Heat Exchanger Simulator: Parallel-Flow vs Counter-Flow (LMTD)
Run a hot and a cold fluid stream through a simple exchanger in parallel-flow or counter-flow, set the inlet temperatures, flow rates and heat transfer area, and compare the outlet temperatures, LMTD and effectiveness live.
Toggle between parallel-flow (both streams enter the same end) and counter-flow (streams enter opposite ends). Change the inlet temperatures, flow rates and heat transfer area, and watch the temperature profiles and outlet temperatures update live.
About the Heat Exchanger Simulator: Parallel-Flow vs Counter-Flow (LMTD)
Free heat exchanger simulator: parallel-flow vs counter-flow (lmtd). Run a hot and a cold fluid stream through a simple exchanger in parallel-flow or counter-flow, set the inlet temperatures, flow rates and heat transfer area, and compare the outlet temperatures, LMTD and effectiveness live. Drag, change the sliders and see the result live. No sign-up, works on phone and computer. Built for engineering, the heat exchanger simulator: parallel-flow vs counter-flow (lmtd) runs instantly in your browser: change a setting or drag an object and the result updates at once, so you learn by trying things out rather than only reading about them.
Run a hot and a cold fluid stream through a simple exchanger in parallel-flow or counter-flow, set the inlet temperatures, flow rates and heat transfer area, and compare the outlet temperatures, LMTD and effectiveness live. Use it to explore engineering ideas at your own pace, then check what you found against the key ideas further down this page.
How to use the Heat Exchanger Simulator: Parallel-Flow vs Counter-Flow (LMTD)
- Use the controls to change Flow arrangement, Hot inlet temperature, Hot mass flow rate, Hot fluid specific heat, Cold inlet temperature, and more. The simulation reacts instantly.
- Pick an option such as Counter-flow, Parallel-flow to switch modes or load an example.
- Press "Reset" to start, reset or change what is happening.
- Where you see a glowing handle, object, weight or atom, drag it with your mouse or finger. Everything responds in real time.
- Watch the readouts and graphs update as you experiment, and compare what you see with the key ideas below.
Things to try
- Keep every input the same and just toggle between parallel-flow and counter-flow, and watch Q, LMTD and the outlet temperatures all change.
- Make the cold flow rate much larger than the hot flow rate so C_min is the hot stream, then swap it around, and see how the effectiveness definition still makes sense either way, checking the 'Limiting stream' readout as you go.
- Increase the area A a lot in counter-flow until the outlet temperatures start to approach each other closely, then try the same area in parallel-flow and compare how close they get.
- Lower U and A until NTU is small, and check that parallel-flow and counter-flow give nearly the same LMTD and Q in that limit.
- Watch the operating point on the effectiveness-vs-NTU chart as you increase the area A: see how it climbs quickly at first, then flattens out even though you keep adding area.
Key ideas you can learn
- A heat exchanger transfers heat between two fluid streams without mixing them, governed by the energy balance Q = m_hot·cp_hot·(Th,in − Th,out) = m_cold·cp_cold·(Tc,out − Tc,in).
- The log mean temperature difference (LMTD) is the correct average driving temperature difference to use with Q = U·A·LMTD, because the temperature difference between the two streams is not constant along the exchanger's length.
- The effectiveness-NTU method solves the outlet temperatures without iterating: NTU = UA/C_min compares the exchanger's size to the smaller stream's heat capacity rate, and a standard formula converts NTU into an effectiveness ε, from which Q = ε·C_min·(Th,in − Tc,in).
- For the same inlet temperatures, flow rates and UA, counter-flow always produces a larger LMTD (and transfers more heat) than parallel-flow, because the temperature difference between the two streams stays closer to constant along the whole length instead of collapsing near the exit.
- In parallel-flow, both outlet temperatures are squeezed toward each other and can never cross; in counter-flow, the cold stream's outlet temperature can actually exceed the hot stream's outlet temperature, which is impossible in parallel-flow.
- C_min, the smaller of the two streams' heat-capacity rates (mass flow rate times specific heat), is whichever stream is limited to the smallest possible temperature change; it always sets the maximum heat the exchanger could ever transfer, no matter how large the area gets.
- The effectiveness-vs-NTU curve rises quickly at low NTU but flattens at high NTU, showing diminishing returns: doubling the exchanger's area (and cost) well past the flat part of the curve buys only a small extra gain in heat transferred, which is why exchangers are usually sized on the curve's knee, not its top.
Where this is used in the real world
Car radiators and engine oil coolers, power plant condensers and boiler feedwater heaters, HVAC coils, industrial process heat recovery, and heat pumps and refrigeration condensers/evaporators are all designed using this same energy-balance and LMTD/effectiveness-NTU analysis.
Who is this simulation for?
Engineering and technology students, makers, robotics clubs and teachers of design and technology. It gives a hands-on feel for how machines behave before you build a real one.
For teachers: project it on the board, let students predict what will happen, then run it together. For students: change one thing at a time and write down what changes.
Frequently asked questions
Why does counter-flow outperform parallel-flow for the same size of exchanger?
In parallel-flow, the temperature difference between the streams is largest at the inlet end and shrinks fast toward the exit, so most of the exchanger area is doing very little work. In counter-flow, the temperature difference stays more even along the whole length, so the same area transfers more heat, giving a higher LMTD and a higher effectiveness.
What does the heat transfer coefficient U represent?
U is the overall heat transfer coefficient, combining the convection on both fluid sides and the conduction through the wall between them, in W/(m²·K). A higher U (better fluids, thinner or more conductive walls, more turbulence) means more heat gets through for the same area and temperature difference.
Why use the effectiveness-NTU method instead of solving LMTD directly?
The direct Q = U·A·LMTD equation needs the outlet temperatures to compute LMTD, but the outlet temperatures depend on Q, so a direct solve requires guessing and iterating. The effectiveness-NTU method sidesteps this by first computing NTU and effectiveness from known quantities, giving Q and both outlet temperatures algebraically in one pass, then LMTD can be computed afterward for display.
What does the effectiveness-vs-NTU chart show and why does it matter?
It plots effectiveness ε against NTU (exchanger size relative to C_min) for the current configuration and capacity ratio, with your current operating point marked. Because the curve rises fast then flattens, it shows that making an exchanger bigger past a certain point gives smaller and smaller improvements in heat transferred — the key idea behind right-sizing an exchanger instead of oversizing it.
How do I tell which stream is C_min?
The 'Limiting stream' readout names it directly: whichever stream has the smaller heat-capacity rate (mass flow rate × specific heat) is C_min, and it is the stream that sets the maximum possible heat transfer, since it can only cool or heat by so much before it runs out of temperature difference to give.
Is the Heat Exchanger Simulator: Parallel-Flow vs Counter-Flow (LMTD) free to use?
Yes. It is completely free, with no signup, no download and no ads inside the simulation. It runs in your web browser.
Does the Heat Exchanger Simulator: Parallel-Flow vs Counter-Flow (LMTD) work on a phone or tablet?
Yes. It uses touch as well as the mouse, so you can drag objects with your finger. A larger screen makes the controls easier to see.