What Is Heat?
Heat is a major concern in every single engineering field, where one of the biggest problems is how to move it around efficiently.
That’s where heat exchangers come in.
In this post, I’m going to teach you what you need to know about these important devices.
Before we talk about transferring heat around, let’s talk about what heat is, exactly.
At its most basic, we look at heat as something that affects the temperature of an object.
Heat is the result of energy being transferred from one object to another, where both objects have different temperatures. The laws of thermodynamics dictate that the direction of the flow of heat is from a hotter object to a colder object.
Speaking of hot and cold:
As humans, “hot” and “cold” are rather intuitive terms to us, but they’re not very useful in the greater schemes of heat. They simply refer to what our biology is capable of handling.
Machines that produce heat might regularly be operating at hundreds of degrees Celsius! In that regard, “hot” and “cold” become very relative terms.
How Is Heat Transferred?
Heat moves between systems through three main methods.
In conduction, two objects of different temperatures are in direct contact, and heat flows from the warmer object to the cooler object until both are at the same temperature.
Heat transfer in conduction is greatly affected by the properties of the materials that each object is made of, specifically their conductivity.
Convection involves liquids and gases, which exhibit a unique behavior – warmer areas of fluid rise when heated, and cooler areas sink.
This creates a circulation pattern or current that produces heat transfer.
Radiation is the emission of electromagnetic waves that can transfer heat to other objects.
All objects emit electromagnetic radiation, and the hotter they are, the more radiation they will emit.
If you’ve ever seen an object glow red when heated, that’s a clear example of radiation.
What Are Heat Exchangers?
Heat exchangers are devices that move heat from one area or object to another.
Most heat exchangers make use of convective heat transfer by using fluids such as water, or special coolant liquids and gases.
Heat exchangers can be active, where a pump circulates fluid around a device; or passive, where heat transfer is left to normal convective and heat dissipation processes.
We’ll mostly be looking at active heat exchangers in this post.
Where Are Heat Exchangers Used?
The most visible application of heat exchangers is in the form of cooling very hot equipment, such as that found in:
These systems contain machines, engines, and devices that produce a lot of waste heat.
If that heat has nowhere to go, it’ll continue to build up within the system, and these systems may fail under high-temperature conditions.
As a result, the machines that make up these systems require heat exchangers to remove heat from them and transfer it into the environment.
For example, the internal combustion engines in almost every car in the world make use of engine coolants and radiator heat exchangers, which pump heat generated from within the engine to the outside.
Another common example is found in computer parts.
Components with high power draw, such as the CPU and graphics card, are attached to heat sinks made of metal vanes that allow heat to radiate outwards.
A fan attached to the heat sink blows air through the vanes, carrying the heat away.
But that’s not all:
Heat exchangers are also used to heat things, not just cool them down.
A good example is found in power plants, which contain heat exchangers that recover heat from exhaust gases and put them back into the system.
By reducing this lost heat, more energy is available to produce the steam that drives turbines and generates electricity.
Another example is swimming pool heat exchangers.
In this setup, a solar heat exchanger, or natural gas or electric heater, warms the water and passes it through conductive pipes that run through a swimming pool.
This transfers heat from the source to the pool, making it warmer.
And more than just talking about hot things:
Even refrigerators and air conditioning units make use of heat exchangers!
I’ve described how the flow of heat is usually from a hotter to a colder object.
This implies that you normally can’t reduce the temperature of an object below the ambient temperature.
This is technically true, but through the use of devices called heat pumps, it’s possible to locally transfer heat from one location to another in what seemingly is the opposite direction of heat flow – from a cooler place to a warmer place.
Refrigerators and air conditioners make use of a refrigeration cycle, which is characterized by special refrigerant fluids that can easily vaporize into gas, and transition back to liquid.
When a gas expands, it heats up, and when it compresses, it cools down.
The refrigeration cycle compresses the refrigerant and passes it through a place that needs to be cooled, absorbing the heat in the process, then pushes the refrigerant through a heat exchange coil outside the cooling area, where it expands and transfers the heat away.
Types of Heat Exchangers by Design
We’ve covered what heat is, how heat is transferred, and what heat exchangers are used for.
Now, the next logical question is:
How exactly do heat exchangers work?
Actually, there’s a lot of different types of heat exchangers, each of which has its own use cases.
Shell And Tube Heat Exchangers
Shell and tube heat exchangers are composed, quite appropriately, of a series of metal tubes, and a shell that encloses the tubes.
Both the shell and the tubes have fluid running through them, and the fluid in both vessels may be running in the same direction, opposite directions, or perpendicularly.
One of the most popular designs around, shell and tube heat exchangers are notable for being able to handle high temperature and pressure, making them suitable for power plants, trains, and other industrial applications.
Shell and tube designs are useful for air to liquid heat exchangers, as well as liquid to air heat exchangers.
Plate Heat Exchanger
A plate heat exchanger makes use of numerous thin conductive plates with small areas where coolant fluid can pass through.
This design maximizes the surface area contacting the device to be cooled, and is also very compact compared to other heat exchanger designs.
Plate heat exchangers are commonly found in the food industry because they can easily be dismantled and cleaned.
They’re also often found in residential heat exchangers for home heating.
Plate heat exchangers are somewhat prone to leakage of coolant fluid.
Brazed Plate Heat Exchangers
A variant of plate heat exchangers, brazed plate heat exchangers reduce the possibility of leakage by brazing the plates together.
Plate Fin Heat Exchanger
Plate fin heat exchangers use a series of fins sandwiched between plates.
Cooling fluid runs through the spaces between the plates, maximizing contact surface area.
This design allows plate fin heat exchangers to be very effective and efficient while staying compact and very lightweight compared to other designs.
Due to these characteristics, plate fin heat exchangers can be found in aircraft, as well as low-temperature applications like cryogenics.
Plate fin designs are suitable for air to air heat exchangers, liquid to liquid heat exchangers, air to liquid heat exchangers, and liquid to air heat exchangers, making them very flexible for a variety of applications.
Types of Heat Exchangers by Purpose
We can also categorize heat exchangers by what purpose they fulfill, rather than how they’re designed.
Two of the most important categories to remember are regenerative and recuperative heat exchangers.
A recuperator is designed to capture heat that might be otherwise wasted in a system.
These are positioned at exhaust vents in buildings or industrial processes, to get heat from the warm outgoing exhaust.
Recuperators work by using counterflowing fluid, or heat exchanger fluid traveling in the opposite direction of heat that is exiting the system.
Regenerators are similar to recuperators, but the exhaust and heat exchanger fluid travel through the same channels.
In a regenerator, exhaust travels through the regenerator heat chamber, where some of the exhaust heat is transferred to the chamber.
Later on, cooler fluid is passed through the same chamber, picking up the heat along the way and cycling it back into the system.
Heat Exchanger Flow
You may have noticed that we frequently mention the direction of fluid flow when discussing how heat exchangers work.
You might, therefore, be asking:
Does the direction of flow have any impact on how a heat exchanger works?
The answer is a resounding yes! Fluid flow direction definitely affects the efficiency of heat transfer.
Counterflow Heat Exchangers
In a counterflow heat exchanger, the two working fluids are pumped in opposite directions to each other.
This is the most efficient heat exchanger design and allows for the largest temperature change in fluid.
Parallel Heat Exchangers
In a parallel heat exchanger, the two working fluids are pumped in the same direction.
While this design is less effective at exchanging heat between the two fluids, it’s useful if both fluids must be equalized in terms of temperature.
Cross Flow Heat Exchanger
A cross flow heat exchanger has the fluids meeting each other at right angles.
Such heat exchangers are mostly used for liquid to air heat exchangers, like in car radiators.
Hybrid Flow Heat Exchanger
Many heat exchanger designs have tubes or channels where working fluids meet each other several times over several passes.
This results in fluid flow that can be cross flow and counterflow at different times.
How Long Do Heat Exchangers Last?
Heat exchangers are naturally under constant environmental stress, due to pressure and heat passing through them all the time.
This raises an important question:
How long do they last?
A good heat exchanger can last decades with the right maintenance and environments.
However, the operative phrase here is “right maintenance!” Many factors can lead to the failure of a heat exchanger.
For example, if a heat exchanger isn’t cleaned properly, impurities can accumulate within the system, affecting efficiency and even clogging pipes and channels. This is known as fouling.
Different kinds of fouling may occur depending on the situation.
A heat exchanger system that has a freezing component may have frozen impurities in parts of the piping.
Another system located near the coast can experience corrosion due to seawater.
There’s also biofouling, which is the accumulation of microorganisms, some of which can even corrode the inside of heat exchangers.
And aside from fouling, if you’re running a heat exchanger beyond its specifications at any point, whether it be fluid pressure flow or operating temperature, you’re reducing its lifespan no matter what!
Thankfully, various maintenance methods are available to many users.
You can use high-pressure water jets to clean fluid channels of any debris and impurities, safely and quickly.
Some heat exchanger types, like plate heat exchangers, can be easily dismantled for quick cleaning.
There are even robotic cleaning tools available for heat exchangers which can inspect and clean as needed.
And finally, some heat exchangers are even self-cleaning to some degree as long as the proper specifications are met, reducing the need for maintenance!
Now It's Your Turn
There’s no such thing as a free lunch, that’s for sure.
Thermodynamics ensures that heat is a constant presence in any kind of work.
But we can get the best lunch possible by having a working knowledge of managing heat and choosing the right heat exchangers for the job.
Now that you’re aware of the various kinds of heat exchangers that are in use daily, try to identify some of them at home.
Imagine a scenario where they’re broken or dysfunctional.
How strongly affected will your life be, by the loss of heat exchanger functionality?
Sound off in the comments and show your appreciation for this critical engineering marvel!