How Engineers Determine HTU for Packed Tower Absorption

In the HTU/NTU method, the required packed height is commonly expressed as:

Z=HTU×NTUZ = HTU \times NTU

where:

• ZZ = required packed height;
• HTU = height of a transfer unit;
• NTU = number of transfer units.

These two terms represent different parts of the packed tower design problem.

NTU describes:

how much mass-transfer duty the process requires.

HTU describes:

how much physical packing height is required to provide one transfer unit under the actual packing and operating conditions.

This distinction is fundamental.

A process may require the same NTU with two different packing types, but the HTU can differ because the packings provide different:

• effective interfacial area;
• gas-side mass transfer;
• liquid-side mass transfer;
• wetting behavior;
• hydraulic conditions.

Therefore:

HTU should not be treated as a universal constant for a packing. It must be associated with the actual mass-transfer basis, packing geometry, fluids and operating condition.

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What Does HTU Mean?

HTU stands for:

Height of a Transfer Unit

It converts mass-transfer capability into an equivalent physical height of packed bed.

A lower applicable HTU generally means:

less packing height is required to provide one transfer unit.

For example:

If:

HTU=0.7 mHTU = 0.7\ m

and:

NTU=5NTU = 5

then:

Z=0.7×5=3.5 mZ = 0.7 \times 5 = 3.5\ m

This is a preliminary mass-transfer height.

It still needs to be coordinated with:

• hydraulics;
• bed segmentation;
• distribution;
• mechanical layout.

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HTU Is Not the Same as NTU

The difference can be summarized simply.

NTU

Controlled mainly by:

• inlet composition;
• outlet target;
• equilibrium;
• operating line;
• required separation.

HTU

Controlled mainly by:

• packing;
• gas and liquid loading;
• mass-transfer coefficients;
• effective area;
• fluid properties.

Therefore:

NTU belongs mainly to the separation duty. HTU belongs mainly to how effectively the packed system performs that duty per unit height.

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1. Select the Correct HTU Basis

Engineers may encounter several HTU definitions.

Common examples include:

• HTUGHTU_G;
• HTULHTU_L;
• HTUOGHTU_{OG};
• HTUOLHTU_{OL}.

These are not interchangeable.

They correspond to:

• gas-side individual mass transfer;
• liquid-side individual mass transfer;
• overall gas-phase mass transfer;
• overall liquid-phase mass transfer.

The HTU definition must match the NTU basis.

For example:

Z=HTUOG×NTUOGZ = HTU_{OG} \times NTU_{OG}

not:

HTUOG×NTUOLHTU_{OG} \times NTU_{OL}

without proper conversion.

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2. Overall Gas-Phase HTU

For many absorption calculations, an overall gas-phase basis may be convenient.

A conceptual relationship can be written as:

HTUOG∝Gas Flow BasisKGaHTU_{OG} \propto \frac{Gas\ Flow\ Basis} {K_G a}

where:

• KGK_G = overall gas-phase mass-transfer coefficient;
• aa = effective interfacial area.

The exact form depends on:

• concentration basis;
• gas-flow definition;
• units;
• correlation.

The important engineering principle is:

Higher volumetric mass-transfer capability KGaK_Ga generally corresponds to lower HTU.

---

3. Overall Liquid-Phase HTU

On a liquid basis, engineers may similarly use:

HTUOLHTU_{OL}

with a relationship involving:

• liquid flow;
• overall liquid-phase coefficient KLK_L;
• effective area.

Again, the units and definitions must remain internally consistent.

The engineer should not compare:

HTUOGHTU_{OG}

and

HTUOLHTU_{OL}

numerically without understanding their bases.

---

4. HTU Depends on Mass-Transfer Coefficients

The gas and liquid phases each contribute transport resistance.

Individual coefficients may include:

kGk_G

and:

kLk_L

These are combined through:

• equilibrium relationships;
• resistance-in-series treatment

to obtain an overall coefficient such as:

KGK_G

or:

KLK_L

Therefore the controlling resistance discussed in #139 directly affects HTU.

---

5. Effective Interfacial Area Is Equally Important

A good mass-transfer coefficient alone is not enough.

The packing must also provide useful gas–liquid interface.

Volumetric mass-transfer performance often depends on:

KGaK_Ga

or:

KLaK_La

Therefore:

Mass-Transfer Coefficient

×

Effective Area

determines much of the usable packed-volume performance.

This is why #138 and #141 are connected but different.

#138 asks:

How much useful interface exists?

#141 asks:

How does the complete volumetric mass-transfer capability translate into height per transfer unit?

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6. Packing Geometry Influences HTU

Packing geometry affects:

• gas turbulence;
• liquid-film development;
• wetting;
• surface renewal;
• effective area.

Important packing characteristics can include:

• size;
• specific surface area;
• void fraction;
• channel geometry;
• surface texture.

Therefore two packings of the same material can have different HTU behavior.

---

7. Packing Size Can Change HTU

Smaller random packing often provides:

• higher specific surface area;
• more gas–liquid contact opportunity.

This may improve mass transfer and reduce HTU.

However, smaller packing may also cause:

• higher pressure drop;
• lower fouling tolerance;
• different capacity.

Therefore:

The packing with the lowest HTU is not automatically the best engineering choice.

---

8. Structured Packing Can Provide Low HTU

Structured packing is often designed to provide:

• high effective surface;
• organized flow channels;
• low pressure drop.

In suitable services, this can result in attractive mass-transfer performance.

But HTU still depends on:

• load;
• liquid properties;
• distribution quality;
• packing geometry.

A catalog efficiency value should not be assumed universal.

---

9. Liquid Loading Influences HTU

Liquid loading affects:

• wetting;
• film thickness;
• liquid turbulence;
• effective area;
• liquid-side mass transfer.

At very low liquid loading:

• effective wetting may decline;
• HTU may increase.

At higher liquid load:

• wetting may improve;
• liquid-side transport can improve.

But excessive liquid load also increases hydraulic resistance.

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10. Minimum Wetting Can Become an HTU Problem

If the tower operates below an effective wetting range:

• large parts of the packing may become underutilized.

Then:

aeffectivea_{effective}

decreases.

This reduces:

KaK a

and can increase HTU.

Therefore low-liquid-load performance should not be evaluated only from pressure drop.

---

11. Gas Loading Also Influences HTU

Gas velocity affects:

• gas-film turbulence;
• gas-side mass-transfer coefficient;
• interfacial behavior.

Increasing gas load may improve:

kGk_G

under some conditions.

But it also moves the tower toward:

• higher pressure drop;
• loading;
• flooding.

Mass transfer and hydraulics must therefore be optimized together.

---

12. HTU May Change Across the Operating Range

The same packing can have different HTU values at:

• minimum load;
• normal load;
• maximum load.

Therefore one nominal HTU should not automatically be used for all operating cases.

Where mass-transfer performance is critical, the relevant operating cases should be evaluated separately.

---

13. Liquid Viscosity Can Increase HTU

Higher viscosity can reduce:

• liquid-side mixing;
• diffusional transport;
• liquid-film renewal.

It may also affect:

• wetting;
• liquid holdup.

Therefore highly viscous systems can exhibit poorer liquid-side mass-transfer performance and higher HTU.

---

14. Surface Tension Influences HTU Through Wetting

Surface tension affects how liquid spreads across packing.

Poorer wetting can reduce:

• effective interfacial area.

That reduces volumetric mass-transfer performance.

Therefore surface tension can influence HTU even though it does not appear directly in every simplified HTU expression.

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15. Diffusivity Is a Key Transport Property

Mass-transfer coefficients depend partly on molecular diffusivity.

Relevant quantities may include:

• gas-phase diffusivity;
• liquid-phase diffusivity.

Low liquid diffusivity can increase liquid-side resistance.

Low gas diffusivity can increase gas-side resistance.

Therefore reliable HTU prediction may require process-specific diffusivity data.

---

16. Temperature Can Change HTU

Temperature affects:

• viscosity;
• diffusivity;
• surface tension;
• gas density;
• equilibrium behavior.

Therefore HTU at:

25°C

may not represent HTU at:

100°C

for the same system.

---

17. Pressure Can Change HTU

Pressure affects:

• gas density;
• gas velocity;
• diffusivity;
• equilibrium;
• hydraulic loading.

High-pressure absorption therefore may require different mass-transfer correlations from atmospheric operation.

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18. Packing Material Can Influence HTU

Packing material affects surface wetting.

Examples include:

• metal;
• ceramic;
• plastic.

The same process liquid may wet these materials differently.

This changes:

• effective area;
• liquid film behavior.

Therefore packing material can indirectly affect HTU.

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19. Distributor Performance Can Change the Effective HTU of the Installed Tower

A mass-transfer correlation may assume reasonable distribution.

If the actual tower suffers severe maldistribution:

• only part of the packing cross-section may be effectively used.

The intrinsic packing may have a good HTU.

But the installed system behaves as if its effective transfer performance is poorer.

Therefore:

Good HTU data do not compensate for bad liquid distribution.

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20. Gas Maldistribution Can Have a Similar Effect

If gas flows preferentially through only part of the bed:

• available packing volume is underutilized.

The tower may require more effective height than expected.

Therefore both:

• gas distribution;
• liquid distribution

affect realization of theoretical HTU performance.

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21. HTU Is Not Simply Geometric Surface Area

Suppose Packing A has:

250 m²/m³

and Packing B has:

200 m²/m³

You cannot conclude:

Packing A HTU must be 20% lower.

Because HTU also depends on:

• film coefficients;
• wetting;
• distribution;
• loading;
• properties.

Geometric area is only one contributor.

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22. HTU Is Not Simply Pressure Drop

A packing can have:

• low pressure drop

but not necessarily:

• lowest HTU.

Similarly, a packing with excellent mass-transfer efficiency may create a higher hydraulic penalty.

Therefore engineers often evaluate:

HTU

and:

Pressure Drop

together.

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23. HTU and Capacity Must Be Evaluated Together

Suppose Packing A provides:

• lower HTU;
• lower hydraulic capacity.

Packing B provides:

• slightly higher HTU;
• substantially higher capacity.

If tower diameter is constrained, Packing B may still be the better design.

Therefore mass-transfer efficiency cannot be optimized independently from capacity.

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24. HTU and Fouling Resistance Can Conflict

Fine, high-area packing may provide attractive clean-service HTU.

But in a dirty process it may:

• foul more readily;
• lose effective area;
• increase pressure drop.

A larger, more open packing may provide worse clean HTU but better long-term operation.

Therefore the correct decision is:

best sustained process performance

not:

lowest clean-test HTU.

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25. HTU Correlations Are Usually Empirical or Semi-Empirical

Packed-column mass-transfer models commonly estimate:

• gas-film coefficient;
• liquid-film coefficient;
• effective area

using experimental correlations.

Different models may use:

• Reynolds number;
• Schmidt number;
• Weber number;
• packing-specific geometry;
• hydraulic variables.

Therefore HTU prediction depends on selecting a correlation applicable to the packing and process.

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26. Correlations Have Validity Ranges

Before using a correlation, engineers should confirm whether it covers:

• random or structured packing;
• packing size;
• gas loading;
• liquid loading;
• fluid-property range.

Extrapolation outside the tested range increases uncertainty.

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27. Avoid Mixing Independent Correlations Without Validation

A common mistake is to use:

• kGk_G from one model;
• kLk_L from another;
• effective area from a third.

This can be acceptable only if the methods are compatible and justified.

Some correlations were developed as integrated models.

Their components should not automatically be mixed.

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28. Vendor HTU Data Need Context

If a packing supplier provides:

HTU = 0.5 m

engineers should ask:

• Which component system?
• Which liquid?
• What gas flow?
• What liquid flow?
• What pressure?
• What temperature?
• Which HTU basis?
• Which packing size?

Without this information, the number is incomplete.

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29. Water–Air Test Data Are Not Universal HTU Data

Hydraulic or mass-transfer testing may use standardized fluids.

Actual process fluids can have very different:

• viscosity;
• diffusivity;
• surface tension;
• equilibrium.

Therefore laboratory data should be used according to their intended basis.

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30. Reactive Absorption Can Reduce Apparent HTU

Chemical reaction can enhance liquid-phase mass transfer.

For some reactive systems:

• dissolved component is consumed rapidly;
• liquid-side driving force remains strong.

This can produce a lower effective transfer height than physical absorption alone.

But the calculation may require:

• enhancement factors;
• reaction kinetics;
• reactive equilibrium.

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31. HTU Can Change as Reactive Solvent Becomes Loaded

Reactive solvent does not necessarily maintain the same chemical strength throughout operation.

As the solvent becomes loaded:

• reaction capacity may decrease;
• liquid-side enhancement may weaken.

Therefore HTU can change with solvent condition.

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32. Different Components Can Have Different HTU

A multicomponent gas may contain:

• H₂S;
• CO₂;
• VOCs;
• other components.

Each component can have different:

• equilibrium;
• diffusivity;
• resistance.

Therefore the relevant HTU may differ by component.

The component controlling packed height should be identified.

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33. HTU Can Change Along the Tower Height

Conditions may change through the bed:

• composition;
• temperature;
• flow;
• solvent loading.

A rigorous rate-based model may therefore calculate local mass-transfer behavior at many elevations instead of assuming one constant HTU.

A single average HTU is a simplified representation.

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34. Preliminary Design Can Use an Average HTU

For suitable systems and early-stage design, an average applicable HTU can support:

• preliminary packing height;
• packing comparison;
• vessel layout.

But the assumed value should be documented.

Final design may require:

• supplier data;
• validated correlations;
• simulation;
• pilot data.

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35. Existing Tower Data Can Be Used to Infer Effective HTU

If an existing tower has:

• known packed height;
• measured inlet composition;
• measured outlet composition;
• known operating conditions;

engineers may estimate effective overall performance.

If required NTU can be calculated from operating data:

HTUeffective=Existing Packed HeightCalculated NTUHTU_{effective} = \frac{Existing\ Packed\ Height}{Calculated\ NTU}

This can help evaluate:

• actual packing performance;
• retrofit options.

But only if the tower is reasonably well distributed and operating data are reliable.

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36. A High Effective HTU May Indicate More Than Poor Packing

If plant-derived HTU is much worse than expected, possible causes include:

• liquid maldistribution;
• gas maldistribution;
• fouling;
• wrong process-property assumptions;
• inaccurate flow measurement;
• equilibrium-data errors.

Do not automatically conclude that the packing itself is defective.

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37. HTU Can Support Packing Comparison

Suppose two candidate packings are evaluated under the same process duty.

Packing A:

HTU=0.55 mHTU=0.55\ m

Packing B:

HTU=0.75 mHTU=0.75\ m

Required:

NTU=6NTU=6

Then:

Packing A:

Z=3.3 mZ=3.3\ m

Packing B:

Z=4.5 mZ=4.5\ m

Packing A may save:

1.2 m of packed height.

But engineers must still compare:

• pressure drop;
• capacity;
• fouling;
• cost;
• internals.

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38. HTU Has Large Impact in High-NTU Services

A small HTU difference becomes more important when NTU is large.

If:

ΔHTU=0.1m\Delta HTU=0.1m

and:

NTU=2NTU=2

height difference:

0.2 m

If:

NTU=12NTU=12

height difference:

1.2 m

Therefore reliable HTU prediction becomes especially important in demanding separations.

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Example 1 — Physical Absorber

A process calculation determines:

NTUOG=4.5NTU_{OG}=4.5

A packing-specific mass-transfer evaluation gives:

HTUOG=0.8mHTU_{OG}=0.8m

Then:

Z=4.5×0.8=3.6mZ=4.5\times0.8=3.6m

This gives a preliminary required packed height.

The design must still verify:

• hydraulic capacity;
• pressure drop;
• distribution.

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Example 2 — Two Candidate Packings

Same process:

NTU=5NTU=5

Packing A:

HTU=0.6mHTU=0.6m

Packing B:

HTU=0.9mHTU=0.9m

Calculated heights:

A = 3.0 m

B = 4.5 m

But Packing A has much smaller flow passages and higher fouling risk.

In a dirty service, Packing B may still provide better lifecycle performance.

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Example 3 — Low Liquid Loading

At normal liquid circulation:

• effective wetting is good;
• calculated HTU is acceptable.

At minimum load:

• effective area falls;
• HTU increases.

The same physical bed therefore provides less separation capability at turndown.

This can define the practical lower operating limit.

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Example 4 — Fouled Existing Tower

An absorber originally met its outlet specification.

After several years:

• pressure drop increases;
• removal efficiency falls.

The process NTU requirement is essentially unchanged.

But actual effective:

HTUHTU

has worsened because of:

• fouling;
• maldistribution;
• reduced usable area.

This is why NTU and HTU should be separated during troubleshooting.

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HTU Determination Workflow

Define Mass-Transfer Basis

↓

Confirm Packing Type and Size

↓

Confirm Gas and Liquid Loads

↓

Confirm Physical Properties

↓

Select Applicable Mass-Transfer Correlation

↓

Calculate Gas-Side Coefficient

↓

Calculate Liquid-Side Coefficient

↓

Estimate Effective Interfacial Area

↓

Combine Resistances on Consistent Basis

↓

Determine Overall Volumetric Mass-Transfer Capability

↓

Calculate HTU

↓

Combine With Matching NTU

↓

Determine Packing Height

↓

Check Hydraulics / Distribution / Fouling

↓

Validate Against Project Data

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HTU Input Checklist

Packing

✓ Packing type✓ Size✓ Geometry✓ Material✓ Surface characteristics

Gas

✓ Flow✓ Density✓ Viscosity✓ Diffusivity

Liquid

✓ Flow✓ Density✓ Viscosity✓ Surface tension✓ Diffusivity

Mass Transfer

✓ Gas-side coefficient✓ Liquid-side coefficient✓ Equilibrium relationship✓ Effective area✓ Overall coefficient basis

Operation

✓ Temperature✓ Pressure✓ Minimum / normal / maximum loads

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Common HTU Calculation Mistakes

Mistake 1 — Treating HTU as a Fixed Catalog Number

Why it fails:

HTU changes with fluid system and operating load.

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Mistake 2 — Mixing HTU and NTU Bases

Why it fails:

HTUOGHTU_{OG} must pair with NTUOGNTU_{OG}, etc.

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Mistake 3 — Using Geometric Area Instead of Effective Area

Why it fails:

Not all physical packing surface necessarily participates effectively in mass transfer.

---

Mistake 4 — Ignoring Controlling Resistance

Why it fails:

Gas- and liquid-side resistances affect the overall coefficient.

---

Mistake 5 — Selecting Packing by HTU Alone

Why it fails:

Pressure drop, capacity, fouling and cost may control the final decision.

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Mistake 6 — Applying Clean-Service HTU to Fouling Service

Why it fails:

Long-term effective area and hydraulic condition may deteriorate.

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Mistake 7 — Using One HTU for Every Operating Case

Why it fails:

Gas and liquid loading can materially change mass-transfer behavior.

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HTU vs NTU vs Packed Height

The relationship is:

Z=HTU×NTU\boxed{Z = HTU \times NTU}

But successful packed tower design requires both:

Mass Transfer

and

Hydraulics

to be acceptable.

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How the DAIER Engineering Assistant Fits Into HTU Evaluation

The DAIER Tower Packing Engineering Assistant can support preliminary organization of:

• packing type;
• gas flow;
• liquid flow;
• tower diameter;
• operating conditions.

https://www.pxdaier.com/tower-packing-engineering-assistant.html

Detailed HTU determination may additionally require:

• gas and liquid diffusivities;
• viscosity;
• surface tension;
• equilibrium data;
• effective-area correlation;
• gas- and liquid-side mass-transfer correlations.

Complex or reactive systems may require:

• rigorous process simulation;
• rate-based modeling;
• pilot or validated supplier data.

The Engineering Assistant should therefore support preliminary packed tower engineering rather than replace detailed mass-transfer design.

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Quick Guide

What does HTU mean in a packed tower?

HTU is the amount of packing height required to provide one transfer unit under a defined packing and operating condition.

Is HTU a fixed property of packing?

No.

It depends on packing, gas and liquid loads, physical properties, mass-transfer coefficients and effective area.

What is the difference between HTU and NTU?

NTU represents the separation requirement. HTU represents how much height the packed system needs per transfer unit.

What lowers HTU?

Higher effective volumetric mass-transfer capability can lower HTU.

Does lower HTU always mean a better packing?

No.

Hydraulic capacity, pressure drop, fouling resistance and cost also matter.

Can HTU change with operating load?

Yes.

Changes in gas or liquid load can change wetting, mass-transfer coefficients and effective area.

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From Packing Performance to Physical Separation Height

The engineering sequence is:

Packing Geometry

•  

Gas / Liquid Loads

•  

Physical Properties

↓

Gas-Side Mass Transfer

•  

Liquid-Side Mass Transfer

•  

Effective Interfacial Area

↓

Overall Volumetric Mass-Transfer Capability

↓

HTU

•  

Required NTU

↓

Required Packed Height

↓

Hydraulic Check

↓

Distribution Check

↓

Mechanical Layout

↓

Final Design

The core engineering rule is:

NTU tells engineers how much separation effort is required. HTU tells them how much real packing height is needed to provide each unit of that effort.