1-2 si units and basic measurements for air,water systems

8/18/2019 1-2 SI Units and Basic Measurements for Air,Water Systems

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PART 1

Module 1/2

SI Units and BasicMeasurements for Air/WaterSystems


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Obecti!es"

• To state the common SI Units used inCommissioning

•Understand how they are used• Calculate areas of ducts and volumeow rates/velocities


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I#TRO$U%TIO# TO MO$U&'

Commissioning of Air and WaterSystems involves the taking of a

numer of measurements to arriveat a ow rate! In this module wewill look at the asic SI Units

involved in calculations which mayarise during commissioning!


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1( T)' SI S*ST'M

The uilding services industryfollows the IS" #InternationalStandards "rganisation$recommendations to ado%t aninternational system of units #theSI System$!


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Base Units"

• Units derived from those aove include

U&IT &A'( S)'*"+

Area

,olume,elocityS%eci-c ,olume.ensityorce0ressure

S1uare 'etre

Cuic 'etre'etres %er secondCuic 'etres %er kilogram2ilograms %er cuic metre&ewton&ewton %er metres or 0ascal

m3

m4

m/s or ms56

m4/kg or m4kg56

2g/m4 or kg m456

&&/m3 or 0a

Unit #ame S*MBO&

+ength'ass Time

'etre2ilogramSecond

m2gs


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Mass + ,olume

• mass of a ody is measured inkilograms #kg$

• !olume of a ody in cuic metres#m-$

• Another measurement of uid volumeis the litre #l$7 which is used in low

airows and also water ow!

1... +IT8(S 9 6 CU*IC '(T8(


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Mass + ,olume lo0 Rates

• Mass ow rates are e:%ressed inkilograms %er second #kg/s$!

• Similarly!olume

ow rates aree:%ressed in cuic metres %ersecond!

m-/s or litres er second l/s3


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$ensity and Seci4c ,olume

• Within the industry fundamentalcalculations are carried out in mass ow#kg/s$!

• ans and %um%s move volumes of air #m4/s$and water #l/s$ res%ectively!

• Commissioning engineers takemeasurements to estalish the volume ow

rate! )ou can not measure mass ow7 youhave to calculate it from volume ow!

T5e lin6 bet0een t5e t0o is density(


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$ensity and Seci4c ,olume%ont7d3

%onsider a cubic metre of 0ater and a cubicmetre of air( T5e 0ater 0ill be considerably5ea!ier and is said to be more dense( $ensityis measured in 68/m- mass er unit !olume3(

• .ensity of water at ;


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,elocity

This is the sim%lest form of motion aody e:%eriences7 moving at aconstant s%eed in a straight line7 this

is said to e a Constant ,elocity!,elocity 9 .istance 9 m

Time s

This e:%ression of velocity #m$#s$

is re%resented in documents as m/s! This ty%eof notation will henceforth e used!


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Pressure

0ressure is force %er unit area!

08(SSU8( 9 orce #&$ 9 &/m3

Area #'$3

• A &ewton %er s1uare metre #&/m3$ istermed a 0ascal #0a$!

• A %ressure of one 0ascal is e:erted whena force of one &ewton is evenly a%%liedover an area of one s1uare metre!


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Pressure %ont7d3

T5is is a !ery small measurement9 5encelar8er units 5a!e been deri!ed( T5ese unitsare t5e 6iloascal9 Bar and Millibar(

In some instances9 ressures are measured in

millimetres of Mercury mm )83 : )8 bein8t5e c5emical symbol for Mercury(

1mm)8 ; 1--(- Pa


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2( AIR&OWTo calculate t5e amount of air >o0in8 t5rou85 a duct it is necessaryto measure a number of arameters(

irstly 0e need to 6no0 t5e cross sectional area of t5e duct in s?uaremetres at t5e oint 0e are measurin8(

1( Ta6e t5e follo0in8 e@amle of a rectan8ular duct( $uct sies are normallydimensioned in mm( Remember 1...mm ; 1m(

*efore you calculate the area you need to turn the dimensions of the duct into metres! i!e!>==mm 9 =!>m and ;==mm 9 =!;m!

Area 9 width : height 9 =!> : =!; 9 =!3;m3

600mm

400m

m


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2( AIR&OW %ont7d3

2(or a circular duct9 you need to 6no0 t5e diameter of t5e duct( Ifunsure about t5e diameter9 measure t5e circumference of t5e ductand di!ide t5at by 05ic5 ; -(12 and is often on a calculator(or e:am%le?5

The circumference of a duct is measured at 6@@mm

.uct diameter 9Circumference 9 6@@ 9BB!Bmmπ 4!6;39 a%%ro:imately >==mm

.uct Cross Sectional Area #m3$ 9 πr3 or πd3

;

r 9 radius of ductd 9 diameter of duct in metresArea 9 4!6;3 : =!>3; 9 =!3@3m3 or =!3@4 to 4 decimal %laces


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2( AIR&OW %ont7d3

-( T5e most comlicated area you are 8oin8 to meet is 05at is 6no0n as `fat oval" and

consists of a rectan8le and t0o semi circles or one circle(

Rectangle

Semi

Circle

Semi

Circle

250mm 250mm

1200m

m

500m

m


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2( AIR&OW %ont7d3

This is naturally a more com%licated sha%e to calculate7 however fromeither the width and de%th of the duct or the de%th and %eri%hery thearea can e calculatedD for e:am%le study the duct aove!

This at oval and duct consist of a circle of ==mm diameter and arectangle measuring ==mm : ==mm! The total area therefore e1uals

the sum of these two areas #rememer all measurements in metres$!

Total Area 9 Area of Circle E Area of 8ectangle

9 4!6;3 : #=!$3 E #=! : =!$

;

9 =!6B> E =!4

Area 9 =!;>m3

COMMISSIONING ENGINEERS MUST ALWAYSMEASURE DUCTS AT THE POINT THEY ARETAKING THE VELOCITY READINGS, THEY MUSTNEVER ACCEPT A DIMENSIONED DRAWING IS

CORRECT !


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2( AIR&OW %ont7d3

,olume lo0 Measurement

•Faving calculated the cross sectional area of the duct at whichthe airow is to e taken7 we need to measure the velocity!

• This is carried out with the aid of measuring e1ui%ment

descried in module 6/ G*asic low 'easurementG!

•(:ce%t in the sim%lest cases7 it is normal to take a series ofvelocity measurements and then calculate the averagevelocity!

• The volume can then e calculated from the following formula!

,olume low 8ate 9 Average ,elocity : Area

m4/s 9 m/s : m3


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2( AIR&OW %ont7d3

'@amleA rectangular duct 6==mm : 63==mm has air owing at an average velocity of;!3 metres %er second!

Calculate the ,olume ow rate

,olume ow rate 9 ;!3 m/s : #6!m : 6!3m$

9 !>m4/s

I o! ala# $!t o!% !&'t# (), # *t+- '& t.* */!at'o& a&0 1&o t.*!&'t# '& .'+. o!% a*% #.o!l0 2*, t.*%* #.o!l0 2* &o 0o!2t o!.av* t.* +o%%*+t a*%# (!&l*## o! .av* )a0* a& *%%o% '& a%'t.)*t'+-

In the aove e1uation we have

,olume ow #m-$ 9 velocity #m$ : Areas #m2$

#s$ #s$

#m4$ 9 #m4$ CorrectH

#s$ #s$


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((8CIS( 6

A circular duct of @=mm diameter hasair owing through it at an averagevelocity of 6=!4m/s!

Calculate the cross sectional area andairow rate!


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((8CIS( 3

A at oval duct with an overall siJe of 6=mm :=mm has air owing through it at an averagevelocity of !> m/s!

Calculate the cross sectional area and air ow rate!

#B(

W5en carryin8 out any e@ercise al0ays s5o0

t5e basic formulae9 units and 0or6in8s(


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A#SW'RS TO 'C'R%IS' 1

Part 1 Cross sectional area

Area of Circular .uct 9=!>@m3

Area multi%lied y ,elocity9 ,olume

Part 2 Airow 8ate,elocity =!>@m3 : 6=!4m/s 9!@=m4/s


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A#SW'RS TO 'C'R%IS' 2

Part 1 Area of Circular section with =mm .iameter 9 =!34@m3

Area of rectangular section 9 =!>>m3

Width =mm #as %er .iameter$

+ength of rectangle 9 overall siJe 6=mm less diameter of circularsection 9 63==mm

Area of lat oval duct 9 =!@B@m3!

Part 2 Airow rate

Area multi%lied y ,elocity 9 ,olume

,olume 9 =!@B@m3 : !>==m/s 9 >!@3m4/s


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WAT'R &OW

• lowrate through %i%ework is generally not measured in the same wayas air through ductwork as water tends to leak through a %i%e whenyou drill holes in itH

• Water ow can e measured ultrasonically and y means of a watermeter7 however the commissioning engineers usual method is tomeasure a %ressure dro% across a restriction of a known characteristic!

• The restriction is normally an "ri-ce7 manufacturers %roduce a chart ofeach ori-ce siJe7 from which water ow rate can e read for ameasured %ressure dro%! The %ressure dro% is normally read with ali1uid or electronic manometer!

• As we will see in later modules #6/ G*asic low 'easurementG and 3/;Glowrate and CirculationG$ %ressure dro% varies as the s1uare of thechange in owrate!

• A ty%ical chart from the Crane catalogue is shown on ne:t %age! It cane seen from this chart that for a %ressure dro% of != k0a there is aow rate of 3!@ l/s!

• Conversley it can e seen that should you re1uire a water ow of 4!=l/s then the %ressure dro% through the ori-ce would need to e !;k0a!


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lo0 Measurement$D.1 E $D11 E $D1

Sie 2 =.mm3

1-2 si units and basic measurements for air,water systems

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