In the intricate network of a process plant, heat exchangers are vital components. However, their optimal performance can be compromised by a critical factor: pressure drop (ΔP). An excessive ΔP is not just a technical concern; it’s a symptom of inefficiency that translates directly into higher operating costs, increased energy consumption, and potentially, a reduced equipment lifespan.
An abnormally high ΔP can exceed 5% of the absolute pressure for gases or two-phase flows, or surpass 0.3 to 0.7 bar (5 to 10 psi) for single-phase liquids. Even for high-viscosity liquids, values above 1.5-2.0 bar (150-200 kN/m²) are a cause for alarm. At JAZAM, we understand that proactive management of pressure drop in heat exchangers: diagnosis, optimization, and strategies to reduce operating costs is essential for the financial and operational health of your plant. In this guide, we will break down the causes, their implications, and the most effective strategies to mitigate it.
The hidden cost of high pressure drop: Impact on your results
We understand that plant management and administration professionals focus on tangible results. A high ΔP, while seeming like a technical detail, has direct and significant consequences on your business indicators:
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Increased OPEX (operating costs): A higher pressure drop means that pumps or compressors must work harder to move fluids through the exchanger. This translates into greater consumption of electrical energy or fuel, significantly inflating operating costs over time. For example, in an industrial pumping system, a sustained increase of 0.5 bar in an exchanger’s ΔP can increase the pump’s annual electricity consumption by thousands of dollars.
Reduced capacity and efficiency: A high ΔP can limit the maximum flow rate that can be processed, creating production bottlenecks. Furthermore, it is often an indication of a low heat transfer coefficient, which reduces the overall efficiency of the system. Imagine a food processing plant where the exchanger responsible for cooling a key product experiences a high ΔP. This could reduce its cooling capacity, forcing a lower production rate, directly impacting delivery and profitability.
- Increased risk of failure and maintenance: A high ΔP can be associated with excessive fluid velocities or the accumulation of deposits, which increases the risk of erosion, flow-induced vibrations, and material fatigue, leading to premature failures and costly unscheduled shutdowns. A recurring case is erosion in the tubes near the inlet nozzles due to excessive velocities, leading to perforations and the need for premature replacement of the tube bundle.
Precise diagnosis: Identifying the root causes of high ΔP
To address the problem, one must first understand its origins. The causes of a high ΔP are diverse and often interconnected:
1. Fouling
This is the most common and recurring culprit. Deposits of scale, biofilms, or corrosion products accumulate on the heat transfer surfaces, reducing the effective diameter of the tubes and shell-side passages. This restriction not only decreases the overall heat transfer coefficient (U) but increases the pressure loss. For a deeper dive into this topic, we invite you to read our full article: The complete guide to heat exchanger fouling: diagnosis, prevention, and robust design.
2. Inadequate flow velocities
Excessively high velocities increase friction and, therefore, the pressure drop. This not only raises pumping costs but can also induce severe vibrations and erosion of the tubes. Vibrations are a significant concern, especially in large exchangers handling gases or steam on the shell side.
3. Suboptimal geometric design
The internal configuration of the exchanger has a direct impact:
Tube arrangement: The layout of the tubes (triangular vs. square) influences the pressure drop and ease of cleaning. A triangular layout allows for greater compactness but makes mechanical cleaning difficult; the square layout is preferable for very dirty fluids.
Baffles: The number and spacing of baffles control the flow on the shell side. Excessive or insufficient spacing can reduce cross-flow efficiency and generate an inadequate ΔP. The pressure drop increases more rapidly than the heat transfer coefficient when reducing spacing. Segmental baffles are used to direct the flow in the shell and improve transfer. The minimum TEMA spacing is the greater of one-fifth of the internal shell diameter or 50.8 mm (2 inches). If you do not have the design and simulation tools to optimize this critical balance, an analysis in ASPEN EDR can provide the precise sizing and specifications for your exchanger.
Nozzles and connecting pipes: Undersized nozzles contribute significantly to pressure loss. The pressure drop in the nozzles should not exceed 5% of the total loss for single-phase fluids and 12% for two-phase fluids. For example, in a large-diameter inlet nozzle, if the velocity exceeds 15-20 m/s for steam or 3-5 m/s for liquids, the pressure drop becomes considerable, impacting overall performance.
4. Accumulation of non-condensable gases
In condensers, the presence of inert gases can block the condensation surfaces, drastically reducing the heat transfer coefficient and, therefore, indirectly affecting the ΔP by requiring more severe operating conditions. A classic example is the accumulation of air in a steam condenser, which forms an insulating layer and reduces heat transfer, forcing operation at a higher pressure to compensate, which in turn increases the overall ΔP.
5. Operation and maintenance issues
Inadequate flow control, sudden changes in temperatures or flows, or lack of purging/draining can exacerbate pressure drop and fouling. Lack of purging on the shell side of a kettle-type reboiler can lead to the accumulation of heavy liquids, decreasing the vaporization zone and increasing the ΔP.
Solution and optimization strategies: Restoring efficiency
Once the root cause has been diagnosed, implementing solutions may require operational adjustments or design modifications.
2.1. Operational adjustments and maintenance: The first line of defense
Periodic cleaning: It is essential to perform cleaning and maintenance when thermal efficiency begins to degrade noticeably, not waiting for it to drop drastically. A 10-15% decrease in the overall heat transfer coefficient can be an early signal to consider cleaning. Methods can be chemical, mechanical, or hydrodynamic. Experience indicates that exchangers may require cleaning in periods of 1 to 1.5 years due to deposit accumulation.
Fluid management:
Dirty/corrosive/hazardous fluid: Whenever possible, the fluid with the highest fouling tendency, most corrosive, or most hazardous should circulate on the tube side. This facilitates cleaning (e.g., with brushes or high-pressure flow to remove layers) and reduces the shell’s exposure to corrosion. A practical example: in the petrochemical industry, crude oil, which is prone to fouling and corrosive, usually goes through the tubes.
High-pressure fluid: It is recommended that the higher-pressure fluid flows on the tube side for better containment and a simpler mechanical design.
High-viscosity fluid: Place the high-viscosity fluid in the shell. If it flows through the tubes, it will require higher pumping power and, therefore, a higher ΔP on that side.
Optimization of operating velocities: For cooling water, design velocities of approximately 2 m/s in the tubes and 0.6 m/s in the shell are suggested. It is crucial to avoid velocities below 0.9 m/s (3 fps) for water in condensers, as they promote fouling. For example, in the analyzed “Case H,” the velocity of 0.44 m/s on the tube side was considered too low, contributing to fouling.
Purging of non-condensables: In condensers, ensure adequate purging of non-condensable gases at the cold end to maintain efficiency.
2.2. Design modifications (when necessary):
When operational adjustments are not sufficient, design modifications become indispensable.
Review of the tube bundle geometry:
Number and diameter of tubes: An increase in the number of tubes or the exchanger diameter can reduce the pressure drop. For example, going from 306 to 350 tubes can decrease the pressure drop, although it increases the overdesign percentage, which can be 22.57% as observed in simulations. Tubes of 3/4″ or 1″ outer diameter are common, and API 660 recommends a minimum of 14 BWG for materials like aluminum and carbon steel.
Length of the exchanger: Increasing the exchanger’s length can reduce the pressure drop, although it also increases the exchange surface area. For example, an 18-foot long exchanger may achieve a lower ΔP than a shorter one with the same transfer area.
Spacing and type of baffles: If the pressure drop is very high, increasing the spacing between baffles can reduce it. However, this could decrease turbulence and, therefore, the heat transfer coefficient, representing a critical balance. Segmental baffles are used to direct the flow in the shell and improve transfer. The minimum TEMA spacing is the greater of one-fifth of the internal shell diameter or 50.8 mm (2 inches). If you do not have the design and simulation tools to optimize this critical balance, an analysis in ASPEN EDR can provide the precise sizing and specifications for your exchanger.
Exchanger configuration: For large temperature differences (greater than 50-60°C) between fluids, it is essential to use exchangers with floating heads or U-tubes to accommodate the differential thermal expansion between the shell and tubes, avoiding excessive stress. Consider multiple passes for more efficient heat transfer. The counter-current configuration is the most thermally effective. If there are temperature crosses or the correction factor FT is low, the shells should be divided in series.
Series or parallel arrangements: Series arrangements are for low flow rates and large heat duties (with risk of high pressure drop), while parallel arrangements are for high flow rates with moderate temperature drops. A practical example: if you need to cool a fluid with a large temperature drop but with a limited flow rate, a series arrangement of two small exchangers might be more efficient than a single large one, allowing for a better ΔTlm and ΔP control.
2.3. The crucial tool: Simulation and optimization with ASPEN EDR
The complexity of the phenomena affecting pressure drop demands precision tools. Software like ASPEN EDR is indispensable for accurate diagnosis and optimization. These tools allow for:
Precise modeling: Simulating the exchanger’s behavior in design, rating, or simulation modes.
Parametric analysis: Virtually adjusting parameters such as the number of tubes, shell diameter, tube length, and baffle spacing, and observing their direct impact on the pressure drop and overdesign percentage. This allows for finding the optimal balance between the required heat transfer and an acceptable ΔP, with a reasonable overdesign to compensate for fouling. For example, simulating the impact of going from a single tube pass to two passes, as analyzed in “Case H,” allowing the prediction that the tube velocity would approximately double, and the pressure drop would increase from 0.04495 bar to 0.3596 bar, still within permissible limits.
Identification of problem areas: Generating temperature and flow profiles that help identify stagnation zones (low velocities and high risk of fouling) or high velocities (risk of erosion and vibrations).
If your current equipment exhibits a high ΔP that affects its operation, do not wait for it to become a costly failure. Request a technical evaluation with our engineers specialized in simulation with ASPEN EDR.
The JAZAM advantage: from simulation to predictive certainty
Tools like Aspen EDR are powerful, but their value depends on the engineer’s expertise. The use modes are explained below to understand their power.
Conclusions and key recommendations
The pressure drop in heat exchangers: diagnosis, optimization, and strategies to reduce operating costs is a vital indicator of your process’s health. A high ΔP is not an isolated problem but a symptom of underlying inefficiencies that affect profitability and reliability.
A proactive approach that combines a precise diagnosis of the causes (fouling, inadequate velocities, suboptimal design), the implementation of operational adjustments (cleaning, fluid management), and, when necessary, design modifications validated by simulation software like ASPEN EDR, is fundamental to ensuring the optimal performance and longevity of these critical assets.
At JAZAM, we are committed to optimizing your heat exchangers. Our team of expert engineers uses the most advanced simulation tools to diagnose, design, and optimize solutions that not only solve pressure drop problems but also improve your energy efficiency and extend the life of your assets.