Htri — Heat Exchanger Design Top

A design that works at 100% load may vibrate or foul at 50% load.

Perhaps the most contentious topic in HTRI design is the Fouling Resistance ($R_f$). It is the "catch-all" safety factor, but it is often misused.

The Top Paradox: The Dirty Shell. When you input a high fouling factor (say, 0.003 $m^2K/W$) into HTRI, the software increases the required surface area. However, it assumes the fouling is uniformly distributed. htri heat exchanger design top

A deep design insight recognizes that fouling is dynamic. If you over-design a reboiler by adding too much surface area to counter fouling, you inadvertently lower the wall temperature. In many crude oil or heavy hydrocarbon applications, lower wall temperatures can actually accelerate fouling deposition (specifically waxing or asphaltene precipitation).

Top-tier HTRI design involves analyzing the Wall Temperature output tab. If the wall temperature is approaching the pour point or cloud point of the fluid, you aren't solving fouling; you are inviting it. You must balance the $R_f$ with velocity. High velocity (high shear) cleans the tubes; high surface area (low velocity) lets dirt settle. The HTRI designer must choose shear over area. A design that works at 100% load may

HTRI provides powerful diagnostic warnings. Pay attention to these:

| Warning | Meaning | Fix | |---------|---------|-----| | Flow-induced vibration | Tubes may fail | Increase baffle spacing, reduce baffle cut, add tie rods | | Temperature cross | ΔTₘ too low | Use multiple shells in series or crossflow | | Low shell-side velocity | Fouling risk | Reduce baffle spacing, use smaller baffle cut (20-30%) | | LMTD correction factor (F) < 0.75 | Inefficient design | Switch to 1-2 pass or multiple shells | | Overdesign >30% | Too large / costly | Reduce area (shorten tubes, fewer tubes) | Expert Insight: If Xist shows a temperature cross

| Side | ΔP (kPa) | Allowable (kPa) | Status | |------|----------|------------------|--------| | Shell | 48 | 70 | ✅ OK | | Tube | 62 | 80 | ✅ OK |

For close temperature approaches (<20°F), a standard TEMA AES or BEM may be impossible without a temperature cross. The "top" solution is switching to:

Expert Insight: If Xist shows a temperature cross on a single-pass shell-side, do not brute-force it with more area. Change the TEMA type.

When you print an HTRI run sheet, a "top" design is immediately recognizable by specific outputs. Here is what to look for and what to avoid.