Understanding the Fouling Factor in Heat Exchangers

Well then, let’s go discuss the fouling factor in heat exchangers. If you’re working with heat exchangers, you understand, they’re vital for transferring heat through your process. But there is a problem: They get dirty. That dirt layer? It’s called fouling. And it totally wrecks your performance.

So, what’s a fouling factor? You can think of it as an engineered number designed to account for the added resistance to heat transfer due to this accumulation on the surfaces in your heat exchanger. When the thermal conductivity or thickness of that crud layer isn’t known — which is most of the time until you shut things down — the fouling factor is how engineers factor it into their design calculations. It’s literally a number, commonly referred to as Rf or Rd, and it can be viewed as the reciprocal of a heat transfer coefficient (1/htc in the imperial units). It’s units tend to appear in tables as ft²-°F-hr/BTU or m²K/W.

Why This Fouling Factor Stuff Even Matters

Picture it: If your heat exchanger is the motor cruising your heat transfer process, fouling is the gunk in the fuel line. It slows everything down.

Here’s the breakdown on what a fouling factor indicates and why it matters:

• Kills Heat Transfer: The first impact is that it decreases your U in general. A smaller U results in a reduced amount of heat crossing the surface.
• Reduces Performance: Without efficient heat transfer, your heat exchanger cannot do its job. Your heat duty drops.
• Increases Pressure Drop: Fouling clogs and decreases the area that your fluid will flow through. That is, the fluid must push harder to get through, which increases pressure drop. Increased pressure drop might also make matters worse by decreasing the flow.
• Can Hide Corrosion:In some instances, the fouling layer can obscure corrosion as it takes place below it or worse, create it. This can reduce the life of your equipment, and potentially cause them to fail.
• Costs You Money: Any and more of these problems lead to lower efficiency and high expenses. You lose production, you waste money on cleaning, and maybe you replace equipment earlier than you’d have to.

Let’s plug in a fouling factor so we can at least try to account for these nasty effects before we even begin building the exchanger.

How Do You Even Get These Numbers?

This is not always easy. The best way to determine fouling factors for your specific application is to use data from your own plants- existing operating units, historical performance data, and cleaning history. The actual fouling factors can be calculated from your production data.

But let’s be honest, sometimes you don’t have the golden data. Or perhaps you’re creating something new. That’s when you fall back on published data — average fouling factors collected over the years for various systems and fluids. Such coefficients can be found in tables and databases, such as those found in standards such as TEMA (Tubular Exchanger Manufacturers Association).

Remember, these average numbers are only estimates. They are a characteristic of the system and the equipment, and they depend on factors including the fluid, the type of heat exchanger, temperatures, velocities and the materials the heat exchanger is made out of.

The fouling resistance (Rd) can be calculated from operating data if you can determine the clean heat transfer coefficient (U) and the fouled U; Rd = 1 / Ud - 1 / U

Some Common Fouling Factors For Reference

Fine, so you need a number for the design. You’ll head to the tables. Remember that the units are crucial here — they don’t always match up. One popular unit you suspect is ft²-°F-hr/BTU. Another is m²K/W or sometimes shown as m²K/kW.

Here are a few examples, drawn from the sources, to offer a sense of the variety:

Fouling Resistance Values (ft²-°F-hr/BTU), Selected

• Fuel Oil #2: 0.002
• Fuel Oil #6: 0.005
• Hydrocarbon Light vapors (clean): 0.001
• Natural Gas Flue Gas: 0.005
• Steam (oil free): 0.0005
• Heavy Fuel Oils: 0.005 - 0.007
• Kerosene: 0.002 – 0.003
• Brackish Water (greater than 125°F, greater than 3 ft/s velocity): 0.002
• Cooling Tower Water (non-chemically treated, greater than 125°F, flow less than 3 ft/s): 0.005
• Sea Water (125F or below, 3 ft/s velocity or higher): 0.0005
• River Water (average, 125°F or less, velocity 3 ft/s or less): 0.003

Some representative fouling resistance (m 2 K/W).

• Alcohol vapors: 0.00009
• Boiler feed water, above 325 K, treated: 0.0002
• Fuel oil: 0.0009
• Seawater below 325 K: 0.00009
• Steam: 0.00009

Typical Fouling Resistance Values for PHEs compared to TEMA (m2K/kW) (Note the * 1000 change in the denominator for the kW!)

• Water Soft (PHE): 0.018
• Water Soft (TEMA): 0.18–0.35
• PHE cooling tower water: 0.044
• Cooling tower water (TEMA) = 0.18–0.35
• Seawater (PHE): 0.026
• Seawater (TEMA): 0.18–0.35

Notice how the values change with fluid, temperature, velocity and even type of exchanger (PHE vs. TEMA)? This is why it is so difficult to choose the right factor.

What Kind of Gunk Are We Even Talking About? The Types of Fouling.

Fouling isn’t one thing — it’s a whole family of nasty deposits. The sources tend to divide it into a few key types:

• Chemical Fouling (Scaling): This is limescale in your kettle. Chemical reactions in the fluid cause stuff to stick to the hot surface. Salts are the most common culprits here, as they tend to fall out of solution as temperature rises. This can be reduced by controlling the wall temperature. Chances are you’ll need either chemicals or mechanical tools to remove it.
• Biological Fouling: Organisms, such as algae or bacteria, multiply in the fluid and adhere to the surfaces. The designer of a heat exchanger can’t always prevent the organisms from being there, but the choice of materials, such as brass (which is toxic to some organisms), can make a difference. Once again, it will generally requires chemical application or brushing.
• Deposition Fouling (Sedimentation): Occurs when suspended particles in the fluid are deposited to the substrate. This happens typically when the flow impedes below a certain value. Good design can reduce this by maintaining velocities. Mounting the exchanger vertically can also be a solution, allowing gravity to draw particles off. The available cleaning approach is based on mechanical brushing.
• Corrosion Fouling: This occurs when corrosion products accumulate, creating an additional layer that hinders heat transfer. The trick is to choose corrosion-resistant materials, such as stainless steel. This is in contrast to corrosion under the fouling layer.
• Reference is also made to other fouling mechanisms such as reaction fouling (e.g., asphaltenes in crude oil) and coking.

What Makes Fouling Worse? Factors Influencing the Rate

It’s not random how quickly fouling accumulates. It’s a result of several causes:

• Fluid Properties: The fluid you’re dealing with and what’s in that fluid will be a big deal.
• Process conditions: Temps (esp wall temps), flows, and pressures all come into play. High wall temperatures can induce chemical fouling or generate such phenomena as film boiling in reboilers, which augments fouling.
• Velocity and Shear Stress: Especially important for single-phase fluids is the high velocity and shear stress at the wall. High shear stress can avoid particle deposition, and the low wall temperature can mitigate scaling. There’s often a tradeoff here, because of course higher velocity also means higher pressure drop (and therefore higher pumping cost).
• Equipment Design: Flow patterns and shear stress are influenced by heat exchanger type (shell-and-tube vs. plate, etc.) and geometry (tube diameter, channel width, baffles). These “stagnant” zones can result in over fouling.
• Materials and Surface: The construction materials are of high importance, in particular with respect to corrosion and biofouling. The texture of the surface is also relevant; smoother surfaces are less foulded.
• Time: Fouling is a time-dependent behavior, it develops over time. And the rate is not always steady.

Implications of Fouling Factor in Design: The “Margin”

This is where the “fouling factor” does its job, or at times serves as a placeholder or “fudge factor”. In heat exchanger design, one uses an adjusted overall heat transfer coefficient – the “fouled” coefficient (Uf).

Heat balance equation is written in an alternate form as Q = U A LMTD (The Basic Principle of Heat Transfer = Heat Duty = Overall Heat Transfer Coefficient Area Log Mean Temperature Difference).

When you design for fouling, you design w Uf instead of with the “clean” coefficient (Uc). The fouled resistance (1/Uf) is the addition of the clean resistance (1/Uc) and the fouling resistance (ff or Rf) respectively:  (38) 1/Uf = 1/Uc + ff (or Rf) 3.2.4 Overall heat transfer coefficient Once the µ value is calculated the total heat transfer coefficient (U) can be estimated using eq. (39).

1/Uf = 1/Uc + ff

Because you’re throwing more resistance into the mix, 1/Uf goes up, which means Uf goes down.

Therefore if Q and LMTD are fixed and you are considering a lower Uf then you need the larger heat transfer area (A) in order to solve equation. This deferred area is called “fouling margin” or “excess surface area margin”. It’s building in a buffer to the design, so that after some crud does build up, the exchanger can still meet its target performance. This is sometimes also referred to as the “cleanliness factor” (cf). Uf requires more area than if designing clean.

The notion is that the exchanger is over-performing when new and preferably settles in down to your target as it fouls, until it becomes time to clean it. Positioning may be required to sustain the predetermined heat duty for a particular “run-length”.

Here’s the Catch: Where It Falls Short and Why It’s Not Perfect

It is common to simply use traditional values of fouling factors from tables, but there are some serious issues with this.

• High level of uncertainty: These numbers come with a “significant level of uncertainty”. Applying them is sometimes known as adding a “safety margin,” but it is frequently a guess.
• Leads to Oversizing: Use of conservative, average values often results in mechanical heat exchangers, which are larger than actually required. In some cases “two or three times more area than is really necessary”.
• Oversizing is Expensive: Larger exchangers are more expensive in terms of both capital and installation costs.
• Oversizing Could Make Fouling WORSE: This is crazy, isn’t it? Yet a larger exchanger requires larger flow paths, and it would slow the fluids down, possibly to the point that other space-efficient technologies, like plate exchangers, make more sense, Heaton says. Reduced flow velocity, particularly in the case of single phase fluids, can cause an increase in the fouling rate. It can also produce dead zones in the shell side.
• Doesn’t Capture Reality: Standard terms fail to represent how the fouling behavior evolves over time and my not accurately reflect the dependence on specific conditions such as velocity and temperature. Divide by the same factor for all velocities doesn’t even make sense.
• Non-Universal: TEMA fouling factors that can be widely used for shell and t ube heat exchangers, cannot be used for (PHEs). TEMA values to PHEs result in over-sized areas and bad designs.
• Absurd Results: If the same fouling thickness taken from standard factor of large tube is used in for small channel in man exchanger, unbelievable increase in pressure drop occurs.

This points to the problem: The conventional method is easy, but it’s a blunt instrument, and can be overly blunt — to the point of reinforcing the very problem you’re trying to solve.

So, How Do You Design to Push Back?

You can’t eliminate all fouling, but design choices can mean a world of difference when it comes to slowing it down and determining how much it will matter.

Here are some strategies:

• Maximize Velocity/Shear Stress (Single-Phase): This is perhaps the most important of all. High fluid velocity to achieve high shear stress at the wall. Try to achieve shear stress above ~10 Pa if you can. Relatively, reducing wall temperatures also restricts particle deposition. You’ll pay for it in pressure drop, but it might pay you back even more in the long run in terms of reduced fouling.
• Control Wall Temperature: In the case of chemical fouling/scaling, minimize the wall temperature. In the boiling systems (reboilers, steam generator), control wall temperature with care and do not generate unnecessary vapor to create dry spots. Try simulating under clean conditions to see what the start-up temperatures are.
• Good Flow Distribution: Ensure that the fluid reaches all the heat transfer surfaces uniformly to prevent any ‘Dead Zones’, in which particles can accumulate. That sticky heat is also important in condensers.
• Select the Proper Materials: For that kind of fouling, use anti-corrosion materials. Smooth surfaces, such as polished stainless steel, also decrease particle sticking.
• Think About Performance Tubes: Corrugated tubes for instance, promote turbulence and increase the rate of heat transfer at lower velocities and can help minimize deposition and chemical fouling, by keeping the wall temperature closer to the bulk fluid temp.
• Installation: If installed vertically it will assist witthe deposition fouling as gravity can be utilised.
• Use Better Models: Some designers use models larger than a table (like Scribbs’s Ebert-Panchal model) that directly consider velocity, temperature, etc. These may also identify situations where fouling will not occur, allowing for reduced capital costs. One can expect these sorts of models to be used more in the future.
• Exploit PHE Benefits: PHE 17 naturally promotes relatively high shear stresses, because of small channels and corrugation of PHE plates. This tends to result in a possibility of a lower fouling margin than with shell-and-tube exchangers. And with gasketed PHEs, you can always add or remove plates later, if you find you need more or less capacity.
• Economics: Evaluate the initial (smaller exchanger / higher pressure drop) versus the operating / maintenance (pumping cost versus cleaning frequency) trade-off. Unless it leads to significantly worse fouling, don’t just settle for a low pressure drop.
• Give Margin its Due: Yes, you do still need margin. But provide it smartly. Not just with big, conservative factors. Too much margin doesn’t buy proportionately longer run-lengths and, in some cases, merely exacerbates fouling. There is some indication either that the high-fouling limit may be 50% excess surface, or that for low fouling it may be much less. Consider size impact for even “clean” fluids.
• Think About Redundancy: Spares are useful even in 2×100% or 3×50% designs that let you keep going when a unit is out for cleaning or servicing.
• Analyze the Fluid: Analyze the specific fluids you are working with in order to obtain a correct fouling factor.
• Design for cleaning: Facilitate access to clean the exchanger.

Bottom Line on Fouling Factors

Fouling is the enemy of a heat exchanger, resulting in poor performance and increased pressure drop, not to mention higher costs. The fouling factor is the conventional method employed in the design to allow for the deposition whether or not it inevitably occurs. This results in designing to a lower overall heat transfer coefficient, since the greater exchanger area simply adds to the “margin”.

Nevertheless, the exclusive use of regular, fixed fouling factors has huge disadvantages: very high uncertainty, overdimensioning costs and even a worsening of fouling because of reduced velocities.

The wise approach is to let these design measures to reduce the frequency of fouling to be complemented by an adequate, not extreme, fouling margin. It may be game changer pushing technologies with higher shear like corrugated tubes or PHEs. Ideally you want to keep your heat exchanger working well enough to not need cleaning for a good amount of time, at a reasonable cost both in terms of capital cost and operating cost and reliability.

FAQ

• What is the fouling factor of a heat exchanger? The fouling factor (Rf or Rd) is the resistance to heat transfer caused by the build-up of a deposit layer on the heat transfer surface. This layer acts as an insulator. It represents an additional resistance included in design calculations to account for reduced performance over time.

• How do you calculate the fouling factor? The best source is calculation from actual plant operating data (comparing clean vs. fouled performance). It can be mathematically determined using the overall heat transfer coefficients of a clean (U) and fouled (Ud) heat exchanger: Rd = 1 / Ud – 1 / U. Fouling factor (Rf) can also relate to deposit thickness (δ) and conductivity (λf): Rf = δ / λf. Tables of typical values are also used when plant data isn’t available.

• What is fouling and what is the formula for it? Fouling is the undesired accumulation of deposits on heat transfer surfaces. It causes a degradation in heat transfer performance by forming an insulating layer. Fouling is a complex phenomenon dependent on time, fluid properties, temperatures, velocities, and exchanger design. There isn’t a single simple formula for fouling itself, but its rate is the difference between deposition and removal rates. The concept is typically integrated into heat transfer calculations by adding the fouling factor as a resistance.

• What is the F factor in a heat exchanger? Based on the sources provided, the “F factor” most likely refers to the fouling factor (also noted as Rf, Rd, or ff). It quantifies the thermal resistance due to the fouling deposit and is used in design to account for performance degradation.

• How do you reduce fouling factor? You reduce the impact of fouling (effectively reducing the operating fouling factor) through design and operation:

  • Increase fluid velocity/shear stress: This minimizes particle deposition and aids removal. Plate heat exchangers naturally create high shear.
  • Control wall temperature: Crucial for reducing chemical scaling. Lower heating medium temperatures help prevent film boiling in reboilers.
  • Ensure even flow distribution: Avoids stagnant zones where deposits can settle.
  • Select appropriate materials: Use corrosion-resistant materials and potentially materials toxic to biological organisms. Smooth surfaces help minimize particle collection.
  • Use corrugated tubes: Enhances turbulence and keeps wall temperatures closer to bulk fluid temperature, reducing deposition and chemical fouling.
  • Maintain design flow rates: Operating below design promotes fouling.
  • Treat fluids: Feed water treatment can prevent corrosion.
  • Avoid excessive oversizing: Using overly large fouling factors in design leads to larger exchangers with lower velocities, potentially increasing fouling.

• How do you detect fouling in a heat exchanger? Key indicators include:

  • Reduced heat transfer performance: This is considered an effective and reliable sign. The unit may not achieve its required heat duty.
  • Increased pressure drop: Deposit build-up reduces flow area, increasing resistance. However, this is considered unreliable as a sole indicator of performance.
  • Calculation from operating data: Determining the actual fouling factor over time from measured temperatures and flow rates.
  • Visual inspection: If the design allows, examining the surfaces can reveal deposits.