What is a Heat Exchanger

Want to learn more about heat exchangers? What they are used for and how they work? This FAQ page is a good introduction to the subject.


What is a Heat Exchanger?

Frequently Asked Questions Index;


  1. What is a Heat Exchanger
  2. What types of Heat Exchanger are available?

Designing a Heat Exchanger

  1. How is a Heat Exchanger designed?
  2. Marine Heat Exchangers
  3. What fluids can a Heat Exchanger operate with?
  4. What is Temperature Cross Over?
  5. What does a Heat Exchanger “Pass” refer to and how many do I need?
  6. How to make a Heat Exchanger design more efficient
  7. Which industry specific Heat Exchangers can Thermex provide?


  1. How to Install a Heat Exchanger
  2. How to increase the lifetime of a Heat Exchanger
  3. What causes a Heat Exchanger to fail in operation?

Heat Exchanger Terminology




1. What is a Heat Exchanger?

Put simply, a heat exchanger is a device which transfers heat from one medium to another, a Hydraulic Oil Cooler or example will remove heat from hot oil by using cold water or air. Alternatively a Swimming Pool Heat Exchanger uses hot water from a boiler or solar heated water circuit to heat the pool water. Heat is transferred by conduction through the exchanger materials which separate the mediums being used. A shell and tube heat exchanger passes fluids through and over tubes, where as an air cooled heat exchanger passes cool air through a core of fins to cool a liquid.


2. What types of Heat Exchanger are available?

There are many different types of heat exchanger available, the three main types that Thermex supplies are;

Shell and Tube Heat Exchanger

Shell and Tube; Shell and Tube Heat Exchangers consist of a large number of small tubes which are located within a cylindrical shell. The tubes are positioned into the cylinder using a tube bundle or "tube stack" which can either have fixed tube plates (permanently fixed to the body) or, in the case of Thermex Heat Exchangers a floating tube stack which allows the tube bundle to expand and contract with varying heat conditions as well as allowing the tube bundle to be easily removed for servicing and maintenance.

Plate Heat Exchanger

Plate Type; Plate Heat Exchangers operate in very much the same way as a shell and tube heat exchanger, using a series of stacked plates rather than tubes. Plate heat exchangers are usually brazed or gasketed depending on the application and fluids being used. Their compact stainless steel construction makes them an ideal choice for use with refrigerants or in food and beverage processing.

Air Cooled Heat Exchanger

Air Cooled; Air Cooled Heat Exchangers are commonly used in vehicles or other mobile applications where no permanent cool water source is available. Thermex designs and supplies combination cooling packs (or combi-coolers) which combine an engine jacket water cooler, oil cooler and charge air cooler into a single unit reducing space requirements and improving efficiency. Cool air is provided either by a fan or by air flow caused by the movement of the vehicle.


Designing a Heat Exchanger

1. How is a Heat Exchanger designed?

To be able to select a heat exchanger, we need to know;

  • Primary circuit fluid type, temperature and flow rate (usually the hot fluid)
  • What you want to take out of the primary circuit (Heat dissipation or a target outlet temperature)
  • Secondary circuit fluid type, temperature and flow rate (usually the coolant)

The fields above are only the basics. When putting an enquiry together you should also make Thermex aware of any pressure loss limitations and any other special requirements.

Please click here to download the Thermex data sheet template which highlights the required fields for heat exchanger selection. The data sheet should also be saved and sent to us if you require a quotation for a heat exchanger.

Please feel free to contact us if you need assistance with the form.


2. Marine Heat Exchangers

The operating principles of a marine heat exchanger are the same as a cooler designed for fresh water use, the main consideration for the designer however is that the marine heat exchanger must be resilient to erosion or corrosion caused by sea water. This means that materials that come in to contact with the sea water must be suitable, such as 90/10 Cupro-Nickel, 70/30 Cupro-Nickel, Bronze and Titanium.

There are other factors which need to be taken in to consideration when a marine heat exchanger is being designed. One is the velocity, if it is too low then there is a risk that sand and other particles will block the tubes. If it is too fast on the other hand then those same particles can rapidly erode the tube plate and tubes.

Additional protection can be provided by installing a sacrificial anode which Thermex can include upon request. This will be installed in to the threaded hole normally used for a drain plug and is in direct contact with the sea water flow.


3. What fluids can a Heat Exchanger operate with?

The suitability of a fluid with a heat exchanger will depend on the type of heat exchanger being used and the materials which are available. Standard Thermex Heat Exchangers are suitable for most fluids including Oil, Water, Water Glycol and Sea Water. For more corrosive fluids such as chlorinated salt water, refrigerants and acids other materials such as Stainless Steel and Titanium will need to be used instead.


4. What is temperature Cross Over?

Temperature cross over is a term used to describe the scenario where the temperatures of both circuits in a liquid cooled heat exchanger begin to cross over. This can be an important factor in a heat exchanger design as the efficiency of a cooler will be significantly reduced when the temperatures cross over. In many cases a plate heat exchanger is the best option for applications where temperature cross over can't be avoided.

  Oil Water
Inlet Temperature 80  30
Outlet Temperature 50   51.5
Flow Rate  25 L/min 15 L/min 

The table above demonstrates that the cooling water outlet temperature is slightly higher than the outlet temperature of the oil. One simple way to combat this and increase the efficiency of the oil cooler is to increase the flow rate of the coolant. In this particular example increasing the water flow rate to 25 L/min would reduce the water outlet temperature to 43°C


5. What is a heat exchanger "pass" and how do I know how many passes I need?

A Heat Exchanger Pass refers to the movement of a fluid from one end of the heat exchanger to the other. For example, when referring to the "through tubes" circuit (usually the coolant); · Single Pass – Fluid enters one end of the heat exchanger, and exits at the other end. · Double Pass – Fluid enters and exists the heat exchanger at the same end. · Triple Pass – Fluid travels the length of the heat exchanger body three times before exiting. The images below will help to demonstrate this;

Single Pass (1 pass)

Diagram of a 1 pass oil cooler

Double Pass (2 pass)

Diagram of 2 pass oil cooler

Triple Pass (3 pass)
Diagram of a 3-pass oil cooler


  • A greater number of passes increases the amount of heat transfer available, but can also lead to high pressure loss and high velocity.
  • With a full set of operational data, Thermex can select the most efficient heat exchanger possible whilst working within the pressure loss and velocity limits.
  • The number of passes on the primary circuit can also be adjusted to optimise thermal performance and efficiency by changing the baffle quantity and pitch.


6. How to make a heat exchanger more efficient.

Heat exchanger efficiency can be defined in many ways, in terms of thermal performance there are several key factors to consider;

Temperature differential - As discussed in point 3 (temperature cross-over) the difference between the hot fluid and coolant is very important when designing a heat exchanger. The coolant always needs to be at a lower temperature than the hot fluid. Lower coolant temperatures will take more heat out of the hot fluid than warmer coolant temperatures. If you had a glass of drinking water at room temperature for example, it is much more effective to cool it down using ice rather than just cool water, the same principle applies to heat exchangers.

Flow rate - Another important factor is the flows of the fluids in both the primary and the secondary side of the heat exchanger. A greater flow rate will increase the capability of the exchanger to transfer the heat, but a greater flow rate also means greater mass, which can make it more difficult for the energy to be removed as well as increasing velocity and pressure loss.

Installation - The heat exchanger should always be installed based on a manufacturers' guidelines. Generally speaking the most efficient way to install a heat exchanger is with the fluids flowing in a counter-current arrangement (so if the coolant is travelling left to right, the hot fluid travels right to left) and for shell and tube heat exchangers the coolant should enter at the lowest inlet position (as shown in the diagrams above) to ensure that the heat exchanger is always full of water. For air cooled heat exchangers it is important to consider the air flow when installing a cooler, any part of the core which is blocked will compromise cooling capacity. To find out more about counter-flow and why it is more efficient, please read our blog post "Why counter flow heat exchangers are more efficient"


7. Which industry-specific Heat Exchangers can Thermex provide?

Sometimes, a specific heat exchanger type can be selected based on the industry it will be used for. Some examples include;

Hydraulics and Industrial Process
Food Process Equipment
Mobile Plant
Power Generation
Spas and Swimming Pools



Commissioning and Installation

1. How to install a Heat Exchanger

The correct method of heat exchanger installation varies depending upon the exact type of unit and the environment of operation. For installations instructions please refer to one of our Installation and Operation Manuals


2. How to increase the lifetime of a Heat Exchanger

Heat exchangers are manufactured from robust materials, have no moving parts and operate at a variety of different pressures and temperatures, therefore if a heat exchanger is used in the correct way then there is no reason why it shouldn't be able to remain operational for many years. To help increase the operational lifetime of a heat exchanger there are several steps that should be taken;

Make sure the design data is accurate - If you are sending data to our engineers for heat exchanger selection, then it is best to make sure that it is as accurate as possible. Not only will this ensure that your heat exchanger is thermally efficient but also that it will be able to operate for a long period of time. If the flow rates are too high then erosion could be a problem, if the pressures are too high then leaks could occur and if there are any unusual chemicals in the fluids (such as acids in coolant water) then please contact us to check the compatibility. If our standard materials aren't suitable then we can usually supply an alternative which is.

Commission and Installation - When installing your heat exchanger, make sure that correct fittings and pipe work are used. If unsure, please check the Thermex Installation and Operation Manual for details. For heat exchangers on applications which use polluted, shallow or brackish sea water as the coolant and have copper alloy tubes, it is recommended to run the heat exchanger in clean sea water for several weeks first, this creates a protective layer over the tube material which helps to protect against corrosion. More information regarding this subject can be found on Where possible, filters should also be used where fluids may contain solid particles to prevent tube erosion from occurring.

Regular Maintenance and Servicing - All Thermex shell and tube heat exchangers are designed to allow for easy maintenance and servicing. The end caps can be removed allowing the internal tube bundle to be removed for cleaning. New seal kits can also be provided from stock and are available to purchase on our Web Shop. For information and tips on servicing your heat exchanger, please see the Thermex Installation and Operation Manual.


3. What causes a heat exchanger to fail in operation?

There are many possible causes for heat exchanger failure, some examples for shell and tube include;

Symptom Possible Cause Possible Solution Prevention
Oil and Water Mixing O Ring failure
Normally a seal kit is all that is required to solve the problem, but temperatures should be checked to make sure that the seals purchased are suitable.

Before purchasing an Oil Cooler, make sure that Thermex are aware of the pressures and temperatures being used.
Split Tube
Erosion on the tube plate face  Water velocity is too high Check water flow rate and advise Thermex. In some cases a higher grade of material such as 70/30 CuNi or Titanium may be required. When using sea water, make sure that the flow rate doesn't exceed the Heat Exchanger's capabilities. Contact us for advice.
Header / End cap split Frozen water expanding within the heat exchanger / Oil Cooler. New header kit and Seal Kit will be required. Drain the water system during the winter if the cooler isn't being used.
Insufficient thermal capacity, or reducing performance.

Operating conditions not matching design conditions Review design conditions and operating conditions with Thermex engineers, a new tube stack or pump may be required. Ensure that operational data provided to Thermex is as accurate as possible.
Sediments, dirt or other blockage inside the heat exchanger Remove headers to check tubes, clean with a suitable fluid or pipe cleaner. Use a filtration system to prevent blockages within the tubes, or increase cooling water flow rate.
Too many plugged tubes New tube stack will be required. Avoid taking action which will block tubes or reduce the number of them in operation.



Heat Exchanger Abbreviations and Terminology

Below are some abbreviations and terms often used in the heat exchanger industry;

  • ETD Extreme Temperature Differential
  • EGC Exhaust Gas Cooler
  • MGO Marine Gas Oil Cooler
  • HTHE Header Tank Heat Exchanger
  • CAC Charge Air Cooler
  • ACOC Air Cooled Oil Cooler
  • PHE / PHEX Plate Heat Exchanger
  • Shell Side Flow/Connections - Shell Side refers to the flow of a fluid, such as oil, over the external surface of the tube stack
  • Tube Side Flow/Connection - Tube Side refers to the flow of fluid through the tubes of the tube stack, usually the coolant Pressure Loss / Drop The level of back pressure generated by pumping the fluids through the heat exchanger circuits 

Product Categories

Shell and Tube Heat Exchangers

Shell and Tube Heat Exchangers

Shell and tube heat exchangers are our most popular and adaptable product range, suitable for Marine, Hydraulic and Industrial applications.

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Finned Tube Heat Exchangers

Thermex Finned Heat Exchangers

High performance finned tube stack heat exchangers, manufactured by Thermex

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Air Cooled Heat Exchangers

Air Cooled Heat Exchangers

Heavy duty Air Cooled Heat Exchangers for use with Agricultural, Construction and other Off-Highway machinery.

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