Welcome back this lecture is a continuation of a earlier lecture. In the last lecture

we have discussed how to estimate frictional and dynamic losses in air conditioning ducts

and I also mentioned briefly the design rules to be followed while designing air conditioning

ducts. So the specific objectives of this particular lecture are to discuss classification

of duct systems.

Discuss commonly used duct design methods namely velocity method equal friction method

and static regain method and then discuss briefly the performance of duct systems and

discuss fans and fan laws and finally discuss briefly the interaction between fan and duct

systems.

And at the end of the lecture you should be able to classify duct systems design air conditioning

ducts using either velocity method or equal friction method or static regain method. Then

evaluate performance of duct systems then discuss and use fan laws finally discuss interaction

between fan and duct systems.

So let us first look at classification of air conditioning duct systems how they are

classified based on the load on duct due to air pressure and turbulence duct systems are

classified as low pressure systems. Where in the air velocity is less than or

equal to ten meter per second and the static pressure is less than or equal to five centimeters

of water column. These systems are known as low pressure systems. Then you have medium

pressure systems where the velocity is still less than or equal to ten meter per second.

But the static pressure is between five centimeter to fifteen centimeter of water then finally

you have high pressure systems where the velocity is greater than ten meter per second and the

static pressure is between fifteen to twenty five centimeters of water column okay. This

how normally the air conditioning systems are classified.

Now high, what is the, you, effect of velocities high velocity in the duct in the ducts results

in smaller ducts and hence lower initial cost and lower space requirement since the velocity

is high for the same flow rate we can reduce the cross section area that how you get smaller

ducts okay. However if you have high velocities you will

find that the pressure drop will be higher as a result the fan power consumption will

be higher okay. So this is the disadvantage of velocity systems in addition to this due

to high velocities there will be increased noise and hence high velocity air duct systems

require a noise attenuation okay. So these are the effects of high velocities. Now the

typical recommended air velocity it is in supply ducts are as follows for residences

normally the velocities are between three to five meter per second and in theatres the

velocity is between four to six point five meter per second whereas in restaurants it

can be between seven point five meter per second to ten meter per second. As I said

one of the important criteria for selecting the air velocity is noise okay. So in residence

normally noise is not tolerated so you have to maintain low noise whereas in places like

restaurants and all whereas such you have high background noise we can effort to have

higher velocities. So because the higher noise generated because

higher velocities is not a problem okay. These are the normally recommended values of however

there are some special applications where one may have to go for still higher velocities

for example in air craft air conditioning systems or in ship air conditioning systems

the air velocities can be as high as about thirty meter per second okay. So here it is

noise is not the main important criteria but the size of the duct is very important okay.

So should you would like to minimize the size as a result people normally use higher velocities

okay. So the velocity is selected based on mainly on noise criteria okay. Also on fan

performance and the power consumption of the fan okay.

Next look at duct design normally what is the input for duct design from load and psychometric

calculations the required supply air flow rates to each conditioned space are known

this we have discussed earlier then from the building layout and the location of the supply

fan the length of each duct run is known. For example, let me show a typical building

layout.

Let us say that this is a typical air conditioning duct layout we locate the, let us say the

plant here. So you have the fan located somewhere here and these are the outlet supply air outlets

that means these are the different zones where your conditioned spaces are maintained. Let

us say so one two three four five okay. So the ABC etcetera are the duct portion. So

you can see that for these kind of a duct layout you have the fan here and the location

of these outlets one two three four five etcetera are fixed by the design of the building okay.

And you also know where from the location of the fan what is the distance for example

from this point to this point okay. Similarly from this point to this point so physical

location is known so you know the distance and once you know the distance you know what

is the length of the ducts. So normally the length of the ducts is known okay.

Then from these given input we have to design the duct and what is the purpose of the duct

design the purpose of duct design is to select suitable dimensions of duct for each run and

then to select a fan which can provide the required supply air flow rate to each conditioned

zone. That means from the taking the input from the cooling load calculations and from

based on the building specifications and the location of the plant room we have to design

a duct system okay. Design of duct system means mainly selecting the dimensions of the

ducts okay. If you are selecting a circular duct what is the diameter if it is rectangular

duct what are the two side okay. Because the lengths are known and once you select the

dimensions you also have to select a suitable fan which can provide the required amount

of air flow rates to each of this conditioned zone okay. So this is the purpose of a duct

design the duct run with highest pressure loss is called as index run okay I will explain

this a little later.

Due to the several issues involved the design of an air conditioning duct system in large

buildings could be sophisticated operation requiring the use of computer aided design

or CAD software now a days. When you have very large buildings requiring say hundreds

of room with different conditioned spaces located at several points then the duct design

or duct layout can be extremely complicated okay. What I have shown is a relative simple

duct layout but a actual duct layout in large buildings can be very complicated. So if you

want to design it optimally then you have to use advanced methods and you may also have

to use some computer aided design software for designing the ducts okay.

However we in this lecture we shall discuss some simpler methods which are normally used

for simple layouts what are these simpler methods. These simpler method first methods

are known as velocity method. Second one is known as equal friction method third one is

known as static regain method. So I shall discuss only these methods in these lecture.

First let us look at velocity method the various steps involved in this particular method are

first select suitable velocities in the main and branch ducts okay that means. Let us say

that again.

Let us go back to this one. Let us say that this is our duct layout so in the velocity

method the first thing we do is we select velocities okay, in different runs, for example

this duct run is known as your main okay, whereas these duct runs are called as branch

branches okay, different branches. So first what we do is we select velocities in main

and in the branches okay. Based on, let us say noise criteria or a based on the space

considerations or fan power consternations etcetera. We have to select suitable velocities

in the main and in the branches this is the first step in velocity method.

Once you select the air velocities we can easily find out the dimensions of the main

air branch ducts from air flow rates and velocities remember that we know the air flow required

air flow rate from psychometric calculations okay, so you know the air flow rate and once

you select the velocities air flow rate is nothing but cross section area multiplied

by velocity. So flow rate is known velocity is known so you can find out what is the required

cross section area so if you are selecting a circular duct let us say then from the cross

section area you can easily find out the diameter. If you are selecting a non circular duct let

us say a rectangular duct even then you know the cross section area. So you have to f ix

either one side or the aspect ratio so that you can find the other side okay. So it is

very easy once you know the flow rate and velocity finding the dimensions of the duct

are easy okay. So this is the second step in the velocity method okay, then the third

step is find frictional pressure losses from velocity and duct dimensions either using

the frictional friction chart or using the frictional pressure drop equation.

This I have discussed in the last lecture as I was telling once you know the flow rate

Q this is known and once you know the dimensions okay. Because as I said Q dot is equal to

A into Velocity. So velocity is known Q dot is known so first find out A once you know

A you can find out D okay. So d is also known to us Q dot is also known to us, so we can

find out the frictional pressure drop using this equation which I have discussed in the

last lecture or you can also use the friction chart for example in the friction chart if

you remember this was discussed in the last lecture we have the frictional pressure loss

per unit length on the x axis flow rate on the y axis and different constant velocity

lines these are the constant velocity lines okay. Once you select the, let us say velocity

okay and if you know the flow rate then you can find out what is the frictional pressure

drop per unit length okay. So this is the third step in the velocity method.

Then we also have to find dynamic losses in each run I have discussed how to find dynamic

losses in each run in the last lecture then we have to find selective fan that can find

that can provide sufficient fan total power fan total pressure for the index run okay

so having estimated the dynamic and frictional pressure losses in each duct run you have

to find out what is the duct run that gives the highest pressure drop okay so that is

known as the index and we have to select a fan which can provide sufficient fan total

pressure for the index run. That means for the maximum possible pressure drop conditions

okay. So for example let us again look at this duct layout.

Let us say that our index run is here okay, like this let us say that means A to G to

H okay, that means the pressure total pressure drop from this point to this point is the

highest compared to other pressure losses okay. Pressure in the other duct runs then

you call this AGH as the index run because the total pressure loss in this particular

duct and is the maximum okay. So we have to select a fan which can provide this maximum

pressure loss.

Okay so this is the method. So first select the velocity is this very simple method as

you can see first select the velocities suitable velocities since flow rates are known from

the velocities find out the dimensions. Once you know the dimensions and the flow rates

you find out the frictional pressure drop and then from the duct layout depending upon

the branches and bends turns etcetera. You find out the dynamic losses then you add up

frictional and dynamic losses for each duct run okay. And find out the duct run which

gives the maximum pressure loss that particular duct run is called as the index run and you

select a fan whose FTP is equal to the total pressure loss of the index run okay. So this

ends the design purpose and the procedure as far as velocity method is concerned okay.

Now what is to be done is, the we have to use balancing dampers in each run and what

is the purpose of balancing dampers. The damper in the index run is left completely open while

the other dampers are throttled to maintain the flow rate at the required design values

okay. So again if you go back to the duct layout let us say that in the index run the

total pressure loss is hundred Pascal's okay. I select a fan which gives me a FTP of hundred

Pascal's but in a, in one particular duct which is not the index run but another duct

run the total pressure drop is ten Pascal. Let us say, so if I am using a fan which develops

hundred Pascal of FTP and it is connected to a duct run which is having ten Pascal only

then you cannot maintain the flow rate. The flow rate will be higher in this particular

shorter duct run okay. So what we have to do is we have to artificially

introduce some friction okay or some resistance so that for each duct run ultimately the total

pressure loss will be same which is equal to the FTP okay. That is the reason why we

have to use dampers and in the index run the total pressure loss is equal to the FTP. So

the damper as got to be completely left open whereas in the other runs where the total

pressure loss is less than the FTP you have to adjust the dampers in such a way that the

additional resistance offered by the damper plus the actual resistance will be equal to

the total pressure loss. So that is how the system has got to be balanced okay. So this

is the in brief the velocity method and what are the typical characteristics of velocity

method as you can see velocity method is very simple. But one important thing is that it

requires experience for fixing suitable velocities in main and branches. So this is the first

step in any velocity method you have to select suitable velocities.

How do you know what is suitable velocity okay, so this depends upon the experience

okay. So depending upon the application depending upon various other considerations you have

to fix suitable velocities. If you do not select proper velocities then you will end

up with a non optimal duct design okay. So success of this method depends upon fixing

the suitable velocities okay next this method is not very efficient because

as dampers in all but index run have to be closed partially leading to noise and losses

so as i have explained we use dampers in all the run okay and the dampers in index run

is completely kept open whereas in the other runs we close the damper so as to introduce

additional resistance okay. This is actually introducing a loss deliberately

okay because by closing the damper partially you are introducing losses okay. And this

will lead to not only loss in efficiency but also the generation of noise okay so the use

of dampers is not very efficient okay. As a result the velocity method which requires

adjustment of dampers for maintaining the FTP same in all duct runs is not very efficient

from this point of view okay.

Next let us look at second method known as equal friction method in this method the frictional

pressure drop per unit length in the main and branch ducts are kept same okay. That

means everywhere.

The frictional pressure drop per unit length, that means delta PF by L is constant okay,

constant, for example this main for this part of the main for this branch for this branch

for this branch etcetera. Every where this parameter is same this is the principle of

equal friction method that is why I call it as equal friction method okay.

So that means delta Pf by L of duct run A is equal to delta Pf by L of duct run B that

is equal to delta Pf by L of duct run C etcetera okay. Then the step wise procedure is as follows

select a suitable frictional pressure drop per unit length. So that combined initial

and running cost are minimized okay. So in the earlier method we have fixed the velocities

where as in this method we have to select a suitable frictional pressure loss per unit

length okay. That is the first step then the equivalent diameter of the main duct is obtained

from the selected value of delta Pf by L and the air flow rate. For example the total air

flow rate through the main is given by this Q dot A is equal to Q dot one plus Q dot two

plus Q dot three plus Q dot four plus Q dot five.

For example if you look at the figure this is the air flow rate Q dot A okay, sorry,

Q dot A through this main duct run A you can see from the mass balance or volume balance

that Q dot A is nothing but Q dot one plus Q dot two plus Q dot three plus Q dot four

plus Q dot five okay. So this is equal to summation of different Qi's okay. So which

is known to us right.

So Q dot A is known from the air flow rates in different condition zones then from the

Q dot A and using the delta Pf by L value the equivalent diameter of the main duct is

obtained okay. Either using the friction chart or using the flowing equation okay. So you

know that I have discussed this equation before we know this and we have fixed this. So you

can easily find out what is the equivalent diameter of duct run A since delta Pf by L

is same for all the duct runs the equivalent diameter of the other duct runs are obtained

from the equation followings equation is used Q dot to the power of one point eight five

two divide by D, D equivalent to the power of four point nine seven three of A of is

equal to this of B which is equal to this of c etcetera okay and we know this we know

okay. We have calculated D equivalent just know

I explained you have calculated this from the frictional pressure drop per unit length

and the Q. So this is known to us and this is also known to us so you can find out this.

That means the equivalently diameter of duct run B can be obtained by exuviating this two

similarly by equating these two we can find out the equivalent diameter of duct run C

okay. So this procedure is followed for finding the equivalent diameter of all other duct

runs okay. So this is as far as the circular duct is concerned if you are using non circular

duct such as rectangular duct the two sides are obtained from the D equivalent value and

a given aspect ratio. I have discussed this earlier if you know the equivalent diameter

of a rectangular duct how to find the two sides either you have to specify the aspect

ratio or you have to specify one side. So that you can find the other side have also

specified an equation or I also given an equation which was derived from the equivalents of

these two ducts, how to find the two sides from the equivalent diameter okay. This was

discussed in the last lecture so the velocity of air through each duct is obtained from

the volumetric flow rate and the cross sectional area.

Now from the dimensions of the ducts in each run the total frictional pressure drop of

that run is obtained by this equation delta Pf A is equal to this is very simple this

is nothing frictional pressure drop in A is nothing but frictional pressure drop per unit

length in A multiplied by length of duct run A LA is the total length of duct run A similarly

in B the frictional pressure drop in B is equal to frictional pressure drop per unit

length in B multiplied by length of B in this particular method delta Pf by L of A is equal

to delta Pf by L of B etcetera okay. So we can find out what is the frictional pressured

drop in each duct run now after you find the frictional pressure drop we have to find what

is the dynamic pressure loss in each duct run okay. As I have explained in the last

lecture the dynamic pressure losses depends upon the velocity and it also depends upon

the duct layout how many bends you have how many whether you have a branch take off or

whether you have any other coil etcetera. But depending upon this information one can

calculate the dynamic pressure losses, so once you find out the frictional pressure

losses and dynamic pressure losses of each duct run the total pressure loss of that particular

duct run is nothing but the sum total of dynamic and frictional pressure losses okay. So that

how we can find out the total pressure loss in each duct run okay. So that is given here

the total pressure drop in for example duct run A is equal to delta Pf A plus. delta Pd

A similarly for B and so on

Next the fan is selected to suit the index run with a highest pressure loss okay. As

before a dampers have to installed in all the duct runs to balance the total pressure

loss okay. So in brief this is the equal friction method. So let me again summarize equal friction

method this equal friction method starts with selecting suitable value of frictional pressure

loss per unit length okay. So this is the first step in this method. So once you know

the frictional pressure drop per unit length then you can find out the duct dimensions

because you know the flow rate okay. So flow rate is known frictional pressure loss per

unit length, is known. So from these two you find out the duct dimensions once you know

the duct dimensions you have find out the velocities from the duct dimensions and the

air flow rate. So once you know the velocities you can find

the dynamic pressure losses in each duct run then you have to sum up the dynamic and frictional

pressure losses of each duct run and you have to find out the index run. That means the

run which gives the highest pressure loss okay. So this is known as index run and you

have to select a fan whose FTP or to the fan total pressure is equal to R greater than

the total pressure loss of the index run okay. Then since the total pressure loss of other

duct runs are less than that of the index run we again have to install dampers and dampers

have got to be adjusted. So that you get the required flow rate in each supply air outlet

okay. So this is the equal friction method now the equal friction method is simple and

is most widely used oaky, among all the conventional method this is the most widely used method.

And this method is better than the velocity method as most of the available pressure drop

is dissipated in the duct runs not in balancing dampers oaky. So this is better than the velocity

method because what we can do is if you have frictional pressure drop available then you

can reduce the dimensions of the duct. So that you can dissipate the available frictional

pressure drop in the ducts itself there by you can go for smaller ducts right, rather

than wasting it in the dampers okay. So that how this method is slightly better than the

velocity method and this method is suitable when the ducts are not too long okay. If the

ducts are very long what happens is the ducts near the fan get over pressured okay. Then

this system is not very good or recommended for this kind of air conditioning systems

okay, and equal friction method can be used for both supply as well as return ducts procedure

is same for both supply as well as return ducts.

Next let us look at the third method that is static regain method. This method is commonly

used for high velocity systems with long duct runs especially in larger systems. That means

large capacity systems we normally use static regain method because you will find that this

method is more efficient compared to velocity method and equal friction method. In this

method the static pressure is maintained same before each terminal or branch that is why

the name is static regain method. And the procedure followed is as given below first

velocity in the main duct leaving the fan is selected. You do not have to select velocities

in all duct runs but you just have to select the velocity in the main duct okay. Then velocities

in each successive runs are reduced such that the gain in static pressure due to reduction

in velocity pressure equals the pressure drop in the next duct section okay, so let me explain

this.

Let us say that this a typical duct run let us say that air is coming here and you have

one branch here and then you have the upstream branch, let us say I am sorry downstream branch

you have upstream here downstream and the branch okay. So what is done in this method

is in the, for example if I am taking about the downstream, you have velocity at the point

two section two is V two. Let us say and the velocity in section one that is upstream is

v one right. So what is done in this method is, V two is reduced that means V two is less

then V one. So when you are reducing velocity what happens there is a conversion of velocity

pressure into static pressure raise. So this I have discussed in the lecture this is what

is known as static regain factor okay. This raise in static pressure due to reduction

in velocity pressure is such that it matches with the frictional and dynamic losses in

the section form point one to two okay. That means whatever is the frictional losses from

point one to two is equal to the static regain factor R multiplied by the velocity pressure

at point one minus velocity pressure at point two okay. Pv one minus Pv two Pv two is always

lower then Pv one that's how you select the dimensions of the downstream duct okay. So

find that because you balancing exactly the drop in velocity pressure with that of the

losses, you find that the static pressure at point one and two will remain same that

means Ps one will be equal to Ps two right. So P what is section two? Section two is nothing

but the upstream for the next downstream section and the branch okay. So you keep the static

pressure here equal to static pressure here similarly if you have further branches and

down streams again at this point also you have to select these duct dimensions such

a way that the static pressure in the downstream will also remain same as that of the upstream

okay. So this is the velocity this the static regain method okay.

Now if section one is the outlet of the fan then its dimensions are known from the flow

rate and velocity okay. What I mean is, let us say that you have the fan here okay, and

then we know what is the total air flow rate here q total is known because this is nothing

but the sum total of all the air flow rate flows rate to each outlet. So this is known

to us right and we are selecting a suitable velocity here. So once you know Q dot and

V then you can find out what are the dimensions here okay. The dimensions are known here,

and then what we have to do is we have to reduce the velocity. Here you may have to

increase the dimensions for that purpose so you reduce the velocity such that the static

pressure remains constant okay.

However since both dimension and velocity at section two are not known before hand we

have to use a trial and error method to obtain the required dimensions of section two.

What it means is, we have to use this equation okay. This equation as got to be used to fix

the dimensions of section two right here you have v two on this side and this side also

you have V two which is not known okay. So we have to use a trial and error method because

we do not know the dimensions since hence we do not know velocity also. Since velocity

appears on right hand side as well as left hand side you may have to use a trial and

error method. So that this equation is finally satisfied okay, so this is the procedure for

static regain method. And this procedure is continued in the direction of air flow and

the dimensions of the downstream ducts are obtained okay. All the downstream ducts are

obtained by following the same method okay.

Keeping the static pressure constant before each branch or terminal okay, so this is the

principle and then the total pressure drop is obtained from the pressure drop in the

longest run and a fan is accordingly selected thus this step is same for all the methods

okay. The fan as got to be selected finally to suit the pressure loss in the largest duct

run longest duct run I am sorry, that means the duct run with the highest pressure loss

okay.

So the you will find that the static regain method yields a more balanced system and it

does not call for unnecessary dampering okay. The dampers are required in velocity method

and equal friction method for balancing whereas in static regain method you are changing the

dimensions in such a way that the static pressure remains same. So you do not really require

dampering okay since dampering is not required efficiency will be higher however there are

certain problems with static regain method what are the problems as velocity reduces

in the direction of air flow the ducts size may increase in the air flow direction okay,

so this one major problem okay. So you have to progressively in reduce the velocity okay.

So as a result the duct size may go up second problem is that as the velocity reduces in

the direction of airflow the velocity at the exit of the longer duct runs may become too

small for proper air distribution ion the conditioned space, okay.

That means, let us say that you start with some velocity in the main okay. Then you go

on reducing the velocities in the direction of the air flow and finally at the outlet

supply air outlet you may find that the air velocity is too small okay. Too small means

for too small for proper air distribution inside the conditioned space okay. Because

you have to reduce the velocity because you are trying to convert the velocity pressure

into static pressure raise okay so that is the principle of this method. So as result

when you are using this method velocity reduces in the direction air flow but if you if it

reduces too much then it may affect the air distribution in the conditioned space okay.

This is another problem of static regain method if such a problem occurs what is to be done

is you have to again go back and increase the velocity of the main duct which you have

selected in the first step okay. So that means it require some kind of a trial

and error or hytracesion method. So initially you select some velocity and make sure that

the velocity at the supply air outlets is sufficient for proper air distribution if

it is not sufficient you have to go back and either increase are decrease the velocity

okay. You have to decrease the velocity if the; if you find that the velocity is too

high or you can you have to increase the velocity if you find that the outlet velocity is too

low okay. So you find that as a result of this the design calculations are more involved

compared to equal friction method okay. Velocity method and equal friction methods are simpler

whereas static regain method is more complicated.

Now let us look at briefly the performance of duct systems for the duct system with air

in turbulent flow the total pressure loss delta Pt is found to be proportional to the

air flow rate okay, total pressure drop okay. Total pressure drop includes frictional as

well as dynamic losses so it is observed that when if the air flow is turbulent in nature

this is proportional to air flow rate square of air flow rate okay. That means you can

write delta Pt is equal to C into Q dot square where C is the resistance offered by the duct

system and for a given duct system C may vary if the filters become dirty and or dampers

position change theoretically, let us say that the filters do not get dirty filters

stay as they are and if you do not change the damper position then the resistance value

remains same okay. For a given duct system given duct system means a duct system with

given dimensions right and given length etcetera. And variation of delta Pt with air flow rate

is parabolic you can see from this equation that if you plot total pressure loss or total

pressure drop verses Q dot you find that you get a parabolla okay. You get a curve of this

nature.

Okay, so for a given duct system as I said this is the delta Pt total pressure loss and

this a air flow rate meter cube per second right. So we have seen that delta Pt is equal

to C into Q dot square so you get a parabolla which passes through the origin okay. That

means when Q dot is zero delta Pt should be zero right. So let us say that this curve

A is under design conditions okay. So that means at the time of design or for design

you have selected this kind of a duct performance curve or duct characteristic curve but however

with usage the filters may become dirty or you may close the damper okay. If the filters

become dirty or if you close the damper you get this curve okay.

That is curve B with dirty filters or closing of dampers what is happen when the filters

become dirty or when you are closing the dampers for the same flow rate the pressure drop increases

obviously okay. Resistance increases so pressure drop increases right now if you open the damper,

let us say that the filters are clean but you have opened the dampers more compared

to the design condition then you find that the duct performance curve is given by this

blue line C, okay. This is with dampers opened right so for when you open the dampers you

find that for the same flow rate the total pressure drop will be less okay. Because the

pressure loss across the damper is reduced because of the opening so as result you get

less total pressure loss okay. So these kinds of curves are known as duct performance curves

okay. What is the use of this curves you can see that since delta Pt as I have written

here is proportional to is proportional to Q dot square if you know the delta Pt value

at one point let us say okay, at one and if you know flow rate at two. Then you can find

out what is the total pressure loss at the second condition two that means if one is

the design condition at design condition you know what is the total pressure loss and the

flow rate and you what find out an off design condition where the flow rate is different,

we can easily find out what is the total pressure loss using this equation okay. So that is

the use of duct performance curves.

Next let us look at system balancing very briefly what do you mean by system balancing

in large buildings after the air handling unit consisting of ducts coils etcetera is

installed it as to be balanced for satisfactory performance. So what do you mean by balancing

system balancing requires measurements of actual flow rates at all supply air outlets

and return air inlet. So you have to do elaborate measurements carry out elaborate measurements

of air flow rates at all air outlet and inlets. Then the dampers have to adjusted so that

the actual measured flow rate correspondence to the specified flow rates and sometimes

as i part of systems balance, balancing, it may also require adjusting the fan speed to

get required temperature drop across the cooling or heating coils and required air flow rates

in the condition zone okay. Ultimately why do we do system balancing let us say that

you have you carried out the cooling load calculation then you designed the ducts you

have selected the fan you have installed everything right. So finally you have to get the required

performance that means you have to get the required temperatures in the conditioned space

oaky. The, this depends upon the temperature of the supply air and also depends upon the

supply air flow rate okay. So you have to make sure after you install everything that

you are getting the right amount air at the right temperature okay. So what is done is

since several assumptions are involved at the time of design it does not happen automatically.

So some kind of adjustment is required to get the required conditions okay.

So this is known as system balancing and this involves several steps and it is carried out

in different manners. So basically what it involves is the measurement of, for example

temperature and air flow rates at all supply air outlets and return air inlets okay. So

that means you have to have accurate instrumentation and you have to measure all these parameters

and if you find that the supply air flow rate is let us say more than required okay. Then

you have to reduce the supply air flow rate then you can reduce the supply air flow rate

either by changing the damper position or by varying the fan speed of course if you

are using a fan for all the same fans for all duct runs then if you are varying the

fan speed then it will affect the air flow rate in other spaces also right.

So fan speed variation may not be possible always normally the option left for us is

to vary the position of the damper in that particular zone to get the required air flow

rate, okay. So you vary the damper position again measure the flow rate and see that you

are getting the required flow rate okay. So this process as to be continued till you get

the required amount of air flow rates in all the conditioned zones okay. So at this point

the position of the dampers are fixed then sometimes it may also require you may also

require the variation of fan speed for example if you are finding that the required temperature

drop across the cooling coil is too small or too high okay. Which may lead to too much

of dehumidification or right or too less of dehumidification you may have to adjust the

air flow rate okay. So again this requires adjustment of fan speed right.

So this procedure is known as system balancing normally system balancing is the last part

of the air conditioning system installation and design and installation and after system

balancing normally the air conditioning plant is handed over to the building owner okay.

Normally large air conditioning systems the system balancing can be very costly business

and it may take lot of time and effort right. Because it requires measurements of temperatures

and velocities precisely at several locations okay. So most of the times in smaller buildings

and all people may skip this procedure all together okay. They may do it in a trial and

error bases if you find it too cold you have close the damper or if you find it too warm

you may open the damper okay, not by taking any measurements okay.

But however in large systems system balancing is always recommended because once you balance

a system properly you get the maximum possible benefit from the system okay. So system balancing

as to be done at least in the large air conditioning systems okay.

Next let us look at fans okay. Very important component of any air conditioning system the

fan is an essential and one of the most important components of almost important components

of almost all air conditioning systems small or big you have to use a fan for circulating

air okay. Fan is a costly component it consumes lot of power okay and it also generates noise

right. So it is source of noise it consumes power it also incurs lot of initial cost so

you have to select the fan properly okay. If you incorrect selection of fan can be give

rise to bad performance or poor performance okay. Normally the centrifugal fan is most

commonly used in all air conditioning systems because it can efficiently move large quantities

of air over a large range of pressures okay. Generally you find that in almost all air

conditioning systems centrifugal fans are used the other type of fan namely the axial

flow type fan is very rarely used in some special applications and the operating principles

of centrifugal fan is exactly similar to that of a centrifugal compressor. We have discussed

the operation principle of centrifugal compressor how a centrifugal compressor increases a pressure

of refrigerant okay. Similarly a centrifugal fan increases the pressure and kinetic energy

of air okay. So the principle is almost same. Normally forward curved blades are used in

low pressure systems okay, whereas air fall type or backward curve blades are used in

high pressure large capacity systems okay. Compared to forward curved blades centrifugal

fans the air fall type or backward curved blades give higher efficiency okay.

Since efficiency is important in large systems normally large systems use air fall type or

backward curved blade fans okay. Normally the air falls blades are costly that mean

the centrifugal fan based on air flow profile is costlier compared to forward curve blades

okay. So in smaller systems forward curved blades are used in larger systems air fall

types or backward curved blades are used.

Next let us look at fan laws these are very important and extremely useful what are fan

laws the fan laws are a group of relations that are used to predict the effect of change

of operating parameters of the fan on its performance okay. If a particular operating

parameter changed how the fan is performing okay, so that is the use of fan laws okay.

And these laws are valid for fans which are geometrically and dynamically similar okay.

If the fans or geometrically and dynamically similar you can apply the fan law if they

are not geometrically or dynamically similar you cannot use. Then for example you cannot

use the fan laws or you cannot use the fan laws for comparing performance of a forward

curved blade with backward curved blades Okay. They are not geometrically similar so

you cannot use fan laws to these two however you can use the fan laws to one forward blade

that of another forward curved blade fan of different size may be okay. This is what is

known as geometrical similarity right. Similarly also have to have dynamic similarity.

That means the force ratios for these two fans have got to be same right so for these

geometrically a dynamically similar fans one can use the fan laws. Now what are the important

operating parameters the important operating parameters are density of air which depends

on the air pressure and the temperature then rotative speed of fan. That means RPM or RPS

of the fan then the size of the fan what is the diameter width of the impeller etcetera.

So these are three operating parameters out of these three size of the fan is important

at the time of design okay. At the time of operation we are mainly concerned with density

of air and rotative speed of fan okay. We would like to know for a given fan okay. How

the variation in density effect the fan performance how the variation fan speed effect the fan

performance okay. So the fan loss that I am going to present are related to these two

operating parameter that is density and the rotative speed of the fan okay.

Now for a given air conditioning system with fixed dimension fittings etcetera. It can

be easily shown that the air flow rate Q dot is proportional to omega okay. Omega is the

rotational speed in let us revolutions per second okay. So remember that this is for

a given air conditioning system that means the dimensions etcetera are fixed the cross

sectional area is fixed okay. So for a given air conditioning system Q dot is proportional

to speed only. Because you know that Q dot is equal to A into velocity so for a given

duct system cross section area is fixed. So the air flow rate depends only on the velocity

of air okay. And the velocity of air directly depends upon the rotative rotation speed of

the fan oaky. As a result you can write Q dot is proportional to rotational speed of

the fan okay. Similarly the static pressure raise delta Ps is proportional top rho av

square by two this is the static pressure raise remember where rho is the density of

air and V is the velocity of air in the duct right.

Next the important performance parameter is fan power input okay. W dot fan power input

is proportional to Q dot into delta Ps plus Q dot into rho V square by two okay. So the

power input to the fan consist of two parts you can see that right hand side you have

two parts the first part is the power input required to raise the static pressure of a

certain amount of air okay. So this part is utilized for raising the static pressure of

the air the second part is this. So this part that accounts for the power input required

to increase the kinetic energy of the air okay, of given flow rate right. So it consist

of static pressure raise part and kinetic energy part so these are the typical relations

which relative the important performance parameters like air flow rate static pressure and the

power input to density and rotative speed. Now using the above relations the fan loss

that relate performance varies to density and fan speed can be obtained okay. So we

use these three relations to arrive at the required fan laws okay.

First law with law fan law one fan law one is applicable when density of air remains

constant and the speed omega varies okay. So this law is valid under these conditions

so when the density of air remains constant and when the speed varies from the previous

relations we find that the air flow rate varies with rotative speed okay. Q dot is proportional

to rotative speed that means if we increase the speed Q dot increases directly okay. Whereas

the static pressure raise is proportional to omega square why it is proportional to

omega square static pressure raise is proportional to V square and V is proportional to omega.

So static pressure raise is proportional to omega square and the power input is proportional

to omega cube remember that omega is the rotational speed okay.

Revolutions per second or revolutions per minute or even radiant are per second okay.

So the power input is proportional to cube root of rotative speed whereas the static

pressure raise is proportional to square off, I am sorry it is cube of rotative speed square

of rotative speed for static pressure raise and directly proportional as far as the air

flow rate is concerned okay. So this is valid for constant air density right. So what is

the use of this law the use of this law is that for example when the density of air is

remaining constant and i increase. The, let us say that I double the velocity okay, the

fan is running at ten RPS ten revolutions per second i keep the density constant and

increase the rotative speed to twenty revolutions per second okay. So I would like to find out

at these new conditions what si the new flow rate.

So from this fan law one states that Q is directly proportional to omega. That means

if you double the rotative speed Q also becomes double okay. And static pressure raise is

proportional to square of the rotative speed so if the rotative speed doubles then the

static pressure raise increases by four times okay. And the power input is proportional

to cube of the rotative speed so if you double the rotative speed the power input increases

by eight times okay. So you can easily find out what happens when the rotative speed varies

using this particular law okay. Next comes second the fan law two this is valid when

air flow rate remains constant and the density varies okay.

So this law is valid for this condition we are keeping the air flow rate constant but

density is varied right. So when you keep the air flow rate constant Q dot remains constant

and density is varying so the static pressure raise is proportional to density. So if the

density becomes double static pressure raise also becomes double if density becomes half

static pressure raise becomes half okay. And the power input is also varies directly with

density okay this relation comes from the, if you remember from the expression of for

example delta Ps is equal to rho v square by two okay, we are keeping Q dot constant.

That means we are keeping the velocity constant okay. If you keep the velocity constant static

pressure raise is directly proportional to density okay, similarly the power input is

also directly proportional to density the third law is a valid under these conditions

where the static pressure raise okay, delta Ps remains constant and density varies okay.

So when static pressure raise remains constant and density varies you find that the air flow

rate varies like this Q dot is proportional to one by square root of density and static

pressure raise remains constant okay. So that is the condition of the law and rotative speed

also varies inversely with square root of density right that is the reason why the flow

rate also varies like this and finally the power input to the fan also varies in this

fashion okay, one by square root of density okay. So these are some typical fan laws you

also can arrive at different fan laws for example if you take the size also into consideration

you can derive more number of fan laws okay. Now let me give a small example which will

show you the usefulness of the fan law.

Okay, example, a fan is designed to operate at a rotative speed of twenty revolutions

per second and at the design conditions the air flow rate is twenty meter cube per second

the static pressure raise is three hundred Pascal and the air temperature is twenty degree

centigrade at these conditions the fan requires a power input of one point five kilowatt.

Keeping the speed constant at twenty RPS if the air temperature changes to ten degree

centigrade what will be the air flow rate static pressure raise and power input okay.

So how do we do this first we are varying here the density varies because air temperature

varies right however since speed remains constant air flow rate remains constant oaky. That

means you have to apply fan law two okay what is fan law two say?

Fan law two is this air flow rate Q dot remains constant and density rho varies so and according

to this law static pressure raise is directly proportional to density and power input is

also directly proportional to density okay.

So using this you can easily find out for example air flow rate remains constant at

twenty meter cube per second and static pressure raise delta Ps at condition two is directly

proportional to density and density is inversely proportional to absolute temperature. So you

can find out delta Ps two is equal to delta Ps one into rho two by rho one which is equal

to delta Ps one into T one by T two T one T two are temperatures absolute temperatures

as I said. So that is equal to three hundred into two

ninety three by two eighty three which is equal to three ten point six Pascal's okay.

Next power input power input is also directly proportional to density that means power input

at new condition two is equal to power input at condition one multiplied by the density

ratio rho two by rho one. So if you substitute the values you find that the required power

input is one point five three kilowatt okay. So you can see that you can see how useful

the fan laws are, for example if you do not have any fan laws it may be required to test

the fan or take measurements to find out what will be the new air flow rate what will be

the new power input etcetera right. But by using the fan laws we can avoid testing under

all possible conditions, it is not practical to test fan under all

possible condition are test all kinds of fans which are geometrically and dynamically similar

okay. So under these conditions you can use these simple fan losses and find out performance

at one condition if the performance ta other condition is known okay. So this is the usefulness

of fan laws. So finally let us look at interaction between

fan and duct systems okay.

This particular curve shows the performance, fan performance curve okay. This is fan performance

curve at rotative speed omega one this a fan performance curve at rotative speed omega

two and this is your duct performance curve okay. Both are plotted verses air flow rate

Q dot in meter cube per second right. As I said the duct performance curve is a parabolla

and fan performance curve of a centrifugal fan varies like this and there is a point

of intersection. For example at a given rotative speed omega one the duct performance curve

intersects the fan performance curve at this point. So this point is known as the balance

point at this flow rate and at this flow rate the total pressure loss is equal to delta

Pt one which is equal to FTP one. That means the fan total pressure developed by the fan

so at this point very thing is perfectly balanced. Let us say that now the, we have to reduce

the air flow rate and if the air flow rate is reduced to Q two okay.

At the reduced air flow rate you find that the total pressure loss of the duct is reduced

okay. That is delta Pt two however if you do not change the fan you find that the fan

total pressure is here, this higher then, this, so if you again want to balance a system

you may have to reduce the speed of the fan so that again you get a new fan performance

curve such that when the intersect the flow rate will be Q dot two and the total pressure

loss is equal to fan total pressure obtained by the fan okay. So if you have this kind

of matching curves you can easily find out the balance points this is the use of these

curves.

Okay, so this I have already mentioned the point of the intersection of the fan performance

curve and the duct performance curve yields the balance for the combined performance of

fan and duct system. If the flow rate is reduced to Q two then the total pressure loss reduces

to delta Pt two to match the reduced flow rate and the reduced pressure loss. The speed

of the fan as to be reduced or the portion of the inlet guide veins of the centrifugal

fan have to be adjusted to reduce the flow rate okay. So you have to balance both fan

as well as the duct at every changed condition okay, at all points they have to got to be

balanced okay. So if you have the performance characteristic curves using them you can find

out the balance points okay. So at this point i end this lecture let me quickly summarize

what we have learned in this lesson.

In this lecture the following points are discussed classification of duct systems commonly used

duct design methods. That is velocity method equal friction method and static regain method

then we have also discussed very briefly the performance of duct systems. Then we have

discussed fan laws and what is the use of fan laws and finally we have discussed very

briefly the interaction between fan and duct systems okay. At this point I end this lecture

and I continue this in the next class. Thank you.

Welcome back, in the last two lectures we discussed transmission of air. Once you transmit

the required amount of air to the conditioned space you have to distribute it properly within

the conditioned space. So that ultimately comfort conditions can be maintained inside

the air conditioned building. In this lecture I shall discuss space air distribution okay.

So the specific objectives of this particular lecture are to introduce air distribution

systems and discuss air distribution performance index, describe factors affecting air distribution,

describe performance aspects of circular and rectangular free-stream jets, describe various

distribution devices, describe airflow patterns, describe selection criteria for air supply

outlets. At the end of the lecture you should be able to explain the importance of air distribution

and air distribution performance index, list factors affecting air distribution, define

blow drop spread and entrainment evaluate performance of circular and rectangular free-stream

jets list various distribution devices draw airflow patterns and finally list selection

criteria for air supply outlets.

So let me give a brief introduction to space air distribution. As I have already mentioned

once a required amount of supply air is transmitted to the conditioned space. It is important

to distribute the air properly inside the conditioned space. Thus it is important to

design or select suitable air distribution system and the air distribution system selected

should create a proper combination of temperature humidity and air motion in the occupied zone

and it should avoid draft in the occupied zone. What do you mean by draft?

Draft is defined as a localized feeling of cooling or warmth draft is measured above

or below the controlled room condition of twenty four point four degree centigrade and

an air velocity of point one five metre per second at the centre of the room the effective

draft temperature