on the compressor and in the output product, indicate that discharge pressure is above the threshold.
| Inspection point | Specific symptom |
|---|---|
| Discharge line | Abnormally hot; felt through a glove, above 90°C (normal operation 70 to 80°C) |
| Compressor sound | Heavy compression sound, metallic knocking from the compression chamber |
| Operating current | 10 to 25% above the rated current on the machine nameplate |
| Output performance | Freezing room failing to reach -18°C, thin flake ice, pull-down time extended by 30 to 50% |
Checking the compressor cylinder head temperature with an infrared thermometer – an important physical check when discharge pressure is suspected to be above the threshold
6 groups of causes of high discharge pressure
High discharge pressure originates from three root groups: problems at the condenser, problems with the refrigerant charge, and degradation of compressor function. Of these, the condenser group accounts for the majority of recorded cases. The groups below are ordered by probability, from most to least common, so operators can work through the checks in a sensible sequence when a fault has just occurred.
Poor condenser heat rejection
Scale on the condenser surface, bent and deformed aluminium fins, and an oil layer settled inside the tubes all reduce the heat transfer coefficient. On evaporative condensers, blocked spray nozzles, algae on the heat rejection panels and lime scale from the supply water are the common culprits.
The chain of causation is: refrigerant heat cannot be rejected to the environment, condensing temperature rises, condensing pressure rises, and discharge pressure follows. In a maintenance project for Ca Mau Seafood Processing and Services JSC, a 2 to 3 mm scale layer on the evaporative condenser was the root cause identified after other factors had been ruled out.
Weak condenser fans and cooling water pumps
A fan motor with a burnt capacitor, a broken blade, or rotation in the wrong direction from incorrect phasing after electrical maintenance reduces airflow through the condenser below the design threshold. A cooling water pump losing power, worn impellers or blocked pipework cause insufficient water flow. A cooling tower short of water because of a faulty float valve, or with fill pack that has fallen away, loses heat rejection capacity locally.
Refrigerant overcharge
Excess liquid refrigerant occupies space in the condenser, reducing the heat exchange area actually available for condensing. Measured subcooling will be higher than normal, typically above 8 to 10 K. This is the distinguishing sign from non-condensable gases, because the symptom of high pressure is the same but the remedy is the opposite. The condition usually appears after a fresh refrigerant charge, especially where no electronic charging scale was used
Non-condensable gases in the system
Non-condensable gases (NCG) are air, moisture or other gases that cannot condense under refrigeration system operating conditions. Sources include: charging without pulling a thorough vacuum, repairs leaving joints open, and suction pressure dropping below atmospheric so air enters through leak points.
NCG accumulate at the top of the condenser, where pressure is highest and refrigerant velocity is lowest, occupying space and reducing the effective condensing area. The quantification method is based on comparing pressure against temperature. For NH3, for example, a head pressure of 180 psig (12.4 bar) corresponds to a condensing temperature of 95°F (35°C). If the measured drain temperature is only 80°F (26.7°C), the system has roughly 12 to 15 psi
Degraded compressor function
Suction and discharge reed valves leaking, worn or broken. Worn bearings and damaged cylinder head gaskets reduce sealing. Refrigerant flow through the compressor falls, so both cooling capacity and condensing capacity drop, yet discharge pressure rises because the system is out of balance.
The typical secondary sign pattern: suction pressure falling in parallel with discharge pressure rising. When the two gauges are seen moving in opposite directions, a technician can immediately suspect the compressor group rather than continuing to check the condenser.
Ambient temperature above design
On hot days above 38 to 40°C in southern and central Vietnam, the actual condensing temperature exceeds the design temperature (typically 45°C for air-cooled condensers). A condenser installed hard against a wall, with poor ventilation, or with an obstruction less than 50 cm away produces a similar effect. Direct solar radiation on the refrigerant receiver causes a local pressure rise.
Telling apart high discharge pressure from overcharge versus from non-condensable gases
The two causes — overcharge and non-condensable gases — produce the same symptom on the high-pressure gauge but call for opposite remedies. Venting refrigerant when the real cause is non-condensable gases leaves the system undercharged afterwards, creating a new fault in cooling capacity.
This is the most common mistake Tan Long sees in technical support requests. A technician sees high pressure and reflexively bleeds off refrigerant without checking subcooling. The five criteria below distinguish the two cases accurately.
| Diagnostic criterion | Refrigerant overcharge | Non-condensable gases |
|---|---|---|
| High-pressure needle behaviour | High, steady | High, vibrating and fluctuating |
| Measured subcooling | Abnormally high (>10 K) | Normal, or falsely high |
| Drain temperature deviation from saturation | Trong dung sai (~5°F) | Large deviation (>10°F) |
| Pressure when the machine is off | Falls slowly towards saturation | Top of the condenser stays abnormally high |
| What to do | Recover some refrigerant | Purge gas from the top of the condenser |
Consequences of leaving high discharge pressure unaddressed
Sustained high discharge pressure affects four groups of indicators: compressor lifespan, cooling capacity, operating cost and occupational safety. Each group carries a quantifiable level of damage, allowing a business to assess the opportunity cost of delaying the fix.
Impact on the compressor and equipment lifespan
Compressor current rises with discharge pressure, leading to winding overheating and the risk of motor burnout. This consequence is especially serious on hermetic and semi-hermetic compressors, where the windings sit immersed in refrigerant, so a single burnout contaminates the entire system.
The pressure difference across the piston (or screw rotor) increases, accelerating wear on bearings, gaskets and reed valves. The design lifespan of an industrial NH3 compressor averages 80,000 to 100,000 hours; repeated high-pressure faults can cut that by 30 to 40%. Replacing a 100 HP NH3 screw compressor costs from 500 million to 1.5 billion VND — many times the cost of prevention.
Impact on operating cost and safety
Specific cooling capacity falls because compression work increases while useful refrigerant flow decreases. The warehouse fails to reach temperature, freezing time extends by 20 to 40%, and the quality of the stored product declines.
Electricity consumption rises by 10 to 20% compared with standard operating conditions for the same cooling output. For a 100 RT seafood plant, that is an increase of tens of millions of VND per month. On NH3 systems, a sudden pressure spike can crack pressure vessels and cause leaks amoniac. NH3 is a toxic refrigerant, causing respiratory burns at concentrations of just 25 ppm under industrial safety standards.
The 5-step procedure for handling a high discharge pressure fault
The procedure for handling a high discharge pressure fault follows the sequence: protect the compressor first, diagnose second, then intervene on the actual cause. Never reset the HP switch before the cause has been identified, because that puts the machine back into operation in a faulty state and pushes the pressure higher still.
The procedure below is the one Tan Long applies on industrial food and seafood projects. An in-house operating team can carry out up to step 4; step 5 requires a specialist refrigeration engineer.
Step 1: Shut down and read the pressure gauges
Switch off power to the compressor set as soon as the HP switch trips, to protect the motor from restarting in a faulty state. Read the discharge pressure at the moment of the fault and the corresponding condensing temperature, and record them in the operating log. Observe the gauge needle for 60 seconds: a vibrating, fluctuating needle points to non-condensable gases; a needle steady at a high value points to a poorly performing condenser or an overcharge.
Step 2: Check the heat rejection chain
Check whether the condenser fans are running and rotating in the correct direction. Check whether the cooling water pump is operating and whether the pump pressure gauge reads the design value. Check whether the cooling tower water level is sufficient and whether the spray nozzles are blocked. This is the most common cause and the quickest to fix, typically resolving 50 to 60% of the sudden high-pressure cases Tan Long records at seafood plants.
Step 3: Clean the condenser
When the heat rejection side is working normally but the pressure is still high, check the cleanliness of the fins, scale on the copper or steel tubes, and oil settled inside the tubes. Use a neutral alkaline specialist chemical clean for NH3 systems, or a pressure wash with water and a lime-scale removal chemical for Freon systems. Cleaning an evaporative condenser requires draining the old water, scrubbing the fill pack and checking the spray system before refilling.
Step 4: Measure subcooling and adjust the charge
After ruling out heat rejection causes, measure subcooling on the liquid line leaving the condenser. Abnormally high subcooling above 8 to 10 K indicates the system is overcharged; some refrigerant must be recovered with a recovery machine rather than vented to the atmosphere. Normal subcooling with pressure still high points to non-condensable gases; purge through the valve at the top of the condenser following the safety procedure specific to each refrigerant.
Step 5: Check the compressor and return line
If all four causes above have been ruled out and the pressure remains high, check the reed valves, bearings and cylinder head gaskets of the compressor. Check whether the return gas stop valve is fully open, whether there is debris in the filter drier, whether the liquid line filter is blocked, and whether the expansion valve is working correctly. This step requires a specialist refrigeration engineer; Tan Long advises businesses to contact a specialist technical team rather than dismantling the machine themselves, because errors during reassembly can
How high discharge pressure faults can be prevented through scheduled maintenance
High discharge pressure faults are prevented most effectively through three simultaneous practices: cleaning the condenser on schedule, checking the refrigerant charge periodically, and monitoring operating parameters continuously. High discharge pressure is one of the common errors of cold storage that businesses need a firm grasp of the inspection procedure for, in order to stay ahead of it.
Cleaning schedule evaporative condenser for food and seafood refrigeration systems should be set at once every 3 to 6 months; air-cooled condensers once every 6 months. Checking subcooling and superheat monthly helps catch deviations early, before pressure reaches the HP threshold. NH3 systems need a non-condensable gas purge once a quarter, especially after each repair or refrigerant charge.
IoT and SCADA systems monitoring pressure, temperature and current in real time provide an alert before the HP switch trips. PLC control cabinet combined with analogue pressure sensors makes it possible to plot pressure day by day, revealing an upward trend before it becomes an urgent fault.
A cost comparison shows the value of scheduled maintenance. Quarterly maintenance on a 100 HP NH3 system costs roughly 5 to 15 million VND. Replacing refrigeration compressor unit when it fails from sustained high pressure costs 500 million to 1.5 billion VND — 30 to 100 times the cost of prevention.
Frequently asked questions about high discharge pressure faults
Should the HP switch be reset immediately when the machine trips?
No. Resetting the HP switch before the cause has been identified is the most dangerous mistake in handling this fault. The HP switch is the last layer of protection before the compressor suffers mechanical damage. Resetting without addressing the cause puts the machine back into operation in a faulty state; pressure continues to rise and can exceed the high-pressure relief valve threshold. The correct procedure is: shut down, read the gauges, diagnose, fix the root cause, and only then reset.
What is subcooling and how is it measured?
Subcooling is the difference between the saturation temperature of the refrigerant at condensing pressure and the actual temperature of the liquid refrigerant leaving the condenser, expressed in K or °C. Measuring it takes four steps: use a pressure gauge to read the pressure on the liquid line, look up the refrigerant's P-T table to obtain the saturation temperature, measure the actual liquid line temperature with a contact thermometer, and subtract the actual temperature from the saturation value. Standard subcooling is 4 to 7 K for most systems; above 10 K indicates an overcharge; below
How do NH3, R404A and R22 differ when high discharge pressure occurs?
NH3 (R717) has lower operating pressures than the Freons but a higher discharge temperature, which can reach 130 to 150°C when pressure is high. The main risk is a toxic leak causing respiratory burns. R404A and R507A operate at pressures 2 to 3 times higher than NH3; the main risk is motor overload and mechanical wear. R22 is being phased out under the Kigali Amendment; high pressure usually accompanies older systems with degraded condensers and a high NCG proportion from leaking joints. The core handling procedure is the same, but the cleaning chemicals,
What tools are needed to diagnose high discharge pressure in the field?
The basic diagnostic tool kit comprises: a manifold gauge set matched to the refrigerant, a contact thermometer covering -40 to 200°C, a clamp meter for current, a refrigerant recovery machine, an empty refrigerant cylinder with an electronic scale, a vacuum pump, a purge valve set for the condenser, and an operating logbook. This is the minimum configuration for working through step 4 of the procedure; step 5 additionally requires specialist compressor dismantling tools.
When a refrigeration system runs at stable discharge pressure
A refrigeration system runs at stable discharge pressure when the pressure sits within the design range for the refrigerant, subcooling is 4 to 7 K, superheat is 5 to 8 K, compressor current is at 80 to 95% of rated current, and the warehouse reaches setpoint within the design time.
This state rests on three foundations: correct design from the outset, installation to technical standards, and complete scheduled maintenance. These are also the criteria the Tan Long technical team has applied to industrial NH3 systems at seafood and food plants nationwide over the past 20 years, from Frescol Tuna Vietnam in Nghe An to Lotte F&G and CP Vietnam.
See also:
- Cold store repair: how to identify faults and the expert troubleshooting process
- How does the Chiller maintenance process take place?
- How to adjust the cold storage throttle valve accurately and effectively
Frequently asked questions about high discharge pressure faults
What is high discharge pressure in a refrigeration system?
It is the condition where pressure on the compressor discharge side exceeds the design value, causing the high-pressure switch to act and stop the machine to protect the system.
What are the signs of high discharge pressure?
The compressor stops repeatedly because the high-pressure switch acts, the high-side gauge reads above the threshold, the discharge is abnormally hot, compressor current rises and the system fails to reach the required temperature.
What causes high discharge pressure?
The most common are a fouled condenser or insufficient heat rejection flow, a failed heat rejection fan or pump, refrigerant overcharge, non-condensable gases in the system, and excessive ambient temperature.
What happens if high discharge pressure is left unaddressed?
The compressor works overloaded, the high discharge temperature degrades the lubricating oil, gaskets and valves fail, electricity consumption rises sharply and the risk of compressor failure grows over time.
In what order should a high discharge pressure fault be handled?
Shut down safely, check the condenser and heat rejection system first, then check the refrigerant charge and non-condensable gases, check the protection devices and sensors, fix the cause, and only then restart and monitor.
How can high discharge pressure be prevented?
Clean the condenser on schedule, maintain cooling water quality, check fans and pumps periodically, charge the correct refrigerant quantity, purge non-condensable gases when needed, and follow the pressure log to spot abnormal trends.
