One-Pass vs Multi-Pass Shell-and-Tube Heat Exchangers: What Changes?

When selecting a shell-and-tube heat exchanger for industrial cooling, refrigeration, chemical processing, or heat recovery, engineers must consider more than heat transfer area and equipment dimensions. One important design parameter is the number of times the fluid travels through the heat exchanger.

This is commonly described as the number of passes.

A one-pass shell-and-tube heat exchanger provides a relatively straightforward flow path. A multi-pass arrangement redirects the fluid through different sections of the tube bundle or, in certain designs, through multiple shell-side flow paths.

Changing the number of passes can influence tube-side velocity, heat transfer coefficients, pressure drop, temperature profiles, pumping energy, and mechanical complexity.

However, more passes do not automatically mean higher efficiency.

A multi-pass exchanger may improve heat transfer under specific operating conditions while creating additional pressure losses. A one-pass design may offer better overall economics when the required thermal duty can already be achieved without additional flow redirection.

This guide explains the differences between one-pass and multi-pass shell-and-tube heat exchangers, how pass arrangements affect performance, and how engineers can select a suitable configuration for industrial applications.

1. What Does One-Pass or Multi-Pass Mean in a Shell-and-Tube Heat Exchanger?

1.1 Understanding Shell-Side and Tube-Side Flow

A shell-and-tube heat exchanger consists of a cylindrical shell containing a bundle of tubes.

One fluid flows through the tubes, while the other flows around the outside of the tubes inside the shell.

Heat transfers between the two fluids through the tube walls without direct mixing under normal operating conditions.

The two flow circuits are identified as:

• Tube side: The fluid flowing inside the tubes.

• Shell side: The fluid flowing through the space surrounding the tube bundle.

Each circuit can have its own flow arrangement.

The number of tube passes and the number of shell passes are separate design parameters.

For a basic introduction to the equipment, see Shell and Tube Heat Exchanger Working Principle Explained.

1.2 What Is a One-Pass Shell-and-Tube Heat Exchanger?

In a single-pass tube-side configuration, the fluid enters the heat exchanger at one end, flows through the tubes once, and then exits at the other end.

The fluid does not flow back through another set of tubes before leaving the heat exchanger.

In traditional straight-tube designs, this configuration typically features an inlet connection at one end and an outlet connection at the other.

The single-pass tube-side configuration is relatively simple, and its hydraulic resistance may be lower than that of comparable multi-pass configurations.

However, actual performance depends on tube diameter, flow velocity, heat transfer area, and operating conditions.

1.2.1 Characteristics of One-Pass Tube-Side Flow

Potential characteristics include:

• A direct tube-side flow path

• A larger number of parallel tubes carrying the total fluid flow

• Relatively low tube-side pressure loss in comparable designs

• Straightforward channel-head arrangements

• Suitability for high-flow or pressure-drop-sensitive applications

A possible limitation is insufficient velocity when the total tube-side flow rate is distributed across too many parallel tubes.

1.2.1.1 Why Low Tube Velocity Can Become a Problem

When the flow velocity on the tube side is too low for the selected operating conditions, the convective heat transfer coefficient may be lower than expected.

For certain fluids, low flow velocities can also increase the risk of deposit buildup or scaling.

This does not mean that low flow velocities will necessarily lead to deposit formation. Scaling behavior depends on the fluid composition, temperature, chemical properties, surface condition, and flow conditions.

Engineers must determine whether the operating flow velocity is appropriate based on the actual process conditions, rather than simply applying generic flow velocity requirements.

1.3 What Is a Multi-Pass Shell-and-Tube Heat Exchanger?

In a multi-pass tube-side configuration, the fluid flows through the tube bundle multiple times.

For example, a double-pass heat exchanger divides the tube bundle into two groups.

The fluid enters the first group of tubes, flows through the heat exchanger, turns at the other end, then returns through the second group of tubes before finally exiting.

Appropriate channel end caps, flow baffles, and tail-end arrangements guide the fluid flow.

Common tube-side configurations include two-pass and four-pass designs, but other arrangements may also be used when mechanical and thermal conditions permit.

1.3.1 How Tube-Side Pass Partitions Work

Baffles divide the tube-side channel head into separate flow paths.

These flow paths prevent the incoming fluid from flowing directly into the outlet chamber without following the intended path.

The baffles must be compatible with the piping layout, tube sheet, gaskets, and channel head design.

1.3.1.1 Why Pass Partition Sealing Matters

Improperly designed or poorly sealed channel partitions may cause fluid to leak between the tube-side compartments.

This is commonly referred to as inter-channel leakage.

Such leakage bypasses part of the intended flow path, thereby reducing the heat exchanger’s efficiency.

Therefore, the integrity of the channel partitions is a key consideration in mechanical design and maintenance.

2. Tube Passes vs Shell Passes: What Is the Difference?

2.1 Understanding Tube-Side Passes

Tube passes describe how many times the tube-side fluid traverses the effective tube length.

A one-pass arrangement provides one traverse.

A two-pass arrangement provides two successive traverses through different tube groups.

A four-pass arrangement provides four traverses.

Increasing the number of tube passes generally reduces the number of tubes operating in parallel within each pass, assuming the same total tube count.

This may increase local fluid velocity.

2.2 Understanding Shell-Side Passes

The shell pass describes the overall flow path of the fluid within the shell.

Traditional TEMA Type E shells use a single-pass shell pass configuration.

TEMA Type F shells utilize longitudinal baffles to form two shell-side passes.

Multi-pass shell passes can also be achieved by appropriately arranging heat exchangers in series.

It is important to note that shell-side baffles and shell-side passes are not the same concept.

2.2.1 Why Baffle Count Does Not Equal Shell Pass Count

Segmented baffles guide the shell-side fluid flow through the tube bundle and support the tubes.

As the shell-side fluid flows through a heat exchanger equipped with baffles, its direction of flow may change repeatedly.

However, these localized changes do not necessarily mean that the heat exchanger is automatically classified as a multi-pass shell-and-tube design.

A traditional single-pass TEMA Type E shell-and-tube heat exchanger may contain numerous segmented baffles.

2.2.1.1 A Common Engineering Misunderstanding

An exchanger with six internal segmental baffles is not necessarily a six-pass shell-and-tube heat exchanger.

The number of shell passes is determined by the overall shell-side flow arrangement, not the number of local flow reversals created by segmental baffles.

2.3 Understanding 1–1, 1–2, 1–4, and 2–4 Arrangements

Shell-and-tube pass arrangements are commonly described by the number of shell passes followed by the number of tube passes.

For example:

1–2 means one shell pass and two tube passes.

It does not mean one inlet and two outlets.

Similarly, 2–4 refers to two shell passes and four tube passes, not four baffles or four separate heat exchangers.

These designations are useful when comparing equipment, but they do not fully describe the mechanical construction.

3. One-Pass vs Multi-Pass Shell-and-Tube Heat Exchangers: Key Differences

The best choice depends on the required thermal performance and the operating limitations of the complete cooling or heating system.

4. How Does the Number of Tube Passes Affect Heat Transfer?

4.1 The Relationship Between Fluid Velocity and Heat Transfer

The heat transfer coefficient on the tube side depends on several factors, including flow velocity, fluid properties, tube geometry, and flow regime.

A simplified exchanger heat transfer equation is:

Q = U × A × ΔTm

Where:

• Q = Heat transfer duty

• U = Overall heat transfer coefficient

• A = Effective heat transfer area

• ΔTm = Appropriate mean temperature difference

The overall heat transfer coefficient reflects the combined effects of tube-side convection, shell-side convection, tube-wall conduction, and relevant fouling resistance.

Increasing tube-side velocity may improve the tube-side convective heat transfer coefficient.

However, the benefit to the overall coefficient depends on which thermal resistance controls the process.

If the shell-side resistance dominates, increasing the tube-side coefficient may produce only a limited improvement in overall heat transfer performance.

4.2 Why Multi-Pass Arrangements Can Increase Tube Velocity

Consider an example heat exchanger containing 80 straight tubes.

Assume that all tubes have the same inner diameter and that the total flow rate on the tube side remains constant.

In a single-pass design, the flow is distributed among the 80 parallel tubes.

In a double-pass design using the same 80 tubes, each pass may contain approximately 40 parallel tubes.

Consequently, the average flow rate per effective tube increases.

4.2.1 Example: Comparing One-Pass and Two-Pass Velocity

Assume the following conditions:

The average velocity in each active tube is approximately:

Tube Velocity = Volumetric Flow Rate ÷ Total Active Tube Flow Area

For the one-pass arrangement:

Velocity ≈ 0.75 m/s

For the two-pass arrangement:

Velocity ≈ 1.49 m/s

Therefore, under these simplified assumptions, the two-pass arrangement approximately doubles the average tube velocity.

4.2.1.1 Why Double Velocity Does Not Mean Double Heat Transfer

The convective heat transfer coefficient does not increase linearly with velocity in every flow regime.

The relationship depends on Reynolds number, Prandtl number, tube geometry, and the applicable heat transfer correlation.

Moreover, the overall heat transfer coefficient includes resistances outside the tube-side fluid film.

Consequently, doubling tube velocity does not guarantee a doubling of cooling capacity or overall heat transfer efficiency.

The example illustrates flow geometry only and is not a performance rating for a particular exchanger.

4.3 How Pass Count Influences Thermal Effectiveness

In certain applications, the flow velocity through the tube bundle channels can be adjusted by changing the arrangement of the channels, thereby improving heat transfer performance.

However, this also alters the overall temperature distribution.

In multi-channel heat exchangers, there may be sections where the fluid flow directions are generally the same, as well as sections where the axial fluid flow directions are opposite.

This makes thermal analysis more complex than that of a simple, ideal counter-current heat exchanger.

Therefore, the actual fluid flow configuration must be taken into account during the design process.

4.4 The Importance of the LMTD Correction Factor

For a heat exchanger operating with ideal countercurrent flow, the log mean temperature difference can be calculated from the temperature differences at the two ends.

For a single-phase process with appropriate terminal temperatures:

LMTD = (ΔT1 − ΔT2) ÷ ln(ΔT1 / ΔT2)

In many shell-and-tube arrangements, the actual flow pattern differs from ideal countercurrent flow.

An LMTD correction factor may therefore be required.

The corrected thermal relationship becomes:

Q = U × A × F × LMTD

Where F is the mean temperature difference correction factor for the specified flow configuration.

4.4.1 Why the Correction Factor Changes With Pass Arrangement

The correction factor depends on the arrangement and terminal temperature conditions.

A 1–2 exchanger and a 2–4 exchanger may have different correction factors for the same specified inlet and outlet temperatures.

This can change the effective temperature driving force and the heat transfer area required.

4.4.1.1 When a Thermal Arrangement Needs Reconsideration

If the calculated correction factor is lower than the design standard, a different heat exchanger configuration may be required.

Possible solutions include changing the number of shell-side passes, connecting multiple heat exchangers in series, or modifying the heat exchanger configuration.

Simply increasing the number of tube-side passes does not necessarily resolve the issue of an unfavorable temperature driving force.

The appropriate configuration should be determined through engineering calculations, rather than assuming that increasing the number of passes will always improve thermal efficiency.

5. How Does Pass Count Affect Pressure Drop and Pumping Energy?

5.1 Why Multi-Pass Designs Usually Have Higher Tube-Side Pressure Drop

When a fluid encounters resistance as it flows through pipes, manifolds, elbows, inlets, and outlets, the pressure drop increases.

Increasing the number of passes on the pipe side typically results in two significant changes:

First, the number of pipes operating in parallel decreases with each pass; therefore, the flow velocity increases while the total flow rate remains constant.

Second, the fluid must travel a longer effective path and may encounter more changes in direction.

The combined effect of these factors leads to a significant increase in pressure loss on the tube side.

5.2 The Relationship Between Velocity and Friction Loss

For a suitable single-phase flow model, the Darcy–Weisbach equation describes frictional pressure drop:

ΔP = f × (L / D) × (ρv² / 2)

Where:

• ΔP = Frictional pressure drop

• f = Darcy friction factor

• L = Flow path length

• D = Hydraulic diameter

• ρ = Fluid density

• v = Average fluid velocity

The equation illustrates why velocity and flow path length are important.

However, the friction factor may change with Reynolds number and surface roughness.

Additional losses from return chambers, tube entrances, exits, and other fittings must also be considered.

Therefore, the actual pressure drop cannot be predicted accurately from pass count alone.

5.3 How Pressure Drop Affects Pumping Energy

A circulation pump must overcome pressure losses throughout the system.

The approximate hydraulic power required across a component is:

Hydraulic Power (kW) = Pressure Drop (kPa) × Volumetric Flow Rate (m³/s)

Actual electrical consumption also depends on pump, motor, and drive efficiency.

5.3.1 Example: Comparing Hydraulic Power

Assume two hypothetical exchanger configurations operate at the same volumetric flow rate of 0.01 m³/s.

The second option requires an additional 0.30 kW of hydraulic power across the exchanger at the specified flow.

This comparison uses assumed pressure drops to illustrate the calculation. It does not claim that doubling the number of tube passes always doubles the pressure drop.

5.3.1.1 Why Continuous-Duty Systems Need Lifecycle Cost Analysis

In a system operating many hours per year, additional pressure drop can create significant cumulative energy consumption.

A multi-pass exchanger may offer a smaller heat transfer area or improved heat transfer under certain conditions.

However, any reduction in exchanger investment should be compared with additional pumping energy and maintenance requirements.

The preferred configuration is the one that meets the process requirements at an acceptable overall lifecycle cost.

6. Does Multi-Pass Design Always Improve Shell-and-Tube Heat Exchanger Efficiency?

6.1 When Additional Tube Passes Can Be Beneficial

Multi-pass designs may be useful when the proposed one-pass arrangement produces insufficient tube-side velocity.

Increasing velocity may improve convective heat transfer and help the equipment achieve the required thermal duty.

Potential applications include:

• Industrial liquid-to-liquid cooling

• Certain oil cooling applications

• Process water heating and cooling

• Heat recovery systems

• Applications with unequal shell-side and tube-side thermal characteristics

The actual benefit depends on fluid properties and the complete equipment design.

6.2 When Additional Passes May Be Unnecessary

A one-pass design may already provide sufficient heat transfer performance.

Additional passes can then increase hydraulic resistance without delivering a meaningful benefit.

This can occur when:

• Tube-side velocity is already suitable

• The tube-side heat transfer coefficient is not the limiting resistance

• Available pumping pressure is restricted

• The process fluid is highly viscous

• The equipment requires a low-pressure-drop flow path

For these applications, additional tube passes may create more disadvantages than benefits.

6.3 Why More Passes Do Not Always Mean a Smaller Exchanger

In some cases, a multi-pass configuration can allow for the use of a smaller heat exchanger by increasing the tube-side heat transfer coefficient.

However, this result is not guaranteed.

The total heat transfer area required also depends on shell-side performance, the average temperature difference, resistance to fouling, and the allowable pressure drop.

If the thermal benefit is small, increasing the number of heat transfer passes may not offset the advantage of reduced equipment size.

Increasing the number of heat transfer passes does not necessarily result in an automatic increase in the physical heat transfer surface area.

7. Tube Passes, Mechanical Construction, and Maintenance

7.1 How Pass Arrangements Affect Channel Heads

Multi-pass exchangers require suitable channel-head compartments and pass partitions.

These components must direct the fluid through the intended tube groups while maintaining appropriate sealing.

Increasing the number of passes may increase the complexity of the channel head and associated gaskets.

The mechanical arrangement should also allow the required level of inspection and maintenance.

7.2 One-Pass vs Multi-Pass in Fixed-Tubesheet Exchangers

Fixed-tubesheet heat exchangers use tube sheets attached to the shell.

Straight tubes can be arranged for one or multiple tube-side passes when the channel-head design permits.

Tube-side mechanical cleaning may be possible with suitable access.

However, because the bundle is normally fixed within the shell, external tube-surface cleaning can be more difficult.

Pass count should therefore be evaluated alongside the expected fouling conditions and maintenance strategy.

7.3 Multi-Pass Arrangements in U-Tube Exchangers

A U-tube heat exchanger uses tubes bent into a U-shape, with both ends connected to a common tube sheet.

The fluid flows through one leg of the tube, turns at the U-bend, and then returns through the other leg.

In a conventional U-tube arrangement, this naturally forms a two-pass tube-side flow path.

In appropriate designs, other flow path configurations may be used, but these require specialized engineering.

U-tube heat exchangers should not be described as traditional single-pass, straight-through tube-side heat exchangers.

7.4 Multi-Pass Arrangements in Floating-Head Exchangers

Floating-head heat exchangers allow differential thermal expansion between the tube bundle and shell.

Depending on the front and rear head designs, the tube-side fluid may be routed through multiple passes.

This construction can provide access for maintenance of the tube bundle and, in suitable configurations, mechanical cleaning of straight tubes.

However, floating-head construction introduces additional mechanical components and potentially more sealing interfaces.

The number of passes should be coordinated with the selected head arrangement.

For a broader understanding of construction differences, visit Aidear's shell-and-tube heat exchanger technical guide.

8. How to Choose Between One-Pass and Multi-Pass Shell-and-Tube Heat Exchangers

8.1 Start With the Required Heat Duty

The initial selection should be based on the actual process requirements.

For single-phase fluids, heat duty can be estimated using:

Q = m × Cp × ΔT

Where:

• Q = Required heat transfer rate

• m = Fluid mass flow rate

• Cp = Specific heat capacity

• ΔT = Fluid temperature change

The calculation must use the appropriate fluid properties.

For phase-change applications, enthalpy differences should be used instead of relying only on a sensible-temperature-change equation.

8.2 Evaluate Both Fluid Circuits

The manufacturer needs information for both the shell side and tube side.

Important parameters include:

• Fluid composition

• Inlet and outlet temperatures

• Flow rates

• Operating pressure

• Design pressure

• Allowable pressure drop

• Viscosity and density

• Thermal properties

• Fouling characteristics

• Phase condition

The choice of which fluid passes through the tubes and which flows through the shell can affect the resulting thermal and hydraulic design.

8.3 Compare Candidate Pass Arrangements

Rather than selecting a multi-pass configuration automatically, engineers should compare suitable alternatives.

These are preliminary design directions rather than universal selection rules.

8.3.1 Recommended Engineering Selection Sequence

A practical selection process involves:

1. Establish the design and operating heat duties.

2. Confirm fluid properties over the expected temperature range.

3. Calculate the required thermal performance.

4. Evaluate a mechanically suitable one-pass configuration.

5. Check tube-side velocity and heat transfer coefficients.

6. Check shell-side thermal and hydraulic performance.

7. Calculate pressure drop on both sides.

8. Evaluate alternative pass arrangements where necessary.

9. Review the temperature correction factor and operating range.

10. Compare maintenance, manufacturing, and lifecycle costs.

8.3.1.1 Why the Final Selection Must Be Based on Both Thermal and Hydraulic Design

An exchanger can meet the specified heat duty while exceeding the available pumping pressure.

Alternatively, it may have acceptable pressure drop but fail to achieve the required outlet temperature.

A technically acceptable selection must satisfy both conditions simultaneously.

For that reason, thermal sizing and hydraulic evaluation should be treated as connected parts of the same engineering process.

8.4 Consider Operating Conditions Beyond the Design Point

Heat exchangers may operate under varying loads, temperatures, and fluid flow rates at different times of the year.

The layout of the heat transfer channels should be evaluated for the relevant operating range, rather than based solely on a single rated operating condition.

Partial-load operation, minimum flow rates, changes in viscosity, and maintenance conditions can all affect the suitability of a particular configuration.

9. Common Mistakes When Selecting Heat Exchanger Passes

9.1 Assuming More Passes Always Mean Better Heat Transfer

Additional passes may increase tube-side velocity, but they also increase hydraulic resistance.

They do not guarantee higher overall efficiency.

9.2 Confusing Tube Passes With Shell Baffles

Segmental baffles influence shell-side flow distribution but do not automatically create additional shell passes.

The distinction should be clear in equipment specifications and drawings.

9.3 Ignoring the LMTD Correction Factor

Different flow arrangements can change the effective mean temperature difference.

A thermal design based on ideal countercurrent conditions may overestimate the available driving force if the actual flow arrangement is not considered.

9.4 Selecting Pass Count Without Checking Pressure Drop

A thermally attractive multi-pass configuration may be unsuitable when the required pumping pressure exceeds the system limits.

Both fluid circuits must be evaluated.

9.5 Assuming Pass Count Determines Cleaning Accessibility

The number of passes does not independently establish whether the exchanger can be mechanically cleaned.

Tube geometry, channel-head access, tubesheet configuration, and bundle construction must also be considered.

10. Frequently Asked Questions About Shell-and-Tube Pass Arrangements

10.1 What Is the Main Difference Between One-Pass and Two-Pass Shell-and-Tube Heat Exchangers?

In a one-pass tube-side exchanger, fluid travels through the tube bundle once.

In a two-pass arrangement, it travels through one tube group and returns through another.

The two-pass arrangement can increase tube-side velocity but generally increases pressure drop for an otherwise comparable design.

10.2 Is a Four-Pass Heat Exchanger Better Than a Two-Pass Exchanger?

Not necessarily.

A four-pass exchanger may provide higher tube-side velocity, but it may also create considerably more hydraulic resistance.

The preferred arrangement depends on the required duty, tube dimensions, fluid properties, allowable pressure drop, and thermal configuration.

10.3 Can a Shell-and-Tube Heat Exchanger Have One Shell Pass and Four Tube Passes?

Yes.

A 1–4 arrangement has one shell pass and four tube passes.

This is distinct from a 2–4 arrangement, which has two shell passes and four tube passes.

10.4 Is a U-Tube Heat Exchanger Single-Pass or Multi-Pass?

A conventional U-tube exchanger has a two-pass tube-side flow path because the fluid travels down one leg of the U-tube and returns through the other.

Its shell-side pass count is a separate design feature.

10.5 Does Increasing Tube Passes Increase Heat Transfer Area?

No.

Increasing the number of tube passes changes how the fluid moves through the available tubes.

It does not automatically increase the physical heat transfer area.

A different pass arrangement may affect how effectively that area is used.

10.6 How Many Passes Should an Industrial Shell-and-Tube Heat Exchanger Have?

There is no universal optimum.

The appropriate number depends on heat duty, fluid properties, temperatures, flow rates, mechanical construction, and allowable pressure drop.

One-pass, two-pass, and four-pass configurations can all be suitable for different applications.

A final selection should be supported by manufacturer-specific thermal and hydraulic calculations.

11. Conclusion: Select the Right Number of Passes for Your Application

The difference between one-pass and multi-pass shell-and-tube heat exchangers is more important than the number of times fluid travels through the tube bundle.

Pass arrangement affects fluid velocity, heat transfer coefficients, hydraulic resistance, temperature profiles, and mechanical construction.

A one-pass exchanger may provide a simpler and more economical solution when the required thermal performance can be achieved with acceptable velocity and pressure drop.

A multi-pass arrangement may improve tube-side heat transfer under suitable operating conditions, particularly when higher channel velocity is required.

However, additional passes can increase pumping energy and mechanical complexity.

The most suitable configuration should balance:

• Required heat duty

• Shell-side and tube-side flow rates

• Tube velocity

• Heat transfer coefficients

• Effective temperature difference

• Allowable pressure drop

• Maintenance accessibility

• Equipment and lifecycle costs

The best shell-and-tube heat exchanger is not the one with the most passes. It is the one that meets the thermal and hydraulic requirements with reliable operation and a practical total cost.

11.1 Explore Shell-and-Tube Heat Exchanger Solutions From Aidear

Changzhou Aidear Refrigeration Technology Co., Ltd. supplies heat exchanger solutions for industrial cooling, refrigeration, HVAC, and heat recovery applications.

Shell-and-tube heat exchangers can be configured using different materials, tube arrangements, and mechanical constructions to meet specific project requirements.

For applications where one-pass and multi-pass designs are being compared, the selected configuration should be confirmed against the actual fluid conditions, thermal duty, pressure limits, and available equipment options.

Explore our Shell-and-Tube Heat Exchangers to learn more about available product solutions.

11.2 Request a Custom Shell-and-Tube Heat Exchanger Quote

Are you selecting a one-pass or multi-pass shell-and-tube heat exchanger for an industrial project?

Share the following information with our team:

• Required heating or cooling capacity

• Hot-side and cold-side fluid types

• Inlet and outlet temperatures

• Flow rates

• Operating and design pressures

• Allowable pressure drop

• Installation and maintenance requirements

Aidear can discuss suitable heat exchanger configurations based on your application requirements and the available product designs.

Request a Custom Quote to find a suitable shell-and-tube heat transfer solution for your project.