How Delta T and Approach Temperature Impact Heat Exchanger Size

In the world of Plate-and-Frame Heat Exchangers (PHEs), there’s always a trade-off: space, cost, and efficiency don’t always align perfectly. One of the key factors that drives this trade-off is the temperature difference (delta T) and the approach temperature of the system.

Let’s look into this in more detail:

The Basics: Delta T vs. Approach Temperature

• Delta T (Temperature Difference): The bigger the difference between the hot and cold fluid temperatures, the smaller the heat exchanger needs to be.
• Approach Temperature: This is the smallest temperature gap between the two fluids. Smaller gaps mean more efficiency but require a much larger heat exchanger to do the job.

Here’s where it gets interesting: when you reduce the approach temperature to maximize energy recovery, you end up needing more plates in the PHE, which means more space, more material, and, yes, more cost.

Real-Life Example

Let’s say your system needs to transfer 1,000 kW of heat, and your heat transfer coefficient (U) is 2.5 kW/m2·K:

1. If your delta T is 10°C, you’ll need about 40 m2 of plates.
2. If your delta T drops to 5°C, you’ll need 80 m2 of plates—double the size of the heat exchanger!

That’s the cost of increasing efficiency.

In district energy applications, lower approach temperatures are common because they maximize energy recovery and efficiency. But it comes at a price: larger, more expensive heat exchangers.

So, what’s the balance?

• If space and cost are tight, a higher delta T might be your friend.
• If efficiency and energy recovery matter more (as they often do in district energy), you’ll need to accept a bigger PHE.

The Takeaway

When designing heat exchangers, it’s all about priorities. Efficiency? Cost? Space? Each decision affects the size, complexity, and long-term performance of the system.

What do you prioritize in your designs?

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