Design of a heat exchanger using HTRI.docx

Design of a heat exchanger using HTRI Mon, 06/11/2012 - 09:14 — webmaster

Body: I am writing this Blog to help modelling Heat Exchanger in HTRI step by step. We can design general shell & tube type heat exchanger, Air cooler, PHE’s, Jackated Pipe exchanger, Hairpin type exchanger, Spiral plate type exchanger, Economiser & Fire heaters by using HTRI but I want to keep this blog restricted to Shell & tube type exchanger only 1.1.

Process Parameters

Input flow rate, temperature & vapour fraction at inlet / outlet conditions and the allowable pressure drop for shell & tube side. For liquids, vapor fraction is “0”; for gas it is “1” and for two phase it is between 0 & 1. 1.2.

Geometry

TEMA type As given in Process data sheet (if not mentioned, then it shall be decided based on type of fluid, condition etc.) Orientation Orientation may be horizontal, vertical or inclined with an angle between 0 and 90 deg. Hot fluid side Hot fluid shall be either on the shell side or on the tube side Tube type Plain or finned (for shell & tube generally plain tubes are used) Tube length In design mode, enter the length & design the exchanger for various shell ID’s. Standard tube lengths available in FPS units are 4, 6, 8, 10, 12, 14, 16, 18, 20, 22 and 24 feet. In MKS unit, tube lengths can be used in steps of 500 mm. Sometimes, it may be required to use FPS standard tube lengths in MKS unit (for e.g. 6096 mm, 3048 mm, etc.) Note1: Sometimes odd tube length would be acceptable for reboiler and small exchangers. Note2: Design of exchanger will be different if selection of grid for tube length is different. If we decide only 2,4 & 6 Meter tube to be used then we have limitation of tube length. Some time we may land up with design lesser DP utilization because of tube length constrain. Now a day’s tubes in any length are available, after discussion with lead engineer we can select odd tube length like 3500 or 3600 mm. At the end we should select tube length such that there will be fewer inventories of tubes in store. Note3: Some time for high pressure exchanger (i.e. D type or C type exchanger for which design pressure is more than 70 to 80 Kg/cm2) calculated tube sheet thickness by HTRI is on lower side than calculated thickness mechanically because some time tube sheet is to be designed full design pressure but HTRI is consider for differential design pressure. For such exchanger we should add additional tube sheet thickness in geometry panel of HTRI during design and we should specify tube length excluding tube sheet thickness on process data sheet. Also we should mention only effective heat transfer area and clearly explain all these points in notes. Effective Tube length This is the value used for heat transfer and need not be entered. HTRI calculates tube sheet thickness and also the effective tube length. Effective tube length = Total tube length – Tube sheet thickness – Tube projection Surface Area Gross Total installed area (Number of tubes x total tube length x p x tube OD) Surface Area Effective Total installed area. (Number of tubes x effective tube length x p x tube OD) Shell ID In design mode this input is not required. HTRI calculates. If an exchanger has to be designed for a fixed shell ID then input the ID and vary tube length by using grid design option. (Refer grid design option 4.2.6) Tube OD Generally 19.05, 25.4, 31.75, 38.1, 50.1 mm Note: If tube OD is specified in mm then tube thickness should be in mm i.e. 1.5 mm,2 mm 2.5 mm, 3.0mm (as per project design basis) And if tube OD is specified in inches then tube thickness will be in BWG standard i.e. 12,14,16,18,BWG etc. We can convert BWG thickness in to equivalent mm. Never specify tube OD in mm & thickness in BWG or tube OD in inches and thickness in mm. Tube Pitch Generally 1.25 times tube OD. Other values can also be specified. Note: Some time to elimate vibration pitch can be more than 1.25 times tube OD. Tube layout Refer Kern Tube passes Not required for design case. (For floating head and U-tube, even number of passes shall be entered, viz. 2, 4, 6, 8 etc.) Note : Some time for floating head exchanger single pass can be accepted. Tube count Not required for design case. Tube material Tube MOC shall be selected from HTRI data bank. Note: For Refinery MOC of baffles, tube supports, tie-rods, spacers: ( same as tube side MOC) but for inorganic chemical plant we can use MOC of baffles tube support, tie-rods spacers same as shell MOC.You should discuss with your senior and finalise if there is any issue regarding MOC. Baffle type Various baffle types are Single, Double, NTIW and Rod. Baffle type need not be specified for design case. Baffle cut Baffle cut is specified with respect to shell inlet nozzle axis and can be either vertical or horizontal (if the baffle cut is perpendicular to the nozzle axis, then the cut is horizontal and if the cut is

parallel to the nozzle axis, then it is vertical). Baffle cut need not be specified for design case. Baffle spacing Not required for design case. (However, the inlet, outlet and central baffle spacing can be varied) No of baffles = (Total no of cross passes – 1) Select no of cross passes such that we will get shell side nozzle at apposite to each other. (Some time process or layout require nozzle at same side.) Parallel pass lane Not required for design case (Refer HTRI help) Need to specify width in order to achieve 2D bend radius or inline cleaning arrangement for square or rotated square pitch. Perpendicular pass lane Not required for design case (Refer HTRI help) Need to specify width in order to achieve 2D bend radius or inline cleaning arrangement for square or rotated square pitch. Sealing strips Not required for design case (Refer HTRI help) Sealing strip reduces leakage in bundle and shell, which increases the cross flow fraction. Shell side inlet / outlet nozzle Standard i.e. ANSI or JIS standards, schedule i.e. Std.Sch. ,Sch 40,Sch 80 etc., nos., ID, type, position i.e. top, bottom etc.

Not required for design case Location: For liquid-liquid exchangers both shell side & tube side inlet nozzle should be from bottom of shell.(If not then there should be provision in piping to ensure exchanger is always full of liquid) For gas or two phase fluid inlet is from top and vapour / non condensable outlet is from top of other side and liquid outlet at bottom. Nozzle RV2 should be less than 2232 kg/mS2.if RV2 is more than 2232 then impingement plate is to be provided for nozzle.At Shell entrance and bundle entrance TEMA limit for RV2 is 2232 kg/mS2. Tube side inlet / outlet nozzle Standard i.e. ANSI or JIS standards, schedule i.e. Std.Sch. ,Sch 40,Sch 80 etc., nos., ID, type, position i.e. top, bottom etc. Not required for design case Select nozzle location such that 1st tube pass is in counter current with respect to shell side pass. Impingement plate Not required for design case For gas and two phase inlet impingement plate is to be provided. Some time instead of plate type impingement plate 2 Rows of rod are used as protection of first few rows of tube. If shell side nozzle inlet / outlet RV² is more than the allowable limit, then HTRI will consider an impingement plate. For gas and two-phase flow, impingement plate is required. For liquid phase, it depends on the value of RV². Generally, rectangular impingement plates are used for exchanger There is some optional data, which is not required for design purpose. However, this data should be corrected at the time of rating and fine-tuning, which is given below. Total tube sheet thickness, floating head support plate, support at U-bend. 1.3.

PIPING

This detail is required only for reboiler type exchanger. 1.4.

Process

Exchanger Duty Enter actual exchanger duty

Duty Multiplier Enter the duty multiplier For e.g. for 10% over design on duty, enter 1.1

Fouling resistance Enter the value mentioned in data sheet. If this value is not available in the datasheet, then the same should be taken from published literature, reference books, like ‘Process Heat Transfer’ by D.Q. Kern. 1.5.



Physical Properties of hot & cold fluid

Physical properties input Three options are available: (a)

Mixture property via grid

(b)

Component by component

(c)

Grid & component.



Heat release curve Three options are available (a)

User specified

(b)

Dew/bubble point specified

(c)

Programme calculated

Composition units

Moles or mass



Flash type Differential (separate and not in contact) or integral (well mixed and in thermal and chemical equilibrium) 1.6.

Grid Design

Geometry a.

Shell ID

Enter the minimum and maximum shell ID and either the number of steps or the step size in mm or inch b.

Baffle spacing

Enter the minimum and maximum baffle spacing and either the number of steps or the step size in mm or inch c.

Tube passes

Enter the minimum and maximum number of passes and ‘odd’ or ‘even’ passes. d.

Tube length

Enter the minimum and maximum tube length and either the number of steps or the step size in mm or inch e.

Pitch ratio

Enter the minimum and maximum pitch ratio and either the number of steps or the step size in mm or inch f.

Tube diameter

Enter the minimum and maximum tube diameter and either the number of steps or the step size in mm or inch g.

Shell type

Select any one of: TEMA ‘E’, ‘F’, ‘G’, ‘H’, ‘J21’, ‘X’, ‘K’ type shell h.

Baffle type

Enter any one from the following options: Single Double No tubes in window (NTIW) Rod None HTRI gives various designs with different shell ID with optimum baffle spacing for given tube length and tube passes. HTRI gives shell ID in standard inch format. It has to be fine tuned to the nearest round number that is divisible by 5. This can be done by putting the programme on ‘rating’ mode. Constraints Vary hot fluid velocity, cold fluid velocity, pressure drop etc. Options Generally we are not changing these options .If required we can vary all options as given in the HTRI. i.e. Allowed over design range, Baffle spacing options, Tube pass options etc. Warnings If you required additional warnings other than HTRI’s standard warnings, then we can specify here. i.e. Warnings for hot stream or cold stream tube wall temperature, Allowed critical velocity ration, Allowed vibration frequency ratio etc. No warning should be neglected while designing the exchanger. Once all the above data listed in sections 1.2. through 1.6 is entered, the programme shall be run by clicking the ‘run’ option on the tool bar.

HTRI common warning message And solutions 1. The physical properties of the hot(cold) fluid have been extrapolated beyond the valid temperature range. check caculated values. Hot (cold) interpolation of physical properties of the fluid temperature exceeds the available temperature range, check the calculated values. You can increase the setting range of temperature or change physical properties package HTRI as VMGThermo, if the design has been set, you can ignore this issue. Because the properties of projection is the interpolation method derived software just to remind out of temperature range, little effect on the results. 2. An internal temperature cross exists in the exchanger. the program handles the reverse heat flow properly in the calculations, but you may want to consider changing the terminal process conditions to avoid the internal temperature cross. Exists within the heat exchanger temperature, software in calculating the appropriate changes in the fluid situation. But you might want to consider changing the process to avoid such problems. When the tube is 1, there is no temperature cross-cutting issues, but for similar problems in multitube heat exchanger indicate fluid when the process fluid is not suitable, try increasing the temperature difference or change the flow rate and other conditions. Not to be missed. 3. The design logic has modified the user specified value for baffle spacing (etc.) is software designed to modify the userspecified data such as distance between baffles. In the proofreading process based on the content of the REPORT for amended remedy. Not to be missed. 4. The Bstream flow fraction is very low.check the design. Points of b flow rate is too small check design. Other disclosure at this time flow is too large, you can enlarge baffle spacing increase b flow; add sealant tape b, c to increase flow reduction; adjust the arrangement of pipes f flow reduction. 5. The inlet baffle spacing is less than the recommended minimum spacing.check the design.this condition may lead to problems when the exchanger is built. Inlet baffle spacing is smaller than recommended minimum baffle spacing, re-check the design, which at the time of construction of the heat exchanger will cause a lot of problems. Try increasing baffle spacing. Not to be missed. 6. Shell exit velocity exceeds critical velocity, indicating a probability of fluide lastic instability and flow-induced vibration damage. If present, fluide lastic instability can lead to large amplitude vibration and tube damage. Shell side outlet velocity exceeds the critical speed, suggesting that flow instability and the possibility of damage caused by fluid vibration. If such a phenomenon exists, can lead to huge tubes and tube vibration

amplitude. Try changing the shell class J12 exit diameter increases or shell. Not to be missed. 7. The inlet unsupported span length (inlet baffle spacing+central baffle spacing)exceeds the TEMAmaximum unsupported span length. Non-support plate spacing length at the entrance (inlet baffle spacing, combined with the Center baffle spacing) exceeds the maximum length without support plate spacing TEMA. Try reducing the baffle spacing. Heat exchanger baffle spacing is generally isometric. Not to be missed. 8. The vapor specific heat of the hot fluid is calculalated in the area of the critical temperature.the linear interpolation of the vapor specific heat and the sensible heat duty may be inaccurate in this region.specified the vapor specific heat and/or the heat release curve for this case. Heat flow calculation of gas compared to heat at critical points. In this range the linear interpolation method is used to get the gas heats heat load may not be accurate. In this case specify the latent heat of vapor or hot fluid curves. Can be ignored. 9. A differential flash is recommended for two-tube pass intube condensation unless a U-tube bundle is used because of potential phase separation in the header. Condensation in the two tube tube recommends using the differential method. Unless you are using a u-tube. Because there may be the potential for vapor at the head part. Change calculation in the hot/cold fluid properties>Flash type type. Not to be missed.