MagiCAD Ventilation Calculations

8/10/2019 MagiCAD Ventilation Calculations

1/20

Va n h a n k i r k o n k a tu 6 , 2 6 1 0 0 R a u m a , F i n l a n d , t e l . + 3 5 8 ( 0 ) 2 8 3 8 7 6 0 0 0 , f a x + 3 5 8 ( 0 ) 2 8 3 8 7 6 0 5 0 , w w w. p r o g m a n . f i

2002-08-23

1(1)

MagiCADVentilationTechnical Specification


2/20


2002-08-23

2(2)

M AGI CAD - VENTI LATI ONTechnical Specif ication

1. I ntroduction

1.1 Pur pose of this document

The purpose of this document is to provide specification of the computing methods that are usedin MagiCAD Ventilation application.

1.2 Scope of this document

This document consists of the following information about MagiCAD: duct sizing method balancing and pressure drop calculations sound calculations

1.3 Quanti ties and uni ts used in thi s docu ment

dp Pa pressure drop

v m/s velocity - single resistance coefficientA m2 aread m diameter m2/s kinematic viscosity - drag coefficient of the flow in the pipek m absolute roughness of the pipeRe - Reynolds number

2. Duct sizing

2.1 Which sizing method is the best one?

To understand the backgrounds of MagiCAD sizing principles, we will first briefly examine whatkind of results we get using different duct sizing methods.

Constant pressure dropMaximum pressure drop

In practice, these two methods are one and same, because both of these duct selection methodsare based on the friction pressure drop defined for the duct(s).Duct sizes in the system are well predictable, as there are no unexpected pressure drops orvelocities. On the other hand, the duct sizes are selected one by one, which means that theremay be more reducers than it is practical.

Constant velocityMaximum velocity


3/20


2002-08-23

3(3)

These two methods are one and same, because both of these duct selection methods are basedon the velocities defined for the duct(s).The advantages and disadvantages are the same as with the pressure drop based sizing.

Static regainStatic regain method is somehow famous, and many designers who have not been workingwith this method would first think it the best one. But in practice, this method has somefeatures that must be taken into account.The velocity drop after branch should be at least 2 - 3 m/s (3 to 5 Pa of dynamic pressure) tocompensate pressure drop in the next part. This means that the size of the main duct willremain the same from the fan to the last branch of the duct route.After all, there will be very low velocities in most of the main ducts, and that means morespace requirements and more costs. Therefore, ductwork designers do not use static regainmethod as much as it is generally thought.

Proportional air flow methodThis method is probably used by a designer who has first tried to use the static regain method.The principle is that sizing proceeds from fan to rooms, and a reducer is used when the air flowhas reduced x % from its original value (user can specify x, usually it is 30 to 60%). Thereasons for using this method may be constructional.If selection of the first duct is based on vmax or dpmax , the velocities are definitely below thelimit, and then there are no problems. But there are situations when reducers appear at wrong

places. For example: we have set x to 30%, and the air flow reduces 20% in the first branchand 1% in the next ten branches each what is the result? A very long and large main duct.The optimal solution in this case would obviously be a reducer at the first branch.

As a summary, we cannot show any single method that could be used in a calculation program toselect optimal duct sizes fast and efficiently in every possible case.

2.2 M agiCA D sizing method

The first step in MagiCAD sizing method is preliminary sizing. Duct sizes are selected one byone from the system-dependent , user-defined criteria.

User can define maximum velocities and maximum friction loss for different duct sizes.MagiCAD scans duct sizes of the series (each duct belongs to some duct size series) and takes thefirst one that is acceptable. With non-circular ducts program calculates the equivalent diameter.


4/20


2002-08-23

4(4)

Which method MagiCAD uses for preliminary sizing? It is up to user, because criteria can bedefined so that we will get maximum/constant velocity- or maximum/constant pressure drop-

based duct sizing.

The second step , which in fact takes place at the same session with preliminary sizing, checksthat there will not be unnecessary reducers. The criterion is the distance between two T-branchesin main duct direction. If the distance is shorter than one metre, the duct size is adjusted to thenearest duct in the fan side of the branch. There is also a rule that a duct can never be smaller thanthe previous duct when going towards the fan.

The third step allows user to customise sizing results. User may specify and lock size of anindividual duct. Because MagiCAD never reduces duct size when going towards fan, it is easy todefine size of main duct for the whole branch: just lock the size of the last duct.

3. Balancing

3.1. Bal ancin g pr inciples

As a default, MagiCAD balances ductwork to the minimum pressure level. As a result we get pressure drop demand for fan selection.

User can specify the minimum pressure drop for flow dampers and air devices, which makes it possible to get higher pressure level than minimum.

When there are several flow dampers in the same flow route, MagiCAD uses first the damperwhich is nearest to the fan. If all the required pressure drop is not achieved, MagiCAD adjusts thenext flow damper(s), and finally the supply/exhaust device if necessary.If some route cannot be balanced, MagiCAD shows the supply/exhaust device where the pressureis out of limits.

MagiCAD has also command, which highlights the most significant flow route.

3.2. Pressur e drop calculations

3.2.1 Suppl y/exhaust devices, flow dampers, silencer s etc.

Pressure drops and balancing are calculated using the manufacturers' product data.


5/20


2002-08-23

5(5)

3.2.2 DuctsFor friction pressure drop in ducts, MagiCAD uses Colebrook equation with the roughness valuewhich can be specified for each duct size series by the user.

The following diagram specifies pressure drops [Pa/m] for roughness value 0.15 mm, which isnormally used in steel ducts.

Circular duct

1

10

100

10 100 1000 10000 100000

qv [l/s]

d p

[ P a

]

10012516020025031540050063080010001250


6/20


2002-08-23

6(6)

[ ]

[ ]

[ ] [ ]36,0

2Re

0,6871,311,0

1Re51,2

71,32

1

2

25,0

10

Pavd

dp

d

d k

d k

Log

=

+=

+=

is iterated with the formula [1]. If the formula does not converge, the approximate formula [2] isused. = drag coefficient of the flow in the pipek = roughnessd = inner diameter of the pipe [m]Re = Reynolds number

d vd v =

=Re

= kinematic viscosity of the air (+20C) = 0,00001511 [m 2/s]v = velocity [m/s]

3.2.3 Cir cul ar bends

For 90 circular bends MagiCAD uses equation that gives values presented in the followingdiagram

Bend, 90

v [m/s]

d p

[ P a

]

1

10

100

0 5 10 15

[ ] Pavdp 8,132,0 =


7/20


2002-08-23

7(7)

v = flow velocity [m/s]

For 45 bend, the values are divided by 2.0 and for 30 bends they are divided by 3.0

3.2.4 Rectangul ar bends

For 90 rectangular bends MagiCAD uses equation that gives values presented in the followingdiagram.

Rectangular bend 90 deg

qv [l/s]

0.10

1.00

10.00

100.00

10 100 1000 10000

300x100

300x200

500x200

500x300

800x300

1000x600

[ ] Pavdp 26,0 =

= single resistance coefficientv = flow velocity [m/s]

Curved inside = 0,33...1,4With curved bend = 0,35With sharp bend = 1,5

For 45 bend, the values are divided by 2.0 and for 30 bends they are divided by 3.0


8/20


2002-08-23

8(8)

3.2.5 Reducti on

v2v1

Reducer

v1 [m/s]

d p

[ P a

]

1

10

0 5 10

v2=4m/s

v2=6m/s

v2=8m/s

v2=10m/s

v2=12m/s

( ) [ ] Pavvdp 9,112146,0 =

v2 = flow velocity at the outlet [m/s]v1 = flow velocity at the inlet [m/s]

The formulas are curve fittings, and match well Flkt's curves. Flkt has separate curves forcircular and rectangular ducts, but they have identical values.

The same equation is used both for circular and rectangular reducers.


9/20


2002-08-23

9(9)

3.2.6 Expansion

v2v1

Expansion

v1 [m/s]

1

10

100

0 5 10 15 20

v2=2 m/s

v2=4 m/s

v2=6 m/s

v2=8 m/s

v2=10 m/s

( ) [ ] Pavvdp 8,121864,0 =


The formulas are curve fittings, and match well Flkt's curves. Flkt has separate curves forcircular and rectangular ducts, but they have identical values.

The same equation is used for circular and rectangular expansions.


10/20


2002-08-23

10(10)

3.2.7 T -branches

For T-branches, the same equations are used for both the circular and rectangular branches:

v2v1

T-branch, supply air

v1 [m/s]

1

10

100

5 10 15

v2=4 m/s

v2=6 m/s

v2=8 m/s

v2=10 m/s

[ ] Pavvv

vv

dp 211

2

2

1

2 6,03785,07619,04408,0

+

=

v2 = flow velocity at the outlet [m/s]

v1 = flow velocity at the inlet [m/s]


11/20


2002-08-23

11(11)

v1v2

T-branch, exhaust air

v1 [m/s]

1

10

100

0 5 10 15

v2=4 m/s

v2=6 m/s

v2=8 m/s

v2=10 m/s

[ ] Pavvv

dp 21

76,0

1

2 6,02,0

=

v2 = flow velocity at the inlet [m/s]v1 = flow velocity at the outlet [m/s]


12/20


2002-08-23

12(12)

v2

v1


v2 [m/s]

1

10

100

0 5 10 15

v1=4 m/s

v1=6 m/s

v1=8 m/s

v1=10 m/s

[ ] Pavvv

dp2

1

5,1

1

2 6,05,04,04,08,007,1

+=

If the contents of the inner brackets is zero or near zero, the following formula is used:

216,08,007,1 vdp =


13/20


2002-08-23

13(13)

v1

v2

T-branch, exhaust air

v1 [m/s]

05

1015202530

35404550

0 5 10 15

v2=4 m/s

v2=6 m/s

v2=8 m/s

v2=10 m/s

[ ] Pavvv

vv

dp2

11

2

2

1

2 6,04,04,07,0 +

=


Note that the negative values are set zero to avoid infeasible situations.


14/20


2002-08-23

14(14)

v2

v1


v2 [m/s]

10

100

0 5 10 15

v1=4 m/s

v1=6 m/s

v1=8 m/s

v1=10 m/s

[ ] Pavvv

dp 211

2 6,00,14,0

+=



15/20


2002-08-23

15(15)

v2

v1

[ ] Pavvv

dp2

11

2 6,06,056,0

+=


3.2.8 F rom duct to distri bution box

v

A1 A2

[ ] Pavdphowever

A A

A A

if A A

A A

if A A

A A

if A A

2

2

1

2

12

1

2

1

2

1

2

1

2

1

6,0

15,46

5,4

1,06,0

5,435,4

315,045,0

322,025,0

2125,0

=

+=

MagiCAD Ventilation Calculations

Documents

Transcript of MagiCAD Ventilation Calculations