A Practical Guide to Shell-and-Tube Heat Exchanger Fluid Allocation

A Practical Guide to Shell-and-Tube Heat Exchanger Fluid Allocation

Fluid allocation in a shell-and-tube heat exchanger is one of the most consequential early design decisions in process engineering. Poor allocation can dramatically increase cost, reduce reliability, and introduce operational hazards.

Incorrect allocation frequently drives:

• Higher shell thickness (cost ↑)
• Exotic alloy requirements (material cost ↑)
• Cleaning downtime (OPEX ↑)
• Thermal performance losses (area ↑)

This guide compiles the fundamental rules of thumb used by major operators and EPCs for reliable, low-cost heat exchanger design.

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Why Getting Fluid Allocation Wrong Costs You Millions

A single wrong decision on tube-side vs shell-side allocation can add hundreds of thousands to millions of dollars in total installed cost and can cause long-term maintenance penalties.

Real examples include:

• A refinery that placed crude oil (fouling + corrosive) on the shell side → required expensive alloy cladding.
• A gas plant that placed high-pressure refrigerant on the shell side → forced uprating the entire shell.
• A polyester plant that put viscous slurry on the tube side → required frequent shutdowns for cleaning.

Correct allocation can avoid 95% of these issues with a simple set of heuristics.

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Master Decision Table (Print & Pin Above Your Desk)

Fluid Characteristic	Preferred Side	Primary Reason (Cost/Safety/Performance)	Typical Savings
Highest pressure (> 20 barg)	Tube side	Tubes + channel cheaper to design for high pressure	15–45 %
Most corrosive / toxic	Tube side	Limits exotic alloy to tubes, tubesheet, and channel	30–70 %
Most fouling / scaling / dirty	Tube side	Mechanical cleaning + higher velocity	40–80 %
Highest temperature (> 350 °C)	Tube side	Protects shell from high metal temperature	10–25 %
Viscous fluid (μ > 50 cP)	Shell side	Better heat transfer at low Re; turbulence from baffles	20–40 %
Lowest volumetric flow rate	Shell side	Avoids excessive tube-side pressure drop	15–35 %
Condensing vapor	Shell side	Gravity drainage + easier venting	25–50 %
Boiling fluid	Shell side	Prevents vapor lock + improves stability	Critical
Contains non-condensables	Tube side	Positive venting at tube outlet	Essential
Risk of freezing on cold startup	Shell side	Freezing in shell is survivable; frozen tubes rupture	Avoid failure

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Quick 60-Second Allocation Flowchart

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Real Industry Examples That Paid Off

Project / Location	Service	Allocation Decision	Result / Savings
NGL Fractionation	High-pressure propane	High-pressure fluid → tubes	Large capital savings
Ethylene Cracker	Corrosive, fouling crude	Crude → tubes; cooling water → shell	Exotic alloy minimized
PTA Plant	Slurry stream (high fouling)	Slurry → tubes (rotatable bundle)	Major extension in cleaning interval
Offshore Platform	Thermal oil reboiler (condensing)	Condensing service → shell	Lower area + stable operation
Caustic Soda Plant	50% NaOH (viscous + corrosive)	NaOH → shell; steam → tubes	Lower ΔP + cheaper metallurgy

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Pro Tips Most Junior Engineers Miss

1. Never put both high pressure AND corrosive fluids on the shell side — metallurgy cost becomes extreme.
2. Cooling water usually belongs on the tube side — easier cleaning and higher velocity minimize fouling.
3. Vacuum streams typically go on the shell side — avoids thick-wall tubes.
4. Rotatable U-tube bundles dramatically reduce cleaning costs when the fouling fluid must be on the tube side.

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Key Engineering Notes

Governing Equations

Overall heat-transfer coefficient:

$$\frac{1}{U A} = \frac{1}{h_t A_t} + R_f + \frac{\Delta x}{k A_m} + \frac{1}{h_s A_s}$$

Tube-side velocity requirement:

$$v = \frac{4 \dot{m}}{\pi D_i^2 \rho N_{tubes}}$$

Shell-side Reynolds number (Bell–Delaware):

$$Re_s = \frac{D_e G_s}{\mu}$$

Where:

• $D_e$ = equivalent diameter
• $G_s$ = mass velocity across tube bundle

These relationships explain why viscous or low-flow-rate fluids often perform better on the shell side.

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Design Example (Worked)

Service: Hot fouling hydrocarbon → cooled by cooling water
Task: Choose tube vs shell allocation

Step 1 — Compare fouling

Hydrocarbon fouling factor:
$R_{f,hc} \approx 0.0007\ \text{m}^2\cdot\text{K}/\text{W}$
Cooling water fouling factor:
$R_{f,cw} \approx 0.0002\ \text{m}^2\cdot\text{K}/\text{W}$

Decision: Put fouling hydrocarbon on tube side.

Step 2 — Material

Hydrocarbon requires 316SS, water requires CS.
Tube-side allocation confines 316SS to tubes only → large cost savings.

Step 3 — Maintainability

Tube-side mechanical cleaning possible → further OPEX savings.

Final allocation: Hydrocarbon → tubes; cooling water → shell.

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Final Checklist Before Freezing the P&ID

• Highest pressure → tubes
• Most corrosive or toxic → tubes
• Most fouling → tubes (or use rotatable bundle)
• Phase-change service → shell
• Viscous or lowest flow → shell
• Non-condensables → tubes
• Freezing risk → shell
• Cooling water (unless seawater + titanium) → tubes

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