Rating Heat Exchangers

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 for

heat exchanger design.

• Determine if an existing heat exchanger will meet the process

specifications.

Prerequisites

Before beginning this workshop you need to have completed the

previous modules.

We ignore the adjust so that

it doesn’t interfere with out

calculations.

Process Overview

First Part

Process Overview

Second Part

Rating Heat Exchangers 5

5

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.

6 Rating Heat Exchangers

6

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)= BalanceError

Q UA(LMTD)Ft Mhot(Hin Hout )

hot

– Qloss Mcold(Hout – Hin)

cold

= = – = – Qleak

Rating Heat Exchangers 7

7

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

LMTD

ΔT1 – ΔT2

Ln(ΔT1 ⁄ ΔT2)

= ------------------------------------

ΔT1 = Thot ,out – Tcold, in

ΔT2 = Thot , in – Tcold,out

8 Rating Heat Exchangers

8

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

as Weighted. 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 exchanger’s material and energy balance. Steady-

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

dynamic simulation.

Figure 2

Rating Heat Exchangers 9

9

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 transfer

coefficient U is constant the specific heats of the streams at both

exchanger sides are constant. The heat curves of both shell and

tube side are linear. The heat exchanger geometry is not taken

into account in this model.

• Weighted Model. This model is particular powerful in dealing

with non-linear heat curve problems such as phase change of

pure components in one or both heat exchanger sides. The heat

curves are divided into a number of intervals and energy balance

is performed in each interval. This model can only be used for

energy and material balance. The heat exchanger geometry is

not taken into account in this model.

• Steady State Rating Model. This model makes the same

assumptions as

...