CEN/TC 110

Date: 2025-03

prEN 18208:2025

Secretariat: DIN

Liquid-to-liquid single-phase heat exchangers — Test procedure for determining performance

Einphasen-flüssig/flüssig Wärmetauscher — Prüfverfahren zur Bestimmung der Leistungskriterien

Echangeurs monophasiques liquide/liquide — Procédure d'essai pour la détermination de la performance

CCMC will prepare and attach the official title page.

Contents Page

European foreword 3

Introduction 4

1 Scope 5

2 Normative references 5

3 Terms and definitions 5

4 Symbols and abbreviations 8

5 Performance of liquid-to-liquid heat exchangers 10

5.1 Typical quantities 10

5.1.1 Derived quantities 10

5.1.2 Rated conditions 10

5.2 Hydraulic performance 12

5.3 Thermal performance 12

5.3.1 Energy balance 12

5.3.2 Heat transfer rate 14

5.3.3 Heat transfer coefficients 14

5.3.4 Heat transfer surface 14

5.3.5 Analytical calculation methods 15

6 Test principle 21

6.1 Test procedures 21

6.1.1 Test bench acceptance criteria 21

6.1.2 Test acceptance criteria 23

6.1.3 Test conditions 25

6.2 Measurements and instrumentation 26

6.2.1 General 26

6.2.2 Measurements 26

6.2.3 Acceptable amplitude of fluctuations 27

6.2.4 Overall uncertainties 27

6.2.5 Liquid quality 29

6.3 Test analysis 29

6.3.1 General 29

6.3.2 Transposition of new/replacement heat exchanger test results to reference conditions 30

6.3.3 Transposition of tests results from heat exchangers already in use under reference conditions 31

6.4 Test report 33

Annex A (informative) Calculation of thermal performance uncertainty k.A.F. according to LMTD method 34

Annex B (informative) Test report template 35

Annex C (informative) Example of test 37

Bibliography 44

European foreword

This document (prEN 18208:2025) has been prepared by Technical Committee CEN/TC 110 “Heat exchangers”, the secretariat of which is held by DIN.

This document is currently submitted to the CEN Enquiry.

Introduction

This document is designed to support a series of European standards dedicated to heat exchangers.

This document provides manufacturers and users with the information needed to present the thermohydraulic performance characteristics of a heat exchanger. These characteristics form the basis for evaluating the state of a heat exchanger, whether it is new/a replacement, or already in operation.

This document also provides a performance-based acceptance test procedure for a liquid-to-liquid single-phase heat exchanger (sensor calibration, acknowledgement of uncertainties, standardization of physical properties of liquids) and the possible transpositions under various rated conditions.

The customer's technical specification, referred to in this document, defines or adapts the level of tolerance. It also defines thermal performance acceptance criteria. Examples are provided in Annex C.

Underlying assumption of this document is that the liquid is a Newtonian fluid in turbulent conditions. This document may be adapted for other liquids or conditions, as agreed by parties involved in the performance test.

This document defines the general terms and the calculations used to determine the thermohydraulic performance of heat exchangers. It includes the general test procedure and related theories.

This document is intended to be used for acceptance-testing heat exchangers in test facilities such as laboratories, manufacturer test facilities and final installation site.

This document specifies three acceptance levels:

— level 1 for minimum tolerances;

— level 2 for nominal tolerances;

— level 3 for maximum tolerances;

This document constitutes an application-specific standard in line with EN 305 and EN 306.

The following documents are referred to in the text in such a way that some or all of their content constitutes requirements of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.

EN 247, Heat exchangers — Terminology

EN 305, Heat exchangers — Definitions of performance of heat exchangers and the general test procedure for establishing performance of all heat exchangers

EN 306, Heat exchangers — Methods of measuring the parameters necessary for establishing the performance

EN ISO 5167 (all parts), Measurement of fluid flow by means of pressure differential devices inserted in circular cross-section conduits running full

ISO/TR 12767, Measurement of fluid flow by means of pressure differential devices — Guidelines on the effect of departure from the specifications and operating conditions given in ISO 5167

For the purposes of this document, the terms and definitions given in EN 247 and the following apply.

ISO and IEC maintain terminology databases for use in standardization at the following addresses:

— ISO Online browsing platform: available at https://www.iso.org/obp/

— IEC Electropedia: available at https://www.electropedia.org/

3.1

liquid

any type of liquid, such as water, used to transfer thermal energy (heat)

Note 1 to entry: There can be a combination of two or more types of liquids, all in the same phase state (single-phase liquid).

3.2

primary liquid

hottest liquid, which serves as a heat source

3.3

secondary liquid

coolest liquid, which serves as a heat sink

3.4

mixture

liquid with two or more elements, all in the same phase state

3.5

heat exchanger

device designed to transfer heat between two physically separated liquids

3.7

thermohydraulic performance of a heat exchanger

heat exchanger thermohydraulic performance that can be established by measuring, or can be calculated using measured parameters, and is expressed according to one or more of the following elements:

— temperature of primary/secondary liquid;

— flow rate of primary/secondary liquid;

— pressure of primary/secondary liquid;

— temperature difference;

— heat transfer coefficient;

— heat transfer rate transferred by primary liquid / captured by secondary liquid;

— pressure drops;

— fouling factors.

Note 1 to entry: These elements are identified per EN 305 and may be established by measuring per EN 306.

3.8

heat exchanger categories

classification of heat exchanger types based on design criteria, physical criteria, or a mix of the two

3.9

heating

temperature increase of a secondary liquid that does not change its phase state

3.10

cooling

temperature decrease of a primary liquid that does not change its phase state

3.11

fouling

deposit of a layer of unwanted materials entailing/causing resistance to the heat transfer

3.12

plugging

obstruction of one or more openings/channels in a heat exchanger where no primary or secondary liquid can circulate, resulting in a decrease in the transfer surface and an increase in pressure drops

3.13

irreversible pressure drop

decrease of static pressure between inlet and outlet not including change of level

3.14

typical inlet

inlet considered the most representative of each heat exchanger, chosen by the manufacturer or in accordance with the installation conditions

3.15

typical outlet

outlet considered the most representative of each heat exchanger, chosen by the manufacturer or in accordance with the conditions of use

3.16

pinch temperature

minimum difference between the primary liquid and secondary liquid temperatures

3.17

customer

user

entity that purchases a good or service from a supplier

Note 1 to entry: For test of used heat exchangers, the user is responsible of the fouling of the heat exchanger. (i.e for cleaning it or defining the fouling resistance)

3.18

maker

manufacturer

supplier

entity that supplies goods to a user and guarantees the performance of the heat exchanger (sourcing, manufacture, production inspection, packaging, storage, transport)

Note 1 to entry: For new heat exchangers, in the case of performance tests, they are also responsible for checking heat exchanger cleanliness (initial cleanliness report), preparing the heat exchanger and its immediate environment (e.g. anchoring, adapters, cradles, access scaffolding) and establishing the thermohydraulic criteria checking report (by transposition if the conditions differ from the reference conditions, per 6.3).

3.19

verifier

laboratory

entity that verifies the equipment performance through one or more tests and ensures the exchanger test measurements (calibration, test bench assembly, bench cleanliness check, sensor installation, liquid analysis report, acquisition chain, test bench/heat exchanger adapters connection, performance test, uncertainty calculations), including measurement uncertainties

3.20

design state

reference conditions

heat exchanger operating parameters that will determine the minimum k.A.F. coefficient required to satisfy the customer's criteria

Note 1 to entry: This state is determined by the manufacturer based on the operating situations required by the customer (mass flow rates and liquid temperatures, heat transfer rate to be transferred, etc.)

3.21

steady-state

condition where the average value of all main variables of the heat exchanger (temperature, pressure, flow rate) remains constant over time, and fluctuations are in the same order of magnitude of the measuring sensor, given that external conditions and inputs are unchanging

Note 1 to entry: Steady-state criteria are expressed in 6.1.3.

3.22

required

thermohydraulic rated conditions according to customer requirements

For the purposes of this document, the symbols, abbreviations and subscripts apply:

NOTE The LMTD correction factor F depends on the type of heat exchanger and the flow configuration. This factor is equal to or less than 1 (5.3.5.2).

When this document serves as the basis for establishing performance characteristics, the parts contained in EN 305 shall be used (see Table 1).

Table 1 — Quantity used to establish performance characteristics

General

The thermohydraulic performance of a heat exchanger shall be defined for rated conditions by:

— type of liquid;

— flow rate (primary and second liquid mass flow rate);

— temperature (at primary and secondary liquid inlet and/or outlet);

— total pressure (at primary and secondary liquid inlet and/or outlet);

— pressure drops (of primary and secondary liquids);

— physical properties of liquid (density, enthalpy and/or specific heat capacity, thermal conductivity, dynamic viscosity) and chemical composition of liquids involved (calculated at primary and secondary mean temperature);

— physical properties of material used at the average temperature of the material;

— fouling state of heat transfer surfaces;

— auxiliary equipment requirements (e.g. purges, level controls, pumps);

— environmental constraints (e.g. ambient temperature, humidity, contamination);

— operating frequency (for regenerative heat exchangers).

NOTE The physical and thermodynamic properties of the liquids use international standards such as IAPWS IF95/97 for standard water and IAPWS 08 for seawater. In the absence of an international standard, an appropriate thermodynamic model to estimate the physical properties of the liquids can be proposed.

Fouling kinetics, whether asymptotic (soft, fragile deposits such as particulate fouling) or not (hard, adhesive deposits such as scale), shall be taken in account at the design phase.

For the single-phase liquid system of a heat exchanger, multiplying the specific heat capacity by the temperature difference on the primary and/or secondary side may be equivalent to the specific enthalpy difference of the liquid in J/kg. It shall be checked according to the fluid properties and the difference between both methods shall be less than aimed uncertainties to use heat capacity method.

EXAMPLE For liquid water, if the temperature is above 250°C, it is better to use the enthalpy balance.

Flow

Flow should be viewed as the mass flow rate q_m relative to the normal entry or outlet.

Temperature

The average inlet temperature shall refer to the heat exchanger inlet. The average outlet temperature shall refer to the heat exchanger outlet.

Pressure

The total inlet pressure shall be expressed as the average absolute total pressure relative to the inlet.

The total outlet pressure shall be expressed as the average absolute total pressure relative to the outlet.

Fouling

The fouling thermal resistance (R_foul1 and R_foul2) of each side of the heat transfer wall is expressed as the composite value R_foul. Fouling can also be expressed in terms of the percentage difference on the overall heat transfer coefficient (k).

Fouling shall be calculated using the mean values obtained through the operating test data in accordance with the following formulae:

— For fouling resistance, see Formula (1):

(1)

— For variations in heat transfer surface (deviation of value k), see Formula (2):

(2)

— For cleanliness factor (C_foul), see Formula (3):

(3)

where

NOTE 1 The cleanliness factor is not constant and depends on test conditions. It is preferable to use the concept of fouling resistance, which is intrinsic to the heat exchanger and does not depend on test conditions.

NOTE 2 In the case of new/replacement heat exchangers, the cleanliness factor is equal to 1.

According to Bernoulli's theorem, the formula is written in the general form between two points A and B of a single flow in a facility; see Formula (4):

= (4)

where

Velocity v of the liquid is calculated using mass flow rate q_m, density ρ and flow section S, with .

The energy balance allows the total energy at the heat exchanger inlet to be compared with the total energy at its outlet, as shown in Figure 1.

This balance should be included in the test reports conducted on the heat exchangers in order to check the measured results.

Figure 1 — Energy balance

In steady-state, the enthalpy variation on the primary liquid side shall be equal to the enthalpy variation on the secondary liquid side assuming no heat losses to the environment nor heat sources inside the component. In general, the power (mechanical or electrical) supplied within the limits of the measurements, and the heat transfer (heat gains or losses) between the heat exchanger and its environment, should be considered.

Based on Figure 1, this relationship can be written in the theoretical form given in Formula (5):

(5)

where

Generally, the previous formula is simplified as follows because heat losses to surroundings and head losses thermal impact are negligible, see Formula (6):

(6)

The heat transfer rate of a single-phase heat exchanger (i.e. when the two liquids remain in liquid state) is characterized by the following formulae: see Formula (7) for primary side and Formula (8) for secondary side.

(7)

(8)

where

In this relationship, the heat exchanger heat balance describes the fact that the heat transferred by the primary liquid is equal to the heat captured by the secondary liquid, adjusted for heat losses or gains to/from the surrounding environment.

The heat transfer can be characterized by various types of heat transfer coefficients that express the heat transfer rate per heat transfer surface area unit and per temperature difference unit.

The heat transfer coefficients shall be given separately for primary and secondary flows and aggregated in an overall heat transfer coefficient. The overall heat transfer coefficient combines the effects of convection and conduction between the two flows and the heat transfer surface of the heat exchanger.

The conditions for establishing the heat transfer coefficient — for example the conditions for establishing the heat transfer surface area and the temperature difference — shall be described.

In this document, the overall heat transfer coefficient should be used. This coefficient is calculated based on the log mean temperature difference (5.3.5.2) and the total heat transfer surface area in contact with either liquid, by adding fins or any other type of additional surface.

Under normal operating conditions, the calculated values of the overall heat transfer coefficient might need to be adjusted using the fouling coefficients from each side of the heat transfer wall, as shown in 5.1.2.5. The value of these coefficients shall be specified at the time of purchase.

The part of the surface of the heat exchanger channel walls that is involved in the heat transfer process is called the heat transfer surface.

The heat transfer surface can have a specific shape with special accessories, e.g. fins, designed to increase the heat transfer coefficient or the transfer surface area.

The method for determining the transfer surface shall be described if there are several possible ways to calculate it. The correlation used to calculate the performance shall be defined accordingly to the chosen heat transfer surface.

General

Two types of thermal calculations can be used to size a heat exchanger according to its use:

— Determination of transfer surface, A, knowing the heat transfer rate transferred and the inlet and outlet temperatures of the two liquids (LMTD method);

— Determination of the liquid outlet temperatures knowing their inlet temperatures and the transfer surface (NTU method).

NOTE ASME PTC 12.5 is based on the LMTD method and ANSI/AHRI Standard 401 is based on the NTU method.

LMTD method

General

The log mean temperature difference (LMTD) shall be calculated as the ratio of the deviation between the temperature differences at the inlet and outlet, and the logarithm of the quotient of these two temperature differences, determined by the temperature difference between the primary and secondary sides along the length of the heat exchanger; see Formulae (9) and (10):

if ≠ (9)

LMTD = = if = (10)

where

Figure 2 — Counter-current flow

NOTE In this figure, the primary liquid is the limiting liquid (q_mc_p)_min.

Figure 3 — Co-current flow

For other types of flows (e.g. cross or mixed flows), a correction factor, called F, strictly less than 1, shall be applied to LMTD calculated using a counter-current flow approach and shall be applied to calculate the heat transfer rate.

The pinch temperature shall be established by determining the difference between the primary circuit outlet temperature and the secondary circuit inlet or outlet temperature, depending on the type of flow (counter or co-current flow). For physical reasons, the final temperature difference, ΔT_o, is positive.

The LMTD method is generally used for heat exchanger design, in order to establish the heat exchanger geometry.

Relations between ϕ, k and LMTD

As a basic heat exchanger principle, heat transfer rate transfer is characterized by the transfer of a certain quantity of heat per time unit, defined by the following formulae:

— For heat transfer rate, see Formula (11):

(11)

where

The mean power on the primary side and secondary side shall be considered; see Formula (12):

(12)

— For the overall heat transfer coefficient, see Formula (13):

(13)

Thermal performance

The definition of the thermal performance of a heat exchanger is as follows; see Formula (14):

(14)

Required thermal performance

The required k.A.F, k.A.F_required (see 6.1.2) is calculated based on reference conditions and the Formula (14). This is the minimum k.A.F that the heat exchanger shall achieve. This k.A.F_required may be increased with margins such as:

— uncertainties provision for calculations;

— uncertainties provision for manufacture;

— uncertainties provision for performance acceptance tests;

— tube plugging margin.

There may be several k.A.F_required if there are several reference conditions (e.g. several operating points, clean or fouled case).

NTU method

General

NTU (number of thermal transfer units) is a number with no dimension, often referred to as a “thermal length” concept.

The NTU of a heat exchanger is calculated using Formula (15):

(15)

The NTU method is generally used for simulations once the heat exchanger geometry is established.

Thermal efficiency

Heat exchanger efficiency is the ratio of heat transfer rate actually transferred relative to the maximum heat transfer rate it is theoretically possible to transfer using ideal equipment (infinite exchange surface, with no fouling), using the same liquids at the same mass flow rates and same inlet temperatures; see Formulae (16), (17), (18) and (19).

The power transferred is at its maximum if one of the liquids undergoes a temperature variation equal to the difference of the inlet temperatures of the two liquids. This maximum variation can only be undergone by the liquid with the lowest q_mc_p value (Figures 4 and 5).

(16)

(17)

(18)

(19)

where

In practice, the efficiency of a heat exchanger is defined for main process liquid, i.e. the heated or cooled liquid, for which q_mc_p is not necessarily the minimum. However, only the efficiency defined by the liquid for which q_mc_p is a minimum has physical meaning.

Figure 4 — Counter-current flow

Figure 5 — Co-current flow

Liquid heat flow ratio

The heat flow ratio of liquids, called y, is defined by the following formulae; see Formula (20).

(20)

— For the primary liquid, see Formula (21).

(21)

— For the secondary liquid, see Formula (22).

(22)

Relationship between ε, NTU and y

In general, ε can be written as given in Formula (23).

= f (NTU, y, flow configuration) (23)

This formula is valid for both liquids.

In the case of simple flows that are exclusively either counter or co-current flows, the formula is as follows:

— exclusively counter-current flow, see Formulae (24) and (25).

for y≠1 (24)

for y = 1 (25)

— exclusively co-current flow, see Formula (26).

(26)

where

Efficiency ε can be included in a diagram according to the NTU for various values of y, as illustrated in Figures 6 and 7.

Figure 6 — Counter-current flow

Figure 7 — Co-current flow

These diagrams are limited to values of y between 0 and 1.

For other types of flows, the formulae can be found in the literature referenced in the bibliography, and the manufacturer should include them in their documentation.

General

This section gives several directives to manufacturers, test laboratories and users for preparing and presenting the test procedures relative to the methods for measuring heat exchanger performance.

A heat exchanger intended to be installed in a given heat transfer system should be tested under similar conditions with the auxiliary equipment required for its operation. Before commissioning, the heat exchanger shall be identified and its conformity with design characteristics shall be checked.

The heat exchanger shall be connected to the test device, filled with the appropriate test liquids. Sealing shall be checked, and its installation shall be guaranteed. Any air trapped in the system shall be purged and sensors shall be installed per current standards.

To improve the heat balance, heat losses/gains to/from the environment should also be avoided. Insulation can be used to avoid these heat exchanger heat losses/gains. It can also be used between the heat exchanger to be tested and the temperature measurement stations.

Lastly, test bench start-up shall not disturb heat exchanger operation or its measurements. For example, a pump installed very close to the heat exchanger can cause recirculating and inadequate distribution in the heat exchanger.

The test bench shall never impact heat exchanger cleanliness, regardless of the test bench chosen (piping material, equipment such as pump, heat exchanger, etc.).

Test method

The heat exchanger test method depends on the purpose of the test and shall therefore be chosen according to whether the heat exchangers are new or already in use (operating).

1. A new heat exchanger is defined as a heat exchanger whose thermal and hydraulic characteristics have not been altered by corrosion or deposits (i.e. clean).

A new heat exchanger intended to be installed in a given heat transfer system should be tested under similar conditions with the auxiliary equipment required for its operation. Before start-up, the heat exchanger shall be identified and its conformity with design characteristics shall be checked.

Sufficient performance data from near the theoretical point should be measured in order to predict the performance characteristics under the reference conditions. The number of measurement points shall be in accordance with EN 305 and EN 306.

2. Inversely, a heat exchanger that is already in use (operating) is a heat exchanger whose thermal and/or hydraulic characteristics might have been altered due to corrosion and/or deposits, and whose performance test is generally performed in situ.

Fouling is produced by several mechanisms including the formation of oxide scale, the calcination of organic matter and the generalized deposit of organic and inorganic matter. It is important to have a reference test of a new heat exchanger beforehand because it is difficult to characterize fouling. Regularly testing heat exchangers that are already operating enable monitoring of their performance and indicate when to perform cleaning or other necessary actions (plugging in the event of perforation), including replacing the heat exchanger with a new device.

Test conditions

Test conditions shall be as close as possible as the reference condition. When it is not possible to match references conditions, the following requirements shall be followed:

— Tests conditions shall include ± 20 % of Reynolds number conditions compared to the reference conditions.

— To increase transposition accuracy, other tests points with varying Reynolds number (primary and secondary sides) shall be included in the test programme.

Maximum transposition accuracy can be achieved by making Prandtl number vary for each Reynolds number tested. (Variation of the test loop power)

Fluids used for the tests shall be as close as possible as fluids used in the plant. If not possible, manufacturers and customers shall reach an agreement on the test fluids and their representativeness.

Test programme

The test program cannot be executed until the client has approved the programme and the procedure, unless otherwise agreed between the parties.

Given that performance tests are conducted with a clean heat exchanger, as part of the test programme, the manufacturer shall establish and provide the customer with a method for transposing the test results in order to obtain the performance in fouled state in design state.

The test programme shall include:

— a risk and opportunity analysis dedicated to the tests (identification, level of exposure with potential consequences and probability of occurrence, as well as treatment, circuit leaks);

— the general description of the test loop (technicians, location, date, description of primary circuit and secondary circuit (equipment), instrumentation positions and settings, etc.);

— the thermohydraulic performance test procedure (test steps, interpretation of results (formulae used, uncertainties, acceptability criteria and approach), performance test report, heat exchanger state before, during and after testing).

The thermohydraulic performance test procedure shall include the following items:

— schedule;

— general order of operations (before testing such as assembly and cleaning of test loop, during testing, after testing such as cleanliness tests and circuit disassembly);

— hydraulic network and instrumentation diagram;

— description of liquids used and/or produced.

Test data other than those required to establish heat exchanger thermohydraulic performance may be supplied for information only.

In all cases, the customer, the manufacturer, and the verifier (if different from the customer or the manufacturer) shall reach an agreement on the method for performing the tests with the appropriate measurements.

General

The acceptability criteria shall be defined jointly by the customer and the manufacturer. Examples of thermal performance acceptance criteria are proposed in 6.1.2.3.

In particular, the heat exchanger shall meet the thermohydraulic performances that satisfy the following criteria with the acceptance level required in 6.2.

Operating parameter conformity

For the operating point to be deemed compliant, the flow rates, pressure drops, pressures, power and temperatures at the primary and secondary liquid inlets shall fall within a range to be determined by the customer and the manufacturer jointly.

If the test supervisor cannot ensure these rated conditions (design state), similarities through testing and/or transposition by numerical simulation should be performed (with or without adjustment of numerical model according to variation with test performed) by agreement between the customer and the manufacturer.

Examples of thermal performance acceptance criteria

Best-estimate criterion

In the context of a best-estimate evaluation, the required k.A.F. shall be equal to the testing k.A.F. plus or minus the absolute uncertainties related to the measurement point as per Formula (27).

(27)

New or replacement heat exchanger with transposition to reference condition and 100 % confidence

Compared to the previous criteria, this criterion is made to accept only performance higher than required.

Satisfying the contractual thermal criterion (performance) is defined as a clean state, with acknowledgement of test uncertainties under test conditions (checking of compliance with specifications).

Formula (28) allows the difference in thermal performance k.A.F to be established between the numerical simulations and the thermal performances given by the tests in clean state, under test conditions.

(28)

It may be required that the heat exchanger is not oversized. In this case, Formula (29) is used instead:

(29)

Satisfying the design thermal criteria is defined in clean and fouled state, with acknowledgement of test uncertainties under reference conditions.

Formulae (30) and (31) ensure that the actual thermal performance of the heat exchanger in design state exceeds the required thermal performance, in clean state and fouled state, respectively.

(30)

(31)

where

Satisfying the contractual hydraulic criterion is defined by the customer (maximum pressure drop allowed).

Heat exchanger already in use with transposition to reference condition and 100 % confidence

Satisfying the design thermal criteria is defined in fouled state, with acknowledgement of test uncertainties under reference conditions.

Formula (32) ensures that the actual thermal performance of the heat exchanger in design state exceeds the required thermal performance in fouled state.

(32)

where

In all cases, if plugging is possible (primary and/or secondary side), the following equality can be checked in fouled state, k.A.F._required = k.A.F._transpo – U_k.A.F., in order to determine the number of tubes that could be plugged.

Satisfying the contractual hydraulic criterion is defined by the customer (maximum pressure drop allowed).

Once stable conditions (steady-state) have been established, the test period shall last from at least 15 min. It shall include recording of the data at a given fixed frequency for each test, e.g. 30 series of measurements. If possible, a second test shall be performed to ensure result repeatability and reproducibility.

The longer the test, the more the experimental standard deviation will be reliable. This experimental standard deviation (random uncertainty) shall be integrated in the overall test uncertainty and combined with systematic uncertainties such as sensor and acquisition chain calibration (quadratic sum and/or Monte Carlo method). The acquisition frequency of all the data (flow rate, temperature, pressure) shall be lower than the acquisition frequency of each sensor (acquisition chain included) to avoid oversampling.

During the test, both requirements below shall be satisfied:

Quality criterion:

The heat transfer rate on the primary liquid side, ϕ₁, and the heat transfer rate on the secondary liquid side, ϕ₂, shall not each differ from their mean (time-based over the length of the test) by the values detailed in Table 2 below.

NOTE This value is adopted from ANSI/AHRI Standard 401.

The relative deviation between the average primary heat transfer rate ϕ₁ and the average secondary heat transfer rate ϕ₂ shall not exceed ± 2 %; see Formula (33):

(33)

Table 2 — Acceptable deviation between primary and secondary heat transfer rate

Stability criterion:

The heat transfer rate test shall also be checked to confirm test stability; see Formula (34):

(34)

where

All measurement instruments used (flowmeters, temperature sensors, pressure sensors) shall be calibrated before testing per ISO/IEC 17025: the corrections indicated shall be applied to satisfy the error limits. The instruments shall be recalibrated according to a regular schedule that is appropriate for each instrument (calibration date should be less than 1 year at time of test). Calibration registers shall be kept.

All measurement instruments shall be used so as to ensure conformity with the precision specified in the test plan. The requirements and procedures described in EN 306 are intended to provide users with general information and practical procedures allowing them to reasonably estimate measurement uncertainty during testing.

The test equipment used shall be documented and the customer shall be able to consult this information on simple request. The equipment shall be calibrated periodically.

General

The customer and the manufacturer may agree to use any level to assess whether a specific heat exchanger will meet a guaranteed point.

Default level

Three acceptance levels have been defined for the heat exchanger test (1, 2 and 3). The customer and the manufacturer may agree to use any level to assess the contractual thermohydraulic performance of the heat exchanger. Unilateral criteria may also be defined by agreement between the customer and the manufacturer.

The default levels are as follows:

Laboratory measurements

If measurements are performed in a laboratory, the heat exchanger can be operated at the theoretical calculation point or very close to it. The measurement uncertainties can be the lowest (acceptance level 1 of test to be achieved).

The tests shall be performed, and the results evaluated solely by individuals who have the necessary skills and expertise.

Factory measurements

It can be harder to achieve low measurement uncertainties than in a laboratory (acceptance level 2 of test to be achieved).

In situ measurements

For in situ measurements, it is generally even more difficult to obtain the required temperatures under reference conditions.

Measurement uncertainties are also harder to achieve (acceptance level 3 of test to be achieved).

If the construction or operation of the test facility is such that large amplitude fluctuations occur, measurements may be performed using a damping device for the measurement instruments or their connection lines, or by smoothing the electronic data, which might reduce the amplitude of the fluctuations relative to the values given in Table 3. A linear and symmetrical damping device shall be used, e.g. a capillary tube, which shall enable integration over at least one complete fluctuation period.

Table 3 — Acceptable amplitude of fluctuations as a percentage of the average value of the measured quantity

General

The acceptable values of overall uncertainties per acceptance level are summarized in the following table and discussed in greater detail in the subsequent sections (see Table 4). Uncertainties can be two-sided or one-sided. These are generic levels of acceptance, and they can be modified upon agreement of all parties involved in the tests.

Table 4 — Acceptable values of overall uncertainties

NOTE Overall thermal performance uncertainties might not be achievable depending on heat exchanger duty (very low temperature difference) and might need to be reconsidered.

Flow

Flowmeters shall be calibrated from ± 0,1 % to ± 1 % depending on the level wanted.

The flowmeters shall be positioned with a certain straight length upstream and downstream of the heat exchanger, in accordance with the applicable part of the EN ISO 5167 series or ISO/TR 12767, to ensure a certain level of measurement uncertainty.

The overall relative uncertainty (including random and systematic uncertainties) of flowmeter measurements (calibration, acquisition chain, etc.) shall be:

— between ± 0,5 % and ± 1 % (level 1 tolerances), such as the use of orifice plates installed as far as possible from singularities/disturbing elements, according to the applicable part of the EN ISO 5167 series;

— between ± 1 % and ± 2 % (level 2 tolerances), such as the use of electromagnetic flowmeters (the liquid shall not be demineralized) or orifice plates, according to ISO/TR 12767;

— between ± 2 % and ± 5 % maximum (level 3 tolerances), such as the use of electromagnetic flowmeters or ultrasound (invasive or non-invasive).

NOTE Values for level 3 tolerances are adopted from ASME PTC 12.5.

It is important to know the position of the installed flowmeters (at heat exchanger inlet and outlet) to estimate the impact of temperature on the thermohydraulic parameters of the liquid as precisely as possible.

Temperature

Temperature sensors shall be calibrated between ± 0,1°C and ± 0,3°C of the level wanted and adapted according to the temperatures (precision) and deviations considered.

NOTE These values are adopted from ASME PTC 12.5.

Temperature sensors shall be located as close to the heat exchanger as possible. Use of static mixer before probes is highly encouraged.

The overall absolute uncertainty (including random and systematic uncertainties) of temperature sensor measurements (calibration, acquisition chain, etc.) shall be:

— ± 0,15 °C (level 1 tolerance),

— ± 0,3 °C (level 2 tolerance),

— ± 0,7 °C maximum (level 3 tolerance).

This data shall be adapted to the temperatures at play. For a low temperature deviation of the liquids, the lowest level should be targeted.

Each measurement should be at least doubled independently in order to decrease the uncertainties (division of uncertainty by , n being the number of sensors per measurement).

Pressure

Pressure sensors shall be calibrated at ± 0,3 %.

NOTE 1 This value is adopted from ASME PTC 12.5.

Pressure sensors shall be located as close to the heat exchanger as possible.

The overall uncertainty (including random and systematic uncertainties) of pressure gauge measurements (calibration, acquisition chain, etc.) shall be a maximum of ± 1 % of the full scale in all cases.

NOTE 2 This value is adopted from ASME PTC 12.5. ANSI/AHRI Standard 401 requires a precision up to a maximum of ± 2 % of the measured quantity.

A differential pressure sensor shall be used to calculate the mechanical energy loss (pressure drops). Otherwise, two absolute pressure sensors upstream and downstream of the heat exchanger may be used, taking the height difference between the two into account.

To maximize the success of the pressure measurements, a combination of a differential pressure sensor and two absolute pressure sensors can also be used if required (to offset outlet signal instability, original offsets, the measurement range difference, etc.).

Hydraulic performance

Hydraulic performance depends in particular on the flow rate and pressure measurements, and on test conditions. The overall relative uncertainty (including random and systematic uncertainties) of hydraulic performance measurements Δ_p depends on the acceptance level conducted:

Thermal performance

The overall relative uncertainty (including random and systematic uncertainties) of thermal performance measurements k.A.F. depends on the acceptance level conducted:

The uncertainty calculation on thermal performance k.A.F using the LMTD method may be performed in accordance with Annex A.

The quality of the liquid shall be validated jointly by the customer and the manufacturer. An analysis of the liquid may be performed as agreed between the customer and the manufacturer before testing (last rinse) and after testing. Liquid resistivity or conductivity may be measured in real time.

To attenuate the risk of contaminating the liquid, several precautions may be taken such as manually degreasing the welds on the test bench, installing one or more filters near the heat exchanger, or closed-loop circulation for a certain amount of time with cleaning of the filter(s) several times, before connecting the heat exchanger to the test bench.

The state of cleanliness shall be the same before, during and after testing: an examination report may be drawn up after testing (e.g. cleanliness tests such as white rag test).

In some cases, in situ measurements cannot be obtained due to local conditions. European standards should be taken into account for the measurement methods and the various types of applications.

To perform a transposition to the thermal and hydraulic level, the Bell-Delaware method for tubular heat exchangers for all liquids can be used as a best-estimate correlation for the outer side of the tube. For inside tubes, Gnielinski correlation is also a best-estimate choice and has a wide range of validity.

For other type of heat exchanger, the manufacturer shall propose the correlations.

A methodology for transposing test results is proposed in 6.3.2 (new heat exchangers) and 6.3.3 (heat exchangers already used), in the event that the test conditions differ from the reference conditions, which might be impossible to achieve.

Thermal

The thermal transposition of test results to reference conditions for new/replacement heat exchangers is proposed below:

1. Measure the flow rates, temperatures and pressures under test conditions (clean state), and their respective uncertainties;

2. Calculate the thermal performance k.A.F according to the LMTD method or the NTU method corresponding to this test, and reduce it by the measurement uncertainties;

3. Compare and adjust, if required, the numerical model (design) with a thermal test coefficient “C_e”, minus test uncertainties (under test conditions with zero fouling resistance), in accordance with the Kern method; see Formula (35):

(35)

where

NOTE For a tubular heat exchanger, the difference between the tube's internal and external diameter between the primary and secondary liquid is considered in the calculation. The overall heat transfer coefficient is generally relative to the external transfer surface.

4. Transpose this model, “adjusted” or not, to the thermal level under the reference conditions in clean state, and under the design state conditions in fouled state, considering the actual liquid (which may be different from the liquid tested, for example between the laboratory test and the site test). Ensure that the thermal correlations are in their validity range.

Another transposition methodology may be proposed by the customer or the manufacturer.

Additional guidelines for thermal transposition on shell and tubes heat exchangers

Uncertainty on conduction is generally low. It is arguable not to apply Ce coefficient on R_mat, especially if conduction resistance is low compared to convection resistances.

Similarly, uncertainty on shell side is generally higher (uncertainty on correlation and manufacturing uncertainty). In some situations, it can be arguable not to apply Ce coefficient on α_tube especially if tube convection resistance is low compared to shell convection resistance. Tube Nusselt number correlations are generally widely tested, known uncertainty and have a lower uncertainty than shell Nusselt number correlation.

When several test points are available, the following guidelines may help reduce dispersion of results:

— When 2 or 3 test points are available it is not advisable to put several correction coefficients or a complex correction law (polynomial…)

— When more test points are available, more complex correction approach can be envisaged such as having several correction coefficient: one for tube side, one for shell side…

Hydraulic

The hydraulic transposition of test results to reference conditions for new/replacement heat exchangers is proposed below:

1. Measure the pressure drops under test conditions (in clean state) and increase them with the measurement uncertainties;

2. Compare and adjust, if required, the numerical model with a hydraulic test coefficient “C_DP”; see Formula (36):

(36)

where

To help with the pressure drop consistency analysis, checking that the results are consistent with the following law is recommended: . In some cases, flow rate exponent could be slightly different from 2.

3. Transpose this model, “adjusted” or not, at pressure drop level under design state conditions, taking the actual liquid into account. Ensure that the hydraulic correlations used are in their validity range.

The pressure drops measured and extrapolated (trend curve) shall be less than or equal to the pressure drops deduced from the design state at the defined heat exchanger terminals (with error bars added). At least 3 points shall be considered to determine a trend curve and perform an extrapolation.

Thermal

The thermal transposition of test results to reference conditions for heat exchangers already in use is proposed below:

1. Measure the flow rates, temperatures and pressures under test conditions (according to actual fouling of heat exchanger), and their respective uncertainties;

2. Calculate the thermal performance k.A.F according to the LMTD method or the NTU method corresponding to this test, and reduce it by the measurement uncertainties;

3. Compare and adjust, if required, the numerical model (design) with a thermal test coefficient “C_e”, minus test uncertainties (under test conditions with design fouling resistance), in accordance with the Kern method; see Formula (37):

(37)

where

For a tubular heat exchanger, the difference between the tube's internal and external diameter between the primary and secondary liquid shall be taken into account in the calculation. The overall heat transfer coefficient is generally relative to the external transfer surface.

4. Transpose this model, “adjusted” or not, to the thermal level under design state conditions, in fouled state. Ensure that the thermal correlations used are in their validity range.

Another transposition methodology may be proposed by the customer or the manufacturer.

Hydraulic

The hydraulic transposition of test results to reference conditions for heat exchangers already in use is proposed below:

1. Measure the pressure drops under test conditions (according to actual fouling of heat exchanger) and increase them with the measurement uncertainties;

2. Compare and adjust, if required, the numerical model with a hydraulic test coefficient “C_DP”; see Formula (38):

(38)

where

To help with the pressure drop consistency analysis, checking that the results are consistent with the following law is recommended: .

3. Transpose this model, “adjusted” or not, at pressure drop level under the required design state conditions. Ensure that the hydraulic correlations used are in their validity range.

The pressure drops measured and extrapolated (trend curve) shall be less than or equal to the pressure drops deduced from the design state at the defined heat exchanger terminals (with error bars added). At least 3 points shall be considered to determine a trend curve and perform an extrapolation.

The test report shall include the date, names of the observers, essential physical data of heat exchanger tested, model number of heat exchanger tested by manufacturer, liquids used, test procedure, test programme, reference to calibrations and instrument calculations, all test measurements, and the determined results.

A full set of records shall be stored in hard copy or electronic form. For design test data, all test records shall be initialled by the representatives of the parties supervising the test. Each party shall receive a copy of all the records.

To be able to re-evaluate questionable measurements, the facility and the instruments should remain assembled and ready to operate until the performance test data are analysed and validated by all stakeholders.

The calculation of thermal performance uncertainty k.A.F. according to LMTD method is presented in Annex A.

A test report shall be drawn up: a test report template is proposed in Annex B.

1. 
   (informative)
   
   Calculation of thermal performance uncertainty k.A.F. according to LMTD method

Calculation of relative uncertainty of thermal performance k.A.F according to LMTD method by quadratic sum is indicated in Formula (A.1):

(A.1)

This uncertainty shall be taken into account in the overall thermal performance of the heat exchanger, from the design phase; see Formula (A.2) and (A.3):

(A.2)

(A.3)

The calculation of heat transfer rate ϕ can be performed directly using the enthalpy or using the specific heat capacity multiplied by the temperature. The uncertainty of all the physical properties of the liquids shall be considered in all cases.

NOTE Formula (A.2) is the mean of uncertainties between primary and secondary sides. Another, more conservative approach, could be to use the maximum of both.

If the uncertainties are provided at 1σ (1 standard deviation), it is necessary to multiply by 1,96 to obtain a two-sided uncertainty at 95 % (2 standard deviations). The calculation of the thermal performance uncertainty of the heat exchanger shall be performed using the Quadratic Sum method and/or the Monte Carlo method and deduced from the actual thermal performance measured.

If there is a single threshold overshoot criterion (e.g. thermal performance of heat exchanger greater than a design requirement, but no minimum), the confidence interval can be considered one-sided at 95 %. For threshold overshoot with one-sided test, the expansion coefficient is taken as equal to 1,645 with Formula (A.3).

If the uncertainty of the overall heat transfer coefficient k is requested, it is necessary to consider the uncertainty of corrective factor F.

One-sided uncertainty = Two-sided uncertainty / 2 × 1.645 (at 95%) (A.4)

1. 
   (informative)
   
   Test report template

An example of a test report is provided below, for information only.

1. General information

a) Identification of heat exchanger tested;

b) Identification of location (laboratory, factory, site, etc.);

c) Name of customer;

d) Name of supplier;

e) Name of laboratory (if laboratory test);

f) Statement from persons who performed and observed the test;

g) Test date(s) and duration(s);

h) Date of first commercial use of heat exchanger;

i) Required reference conditions of heat exchanger;

j) Statement of test bench acceptance criteria;

k) Statement of heat exchanger's thermohydraulic performance acceptance criteria;

l) Number of tests to be included in report.

2. Purpose(s) of test;

3. Background;

4. Test methods and procedures;

5. Test data and results under test conditions (flow rates, temperatures, pressures, pressure drops, power, LMTD, primary and secondary side performance);

6. Test result under reference conditions (flow rates, temperatures, pressures, pressure drops, power, LMTD, primary and secondary side performance);

7. Comparison of several series of tests (if conducted);

8. Conclusions;

9. Appendices:

a) Calculation example;

b) List of instrumentation and calibrations;

c) List of personnel participating in tests;

d) Uncertainty analysis;

e) Mechanical data, specification sheets and drawings;

f) Raw test data.

1. 
   (informative)
   
   Example of test
   1. General

This Annex shows an example of application of the requirements and guidelines contained in this report.

• 1. Thermal performance of a water/water shell-and-tube heat exchanger
     1. General

This subclause evaluates thermal performance of a water-to-water TEMA AEL heat exchanger. It has 2 passes tube side and 1 pass shell side, segmental baffles, and no tubes in window shell, as in Figure C.1.

This heat exchanger is installed on a power plant and tests were performed on-site assuming a clean heat exchanger.

Test thermal acceptance criteria has been defined as per 6.1.2.3.2. The aim of this criteria is to ensure the heat exchange will outperform the required thermal performance. However, a variation of the conclusions with a test acceptance criterion as per 6.1.2.3.1 is also proposed.

Key

Figure C.1 — Test heat exchanger description

• • 1. Heat exchanger reference data

This heat exchanger has been designed to ensure the following performance in Table C.1.

Table C.1 — Heat exchanger design data

T₁₁ and T₂₁ can be determined using Formulae (C.1) and (C.2) according to 5.3.2, neglecting losses:

(C.1)

(C.2)

ΔT_i, ΔT_o and LMTD can then be determined according to 5.3.5.2.1. In the case of a mixed flow heat exchanger, the counter-current configuration is used and will be corrected by a LMTD correction factor:

ΔT_i = T₁₁ – T₂₂ = 44,58 - 35,47 = 9,11 K

ΔT_o = T₁₂ – T₂₁ = 35 – 26 = 9 K

LMTD = 9,05 K

The minimum required k.A.F can be determined using Formula (C.3) according to 5.3.5.2.2:

k.A.F_required = (C.3)

• • 1. Test conditions and data

Considering the high heat transfer rate, this heat exchanger cannot be tested in a lab. It is tested on the power plan where it is installed but it cannot be tested at full heat transfer rate conditions. Therefore, a transposition will be necessary to check that performance of the heat exchanger is above k.A.F_required calculated before.

The heat exchanger is tested with reference conditions liquids and is not thermally insulated.

The aimed level of uncertainties for this performance test is level 2 (see 6.2.4.1) and the sensors are installed on site are described in Table C.2 Each sensor’s uncertainty is determined by combining both sensor’s uncertainty and the signal treatment system uncertainty. When n several sensors are measuring the same information and can be considered as independent, the overall measurement uncertainty is determined by the following quadratic sum (see JCGM 100:2008):

(C.4)

Table C.2 — Number of sensors and uncertainties

Data acquisition has begun once stable conditions have been reached (stable power during 30 min, see 6.1.3). The data acquisition lasted 40 min and each measurement data was averaged over this duration. Output data and calculations results are detailed in Table C.3 below:

Table C.3 — Thermal test measurements (values in italic are computed)

Each value is computed by the same manner as in C.2.2 using formulae from 5.3.

Uncertainties of Φ, LMTD and then k.A.F are evaluated using formulae:

= 2066 kW

0,3 K

= 0,0371

248,9 kW/K

This test enabled to have a 1-sided k.A.F uncertainty of 5 % and is therefore judged compliant with the level 2 acceptable values of overall uncertainties.

• • 1. Test results transposition

A digital model of the heat exchanger has been made to simulate the heat exchanger using a spreadsheet, DTLM method, usual correlations for Nusselt number and IAPWS water properties. A specialized software could also have been used.

In test conditions, the digital model estimates the k.A.F coefficient to 4 353 kW/K. However, the results from the test minus uncertainties are 3 882 kW/K in the best case (1 sided uncertainty). The digital model shall be adjusted to fit the reality. Coefficient C_e [see Formula (35)] shall be calculated iteratively to be accurate:

1. C_e_1 = 3 882 / 4 353 = 0,89. New calculation gives k.A.F = 4 038 kW/K

2. C_e_2 = 3 882 / 4 038 * C_e_1 = 0,96*0,89 = 0,85. New calculation gives k.A.F = 3 913 kW/K

3. C_e_3 = 3 882 / 3 913 * C_e_2 = 0,99*0,85 = 0,84. New calculation gives k.A.F = 3 881 kW/K

4. C_e_4 = 3 882 / 3 881 * C_e_3 = 0,841. New calculation gives k.A.F = 3 882 kW/K

In this case the digital model is adjusted with a C_e coefficient of 0,841.

A calculation is performed with the reference data (see C.2.2), results are as follow:

— k.A.F_calculated = 2 732  kW/K < k.A.F_required = 3 978 kW/K

— Φ_calculated = 24 662 kW < Φ_required = 36 000 kW

• • 1. Test conclusion

The thermal test performance conclusion is that the heat exchanger is undersized by 31 % either on the k.A.F criterion or on a heat transfer rate criterion.

• • 1. Thermal results according to best estimate criterion

The thermal test performance acceptance criterion of 6.1.2.3.1 is calculated below considering in this case that the reference conditions are the following ones, as detailed in Table C.4:

Table C.4 — Heat exchanger design data – Best estimate

NOTE These references conditions involve a lower required thermal performance than in the previous paragraphs.

— In the same manner as C.2.2, k.A.F_required = 4 195 kW/K

— k.A.F – U(k.A.F) (1 sided) is 3 882 kW/K

— k.A.F + U(k.A.F) (1 sided) is 4 291kW/K

k.A.F_required is between tests results considering uncertainties so criteria is validated. Considering k.A.F is lower than k.A.F_required, the heat exchanger might be underperforming a bit.

• 1. Pressure drop of a water/water shell-and-tube heat exchanger

The same heat exchanger as in C.2 pressure drops has been measured and are detailed in Table C.5.

Table C.5 — Pressure drop test measurements (values in italic are computed)

For both sides, pressure drops respect the design requirement, so the pressure drop criterion is satisfied.

NOTE Another acceptance criteria could have been defined in the same way as the best estimate criterion for thermal performance defined in 6.1.2.3.1.

The digital model of the heat exchanger had the following estimates for hydraulic resistances at the same flow rates, temperatures, and pressures as the test:

— Primary: 72,3 .10⁻⁹ barg/ (m³/h)²

— Secondary: 68,8 .10⁻⁹ barg/ (m³/h)²

Transposition coefficients are calculated according to 6.3.3.2 to make digital model better fit the reality:

— C_{DP_primary} = 84,4/72,3 = 1,17

— C_{DP_secondary} = 59/68,8 = 0,86

Bibliography

[1] IAPWS-IF95, Revised Release on the IAPWS Formulation 1995 for the Thermodynamic Properties of Ordinary Water Substance for General and Scientific Use

[2] IAPWS-IF97, Revised Release on the IAPWS Industrial Formulation 1997 for the Thermodynamic Properties of Water and Steam

[3] IAPWS-08, Release on the IAPWS Formulation 2008 for the Thermodynamic Properties of Seawater

[4] JCGM 100:2008, Evaluation of measurement data — Guide to the expression of uncertainty in measurement

[5] JCGM 101:2008, Evaluation of measurement data — Supplement 1 to the “Guide to the expression of uncertainty in measurement” — Propagation of distributions using a Monte Carlo method

[6] EN ISO/IEC 17025, General requirements for the competence of testing and calibration laboratories (ISO/IEC 17025)

[7] ASME PTC 12.5, Single Phase Heat Exchangers, Performance Test Codes, An American National Standard, The American Society of Mechanical Engineers, 2005

[8] ANSI/AHRI Standard 401, Standard for Performance Rating of Liquid to Liquid Heat Exchangers, 2015

[9] RCC-M, Design and Conception Rules for Mechanical Components of PWR Nuclear Islands, AFCEN

[10] GRETh Technical Manual – TM 24 – Heat exchange and pressure drop correlations outside a bundle in a shell-and-tube exchanger: Bell-Delaware method, 1999

[11] Kern D.Q. Process Heat Transfer. Tata McGraw Hill Publishing Company Ltd, 1965

[12] EN 307, Heat exchangers — Guidelines to prepare installation, operating and maintenance instructions required to maintain the performance of each type of heat exchangers