Welcome back in the last lecture we discussed
the selection of inside design conditions
based on the comfort criteria.
In this lecture I shall discuss the selection
of outside design conditions and then selection
of supply conditions based on the psychometric
of air conditioning systems.
So specific objectives of this particular
lecture are to discuss criteria for selection
of outside design conditions, discuss selection
of supply conditions and psychometric of air
conditioning systems, analyze summer air conditioning
systems namely simple system with hundred
percent, re-circulated air system with outdoor
air then system for high latent load applications.
At the end of the lecture you should be able
to select suitable outdoor design conditions
based on weather data and applications find
the supply conditions based on the psychometric
of air conditioning systems and analyze various
summer air conditioning systems.
So first let us look at how to select outside
design conditions. So the ambient term as
you know very well the ambient temperature
and moisture content vary from hour to hour
and from day to day and from place to place.
For most of the major locations of the world
meteorological data are available in the form
of mean daily or monthly maximum. And minimum
temperatures and corresponding relative humidity
or wet bulb temperature this data is recorded
and it is available. Now selection of the
worst possible conditions lead to excessive
large cooling or heating equipment as worst
conditions prevail only for a few hours in
a year.
That means if you look at the meteorological
data and if you select the worst possible
condition. That means if you are designing
a summer air conditioning system you take
the maximum possible temperature dry bulb
temperature in maximum possible wet bulb temperature
and can do the cooling load calculations.
And if you decide your capacity of the cooling
system accordingly. Then you will find that
the installed capacity is really excessive
because the worst conditions generally prevail
only for a few hours throughout a year okay.
So it is not really economical so there must
be some other criteria for fixing the outside
design conditions. So let us look at these
criteria.
First let us look at outdoor design conditions
for summer outdoor design conditions for summer
or chosen based on the values of dry bulb
and mean coincident wet bulb temperature.
That is equal or exceeded one percent two
point five percent or five percent of total
hours from June to September. From June to
September the total number of hours will be
about twenty-nine twenty-eight. That means
two thousand nine hundred twenty-eight hours.
So what it basically means is you have to
select let us say that, I am selecting one
percent value okay. That means one percent
of the time the outside design conditions
will be higher than the selected condition.
Okay. One percent of two thousand nine hundred
twenty-eight is roughly about thirty hours
okay. So only during thirty hours the outside
conditions will be hotter in the humid air
than the selected conditions okay. That means
the system capacity may be inadequate only
for thirty hours in one year okay.
If you are selecting let us say two point
five percent of data then due for seventy-five
hours the system capacity will be slightly
lower than required similarly for five percent
data. It will be about one fifty hours the
outside conditions will be slightly more than
selected conditions okay. This is the meaning
of selecting the outside design condition
based on one percent or two point five percent
or five percent value of the dry bulb. And
one more thing you have to notice here is
you are taking the maximum wet bulb temperature
you are taking the coincident wet bulb temperature.
That means what is the wet bulb temperature
when the dry bulb temperature is at one percent
value or two point five percent value or five
percent value okay. This is the criteria suggested
for selecting the outside design conditions.
These values are major locations in the world
or available in data books. For example ASHRAE
data book gives the one percent value or two
five percent value or five percent value for
major locations in the world and in the absence
of any special requirements ASHRAE recommends
the two point five percent value for summer
outdoor design conditions. So a special requirements
means there are some situations where the
inside temperature is very critical. That
means even for ten hours of the in throughout
a year you do not want the temperature to
exceed a particular comfort value okay. That
means inside condition should not exceed a
particular value under any circumstances then
you have to go for one percent value instead
of going for a two point five percent value.
On the other hand there are some applications
where the temperature is not so critical okay,
too critical is variation is allowed then
instead of going for two point five percent
one can go for five percent value okay. So
there by you will be saving the system cost
okay. Both installed cost as well as the running
cost but in the absence of any recommendation
one can go for the two point five percent
value okay.
Next comes the selection of outdoor design
conditions for winter just like summer outdoor
design conditions for winter are chosen based
on the values of dry bulb temperature. That
is equal or exceeded ninety-nine percent ninety-seven
point five percent or ninety-five percent
of total hours from December to February December
to February is the typical winter time. So
if you count the number of hours you find
that from December to February there will
be about twenty-one sixty hours. So if I am
selecting let us say ninety-nine percent value
dry bulb temperature value.
That mean one percent of the time I mean,
that mean, one percent of twenty-one sixty
hours that is about twenty-one hours the outside
temperature will be lower then what is required
okay. That means your heating system will
be inadequate for one percent of the total
duration of two thousand one hundred sixty
hours. Similarly if it ninety-seven point
five percent means two point five percent
of the time the outside conditions will be
lower than the design condition. That means
the system will be inadequate only for two
point five percent of the time okay. So this
is how the winter design conditions fixed
similar to summer design condition winter
design conditions are also available in data
books such as ASHRAE data books okay.
And generally the ninety-seven point five
percent value is recommended if there is no
special recommend special requirement exists.
But if the building is made of light weight
materials if the building is poorly insulated
or if it has considerable glass are the space
temperature is critical then the ninety-nine
percent value is recommended okay. So when
some special situations exist then we can
go for ninety-nine percent value okay. Ninety-nine
percent value means the installed capacity
will be slightly higher than that of ninety-seven
point five percent value okay.
Now let us look at the psychometric of air
conditioning systems from known cooling and
heating loads on the building and design inside.
And outside conditions psychometric calculations
are performed to find supply air condition
by supply air conditions we mean air flow
rate. That mean mass flow rate of the supply
air what is the supply dry air dry bulb temperature
what is its humidity ratio and enthalpy. All
these are known as supply air conditions second
thing is we can also find out the coil specifications.
That means what is the latent and sensible
load from the coil what is the coil required
coil ADP that is a due point and what is equal
to the required coil bypass factor. Normally
depending upon the ventilation requirements
of the building the required outdoor or fresh
air is specified okay. This something which
is specified based on the ventilation requirements
now let me show a typical summer air-conditioning
system.
Okay, this can be any air conditioning system.
But let me take the case of summer air conditioning
in summer air conditioning, let us say that
the required inside conditions are say at
forty-five degree centigrade and required
humidity ratio. Let say is ten grams per kg
of dry air okay, and the enthalpy is the corresponding
enthalpy. Let us say these are the required
inside conditions which correspond to our
comfort requirement. Since it is summer outdoor
conditions will be hotter and humidor. That
means let us say that the outside temperature
is thirty-five degree centigrade and outside
humidity is let us say thirty grams per kg
of dry air since outdoor temperature is greater
than inside temperature outside humidity is
greater than inside humidity there will be
and the building is not perfectly insulated.
So there will be sensible heat transfer from
outside to the inside of the building there
will also be latent heat transfer from outside
to the inside of the building okay. This could
be single room or it could be huge building
consisting of many rooms right.
So we want to maintain the inside conditions
at these values and there is a continuous
addition of sensible as well as latent heats
to the building. So to maintain the room at
these particular conditions we have to take
out this heat at the rate at which it is entering
into the condition space this is achieved
by using an air conditioning system. In the
air conditioning system what we do is we supply
for this case. For example a supply air which
is cold and dry okay. So cold and dry at the
required mass flow rate is supplied to the
building. This cold and dry air flows through
the conditioned space and as it flows through
it picks the sensible heat and it picks the
latent heat. And we assume that when it comes
out of the building or comes out of the conditions
space its conditions will be almost same as
the conditioned space conditions okay.
So what enters into the condition space is
cold and dry air and what comes out is relatively
warm and humid air okay. At the same conditions
as that of the conditions with that means
at twenty-five degree centigrade. And let
us ay and the humidity of ten grams per kg
okay. So at this condition the air comes out.
Now the air goes out flows through the ducts
these are all the ducts the lines okay. And
some part of the air is thrown out that is
exhaust air is thrown out and rest is re circulated
this is the re circulated air. Now this air
is thrown out because we continuously want
to add some outdoor air to cater to the ventilation
requirements okay. So we supply some outdoor
air so at this point this re circulated air
and outdoor air may get mixed and you have
a mixed air here and this mixed air is quite
warm and humid okay. So it has to be made
again cold and dry. So it is made to flow
through a cooling coil as it flows through
the cooling coil it rejects sensible and latent
heats to the cooling coil and in this in that
process the air becomes cold and dry okay.
So this cold and dry air is again supplied
to the building. This is cycle continuous
okay. So ultimately you see that the building
you have latent and sensible heat transfer
to the conditions space and the air is picking
up these heats. And it is rejecting that heat
to the cooling coil this cooling coil could
be the evaporator of a refrigerant system.
So the refrigerant evaporates and ultimately
this heat is rejected to the ambient at the
condenser okay.
So this is the typical summer air conditioning
system okay. And the psychometric we do the
sub performance psychometric calculations
first to find out what should be the required
mass flow rate of supply air okay. What is
the required mass flow rate and what should
be its temperature and what should be its
humidity ratio okay. So all these things we
have to fix by performing psychometric calculations
right and we also have to know what should
be the required cooling capacity of the coil
okay. How much sensible heat as to rejected
at the coil how much latent heat as to be
rejected at the coil what is the required
bypass factor of the coil and what should
be the a coil ADP okay. That is apparatus
due point okay. So all these things will be
will be obtained by performing the psychometric
calculations okay.
Now let us begin with summer air conditioning
systems first let us take a simple system
with hundred percent re-circulated air okay.
Let me first describe the system.
So as I said this simple system with hundred
percent re-circulated air. That means we are
throwing any air nor we are adding any outside
air. So the flame air is flowing through the
system again and again. So you can see that
is a close system as far as air is concerned
okay. Again you have the sensible and latent
heat loads on the building. This is the i
is the condition space state. And let us say
this s is same as this is the state of the
supply air and the state of the air at the
exit of the condition space is same as the
condition space i okay. And so if you are
assuming that there is no heat transfer in
the duct and if the fan power is negligible
then you find that at this point. That means
at the inlet to the cooling coil also the
conditioned space conditions exist. That means
you have the status i itself. So in the cooling
coil light under goes cooling and humidification.
So at point s you have cold and dry air okay
so when the same cold and dry air so goes
to the building.
So we have a supply fan here you have also
have a return fan okay. Return fan is an option
and as I said all these lines are the ducts
through which the air flows okay. And the
same process is shown on a psychometric chart
here. So you have the dry bulb temperature
here on psychometric chart as you know on
the y axis we have the humidity ratio. And
this as you know is the hundred percent relative
humidity or saturation curve okay. And point
i is the state of the conditioned air in the
inside the building. And point s is state
of the air at the exit of the cooling coil
or at the inlet to the condition space okay.
So as I said cold and dry air is supplied
to the room and the air that leaves the conditioned
space is assumed to be at the same conditions
as that of the conditioned space. The supply
air condition should be such that as it flows
through the conditioned space it can counteract
the sensible and latent heat transfers taking
place from the outside to the conditioned
space. So that the space can be maintained
at required low temperature and humidity I
have explained this already.
Now assuming no heat gains in the supply and
return ducts and no energy addition to the
fans energy addition due to the fans and applying
energy balance across the room the different
loads on the room are okay. So let us look
at the room.
What we are doing is taking a control volume.
Let us say I am taking a control volume okay.
And if you are assuming a steady state and
if you are applying energy balance across
this control volume you find that the rate
at which the sensible heat is entering into
the building must be the rate at which it
is leaving. Because the air flow that means
Qsr should be equal to oh supply air flow
rate into Cpm. That is the moist air, so humidity
specific heat of the humid air into Ti minus
Ts okay. This is the energy balance for the
sensible heat similarly you can perform an
energy balance for the latent part and you
can also perform an energy balance for the
total heat transfer rate okay. So that is
what is done here.
And you get this kind of equations of for
the sensible heat transfer rate this is the
equation Qsr is the sensible cooling load
on the building okay. S stands for sensible
r stands for the room or building that is
equal to ms where ms is the supply air mass
flow rate Cpm as I said is the specific heat
of the moist air Ti is the condition space
temperature Ts is the supply air temperature.
Similarly the latent load on the building
Qlr that is equal to mass flow rate of the
supply air ms into hfg the latent heat of
vaporisation multiplied by humidity ratio
difference wi is the humidity ratio inside
the conditioned space ws is the humidity ratio
of the supply air okay.
So these two are for the sensible and latent
heat loads or co sensible and latent heat
transfers. Similarly you can draw a write
an equation as I said for the total heat transfer
rate Qtr is nothing but sensible heat transfer
rate plus latent heat transfer rate. So that
is nothing but mass flow rate of the supply
air multiplied by the enthalpy raise of the
air as it flows through the conditioned space
here hi is the exit enthalpy of the air and
hs is the inlet enthalpy of the air or supply
air okay. And the room sensible heat factor
as you have seen RSHF is simply defined as
the ratio of the sensible heat transfer to
the total heat transfer okay. Qsr divided
by Qtr.
Now from the RSHF value one can calculate
the slope of the process line undergone by
the air as it flows through the conditioned
space.
That means slope of this line okay. Let us
say the, if this is theta okay, the tan theta
is the slope of this line this we can obtain
if you know the RSHF or the sensible heat
factor this we have discussed in the last
lecture okay.
And we have seen in the last lecture that
the expression for the slope is given by this
value that is tan theta is one by two four
five one multiplied by one minus RSHF divided
by RSHF where RSHF is the room sensible heat
factor which is known to us okay. And one
very important thing to be noted here is that
for the given room sensible and latent cooling
loads. The supply condition must always lie
a on this line. So that it can extract the
sensible and latent loads on the conditioned
space in the required proportions.
That means we know that as I said let me quickly
repeat what I said. We know this conditioned
space state i based on the comfort criteria
this is known to us okay. So we can place
this point on the psychometric chart and the
slope of this line can be obtained. If you
know the value of RSHF okay for the timing
being let us say we know the latent and sensible
heat loads on the building right. So we can
calculate RSHF and from RSHF we can find out
what is slope of the line once you know the
slope of the line and this inlet state I then
you can draw this line okay. This RSHF line
and the supply air condition must lie somewhere
on this line okay. It cannot lie anywhere
else if it is lying anywhere else that, then
it cannot take the sensible and latent heat
loads in the required proportion okay. So
if it has, if the air, this air has to take
this sensible and latent heat loads in this
required proportion then this must always
lie on this RSHF line okay.
So that is okay. Now since condition I is
known say some thermal comfort criteria knowing
the slope one can draw the process line s-i
through i we have already explained. This
the intersection of this line with saturation
curve gives the ADP of the cooling coil we
also know this.
If i extend this line let us say that this
line we are able to draw this line okay. We
are able to draw this line because we know
the slope of the line and we know one point.
So if you draw this line and if you extend
this line where this line extend this saturation
curve that temperature will give you the coil
ADp okay, ADp of the cooling coil.
For case of hundred percent re-circulation
the process that the air under goes as it
flows through the cooling coil. That means
process i-s will be exactly opposite to the
process under gone by air as it flows through
the room that is process s-i okay.
That means what is happening for the air as
it flows to the conditioned space. That means
as it moves from this point to this point
will be exactly opposite to what is happening
to the air as it flows through the cooling
coil okay. That means process i-s will be
exactly equal and opposite to process s-i.
For this special condition of hundred percent
re-circulated air that why you have a single
line here okay. So as the air flows through
the room the process will be like this and
as air flows through the cooling coil the
process will be like this. That means as the
air flows through the condition space air
picks up sensible heat and it also picks up
latent heat that means its temperature increases
and its moisture content increases okay. As
it flows through the condition space and the
same air as it flows through the cooling coil
it its temperature reduces and its humidity
ratio also reduces okay. Because that mean
we have to go for a cooling and dehumidification
process in the cooling coil okay. So as far
as the condition space is concerned it is
heating and humidification. And as far as
the cooling coil is concerned it is cooling
and dehumidification process and if you plot
these two processes on the psychometric chart
they exactly coincide only thing is that the
direction will be opposite okay.
Hence the load on the cooling coil is exactly
equal to the load on the room okay.
That means this Qsr is exactly equal to Qsc
whatever sensible heat is being transferred
to the room as to be rejected at the cooling
coil similarly water latent heat is transferred
to the room as to be rejected at the cooling
coil okay. That means Qsr is equal to Qsc
that is coil load and Qs Q sorry Qlr is equal
to Qlc okay. This is not always valid this
is valid only for this particular system with
hundred percent re-circulated air.
Now we have to fix the supply conditions how
we fix the supply condition the supply conditions
have to be fixed from the equation for different
room loads what are those equations we have
three equations.
One equation for sensible heat transfer one
for latent heat transfer one for total heat
transfer. And if you look at these equations
we know t i we know wi we know hi okay. Because
these are fixed based on your inside conditioned
space requirements. For example based on comfort
criteria okay. So these are known and these
will be estimated by performing cooling load
calculations okay. This we will discuss a
little later for the time being let us assume
that we know these things also for cooling
load calculations okay. So these things are
known to us ti wi and hi are known.
And let us assume that Cpm is also known to
us that is the specific heat is known latent
heat of vapour is also known. So what are
the unknowns are mass flow rate of the supply
air temperature of the supply air humidity
ratio of the supply air and enthalpy of the
supply air. So there are four unknowns ms
ts ws and hs and there are three equations
okay.
Since there are four unknowns and three equations
one parameter as to fixed to fine the other
three unknown parameters obviously. So if
bye pass factor x of the cooling coil is known
then from the definition of bye pass factor
we know that for the cooling coil bye pass
factor is given like this x is equal to ts
minus tADP divided by ti minus t ADP where
ts is a exit temperature of the cooling coil.
That mean exit temperature air at the cooling
coil tADP is the apparatus due point of the
coil ti is the air temperature at the inlet
to the cooling coil okay. So this is based
on the definition of bye pass factor from
this equation you can write ts is equal to
tADP plus x into ti into tADP now x is known
to us it is specified by the manufacturer.
Let us say so this is known to us ti is known
to us tADp we have obtained by extending the
line and from intersection of that RAHF line
with the saturation curves this is also known
to us. So we can find out what is the required
supply temperature okay.
So once we know the supply temperature then
the mass flow rate of supply air is simply
obtained from this equation ms is equal to
Qsr divided by Cpm into ti minus ts ti minus
ti can be written in terms of coil ADP. And
the bypass factor that is ti minus tADP into
one minus x at and the supply air humidity
ratio and enthalpy are obtained from the equations.
For the latent load and the total load okay.
So once we know this is known to us this is
known to us everything is known on this side.
So we can find out what should be the supply
air enthalpy what should be the supply air
humidity ratio okay. So once we know one parameter
that is let us say to the supply air temperature
all other things can be obtained.
Now from the equation it is clear that the
required mass flow rate of the supply air
decreases as the bypass factor x increases.
That means from this equation you can see
that everything is fixed as the bypass factor
x increases this also increases okay.
In the in the limiting case when the bye pass
factor is zero okay. That means on the other
side when the by-pass factor is zero the minimum
amount of supply air flow rate required is
given by this equation. This the minimum amount
of supply air required that is equal to sensible
heat load on the building divided by specific
heat of the moisture multiplied by ti minus
Tadp. So this is the minimum amount of supply
air required okay. Thus with hundred percent
re-circulated air the room ADP is equal to
coil ADP and the load on the coil is equal
to the load on the room okay. This is what
we have to keep in mind okay. Normally this
simple system with hundred percent re-circulated
air is not practical means we always have
to have some kind of some amount of outdoor
air because ventilation is required okay.
So but this is taken first because this is
simple and rest of the systems can be developed
from this system okay.
So now let us look at the system with outdoor
air for ventilation. As I already as I have
already explained in actual air condition
system some amount of outdoor air or fresh
air is required to take care of the ventilation
requirements. The require outdoor air for
ventilation purposes is known from the occupancy
data and the type of the building. For example
hospitals. First of all why do we require
outdoor air as I have already told we require
it for ventilation purpose. That means there
are large numbers of people inside the condition
space. So gradually the oxygen concentration
digests because of the respiration process
that because we breathe in oxygen and we breathe
out carbon dioxide okay. So after sometime
the oxygen level may drop okay. So this requires
a continuous supply of fresh air to take care
of the oxygen requirements more importantly
it is found that the outdoor air is required
to dilute the orders okay. Because a people
continuously emit body orders so so that will
affect the quality of the indoor air okay.
So you have to continuously add some fresh
air which will dilute the orders okay. So
in all actual systems fresh air is required
right now some fresh air is required depend
depends upon how many people are there of
example in the conditioned space okay. Obviously
more the number of the people in the conditioned
space higher will be the amount of ventilation
air required there are several standards which
specify what should be the ventilation air
required for person per second okay. So many
meters per person per second again this also
depends upon the application. For example
in some applications the ventilation requirements
may be high. For example in areas where there
is heavy smoking okay. Then to dilute the
orders of the smoke you have to supply more
amount of fresh air. That means the required
ventilation will be more also required ventilation
will be more in areas like hospitals and all
where the purity of air is very important
okay. In hospitals and all in some cases all
the air is throughout and you continuously
take hundred percent fresh air okay. So what
I am trying to say is the ventilation requirement
depends upon all this data okay.
The occupancy data and type of the building
etcetera okay. Generally this kind of data
is available before we design the air condition
system okay. Because we know what is the occupancy
and we know what purpose the building is begin
used okay. So we can estimate the ventilation
requirement and at the beginning itself right.
So as I said normally either the quantity
of outdoor air required is specified in absolute
values or it is specified as a action of re-circulated
air okay, whatever be the this thing we know
how much is required.
Okay, we let us take a simple case first the
case one is where the bypass factor of the
coil is zero. That means we are talking about
a perfect coil so from the okay. First let
me show the system and then explain the working
principle.
So if you compare this with earlier one we
did not in the earlier system we did not have
this arm okay. It was a hundred percent close
system the same air is being be circulated
but in this system you find that some amount
of air is continuously thrown out and this
is what is known as exhaust air and some amount
of air is continuously added. This is if air
is taken from outside so it as higher oxen
level or it is purified and supplied okay.
And if you look at the mass balance you find
that the exhaust air mass flow rate is always
equal to outdoor air mass flow rate okay.
Because you have to ultimately balance the
mass. So this outdoor air and the re-circulated
air which is assumed to be at the same condition
as that of the conditioned space or mixed
at this point okay.
So what you have at this point is a mixed
air this mixed air flows through the cooling
coil it gets cooled and humidified and it
comes out at the condition s and at this condition,
I am sorry, should be ts ws and hs at this
condition. It is supplied to the conditioned
space. So that it can take care of the sensible
and heat and latent heat loss on the building
okay. Now if you draw this process on the
psychometric chart this condition o is that
of the outdoor air outdoor air is much hotter
and humid air. So you have the, you can see
that this is at this point and a point i is
the conditioned required inside the building
this is i this is o okay. And at this point
this outdoor air and the inside air are mixed.
And let us say that the resultant mixed condition
m is here.
And we have discussed this earlier the resultant
condition will be lying on this on a line
joining these two points and the exact location
of the point depends upon the ratio of mo
to m re-circulated depending upon the at this
point will be vary. Let us for time beginning
assume that we have this point lies at this
point. So at this point it enters the cooling
coil and as the air flows through the cooling
coil it under goes cooling and humidification.
So its temperature drops and its humidity
also drops okay. So this is the humidity drop
and this the temperature drop okay. Since
this is the case with of zero by pass factor
you find that the exit condition s coincides
with the apparatus dew point of the coil okay.
So the exit condition is somewhere here okay.
So the psychometric on the psychometric chart.
The process looks like this okay. So outdoor
air is mixed outdoor and the indoor airs are
mixed then this mixed air flows through the
cooling coil. So process ms is what happens
in the cooling coil okay. And at this point
it enters into the condition space so process
si is what happens in the conditioned space
right.
So again from the room loads and the inside
condition the process line is drawn.
So we can drawn the fulfil assuming that we
know these sensible and latent heat loads.
Let say we know these line load so we can
find out the RSHF once you find the RSHF you
can draw this RSHF line. Because we know the
point i okay. So you know the slope of this
line and you know this point. So you can draw
this line where this line intersects the saturation
curve that is your coil ADP and for zero bypass
factor that is also the supply air temperature.
So that means we all also know this temperature
from this point okay. And once you know this
temperature then we can find out the mass
flow rate of air okay, ms, how you find out
the mass flow rate of air from energy balance
for sensible heat portion. We have seen that
mass flow rate of air is seen nothing but
Cpm into ti minus ts Qsr divided by CPm into
ti minus Ts everything is known to us Qsr
is known, Cpm is known, ti is known, ts is
known. So we can find out the mass flow rate
of supply air once we know the mass flow rate
of supply air then again performing latent
cooling load balance and the total cooling
load balance we can find out the supply air
humidity ratio and supply air enthalpy okay.
So we can fix the state of the air and that
in fight for this simple case once you locate
this point you can directly read this values
from the psychometric chart okay.
So okay, this is already is explained to you.
And from mass balance of air that means at
this point if you do the mass balance at this
point if you are doing the mass balance you
find that m dot o plus m dot re-circulated
is equal to m dot s okay. Because mass is
entering like this mass is coming from this
side and its getting mixed and the total mixed
air is going in this direction.
Where m subscript rc is the re-circulated
air flow rate and m subscript o is the outdoor
air flow rate okay. Since either m subscript
o are the ratio of m mo by mrc are specified
one can calculate the amount of re-circulated
air. Because we have calculated this and from
ventilation requirements we know this. So
you can calculate what this, the re-circulated
air and the amount of air exhausted from the
system is equal to the amount of outdoor air
as I have already explained.
So now we have to find out the coil loads
from energy balance across the cooling coil
okay.
That means now you have to perform energy
balance across the cooling coil. That means
let us say take a control volume across the
cooling coil and apply energy balance across
the cooling coil energy balance means you
can write down an energy balance equation
for the sensible heat load only okay. That
means Q sensible of the coil that is equal
to mass flow rate of supply air into Cp of
the supply air into the temperature difference
temperature difference is nothing but inlet
temperature minus outlet temperature inlet
temperature is tm outlet temperature is ts.
So this multiplied by tm minus ts okay. Similarly
you can write an equation for the latent load
and you can also write an equation for the
total load.
So if do that, then you will find that for
the coil these are the different loads this
is the sensible load this is the latent load.
And this is the total load okay and here as
I have already explained to you m refers to
the mixing condition which is result of mixing
of re-circulated air with outdoor air.
And applying mass and energy balance to the
mixing process.
So this we seen in the earlier lectures how
to apply the when two air streams are mixed
how do we find the exit condition okay. So
here we have mixing of two air streams okay.
So this one air stream and if this second
air stream these two air streams are getting
mixed this air stream is coming from the conditioned
space. At the condition same as that of the
space and this air stream is coming from outdoor
okay. That means this one and this one both
are getting mixed and we would like to find
out what do what should be this point okay.
Where should be this point so that is obtained
by again applying mass and energy balance
across this point okay.
So from mass and energy balance across the
point you can easily show that the ratio of
outdoor air divided by supply air that is
mo divided by ms is equal to wm minus wi by
wo minus wi where wm is the humidity ratio
of the mixed air and wi is the humidity ratio
of the conditioned space wo is the humidity
ratio of the outdoor air this also equal to
the enthalpy ratios okay. And if you are assuming
that the specific heat remains constant this
is also approximately equal to the temperature
ratios okay. And since we know this we also
know this we know this. So you can find out
wm similarly in this equation we know hi we
know ho so we can find out hm similarly we
can know we know ti to you can find out tm
using this equation okay.
And since mo by ms is always zero. Because
you are always supplying some amount of outdoor
air from the above equation it is clear that
wm will be greater then wi so hm will be greater
than hi and tm will be greater then ti okay.
This implies that ms into hm minus hs is always
greater. Then ms into hi minus hs now what
is ms into hm hm minus hs they are nothing
but the load on the cooling coil and what
is ms into hi minus hs that is load on the
room. So you find t hat whenever you are using
some amount of outdoor air the load on the
cooling coil is always higher than the load
on the room okay, load on the conditioned
space this is obvious. Because what you are
doing by adding outdoor air is you are adding
some energy to the system.
And at the same time you are throwing out
some air which is cold and dry I mean compared
to the outdoor air okay. So if you throwing
out some cold air and you are taking in some
hot air so you have to pay some penalty for
this. So this penalty is paid at the cooling
coil by the difference between the loads on
the coil and the building okay.
The difference between the cooling load and
the coil and cooling load on the conditioned
space can be shown to be equal to Qsc minus
Qsr that is equal to mo into Cpm into to minus
ti okay. This is the difference in the sensible
cooling loads of the coil and the room and
this is a latent heat loads of the coil and
the room this is a total heat load. So this
you find that is proportional to the amount
of outdoor air added means also proportional
to the temperature difference or humidity
ratio difference or enthalpy difference okay.
So from the above equation it is clear that
the difference between cooling coil and conditioned
space increases as the amount of outdoor air
increases.
So you can see that as mo increases this difference
also increases or as the outdoor air becomes
hotter and more humid okay. This is the reason
why in air conditioned buildings and all normally.
For example air conditioned theatres and all
they don't allow you to smoke okay because
once people smoke inside the conditioned space
then the required outdoor air will be more
okay you have to supply more amount of ventilated
air or outdoor air to take care the odours
because of the smoke okay. So a more amount
of outdoor air means more amount load on the
cooling coil more amount of load on the cooling
coil means the cooling coil capacity as to
be higher. So initial cost will be more and
running cost also will be more okay. So smoking
etcetera in the conditioned space is not good
from this point of view okay.
The line joining the mixer condition m with
the coil ADP is the process line under gone
by the air as it flows through the cooling
coil the slope of this line depends on the
cooling coil sensible heat factor okay. So
let me just show the this thing.
All I am talking about the, this line okay.
This as we know this line is your RSHF line
this line is nothing but the process line
as the air flows through the cooling coil
process ms okay. This depends upon what is
the ratio of sensible and latent heat loads
at the coil okay. So if you can define what
is known as the coil sensible heat factor
CSHF as the sensible heat load on the coil
divided by total heat load on the coil okay.
So that will give you the coil sensible heat
factor and then you can find out the slope
of this line. So slope of this coil line as
let say that this is theta one. Let us say
then tan theta one is equal to one by two
four five one we can seen this one minus CSHF
divided by CSHF okay. So this we can find
out from the coil loads so you can find out
the slope of this line.
Okay so that is what is given here.
Mow let us look at another case two coil by
pass factor x is greater the zero for actual
cooling coils the bypass factor will be greater
than zero as it okay. Because it is not possible
to have a perfect coil. So you will always
have some amount of bypass factor as result
the air temperature of the exit of the cooling
coil will be higher than the coil ADP and
the process gets modified.
So when you have a bypass factor having a
finite by pass factor of the cooling coil
is equivalent to saying that some amount of
air. That means this amount of air comes in
perfect contact with the cooling coil. And
this amount of air is completely by passing
the cooling coil. So ultimately the mixer
condition you get here is the result of the
air which is completely by passing this and
the amount of air that is completely in contact
with the cooling coil okay. So again if you
apply the mixing rule then you can find what
is the mixed condition okay. This is a system
with by pass factor greater than zero so what
is the change in the psychometric chart.
You find that on the psychometric chart there
is a difference okay, since by pass factor
is greater than zero you find that the exit
condition of the air from the cooling coil
ts is greater then Tadp. because form the
definition of bypass factor we know that x
is equal to ts minus tADP divided by tm minus
tADP okay. m is the inlet condition to the
cooling coil this is greater than zero and
that means ts as to be greater than TADP okay.
So when x is greater than zero the room ADP
will be different from the coil ADP that is
also clear form here.
You can see that the coil ADP is this temperature
room ADP is somewhere here where this is your
room sensible heat factor line okay. This
is you coil sensible heat factor line so the
point where this line intercepts the saturation
curve is your coil ADP and the point where
the room sensible heat factor line intercepts
the hundred percent saturation curve somewhere
here let us say this is your room ADP okay.
This is your room ADP and they are same here
okay.
Normally in actual systems either the supply
temperature ts or the temperature raise of
air as it flows to the conditioned space ti
minis t ts will be specified okay. Based on
some other criteria which we will discuss
little later generally in these values are
specified either the supply temperature or
what is the temperature raise of the air as
it flows through conditioned space that is
ti minus ts okay. These values are specified
not the exact values will be specified. But
the range is given okay for example it generally
specified that ti minus ts should be between
eight to fifteen degrees okay. So you can
choose any equal between eight to fifteen
okay.
So since the supply temperature is specified
one can calculate the required supply air
flow rate and supply conditions okay.
So what I am saying is we know this temperature
this temperature is known to us this temperature
is known to us we also know load on the sensible
load on the building. So we have seen this
equation before this is equal to ti minus
ts so ts is specified ti is known to us Cpm
is known to us this is knows. So you can find
out what is ms similarly you we have seen
the other equations sensible heat load of
this thing is hfg into wi minus ws so ms is
calculated hfg is known to us wi is known
to us ws can be obtained. So you can fix this
condition also okay.
One can draw the line i-s from the known values
of supply temperature and RSHF the intersection
of this line. As I have already told you with
the saturation curve gives the room ADP condition
of air after mixing that is point m is obtained
from known values of ms and m naught using
mixing rule okay. So this again I have explained.
This condition okay, this in the mixing rule
because we know the air flow rate and we also
know this two conditions. So we can find out
this point.
Joining points m and s gives the process line
of air as it flows through the cooling coil
the intersection of this line with the saturation
curve gives the coil ADP.
If the coil ADP and coil by pass factor are
given instead of the supply air temperature
then a trial and error method has to be employed
to obtain the supply air condition okay. Instead
of specifying the supply air temperature if
the coil by pass factor is given then we have
to go for a trial and error method okay. Because
all that we can do is to find the room ADP
and draw the room sensible heat factor line
okay. We do not know what is the supply air
temperature okay.
But the coil ADP is specified coil by pass
factor is specified okay. So can put the draw
the coil ADP or locate the coil ADP on the
psychometric chart and using the bypass factor
and using the trial and error method we can
fix the supply condition okay. The system
described. So for is required for applications
where the sensible heat load is dominant.
That means your room sensible heat factor
is greater than about point seven five when
the latent load on the building is high due
either to high outside humidity or due to
large ventilation requirements. For example
in hospitals or due to high internal latent
loads. For example your conditioned space
consists of a kitchen or a laundry then the
simple system discussed above leads to very
low coil ADP okay. So what are the systems
we have been discussing so for that is fine
as long as your sensible load is more than
three times that of the latent load on the
building. That means your room sensible heat
factor is greater than about point seven five
which is normally the case.
But there are some special cases where the
latent load on the building could be quiet
high okay. So some of the examples are where
the outside conditions are highly humid okay.
For example in coastal areas or where there
is lot of internal latent heat load. As I
said,
for example you have a laundry or kitchen
inside the condition space a lot of moisture
is continuously added okay. So under ofr these
conditions you find that the room sensible
heat factor will be low because the latent
heat load is high okay. Low means it could
be lower than point six or so okay, for these
special circumstances if you are using the
simple system you find at the required coil
ADP. That means required surface temperature
of the coil will be very low now the surface
temperature of the coil is connected to the
evaporated temperature of the system okay.
Since it is connected to the evaporated temperature
of the system refrigerant system as to be
operated at very low evaporated temperature
okay. And as you know when you operate the
refrigerant system at low evaporative temperature
the COP of the system reduces okay. That means
you are running cost of the system goes up
okay. So normally of high latent heat load
applications some modifications are done.
So that you get reasonably high COP of the
refrigerant system okay. So one of the modifications
is what is known as the system with reheat
coil.
Okay, so low coil apparatus due point indicates
operation at refrigerant system at low evaporator
temperatures these all reputation of whatever
I have said operating the system at low evaporative
temperatures decreases the COP of the refrigerant
system. Hence a reheat coil is sometimes used.
So that the cooling coil can be operated at
relative high ADP and at the same time. The
high latent load can also be taken care of
okay. I will explain this system shortly.
When a system with reheat coil air is first
cooled and dehumidified in the cooling coil
and is then reheated sensibly to the required
supply temperature ts using the reheat coil
okay.
So that means a first let me explain the system
hardware. So you again have the conditioned
space here where we require ti and wi this
is supply state here we require this much
amount of air flow rate at this temperature
and this humidity in this enthalpy. At this
condition this is supplied to the conditioned
space and f again we have the sensible heat
load on the room latent heat load on the room
okay. So we have the cooling coil here in
addition to the cooling coil we have also
a heating coil okay. So this is the heating
coil okay. So first what is done here is the
air is first cooled and humidified in the
cooling coil. Then it is sensibly heated in
the re heat coil okay. So first you cool the
air and remove the moisture in this process
that means it is a cooling and dehumidification
process. And once you remove the moisture
then what do you is you sensibly heat the
air by passing it through the reheat coil
so that you get the required supply air condition
okay. So this is shown on the psychometric
chart like this.
Okay, let us begin at this process we have
the indoor air and outdoor air both are mixed
and we get a mixed condition this mixed condition
flows through the cooling coil cooling and
dehumidification coil okay. This is cooling
and dehumidification coil in this process
you can see that the required amount of moisture
is extracted okay. So this is the amount of
moisture to be taken out. So that is achieved
in this process but you find that this temperature
or this point does not lie on the RSHF line
okay. If this is this is your RSHF line and
this RSHF will be quite low because the high
latent load let us this less then point six
okay. Since this point does not lie on this
line if you leave air at this condition with
conditioned space it cannot take the required
amount of sensible and latent heat loads.
So okay, somehow you have to bring this point
on to the room sensible heat factor line okay.
So what for that purpose what is done is at
this point air flows through the reheat coil
and in the reheat coil it is sensibly heated
okay. So in the reheat coil the temperature
increases from tc to ts. So that the point
s lies on the room sensible heat factor line
so that it can take care of the latent and
sensible and heat loads okay. Since it is
sensible process tc will be lower then ts
okay and wc will be equal to ws okay. So no
change in the humidity ratio.
And we have to find out now the capacity of
the reheat coil the capacity of the reheat
coil is obtained from energy balance across
the coil very simple.
All that you have do is take this as a control
volume and perform the energy balance. So
okay, from that you find that only sensible
heat transfer is taking place. So then reheat
coil Qr is equal to mass flow rate of the
supply air into specific heat of the air multiplied
by the temperature difference ts minus tc
okay.
So Q reheat is given by ms into hs minus hc
since this sh a pure sensible heat transfer
process hs minus hc is written as Cpm into
ts minus tc okay. So with reheat the system
operates at higher coil ADP that means you
can get expect high COP of the refrigerant
system how do the required mass flow rate
increases okay. So the benefit of the cool
reheat coil is that you can operate your refrigerant
system at a higher evaporated temperature.
But there is also a disadvantage. Because
first disadvantage we are unnecessarily cooling
the air to a lower temperature and then you
are heating it so energy is required first
for cooling and energy is also required for
heating okay. It is actually wasteful use
of energy second thing is that because of
this reheating process the temperature difference
ti minus ts reduces okay. Once ti minus ts
reduces the required amount of supply air
increases. These are the two disadvantages
of the reheat coil okay. And the benefit is
higher COP so ultimately in an actual system
one as to perform calculations and see whether
reheat coil is justified or not okay.
And if possible energy for reheat can be obtained
from a waste heat source such as condenser
okay. That means instead of using let us say
an electrical heater for other reheat coil
we can also use a waste heat source one of
the waste heat sources is the heat rejected
at the condenser okay. If it is possible you
can use that heat for heating the air in the
reheat coil okay. That is how you can reduce
the, a energy consumption okay. So at this
point is stop it and let us conclude.
What we have learned in this lecture in this
lecture the following topics are discussed
selection of outdoor design conditions psychometric
of summer air conditioning systems with and
without outdoor air and systems with reheat
okay. We will continue this lecture in the
next class okay.
Thank you.
