An increasing pressure drop across a heat exchanger can be an early indication that the system is no longer operating under its original conditions. Fouling is a common cause, but it is certainly not the only one. A higher flow rate, changes in fluid properties, a blocked strainer, incorrect assembly or unsuitable sizing can also increase the pressure drop.
Pressure drop refers to the decrease in pressure as a fluid passes through the plates, tubes, connections and flow channels of a heat exchanger.
Identifying the cause of a high pressure drop therefore requires more than inspecting the heat exchanger alone. The development of the pressure difference, the operating conditions and the performance of the entire system provide the most reliable overall picture.
What does pressure drop across a heat exchanger mean?
Pressure drop is the difference between the pressure at the inlet and the pressure at the outlet of a heat exchanger. A certain amount of pressure drop is normal and unavoidable. As soon as a liquid or gas flows through pipes, a plate pack or a tube bundle, it encounters resistance.
The measured pressure difference is also referred to as differential pressure or ΔP in technical documentation. Although pressure drop and differential pressure are often used interchangeably, they do not mean exactly the same thing. Pressure drop describes the reduction in pressure through the heat exchanger, while differential pressure is the measured difference between two measuring points.
The normal pressure drop depends on factors including:
- the flow rate;
- the temperature and viscosity of the fluid;
- the dimensions of the channels or tubes;
- the length of the flow path;
- the size of the connections;
- the plate geometry;
- the distribution of the fluid across the channels.
A narrow or heavily corrugated channel generally creates more resistance, but it may also generate greater turbulence and improve heat transfer. Pressure drop is therefore always part of the technical balance when selecting or designing a heat exchanger.
A high pressure drop does not automatically indicate a fault. It becomes a concern when the value differs from the design value or from previous measurements taken under comparable conditions. In particular, watch for an increasing pressure difference, reduced flow, declining cooling capacity or additional pump power being required to maintain the same flow rate.
Causes of pressure drop in a heat exchanger
1. Fouling and deposits
The accumulation of deposits inside a heat exchanger is commonly known as heat exchanger fouling. Limescale, salt deposits, sludge, sand, corrosion products, oil, grease and other process residues can build up on the heat-transfer surfaces.
These deposits narrow the plate channels or tubes. This reduces the available flow area and requires a greater pressure difference to maintain the same flow rate. At the same time, the fouling layer creates additional resistance to heat transfer. A fouled heat exchanger can therefore perform poorly both hydraulically and thermally.
Several types of contamination may be present at the same time. A deposit may, for example, consist of limescale, sludge and corrosion products. In marine systems, biological growth may also be present. The composition of the fouling helps determine which cleaning method is suitable.
2. Partial or sudden blockage
A sudden increase in pressure drop is more likely to indicate an acute blockage than a deposit that has gradually built up over time. Possible causes include:
- pieces of rust;
- gasket fragments;
- welding debris;
- fibres;
- pieces of wood or vegetation;
- shell fragments;
- process waste;
- materials left behind following maintenance.
In a plate heat exchanger, the free channel openings can be relatively narrow. One larger object may therefore partially block several flow channels.
When a sudden change occurs, do not inspect the heat exchanger alone. Also check the filters, strainers and pipe sections upstream of the unit. A blocked strainer can cause symptoms similar to those of a blocked heat exchanger.
3. A higher flow rate
When more fluid passes through the same heat exchanger, the flow resistance increases. A high pressure drop may therefore result from a changed pump setting, a different control strategy or a change in the distribution between heat exchangers operating in parallel.
Pressure measurements should only be compared when the flow rate and other operating conditions are approximately the same. Without a flow measurement, it can be difficult to determine whether the higher pressure drop is caused by fouling or by a greater system load.
A larger pump does not automatically improve performance. If the heat exchanger was not designed for the higher flow rate, the additional flow can cause a substantial increase in pressure drop and unnecessary energy consumption.
How can we help YOU?
Briefly describe your situation, and we will contact you soon.
4. Prolonged operation at a low flow rate
A low flow rate generally creates a lower pressure drop at that particular moment. However, prolonged operation at an insufficient flow velocity can accelerate fouling.
As the flow velocity decreases, turbulence also decreases. Solid particles are then less likely to remain suspended and can settle more easily. Poorly flushed areas may also develop, allowing sludge, process residues or biological contamination to accumulate.
This may occur in situations involving:
- prolonged operation at partial load;
- oversized heat exchangers;
- incorrectly balanced systems;
- parallel heat exchangers with uneven flow distribution;
- processes that frequently start and stop.
A low pressure drop today can therefore contribute to a higher pressure drop at a later stage.
5. Changes in the fluid
Flow resistance is not determined solely by the heat exchanger. The properties of the fluid also play an important role.
Higher viscosity generally creates more resistance. Oil or a glycol mixture may, for example, become more viscous at a lower temperature. This can increase the pressure drop even when the heat exchanger itself is clean.
Other relevant changes include:
- a higher concentration;
- a change in composition;
- a higher solids content;
- a change in process pressure;
- a different pH value;
- chemical reactions or precipitation;
- switching to a different process fluid.
Substances that were previously dissolved may precipitate as the temperature, pressure or concentration changes. A change in the process can therefore eventually cause actual fouling as well.
6. Design and sizing
If the pressure drop has been high since commissioning, the cause may be related to the design or sizing of the heat exchanger. Relevant factors include:
- connections that are too small;
- narrow flow channels;
- too few parallel channels;
- a long flow path;
- a plate pattern with relatively high flow resistance;
- operation outside the original design conditions.
A high pressure drop does not immediately mean that the design is incorrect. Heat exchanger design involves balancing heat transfer, pumping power, unit size and fouling sensitivity.
Problems may arise when operating conditions change at a later stage. A heat exchanger that was originally sized correctly may fall outside its hydraulic design conditions following a production expansion, process change or adjustment to the pump settings.
7. Assembly and maintenance errors
If the pressure drop changes immediately after maintenance, check the assembly and the positions of the valves.
In a gasketed plate heat exchanger, an incorrect plate sequence, altered flow arrangement or incorrect tightening dimension can affect the distribution of the fluid.
Other possible causes include:
- contamination left inside the heat exchanger;
- incorrectly positioned gaskets;
- deformed gaskets obstructing an opening;
- partially closed valves;
- incorrect pipe connections;
- incorrectly reinstalled components;
- a plate pack that was not fully cleaned.
A damaged gasket primarily causes a sealing problem. It will only restrict the flow if it is incorrectly positioned or deformed and partially obstructs a port or channel.
8. Uneven flow distribution
When several heat exchangers operate in parallel, the flow is not always distributed evenly. Differences in pipe resistance, valve positions, control settings or fouling levels may cause one heat exchanger to receive more fluid than another.
The most heavily loaded heat exchanger may consequently develop a high pressure drop. The heat exchanger receiving the lowest flow may become more susceptible to settling and fouling because of the reduced flow velocity.
Correct balancing and separate measurements for each heat exchanger are therefore important.
Pressure drop in marine heat exchangers
In marine heat exchangers, the seawater side requires particular attention. Seawater can carry sand, sludge, shell fragments, plant material and other debris into the cooling system.
Biofouling, or biological growth, can also develop due to bacteria, algae, mussels, barnacles and other organisms. This growth can narrow the flow channels and make the surfaces rougher, increasing flow resistance.
These issues can occur in:
- plate coolers;
- oil coolers;
- engine coolers;
- central coolers;
- condensers;
- seawater coolers.
Corrosion products originating from the pipework may also enter the heat exchanger.
However, the source of the pressure loss is not always the heat exchanger itself. The sea chest, suction screens, strainers, pumps, valves and pipes are all part of the seawater circuit. A contaminated sea chest or blocked strainer can restrict the available flow before the water even reaches the heat exchanger.
Where possible, measure the pressure directly upstream and downstream of the heat exchanger. The entire route from the seawater inlet should also be inspected. This prevents the unnecessary dismantling of a heat exchanger when the actual blockage is located elsewhere in the system.
Pressure drop in industrial heat exchangers
In industrial installations, the nature of the fouling depends heavily on the process and the fluid being used.
Cooling-water systems may be affected by limescale, sludge, corrosion products and biological growth. In oil and petrochemical processes, hydrocarbons, sediment and chemical reaction products can form deposits.
Other examples include:
- fats, proteins and product residues in the food industry;
- fibres and organic matter in wastewater;
- sand and suspended solids in surface water;
- corrosion products in closed-water systems;
- mineral precipitation in chemical processes;
- oil deposits in lubrication and hydraulic systems.
There is therefore no single cleaning method suitable for every industrial heat exchanger. The correct approach depends on the type of heat exchanger, its materials, the nature of the contamination and the accessibility of its components.
What can the development of the pressure drop reveal?
| Observed development | Possible direction for investigation |
|---|---|
| Pressure drop increases gradually | Fouling, mineral deposits, sludge, corrosion products or biofouling |
| Pressure drop increases suddenly | Loose debris, an acute blockage or a blocked strainer |
| Pressure drop changes immediately after maintenance | Assembly, plate sequence, valve position or material left behind |
| Pressure drop varies with production | Changes in flow rate, temperature, viscosity or process load |
| Parallel heat exchangers show major differences | Uneven flow distribution or different levels of fouling |
| Pressure drop has been high since commissioning | Sizing, connection dimensions, plate selection or design conditions |
Use these signals as a starting point, not as a final diagnosis. Always assess the pressure difference together with:
- the current flow rate;
- the inlet and outlet pressures;
- the inlet and outlet temperatures;
- the cooling or heating capacity;
- the pump settings;
- the energy consumption;
- the fouling and maintenance history.
Also verify that the pressure gauges are reliable and installed at the correct measuring points.
Consequences of excessive pressure drop
Increasing flow resistance can restrict the amount of fluid passing through the heat exchanger. As a result, the unit may transfer less heat.
Possible consequences include:
- reduced cooling or heating capacity;
- higher energy consumption;
- a greater load on pumps;
- increasing process or engine temperatures;
- unstable operating conditions;
- higher operating costs;
- accelerated wear;
- production losses;
- unplanned downtime;
- damage to system components.
Thermal performance loss and pressure drop do not always develop at the same rate. A thin insulating layer of contamination may already reduce heat transfer without causing a significant increase in differential pressure. Monitoring pressure drop alone is therefore not sufficient.
Investigate first, then clean effectively
Cleaning should not automatically be the first response to a high pressure drop. Begin by checking:
- whether the flow rate or pump setting has changed;
- whether the temperature, composition and viscosity are unchanged;
- whether the valves are fully open;
- whether the filters, strainers and pipes are clear;
- whether the measuring instruments are functioning correctly;
- whether the heat exchanger has been assembled correctly;
- whether the installation is operating within its design conditions.
If fouling is identified as the cause, the heat exchanger may be cleaned chemically, ultrasonically or mechanically, depending on the situation.
During Cleaning in Place, commonly abbreviated to CIP, a cleaning fluid is circulated through the heat exchanger without fully dismantling it. This method is not suitable for every type of contamination or heat exchanger construction. Stubborn, mixed or inaccessible deposits may require dismantling and separate cleaning of the individual components.
In gasketed plate heat exchangers, the plates, gaskets and flow arrangement can be inspected after dismantling. Damaged components should be repaired or replaced before the heat exchanger is correctly reassembled and tested.
Regularly recording pressure, flow rate and temperatures makes trends visible. This allows maintenance to be scheduled before the capacity falls too far.
Heat exchanger inspection, cleaning and overhaul by Blue Orange
Blue Orange maintains industrial and marine heat exchangers. Depending on the situation, heat exchangers can be inspected, dismantled, cleaned, repaired, reassembled and tested.
The choice between chemical, ultrasonic and mechanical cleaning is based on:
- the type of contamination;
- the heat exchanger material;
- the construction;
- the operating conditions;
- the condition of the components.
If cleaning alone is not sufficient, the heat exchanger may require an overhaul. Plates, tubes, bundles and gaskets are then inspected and repaired or replaced where necessary.
By first investigating the cause of the pressure drop, it is possible to determine more accurately which work is actually required.
Conclusion
Pressure drop across a heat exchanger is not, by itself, proof of fouling. An abnormal pressure drop may also be caused by a higher flow rate, changes in fluid properties, a blocked strainer, incorrect assembly, uneven flow distribution or altered operating conditions.
The most reliable diagnosis is achieved by assessing hydraulic and thermal data together. Do not look at ΔP alone; also consider the flow rate, temperatures, capacity and the surrounding system.
Are you experiencing an increasing pressure drop, reduced flow or declining cooling capacity? Blue Orange can inspect, clean, test and overhaul your heat exchanger. By identifying the cause first, the appropriate treatment can be selected more effectively.
Biofouling cannot be completely prevented in marine environments. Seawater naturally contains microorganisms that attach to wet surfaces and gradually form biofilms. However, regular inspections, preventive maintenance and timely cleaning can significantly reduce the build-up of biofouling and help maintain the thermal efficiency of a marine heat exchanger.
Biofouling mainly affects heat exchangers that operate with seawater, either directly or indirectly. This includes plate heat exchangers, shell & tube heat exchangers, Jacket Water Coolers, Lube Oil Coolers and heat exchangers used in Central Cooling Systems. The rate of fouling depends on factors such as water quality, operating conditions and maintenance intervals.
The cost of heat exchanger refurbishment depends on several factors, including the type and size of the heat exchanger, the extent of fouling or damage, and whether components such as plates or gaskets need to be replaced. In many cases, refurbishment is a more cost-effective alternative to replacing the entire heat exchanger. Following an inspection, the required work can be assessed and an accurate quotation can be provided based on the actual condition of the unit.
Yes. As biofouling reduces heat transfer, cooling systems become less efficient. Pumps may have to work harder, cooling capacity decreases and engines can operate at higher temperatures. Depending on the system and operating conditions, this can lead to increased energy use and higher fuel consumption.
If left untreated, biofouling can increase the risk of localized corrosion beneath the biofilm. Over time, this may damage plates, tubes or gaskets, eventually leading to leaks or reduced reliability. Timely inspection and cleaning help minimise the risk of permanent damage and extend the service life of the heat exchanger.
Not every type of fouling requires the same treatment. An inspection helps identify the nature and severity of the contamination, allowing the most appropriate cleaning method to be selected. This ensures effective cleaning while avoiding unnecessary wear or damage to the heat exchanger.
Need Help with Heat Exchanger Refurbishment?
Not sure whether your heat exchanger requires refurbishment? Or would you rather address a developing issue before it leads to unplanned downtime?
At Blue Orange, we combine practical experience with technical expertise. We provide inspections, technical advice and refurbishment solutions tailored to marine heat exchangers, your installation and your maintenance schedule. No one-size-fits-all approach—just clear recommendations based on the actual condition of your equipment.
Feel free to contact us to discuss the condition of your heat exchanger. Together, we’ll determine the most suitable solution to keep your cooling system operating reliably—both now and in the long term.