Heat exchangers · one setting on one block

The flow direction that decides how big your exchanger is

Hook a heat exchanger up with both streams running the same way and nothing obviously breaks. The drawing still looks like a heat exchanger, the block still solves, and the outlet temperature comes back worse than the arithmetic promised, so you add area to chase it. Here is the ten-second check that tells you which arrangement you are looking at, and why the same duty costs more steel when the two streams run together.

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01

Two ways to point the same two streams

A heat exchanger has a hot stream and a cold stream and two ways to arrange them. Co-current runs both the same way, so both enter at the same end. Counter-current runs them against each other, so the hot stream enters where the cold stream leaves. On a drawing the pipework looks close to identical, and the two arrangements behave nothing alike.

Plot temperature against length and the difference shows up straight away. Both charts below use the same pair of streams with the same heat capacity flow rates: hot in at 150 °C, cold in at 20 °C.

Co-current

Temperature against length for a co-current heat exchanger Both streams enter at the left. The hot curve starts at 150 degrees Celsius and falls, the cold curve starts at 20 degrees Celsius and rises, and the two converge toward a common value near 85 degrees without ever crossing. The hot stream leaves at 95 degrees and the cold stream leaves at 75 degrees, both at the right hand end. hot stream cold stream 150 °C 20 °C 95 °C 75 °C both inlets both outlets length along the exchanger temperature
Both streams enter at the left. The gap between them opens at 130 °C and collapses as they run, and the two curves chase the same value from opposite sides. However long you make the exchanger, the best they can do is meet, so the cold stream always leaves below the hot stream.

Counter-current

Temperature against length for a counter-current heat exchanger The hot stream enters at the left at 150 degrees Celsius and leaves at the right at 60 degrees. The cold stream enters at the right at 20 degrees and leaves at the left at 110 degrees, so its outlet is drawn at the left hand end. The two lines run parallel with a constant 40 degree gap, and the cold outlet at 110 degrees sits well above the hot outlet at 60 degrees, which is a temperature cross. hot stream cold stream cross 150 °C 110 °C 60 °C 20 °C hot in, cold out hot out, cold in length along the exchanger temperature
The hot stream enters at the left and the cold stream enters at the right, so the cold stream’s outlet sits at the left of the plot. The gap holds at 40 °C the whole way. The cold stream leaves at 110 °C, above the 60 °C the hot stream leaves at, and that crossover is only available in this arrangement.

Both charts are teaching sketches on one set of assumptions: equal heat capacity flow rates on the two sides, which is what makes the counter-current lines straight and the gap constant, no phase change, no pressure drop and a constant overall coefficient. The shapes are what change with the arrangement. The numbers are illustrative and size nothing.

02

The ten-second check

Read the two outlet temperatures and compare them. If the cold stream leaves at or above the temperature the hot stream leaves at, you have a temperature cross, and only a counter-current arrangement can deliver one.

Then look at what the flow direction is set to, because the two answers have to agree.

  • A model that shows a cross while the direction is set to co-current is wrong. Most simulators will refuse to converge rather than hand you the number, so a block that will not solve is often telling you this and nothing else.
  • A unit that is plumbed co-current out on the plant will never give you a cross, and adding area will not change that. The most a co-current exchanger can do is bring the two streams to the same temperature, and it only gets there at infinite length.

Two numbers and one dropdown, and it catches the most expensive mistake available on the block.

03

What it costs in area

Same duty, same two streams, same steel: a co-current exchanger wants more area every time. The reason is in one line.

A = Q / (U × ΔTlm)

The driving force is the temperature difference between the two streams along the exchanger, and the area you need is the duty divided by the overall coefficient times the log mean of that difference. Hold the duty and hold U, and the mean temperature difference is the only term left to move. Counter-current keeps the gap even along the whole length. Co-current spends the gap early, the two curves converge, and the last stretch of tube is working against almost nothing. For the same four terminal temperatures the counter-flow log mean temperature difference always exceeds the parallel-flow one, and a smaller mean difference buys you a bigger exchanger.

How much bigger depends on how close the terminals sit. In the comfortable cases it is a few percent, and a worked comparison of the two arrangements on one duty comes out about 4% bigger in area, 4.3% in that particular case. That figure is one worked comparison and not a general law. Push toward the temperature cross above and the penalty stops being a percentage at all, because no amount of area gets a co-current unit there.

04

Where the setting lives

Every exchanger block in a process simulator carries a flow-direction setting. It is one field on one block, it sits beside a dozen fields that matter less, and it is easy to leave on whatever it came with. When the outlet temperature will not reach its target, or the area the model asks for looks wrong by a third, open that setting before you touch anything else.

The model should be the thing you described

A heat exchanger that will not hit its target is usually a driving force problem, and the driving force is decided by how the two streams are pointed. Reflux drives your simulator from plain English so that the arrangement in the model is the one you said out loud, and so that asking what happens if the streams are turned around is a question rather than an afternoon of rebuilding.

We’re opening a small trial to people who ask for it, and we are not precious about which software you run. Whatever you use, we’ll build for it.

Nathan Ruberto · Co‑Founder, CEO

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