thermal efficiency equation

What is thermal efficiency? A conversation with a Sterling TT expert

Thermal efficiency is a concept that often floats around in discussions about heat exchangers and energy systems. However, its true meaning and significance can be elusive, especially for those who aren’t steeped in technical jargon.

In this enlightening conversation with Nic Zeoli, Director of Engineering and a seasoned mechanical engineer at Sterling TT, we aim to provide a clear and insightful perspective on thermal efficiency, exploring what it means and how it impacts the design of heat exchangers.

Nic Zeoli, Director of Engineering at Sterling Thermal Technology
Nic Zeoli, Director of Engineering at Sterling Thermal Technology

Introduction to thermal efficiency for heat exchangers

Before we get into the talk with Nic, let’s review what we mean by thermal efficiency when it comes to heat exchangers.

Nic emphasised that efficiency is often a measure of what you achieve compared to what could theoretically be achieved in an ideal world with no losses.

In simpler terms, if you use 8 litres of a resource out of 10 available, your efficiency is 80%, with the other 2 litres being wasted.

“Efficiency always measures something that happened against what would occur in an ideal world when there are no losses.”

A heat exchanger transfers heat from one fluid to another, heating the colder fluid. Thermal efficiency tells us how well the heat exchanger achieves this goal. You can think of it as the performance of the heat exchanger.

Thermal efficiency can be measured by comparing the actual heat transferred, usually measured in joules or British thermal units, to the theoretical “ideal” heat transfer:

E = Q / Qmax

Thermal Efficiency (%) = (Actual Heat Transfer / Maximum Possible Heat Transfer) x 100

An essential equation for calculating the actual and maximum heat transfer to a body (excluding phase change) is:

Q = m Cp dT

Heat Transfer = Mass Flow Rate x Specific Heat Capacity x Temperature Change

This takes into account the amount of fluid that is moving (mass flow rate), the thermodynamic properties of the fluid (specific heat capacity) and the change in temperature of the fluid to tell us the heat transfer rate.

As we look at more complex heat exchanger configurations, we need other equations to calculate actual heat transfer and maximum heat transfer, and to optimise the various parameters of the heat exchanger. Two industry-standard methods are LMTD (Logarithmic Mean Temperature Difference) and NTU (Number of Transfer Units), though these calculations are outside the scope of this article. Consider visiting these pages for more information:

Arithmetic and Logarithmic Mean Temperature Difference The Engineering Toolbox

E-NTU Heat Transfer MathWorks

How important is thermal efficiency?

Nic expressed some scepticism about the term “efficiency,” especially in the context of heat exchangers, where he finds it less meaningful. The fundamental question is: “What is the ideal heat exchanger to compare with?”

He argued that while thermal efficiency might make sense in some processes, he stressed that the primary goal for Sterling TT is to create the most optimised, cost-effective, and environmentally friendly heat exchangers possible, and that can’t be achieved only by thinking about efficiency.

“The efficiency equations are the same for everybody. Those are a part of the physics of things. And, at Sterling TT, we use those equations to make an optimised heat exchanger.”

Nic’s insights emphasised the importance of considering real-world factors in heat exchanger design. He highlighted that software tools like Aspen and HTRI assist in the design process but only account for some of the nuances and experience required. For instance, deciding which fluid to pass through the tubes and which to keep on the shell side can significantly impact the longevity and efficiency of a heat exchanger.

Nic explained that, as well as thermal efficiency, it was crucial at Sterling TT to maximise the usage of materials and minimise resistance. By optimising tube wall thickness and other design factors, we aim to reduce both the cost and environmental impact of our heat exchange solutions.

This decision-making process relies heavily on the expertise of Sterling TT’s engineers.


On the surface, thermal efficiency seems to be the most important factor when it comes to heat exchangers, but Nic gives a subtly different perspective.

Instead of focusing solely on efficiency metrics, Sterling TT takes a holistic approach, striving to create customised, optimised, and environmentally conscious heat exchangers. By understanding the intricacies of real-world applications and harnessing their wealth of experience, Sterling TT continues to be a trusted partner in the world of heat exchanger design and manufacturing.

Nic’s approach is a refreshing take on the subject and helps explain how Sterling TT has kept at the top of its industry.

Learn more on Sterling TT’s heat exchanger conceptual design page, or contact us today.

Do you need our expertise? Contact us

    This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.