PV Elite: Designing a Heat Exchanger – the Pitfalls

Intergraph CAS

Ray Delaforce

We get many questions from users wanting to know why their heat exchangers refuse to produce

a report. This article does not teach you how a build a heat exchanger. Its purpose is to diagnose

the reasons why, when you have built one, errors are reported.

Build the Heat Exchanger with the first tubesheet on the left (or bottom)

Consider these two pictures:

In the left illustration, the tubesheet is on the left hand side, and in the right illustration, the

tubesheet is on the right hand side. PV Elite, by default builds the heat exchanger with the

tubesheet on the left. Shown above is a U-Tube heat exchanger; however this could represent

any type of heat exchanger.

PV Elite enables you to build the heat exchanger either way, but it is less problematic if you put

the tubesheet on the left. If you put the tubesheet on the right, you will have to juggle with the

input to get the 3D model to look right.

In the PV Elite heat exchanger dialog screen, this is where you have the choice of putting the

tubesheet on the right or the left.

Instead of a horizontal heat exchanger, you may have a vertical heat exchanger. The same

principle applies. We strongly recommend you place the tubesheet on the bottom to avoid the

problems discussed above.

The basic types on Heat Exchanger

Whether you are building a heat exchanger with two tubesheets or one (in the case of a U-Tube

or floating head type), the first tubesheet is what you enter first. There are four basic

configurations. Here they are:

A

The tubesheet is welded to both the channel side and the

shell side.

B

The tubesheet is bolted to the channel flange, and welded to

the tubesheet.

C

The tubesheet is ‘sandwiched’ between two flanges,

gasketed both sides.

D

The tubesheet is bolted to the shell flange and welded to the

channel. This is only available for a U-Tube or floating

head exchanger.

We shall deal with these tubesheet configurations one by one. A very important point is to

determine which are the tube side (or channel side) components, and which are the shell side

components. This is the most common pitfall where users make mistakes.

Give all your exchanger components meaningful names

One of the most common mistakes made by users when they send a file for us to diagnose is that

they do not give the components of the heat exchanger meaningful descriptions. This field in PV

Elite is put there for a purpose.

Although naming components is not necessary for PV Elite to function properly, not doing so is

a bad practice. This field is put there as an aid to the user.

Naming the components is a very useful practice, especially in the case of heat exchangers

because it is very important that PV Elite ‘knows’ which components are on the tube side, and

which components are on the shell side.

We tell PV Elite that we are designing a heat exchanger here, but PV Elite has no idea what are

shell side components and what are tube side components. So, this is the starting point of heat

exchanger design:

Assigning the correct component to the Shell side of the exchanger

Look carefully at this illustration:

1. Every pressure envelope component is given a description

2. We know immediately what elements are on the shell side, and what elements are on the

tube side.

For what follows, you must know about Node Numbers. If you do not understand Node

Numbers, you can find information on the subject in the Knowledge Base. Go there if you are

not familiar with this subject, and then come back to this article.

Look again at how PV Elite assigned the Node Numbers:

Now look at the Description for Node 10-20:

We now have the descriptions and node (The From Node and the To Node) numbers all

packaged together.

Assigning the Shell Side Components

Just by taking a little care in building our model we can now easily discern the shell side

components, and check the appropriate boxes here:

Instead of just seeing Node number, we now have descriptions to work with.

A Common Mistake

Suppose you were to have this situation:

Here, the tubesheet exists in one component – the main shell. Part of that shell is on the tube

side, and part of that shell is on the shell side. PV Elite cannot take one component and split it

into a shell side and the tube side. The correct way to build this model is to have a separate

channel, and separate shell like this:

Now, PV Elite can see a tube side component, and a shell side component.

Attaching the Tubesheet to an Exchanger Component

The most mistakes in building the exchanger occur when it is time to attach the tubesheet to

something. As a general rule, it is better to attach the tubesheet to a channel component. This is

important when tubesheets are attached to flanges. However, we shall treat the tubesheet types

one at a time.

Tubesheet integral with the shell and the channel

We are going to attach this to the Channel. There are two important

points concerned with the channel:

1. The channel ‘From Node’ is 30 (important)

2. The channel is 20 inches long (also important)

We have to convey this information to PV Elite. Note how we do this:

This simple method will ensure PV Elite knows exactly where to place

the tubesheet, and your 3D model will look right. Also we need to tell

PV Elite the configuration of this tubesheet:

Tubesheet Welded to the shell Bolted to the Channel Flange

A very important decision has to be made here. The tubesheet

MUST be attached to the channel flange for two reasons:

1. The tubesheet has to be the dividing point between the shell

side and the tube side, and,

2. The bolting load (force) has to be transferred from the

flange to the tubesheet.

Also note these important points:

1. The flange From Node is Node 40 – the connection point

2. The flange length is 7.19 inches.

This is how we convey this information to PV Elite:

We must also tell PV Elite that the tubesheet is extended as a

flange, and that the bolt load (force) is transferred to the tubesheet:

PV Elite now knows all the important points about the placement

of the tubesheet.

We must also tell PV Elite what the configuration is:

Tubesheet situated between two flanges (gasketed both sides)

If you look carefully at the way the tubesheet is ‘sandwiched’ between the two flanges,

those gaps between the flange and the tubesheet tell us the bolt load is not transferred

from either flange to the tubesheet. The bolt load is only transferred from one flange to

the other flange.

The channel is item 10-20 and the shell is item 40-50. The usual

convention in the case of the Tubesheet sandwiched between two flanges

is to attach the tubesheet to the channel flange, item 20 (Nodes 20-30).

1. The channel flange is ‘From Node’ is 20

2. The flange is 7.19 inches long.

This how we convey this information to PV Elite:

We must tell PV Elite the tubesheet is extended as a flange, but, the bolt

load is not transferred to the tubesheet:

And:

Tubesheet Welded to the channel, Bolted to the Shell Flange

The configuration only applies to U-Tube and Floating Head Exchangers.

Notice, the tubesheet is located on the left side of the flange. The

channel item is node 20-30 and the shell is node 40-50. However, we

are attaching the tubesheet to the Flange, item 30-40.

1. The tubesheet is attached to the flange whose ‘From Node’ is 30

2. The bolt load is transferred to the flange.

We first let PV Elite know the configuration of this exchanger:

Also, the bolt load is transferred to the tubesheet. We need to let PV

Elite know this:

Notice the strange distance from the ‘From Node’:

Whenever you have a tubesheet attached to the left of any item, you will

have to play around with this length until the 3D model looks right.