1.3.9 Rating Heat Exchanger

8/3/2019 1.3.9 Rating Heat Exchanger

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Rating Heat Exchangers 1

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Rating Heat Exchangers

2002 Hyprotech Ltd. All Rights Reserved1.3.9 Rating Heat Exchangers.pdf


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2 Rating Heat Exchangers

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Workshop

A heat exchanger is a vessel that transfers heat energy from one process

stream to another. Until now, we have not considered the physical

parameters of the heat exchangers we have modeled. In this module we

will be entering this additional information regarding our shell and tube

heat exchanger and allowing HYSYS to determine whether or not it will

suit our needs.

Learning Objectives

In this workshop you will learn how to:

Use the Heat Exchanger Dynamic Rating Method in HYSYS forheat exchanger design.

Determine if an existing heat exchanger will meet the processspecifications.

Prerequisites

Before beginning this workshop you need to have completed theprevious modules.We ignore the adjust so thatit doesnt interfere with outcalculations.


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ProcessOverview

FirstPart


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ProcessOverview

SecondP

art


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Building the Simulation

We will be modifying the heat exchanger modeled in the Heat

Integration module. Open the case saved at the end of module and

ignore the Adjust operation.

Modeling Heat Exchangers

In this workshop, we will examine a heat exchanger from the Pre-Heat

Train. Heat exchangers are modelled in HYSYS using one of three

configurations:

Shell and Tube

Cooler/Heater

Liquified Natural Gas (LNG) exchanger

The Cooler/Heater operations are single-sided unit operations where

only one process stream passes through the operation. The LNG

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 module, a shell and tube exchanger of given dimensions will be

rated to see if it will meet the requirements of the process.


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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.

where: M = Fluid mass flow rate

H = Enthalpy

Qleak= Heat Leak

Qloss= Heat Loss

The Balance Error is a Heat Exchanger 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.

The Heat 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:

where: U = Overall heat transfer coefficient

A = Surface area available for heat transfer

LMTD = Log mean temperature difference

Ft= LMTD correction factor

(1)

(2)

Mcold

Hout

Hin

( )cold

Qleak

( ) Mhot

Hin

Hout

( )hot

Qloss

( ) BalanceEr r or =

Q U A LM TD( )Ft

Mhot

H(in

Hout

)hot

Qloss

Mcold

Hout

Hin

( )cold

Qleak

===


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Log Mean Temperature Difference (LMTD)

The LMTD is calculated in terms of the temperature approaches

(terminal temperature differences) in the exchanger using the following

equation:

where:

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 on

the next page.

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.

(3)

Figure 1

LMTDT

1T

2

Ln T1

T2

( )-------------------------------------=

T1 Tho t, ou t Tcold, in=

T2 Tho t, in Tcold,ou t=


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The following curve represents a superheated vapour being cooled and

then condensed. Note that it is not linear because of the condensation

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. Break the heat loss curve into regions at point B.

2. Calculate the terminal LMTD for each region.

3. Sum all of the LMTDs to find the overall LMTD.

HYSYS will do this automatically if the Heat Exchanger model is chosen

asWeighted. Therefore, if condensation or vaporization is expected to

occur in the exchanger, it is important that Weighted is chosen as the

model.

Available Heat Exchanger Models

There are five shell and tube heat exchanger models available in HYSYS.

The End Point and Weighted models can be used for material and

energy balance for any two-sided heat exchangers. They can also be

used for shell and tube exchangers material and energy balance. Steady-

State Rating model is used for rating in steady-state mode as well as in

dynamic simulation.

Figure 2


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The basics of each model:

End Point Model. This model is based on Q = UAFt(LMTD). The

main assumptions behind this model are the overall heat transfercoefficient U is constant the specific heats of the streams at bothexchanger sides are constant. The heat curves of both shell andtube side are linear. The heat exchanger geometry is not takeninto account in this model.

Weighted Model. This model is particular powerful in dealingwith non-linear heat curve problems such as phase change ofpure components in one or both heat exchanger sides. The heatcurves are divided into a number of intervals and energy balanceis performed in each interval. This model can only be used forenergy and material balance. The heat exchanger geometry isnot taken into account in this model.

Steady State Rating Model. This model makes the sameassumptions as the End Point Model. It simply an extension of theEnd Point model which incorporates a rating calculation. Ifdetailed geometry information is provided, the exchanger can berated using this model. For linear or nearly linear heat curveproblems, this model is a good choice because it is much fasterthan the dynamic rating-detailed model.

Dynamic Basic Model. The Basic Model is based on Q =UAFt(LMTD) and makes the same assumptions as the End Pointmodel. This model was originally developed for dynamic modebut was extended for rating in steady state. This model issomewhat oversimplified in that geometry configurations are nottaken into account. Therefore, this model has limited functionality.When using this model, both pressure drops and the overall UAmust be specified.

Dynamic Detailed Model. The Detailed Model divides the entireheat exchanger into a number of heat zones. In each heat zonethere is a shell hold-up and one or more tube hold-ups, accordingto the number of tube passes per shell pass. It is a goodcounterpart to the Weighted Model. The Dynamic Detailed Modelis used both in steady state and in dynamic operation and isdesigned to solve any linear and non-linear heat curve problems.

Heat Exchanger Rating

Providing detailed heat exchanger to a HYSYS heat exchanger with the

Shell and Tube inlet streams fully defined, HYSYS can calculate the

conditions of the outlet streams. HYSYS iterates on the outlet

temperatures until the heat balance is satisfied. Pressure drops are

determined from the geometry.


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We need to break the Kerosene_PA return connection to the Heat

Exchanger.

1. Break the connection of the Kerosene_PAReturn stream to the HeatExchanger.

2. Go to the Property View of the Heat Exchanger and complete theOutletShell with a stream called Kerosene Out.

3. Go to the Parameters tab. See that the pressure drop from the shellside has been deleted. Remove the one from the tubes because,

rating a Heat Exchanger, HYSYS, uses its own correlation to calculatethis value.

4. The Rating option can be chosen by selectingDynamic Rating fromthe Heat Exchanger Model drop-down menu on the Parameterspage on the Design tab. Note that once this model is chosen, allinformation on this page disappears. This is because with this typeof model the required information must be specified elsewhere.

Some of the physical design specifications of an exchanger must be

supplied on the Sizing page of the Rating tab.

When using the Rating mode the Duty can not be fixed, that means thatthe Streams entering the Heat Exchanger can not be fixed by theColumn, as they are when exporting the Pump Around.

Figure 3


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The Rating tab has the following aspect.

The radio button selection in the Sizing Data 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 include;

Overall. Required information about the entire exchanger. Most

of the information entered here is used only in dynamicsimulations.

Shell. Required information concerning the shell side of theexchanger. All variables must be specified.

Tube. Required information concerning the tube side of theexchanger. All variables must be specified.

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. 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.

Figure 4


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TEMA - Front End Stationary Head Types

TEMA - Shell Types

TEMA - Rear End Head Types

TEMA Types Description

A Channel and Removable Cover

B Bonnet (Integral Cover)

C Channel Integral with TubeSheet and RemovableCover (removable tube bundle only)

N Channel Integral with TubeSheet and RemovableCover

D Special High Pressure Closure

TEMA Description

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 Types Description

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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Rating Parameters

Brief explanations are provided below for each Simple Rating parameter.

The parameters are categorized according to the radio buttons in the

Sizing Data group box. Most of these parameters are only available when

the mode is chosen as Detailed as opposed to Basic.

Overall Information:

Tube Volume per Shell. The volume inside the tubes, used onlyin dynamic simulations

Shell Volume per Shell. The volume inside the shell, used only

in dynamic simulations Heat Trans. Area per Shell. The total area available for heat

transfer, calculated from the specified geometry.

Elevation. The height of the base of the exchanger, used only indynamic simulations.

Tube Passes per Shell. The number of tube passes per shell.

Orientation. The orientation of the exchanger, used only indynamic simulations.

Number of Shells in Series. The number of shells in series.

Number of Shells in Parallel. The number of shells in parallel.

TEMA Type. Described earlier.

Shell Side Required Information:

Shell Diameter. Can be specified or calculated from inputtedgeometry.

Number of Tubes per Shell. The number of tubes in one shell

Tube Pitch. The shortest centre to centre distance between 2tubes

Tube Layout Angle. A choice between four differentconfigurations.

Shell Fouling. The fouling factor on the shell side.

Baffle Type. A choice of single, double, triple, or grid.

Baffle Orientation. A choice between horizontal or vertical. Baffle Cut (% Area). The percent of the cross-sectional profile

unobstructed by the baffle.

Baffle Spacing. 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 Thickness. Usually calculated from the two numbersinputted above.

Tube Length. The tube length per shell (one side for a U-tube).

Tube Fouling. The tube side fouling factor.

Tube Thermal Conductivity. The thermal conductivity of thetubes, used in determined the overall heat transfer coefficient, U.

Tube Wall Cp, and Tube Wall Density. Two physical propertiesof the tube material, used only in dynamics.

If you want HYSYS to use general correlations to determine the shell and

tube side pressure drops and heat transfer coefficients, select theDetailed model on the Parameters page. This will allow HYSYS to

calculate the desired terms.

We are going to use some of the values provided by HYSYS by defaultand change others.

5. Introduce the following data where it corresponds:

6. Go to the Parameters tab and check the Detailed radio button. Notethat the data needed for the simulation changes.

The Rating model in HYSYS uses generalized correlations for heattransfer coefficients and pressure drop. These correlations are suitablefor approximate results in most cases but may not be valid for everyexchanger. For more accuracy, a rigorous model may be required.Please contact your Hyprotech representative for a list of available thirdparty heat exchanger packages that are compatible with HYSYS throughOLE Extensibility.

In this cell... Enter...

Tube Passes per Shell 1

OD (mm) 25

ID (mm) 21

Tube Pitch (mm) 30

Baffle Type Double


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Make sure that both pressure drop cells are empty, as we said before,

HYSYS calculates the values.

Compare the Kerosene Out stream with the Kerosene_PA Return.

Completing the heat integration

From the previous exercise, we know that the Kerosene Out and the

Kerosene_PA Return streams have different values, this is because one is

calculated by the Heat Exchanger and the other by the Column,

respectively. To complete the heat integration and close the loop you

need to break the Pump around, since, as said before in Heat Exchange

rating, the outlet streams can not be fixed.

Once you've broken the Pumparound, the data in the Kerosene Streams

will be deleted, it's a good idea to keep the values of Kerosene_PA Return

to provide the Outlet Recycle stream with a starting point. Create a

stream called Kerosene_PA Return Copy and defined it fromKerosene_PA Return.

1. Go to the Atmosphere Column Property View.

2. Enter the Column Environment.

3. Delete the Kerosene SS Cooler. You will be asked if you want todelete all elements associated with the Pump Around. Answer NO.

You are also asked if you wish to delete the Kerosene Pa_Rate(Pa)since it is not required any more. AnswerYES.

4. Return to the Main Property View. AnswerYES to deleting theKerosene PA_Q Cooler.

5. You need to change the specifications for running the column later.Move to the Monitor page, and click in the Kerosene_PADuty(Pa)check box to deactivate it.

6. Add a Kerosene PA_Draw Flowspec of300 m3/h (45,000 barrels/day)

What is the new temperature of stream To Desalter? ______________________

Kerosene Out Kerosene_PA Return

Temperature

Pressure


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7. Add a Recycle unit and connect the Kerosene Out as the Inlet streamand the Kerosene_PA Return as the Outlet.

8. In the Numerical page of the Parameters tab, change the MaximumIterations to 20.

9. You need to provide the Outlet Recycle stream with an initial guess,so that all the streams entering the Column are defined, define itfrom the Kerosene_PA Return Copy you previously created.

10. Run the Atmosphere column.

Exercise 1In the previous module, we learned where to place Recycles. It is also

important to minimize the number of recycles used in the flowsheet.

Look at your simulation and decide where to place a single recycle to

converge the case.

What is the Total NL-Solver Iterations? _________________________________This data is shown in the window in the right down corner.

How many iterations did you need? ___________________________________

Tip: A helpful way of doingthis is to place the Recyclementally in differentlocations and imagine theHYSYS calculationsequence.


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Exercise 2

You are asked to find a heat exchanger to replace the existing one.

However, since you are on a very restricted budget, you can only

consider used equipment. A heat exchanger has been found in a nearby

plant. If the critical process parameter is to maintain the To Desalter

temperature of at least 85C (185F), can this heat exchanger be used?

The TEMA definition of this exchanger is A.E.L. The dimensions are

given here:

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 0.0001C-h-m2/kJ.

Tube length (m) 5

Number of Tubes 150

Baffle Type single

All other parameters are the HYSYS default values

What is the temperature of To Desalter using this exchanger? _____________

What will the temperature of To Desalter be after 6 months of service? ______

Will this exchanger be adequate after 6 months of service? _______________


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1.3.9 Rating Heat Exchanger