the standard module - ventilation systems - chemcad.no · the standard module - ventilation systems...

Chapter 3

The Standard Module - Ventilation Systems

3.1 Introduction

The PIPENET Vision Standard module is a tool for steady state flow modelling of networks of pipes

and ducts. It can model incompressible and compressible flow networks. It is widely used for modelling

ventilation systems in the nuclear and other industries. Such calculations are central to the design process

in some industries because good design of ventilation systems is essential for safety.

Two examples are considered in this section of the document.

• A simple extraction system.

• Balancing a system which has fans on the inlet and extract sides of the system.

The first of the above examples will be covered in more detail with several figures of dialog boxes. The

other examples are equally important but will show fewer dialog boxes

1

CHAPTER 3. THE STANDARD MODULE - VENTILATION SYSTEMS

3.2 Example 1: Machine shop air extraction system

3.2.1 Objectives

In this example, we will look at an extract system of the type you might get in a machine shop. The

objective of the exercise is to take a system that has been manually designed such that all the duct sizes

and fan curves are known. We wish to verify that the system would work as required.

3.2.2 Initialisation of data

The network

The network as it is drawn in PIPENET Vision is shown in figure 3.1.

Figure 3.1: Network for example 9, a machine shop air extraction system.

Property Unit

Length m

Diameter mm

Pressure in H2O gauge

Temperature ◦CVelocity m/s

Flow rate type Volumetric

Flow rate m3/s

Density kg/m3

Viscosity Pa s

Table 3.1: User-defined units for example 9.

Options

Standard Options Select the Colebrook-White formula for

the pressure model, and select proceed with calculation for

the warnings control.

Units The units used are user-defined, as shown in table 3.1.

Fluid This is input through the Init | Fluids option. We

wish to use air at 15 ◦C, and this is done by entering data into

the dialog box shown in figure 3.2(a).

Defaults Setting default values can save input time, and are

pre-entered into the dialog box shown in figure 3.2(b). These

values can be changed for individual items, and it is possible

to go back to change default values after the network has been

partially input. Any part of the network which is input after-

wards will have the new default values. Here we use a default

roughness of 0.005 mm.

2 SSL/TM/0001/01 - c©2006 Sunrise Systems Limited


(a) Fluid to be used within our ventilation system.

(b) Default values for the ventilation system.

Figure 3.2: Fluid (above) and defaults (below) dialog boxes.



Fitting name K-factor

PBEND 0.20

D-IN 3.20

D-TEE 0.90

DBEND 0.27

FANIO 2.00

GRILL 5.00

P-IN 0.95

P-TEE 0.48

SEP 20.00

BAG 3.50

HEPA 3.00

Table 3.2: Fittings to be entered for the ventilation system, using

the dialog box shown in in figure 3.3.

Pipe Type This is generally only used when

pipes are to be sized. For the purpose of this ex-

ample, we skip this section.

Libraries

Fittings This dialog box (figure 3.3) is used

when it is desired to remove certain fittings from

the library during the problem input. This would

avoid having to scroll up and down a long list of

fittings when the network is defined. The dialog

box can be accessed via Library | Fittings. The

complete set of user-defined fittings which need to

be input is shown in table 3.2.

Figure 3.3: The fittings dialog box, from which the data in table 3.2 may be entered.



Flow rate Pressure generated

(m3/s) (inch water Gauge)

1.8 24.5

2.3 22.5

2.7 20.5

3.0 18.0

3.3 16.0

Table 3.3: Fan data for example 9, which is entered into the

dialog box shown in figure 3.4.

Fan Curve The fan plays a crucial role in the

performance of the ventilation system. PIPENET

Vision fits a quadratic function to the data points

input for the fan performance curve. This is done

by a program called the ’Pump/Fan module’. In

order to invoke this program we use the command

Library | Pumps. The fan data for this problem

is shown in table 3.3, and is entered into the dialog

box shown in figure 3.4. This data will be saved in

a library when ’OK’ is selected.

Figure 3.4: The pumps - coefficients unknown dialog box, from which the fan data in table 3.3 may be entered.



The Network

To input the network, choose the orthogonal grid option. Then draw the network shown in figure 3.5.

The attributes and fittings for the network items are given in table 3.4.

Figure 3.5: Network for example 9.

Pipe Diameter Length Elevation Fittings

label (mm) (m) (m)

1 53.5 12 0 P-IN

2 53.5 12 0 P-IN

3 53.5 6 0 P-TEE

4 53.5 12 0 P-IN

5 53.5 6 0 P-TEE

6 53.5 12 0 P-IN

7 53.5 6 0 P-TEE

8 360 2 0 0

9 53.5 12 0 P-IN

10 380 0.5 0 0

11 53.5 12 0 P-IN

12 380 0.5 0 0

13 380 1 0 BAG

14 600 8 8 PBEND

15 380 1 0 HEPA

16 600 1.2 0 FANIO

17 600 1.2 0 FANIO

(a) Data for circular ducts (pipes) within the network.

Duct Height Width Length Elevation Fittings

Label (mm) (mm) (m) (m)

18 380 150 1 0 D-TEE,

DBENDx2

19 300 300 0.05 0 GRILL

20 380 150 1.3 0 D-TEE

21 300 300 0.05 0 GRILL

22 380 150 0.6 0 0

23 300 300 0.05 0 GRILL

24 380 150 4.1 0 D-TEE x 2,

DBEND x 2

(b) Data for rectangular ducts within the network.

Table 3.4: Data used in the network.



Note the following:

1. Items 1 to 17 are circular ducts and are input as pipes.

2. Items 18 to 24 are rectangular ducts and are input as ducts.

In PIPENET Vision, ducts and pipes are different items and should be input by using different

items from the palette.

Circular ducts

The attributes for a pipe are input by pointing the cursor at the pipe, and data can be entered in the

properties window. The property window can be accessed by going to View | Properties. An example of

a dialog box which has been completed is shown in figure 3.6.

Figure 3.6: Properties for pipe labelled 1.

This data can be copied and pasted on to pipes 2, 4, 6, 9 and 11. This is done by first pointing the cursor

to the source, right clicking on it, clicking on ’copy’, then pointing the cursor to the target, right clicking

on it and clicking on ’paste’.



Rectangular Ducts

Figure 3.7 shows the properties window for a duct (in this case, duct labelled 18). Note that the

properties window for a duct is slightly different to that of a pipe (figure 3.6), where the width and height

of the duct are specified, as opposed to the pipe radius.

Figure 3.7: Properties for duct labelled 18.

Specifications

All the inputs and the single output are assumed to be at 0 in H2O G. They can be input by giving the

value for one input node, then copying and pasting it on to the others.

The specification for node 1 is shown in figure 3.8(a).

Fan Characteristics

The only other item we need to input is the fan type. This has already been set up and stored in the

library. It is simply a matter of selecting the fan from the library. The properties window for our fan is

shown in figure 3.8(b).



(a) Properties window for node 1. (b) Properties window for the fan.

Figure 3.8:

3.2.3 Calculation and results

Figure 3.9: Output options.

The data for the problem has now been input completely and

so we can proceed to perform a calculation. It is advisable to save

all data and perform a check before a calculation is performed.

A calculation is performed by using the command Calculation |Calculate.

The results can also be seen through the browser or Word. If it

is examined in Word, all the facilities of Word would be available,

including cut and paste etc. The above options can be reached by

the command Calc | Browser, which leads to the dialog box shown

in figure 3.9.

Results can be directly displayed in the schematic or in the Prop-

erties window. For a detailed excel format output, use the Data

window. There is a facility to copy the data from the Data window

to an excel spreadsheet (copy/paste command). The Data Window

option is ideal for fine tuning the design by looking at the results,

making changes to the system and calculating again. Results for

the pipes and the ducts are shown in figures 3.10 and 3.11.



Figure 3.10: Results for pipes.

Figure 3.11: Results for ducts.



Modelling a Leak

Once the initial network has been input, it is easy to perform many calculations to study the various

types of failure that can occur in the system. One of these calculations might be to predict what would

happen if there was air in-leak due to a perforation.

Let us suppose there was a small perforation 20 mm in diameter, exactly half way along duct 20 (see

figure 3.12). Let us also suppose that the wall thickness of the duct material is 2 mm. PIPENET Vision

can be used to model this by using the following steps:

1. Create an additional node half way along duct 20.

2. Attach a pipe 20 mm diameter, 2 mm (0.002 m) long to this node.

3. Set the pressure at the free end of the new pipe to 0 in. H2O G.

Now perform a calculation. It can be seen that this only makes a minor difference to the extract flow

rates.

Figure 3.12: Network, now with a new pipe to represent a leak within the system.

Obviously the perforation size can be changed and more calculations performed.



3.3 Example 2: Balancing a Ventilation System

3.3.1 Objectives

The objectives of this example are:

• Select a suitable fan to drive the system.

• To find ways of ensuring that the pressure remains negative within the compartments. We must also

ensure that the direction of flow is from less contaminated areas to more contaminated areas.

• We also consider what happens if there are doors in between the compartments, which could poten-

tially leak.

• Finally, we consider the case in which one or more doors are left open.

3.3.2 Initialisation of data

The Network

An overall arrangement of the system is shown in figure 3.13. The compartments are divided into two

Figure 3.13: Network arrangement for example 10.

because we will introduce interconnecting doors between the nodes in other simulations later.



Pipe Width Height Length Elevation Roughness

Label (m) (m) (m) (m) (m)

1 1.5 1.5 30 0 4.57×10−5

2 1 1 30 0 4.57×10−5

3 10 10 30 0 4.57×10−5

4 10 10 30 0 4.57×10−5

5 1 1 20 0 4.57×10−5

6 1 1 6 0 4.57×10−5

7 10 10 30 0 4.57×10−5

8 10 10 30 0 4.57×10−5

9 1 1 6 0 4.57×10−5

10 1 1 20 0 4.57×10−5

11 10 10 30 0 4.57×10−5

12 10 10 30 0 4.57×10−5

13 1 1 30 0 4.57×10−5

14 1.5 1.5 30 0 4.57×10−5

Table 3.5: Duct attributes for example 10.

Ancillary Data

The medium is air at 20◦C (use the

ideal gas approximation).

The units to be used are Pa (G), m3/s,

m (diameter), m (length). The other units

can be chosen as desired.

Duct attribures

For the sake of simplicity, no fittings

are considered. The network data also

has a high degree of symmetry so that

the copy and paste of attribute data can

be used to the full. The data for the ducts

is shown in table 3.5.

Fan Selection

In order to select the fans, the first calculation is done without the fans and then the fans will be input.

Assume that there are fans at the input and the output. The pressure that needs to be generated at the input

is 20 Pa and, at the output, -20 Pa is needed. The pressure specifications are shown in figures 3.14(a) and

3.14(b).

(a) Specifications for node 1. (b) Specifications for node 13.

Figure 3.14: Input and output nodes for example 10. The fan curves can be calculated by doing a calculation with these initial

pressure specifications.

PIPENET Vision calculations show that the fans need to generate a flow rate of around 22.3 m3/s.

Therefore, select a fan with the characteristics shown in table 3.6, bearing in mind that the fans need to

be slightly bigger than the minimum required performance. The dialog box for the fan curve is shown in

figure 3.15.



Flow rate Pressure

(m3/s) (Pa)

20 22

25 20

30 17

Table 3.6: Fan data for generating the fan curve.

Figure 3.15: Generated fan curve, using the data from table 3.6.



Modelling the System with Fans

In order to achieve this, we remove the old specifications (on nodes 1 and 13), add the fans, select the

fans from the library (by using the properties dialog box) and set the input and output nodes to 0 Pa. The

new input and output nodes are 14 and 15 respectively.

The calculation yields the results shown in figure 3.16.

Figure 3.16: Results for the initial calculation, including fans.

Balancing the System

We note that the flow rate in the middle compartment is a little high and the pressure in the third

compartment is positive. (Note that ducts 10 and 13 are slightly shorter than ducts 2 and 5). Our objective

now is to find ways of rectifying this. In other words, we wish to reduce the flow rate through the second

compartment and reduce the pressure in the third compartment.

A suggested solution is to place dampers which are set to drop 6 Pa at 6 m3/s. The dialog box in figure

3.17 shows how to do this. The dampers are placed on ducts 6 and 10. It can be seen that the pressure is

negative in the sensitive parts of the system and that the flow is more balanced. So, the dampers can be

left at the above setting. The results of inlcuding these dampers are shown in figure 3.18.



Figure 3.17: Fittings dialog box, from where the parameters for the dampers can be set.

Figure 3.18: Results for the network including the fittings on ducts 6 and 10.



Figure 3.19: Properties window for one of the 2 ducts used to

model a leaky door.

Modelling a Leaky Door

It is of interest to evaluate the effect of a leaky

door between the compartments. We assume that

the door is 2m x 2m in size and has a gap of 2mm

around three sides. The door is 50mm thick. We

can model this by placing a duct of 6m x 0.002m

and 0.05m long between the compartments. The

properties window for such a duct is shown in fig-

ure 3.19. In this particular example, the two ducts

used to model the leaky door are labelled 15 and

16.

The direction of the arrows represents the direc-

tion in which the ducts were input. A positive flow

means it is in the direction of the duct, and a neg-

ative flow means it is opposite to the direction of

the duct. Whether we accept this or not depends

on which of the compartments are more contami-

nated. If the direction of flow is not acceptable then

the dampers may have to be reset. With a relatively

small flow such as this, what is of most concern is

the direction of flow.

The results are shown in figure 3.20.

Figure 3.20: Results with the inclusion of a leaky door.



Figure 3.21: Properties window for one of the 2 ducts used to

model an open door.

Modelling Open Doors

Finally, it is of interest to see what happens if

the doors are left completely open. In order to do

this, we simply connect the compartments with 6

m x 6 m ducts of length 0.3 m (assuming that the

walls are 300 mm thick). The properties window

for such a duct is shown in figure 3.21.

We note from the results shown in figure 3.22

that the pressures equalise between the compart-

ments. However, the pressures still remain nega-

tive and are acceptable. The direction of flow be-

tween the compartments may or may not be accept-

able depending on which compartments are more

contaminated.

Figure 3.22: Results with the inclusion of an open door.


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