Rating Heat Exchangers docx

A common physical configuration for heat exchangers is a shell and tube exchanger, where a bundle of tubes sits inside a shell.. Learning Objectives In this workshop, you will learn how

Rating Heat Exchangers 1

1 Rating Heat Exchangers

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EA1000.32.02

A heat exchanger is a vessel that transfers heat energy from one 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 the Heat Exchanger Dynamic Rating Method in HYSYS for heat exchanger design

• Determine if an existing heat exchanger will meet the process specifications

Prerequisites Before beginning this workshop, you need to:

• know how to install and converge simple Heat Exchangers

• understand the principles of Heat Exchanger design

Process Overview

Modelling Heat Exchangers

In this 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:

• 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

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

Q leak = Heat Leak

Q loss = 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

(1)

Mcold Hout Hin ( – )cold– Qleak

  – Mhot Hin Hout ( – )hot– Qloss  BalanceError=

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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

F t = LMTD correction factor

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

(2)

(3)

Ln -(∆T1 ∆T2⁄ )

=

∆T1 = T hot,out–T cold,in

∆T2 = T hot,in–T cold,out

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

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

Figure 1

Figure 2

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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

as Weighted Therefore, if condensation or vaporization is expected to occur in the exchanger, it is important that Weighted is chosen as the

model

Heat Exchanger Specifications

As with all other unit operations in HYSYS, the 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:

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

between the hot and cold stream at any point in the exchanger, i.e not necessarily at the inlet or outlet

be used to rate existing exchangers

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 impractical This may result in a Heat Exchanger that will not solve

Typical specifications for most

heat exchangers are Pressure

Drops, and one of either,

Temperature, Minimum

Approach, Duty, or UA.

Specifications are added on the Specs page of the Heat Exchanger

Property view Enough specifications must be added to ensure that the Degrees of Freedom equals 0

Heat Exchanger Performance

A summary of the Heat Exchanger’s performance can be viewed on the

Details page of the Performance tab:

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 the HYSYS case, Gas-Gas.hsc on the disk that was supplied

with this module

2 Double-click the Gas-Gas heat exchanger, and answer the following

questions

Figure 3

What is the UA value of the Gas-Gas Exchanger? _ What is the resulting minimum approach temperature if the UA is fixed at

15 000 kJ/C-h (8000 BTU/F-Hr)? _ What are the temperatures of streams Gas to Chiller and Sales

Gas? and _

Typically, heat exchangers are

solved using delta T minimum

approach and UA target

values.

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Heat Exchanger Rating

The Rating option can be chosen by selecting Dynamic Rating from the Heat Exchanger Model drop-down menu on the Parameters page on the Design tab Delete the Delta P on both the tube and shell side This is

because with this type of model the required information must be

specified elsewhere

Dynamic Rating Model

The physical design specifications of an exchanger must be supplied on

the Sizing page of the Rating tab.

1 Firstly, specify the TEMA type to match the desired conditions

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

Figure 4

The radio buttons in the Sizing Data group include:

of the information entered here is used only in dynamic simulations

exchanger

exchanger

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.

TEMA - Front End Stationary Head Types

TEMA – Shell Types

TEMA Type Description

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

TEMA Type Description

F Two Pass Shell with Longitudinal Baffle

K Kettle Type Reboiler

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TEMA - Rear End Head Types

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 Some of these parameters are

only available when the model on the parameters page is selected as

Detailed.

Overall Information

only in dynamic simulations

dynamic simulations

Shell Side Required Information

geometry

TEMA Type 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

W Externally Sealed Floating TubeSheet

• Tube Pitch The shortest centre to centre distance between 2

tubes

configurations

unobstructed by the baffle

Tube Side Required Information

inputted above

tubes, used in determined the overall heat transfer coefficient, U

of 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 the

Detailed model on the Parameters page This will allow HYSYS to

calculate the desired terms

The Rating model in HYSYS 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 are asked to find a heat exchanger that will serve as the Gas-Gas

exchanger 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 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 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 Length = 1.5 m

• Number of tubes = 300

• Tube Pitch = 30 mm

• Baffle Type = Double

• Baffle Orientation = Vertical

• Baffle Cut (% Area) = 15%

• Baffle spacing = 100 mm

• All other parameters are the HYSYS default values

Use the Dynamic Rating mode to determine if the exchanger is suitable;

on the Rating tab, Parameters page, use the Detailed Model in HYSYS

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 0.0001 °C-h-m2/kJ

What is the temperature of the Sales Gas using this exchanger? _

What will the temperature of the Sales Gas be after 6 months of service?

Will this exchanger be adequate after 6 months of service?

Save your case!

Challenge Why was the Recycle needed in this Flowsheet?

For an interesting challenge, disconnect the recycle operation and

stream 1 Connect the stream LTS Vap in place of stream 1.

What one piece of information is stopping the Exchanger from solving

? _

Apart from putting back the Recycle, how else could this be resolved