IPE-TM-400-10 — Evaluating Heat Exchangers for Revamps

1. Table of Contents

1. Table of Contents 1

2. Purpose 2

3. General 2

4. Information Required for Evaluations 2

4.1 Revamp Operating Conditions and Fluid Properties 2

4.2 TEMA Data Sheet or Air Cooled Exchanger Data Sheet 2

4.3 Exchanger Drawings 4

5. Overall Exchanger Evaluation 4

5.1 Design Pressures and Temperatures 4

5.2 Materials of Construction 4

5.3 Pressure Drop 4

5.4 Surface Area 5

6. Thermal Rating Methods 5

6.1 Constant UA Method 5

6.2 Using Key Variable Relationships (GPSA Method) 5

6.3 Inflection Point Engineering Revamp Tools 6

6.4 HTRI or HTFS Computer Programs 7

7. Rating Procedures 7

7.1 Water Cooled Exchangers 7

7.2 Process-Process Exchangers 8

7.3 Air Cooled Exchangers 8

7.4 Horizontal Thermosiphon and Kettle Type Reboilers and Vaporizers 10

8. Pressure Drop Estimation 10

9. Use of Operating Data 12

10. Possible Recommendations 12

10.1 Use Existing Exchanger 12

10.2 Replace Existing Exchanger 12

10.3 Add Additional Exchanger Shell(s) or Air Bay(s) 12

10.4 High Flux / High Cond Tubing 13

10.5 Rerating Existing Exchangers 13

10.6 Rearrangement and Other Modifications 14

11. Special Exchanger Services 14

11.1 Vertical Combined Feed Exchangers 15

11.2 Packinox Welded Plate Type Exchangers 15

11.3 Strength-Welded Tube to Tubesheet Joints 15

12. Overpressure Protection 15

13. Responsibilities and Liabilities 15

Attachment 1 Sample (Completed) TEMA Data Sheet 16

Attachment 2 Tube Wall (Metal) Resistances (h-ft²-F/Btu) 17

Attachment 3 Water Cooled Exchanger - Example Rating 18

Attachment 4 Process-Process Exchanger - Example Rating 20

2. Purpose

This procedure provides guidelines for evaluating existing heat exchangers for revamp projects.

General

The extent to which the Design Engineer will evaluate an exchanger depends upon the engineering product for which the evaluation is being conducted. Process Revamp Studies and Revamp Schedule A's, for instance, require more detailed analyses than Revamp Feasibility Studies, which are quicker evaluations intended to determine the viability of a potential revamp through less than rigorous methods.

Information Required for Evaluations

Use the following information to determine if an existing heat exchanger suits a new or modified service.

4.1 Revamp Operating Conditions and Fluid Properties

The flow rates and physical properties of fluids greatly influence the performance of heat exchangers. The following key data are needed to evaluate an exchanger:

duty and operating temperatures
mass flows
fluid densities
thermal conductivities
viscosities
heat capacities

4.2 TEMA Data Sheet or Air Cooled Exchanger Data Sheet

Inflection Point Engineering Project Specification sheets do not provide all of the data needed to evaluate an exchanger. Therefore, supplement these sheets with the vendor's Tubular Exchanger Manufacturers Association (TEMA) data sheet or equivalent (see Attachment 1).

TEMA data sheets provide the following information:

Number of shells (in series and in parallel)
Surface area (per shell and total)
TEMA type designation (AES, BEU, etc.)
Original process data and stream properties (flow rates, temperatures, etc.)
Number of tube passes
Fluid velocities
Pressure drop (allowed and calculated)
Fouling factors
Duty
Corrected MTD
Heat transfer rate, U (clean and dirty/service)
Design temperatures and pressures
Tube data
Shell diameter
Baffle data
Materials of construction
Nozzle sizes and flange ratings

Typical Air Cooled Exchanger (API/ISO) data sheets provide the following information:

Number of units and number of bays per unit
Surface area (bare tube and finned tube)
Original process data and stream properties (flow rates, temperatures, etc.)
Number of passes
Air inlet and outlet temperatures
Mass air flow rate
Pressure drop (allowed and calculated)
Fouling factors
Duty
Corrected MTD
Heat transfer rate, U (bare tube and finned tube)
Design temperature and pressure
Tube and tube bundle data
Fin data
Header data
Materials of construction
Fan data
Nozzle sizes and flange ratings

If vendor data sheets are unavailable and Inflection Point Engineering originally specified the exchanger, use the Inflection Point Engineering Project Specification and Inflection Point Engineering's estimated exchanger sizing to make rough evaluations. Qualify any comments regarding the exchanger's suitability by indicating that Inflection Point Engineering's evaluation is based on limited information and requires confirmation. For a Revamp Schedule A, a standard note is added to address this situation.

4.3 Exchanger Drawings

Usually, if the HTRI computer programs will be used for thermal rating or pressure drop calculations (see Section 6.4), vendor drawings are needed to evaluate shell & tube heat exchangers. The TEMA data sheets often lack adequate data on the baffle arrangements that are required input for these programs. In such cases, obtain baffle data from the exchanger arrangement drawings.

5. Overall Exchanger Evaluation

Review the following items to determine whether an existing exchanger suits a particular revamp service.

5.1 Design Pressures and Temperatures

The operating pressures and temperatures for the revamp service must not exceed the respective design pressures and temperatures of the existing exchanger (both shell and tube sides). Sometimes, depending on the metallurgy, flange ratings, and design pressure, it is possible to re-rate an existing exchanger for a higher design temperature. This likely requires a corresponding decrease in design pressure. The original manufacturer is expected to re-rate existing exchangers, which typically necessitates a new American Society of Mechanical Engineers (ASME) Pressure Vessel Code stamp.

5.2 Materials of Construction

The existing exchanger's materials of construction must suit the revamp service. Thus, changes in the process stream composition and/or temperature may require a different metallurgy than that of the existing equipment. To determine if the metallurgy of the existing exchanger is acceptable, review any significant changes in operating temperature, sulfur content, H2S content, hydrogen partial pressure, etc.

5.3 Pressure Drop

Estimate the pressure drop for the revamp service and evaluate how it affects the process unit hydraulics. Replacing or augmenting an exchanger may be less expensive than replacing a compressor or multi-stage pump. Also, evaluate the pressure drop on the water side of a water cooled exchanger and compare it to the available pressure drop of the existing cooling water system. Conduct this same type of analysis on the hot oil side of hot oil exchangers.

5.4 Surface Area

The revamp service needs sufficient heat transfer surface area. Although this may be obvious by inspection, it may require a detailed analysis. If analysis is required, there are several ways to evaluate the adequacy of the existing surface area. The method employed simply depends on the type of exchanger in question, the magnitude of change in the operating conditions, and the level of confidence required by the type of revamp engineering undertaken.

6. Thermal Rating Methods

6.1 Constant UA Method

Starting with the Fourier equation for heat transfer, Q = UA(CMTD), the thermal rating may be simplified greatly if the U and A terms are assumed to be constant. If this assumption may be made, then the heat transferred, Q, is directly related to the corrected log mean temperature difference, CMTD. Obviously, if an existing exchanger will be used, the area, A, remains constant.

If the mass flows equal or exceed the design mass flows, and the fluid properties are nearly the same, the overall U for the revamp operation will equal or exceed that of the original design. In such cases, make a preliminary evaluation of the existing exchanger by assuming a constant UA. Do not assume a constant overall U value if the mass flows are less than the design mass flows, or if the fluid properties are significantly different from those of the original design. Instead, perform a more rigorous analysis to estimate the U expected for the revamp operation.

In many cases, the constant UA method sufficiently determines whether an existing exchanger has adequate surface area. Use this method of evaluation, when applicable, for Revamp Feasibility Studies, or Process Revamp Studies.

6.2 Using Key Variable Relationships (GPSA Method)

This method is based on determining the change in one or more of the five resistances that comprise the overall resistance to the heat transfer, R.

R = r film (shell) + r fouling (shell) + r tube wall + r fouling (tube) (Ao/Ai) + r film (tube)(Ao/Ai)

The reciprocal of R is the overall heat transfer coefficient, U.

Although this method technically applies only to sensible heat transfer, i.e., when there is no phase change, it may be used if the phase change of the revamp nearly equals that of the original design. If an exchanger will be used for the same service, assume that the fouling resistances are the same as those of the original design (unless operating data indicates otherwise). If the exchanger will be used in a different service, review and change the fouling resistances as necessary. Consult the Heat Exchanger Specialists for appropriate values. Include the tube wall resistance in the calculation. Attachment 2 lists resistance values for typical tube sizes and materials.

Next, estimate the changes in the film resistances outside and inside the tubes. The film coefficients, ho and hi, are the inverse of the film resistances, ro and ri. Depending on the particular service of the exchanger in question, there are several ways to estimate the film coefficients. Section 7 explains how to rate various types of exchangers.

When one of the film resistances used for the original design is determined (or estimated), it is then possible to approximate the film resistances for the revamp operation by using the following relationships of key heat transfer variables:

Tube side:

h2/h1 = r1/r2 = (G2/G1)0.8 (k2/k1)0.67 (Cp2/Cp1)0.33 (µ1/µ2)0.47

Shell side:

h2/h1 = r1/r2 = (G2/G1)0.6 (k2/k1)0.67 (Cp2/Cp1)0.33 (µ1/µ2)0.27

Where: G = mass velocity

k = thermal conductivity

Cp = heat capacity (specific heat)

µ = absolute viscosity

subscript 1 = original design

subscript 2 = revamp operation

If the exchanger's geometry stays the same, substitute mass flows for mass velocities. Use this method, when applicable, for Revamp Feasibility Studies, Process Revamp Studies, or Revamp Schedule A's.

6.3 Inflection Point Engineering Revamp Tools

Inflection Point Engineering Revamp tools and are available for screening existing heat exchangers to determine if they are thermally and hydraulically suitable for the new service. The limitations of the analysis are described in the tool documentation and . The tools are designed for exchangers with linear heat release curves, similar service applications, and physical properties that do not vary significantly from the original design. If the system requirements are outside of the limits of these tools, an HTRI analysis is required. Contact a Heat Transfer Specialist if in doubt about the suitability of the tools.

6.4 HTRI Computer Programs

The HTRI suite of programs are available on the computer network and should be loaded on all Engineering Department computers. It can be installed on the PC by login to , going to the SBG program tab, and finding the HTRI program or service pack (Program Updates). Use these programs to conduct more rigorous analyses of Process Revamp Studies or Revamp Schedule A's if the exchanger, based on the above methods, is found to be marginal, or the Design Engineer wants to confirm preliminary findings.

Consult the Heat Transfer Specialists if an Engineer wants to evaluate an exchanger using one of these programs or if one of the HTFS/ASPEN programs is needed.

7. Rating Procedures

7.1 Water Cooled Exchangers

Use the following formula to approximate the tube side film coefficient of a water cooled exchanger (with water on the tube side):

hi = 306 (Vw) 0.8

Where: Vw = water velocity, fps

The design shell side film resistance may be back-calculated, provided that the design tube side film coefficient, the design fouling factors, the tube wall resistance, and the design U are known. Use the shell side relationships shown in Section 6.2 to calculate a new shell side film coefficient and a new tube side (water) film coefficient. This makes it possible to determine the overall resistance, R, and the overall U value for the revamp.

The existing exchanger will suit the revamp if the calculated U value equals or exceeds the required U value. Attachment 3 provides an example of this calculation. Initially, the cooling water rate is assumed to remain the same as the original design. This is done to maintain proper minimum velocity in the tubes. An evaluation of the cooling water system hydraulics is required to determine if additional cooling water can be provided.

For water cooled exchangers, the shell side film coefficient controls the overall U value because it contributes more to the overall resistance, R. The hi for water ranges from approximately 750 to 1250 Btu/h-ft²-°F, which is 3 to 4 times greater than the shell side film coefficient for a hydrocarbon stream.

7.2 Process-Process Exchangers

The following equation applies to turbulent flow in tubes. Use it to estimate the tube side film coefficient for a process-process exchanger.

hi = 0.023 G0.8 k0.67 Cp0.33

µ0.47 D0.2

Where: G = mass velocity (lb/h-ft²)

k = thermal conductivity (Btu/h-ft-F)

Cp = heat capacity (Btu/lb-F)

µ = viscosity (lb/ft-h) = cP x 2.42

D = tube ID, ft

For exchangers with similar fluids on both sides, e.g., combined feed-effluent exchangers or fractionator feed-bottoms exchangers, estimate the change in the magnitude of the film coefficients for the revamp operation by making a simplification. If the tube side and shell side film coefficients are assumed equal, determine a value for the original design given the original U value, fouling factors, and tube wall resistance. Then, estimate the revamp film coefficients using the property relationships given in Section 6.2. Attachment 4 contains an example of this calculation.

If the fluid properties and flow rates on both sides of the exchanger are similar, relate the revamp's U value to that of the original design by using the following relationship of mass flows:

Where: W = mass flow rate

The flow rates for G2 and G1 may be tube side or shell side flows, as long as they are consistent.

7.3 Air Cooled Exchangers

Air cooled exchangers are typically limited by the amount of air that the fans can deliver. Thus, to evaluate an existing air cooled exchanger, first determine the air side flow and temperatures. This involves obtaining the design mass air flow rate from the vendor data sheet and calculating the outlet air temperature, given the inlet air temperature and desired revamp duty, using the basic equation for sensible heat transfer:

Q = W Cp T

Where: W = air mass flow rate, lb/h

Cp = heat capacity, Btu/lb-F

= 0.24 Btu/lb-F for air below 300F

T = air side temperature rise, F

Calculate the air side T by rearranging the equation:

T = Q

0.24W

Use the calculated outlet air temperature to calculate the linear MTD for the revamp operation. If a phase change occurs, e.g., condensing service, determine the weighted MTD. Determine the MTD correction factor, f, by calculating the P and R values, knowing the configuration of the air cooler (number and type of passes) and using Figures 10-8 and 10-9 from the GPSA Engineering Data Book, tenth edition, 1987. Use Figure 10-8 for single pass units or any air cooler with side-by-side passes. Use Figure 10-9 for 2-pass units where the passes are over and under each other. Over and under passes are the most common. Assume a correction factor of 1.0 for units with three or more passes.

After determining the CMTD, use the duty required and the available surface area to calculate the required U value. Compare the U value required for the revamp with the U value used for the original design, which appears on the vendor data sheet. The Design Engineer must consistently use either the bare or "finned" surface area and the corresponding U value. The existing air cooler will suit the revamp if the required U value is less than or equal to the U value of the original design.

Because the air side film resistance is controlling, changes in the process side flow and fluid properties have less impact than they do in a typical shell & tube heat exchanger. Determine the effect of these changes using the same type of analysis that is used for S&T exchangers. First, estimate the air side film coefficient by calculating the air side "face mass velocity" and using Figure 10-17 on pages 10-15 of the GPSA Engineering Data Book. Then, determine the face mass velocity by dividing the lb/h of air by the air cooler face area in sq. ft, both of which are usually included on the vendor data sheet.

7.4 Horizontal Thermosiphon and Kettle Type Reboilers and Vaporizers

The overall heat transfer coefficient for steam heated (condensing steam) horizontal thermosiphon reboilers and kettle type reboilers and vaporizers does not change significantly with changes in the process side or steam side flow rates. Thus, the extent of possible capacity/duty increase is, for the most part, a function of the change in the LMTD of the original design. Determine the LMTD for these exchangers according to the guidelines provided in Procedure IPE-TM-400-11, “Design and Specification of Thermosiphon Reboilers”.

Changes in the flow rate of hot oil will affect the overall heat transfer coefficient for hot oil heated reboilers. Use the methods described in Section 6.2 to estimate the change in the tube side (hot oil) film coefficient.

The Design Engineer must check the heat flux rate and the approach temperature for the revamp conditions to ensure that they are within acceptable limits. See IPE-TM-400-11, “Design and Specification of Thermosiphon Reboilers” for guidelines for minimum heat flux rates and minimum approach temperature differences in thermosiphon reboilers. Consider using High Flux tubes for lower approach temperatures or higher flux rates (see Section 10.4).

Normally, 33% of the liquid entering horizontal or vertical thermosiphon reboilers vaporizes which prevents an excessive pressure drop and unstable operation. For a revamp, up to 50% vaporization is permissible, provided that the consequent higher pressure drop across the exchanger can be tolerated. See IPE-TM-400-11, “Design and Specification of Thermosiphon Reboilers” for additional guidelines for maximum percent vaporization in a thermosiphon reboiler.

For Process Revamp Studies and Revamp Schedule A's, evaluate the hydraulics of existing horizontal thermosiphon reboilers using . Hydraulics are not usually evaluated for Revamp Feasibility Studies. However, the Feasibility Study report is expected to indicate that the reboiler hydraulics require evaluation to confirm the suitability of the existing exchanger. Procedures IPE-TM-400-11, “Design and Specification of Thermosiphon Reboilers” and IPE-TM-400-02, “Stabbed in Reboilers” discusses the design and specification guidelines for reboilers in greater detail.

8. Pressure Drop Estimation

When conducting Process Revamp Studies and Revamp Schedule A's, estimate the pressure drop of an existing exchanger at the revamp operating conditions in terms of how it will affect the hydraulics of the circuit in which it is contained.

When conducting Revamp Feasibility Studies, perform a preliminary hydraulic analysis of the circuits which contain major equipment (e.g., compressors or multi-stage pumps). This is often accomplished by estimating the pressure drop of the whole circuit rather than that of each piece of equipment.

If the overall circuit pressure drop is excessive, consider modifying or replacing an exchanger that accounts for much of the pressure drop. For single phase flow, estimate the pressure drop for shell & tube exchangers based on the original calculated pressure drop from the vendor data sheet and by using the following relationship:

Tube side:

P2 = P1 (G2/G1)1.8 (1/2) (µ2/µ1)0.20

Shell side:

P2 = P1 (G2/G1)1.85 (1/2) (µ2/µ1)0.15

Where: = average fluid density

Ignore the viscosity term if the fluid viscosities for the revamp operation barely differ from the original design.

The tube side relationship also applies to the process side of air coolers. Use the above relationships for two-phase flow when the vapor/liquid ratio for the new conditions approximately equals that of the original design.

By using the following relationship for two-phase flow, the ratios of the revamp to the original exchanger inlet and outlet line P's may be used to determine the exchanger P for the revamp:

Avoid excessive pressure drops caused by high fluid velocities, even if they are tolerated by the circuit hydraulics. High fluid velocities may cause vibration and/or erosion problems.

For water and similar liquids, velocities of 10 ft/sec and above are considered excessive.

For Schedule A revamps, the pressure drop should be calculated using the more rigorous methods of HTRI software.

9. Use of Operating Data

If operating or test run data for the unit is available, review it with respect to the performance and pressure drop of major heat exchangers. The operating data may indicate that the U value or pressure drop for a particular exchanger is different than what is shown on the vendor data sheet. This may be due to excessive fouling, maldistribution, conservative design, etc. If the operating data seems reliable, factor it into the evaluation. However, if the operating data indicates that an exchanger has excessive fouling, discuss and verify this with the customer, if possible.

Based on discussions with the customer, or input from Technical Service or Engineering Specialists, the Design Engineer may choose to rate the exchanger based on the design condition shown on the TEMA data sheets, or the fouled condition indicated by the operating data. In this event, the Revamp Feasibility Study report or the Process Revamp Study report is expected to state the assumptions and the rating basis.

10. Possible Recommendations

An evaluation of an existing exchanger may result in Inflection Point Engineering making one of the following recommendations.

10.1 Use Existing Exchanger

If the design pressure and temperature, metallurgy, heat transfer surface area and pressure drop satisfy the new process conditions, use the existing exchanger without modification.

10.2 Replace Existing Exchanger

The existing exchanger may need to be replaced to satisfy the new process conditions. This may be due to inadequate design conditions, unacceptable metallurgy, excessive pressure drop, lack of surface area, or some combination of the above. Use of compact heat exchanger technologies, such as plate type heat exchangers, may be considered.

10.3 Add Additional Exchanger Shell(s) or

If the design conditions and metallurgy are acceptable, but the existing exchanger has insufficient surface area, recommend an additional shell, or in the case of an air cooled exchanger, an air bay. Add the additional shell in series or in parallel with the existing exchanger. If the pressure drop permits, it is better to place the new shell in series with the existing exchanger shells. This eliminates flow distribution concerns and it may increase the MTD correction factor. However, in many cases pressure drop considerations require that the additional shell is added in parallel. In this event, use a duplicate of the existing exchanger and symmetrical piping to provide for good flow distribution. This is particularly important for 2-phase flow to the exchanger. For air cooled exchangers, air bays are almost always added in parallel using symmetrical piping to provide good flow distribution.

10.4 High Flux / High Cond™ Tubing

For boiling services where the design conditions and shell metallurgy are acceptable, but the exchanger has insufficient surface area for the revamp, it may be economical to replace the existing tube bundle with Inflection Point Engineering High Flux tubes. This may cause an overall U value that is 2 to 4 times greater than that obtained with conventional bare tubes. Installing High Flux tubes in an existing shell does not require additional plot space and it minimizes downtime.

Examine the pressure drop and outlet nozzle size of reboilers revamped with High Flux tubes.

For condensing services, it may be economical to replace the existing tube bundle with High Cond tubes to increase capacity of the heat exchanger. The special surface characteristics of these tubes improve the heat transfer performance of the exchanger.

The PTE Separation & Heat Transfer Technology Group in Tonawanda can provide sizes and ratings of exchangers with High Flux / High Cond tubing. Consult with the Heat Transfer Specialists about the applicability of High Flux / High Cond for specific revamp services. See tool ” to effectively transfer the data to Tonawanda.

10.5 Rerating Existing Exchangers

When the heat exchanger evaluation shows that an existing exchanger has acceptable metallurgy, sufficient surface area, and acceptable hydraulic performance but the design conditions are exceeded, it may be possible to rerate the heat exchanger to new higher design temperatures and/or pressures. The actual rerating is performed by the exchanger vendor or engineering contractor and not by Inflection Point Engineering.

Rerating involves detailed mechanical design review to determine if the actual thicknesses of the parts of the exchanger are suitable for the new design conditions. If any part is not sufficient, then the impacted part would have to be replaced.

There is no way to determine if a rerating is possible without actually performing the calculations. However, some approximate guidelines are provided for guidance:

To increase the design temperature – Possible if the allowable stress does not drop from the original to revamp design temperature. For example, carbon steel has the same allowable stress from ambient to 650ºF. Rerating is unlikely if the flange rating is higher at the new conditions.

To increase the design pressure – Possible for a increase of <5%. It may be necessary to reduce the corrosion allowance for the impacted side. It is more likely possible to rerate if the increase in design pressure is offset by a decrease in design temperature.

Contact a Heat Transfer Specialist to review if a individual heat exchanger is a good candidate for rerating.

10.6 Rearrangement and Other Modifications

a. Consider using existing exchanger shells (or air bays) in a different service if they do not suit the revamp of their previous service.

b. Consider rearranging the existing shells from series flow to parallel operation when the pressure drop is excessive and the required duty and operating temperatures permit such a change.

c. If sufficient water is available, reduce the water side pressure drop for water cooled exchangers with multiple shells in series by changing the water side from series flow to parallel flow.

d. If the surface area is adequate, but the pressure drop on the tube side of a shell & tube exchanger is excessive, try to reduce the number of tube passes by removing one or more pass partitions from the channel. This may be suggested in the study phase of a project but the contractor or vendor must confirm its practicality.

e. Replacing existing fans with higher efficiency fans may increase the capacity of older air cooled exchangers by 10 to 15 percent. Again, this may be suggested in the study phase of a project but the contractor or vendor must confirm its practicality.

11. Special Exchanger Services

Certain heat exchangers require special consideration when they are evaluated for a revamp service. Some examples are listed below. The Design Engineer shall consult the Heat Exchanger Specialist if they are unsure of an exchanger's suitability, or questions any special features required by a given exchanger service.

11.1 Vertical Combined Feed Exchangers

These exchangers have a single tube-side pass, which means they approach "true counter-current" flow. However, for no-tube-in-the-window (NTIW) type designs, the long baffle spacing makes it necessary to apply a correction factor of approximately 0.9 to the weighted MTD. This will account for the cross-flow nature of the shell-side fluid. Also, if the combined feed liquid/vapor mixture is distributed unevenly to the tubes, these exchangers may suffer from poor performance with operating heat transfer coefficients that are less than that of the design.

11.2 Packinox Welded Plate Type Exchangers

Consider using Packinox type combined feed-effluent exchangers in revamps of Platforming, Tatoray, Pacol, and Isomar Process Units. A single Packinox exchanger can replace existing shell & tube exchangers while providing greater heat transfer efficiency and reducing reactor circuit pressure drop. In many revamp scenarios this makes it possible to reuse the existing recycle gas compressor.

11.3 Strength-Welded Tube to Tubesheet Joints

Certain exchanger services require strength-welded tube to tubesheet joints (SWTTJ) to prevent or minimize the possibility of cross leakage. Reference Procedure and Process Specific Guidelines for each service for a listing of the services and requirements. If an existing exchanger without SWTTJ will be used in a service that requires SWTTJ, replace the tube bundle with a strength welded U-tube bundle

12. Overpressure Protection

Consider whether exchangers need overpressure protection when the design pressure of the low pressure side is less than that of the high pressure side. Follow the guidelines of Procedure .

13. Responsibilities and Liabilities

When Inflection Point Engineering reviews existing exchanger data sheets to determine the suitability of existing equipment, include the revamp notes from Procedure .

Attachment 1

Sample (Completed) TEMA Data Sheet

Attachment 2

Tube Wall (Metal) Resistances

(h-ft²-°F/Btu)

Tube O.D., inches	3/4	3/4	3/4	1	1	1
Tube Wall, inches	0.109	0.083	0.065	0.109	0.083	0.065
Admiralty	0.00015	0.00011	0.00008	0.00015	0.00011	0.00008
Carbon Steel or 90-10 Cu-Ni	0.00036	0.00027	0.00020	0.00035	0.00026	0.00020
1¼ or 2¼Cr-1Mo	0.00049	0.00036	0.00027	0.00047	0.00035	0.00027
70-30 Cu-Ni	0.00059	0.00043	0.00033	0.00057	0.00042	0.00032
Titanium	-----	0.00065	0.00050	-----	0.00063	0.00048
Type 316	0.00127	0.00093	0.00071	0.00121	0.00090	0.00069

Attachment 3

Water Cooled Exchanger - Example Rating

Exchanger Data (See Appendix 1, TEMA data sheet for Naphtha Sidecut Cooler)

2 shells in series, type AES

78 tubes, 1" O.D., 0.083 inch, Admiralty brass, 20 ft. long

Total surface area = 820 ft²

2 tube passes

Linear heat release curve is assumed.

Design U value (service) = 80 Btu/ft²-h-°F

Design CMTD = 42°F

Process Data

Original Design Revamp

Duty, MM Btu/h 2.74 4.70

Process inlet temp., °F 230 275

Process outlet temp., °F 100 100

Water inlet temp., °F 88 88

Water outlet temp., °F 115 115

LMTD Calculation

275 -----> 100 R = (275-100)/(115-88) = 6.48

115 <----- 88 P = (115-88)/(275-88) = 0.14

160 12

LMTD = 160 – 12 = 57.1°F

ln (160/12)

From TEMA Figure T-3.2B for 2 shell passes, LMTD correction factor f = 0.95

Corrected MTD = 57.1 x 0.95 = 54.2°F

U required = 4.70 x 106 Btu/h = 106 Btu/h-ft²-°F

820 ft² x 54.2 F

Determine original design tube-side and shell-side film coefficients

Original Tube-side film coefficient

Water flow rate = 101,500 lb/h

101500 lb x 1 h x ft³ = 0.454 ft³

h 3600 sec 62.1 lb sec

78 - 1 in. O.D., 0.083 in. tubes CSA = 0.296 ft² = 0.148 ft²/pass

2 passes

Water velocity = 0.454 ft³/sec = 3.1 ft/sec

0.148 ft²

hi = 306 (3.1)0.8 = 756 Btu/h-ft²-°F

Attachment 3

Water Cooled Exchanger - Example Rating (continued)

Original Shell-side film coefficient

r film(shell) = 1/U - r fouling(shell) - r tube wall - r fouling(tube)(Ao/Ai) - r film(tube)(Ao/Ai)

= 1/80 - 0.0015 - 0.00011 - 0.003(1.199) - (1/756)(1.199)

= 0.00571

ho = 1/r = 175 Btu/h-ft²-°F

Revamp Shell-side film coefficient

h2/h1 = r1/r2 = (G2/G1)0.6 (k2/k1)0.67 (Cp2/Cp1)0.33 (µ1/µ2)0.27

h2/h1 = (54200/45071)0.6 (0.068/0.074)0.67 (0.52/0.47)0.33 (0.58/0.45)0.27

h2 = 1.169 x 175 = 204.5 Btu/h-ft²-°F

Revamp Tube-side film coefficient

Water flow rate req'd = 4.70 x 106 Btu/h = 174,074 lb/h = 0.779 ft³/sec

(115-88) F x 1.0 Btu/lb-°F

It is assumed that this extra water is available and the distribution system can provide the flow. The calculations should also be done assuming no extra water is available.

Water velocity = 0.779 ft³/sec = 5.3 ft/sec

0.148 ft²

hi = 306 (5.3)0.8 = 1162 Btu/h-ft²-°F

Revamp U

R = r film (shell) + r fouling (shell) + r tube wall + r fouling (tube)(Ao/Ai) + r film (tube)(Ao/Ai)

R = 1/204.5 + 0.0015 + 0.00011 + 0.003(1.199) + (1/1162)(1.199)

R = 0.0111

U = 1/R = 90 Btu/h-ft²-°F

The calculated U of 90 for the revamp is less than the required U of 106; thus, the existing exchanger does not have sufficient surface area for the revamp operation. To achieve the required battery limit temperature of 100F, provide additional surface area. In this case, because process side pressure drop is available, add an additional shell in series. This shell does not need to be identical to the existing shell, but it can be sized to provide the incremental surface area required. Inflection Point Engineering may do a preliminary sizing for EFCEST purposes, but ultimately the contractor is responsible for rating the existing exchanger and sizing the new shell.

Attachment 4

Process-Process Exchanger - Example Rating

Service Stripper Feed - Bottoms Exchanger

Exchanger Data

3 shells in series, type AES

389 U-tubes per shell, 1" O.D., 0.109 inch, carbon steel, 20 ft. long

Total effective surface area = 6,000 ft² - (2,000 ft² per shell)

2 tube passes

Design U value (service) = 49.1 Btu/h-ft²-°F

Process Data

Original Design Revamp

Duty, MM Btu/h 28.82 41.92

Hot-side inlet temp., °F 529 580

Hot-side outlet temp., °F 253 271

Cold-side inlet temp., °F 131 135

Cold-side outlet temp., °F 425 475

Hot-side flow rate, lb/h 165,897 208,880

Cold-side flow rate, lb/h 174,555 214,233

Corrected MTD, °F 97.9 101.3

Fouling Factors (from TEMA data sheet)

Shell side = 0.002 ft²-h-°F/Btu

Tube side = 0.002 ft²-h-°F/Btu

Design Film Coefficients

Assume h0 = hi

R = 1/U = 1/49.1 = 0.0204 ft²-h-F/Btu

R = r film (shell) + r fouling (shell) + r tube wall + r fouling (tube)(Ao/Ai) + r film (tube)(Ao/Ai)

Let r film (shell) = r film (tube) = x

x + (Ao/Ai)x = R - r fouling (shell) - r tube wall - r fouling (tube)(Ao/Ai)

Ao/Ai = Do/Di = 1.0/0.782 = 1.2788

2.2788x = 0.0204 - 0.002 - 0.00035 - 0.002(1.2788)

x = 0.00680

h0 = hi = 1/x = 147.1 Btu/h-ft²-°F

Attachment 4

Process-Process Exchanger - Example Rating (continued)

Calculate Revamp Film Coefficients

Using the GPSA relationships from section 6.2, the revamp film coefficients can be estimated as a function of the mass flow rates and key properties:

Tube side:

h2 = h1 (G2/G1)0.8 (k2/k1)0.67 (Cp2/Cp1)0.33 (µ1/µ2)0.47

h2 = 147.1 (214233/174555)0.8 (0.063/0.064)0.67 (0.600/0.576)0.33 (0.634/0.677)0.47

h2 = 168.5

Shell side:

h2 = h1 (G2/G1)0.6 (k2/k1)0.67 (Cp2/Cp1)0.33 (µ1/µ2)0.27

h2 = 147.1 (208880/165847)0.6 (0.068/0.060)0.67 (0.507/0.632)0.33 (0.394/0.512)0.27

h2 = 159.2

Calculate Revamp U

R = r film (shell) + r fouling (shell) + r tube wall + r fouling (tube)(Ao/Ai) + r film (tube)(Ao/Ai)

R = 1/159.2 + 0.002 + 0.00035 + 0.002(1.2788) + (1/168.5)(1.2788)

R = 0.01878

U = 1/R = 53.3 Btu/h-ft²- F

Determine Surface Area Required for Revamp Operation

A = Q = 41.92 x 106 = 7764 ft²

UT (53.3) (101.3)

The existing surface area of 6000 ft² is insufficient for the revamp. One additional duplicate shell in series will provide a total of 8000 ft², which is more than adequate.

Alternately, the Design Engineer may decide not to add surface to the Feed - Bottoms Exchanger, but to instead make up the additional heat requirement in the reboiler. In this case, use the revamp U value determined above, along with the surface area of the existing exchanger, to estimate a UA value that can be used in some process models to arrive at a revised duty (and outlet temperatures) for the existing shells.

Some process models allow the input of the U value and surface area, or UA, for an exchanger, so the available duty and operating temperatures of the existing exchanger can be incorporated into the process model.