IFTPS Holding Tube Design Guideline

Holding Tube Design Guideline

The Institute for Thermal Processing Specialists is a non-profit organization established exclusively for the
purpose of fostering education and training for those persons interested in procedures, techniques, and
regulatory requirements for thermal processing of all types of food or other materials, and for the
communication of information among its members and other organizations.

Part of the mandate of IFTPS Technical Committees is to develop protocols to be used as guides for
carrying out the work of thermal processing specialists. This guideline was prepared by an ad hoc
committee of Institute and reviewed extensively by members of the Institute.

This document may be photocopied in its entirety for use by interested parties. ©IFTPS 2022

Institute For Thermal Processing Specialists

3-304 Stone Rd. W., Ste. 301, Guelph. ON N1G 4W4
P: 519 824 6774 F: 519 824 6642 info@iftps.org
www.iftps.org

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IFTPS Holding Tube Design Guideline

Contents
1 Introduction .......................................................................................................................................... 3
2 Scope ..................................................................................................................................................... 3
3 Definitions ............................................................................................................................................. 4
4 Basis for Holding Tube Design............................................................................................................... 6
5 Holding Tube Calculations..................................................................................................................... 7
6 Design and Construction of Holding Tubes......................................................................................... 14
7 Associated Instrumentation ................................................................................................................ 19
8 Documentation ................................................................................................................................... 23
9 Verification of Hold Tube .................................................................................................................... 25
10 References .......................................................................................................................................... 28
11 Examples ............................................................................................................................................. 29

ACKNOWLEDGMENTS
This guidance document was written and edited by an ad hoc committee whose members included:
Jerome Auvinet, Dave Bresnahan, Pablo Coronel, Becky Douglas, Elsa Fakhoury, Paul Gerhardt, David
Guntrip, Graham Hicken, Aakash Khurana, Yuqian Lou, André Rehkopf, Craig Reinhart, Cory Riggen,
Marcio Scucuglia, Rich Szyperski, and Julie Willette.

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IFTPS Holding Tube Design Guideline

The following recommendations are to be considered voluntary guidelines. While this does not preclude
the application of other methods and equipment, these guidelines have been developed by consensus
of the Institute for Thermal Processing Specialists and should be given serious consideration for
adoption as methodology for design, construction, and verification of holding tubes.

1 INTRODUCTION
Many processes used in the food processing industry involve holding a product for a specific amount of
time at a given set of conditions. This includes blanching, cooking, starch blooming, caramelization,
protein denaturization, hydration, enzyme deactivation, and reduction of microorganisms. These
processes can be done either using batch or continuous processing.

A holding tube is a fixed length of piping or tubing. In continuous processes a holding tube is typically
used to ensure a product is held at a set of conditions for a specific amount of time. The objective of this
guideline is to define best practices and key regulatory requirements regarding design of holding tubes.

2 SCOPE
This guideline covers design of holding tubes for homogeneous (or near homogeneous) product and
consistent conditions (temperature and flow profile), across and along the tube as well as over time.
Assumptions generally include:

● Temperature – consistent across tubing starting at entrance and minimal heat loss.
● Composition – any reactions (e.g., starch blooming, enzyme deactivation, protein denaturing) do
not affect any change to temperature, holding time requirements or flow regime and are the
same for every portion of the product.
● Rheology – worst case (normally most viscous) or changes do not affect flow regime.
● Particles – small enough to not effect a change to flow regime and heat penetration is not a
factor.

Various methods may be employed to account for inconsistent conditions such as variation in
temperature, flow regime and/or residence time. These methods can include computational fluid
dynamics (CFD) analysis, use of static mixers, or location of monitoring instruments. Each application
should be evaluated by a subject matter expert with experience in holding tube design and construction
for the magnitude of the variations and the assumptions and methods used.

This guideline is focused on the design of holding tubes for food safety purposes (i.e., reduction of
microorganisms.) It may also be used for a holding tube or the portion of a holding tube that is for non-
food safety purposes (e.g., the extended holding of yogurt milk.)

This guideline does not cover design of holding tanks, which may be used for extremely long holding
times.

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IFTPS Holding Tube Design Guideline

3 DEFINITIONS
Average velocity (vavg) – the velocity based on the physical dimensions of the holding tube (inside cross-
sectional diameter) and the product volumetric flow rate.

Average (volumetric) residence time (tavg) – residence time based on the average velocity and holding
tube length.

Correction factor (CF) – A dimensionless factor used to calculate the fastest particle velocity or residence
time from the average.

Cut-out – the value at which an automated system takes an action [e.g., maximum (cut-out) flow or
minimum (cut-out) temperature.]

Fastest particle – The particle of product that has the highest velocity (v) and therefore the lowest
(minimum) residence time (t) in the holding tube.

Fastest particle velocity (vmax) – The velocity of the fastest particle.

Fastest particle residence time – see minimum residence time.

Flow profile or pattern – The shape of the velocity profile in the holding tube based on product
characteristics, hold tube dimensions, and flow rate. Used to determine one of the correction factors.

Holding tube – a length of pipe or tubing without any additional external heating that is used to give
continuous holding of every particle of food for at least the minimum residence time required to achieve
the objective of a (thermal or other) process.

Holding time – see residence time.

Required length (Lreq) – Minimum length of the holding tube needed to meet the required residence
time (treq) at fastest particle’s velocity (vmax).

Minimum residence time (tmin) – the residence time in the holding tube of the fastest particle based on
its velocity (including applicable correction factors) and the holding tube length. This should be higher
than the required residence time (treq).

Required residence time (treq) – The minimum time required for each particle of food to achieve the
objective of the (thermal or other) process. Also referred to as minimum required residence time.

Residence time (holding time) (t) – the time (minimum, required or actual) a particle of food is exposed
to a set of process conditions (e.g., temperature) in the holding tube.

Validation – the collection and evaluation of data, from the process design stage through commercial
production, which establishes scientific evidence that a process is capable of consistently delivering
quality product. (Guidance for Industry Process Validation: General Principles and Practices, FDA,
January 2011)

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Verification – Confirmation by examination and provision of objective evidence that specified

requirements have been fulfilled. (NFPA Bulletin 43-L, 2nd ed. Validation Guidelines for Automated Control of
Food Processing Systems Used for the Processing and Packaging of Preserved Foods, National Food Processors
Association, 2002)

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IFTPS Holding Tube Design Guideline

4 BASIS FOR HOLDING TUBE DESIGN

The basis for holding tube design is that the volume of a given length of tubing results in a specific
“dwell” or residence time for a particle or finite volume of product. This ensures that the product is
exposed to the specific conditions for the required time (treq). Mathematical models have been
developed and are accepted to calculate the minimum residence time of the product in the holding
tube. The volume may be accurately calculated based on measurements of the diameter and length of
the tubing inside. The time the product is in the tube (residence time) can then be calculated using the
known volume, the flow rate of the product, and correction factors for specific operating conditions.

The required residence time will vary depending on the desired results of the process and the operating
conditions (e.g., the scheduled process, process letter, regulatory requirement). Generally, higher
operating temperatures will require less holding time to achieve the same results. The required
residence time is normally calculated based on generally accepted formulas and criteria. It may also be
taken from published tables (e.g., US Grade “A” Pasteurized Milk Ordinance (PMO)), which were most
likely derived from similar calculations or individual tests.

An example is the formula used for determining the process lethality.

)⁄ )
𝐹 = 𝑡 × 10((

FTR: time required at a reference temperature to achieve a given log reduction of the target
organism or other aspect (e.g., enzymes) [min]

treq: minimum residence time required for every “particle” at the operating temperature
[min]

T: minimum operating temperature [°C, °F]

TR : reference temperature [°C, °F]

z: temperature coefficient [°C, °F]

FTR is based on an expected starting concentration and a food safety objective end point. F TR and z are
both based on experimental data.

Note: The required residence time (treq), as well as other design criteria noted below, should be specified
by a subject matter expert with experience in the required process and in holding tube design and
construction. The required time should follow local regulations where the product will be sold.

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IFTPS Holding Tube Design Guideline

5 HOLDING TUBE CALCULATIONS

Calculations surrounding a holding tube are based on the relationship:

𝐿 = 𝑣 × 𝑡 × 𝐶𝐹 (eq. 1)

Where:

L: Holding tube length

v: Velocity in the holding tube

t: Residence time

CF: Correction factor (see below)

The velocity of the fastest particle in the tube is determined using the holding tube diameter, the
volumetric flow, and several correction factors which are detailed in the following sections. From this
velocity, one can calculate either the required length (from the required residence time) or the
residence time (from the actual holding tube length).

The residence time is sometimes referenced by just an average or volumetric time. However, to ensure
proper calculation of the residence time or holding tube length, the time should be specified as being
based on specific correction factors. For example, the specification should indicate if the required
residence time is for the fastest particle or the average time for the bulk product – e.g., 4 seconds
laminar corrected (fastest particle); 15 seconds volumetric (average time).

Examples of the calculations and comparisons of different options are in Section 11.

5.1 CALCULATIONS STEPS

The minimum holding time calculation normally is executed in five steps:
1- Determination of volume correction factors (e.g., product thermal expansion, direct steam
addition),
2- Calculation of the average particle velocity without flow profile correction factor,
3- Determination of required flow pattern correction factors,
4- Calculation of fastest particle velocity (Vmax)
5- Calculation of required holding tube length (Lreq) or residence time of the fastest particle (tmin).
It is critical to review the dimensions / units of all parameters for consistency.

5.2 STEP 1A: DETERMINING PRODUCT THERMAL EXPANSION CORRECTION FACTORS:

The product density is dependent on temperature. In food products, the density normally decreases with
increasing temperature. As consequence, in the continuous thermal process, the increase of temperature
will result in an increase of product volume in the pipes. The increase of product volume will result in
acceleration of the product particles in the pipe. Since the product flow is measured at the flow meter or
metering pump and calculations are based on the product velocity in the holding tube, the flow correction

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factor to calculate the maximum velocity the ratio of the density at the temperature of the product at the
flow meter or metering pump to the density at the temperature of the product in the holding tube as in
equation 2.
CFTherm = ρ flow meter temperature / ρ holding tube temperature (eq. 2)
Where:
ρ: density of the product at the given temperature
As the main component of most liquid foods is water, it is acceptable to calculate the thermal expansion
correction factor using water density at both points. This information can come from steam tables.
When the flow meter is located at the holding tube inlet, the correction factor would be equal to one
(CFTherm = 1.0).
Notes:
1- The product thermal expansion is typically ignored (CFTherm = 1.0) for volumetric calculations,
which are used for some markets, products and/or organizations (e.g., for acid foods).
2- A default value of 6% (CFTherm = 1.06) is used by some organizations for product thermal expansion
based on a temperature increase of water of 160-290 F. Use of default values instead of
calculations should be evaluated by a subject matter expert.

5.3 STEP 1B: DETERMINING DIRECT STEAM HEATING CORRECTION FACTORS:

For direct heating system applications (i.e., steam injection or infusion), the addition and condensation of
steam will result in a larger amount of fluid flowing across the pipe, increasing the velocity. For direct
heating systems it is appropriate to include a correction factor for the volume of steam added (CFSteam)
calculated as in equation 3a.
×
𝐶𝐹 = 1+ (eq. 3a)

Where:
ΔT: Temperature rise due to the addition of steam [°C, °F]
Cp : Specific heat of the product [J/kg °C, BTU/lb °F]
ΔHvap: Heat of evaporation of steam [J/kg, BTU/lb]
Because the heat capacity (energy that is added to heat the material) of water is higher than most foods,
it is acceptable to use heating of water to determine the correction factor for steam addition. It is also
accepted to use the properties of saturated steam at the holding tube temperature. Both proprieties can
be obtained from steam tables. For water heated to approximately 140 °C (284 °F) the correction factor
is approximately 1% per 10°F (0.56°C):
. ∗
𝐶𝐹 = 1+ (°𝐹) 𝑜𝑟 1 + (°𝐶) (eq. 3b)

For indirect heating systems the correction factor would be equal to one (CF Steam = 1.0).

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IFTPS Holding Tube Design Guideline

Note:
1- Steam for direct heating should be saturated and dry (no condensate or superheat).
2- A default value of 12% (CFSteam = 1.12) is used by some organizations for the amount of steam
added. This is based on the amount of 80 PSIG saturated steam needed to increase the
temperature of water of 180-290 F. Use of default values instead of calculations should be
evaluated by a subject matter expert. The holding tube sizing tables in the US PMO for direct
heating are based on this default value.

5.4 STEP 1C: HOLDING TUBE THERMAL EXPANSION

The dimensions of the holding tube are normally measured at ambient temperature. The holding tube
material will expand in both length and diameter when at operating temperature. This will increase the
volume decreasing the velocities and increasing the residence time. Because calculating this increase
adds complications to the calculations, it is usually considered to be a safety factor and not included in
calculations.

5.5 STEP 2: CALCULATION OF AVERAGE VELOCITY

The average holding time is calculated based on volumetric flow rate and holding tube physical
dimensions.
Average flow velocity (vavg) is calculated using equation 4:
×
𝑣 = × 𝐶𝐹 × 𝐶𝐹 (eq. 4)
×( )

Where:
vavg: average particle velocity [m/s; ft/min]
Q: product maximum* volumetric flow rate [l/h, m 3/s, gal/min]
Di: holding tube internal diameter [cm, mm, in]
*Note: the product maximum flow rate refers to the maximum allowed product flow (cut-out
flow) rate in the equipment.
Note: Some markets, products and/or organizations use the average residence time and velocity to
calculate the holding tube size. It is important to clarify that the average flow velocity does not represent
the maximum (fastest) particle speed in the product and average residence time does not represent the
minimum particle residence time in the holding tube, even if the maximum flow rate was used.

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IFTPS Holding Tube Design Guideline

5.6 STEP 3: DETERMINING FLOW PATTERN CORRECTION FACTORS

The calculation of the average velocity is based on a “ideal” flat velocity profile (plug flow) where all
particles move at the same velocity in the pipe (see figure 5.1) However, this does not consider the impact
of the product properties or pipe dimensions on the flow pattern,

Fig 5.1 Velocity profile for a plug flow (source: Nelson 2010; Principles of Aseptic Processing & Packaging)
In the actual food matrix, the particles move with different velocities, resulting in a flow pattern which is
different from plug flow. The fastest moving particles are considered as the worst-case (lowest residence
time) scenario from a calculation perspective.
The mechanics of fluids defines the flow of any fluid through a pipe as occurring in one of 2 ways:
1- Laminar Flow (or Streamline flow): the elements of fluid flow as concentric shells in an organized
pattern, with lower resistance to the flow in the center. This pattern results in a parabolic flow
profile.
2- Turbulent Flow: there is radial mixing across the pipe, partially flattening the flow profile.
The two-flow pattern profiles are represented in figure 5.2:

Fig 5.2: Comparison of velocity profiles of laminar and turbulent flow (source: Lewis & Heppell 2000;
Continuous Thermal Process of Foods)
Quantification of the flow regime is critical for the determination of the processing temperatures and
holding times. Dimensionless Reynolds Number (NRe) is used to predict and identify the flow regimes.
This number is defined as the ratio of fluid momentum force (inertial forces) to viscous shear forces. The

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IFTPS Holding Tube Design Guideline

point at which the flow regime changes from Laminar to Turbulent can be predicted by using the
Reynolds Number (eq 5):
× ×
𝑁 = (eq. 5)
µ

Where:
NRe or Re: Reynolds Number [Dimensionless]
ρ: product density [kg/m3, lb/ft3]
vavg: average flow velocity (same calculated in step 1) [m/s; ft/min]
Di: holding tube internal diameter [cm, mm, in]
µ: fluid viscosity [cps, mPa sec]
As consequence of these flow patterns, product particles have different velocities across the pipe.
Therefore, a correction factor should be applied to the average velocity (V avg) to account for these
different velocities. Mathematical modeling based on the empirical power law velocity profile (e.g., Bird,
et al. 1960) has led to the acceptance of the following flow profile corrections factors (CF profile):
Laminar Flow: maximum particle velocity will be twice the average velocity.
CFprofile = 2.0 (eq. 6a)
Turbulent Flow: maximum particle velocity will be 20% higher than the average velocity.
CFprofile = 1.2 (eq. 6b)
Classical mechanics of fluids defines the end of the Laminar Flow profile when N Re < 2100, and the
beginning of the Turbulent Flow when NRe > 4000. Fluids with NRe between 2100 and 4000 are
considered to be in a transition state, with part of the particles with behavior close to laminar flow
pattern and part with close to turbulent flow pattern. As there is a possibility some particles have a
maximum velocity as in laminar flow pattern, the correction for the laminar flow pattern should be used
for fluids with NRe < 4000 to ensure the worst case was considered.
Notes:
1- The fastest particle correction is ignored (CFprofile = 1.0) for volumetric calculations.
2- Many food products exhibit non-Newtonian rheologic properties. However, most products are
either Newtonian or are nearly Newtonian at the temperatures in the holding tube. More complex
calculations and considerations apply to non-Newtonian fluids. Each application should be
evaluated by a subject matter expert with experience in holding tube design and construction for
the magnitude of the variations and the assumptions and methods used.
3- Determination of product density and rheological properties should use appropriate
methodologies. Both are dependent on the product temperature and density and may be
dependent on pressure if any gas is entrained in the product. The methodology should be
consistent with intended use. Regulatory agencies or Process Authorities could have specific
requirements to validate the product density or viscosity as critical limits.

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IFTPS Holding Tube Design Guideline

4- Some regulatory agencies and Process Authorities have different requirements for the minimum
NRe for turbulent flow. For example, the US Food and Drug Administration (US FDA) specifies a
Re of >4000 for turbulent flow while the European Hygienic Engineering & Design Group
(EHEDG) specifies >2300.
5- Specific corrective factors could be required by Regulatory Agencies for some applications.

5.7 STEP 4: CALCULATING MAXIMUM VELOCITY

Applying the correction factors above to the average velocity we achieve the velocity of the fastest
moving particle (vmax) as in equation 7:
𝑣 = 𝑣 𝑥 𝐶𝐹 (eq. 7)

Using the normal flow profile correction factors, the calculation is:
Laminar flow:
𝑣 =𝑣 × 2.0 (eq. 7a)

Turbulent flow:
𝑣 =𝑣 × 1.2 (eq. 7b)

5.8 STEP 5A: CALCULATING THE MINIMUM RESIDENCE TIME:

The residence time for the fastest moving particle (tmin) in a holding tube is obtained by dividing the
measured holding tube length (L) by the velocity of the fastest moving particle (vmax):

𝑡 = (eq. 8)

Average (volumetric) residence time (tavg) can be calculated using equation 8a. This is the same as not
applying the flow profile correction factor in steps 3 and 4.

𝑡 = (eq. 8a)

Where:
tavg: average (bulk) residence time [min, sec]
Lact: actual holding tube length [m, ft, in]
The residence time of the fastest moving particle (tmin) can also be calculated using the flow profile
correction factors and the average residence time (tavg) as below:
𝑡 =𝑡 / 𝐶𝐹 (eq. 9)

Using the normal flow profile correction factors, the calculation is:
Laminar Flow:
𝑡 =𝑡 × 0.50 (eq. 9a)

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IFTPS Holding Tube Design Guideline

Turbulent Flow:
𝑡 =𝑡 × 0.83 (eq. 9b)

5.9 STEP 5B: CALCULATING REQUIRED HOLDING TUBE LENGTH

During design of the holding tube, the required minimum length of the holding tube (L req) is obtained by
multiplying the required residence time for the fastest moving particle (t req) by the velocity of the fastest
moving particle (vmax).
𝐿 =𝑡 ×𝑣 (eq. 10)

For volumetric flow calculations, the minimum required length (Lvol) can be calculated using equation
10a. This is the same as not applying the flow profile correction factor in steps 3 and 4.
𝐿 = 𝑡 × 𝑣 (eq. 10a)

Where:
Lvol: required holding tube length for volumetric calculations [m, ft, in]
The actual holding tube length (Lact) should always be longer than the required length (Lreq or Lvol).

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IFTPS Holding Tube Design Guideline

6 DESIGN AND CONSTRUCTION OF HOLDING TUBES

6.1 HOLDING TUBE SHAPE

Holding tubes are typically either trombone shaped (straight sections connected by u-bends) or spiral
shaped (coiled). Other shapes, such as going along the walls of a room, are acceptable if they meet the
applicable requirements of this guideline.

6.2 HOLDING TUBE CROSS SECTION

The cross section of the holding tube (i.e., diameter) should be chosen to ensure homogeneous
conditions across the holding tube. Consideration should be given to accumulation of air or other gasses
and to settling of suspended solids (e.g., undissolved powders).

Note: For some products, product buildup may effectively reduce the diameter of the holding tube and
may be considered in selection of the diameter used for calculations and in operation (cleaning
frequency).

When a holding tube has sections with different diameters, the calculations of holding time or length
should be done separately for each section. Consideration should be given to slope (i.e., accumulation
of air or other gases) and drainability in the transitions between sections while considering possible
regulatory requirements.

Static mixers or corrugation of the holding tube have been used to mitigate separation or striation
within the product. However, there is no consensus on the effect on flow pattern (plug, laminar or
turbulent flow) or velocity (cross sectional area).

6.3 SLOPE
The calculations for holding time or holding tube length are based on the tubing being filled with the
fluid (product) flowing through it. If there is something (i.e., air) in the tube that takes up part of the
diameter, the velocity of the fluid will increase and the residence time will decrease. Holding tubes
should be designed to encourage any material, usually gases (i.e., air), that is less dense than the
product to flow up and out of the holding tube. In a properly designed holding tube, a small number of
entrained bubbles in the product is assumed to travel at the same velocity as the product flow and not
accumulate in the holding tube. Therefore, they will not significantly affect the residence time.

To avoid accumulation of air or other gasses when a holding tube has sections with different diameters,
the transitions between the sections should have the same or higher slope on the top as the rest of the
sections. This can be accomplished using eccentric reducers with the “flat” side on the top. Also, placing
the smaller diameter pipe/tube at the outlet end of the holding tube can facilitate the design to address
air or other gasses accumulation, particle settling and drainability.

The optimum configuration is to have a continuous upward slope from the beginning (inlet) of the
holding tube to the end (outlet). This will minimize the risk of air accumulating in a section of the
holding tube. However, this will result in a taller structure.

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Holding tube sections that are designed to be flat (not have a slope) may be acceptable in some
applications, jurisdictions, or organizations. However, they have a risk that the physical installation will
have high points where gases (i.e., air) will accumulate. This could be due to problems in construction or
installation, or to sagging of the lengths.

Holding tubes can be designed and constructed with interconnections that slope downward between
multiple sections of tubing that are flat or sloped upward. This is often done for multiple length holding
tubes or to reduce the overall height of the structure. Because of the likelihood that, in a downward
sloping section, any air will travel at a different velocity than the product, any downward sloping section
should not be included in the holding tube length measurement.

At a minimum, the holding tube should have a net positive slope – the outlet should be higher than the
inlet.

6.4 ADJUSTABLE (MULTIPLE) LENGTH HOLDING TUBES

A system may require multiple holding tube lengths (different minimum residence times) for different
flow rates or products. This can be accomplished by two methods. Each section or combination should
meet the applicable requirements of this guideline.

6.4.1 Replacement
There is a complete holding tube for each required residence time and flow combination. Each holding
tube will be of different lengths and possibly of different diameters. The holding tubes will have a set of
swings connections (elbows or flow plate) at the inlet and the outlet. The inlet instrumentation (e.g.,
temperature, flow) will be before the inlet swings and the outlet instrumentation (e.g., temperature and
pressure) will be after the outlet swings.

The same holding tube may be used for more than one combination of flow and time if the proper
correction factors (e.g., laminar fastest particle correction) are applied to each. For example, a single
holding tube can be used of both 2000 lph at 4 seconds and 4000 lph at 2 seconds.

Each holding tube should be validated for the required length.

6.4.2 Add-on
There are multiple sections of holding tube that can be combined with swing connections to yield the
different required residence time. The various sections are typically the same diameter but may have
different diameters.

The smallest (shortest) section is usually always used and therefore is fixed in place at the outlet of the
holding tube. Additional sections are added to achieve the required residence time. For example:

A (fixed) = 2000 lph @ 2 sec.

A + B = 2000 lph @ 4 sec. or 4000 lph @ 2 sec.
A + B + C = 2000 lph @ 8 sec. or 4000 lph @ 4 sec.
A + B + C + D = 3600 lph @ 8 sec.

Each combination of holding tubes should be validated for the required length.

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6.4.3 Swing connections and reducers

Swing connections and reducers, when used, should not be included in the length measurement unless
it meets all applicable portions of this guideline.

6.5 BENDS
Bends should be included as part of the holding tube length. Any turbulence created by the change in
direction is not considered in the residence time calculation.

The center line of the bend should be used for measuring the length of a bend. The length of a bend will
depend on the center line radius (CLR) of that bend and the angle. The CLR of a bend is typically
described as a multiple of the tubing diameter. Typical commercial elbows are short radius (1.0
diameter radius), long radius (1.5 diameter radius) and sweep (often 3 diameter radius). Tables with
dimensions are available from fitting manufacturers for the bends supplied by them. Identify whether
the lengths specified are for the bent portion only or include any straight ends.

Note: Some organizations use the inner curve or other calculations for a more conservative equivalent
length.

6.6 MATERIAL
The holding tube should be made from material that is suitable for the intended use including the
product(s), sanitation or sterilization and cleaning. The material should be non-toxic, and components
should not migrate to the product. The material finish should be suitable for the intended use.

Consideration should be given to corrosion from product and CIP chemicals. Higher grade materials
(e.g., low carbon, higher grade alloys, etc.) should be used where corrosion is a concern. Note that in
some cases, it may be preferable to use a less expensive, lower grade of material and replace the
holding tube periodically. This should only be done if the corrosion does not affect the cleanability of
the holding tube.

6.7 WELDS
The holding tube assembly should be welded where possible. Unions (e.g., clamps) should be used only
when required to meet design requirements (e.g., instrument insertion, sample or inspection
connections, and interconnection of sections for varying the length).

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The weld quality (e.g., alignment, finish) should be suitable for sanitary use. Automatic welding should
be used where possible. Welders should be qualified on the equipment used. Test coupons should be
made and inspected daily. Grinding and polishing of the exterior of weld is not required. The exterior,
and interior, when possible, of all welds should be inspected before any grinding and polishing is
performed.

6.8 SUPPORTS
Supports for holding tubes should be provided to maintain all parts of the holding tube in a fixed
position, free from lateral or vertical movement. The supports should be designed to allow thermal
expansion of the holding tube and to avoid erosion due to vibration.

The individual coils of spiral coiled holding tubes may rest on each other without separate supports.
Guides should be supplied to keep the coils aligned.

6.9 HEAT LOSS/GAIN AND INSULATION

The holding tube should be designed and installed so no portion between the inlet and the outlet is
heated. If the holding tube is in the same cabinet with a heating section, the holding tube should be
located so the heater does not add heat to the holding tube.

The holding tube should be designed and installed such that heat loss from the surface of the holding
tube is minimized or there is sufficient cross-sectional mixing in the holding tube to ensure that the
outer portion of the product is not at a significantly lower temperature than the rest of the product.
There should be no condensate drip on the tube and the tube should not be subjected to drafts or cold
air.

Holding tubes are sometimes insulated and/or shrouded (a covering structure) to reduce heat loss or for
personnel protection. Removable shrouding, with or without insulation, is acceptable. Pipe insulation is
NOT recommended. Pipe insulation makes it difficult to inspect the holding tube and detect issues such
as possible leaks. Inspection should therefore be performed before installation of insulation materials.
Periodic inspection may be required by regulatory authorities to ensure it continues to meet
requirements. Also, pipe insulation can be easily damaged, resulting in a non-hygienic installation due
to harboring of vermin or water saturation allowing microbial growth. Water saturation can also result
in additional heat loss.

6.10 REGULATORY REQUIREMENTS

Many regulations, standards, and guidelines, including those from US FDA (21 CFR 113, US PMO), the
Codex Alimentarius Commission (CODEX), 3-A Sanitary Standards Inc (3-A) and EHEDG, contain design

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and construction requirements. The applicable local regulations should apply and may override the
recommendations in this guideline. Examples are:

● Holding tube materials and finishes should meet the local requirements for food contact
materials.
● A holding tube for food safety purposes should have a continuous upward slope of not less than
2.1 centimeters per meter (0.25 inches per foot).
● Supports for holding tubes should be provided to maintain all parts of the holding tubes in a
fixed position, free from any lateral or vertical movement.
● No device should be permitted for short-circuiting the portion of the holding tube required for
food safety purposes.
● The holding tube should be so designed that no portion between the inlet and the outlet is
heated.
● The holding tube should be drainable.

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7 ASSOCIATED INSTRUMENTATION
Instruments associated with the holding tube are critical for proper application of a thermal process.
This includes instruments to measure flow, temperature, and pressure. Care should be taken in the
selection, location, installation, calibration, and operation of these instruments. The facility should
establish procedures, including frequency, for calibration. These instruments, calibration, locations, etc.
should meet any local regulatory requirements. For example, US FDA 21 CFR 113 requires calibration at
least annually.

7.1 FLOW – TIMING PUMP

7.1.1 Specification
Since product velocity and consequently the residence time are determined by the product flow, the
pump used for this purpose is called the “timing” pump. The timing pump is used to limit the maximum
flow rate through the holding tube. The pump should be of a positive displacement type. The pump
should be designed to effectively limit the amount of product that slips past the pumping elements
(rotors or pistons). It should be suitable for exposure to conditions (media, temperature, pressure, etc.)
in all phases of operation, including cleaning and sterilization or sanitization.

7.1.2 Location
The timing pump may be located at any point upstream or downstream of the holding tube. The
preferred location is upstream of the holding tube. It should not be possible to bypass the timing pump
fully or partially. There should not be a tank or other buffer between the flow meter and holding tube
that could cause the flow through the holding tube to be different from that through the flow meter.

If product at the flow meter is at a different temperature than at the holding tube, the expected density
difference should be included in the sizing calculations (q.v.).

7.1.3 Installation
Timing pumps should be installed as recommended in the manufacturer’s installation manual and to
meet any specific regulatory requirements. Pulsation dampers may be used. Pulsation dampers are
typically installed with piston type pumps.

7.1.4 Calibration
Maximum timing pump flow should be verified before the first use and periodically thereafter. This can
be accomplished using the rate of change in volume in a tank. This can be done using the level change
in the system balance tank or at the discharge using a tank or drum (catch tank) with an accurate level
or weight measurement. This verification should be done both with and without heating to account for
density differences.

Verification of timing pump flow should be done with all other flow promoting or controlling
components set to achieve maximum flow through the holding tube – pumps at full speed and valves
open.

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7.2 FLOW – FLOW METER

A flowmeter-based timing system may be used to monitor the flow rate and ensure that the flow rate
through the holding tube does not exceed the maximum allowed flow. If so, the flow control device
(valve or pump) is controlled by the signal from the flowmeter and may be located either upstream or
downstream of the holding tube. A recorder is typically included.

7.2.1 Specification
Flow (timing) meters used to measure the flow rate through the holding tube should have sufficient
accuracy and repeatability to ensure that the measured flow rate is correct. The flow meter should be
selected based on accuracy in the entire range of potential flow. It should be suitable for exposure to
conditions (media, temperature, pressure, etc.) in all phases of operation, including cleaning and
sterilization or sanitization.

Flow meters that measure the volumetric flow rate (velocity) directly are preferred. Magnetic flow
meters are most used due to their high accuracy. Mass flow meters may be used if the flow
measurement, corrected for product density, is as accurate as a volumetric flow meter.

NOTE: Mass flow (Coriolis) meters are not accepted for use in pasteurizers for dairy products under the
US Pasteurized Milk Ordinance.

7.2.2 Location
The flow meter may be located at any point upstream or downstream of the holding tube. The
preferred location is upstream of the holding tube. It should not be possible to bypass the flow meter
fully or partially. There should not be a tank or other buffer between the flow meter and holding tube
that could cause the flow through the holding tube to be different from that through the flow meter.

The flow meter should not be located after an injection heater due to the high turbulence (see
Installation below).

NOTE: For infusion-based heating, maintenance of a constant level in the infuser should be a critical
parameter.

If product at the flow meter is at a different temperature than at the holding tube, the expected density
difference should be included in the sizing calculations (see above).

7.2.3 Installation
Flow meters should be installed as recommended in the manufacturer’s installation manual and to meet
any specific regulatory requirements. Care should be taken to ensure that the meter is not affected by
turbulence caused by the piping or other appurtenances. To avoid air entrapment, the meter should be
installed in a vertical or sloping pipe with the flow going up.

7.2.4 Calibration
Flow instrumentation should be calibrated so the transmitter (flow meter) and all receivers (recorder,
indicators, PLC/HMI, switches, etc.) read the same value.

Flow meter calibration should be verified before the first use and periodically thereafter. This can be
accomplished using the rate of change in volume in a tank. This can be done using the level change in
the system balance tank or at the discharge using a tank or drum (catch tank) with an accurate level or

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weight measurement. This verification should be done both with and without heating to account for
density differences.

7.3 TEMPERATURE
7.3.1 Specification
Temperature instruments used to measure the temperature in the holding tube should have sufficient
accuracy and repeatability to ensure that the measurement is correct. Resistance Temperature
Detectors (RTDs) are typically more accurate than thermocouples and are suitable for the temperature
ranges normally used in thermal processing. The sensor should be suitable for exposure to conditions
(media, temperature, pressure, etc.) in all phases of operation, including cleaning and sterilization or
sanitization. It should have sufficient probe length and a reduced tip for faster response and less effect
from conduction through the pipe wall.

The transmitter/converter may be included in the sensor head or located elsewhere (e.g., panel mount,
PLC input or recorder input).

Dual sensing elements may be used to detect equipment failure or drift. The signals from the two
elements are compared and used to alarm if they vary by more than an allowed amount.

7.3.2 Location
The sensing portion of the temperature element should be located to measure the lowest product
temperature. This is generally in the center of the product pipe. It is preferred to locate it in a tee or
elbow so turbulence will minimize the response time.

The temperature should be measured at the outlet of the holding tube. The end of the holding tube
should be taken as the location of the temperature sensor (see Verification section and examples.)

NOTE: Some local regulations require the use of two measurements at the holding tube outlet– one for
indication and one connected to a recorder. Associated control can be based on either the indicator or
the recorder. (See calibration.)

The temperature may also be measured at the inlet of the holding tube to detect variations in the
temperature that could affect the thermal process. This sensor is often also the heater outlet
temperature control sensor.

7.3.3 Installation
Temperature sensors should be installed so they can be removed and reach a common point for easy
calibration and verification. Temperature sensors can be inserted in a well to allow easy removal and
replacement.

7.3.4 Calibration
Temperature instrumentation should be calibrated so the transmitter and all receivers (recorder,
indicators, PLC/HMI, switches, etc.) read the same value. Temperature transmitter calibration should be

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periodically verified using a hot water or oil bath and a thermometer traceable to local/national
calibration standards (e.g., the US National Institute of Standards and Technology (NIST)).

NOTE: When two measurements are used at the holding tube outlet, one (typically the indicator) should
be calibrated to the standard. The other (typically the recorder and associated controller) may be
calibrated to read equal to or lower than the other. US FDA (21 CFR 113, US PMO) requires that the
recorder be adjusted to agree as closely as possible with, but in no event higher than, the Temperature
Indicating Device.

7.4 PRESSURE
Pressure in the hold tube is monitored to ensure that flashing has not occurred, changing the liquid
volume and the holding time. The pressure should be above the saturated vapor pressure (boiling
pressure) of the product at its maximum temperature in the holding tube. The amount above that
pressure may be specified by the regulatory agency (e.g., 10 psi by US PMO).

7.4.1 Specification
Pressure instruments used to measure the pressure in the holding tube should have sufficient accuracy
and repeatability to ensure that the measurement is correct. The sensor should be suitable for exposure
to conditions (media, temperature, pressure, etc.) in all phases of operation, including cleaning and
sterilization or sanitization.

7.4.2 Location
The pressure sensor should be located such that it indicates that there is sufficient pressure in the
holding tube and final heat exchanger. This is typically located, and may be required by local
regulations, at the holding tube outlet (the point of the lowest pressure in the holding tube). However,
it can be located after the holding tube if it is before any pumps or valves, and static and dynamic (flow)
pressure changes are accounted for in determining the required pressure.

7.4.3 Installation
Pressure sensors should be installed so they can be removed and reach a common point for easy
calibration and verification.

7.4.4 Calibration
Pressure instrumentation should be calibrated so the transmitter and all receivers (recorder, indicators,
PLC/HMI, switches, etc.) read the same value. Pressure transmitter calibration should be periodically
verified using a pressure gauge/indicator traceable to local/national calibration standards (e.g., US NIST)

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8 DOCUMENTATION
Documentation for a given holding tube should contain the design calculation(s), construction
drawing(s), and configuration drawing(s) in the event of multiple holding tube configurations. Change
control should require revision to these documents to reflect any changes to the holding tube.

8.1 DESIGN CALCULATIONS

Design calculation sheets can take one of two forms. The calculation from the manufacturer will start
from the fastest particle minimum holding time and calculate the minimum required length. The
calculation used as a record to verify the holding tube construction and support the scheduled process
will start from the holding tube length measured in the field and calculate the effective holding time for
the fastest particle. A separate calculation sheet should be made for each configuration.

The design calculation sheet should show:

● Volume or dimensions to determine volume.

o This is typically some form of diameter and overall length.
o Diameter may be inner diameter or the outer diameter along with the thickness.
● Volumetric flow rate.
● All applicable correction factors and basis information. The factors for consideration are listed
in the Residence Time Calculation section of this guideline.
● Holding time (required or effective) for the fastest particle.

8.2 CONSTRUCTION DRAWING

The construction drawing should show:

● Isometric view of the holding tube

● Dimensions of all straight lengths
● Dimensions of all bends with equivalent length
● Designed slope
● Location of instrumentation located at or in the holding tube
● If there is only one configuration, the items listed for the configuration drawings as described in
the Appendix.

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8.3 CONFIGURATION DRAWINGS

If multiple configurations are possible, a configuration drawing or view should be supplied for each
configuration. For example, one drawing/view depicting the correct configuration for 60 gpm @ 4 s and
a separate drawing/view depicting the correct configuration for 90 gpm @ 4 s. Each configuration
drawing or view should show:

● Correct connections including applicable swings and monitoring equipment for that
configuration.
● Design volumetric flow rate and required holding time for the fastest particle.
● Total length from construction drawing dimensions.
● Optionally:
o Calculated minimum required length.
o Fastest particle correction (laminar or turbulent flow) and/or other correction factors.

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9 VERIFICATION OF HOLD TUBE

To attain the minimum required holding time it is critical to verify that the construction and installation
of the holding tube meets the criteria listed in this Guideline. The purpose of verification is to ensure
that design elements that effect the holding time, including some factors outside of the holding tube
itself, are met. Specific verification criteria and activities may be required by local regulations.

Note: Verification of the holding tube design and construction does not validate that the holding tube
will accomplish its purpose (e.g., microbial reduction, protein denaturization). This guideline does not
address validation of holding tubes.

9.1 FREQUENCY:
The holding tube should be verified:

1. Upon installation, typically as part of the Installation Qualification (IQ).

2. Annually thereafter to detect any changes. Any changes should be reviewed and approved
through a Change Management program.
3. After any alteration is made which may affect the holding time, including modification of the
holding tube or replacement, maintenance or calibration of the timing pump or flow meter.
4. If a flow regime correction other than fully developed laminar flow has been used, the holding
time calculation (velocity of the flow and Reynold’s Number) should be verified for each new
product.
5. After a seal on a speed setting has been broken.
6. Whenever a check of the capacity (i.e., as found testing of the flow meter or pumping rate)
indicates a speed up.

9.2 DOCUMENTATION:
● Verify that the flow meter or timing pump flow has been calibrated to the specified parameters.
● Keep a record of all activities carried out and criteria verified during the verification of the hold
tube.
● Keep records on file per regulatory requirements or for a period beyond the shelf life of the
product, including supporting calculations and calibration certificates for the flow meter,
temperature transmitters, recorders, and all other critical instruments.

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9.3 VERIFICATION CRITERIA

The following criteria should be considered for verification of a holding tube. The criteria to be verified
should be determined for each verification (i.e., initial or follow-up) prior to starting the verification.

9.3.1 General:
● The holding tube sizing calculations (minimum required length) are correct.
● The construction drawings and other design documentation meet the sizing calculations, any
filed scheduled process, and other documents. Note that criteria should be verified to meet the
criteria listed on a record copy of the construction documents.
● The beginning and end of the holding tube are marked on any drawings used as records of the
verification. These points can be marked on the holding tube as a reference.
o The beginning of the holding tube is typically at the later of a) the start of the upward
slope or b) an inlet temperature sensor. For steam injection, the beginning should be
after the steam has fully condensed.
o The end of the holding tube is typically at the earlier of a) the end of the upward slope
or b) the indictor or recorder sensor element.

9.3.2 Construction:
● The holding tube diameter conforms to that listed in the calculations, on the construction
documents and in any filed scheduled process.
● The material of construction of the holding tube, gaskets, insulation, etc. correspond to the
construction drawings and specifications and are suitable for use in all phases of operation,
including cleaning and sterilization or sanitization.
● Surface finishes, including welds, meet the specifications and are suitable for hygienic
applications.
● Supports for the holding tube are provided to maintain all parts of the holding tubes in a fixed
position, free from any lateral or vertical movement other than thermal expansion.
● Heat is not applied to any portion of the holding tube.
● The holding tube is located, or provisions are in place, to prevent excessive or spot cooling such
as by material dripping or air blowing on the holding tube.
● Insulation materials can be easily removed for inspection of the holding tube.
● For variable length holding tubes:
o Each configuration should be verified.
o Each configuration can be connected without excessive force.
o When supplied, position detection (proximity switches) indicate that the holding tube is
connected correctly. When interlocks through an automated control system are
supplied, the interlocks prevent operation if the configuration is not correct.

9.3.3 Slope:
● The holding tube slope is per the construction drawing and other documents.
● The entire hold tube consists of the same diameter.
o Alternatively, if varying diameter tubing is used, the installation does not trap air AND
allows for proper drainage.

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9.3.4 Length:
● The length should be measured between the documented beginning and end of the holding
tube (see above) and exclude any portion with an incorrect slope (see above).
● One way to measure the holding tube length with fittings (90-degree bends, elbows, etc.) is
using a tape measure or a string at the top of the pipe along the centerline. Alternatively, a
string or wire inside the holding tube can be used.
● The total length of the holding tube also can be determined by adding the equivalent lengths of
the fittings to the measured lengths of straight pipe. The centerline length of the fitting should
be treated as an equivalent length of straight pipe. The centerline length may be determined
from published fitting dimensions or measured by forming a flexible steel tape or a string along
the centerline of the fitting.
● The total length can also be determined by measuring the volume (measure the liquid to fill) and
dividing by the inside area.

9.3.5 Direct holding time measurement

● In lieu of measuring the holding tube length, means exist where chemical (e.g., salt) or
radiological tracers are injected into the product stream to measure product flow and residence
time of the fastest particle. However, these methods are normally not used on a routine basis to
verify holding times or product flow rates.
● These methods are typically used for holding times of 15 seconds or more.
● For systems using a timing pump to control (limit) flow, the holding time should be determined
in both divert and forward flow.
● The test results, which are based on the flow rate of water, should be converted to the holding
time for the products processed.
● An example of this method can be found in the US PMO Appendix I, Test 11.

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10 REFERENCES
3-A Sanitary Standards Inc. 2005. 3-A Accepted Practices for the Sanitary Construction, Installation,
Testing, and Operation of High-Temperature Short-Time and Higher-Heat Shorter-Time Pasteurizer
Systems, Number 603-07. McLean, VA.

Bird, R. Byron, Warren E. Stewart & Edwin N. Lightfoot. 1960. Transport phenomena, John Wiley and
Sons, Inc., New York

Campden BRI. 2007. Guideline G53, Guidelines on the safe production of aseptically processed and
packaged foods. http://www.campdenbri.co.uk/publications.

Code of Federal Regulations, Title 21, Part 113 – Thermally Processed Low-Acid Foods Packaged in
Hermetically Sealed Containers.
http://www.accessdata.fda.gov/scripts/cdrh/cfdocs/cfcfr/CFRSearch.cfm?CFRPart=113.

Codex Alimentarius Commission. 1993. Code of Hygienic Practice for Aseptically Processed and
Packaged Low-Acid Foods CAC/RCP 40-1993. http://www.fao.org/fao-who-codexalimentarius/sh-
proxy/en/?lnk=1&url=https%253A%252F%252Fworkspace.fao.org%252Fsites%252Fcodex%252FStandar
ds%252FCXC%2B40-1993%252FCXP_040e.pdf

Consumer Brands Association Science and Education Foundation. 2020. Canned Foods: Principles of
Thermal Process Control, Acidification and Container Closure Evaluation (9th Edition).

European Hygienic Engineering & Design Group. 2017. EHEDG Guidelines, Document 6, The
Microbiologically Safe Continuous Flow Thermal Sterilization of Liquid Foods. Frankfort, Germany.

Lewis, Michael J. & Neil J. Heppell. 2000. Continuous Thermal Processing of Foods: Pasteurization and
UHT Sterilization (Food Engineering Series). Springer.

Agriculture and Agri-Food Canada. National Dairy Code – Part II. 2005.
https://agriculture.canada.ca/en/canadas-agriculture-sectors/animal-industry/canadian-dairy-
information-centre/acts-regulations-codes-and-standards/national-dairy-code-part-ii-and-iii

National Food Processors Association. 2002. NFPA Bulletin 43-L, 2nd ed. Validation Guidelines for
Automated Control of Food Processing Systems Used for the Processing and Packaging of Preserved
Foods. Washington, DC.

Nelson, Philip E. 2010. Principles of Aseptic Processing & Packaging, 3rd ed. Purdue University Press

Public Health Service/Food and Drug Administration. Grade “A” Pasteurized Milk Ordinance, 2019
Revision.

U.S. Food and Drug Administration. 2005. Guide to Inspection of Aseptic Processing and Packaging for
the Food Industry. https://www.fda.gov/inspections-compliance-enforcement-and-criminal-
investigations/inspection-guides/aseptic-processing-and-packaging-food-industry

U.S. Food and Drug Administration. 2011. Guidance for Industry Process Validation: General Principles
and Practices. https://www.fda.gov/files/drugs/published/Process-Validation--General-Principles-and-
Practices

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11 EXAMPLES
The worksheets used in this document are for example only. Prior to use, all formulas and values should
be confirmed by a subject matter expert with experience in holding tube design and construction.
Formulas for steam and water physical properties are regressions of steam table data.

11.1 CALCULATIONS
Table 11.1 a, b and c compare the potential differences in holding tube design variety of processes using
various assumptions and correction factors. Example calculations (Tables 11.1.1 thru11.1.4) are
included for typical conditions. The selection of method and the calculation should be made by a
subject matter expert with experience in holding tube design and construction.

In the calculations and tables, the times are based on the following velocities:

 Laminar and turbulent– fastest particle velocity corrections

 Volumetric – average velocity (no flow pattern correction)

Tables 1.1 a-c Comparison tables

a) 100 gpm system nominal flow rate @ 4 seconds, indirect heating, no thermal expansion,
flowmeter-based timing [maximum (cut-out) flow rate= 110% of nominal flow rate]

Volumetric Turbulent Laminar

262“ 315“ 524“*
*see Table 1.1.1

b) 100 gpm system nominal rate @ 4 seconds, laminar flow correction, thermal expansion,
flowmeter-based timing [maximum (cut-out) flow rate = 110% of nominal flow rate]

Indirect Heating Direct Heating

556“ 622“*
*see Table 1.1.2

c) 100 gpm system nominal flow @ 2 seconds, laminar flow correction, no thermal correction,
flowmeter-based timing [maximum (cut-out) flow rate = 110% of nominal flow rate], US PMO Tables &
Calculation

Indirect Heating Direct Heating

262“ 294“
see Table 1.1.4

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Table 11.1.1 Example Holding Tube Calculation – 100 gpm @ 4 seconds – indirect heating, laminar flow
correction

Flowmeter based timing [maximum (cut-out) flow rate = 110% of nominal system flow rate], no product
thermal expansion

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Table 11.1.2 Example Holding Tube Calculation – 100 gpm @ 4 seconds – direct heating, laminar flow &
thermal expansion correction

Flowmeter based timing [maximum (cut-out) flow rate = 110% of nominal system flow rate], product
thermal expansion

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Table 11.1.3 Example Holding Tube Calculation – 100 gpm @ 15 seconds – indirect heating, volumetric
(no) flow correction

Timing pump (no maximum flow adjustment), no product thermal expansion

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Table 11.1.4 Example Holding Tube Calculation – 100 gpm @ 2 seconds – US PMO calculation

Flowmeter based timing [maximum (cut-out) flow rate = 110% of nominal system flow rate]

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11.2 HOLDING TUBE DOCUMENTATION EXAMPLES

Drawings with calculations

11.2.1 HTST Style

11.2.2 Multiple – Dedicated Style with config views

11.2.3 Multiple – Add-on Style with config views

11.2.4 Spiral Style – non-legal, extended

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Figure 11.2.1 HTST Style Holding Tube

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Figure 11.2.2 Multiple Holding Tubes – Dedicated Style Holding Tube with config views

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Figure 11.2.4 Multiple Holding Tubes – Add-on Style with config views

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Figure 11.2.4 Spiral Style – non-legal, extended

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