Let us continue with our discussion on the
psychrometric or psychometric?
processes that are common in air conditioning
application.
If you remember, we were discussing the by-pass
factor of a cooling coil.
What I said was that a cooling coil is a heat
exchanger surface and basically, it is a cross-flow
heat exchanger where we will have the flow
of cold refrigerant through the inside of
the coil and it makes the coil cool at a very
low temperature and air passes over the external
surface of the coil.
The air which comes in contact with the cooling
coil will attain its dew point temperature.
There is a certain mass flow of air past the
coil or across the coil.
The entire amount of air will not come to
the…
The surface temperature will not come down
to the surface temperature of the cooling
coil.
If the surface temperature of the cooling
coil is ts and if the temperature of air with
which it enters the cooling coil is ta, then
the temperature of the outgoing air will be
somewhere in between ts and ta.
This process is idealized in a different manner
in air conditioning.
It is assumed that certain portion of the
air makes perfect contact with the cooling
coil and has a temperature of ts, while the
other portion of air or the rest of the air
does not contact the cooling coil at all and
so it has a temperature ta itself.
Then, there is mixture of these two air streams
and so we will have the condition of the air
at the exit.
On a psychrometric chart, if we represent
the process, we will have something like this.
Let us say this is the dew point temperature
of the cooling coil.
It is known as apparatus dew point temperature
and generally, it is represented by s because
it is at a saturated condition.
Let us say this is the temperature at which
or this is the condition at which the air
is drawn from outside to the cooling coil.
We can represent this condition by any suitable
notation; let us say we are indicating it
as 1.
We are assuming that certain air is there
at condition 1 and another stream of air is
there at condition s.
These two streams of air are mixing adiabatically
to give condition 2, which is the actual exit
condition of air.
If we represent it schematically, let us say
this is your cooling coil.
This is where the refrigerant is entering,
this is where the refrigerant is coming out
and then, air is entering at condition 1,
air is going out at condition 2 and everywhere
on the body of the cooling coil, we have got
condition s.
One can have, this is ts, this is t2 and this
is t1.
Similarly, one can have ws, w2 and w1.
The by-pass factor is generally represented
by capital X, which is equal to t2 minus ts
by t1 minus ts.
It can be obtained by w2 minus ws by w1 minus
ws.
This is the amount of air which is bypassing
the coil divided by the total amount of air.
So, the coil by-pass factor depends on a number
of parameters.
One can write, by-pass factor or sometimes
it is called coil by-pass factor, it depends
on a number of parameters.
What are these parameters?
These parameters are generally coil design
and in coil design, the diameter of the coil
and the number of passes of the coil will
come.
Then the fin specification is very important;
whether you have got only a few fins or you
have got a large number of fins very closely
spaced, what is the fin material, etc., that
is important, and the air velocity.
The by-pass factor depends on these parameters.
If you have got a very compact coil, which
means that if you have got a large number
of tube turns, large number of fins of highly
conducting material and if you have got high
air velocity, you will have a low by-pass
factor, whereas if air velocity is low, I
mean the fin density is not high, then one
can expect a high by-pass factor.
This by-pass factor is generally specified
by the manufacturer of the cooling coil.
It is a parameter which one needs for design
and estimation.
Knowing the by-pass factor and the apparatus
dew point temperature and the temperature
of the incoming air, one can determine what
will be the temperature of the outgoing air
and how much moisture will get separated.
These useful quantities can be determined
by knowing the by-pass factor of the cooling
coil.
Then, we are going for the analysis of summer
air conditioning.
As I have said, I like to discuss the air
conditioning process for different situations.
Though there could be air around the air conditioning
system, the processes of air conditioning
for summer and winter are different.
Let us first discuss summer air conditioning.
Let us see summer air conditioning, particularly
for a hot and humid climate, which is the
situation in a large number of places in India.
We will have a cooling coil through which
we will have the evaporating refrigerant.
Then, we should have a vapor compression refrigeration
system.
The evaporator of it will be used as the cooling
and dehumidifying coil.
Let us see how one can achieve summer air
conditioning for a hot and humid climate.
Let us say that the condition of the cooling
coil, the apparatus dew point temperature
is somewhere here.
The comfort condition will be somewhere here
and then the outside air condition is somewhere
here – high temperature, high humidity,
outside air condition will be somewhere here.
Basically what is done is, let me draw a figure,
a schematic diagram and then it will be understandable.
We will have a room; this is the room.
In the room, cold and dry air will be supplied.
This is the room where cold and dry air will
be supplied.
This air has to come from a cooler or cooling
and dehumidifying coil.
To have this, one should have a fan and then
from the room, this air has to be extracted.
Part of it will go to the atmosphere and part
of it will be recirculated back.
Then from the open atmosphere, one has to
take certain amount of air.
Generally, in this path, one will have dampers
to control the volume flow; one can have number
of dampers.
This is how one can show schematically the
arrangement.
This is called supply air SA, this is the
return air RA, this is exhaust air, let us
say EA, this is recirculated air REA and this
is fresh air FA.
We can give different names also.
This is 1, 2, 3.
I have drawn a very simple diagram.
1 is the room, 2 is the cooler and then 3
is the fan.
I have shown only a few things.
There could be a number of other items, which
we will discuss later on.
This is a summer air conditioning process,
which I like to also describe with the help
of the psychrometric chart.
1 is the room and let us also represent the
room air condition as 1.
Actually, I said that this is the comfort
condition, if you remember.
While I was describing this diagram, I told
that this is the comfort condition.
It is like this.
When we supply cold air to the room or to
the condition space, we supply it at a very
low temperature and at a very low humidity,
low absolute humidity or low humidity ratio;
we supply the air at a very low temperature
and at a very low humidity.
That is obvious.
We have got experience that if we stand just
beside the exhaust of the air conditioner,
we feel the gust of the air which is coming
out of the air conditioner to the room and
the air is cold enough – it is not comfortable,
it is cold enough.
So, we have to supply cold air because inside
the room, there is enough heat load.
We have to pick it up and that is why you
have to supply cold air.
But where human beings or occupants are there,
when the air comes to that place, it will
be at the comfort condition or it should attain
the comfort condition.
It is also assumed that when the air goes
out of the room, it is having the comfort
condition; that means the air that goes out
of the room is having the properties corresponding
to the comfort condition.
This means that if we design the room air
conditioning system for let us say 25 degree
Celsius or 26 degree Celsius and 60% relative
humidity, the air which will be going out
of the room, will also have 26 degree Celsius
and 60% relative humidity.
This will be the room air condition and it
will also be the condition of the air which
is leaving the room.
Here, what we can see is that part of the
room air is taken for recirculation and is
mixed with fresh air, which is nothing but
outside air.
This is fresh air.
If these two air streams mix together, we
will have the mixture condition somewhere
in between.
This air will now pass through the cooling
coil, something like this (Refer Slide Time:
19:49); this air will now pass through the
cooling coil and then, it will be somewhere
here.
This cooling coil has some sort of a by-pass
factor and so the supply air condition will
be somewhere here and it will go to the room.
How to represent it?
Let us say this is 2 and this is 3.
For this diagram, let us say this is temperature
t and omega.
We can call 1 as room air, 2 as recirculated
air which is again being re-circulated over
the machine.
Let us call it, mixture of recirculated air
plus fresh air, so mixture of recirculated
air plus fresh air.
This is condition 2.
Then 3 is the supply air, SA.
What we can see is that 2 to 3 is a cooling
and dehumidifying process which is taking
place in the cooling coil.
The mixture of recirculated air plus the fresh
air goes through the cooling coil and experiences
a cooling and dehumidifying process.
Then, it is supplied to the room.
In the room, it undergoes a heating and humidification
process.
In the room itself, in general, there will
be both sensible load and latent load.
This will be picked up by the air and so,
we will have heating plus humidification when
the air passes through the room.
There will be a mixture of fresh air and recirculated
air and when it will be supplied to the machine
or cooling coil, it will have a still higher
temperature and higher absolute humidity.
That is what we will have in our summer air
conditioning.
The recirculated air, actually this comes
in the details of air conditioning, why is
it needed?
The ……. first thing that inside human
being will be there and so due to their respiration,
one will have depletion of oxygen and increase
in carbon dioxide.
The need for recirculated air is not only
for that but also there is generation of staleness
inside the room and so air has to be freshened
up.
That is why certain amount of air has to be
put outside as exhaust and instead, we have
to take some amount of fresh air.
That is what we have in summer air conditioning.
But here, you see in the summer air conditioning,
mainly we have one equipment – that is your
cooling and dehumidifying coil.
We have only one process – this is 2 to
3 and by this process, we are controlling
both temperature and humidity.
This task sometimes becomes very difficult.
Sometimes, it is like this – let us say
it is the rainy season and the outside air
is heavily loaded with moisture.
We have got a lot of moisture and we have
to get rid of it.
What can we do?
Then we have to cool the air to a very low
degree or the apparatus dew point temperature
will be very very low.
If the apparatus dew point temperature is
very, very low, then only, this is the condition
of air w2 and this is ws or let us say, taking
the by-pass factor into consideration, this
is w3.
So w2 minus w3 has to be quite large or in
other words, w2 is fixed and we have to make
w3 very large.
That happens in a number of cases when we
are having a lot of moisture in the air that
goes through the cooling coil or may be inside
the room itself and it has to pick up lot
of latent heat.
Inside the room, it will pick up a lot of
moisture and so in those cases, it happens.
We have to lower down the dew point temperature.
If we lower down the dew point temperature,
we can get rid of moisture but at the same
time, the supply air temperature will also
fall down.
That may not be permissible or that may not
agree with our comfort condition.
What do we have to do there?
After getting rid of the moisture, we have
to go for heating the air.
This is called reheating of air.
In summer air conditioning, particularly when
it is highly humid condition, we have to go
for reheating.
Reheat control; sometimes, it is called reheat
control.
Why is it called reheat control?
It is because we are controlling the temperature
of the supply air by the process of reheating
and that is why it is called reheat control.
Let me draw the psychrometric diagram first
and then we will go for the other diagram.
This is the apparatus dew point temperature
and this is where air will be supplied.
From here we will have a reheat process, which
means that we will have increase in temperature
without any change in the humidity and then
we will have change in the room where both
temperature and humidity will increase.
Then, we will have outside air condition OA
and this is 2.
We can have 1, 2, let us call this 3, let
us call this 4; 3 to 4 is the reheat condition.
If we want to represent it schematically,
we will have the room and this is supply air.
Then part of it will go out and the rest will
be taken as recirculated air.
It will be mixed with fresh air, then we are
having the fan, we can have the cooling coil,
we can have the reheater RH and then the air
will go like this.
What we can have here is this is outside air
OA and room air is coming at condition 1,
so this is 1.
1 and OA are mixing, which is giving 2.
Here, it is 3 and here it is 4.
This is what we will get in a physical system.
No, we are not assuming.
You are asking whether we are assuming that
in the room, the temperature difference is
less, but there is no need of assuming it.
Only thing is that from the comfort requirement,
we need to supply the air at certain temperature
and also, that is necessary because the room
has got certain load.
So it is also necessary to supply it at a
particular temperature.What happens is that
sometimes, particularly during humid climate,
we can have lot of humid load.
In that case, to take care of this humid load,
one has to overcool the air so that some good
amount of moisture can be separated out.
If we do that, if we overcool the air also,
that means its temperature is lowered to a
very large extent and that may not be practicable.
That is why we have some method for balancing
it.
We heat the air after it is cooled or after
the moisture is separated.
This process is known as reheating and this
heater is known as a reheater.
One can look into the whole situation from
a different angle.
Already, I have introduced the sensible heat
factor.
The sensible heat factor SHF is equal to sensible
heat load SHL, we call it sensible heat load,
divided by sensible heat load plus latent
heat load.
Sometimes, what happens if the latent heat
load is very high, then the sensible heat
factor becomes very low.
For all the processes, there is some permissible
value of sensible heat factor, but our latent
heat load is so large that it is lowering
down and it is bringing it below the permissible
level of the sensible heat factor.
Then artificially, I have to bring the sensible
heat factor to the acceptable value.
What can I do?
I can introduce artificially some amount of
sensible heating.
That is what has been done by the reheating
process.
Clear?
There must be a certain level at which this
facility is used.
That means the humidity level has to be at
certain level.
Yeah.
When humidity is very high then only we can
do.
What is that high level?
Actually, all this depends on a number of
things and a number cannot be told.
It depends on a large number of things.
It is like this.
Let us say the outside humidity is not very
high, not extremely high.
But inside the room, we have a lot of humid
load.
Then, what we have to do is we have to go
for it.
Generally, a good air conditioner will have
provision for reheating and we can control
the room temperature with the help of reheating.
This is a very convenient method of doing
so.
But again from the energy point of view, this
is not a very good technique.
Because what we are doing is we are artificially
creating heat load and again that heat load
is being taken care of by our machine.
We need to have a heater and to consume that
heat load we have to have some sort of an
oversize machine.
But in certain cases, we have to live with
it; we cannot avoid it.
This is what is called as reheat control or
reheating process or controlling the air condition
by the heating process.
So it is called reheat control.
In certain places, one can have hot and dry
climate.
In hot and dry climate, one can have the cooling
coil.
That means a vapor compression refrigeration
system and the evaporator in the form of a
coil; that can be done.
But whenever you are having the cooling, it
will again take out some amount of moisture
from the air and you probably need some sort
of water sprinkling in the air itself to get
back the humidity.
There is another technique which can be used.
One can go for evaporative cooling.
As far as comfort air conditioning is concerned
in India, in a number of places, those air
coolers are sometimes called reject coolers
are there, where one can do the cooling with
water stream.
There are mechanized systems also, sophisticated
mechanized systems, those are known as air
washers.
With air washers, we can control the desert
type of climate also very easily, because
the air there is going through some sort of
a spray of water which is pumped and not only
that, that water condition can be monitored
either by cooling or heating the water.
That is why I said that it is a rather sophisticated
equipment and with that, we can tackle the
desert climate or hot and dry climate.
Can you repeat your question?
Very quickly, let me draw it so that it will
be clear.
It is like this.
You have a system like this.
This is your air washer, maybe you can have
some sort of a ducting and some sort of a
fan for drawing air – something like that
and you have got a pump.
This is air and these are 
number of nozzles.
Here, you have the cooling coil and the heating
coil.
You are making contact between air and water,
but you can monitor the water condition.
Again, here, one can have the reheater etc.,
– reheater or preheater; those types of
things are possible.
Here, as you are using water, if any evaporation
is needed to increase the moisture content
of air, that is possible.
But in India, the use of this air washer is
limited.
There are certain systems; particularly the
air washers are suitable for large systems
and so for large systems in certain establishments,
air washers are used.
This takes care of the desert climate or hot
and dry climate.
[Conversation between student and professor
- Not Audible (40:27 min)]
That is what I told.
What you are telling is that heating coil
or cooling coil is not there.
Yes, that is a very cheap system.
In large parts of north India, we get this
and we call it air cooler or desert cooler.
That is there; but what I am telling is that
one can have a sophisticated, good system
where temperature control, humidity control
is possible with the help of air washer.
In fact, the air washer is a much versatile
machine compared to our vapor compression-based
air conditioning system because of its versatility
and because it can go for evaporative type
of cooling, it is a much versatile device
compared to the conventional air conditioner.
But then it is more bulky, it is noisy, it
is leak-prone – has leakage problem; that
is why it is not used that much.
We have discussed regarding summer air conditioning.
Now, winter air conditioning, I should say,
is not that critical, particularly in our
condition.
Mostly, we need heating and in some extreme
conditions, we need heating plus humidification
– not in all cases but in extreme cases,
heating plus humidification.
Heating is rather easier.
Basically, you need a blower for circulating
the air and you need some heating and for
a compact system, electrical heating is there.
But people may argue that that is high-quality
energy, I mean heating from low-grade sources
like steam or hot water heating is also there.
Particularly in cold countries where room
heating is a must, they will have a boiler
to have the steam form and then it is circulated
throughout the building.
That is why they will have a centralized heating
mechanism for the room and by that, the entire
room will be supplied or there can be a district
heating system where steam will be generated
at one place and it will be supplied to a
number of consumers – that is possible,
or one can have electric heater.
But when heating plus humidification is needed,
in that case we are having two or three different
options.
One option is water injection – that means
you can increase the moisture content by injecting
water.
Now, if you have to do this, it is always
advisable to increase the air temperature
first and then inject water because at high
temperature, it will have higher ability to
retain moisture.
The process will probably be 
some type of process like this.
One can think of a process like this – 1,
2, 3, 4.
This side is t, this side is w.
Winter air conditioning; 1 to 2 is called
preheating.
Generally, if any heating is there before
evaporation or condensation, we call it preheating.
Then, 2 to 3 is water injection.
If we inject water, what will happen?
Water will evaporate.
During that process, the moisture content
or absolute humidity will increase, but there
will be decrease in temperature.
This process, this curve will be similar – may
not be parallel to your constant wet bulb
temperature curve, but it may be at an angle
with the constant wet bulb temperature curve.
In some cases, if you control the process,
then it can be parallel to the constant wet
bulb temperature curve.
So 2 to 3 is water injection and 3 to 4 is
again reheating.
In winter air conditioning, you can have these
three or maybe these two: preheating and water
injection.
Depending on the situation, you can have this
type of a process.
One can have another option also for winter
air conditioning.
The condition during winter is somewhere here;
our comfort condition is somewhere here.
One can have steam injection.
If you inject steam, then you will increase
the moisture content and you will increase
the temperature also; here, t and w.
This is 1 and 2.
1 to 2 is steam injection.
This is also possible.
Actually, I could get what you told.
See, it 
is again just like our cooling and dehumidifying
coil.
Here also, with a single process, I want to
control two parameters, temperature and humidity.
Here, problems will be there or may be there.
What is done is sometimes or most of the time,
with steam injection we also have reheating
or preheating.
That means temperature is controlled by at
least two methods and steam injection is there
to supplement humidity.
One thing is there; for humidity, one can
allow a larger range for comfort feeling – that
is there.
Control does not become very critical for
comfort condition, but for other applications
like electronic fabrication etc., humidity
control has to be done precisely.
There, some sort of chemical processes are
also used to take care of moisture control.
Some sort of desiccation etc., are also used
for the control of moisture content but for
comfort condition, it is not very critical.
In some cases, you may need to inject steam
because it is giving heating plus humidification
– that is possible.
I think I will stop our discussion on air
conditioning with this and we can go to other
topics.
