Pressure drop is one of the most important design considerations in a shell and tube heat exchanger. An exchanger may provide excellent heat transfer performance, but if it creates too much resistance to flow, it can negatively affect pumps, compressors, production rates, energy consumption, and the surrounding process.
At the same time, simply designing for the lowest possible pressure drop is not necessarily the answer. Fluid velocity helps drive heat transfer, which means heat exchanger design often involves balancing thermal performance against acceptable pressure loss.
For engineers and industrial buyers, understanding this relationship can make it easier to evaluate a heat exchanger specification and provide the information a manufacturer needs to develop the right design.
What Is Pressure Drop in a Heat Exchanger?
Pressure drop is the reduction in fluid pressure between the inlet and outlet of a heat exchanger.
As fluid moves through tubes, nozzles, fittings, baffles, and other internal flow passages, friction and changes in flow direction consume energy. The result is a lower fluid pressure at the outlet than at the inlet.
Pressure drop is commonly expressed as:
- psi
- kPa
- bar
In a shell and tube heat exchanger, pressure drop must normally be evaluated separately for the shell side and the tube side.
Why Does Heat Exchanger Pressure Drop Matter?
Every process system has limits on how much pressure it can afford to lose while moving fluid through equipment.
If a heat exchanger creates excessive pressure drop, the system may require additional pumping or compression power to maintain the required flow rate. In some cases, excessive resistance can prevent the process from reaching its intended flow rate altogether.
Too much pressure drop can contribute to:
- Reduced process flow
- Higher pump or compressor energy consumption
- Difficulty reaching production targets
- Operating problems elsewhere in the system
- Increased utility costs
- Changes in heat exchanger performance
This is why allowable pressure drop should be established early when getting a heat exchanger for a new process or replacing an existing unit.
What Causes Pressure Drop in a Shell and Tube Heat Exchanger?
Pressure loss comes from several sources throughout the exchanger.
Friction Along the Tubes
As tube-side fluid moves against the interior walls of the tubes, friction creates resistance.
The amount of pressure lost depends on factors such as:
- Tube length
- Tube inside diameter
- Fluid velocity
- Fluid density
- Fluid viscosity
- Surface condition
- Number of tube passes
Longer tubes and higher velocities generally create more resistance to flow.
Inlet and Outlet Nozzles
Pressure can also be lost as fluid enters or exits the exchanger through its nozzles.
Nozzle diameter and velocity must therefore be considered as part of the hydraulic design rather than treating the tube bundle as the only source of resistance.
Tube Passes
Many shell and tube heat exchangers use multiple tube passes. Instead of moving straight through the exchanger once, the fluid changes direction and travels through different sections of the tube bundle.
Increasing the number of passes can increase tube-side velocity, which may improve heat transfer. However, it also increases resistance because the fluid travels farther and changes direction additional times.
Shell-Side Baffles
Baffles direct shell-side fluid across the tube bundle rather than allowing it to travel directly from the inlet to the outlet.
This cross-flow can improve heat transfer by increasing fluid movement across the tube surfaces.
However, every change in direction creates additional resistance.
Baffle spacing, baffle cut, shell diameter, tube layout, and fluid properties can all influence shell-side pressure drop.
The Relationship Between Pressure Drop and Heat Transfer
One of the key challenges in heat exchanger design is that pressure drop and heat transfer performance are closely connected.
Higher fluid velocity can improve heat transfer by increasing turbulence and reducing the thickness of the fluid boundary layer at the heat transfer surface.
But increasing velocity also increases friction.
That creates an engineering tradeoff:
More velocity can improve heat transfer, but it usually increases pressure drop.
The goal is not necessarily to minimize pressure drop. The goal is to achieve the required thermal performance without exceeding the amount of pressure loss the process can tolerate.
Tube Diameter Can Affect Pressure Drop
Tube diameter is one of the variables manufacturers can evaluate when balancing thermal performance and pressure loss.
Smaller flow passages can increase fluid velocity for a given flow rate. That can improve heat transfer, but it may also increase pressure drop.
Larger flow passages may reduce velocity and pressure loss, but reducing velocity too much can decrease heat transfer performance or allow suspended solids to settle in some applications.
The correct tube size therefore depends on more than pressure drop alone.
How the Number of Tube Passes Changes Pressure Drop
The number of tube passes can significantly influence tube-side hydraulics.
Consider a simplified example.
If the entire tube bundle is used for one pass, the total tube-side flow is distributed across a relatively large number of tubes.
If the exchanger is divided into multiple passes, fewer tubes carry the flow during each pass. This increases velocity through those tubes.
Higher velocity may improve thermal performance, but it also creates greater pressure loss.
More passes are not automatically better.
A manufacturer must balance tube velocity, heat transfer requirements, pressure drop, erosion risk, fouling behavior, and mechanical configuration when determining the appropriate pass arrangement.
How Baffle Design Affects Shell-Side Pressure Drop
On the shell side, baffles are used to control how fluid moves around the tube bundle.
Closer baffle spacing generally forces the shell-side fluid to change direction more frequently. This can increase velocity across the tubes and improve heat transfer.
It can also increase pressure drop.
Wider baffle spacing may reduce resistance, but excessive spacing can reduce shell-side velocity and affect heat transfer performance.
Other considerations include:
- Baffle cut
- Baffle type
- Tube spacing
- Tube layout
- Shell diameter
- Bypass flow
- Potential vibration
These factors are evaluated together during thermal design rather than selecting baffle spacing based on a single rule.
Fluid Properties Matter
The same exchanger geometry can produce very different pressure drops depending on the fluid moving through it.
Viscosity
More viscous fluids generally create greater resistance to flow than lower-viscosity fluids.
Viscosity can also change considerably with temperature, so accurate operating temperatures are important when evaluating hydraulic performance.
Density
Fluid density affects velocity and pressure relationships and is another important input in exchanger calculations.
Phase
Gas, liquid, condensing vapor, boiling fluid, and two-phase mixtures behave differently.
Applications involving evaporation or condensation generally require additional analysis because fluid properties and flow conditions can change as the stream travels through the exchanger.
Fouling Can Increase Pressure Drop Over Time
Pressure drop is not only a design consideration. It can also be an important operating indicator.
Scale, sludge, biological material, corrosion products, or process deposits can gradually restrict flow passages inside an exchanger.
As those passages become restricted, resistance increases.
An exchanger that originally operated with an acceptable pressure drop may therefore develop a much higher pressure drop after months or years of service.
A Rising Pressure Drop Can Be a Warning Sign
If operating flow conditions have remained relatively consistent but pressure drop begins increasing, fouling or another internal restriction may be developing.
Operators should evaluate pressure drop alongside other performance indicators such as:
- Inlet and outlet temperatures
- Flow rates
- Heat transfer performance
- Pump operation
- Process throughput
A changing pressure drop is one of several indicators discussed in our guide to early warning signs that a shell and tube heat exchanger needs maintenance.
What Is an Acceptable Pressure Drop?
There is no single acceptable pressure drop that applies to every shell and tube heat exchanger.
The allowable value depends on the surrounding process.
For one application, several psi of pressure loss may be insignificant. In another application, even a relatively small additional pressure loss may interfere with process flow or equipment upstream and downstream.
Important considerations include:
- Available pump head
- Compressor capability
- Required process flow rate
- Upstream pressure
- Required downstream pressure
- Energy consumption
- Control valve requirements
- Pressure available elsewhere in the system
Instead of asking a manufacturer to simply make the pressure drop “as low as possible,” the better approach is to provide the actual maximum allowable pressure loss for each stream.
Shell-Side and Tube-Side Pressure Drop Should Be Specified Separately
The two process streams usually have different hydraulic requirements.
For example, the shell-side process may tolerate a relatively large pressure loss while the tube-side process has very little pressure available.
Your specification should therefore identify:
- Maximum allowable shell-side pressure drop
- Maximum allowable tube-side pressure drop
KAM Thermal’s shell and tube heat exchanger RFQ checklist includes allowable pressure drop among the process information that should be supplied when requesting a quote.
What Happens If the Allowable Pressure Drop Is Set Too Low?
It can be tempting to specify an extremely low allowable pressure drop because less resistance sounds better.
But an unnecessarily restrictive pressure-drop requirement can make the exchanger larger, more complex, or more expensive than the process actually requires.
Reducing pressure drop may require design changes such as:
- Increasing tube diameter
- Increasing the number of tubes
- Reducing tube-side passes
- Increasing shell diameter
- Changing baffle spacing
- Increasing nozzle size
- Changing the overall exchanger configuration
These changes can affect equipment size, thermal performance, materials, fabrication requirements, and cost.
The allowable pressure drop should therefore reflect the real limitations of the process rather than an arbitrary preference for the smallest possible number.
What Happens If the Pressure Drop Is Too High?
When actual pressure drop exceeds the value the process can tolerate, several operating problems may appear.
Reduced Flow
The pump or compressor may not be capable of overcoming the additional resistance while maintaining the required process flow.
Higher Energy Consumption
Additional pumping or compression power may be needed to maintain flow.
Reduced Production
If the required flow rate cannot be maintained, the heat exchanger may become a bottleneck in the process.
Changes in Thermal Performance
Reduced flow can alter exchanger performance, potentially preventing the process from reaching the required outlet temperature.
Maintenance Issues
If rising pressure drop is being caused by fouling or obstruction, continuing to operate without addressing the underlying issue may allow deposits to become more severe.
Pressure Drop in Replacement Heat Exchangers
Pressure drop deserves particular attention when replacing an existing exchanger.
It is not enough to know only the shell diameter, tube length, nozzle sizes, and connection locations of the old equipment.
Whenever possible, gather historical operating information such as:
- Normal inlet and outlet pressures
- Original design pressure drop
- Current operating pressure drop
- Flow rates
- Operating temperatures
- Changes in production capacity
- Fouling history
If an existing exchanger has always created excessive hydraulic resistance, simply duplicating the original design could reproduce the same problem.
A custom replacement provides an opportunity to evaluate whether changes to tube count, passes, nozzles, baffles, shell geometry, or other design variables could better match current process requirements.
Pressure Drop and TEMA Heat Exchanger Design
Shell and tube exchanger specifications commonly use standards developed by the Tubular Exchanger Manufacturers Association.
TEMA provides industry-recognized standards and specification resources used in the design and manufacture of shell and tube heat exchangers.
Additional information is available through the official TEMA standards website.
Information to Give Your Heat Exchanger Manufacturer
Accurate hydraulic design depends on accurate process data.
When requesting a shell and tube heat exchanger, provide information for both streams whenever possible, including:
- Fluid name and composition
- Flow rate
- Inlet temperature
- Required outlet temperature
- Operating pressure
- Design pressure
- Maximum allowable pressure drop
- Density
- Viscosity
- Specific heat
- Thermal conductivity when available
- Fouling characteristics
- Solids or contaminants
- Phase changes
Providing realistic operating data allows the manufacturer to balance thermal duty with the hydraulic limitations of the process.
Pressure Drop Is Part of the Complete Heat Exchanger Design
Pressure drop should never be evaluated in isolation.
It interacts with:
- Heat transfer area
- Fluid velocity
- Tube diameter
- Tube length
- Number of passes
- Tube layout
- Baffle spacing
- Shell diameter
- Nozzle size
- Fouling tendency
- Erosion risk
- Equipment size and cost
A good design balances all of these factors rather than optimizing one at the expense of the rest of the system.
Custom Shell and Tube Heat Exchangers for Industrial Applications
KAM Thermal Equipment is a custom shell and tube manufacturer specializing in heat exchangers for industrial applications.
Our team evaluates thermal duty, flow rates, allowable pressure drop, fluids, materials, operating conditions, maintenance requirements, and mechanical constraints when developing equipment for demanding industrial processes.
As a fourth-generation fabricator of specialized industrial products, KAM Thermal Equipment has been designing and manufacturing heat transfer equipment since 1906.
Need Help Evaluating Your Heat Exchanger Requirements?
If you are designing a new system, replacing an existing exchanger, or dealing with excessive pressure drop in current equipment, KAM Thermal can review your process requirements and help develop the appropriate shell and tube configuration.
Call KAM Thermal Equipment at (631) 348-4880 to discuss your heat exchanger application.