A Technical Guide: Hold Tube Design

The holding tube is often considered the "heart" of a continuous flow thermal processing system (Fig. 1). It is the critical section where the required lethality (F-value) is delivered to ensure a microbiologically safe final product. Proper design of the holding tube — including length (L), internal diameter (ID), slope, and flow rate (Q) — directly determines product safety, process compliance, and operational efficiency.

This article provides a comprehensive, science-driven overview of holding tube design based on the latest regulatory guidelines, engineering best practices, and validated process authority recommendations.

Figure 1. Illustration of the main components of an aseptic system.

1. Regulatory & Instrumentation Requirements for Holding Tube Design

Designing a compliant holding tube requires strict adherence to international regulations, including FDA 21 CFR Part 113, the Pasteurized Milk Ordinance (PMO) for dairy, and global guidelines from EFSA and Codex Alimentarius. The holding tube must remain unheated along its entire length (but it can be insulated) and be installed with a minimum slope of 0.25 in/ft to: 1) prevent air entrapment and 2) ensure proper drainage (FDA, 2025). It must be constructed from food-grade stainless steel, be fully cleanable, and compatible with CIP/SIP systems. Smooth internal finishes (internal surface roughness (Ra): ≤ 0.8 µm) and the elimination of dead zones, bypasses, and short-circuits are critical to ensure that even the fastest-moving particle achieves the validated residence time required for delivering the scheduled lethality (F-value). Both the length (L) and internal diameter (ID) must be scientifically determined and verified during installation. Regulations also mandate precise monitoring using validated instrumentation (FDA, 2025):

• A Temperature Indicating Device (TID) — the legal reference thermometer — is placed at the holding tube exit, must be calibrated and certified, and serves as the official compliance point.

• A Temperature Recording Device (TRD) which continuously records product temperature; it must never read higher than the TID. Any deviations automatically trigger flow diversion.

• Flow meters that measure volumetric throughput upstream of heating and are required to accurately calculate residence time; they must be routinely calibrated.

• Pressure gauges are mandatory, to monitor any “flashing” event within the hold tube; furthermore, in direct steam injection systems, they ensure that product pressure remains above steam pressure to prevent dilution or unsafe condensation.

• Chart recorders or digital equivalents that must document temperature, flow rate, and pressure profiles continuously. These records are FDA-mandated, must be permanent, dated, and signed to ensure traceability and audit compliance.

In their totality, these design principles and monitoring controls form the foundations of a validated, FDA-compliant holding tube system, ensuring process reliability, food safety, and regulatory adherence.

2. Engineering Principles of Holding Tube Design

The goal: ensure that every food particle, including the fastest, remains in the holding tube for at least the required minimum residence time, at the minimum required temperature to achieve the desired microbial lethality and safety.

2.1. Lethality: F-value

For continuous-flow thermal processing systems, the entire scheduled lethality (F-value, in minutes) is delivered within the holding tube. The F-value is determined by a qualified process authority based on thermobacteriological data, microbial inactivation kinetics, and the final safety status of the product —whether shelf-stable or refrigerated. Product characteristics such as pH, water activity (a_w), viscosity, and the presence of particulates or multiphase components also influence the F-value determination.

In continuous-flow systems, the F-value is calculated using Equation (1), assuming the minimum required product temperature (T, in °F or °C) remains constant throughout the holding tube. The reference temperature (T_ref) and the z-value (the temperature change required for a 10-fold change in microbial death rate (D-value)) are derived from validated thermobacteriological data and are directly tied to the targeted safety level of the specific product (Chandarana et al., 2010; David et al., 2023).

Since thermal processing combines time and temperature, the parameter t_res —representing the minimum residence time of the fastest-moving particle or least thermally treated portion of the product— is critical to ensuring process safety. For homogeneous liquids under conventional heating conditions, this fluid particle is typically located at the geometrical center of the holding tube, where flow velocity is highest under flowing conditions (Chandarana et al., 2010; David et al., 2023).

2.2. Residence Time Calculation

The holding tube must be carefully designed to ensure that every product particle, including the fastest-moving one, remains inside the tube long enough to achieve the required microbial lethality (F-value) (Chandarana et al., 2010; David et al., 2023). This requirement is validated by calculating the minimum residence time (t_res) using the measured holding tube length (L, in m, ft or in) and the maximum particle velocity (uₘₐₓ, in m/s) — the speed of the fastest-moving particle — while also accounting for Correction Factors (CF) for the flow regime, thermal expansion, and volumetric steam expansion effects (Chandarana et al., 2010; David et al., 2023; IFTPS, 2023):

To ensure food safety in continuous-flow thermal processing, the holding tube must be designed so that even the fastest-moving particle remains in the tube long enough to receive the scheduled thermal lethality. This guarantees that the required F-value is delivered under the worst-case flow scenario (Chandarana et al., 2010; David et al., 2023; IFTPS, 2023).

However, before estimating the minimum residence time, several flow parameters must first be calculated. The critical steps include:

1. Calculating the average velocity (uₐ_vg, m/s) based on the operating flow rate (Q, gpm) and the inner diameter, ID (D, in or m) of the holding tube.

2. Determining the flow regime (laminar vs. turbulent) using the Reynolds number (N_Re), which significantly impacts velocity distribution and safety margins.

3. Applying thermal expansion corrections when the operating temperature reduces product density and increases velocity.

4. Applying volumetric steam expansion corrections when direct steam injection or steam infusion increases product volume, further affecting residence time.

By incorporating these factors, the calculation of uₘₐₓ — the velocity of the fastest-moving particle — becomes accurate and conservative, ensuring that even under challenging flow conditions, the holding tube delivers the scheduled lethality and maintains regulatory compliance (Chandarana et al., 2010; David et al., 2023; IFTPS, 2023).

2.3 Calculate Average Velocity

Where:

• u_avg = Average fluid velocity [m/s]

• Q = Volumetric flow rate [gpm or m³/s, etc.]

• A = Cross-sectional area of the holding tube [m²]

• D = Internal diameter of the holding tube [in or m]

  
  

2.4 Determine Flow Regime (Reynolds Number)

The Reynolds number (N_Re) is a dimensionless value used to determine whether the product flow inside the holding tube is laminar or turbulent. It is calculated using Equation (4), assuming the product behaves as a Newtonian fluid:

Where:

• ρ = Product density [kg/m³]

• u_avg = Average velocity [m/s]

• D = Internal diameter [m]

• μ = Dynamic viscosity [Pa·s]

  
  

In this blog, we assume a Newtonian fluid model to maintain consistency with FDA filing forms and 21 CFR Part 113 requirements. However, if your product exhibits non-Newtonian behavior — scientifically determined through rheological testing of shear rate vs. shear stress — the effective Reynolds number must be calculated using the proper viscosity model for that product. The Reynolds number plays a critical role in residence time calculations, as it defines the flow regime, which directly impacts the maximum particle velocity (uₘₐₓ) and, ultimately, the minimum residence time needed to achieve the scheduled lethality.

Flow regime classification:

• N_Re ≤ 4000 → Laminar flow (velocity profile sharply peaked; CF = 2.0)

• N_Re > 4000 → Turbulent flow (velocity distribution more uniform; CF = 1.2)

Although fluid mechanics literature defines a transitional region between N_Re ≈ 2100–4000, this transition zone is ignored for food safety purposes. Under FDA-aligned food engineering practices, the entire region below N_Re = 4000 is treated as laminar flow. This approach ensures a worst-case design scenario, meaning residence time is always calculated assuming the highest possible particle velocity, minimizing the risk of under-processing. This methodology is consistent with classic food engineering design principles and is the standard approach used by FDA filings, and process authorities worldwide.

Note: For products with high starch content (e.g., potato-based formulations), a higher critical Reynolds number than the standard 4000 may be required. This is because starch gelatinization occurs at typical processing temperatures, significantly increasing viscosity and altering flow behavior. In such cases, always consult a qualified process authority to ensure accurate flow characterization and regulatory compliance.

2.5 Calculate Maximum Velocity

The maximum particle velocity (uₘₐₓ) represents the speed of the fastest-moving particle inside the holding tube — typically located along the geometrical centerline. Equation 5 describes the relationship of uₘₐₓ with u_avg and the parameters that may affect the flow within the hold tube (Chandarana et al., 2010; David et al., 2023; IFTPS, 2023):

1. The flow regime (laminar vs. turbulent)

2. Thermal expansion effects (density decreases at high temperatures)

3. Volumetric steam expansion effects (product volume increase from steam condensation in direct steam injection systems)

Where:

• u_avg = Average velocity of the product (m/s)

• CF = Correction factor based on flow regime

• CFₜₕₑᵣₘ = Thermal expansion correction factor (adjusts for density reduction at higher temperatures)

• CFₛₜₑₐₘ = Volumetric steam expansion correction factor (applied when direct steam is injected into the product)

Key points to ensure process safety:

• When designing for laminar flow, residence times are typically conservative because uₘₐₓ is highest — this is considered the worst-case scenario for lethality.

• If a process assumes turbulent flow, it becomes critical to verify the minimum operating flow rate. When the flow drops below the critical Reynolds number, the system transitions into laminar flow, significantly increasing uₘₐₓ and reducing residence time.

• Any process relies on turbulent assumptions must be validated to ensure that residence time — and, therefore, lethality — remains within safe limits even under low-flow conditions.

2.5.1 Flow regime

In laminar flow, the fluid particle traveling at the geometrical center of the holding tube moves significantly faster than the average fluid velocity. This requires applying a correction factor (CF) of 2.0 to estimate the maximum particle velocity (uₘₐₓ) (Chandarana et al., 2010).

In turbulent flow, velocity distribution is more uniform across the tube’s cross-section, and a lower correction factor of 1.2 is typically used. Because uₘₐₓ is highest in laminar flow, designing the process based on laminar conditions represents the most conservative, worst-case scenario when establishing a validated thermal process.

However, if turbulent flow is assumed for process design, it becomes critical to identify the minimum operating flow rate — the point at which the Reynolds number (N_Re) drops below the critical threshold and the system transitions to laminar flow. Operating below this limit reduces the residence time of the fastest-moving particle, which can compromise microbial lethality and overall product safety.

Therefore, any process designed assuming turbulent flow must be validated at all potential flow rates to ensure food safety.

⚠️ Recommendation: Advanced Food-Tech Solutions advises using the laminar correction factor (CF = 2.0) unless turbulent flow is scientifically demonstrated, validated, and documented.

2.5.2 Thermal Expansion Correction

At high processing temperatures—common in aseptic processing and UHT systems—the density of the product decreases as temperature increases, and hence food volume increases (Lewis and Heppell, 2000). This density reduction leads to an increase in volumetric flow rate within the holding tube, which directly increases particle velocity and can shorten residence time.

Because residence time depends on the velocity of the fastest-moving particle, thermal expansion must be carefully considered when designing or validating the holding tube. To correct for this effect, the thermal expansion correction factor (CFₜₕₑᵣₘ) is applied (Eq. 7).

Where:

• ρ_fm → Product density measured at the flow meter or pump temperature

• ρ_ht → Product density measured at the holding tube operating temperature

Since most liquid foods are largely water-based, it is standard practice to calculate CFₜₕₑᵣₘ using water density data from steam tables unless accurate product-specific density data is available.

When Thermal Expansion Can Be Ignored

In many cases, particularly for volumetric flow-controlled systems, thermal expansion has minimal effect and can be assumed negligible by setting, CFₜₕₑᵣₘ = 1.00:

This assumption is typically valid for:

• Acid foods and low-temperature processes

• Situations where the flow meter is installed at the holding tube inlet

• Indirect heating systems, where the cumulative lethality achieved in heating and cooling heat exchangers compensate for small residence time variations

This approach is widely accepted by process authorities when validated under normal operating conditions.

When Thermal Expansion Correction Must Be Applied: For high-temperature processes—such as direct steam injection or high-temperature aseptic treatments—thermal expansion can significantly reduce the effective residence time.

In such cases (Lewis and Heppell, 2000; IFTPS, 2023):

• Measured density data at both the metering point and the holding tube is strongly recommended.

• If measured data is unavailable, it is a common assumption in industry to apply a default correction factor of 6%, CFₜₕₑᵣₘ = ≈1.06. This default value assumes a worst-case expansion scenario for products processed within the range of aseptic processing temperatures, where change of food volume occurs (100 °C to 160 °C).

This default assumption 6% is based on two key principles:

1. Saturated steam properties are assumed, meaning the volume expansion of food behaves similarly to that of saturated water under typical aseptic system conditions.

2. In the critical aseptic processing temperature range of 100 °C to 160 °C, experimental and theoretical data show that the food volume expansion behaves approximately linearly, with an increase of 1% of volume every 10 °C increase.

Because of this near-linear relationship, using a 6% correction factor is widely accepted by many process authorities as a conservative worst-case assumption — especially when actual product-specific density data is unavailable.

If thermal expansion has a significant impact on residence time, a documented CFₜₕₑᵣₘ calculation or scientific justification must be included in the scheduled process filing. This is particularly critical for direct steam injection systems and other high-temperature aseptic processes, where neglecting thermal expansion could lead to overestimating lethality and non-compliance during audits or FDA process reviews.

2.5.3 Volumetric Steam Expansion (Direct Steam Injection Correction)

In direct steam injection (DSI) or steam infusion systems, steam is introduced directly into the product stream, where it condenses and transfers heat rapidly. This condensation increases the product’s total volume within the holding tube, resulting in a higher particle velocity and a reduced residence time if not accounted for in design calculations.

To correct for this effect, the volumetric expansion correction factor (CFₛₜₑₐₘ) is applied using the following formula (IFTPS, 2023):

Where:

• ΔT = Product temperature rise (°C or °F) between the product temperature in the last heater prior to steam injection an the product temperature at the exit of holding tube (after steam injection)

• Cₚ = Specific heat capacity of the product (J/kg·°C)

• ΔHᵥₐₚ = Latent heat of vaporization of steam (J/kg)

Because most liquid foods have thermal properties similar to water, it is acceptable to use water properties to determine the volumetric expansion correction factor when calculating the impact of steam addition. It is also standard practice to use saturated steam properties at the holding tube operating temperature. These values can be obtained from published steam tables. For example, for water heated to approximately 140 °C (284 °F), the correction factor is approximately 1% per 10 °F temperature rise (Eq. 8b). This simplified form provides a quick estimation when accurate Cₚ and ΔHᵥₐₚ data are unavailable (Lewis and Heppell, 2000; IFTPS, 2023).

When Volumetric Expansion Can Be Ignored: In indirect heating systems (e.g., heat exchangers) where steam does not directly enter the product stream, volumetric expansion can be neglected, CFₛₜₑₐₘ = 1.00.

When Volumetric Expansion Must Be Applied: In all direct steam injection system. Also, there is a mandatory requirement by FDA 21 CFR Part 113, that volumetric expansion effects be addressed and documented when using steam injection systems.

3. Estimating the Minimum Required Temperature

Thermal processing is fundamentally based on the time–temperature relationship. Using Equation (9), the minimum required process temperature (T) at the exit of the holding tube can be calculated when the target lethality (F-value) and the minimum residence time (t_rₑₛ) are known. Conversely, if the required temperature is predetermined, the equation can also be rearranged to calculate the minimum residence time needed to achieve the scheduled lethality (Chandarana et al., 2010). The F-value represents the equivalent time at a defined reference temperature (T_ref) required to achieve a specific logarithmic reduction in the target microorganism population. This relationship depends on the z-value, which represents the temperature change required to achieve a 10-fold change in the D-value.

Where:

• D-value (decimal reduction time): The time (minutes or seconds) required at a constant temperature to reduce the microbial population by 90% (i.e., one log₁₀ cycle).

• z-value: The temperature change (°C or °F) required to shift the D-value by one log cycle.

• F-value: The total lethality delivered, expressed as the equivalent time at T_ref needed to achieve the target microbial inactivation.

For low-acid foods (pH ≥ 4.6), F₀ is used, referring to a T_ref = 250 °F and z = 18 °F, based on Clostridium botulinum spores as the target organism.

The relationship between F-value, T, Tref, and z is given by (Chandarana et al., 2010):

4. Practical Example

Process low-acid food, a mix of potato and sweet potato puree in an indirect aseptic system. If the scheduled lethality selected by process authority is an F₀ of 5 minutes; and the holding tube dimensions are L = 60 ft and ID of 1.87 in, while the product flowing with a maximum flow rate of 50 gpm); what should be the minimum required temperature at the exit of hold tube? Note: Since is indirect system it is safe to assume CFₜₕₑᵣₘ = CF_steam = ≈1.00; no viscosity data available. (correct answer: 282 °F (Roundup the estimated value))

5. Closing remarks

Designing a holding tube for continuous thermal processing requires precise engineering and scientific validation to ensure both food safety and regulatory compliance. Since the holding tube is the critical zone where the scheduled lethality (F-value) is achieved, every parameter—residence time, flow regime, temperature, and system expansion effects—must be accurately determined and verified.

Key engineering priorities:

• Residence time — Ensure the fastest particle meets or exceeds the required treatment.

• Flow regime — Correct Reynolds number determination impacts umax and lethality.

• Thermal & volumetric steam expansion — Properly correct for density changes and steam injection volume.

• Temperature control — Product must exit the holding tube at or above the validated process temperature.

• Instrumentation — Proper calibration and verification of TID, TRT, flow meters, and pressure gauges.

Validation & Documentation: Holding tube validation is a mandatory step for regulatory compliance:

• Taping method — Applying heat-sensitive tapes or time-temperature indicators at tube entry to confirm tube length.

• Dye injection tests — Tracking residence time distribution using approved tracers.

• Flow interruption verification — Ensuring minimum residence time under worst-case flow rates.

• Documentation — Regulatory bodies  require detailed reports including holding tube dimensions, flow calibration, instrumentation verification, and validation results.

By integrating accurate engineering calculations with scientifically validated performance tests, manufacturers can confidently design, validate, and document holding tubes that meet FDA 21 CFR Part 113, PMO, and international standards, ensuring both product safety and process efficiency.

References

Chandarana D.I, Unverferth J.A., Knap R.P., Deniston M.F., Wiese K.L, Shafer B. 2010. Chapter 7: Establishing the Aseptic Processing and Packaging Operation, Principles of Aseptic Processing and Packaging (3rd edition), Purdue University Press, West Lafayette, IN, USA. p. 135-150.

David J.R.D., Coronel P.M., Simunovic J., 2023. Handbook of Aseptic Processing and Packaging 3rd Edition, CRC Press, Boca Raton, FL.

FDA, 2025. Acidified & Low-Acid Canned Foods Guidance Documents & Regulatory Information. Accessed on September 1, 2025. https://www.fda.gov/food/guidance-documents-regulatory-information-topic-food-and-dietary-supplements/acidified-low-acid-canned-foods-guidance-documents-regulatory-information

IFPS, 2023. Holding Tube Design Guideline. Available at: IFTPS.org. Accessed, August 8, 2025.

Lewis M., Heppell N. 2000. Continuous thermal processing of foods pasteurization and UHT sterilization. Gaithersburg, Maryland, USA: Aspen Publishers, Inc.