The Standard Module - Ventilation Systems · The Standard Module - Ventilation Systems ... The...
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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
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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.
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(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.
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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.
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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.
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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.
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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’.
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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).
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(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.
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Figure 3.10: Results for pipes.
Figure 3.11: Results for ducts.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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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