Rating Heat Exchangers
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Rating Heat Exchangers
© 1999 AEA Technology plc - All Rights Reserved ADV 5_1.pdf
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Rating Heat Exchangers
Workshop A heat exchanger exchanger is a vessel that that transfers heat heat energy from from one process process stream to another. A common physical configuration for heat exchangers is a shell and tube exchanger, where a bundle of tubes sits inside a shell. There is no mixing of fluid between the shell and the tubes.
Learning Objectives In this workshop, you will learn how to:
• Use th the e Heat Exchang Exchanger er Simple Simple Rating Rating Method Method in in HYSYS HYSYS for heat exchanger design • Determine Determine ifif an existing existing heat heat exchanger exchanger will will meet meet the process process specifications
Prerequisites Before beginning this workshop, you need to know how to:
• Install Install and converge converge simple simple He Heat at Exchangers Exchangers • Understand Understand the p princi rinciples ples of Heat Heat Exchanger Exchanger design design
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Process Overview
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Rating Heat Exchangers
Modelling Heat Exchangers In this workshop, workshop, we will examine a gas to gas heat exchanger from a Refrigerated Gas Plant. Heat exchangers are modelled in HYSYS using one of three configurations:
• Shel Shelll and and Tube ube • Cool Cooler er//Heat Heater er • Liquif Liquified ied N Natu atural ral Gas Gas (LNG) (LNG) excha exchange ngerr The Cooler/Heater operations Cooler/Heater operations are single-sided unit operations where only one process stream passes through the operation. The LNG Exchanger allows Exchanger allows for multiple (more than two) process streams. A shell and tube heat exchanger is a two-sided unit operation that permits two
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Rating Heat Exchangers
Modelling Heat Exchangers In this workshop, workshop, we will examine a gas to gas heat exchanger from a Refrigerated Gas Plant. Heat exchangers are modelled in HYSYS using one of three configurations:
• Shel Shelll and and Tube ube • Cool Cooler er//Heat Heater er • Liquif Liquified ied N Natu atural ral Gas Gas (LNG) (LNG) excha exchange ngerr The Cooler/Heater operations Cooler/Heater operations are single-sided unit operations where only one process stream passes through the operation. The LNG Exchanger allows Exchanger allows for multiple (more than two) process streams. A shell and tube heat exchanger is a two-sided unit operation that permits two process streams to exchange heat. In this model, a shell and tube exchanger of given dimensions will be rated to see if i t will meet the requirements of the process.
Heat Exchanger Calculations The calculations performed by the Heat Exchanger are based on energy balances for the hot and cold fluids. The following general relation defines the heat balance of an exchanger. (Mcold(Hout-Hin)cold-Qleak )-(Mhot(Hin-Hout)hot-Qloss)=Balance )=Balance Error where: M H
= Fluid mass flow rate = Enthalpy
Qleak = Heat Leak Qloss = Heat Loss The Balance Error is a Heat Exchanger Specification Specification which, for most applications, will equal zero. The subscripts "hot" and "cold" designate the hot and cold fluids, while "in" and "out" refer to the inlet and outlet.
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Rating Heat Exchangers
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The Heat Exchanger Exchanger duty may also be defined in terms of the overall heat transfer coefficient, the area available for heat exchange and the log mean temperature difference: Q = UA(LMTD)Ft = Mhot(Hin-Hout)hot-Qloss = Mcold(Hout-Hin)cold-Qleak where: U A
= Overall heat transfer coefficient = Surface area available for heat transfer transfer
LMTD = Log mean temperature difference difference Ft
= LMTD correction factor
Log Mean Temperature Difference (LMTD) The LMTD is calculated in terms of the temperature approaches approaches (terminal temperature differences) in the exchanger using the following equation: ∆ T 1 – ∆ T 2 LMT D = ----------------------------------ln ( ∆ T 1 ⁄ ∆ T 2 )
where: ∆ T 1
= T ho t , ou t – T c o l d , in
∆ T 2
= T ho t , in – T c o l d , ou t
The LMTD can be either terminal or weighted. This means that it can be calculate over the exchanger as a whole (terminal) or over sections of the exchanger (weighted). The need for this type of calculation is shown below. The following plot is a heat loss curve for a single phase stream. It compares the temperatures of the process streams with the heat flow over the entire length of the exchanger. For single phase streams, these plots are linear.
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Rating Heat Exchangers
The following curve represents a superheated vapour being cooled and then condensed. Note that it is not linear because of the c ondensation that takes places inside the exchanger.
If the LMTD is calculated using the hot fluid temperatures at points A and C, the result would be incorrect because the heat transfer is not constant over the length of the exchanger. To calculate the weighted LMTD: 1. 2.
Brea Break k the the heat heat los losss curv curve e into into reg regio ions ns at at point point B. Calc Calcul ulat ate e the the term termina inall LMTD LMTD for for each each reg regio ion. n.
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Sum Sum all all of of the the LMTD LMTDss to find find the the ove overa rall ll LMT LMTD D.
HYSYS will do this automatically if the Heat Exchanger model is chosen as We as Weighted ighted.. Therefore, if condensation or vaporization is expected to occur in the exchanger, exchanger, it is important that Weig that Weighted hted is is chosen as the model.
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Rating Heat Exchangers
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Heat Exchanger Specifications As with all other unit unit operations in HYSYS, the Heat Heat Exchanger is assumed to adequately meet the process requirements. There are several choices for specifications for the heat exchanger. The choices are given here: Typical specifications for most heat exchangers are Pressure Drops, and one of either, Temperature, Minimum Approach, Duty ,or UA. UA.
• Temperature Temperature - The temperature of any stream attached to the Heat Exchanger. The hot or cold inlet equilibrium temperature may also be defined. The temperature difference between the inlet and outlet between any two streams attached to the Heat Exchanger can also be specified. • Minimum Approach Approach - The minimum temperature temperature difference between the hot and cold stream at any point in the exchanger, i.e. not necessarily at the inlet or outlet. • UA - The overall UA can also be specified. This specification can be used to rate existing exchangers. • LMTD - The overall log mean temperature difference. • Pres Pressu surre Dro Drop ps - The pressure drops on both the shell and tube sides on the exchanger are important specifications that should not be ignored. If the pressure drops are not known HYSYS may be able to estimate them. Care must be taken when choosing specifications because it is possible to select specifications that are either infeasible or i mpractical. This may result in a Heat Exchanger that will not solve. Specifications are added on the Specs page Specs page of the Heat Exchanger Property view. view. Enough specifications must be added to ensure that the Degrees of Freedom equals 0.
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Rating Heat Exchangers
Heat Exchanger Performance A summary of the Heat Exchanger’s Exchanger’s performance can be viewed on the page of the Details Performance tab: Performance tab:
Typically, Typically, heat exchangers are solved using delta T minimum approach and UA target values.
Heat exchangers are sometimes compared on the basis of UA values, i.e., for a fixed surface area, what is the amount of heat (duty) that can be exchanged? 1.
Open pen the HYSYS case, A:\Gas-Gas.hsc A:\Gas-Gas.hsc on on the disk that was supplied with this module.
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Double-click the Gas-Gas heat Gas-Gas heat exchanger, and answer the following questions. What is the UA value of the G as-Gas Exchanger?_________ What is the resulting minimum approach temperature if the UA is fixed at 15 000 kJ/C-hr (8000 BTU/F-Hr)? __________ What are the temperatures of streams Gas to Chiller and Sales Gas?__________
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Heat Exchanger Rating The Simple Rating option Rating option can be chosen by selecting Simple Rating from the Heat Exchanger Model drop down menu on the Parameters page on the Design tab. Design tab. Note that once this model is chosen, all information on this page disappears. This is because with this type of model the required information must be specified elsewhere.
Simple Rating Model The some of the physical design specifications of an exchanger must be supplied on the Sizing page of the Rating tab. Rating tab. 1.
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Firstly, sel sele ect the Mode as Mode as Detailed or Detailed or Basic. Basic. The type of information that the rating routine requires depends on whether Basic or Detailed is chosen on this page. This should be the first step every time. Next Next,, specif specify y the the TEMA TEMA type type to matc match h the the desire desired d condi conditio tions. ns.
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Rating Heat Exchangers
For the Simple Rating model, Rating model, the radio button selection in the Sizing Data group group will dictate the type of information shown at any given moment. Each parameter will be defined later on in this module. The radio buttons in the Sizing Data group group include;
• Overall Overall - required information about the entire exchanger. Most of the information entered here is used only in dynamic simulations. Shell - required information concerning the shell side of the • Shell exchanger. All variables must be specified. • Tube Tube - required information concerning the tube side of the exchanger. All variables must be specified.
Any information information inputted into the Optional/Calculated group will be used in the calculations instead of HYSYS calculated values.
The TEMA Type is selected as part of the Overall sizing data. There are three drop down lists which allow you to specify the geometry of the front end stationary head type, the shell type and the rear end head type for the exchanger. exchanger. The following tables provide brief descriptions for each designated TEMA Type letter. Drawings of the various TEMA types can be found on page 11-4 of Perry’s Chemical Engineers Handbook, Sixth Edition.
TEMA - Front End Stationary Head Types
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TEMA Type
Descr iption
A
Channel and Removable Cover
B
Bonnet (Integral Cover)
C
Channel Integral with TubeSheet and Removable Cover (removable tube bundle only)
N
Channel Integral with TubeSheet and Removable Cover
D
Special High Pressure Closure
Rating Heat Exchangers
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TEMA – Shell Types TEMA Type
Descr iption
E
One Pass Shell
F
Two Pass Shell with Longitudinal Baffle
G
Split Flow
H
Double Split Flow
J
Divided Flow
K
Kettle Type Reboiler
X
Cross Flow
TEMA - Rear End Head Types TEMA Type
Descr iption
L
Fixed TubeSheet like ‘A’ Stationary Head
M
Fixed TubeSheet like ‘B’ Stationary Head
N
Fixed TubeSheet like ‘N’ Stationary Head
P
Outside Packed Floating Head
S
Floating Head with Backing Device
T
Pull Through Floating Head
U
U-Tube Bundle
W
Externally Sealed Floating TubeSheet
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Simple Rating Parameters Brief explanations are provided below for each Simple Rating parameter. parameter. The parameters are categorized according to the radio buttons in the Sizing Data group group box. Most of these parameters are only available when the mode is chosen as Detailed as Detailed as opposed to Basic. Basic.
Overall Information: • Tube ube Vol Volum ume e - the volume inside the tubes, used only in dynamic simulations. • Shel Shelll Vol Volum ume e - the volume inside the shell, used only in dynamic simulations. • Heat Heat Trans rans.. Are Area a - the total area available for heat transfer, calculated from the specified geometry. • Elevation - the height of the base of the exchanger, exchanger, used only in dynamic simulations. • Numb Number er of of Tub Tube eP Pas asse ses s - the number of tube passes per shell. Usually equal to 2*n, where n is the number of shells. • Orien rienttatio ation n - the orientation of the exchanger, exchanger, used only in dynamic simulations. • TEMA Type - described earlier. Shell Side Required Information: • Shel Shells ls in Seri Series es - the number of shells in series. • Shel Shells ls in Para Parall llel el - the number of shells in parallel. • Shel Shelll Di Diam amet eter er - can be specified or calculated from inputted geometry. • Shel Shelll Fou Fouli ling ng - the fouling factor on the shell side. • Baff Baffle le Type ype - a choice of single, double, triple, NTIW or grid. • Baff Baffle le Orie Orient ntat atio ion n - a choice between horizontal or vertical. • Baff Baffle le Cut Cut (% (% Are Area) a) - the percent of the cross-sectional profile unobstructed by the baffle. • Baff Baffle le Spac Spacin ing g - the distance between adjacent baffles.
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Tube Side Required Information: • Tube OD - the outside diameter of the tubes. • Tube ID - the inside diameter of the tubes. • Tube ube Thic Thickn knes ess s - usually calculated from the two numbers inputted above. • Tube ube Leng Length th - the tube length per shell (one side for a U-tube). • Tubes ubes Per Per She Shell ll - provide the total number of holes per shell; HYSYS will determine the appropriate number of tubes based on the input number of tube passes. • Tube ube Pit Pitch - the shortest centre to centre distance between 2 tubes • Tube ube Lay Layou outt Angl Angle e - a choice between four different configurations. • Tube ube Fou Foulling ing - the tube side fouling factor. factor. • Tube Therma Thermall Conduc Conducti tivit vity y - the thermal conductivity of the tubes, used in determined the overall heat transfer coefficient, U. • Tube ube W Wal alll Cp, Cp, and Tube Wall Density Density - two physical properties of the tube material, used only in dynamics.
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More More Information about the exchanger can be entered on the Parameters page Parameters page of the Ratings tab. Ratings tab. If your are entering actual size information about the exchanger and want HYSYS to calculate heat heat transfer coefficients and pressure pressure drops, use the drop-down bar to specify the Heat Transfer Coefficient Calculation as Calculation as Shell & Tube, Tube, also specify both the Shell Pressure Drop Calculator and Calculator and Tube Pressure Drop Calculator as Shell & Tube DP Calc. Calc. This will allow HYSYS to use the specified exchanger geometry and correlations to determine the shell and tube side pressure drops as well as the heat transfer transfer coefficients on both sides of of the exchanger. exchanger.
The Simple Rating model uses generalized correlations for heat transfer coefficients and pressure drop. These correlations are suitable for approximate results in most cases but may not be valid for every exchanger. For more accuracy, a rigorous model may be required. Please contact your Hyprotech representative for a list of available third party heat exchanger packages that are compatible with HYSYS through OLE Extensibility.
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Exploring with the Simulation You You are asked to find a heat exchanger that that will serve as the Gas-Gas exchanger. exchanger. However, However, since you are on a very strict budget, you can only consider used equipment. A heat exchanger has been found in the surplus supply supply of a nearby plant. If the critical process parameter is to maintain a Sales Gas temperature of at least 10 °C (50 °F), can this heat exchanger be used for the Gas-Gas service? The surplus exchanger has been thoroughly cleaned. The TEMA definition of this exchanger is A,E,L. The pressure pressure drops on both sides of the exchanger should be deleted; this will allow HYSYS to calculate these parameters. The dimensions of the exchanger are given here:
• • • • • • • •
Tube ube Len Lengt gth h = 1.5 1.5 m Numb Number er of tube tubes s = 300 300 Tube ube P Pit itch ch = 30 30 mm mm Baff Baffle le Type ype = Doub Double le Baffle Baffle Ori Orient entati ation on = Vert Vertica icall Baff Baffle le Cut Cut (% (% Area Area)) = 15 15 % Baff Baffle le spac spacin ing g = 100 100 mm mm All other other parame parameters ters are the the HYSYS HYSYS default default values values
Use the Simple Rating mode Rating mode in HYSYS to determine if the exchanger is suitable. What is the temperature of the Sales Gas using this exchanger? __________ Previous experience has shown you that after about six months in operation, the exchanger becomes fouled and the fouling factor for both shell-side and tube-side is 0.1 °C-h-m2/kJ. What will the temperature of the Sales Gas be after 6 months of service? __________ Will this exchanger be adequate after 6 months of service? __________
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Challenge Why was the Recycle needed in this Flowsheet? For an interesting challenge, remove the recycle operation and stream 1. Connect the stream LTS Vap in Vap in place of stream 1 stream 1,, and try to solve the exchanger. exchanger. You will have to add just one more specification. Which one? Well, Well, here are are two hints. hints. It is on on the Parameters page, Parameters page, and it was a value that was used before but was not needed after t he exchanger was rated.
Which specification did you add? __________ Why was this value necessary? __________
Another Good Question What is the vapour fraction of the Gas to Chiller stream? __________ Does condensation occur on the tube side of this exchanger? __________ Look at the Temperature vs Heat Flow plot on the Plots page Plots page of the Performance tab. It was mentioned earlier that condensation inside an exchanger may cause one of these lines to be bent, or non-linear. However, it can be seen on this plot that the lines here are linear. linear. Why are both lines linear when they "should" be bent? __________ The UA for this exchanger is defined from the physical parameters of the exchanger. The duty is then calculated as the product of the UA and the terminal LMTD. It is not necessary f or HYSYS to examine individual intervals within the exchanger; therefore, therefore, the lines are drawn as linear.
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