Welcome back in the last few lectures. I have
discussed how to estimate heat transfer through
buildings like through fake walls through
fenestration and heat transfer due to infiltration
ventilation etcetera. So in this lecture I
shall discuss the methods of estimation of
cooling and heating loads on a particular
building and how to estimate the required
cooling or heating capacities using the information
provided in the last lectures okay.
So the specific objectives of this particular
lecture are to discuss estimation of internal
and external sensible and latent cooling,
loads on the building estimation of required
capacity of the cooling system difference
between cooling and heating load calculations.
At the end of the lecture you should be able
to estimate sensible and latent cooling loads
on the buildings due to external and internal
sources using CLTD SHGF SC and CLF tables
and design data and building specifications
and estimate cooling capacity of the system
from the above information and differentiate
between cooling and heating load calculations
and finally estimate heating loads assuming
steady state conditions.
So let me give a brief introduction. Heating
and cooling load calculations involve a systematic
stepwise procedure to estimate the required
system capacity considering all the building
energy flows. As you know cooling load calculations
are carried out to estimate heat gain during
summer and to find out the required cooling
capacity of the system. Whereas heating load
calculations are carried out to estimate heat
loss in winter and find out the required heating
capacity and load calculations are carried
out based on a set of known input data.
Heating load calculations can be carried out
assuming steady state conditions as the peak
heat load occurs before sunrise and outside
temperatures remain fairly constant during
winter months. And also you can neglect internal
heat sources while estimating the heating
loads. Because these internal heat sources
partly offset the heat losses. However for
cooling load calculations you must consider
the unsteady state. Because of the presence
of sun and varying outdoor conditions and
we also have to consider the internal heat
sources. As they increase the heat gain as
a result you find that the cooling load calculations
are invariably more complicated than heating
load calculations. So in this lecture I shall
first discuss in detail the cooling load calculations
and the procedure for heating load calculations
are almost similar and it is much simpler
than this okay.
Now let us look at the methods of estimating
cooling loads. The easiest method is what
is known as based on rules of thumb using
CLTD and CLF method using transfer function
method etcetera. First let us look at rule,
rules of thumb. For example this particular
table shows the required cooling capacity
for various applications. For example you
can see here that for an office building and
if it is an external zone and if it has twenty-five
percent glasses the required cooling capacity
is about three point five TR okay. For thousand
square of floor area okay, thousand feet square
of floor area.
If the percentage of glass is fifty percent
then the required cooling capacity is about
four point five tonnes. And so on, similarly
for other applications, for example, computer
rooms the required cooling capacity lies in
between six to twelve tonnes per thousand
feet square of floor area. Similarly for other
applications like hotels, bed rooms, departmental
stores, shops, banks, theaters etcetera okay.
So these are based on the long years of experience
and these values are arrived at as I said
a many years of experience okay.
So this at what is known as estimating the
cooling capacity based on rules of thumb.
You will notice that rules of thumb they are
very useful for preliminary estimation however
they are not recommended as they do not consider
design aspects of the specific building okay.
So rules of thumb are used only for preliminary
estimation not for final calculations the
CLTD CLF method which is basically suggested
by ASHRAE it is widely used as, it is simple
and it also consumes less time. The transfer
function method is more accurate but it is
more time consuming hence it is generally
used for large commercial buildings. In this
lecture I shall discuss mainly the CLTD CLS
method.
So cooling load calculations based on CLTD
CLF method. The cooling load experienced by
a building varies in magnitudes it does not
remain constant. It varies in magnitude and
the variation can be from zero. That means
no cooling is required to a maximum value
and the design cooling load is a load near
the maximum magnitude but is not normally
the maximum okay. It is near the maximum but
not the maximum design cooling load takes
into account all the loads experienced by
a building under a specific set of assumed
conditions okay. So what are the conditions
based on which the cooling load calculations
are carried out. First the designs outside
conditions are selected from a long term statistical
database.
The conditions will not necessarily represent
any actual year but are representative of
the location of the building next the load
on the building due to solar radiation is
estimated for clear sky conditions. That means
we do not take into account clouds okay. The
third point the building occupancy is assumed
to be at full design capacity and finally
we assume that all building equipment and
appliances are considered to be operating
at a reasonably representative capacity. So
these are the conditions based on which we
carry out the cooling load calculations and
each and every element of the building that
contributes to the building energy flow must
be considered for load calculations. So we
have to take into account all the elements
of the building.
The total building cooling load consists of
external loads and internal loads. Again both
external and internal loads consist of sensible
as well as latent components. Let me show
this with the help of a schematic, for example,
what is shown here is a typical building.
So it is subjected to solar radiation. So
heat transfer takes placed to the building
because of solar radiation through opaque
surfaces through fenestration okay. Similarly
heat transfer also takes place from the ground.
Heat transfer takes place due to infiltration.
Infiltration heat transfer consist of both
latent as well as sensible okay, latent plus
sensible these are the external loads. In
addition to this we also have internal heat
sources for example the people inside the
condition space they add load to the building.
Okay.
So these are what is known as internal heat
sources in addition to people we may also
have several appliances equipment etcetera
okay. So all these constitute internal heat
sources. So you can see that a building is
subjected to external loads as well as internal
loads. Buildings in general may be either
externally loaded or internally loaded. So
what do we mean by externally loaded building
or an internally loaded building. In externally
loaded buildings the cooling load on the building
is mainly due to heat transfer between the
surroundings and the internal condition space
since the surrounding conditions are highly
variable. For example outside solar radiation
outside temperature varies widely in a given
day the cooling load of an externally loaded
building varies widely okay.
So this is the typical characteristic of an
externally loaded building in internally loaded
buildings. The cooling load is mainly due
to internal heat generating sources such as
occupants or appliances or processes since
the load does not depend very much on the
highly variable outdoor conditions. The cooling
load of an internally loaded building does
not vary widely for example consider a theatre
okay. A theatre can be treated as an internally
loaded building because the heat generation
due to the occupancy inside the building generally
is much higher than the external loads. So
you find that irrespective of the outside
conditions the load on the building remains
more or less constant okay, which depends
upon the occupancy right. So the knowledge
of whether the building is externally loaded
or internally loaded is essential for effective
system design it helps if you know before
and whether it is externally loaded or internally
loaded.
Now let us look at estimation of external
loads. First we take, as I said we have to
consider all the elements. First we take heat
transfer through opaque surfaces. Opaque surfaces
means all the walls roof floor doors etcetera
okay. So the heat transfer rate through this
opaque surfaces is sensible heat transfer
only. You do not have any latent component
and the heat transfer rate is given by Q is
equal to U into A into CLTD where U is the
overall heat transfer coefficient of that
particular element. A is the area of that
particular element and CLTD is the cooling
load temperature difference as we have seen
in the last lecture. And for sunlit walls
and roofs CLTD has to be obtained from CLTD
tables. This we have discussed in the last
lecture how to estimate the cooling load temperature
differences.
So using the tables let me show a typical,
l here, for example this particular table
shows the CLTD values in degrees centigrade
or degrees Kelvin for a D type vertical wall.
I have defined what is a D type vertical wall
in the last lecture okay. So you can see here
that here the cooling load temperature difference
is given at different solar times okay. Starting
with seven o clock in the morning to about
eight o clock in the evening and for different
orientations of the wall. For example if it
is north facing, north east facing, east facing,
south east facing etcetera okay. And you can
see here that for a particular orientation
the cooling load temperature difference varies
with time. For example for a north facing
wall it increases okay it reaches the maximum
of eleven degrees at about eight pm okay.
Whereas for a north east face wall again the
temperature difference varies and it reaches
peak between five to seven right and for an
east facing wall the peak is occurs around
noon right. Similarly for other orientations
and in this table what is given is CLTD maximum
is also given okay. For a particular orientation
what is the maximum value of a cooling load
temperature difference? Now if you want to
estimate the heat transfer rate through the
building walls okay, a building may consist
of four walls. Let us say and I would like
to find out what is the, and let us say that
all these four walls are external walls. That
means they are all expose to outdoors that
means there sunlit walls and they are also
exposed to outdoor air. So if you, I would
like to find out what is the total heat transfer
through all these walls.
As you seen have from the table the CLTD values
for a particular orientation varies with solar
time and it reaches a maximum at a particular
time for a particular orientation and the
maximum CLTD value for all walls does not
occur at the same time, obviously for east
facing wall it occurs much earlier compared
to a west facing wall. So it is generally
advisable to calculate the heat transfer rate
at different times. For example start the
calculation at eight am let us say. So at
eight am I find out what is the cooling load
temperature difference for all the sunlit
walls and multiply that into UA of the respective
wall and find out the total heat transfer
rate through all the walls.
Then I also do this calculations at nine am
at ten am like that okay. Like that I continue
the calculation may be till six or seven pm
in the evening. Then I add up the total heat
transfer rates. That means I find out what
is the total heat transfer rate at eight o
clock in the morning nine o clock in the morning
three pm three pm four pm like that okay.
And obviously these values will be different
for different times. So what I have to do
is, I have to select the maximum value for
a fixing the system capacity okay. The maximum
value not necessarily occurs when all of them
are maximum okay. Because all of them do not
reach maximum value at the same time okay.
So generally it is, you have to prepare some
kind of a spread sheet okay. And calculate
for east facing wall eight am nine am ten
am, what is the heat transfer rate for west
facing wall, what is the heat transfer rate
at different times add up and see what is
the maximum heat transfer rate and that is
taken as the design cooling load on the building
through the walls okay.
So this CLTD values and CLTD tables have to
be used for all sunlit walls and roof and
how about other elements. For example which
are not sunlit for other elements which are
not sunlit or which have very small thermal
capacity for such as doors windows internal
walls floor etcetera. The CLTD is simply equal
to the temperature difference across the element.
So here you do not have to consider the solar
radiation aspects etcetera. Because either
they are not sunlit or they have very small
thermal capacity hence they do not store much
energy okay. So for these elements the heat
transfer rate is simply is equal to UA into
temperature difference. For example for an
external door Q is equal to U into A into
T out minus Ti where T out is the design outdoor
temperature and Ti is the design indoor temperature
okay. That is how you have to find out heat
transfer rate through all the opaque elements
of the building.
Next comes heat transfer rate through fenestration.
As I said is a transparent surface such as
windows etcetera for heat transfer through
fenestration the, this includes heat transfer
by conduction due to temperature difference
across the window and also heat transfer due
to solar radiation through the window. So
it consists of two components again this is
sensible in nature okay. So for heat transfer
by conduction is simply given by Q is equal
to U into A into T naught minus Ti where U
is the overall heat transfer coefficient for
that particular window and A is a surface
area of the window and T naught and Ti are
the design outdoor and indoor temperatures.
This is only one part the second part is heat
transfer due to solar radiation. This is given
by Q is equal to A unshaded into SHGF max
into SC into CLF this these aspects. We have
discussed in the last lecture in this particular
expression A subscript unshaded is the unshaded
area of the window okay.
If it has any external shading devices such
as overhang, so to find out the unshaded area
and you have to use that area not the total
area and SHGF max is the solar heat gain factor
maximum. Solar heat gain factor SC is the
shading coefficient which takes into account
that different types of glass and also any
internal shading devices such as curtains
etcetera. And CLF is what is known as cooling
load factor. And we have seen in our earlier
lecture that the values of SHGF max and shading
coefficient FC are available in the form of
tables. Let me show typical tables which I
have already shown.
Earlier, for example this particular table
gives the maximum solar heat gain factor for
sunlit glass located at thirty two degrees
north latitude and the units are watt per
meter square okay. Here the maximum value
is given for different months for December
from the starting from January to December
for different orientations of the window okay.
For example if the window is north facing
or if it is in shade these are the values
to be used okay. And for other orientation
these are the typical values to be used if
you are carrying out the calculations for
summer typically we consider these months
either from May to July okay. So you have
to take either one of these two values depending
upon where the design temperature is likely
to be maximum okay. This is a table for SHGF
max and such tables for different latitudes
are available in ASHRAE hand books.
Next table shows the shading coefficient values.
As I have already said the shading coefficient
considers the type of the glass okay. Whether
it is a single regular glass or whether it
is heat absorbing glass or double glass right
and for different thicknesses. And if they,
if you do not have any internal shading this
is the shading coefficient value and if you
have any internal shading such as initial
blinds or roller shades these are the shading
coefficient values to be used okay. So this
is as for as your shading coefficient and
SHGF max is considered then what is cooling
load factor.
The cooling load factor CLF accounts for the
fact that all the radiant energy. That enters
the conditioned space at a particular time
does not become a part of the cooling load
instantly. Radiation heat transfer introduces
a time lag depending upon the dynamic characteristics
of the surface. So what happens when radiation
enters into a conditioned space? Let us say
that you a window expose to the outside and
solar radiation enters into the conditioned
space through the window okay. Since it is
radiation the air inside the building does
not absorb all these radiation okay. Only
a small fraction of this is absorbed by the
air most of it is absorbed by the surrounding
surfaces. For example the walls floor roof
if any furniture is there that also absorbs
its radiation. So first the radiation is absorbed
by the walls and the other surfaces and not
by the air okay.
So as a result when radiation first enters
into the conditioned space the conditioned
air temperature does not increase immediately
okay. So what happens is first all this surfaces
absorb solar radiation as a result their temperature
increases depending upon their thermal capacity
okay. When the air temperature goes beyond
the conditioned space temperature then there
will be heat transfer by convection from the
internal surfaces to the conditioned air only
when this a heat is transferred to the conditioned
air then only it becomes a part of the cooling
load on the building not when it enters into
the building okay. So that means you can see
that there is a time lag that time at which
the solar radiation enters into the building
and the time at which it is transferred to
the air okay.
So this time lag is coming because of the
radiation factor okay. So this time lag is
taken into account by introducing a factor
called as cooling load factor okay. The radiation
may also introduce a decrement for example
if the surrounding surfaces are absorbing
the radiation and a part of it may be transferred
to the outside air not to the inside air okay.
So all these aspects are clubbed into a single
factor called as cooling load factor if you
do not consider cooling load factor or if
you take the cooling load factor as one that
means you will be over estimating the heat
transfer or over estimating the cooling load
okay another peculiar aspect of this radiation
heat transfer is that even when the source
of radiation is removed still you feel the
effect of the source. That means even when
sunsets still inside the building it will
be warm okay because of the time lag okay.
So this is the typical characteristics of
radiation now due to the time lag the effect
of radiation will be fell even when the source
of radiation in this case the sun is removed
okay. As I have already explained to you the
CLF values for various surfaces are available
in the form of tables okay. So these tables
have been obtained from experimental measurements
etcetera.
So for example this shows a typical table
for cooling load factors for glass with interior
shading and this is applicable to north latitudes
okay. So you can see that for glass with interior
shading and for north latitude okay. So here
again you can see that the CLF these are all
the CLF values okay, you can see that everywhere
it is less than one right and this is the
function of the solar time okay. So it varies
with solar time and at also varies with the
orientation of the window okay. If you are
calculating radiation heat transfer just like
CLTD what you have to do is you have to calculate
heat transfer rate at different times. Let
us say at nine am ten am eleven am etcetera
okay. Then you have to calculate different
times for all the windows that means windows
in different orientations. And then you have
to calculate what is the total heat transfer
rate due to solar radiation at nine am ten
am eleven am etcetera. And you have to select
the maximum value okay for system capacity
estimation.
Okay. As I said this kind of tables are available
in several handbooks such as ASHRAE handbooks
and all for different conditions okay. For
example as I said this is with interior shading
okay. Without interior shading you will have
different values for cooling load factors
for glass okay.
Next comes heat transfer due to infiltration.
Heat transfer due to infiltration consists
of both sensible as well as latent components.
When outside air enters into the building
it brings along with it both sensible heat
as well as latent heat in the form of moisture
okay. So due to due to ventilation as well
as infiltration you have sensible and cooling
loads being added to the building okay. So
we have to find out what is the amount of
sensible heat transfer to the building because
of infiltration and we also have to find out
what is the latent heat transfer to the building
because of infiltration okay.
The sensible heat transfer rate due to infiltration
is given by Q subscript s infiltration is
equal to m m dot subscript o into Cpm into
T naught minus Ti which is a, which is written
in terms of volumetric flow rates. This is
your volumetric flow rate. So many meter cube
per second okay. Multiplied with density of
the air Cpm is the specific heat of the moisture
and T naught and Ti are the outdoor and indoor
design temperatures okay.
So if you know the design outdoor and indoor
temperatures and if you know what is the infiltration
rate you can find out what is the heat transfer
rate sensible heat transfer rate due to infiltration.
Similarly the latent heat transfer rate due
to infiltration is given by this expression.
This is a latent heat transfer rate due to
infiltration which is equal to infiltration
mass flow rate of infiltrated air multiplied
by the latent heat of vaporization for water
multiplied by the humidity ratio difference
where this is the outdoor humidity ratio,
this is the indoor humidity ratio okay. Again
this is written in terms of the volumetric
flow rates right. So if you know the infiltration
rate then you can easily calculate what is
the heat transfer rate due to infiltration.
Of course the main problem is how to estimate
the infiltration rate.
The infiltration rate is obtained by either
the air change method or the crack method
okay. This also I have discussed in the last
lecture in one of the lectures infiltration
by air change method is given by this formula
infiltration rate okay, in meter cube per
second right is given by ACH multiplied by
V divided by three thousand six hundred where
ACH is the number of air changes per hour
and V is the gross volume of the conditioned
space in meter cube okay. Since air changes
are per hour where we have to divide by three
thousand six hundred to get meter cube per
second okay.
And these ACH values it is observed that the
air change values vary from point five ACH
for tight and well sealed buildings to about
two for loose and poorly sealed buildings
okay. And it can be as low as point two for
modern buildings which are very tight okay.
The modern buildings are generally designed
not to allow any outdoor air that means they
have very small infiltration rates okay. So
knowing the infiltration rate from air change
method from air changes you can calculate
what is the heat transfer rate due to infiltration
of course how do you decide whether the building
is well built tightly sealed or what should
be the value of ACH to be used okay. Generally
the value of ACH depends upon the condition
of the building the building is old okay.
And if it not poorly if it is not properly
sealed that means windows and all there will
be lot of air gaps then obviously the ACH
value will be higher okay. So you have to
use some experience in choosing a proper value
for ACH okay the next method.
As I said is what is known as infiltration
rate by the crack method okay. So this is
given by again the infiltration rate here
this is the infiltration rate in meter cube
per second this is equal to A into C into
delta P to the power of n okay. And it is
in meter cube per second what is A? A is what
is known as effective leakage area of the
cracks okay. The units are meter square and
C is the flow coefficient which depends upon
the type of flow that means type of the air
flow through the cracks okay. So depending
upon the type of flow the value of C varies
and n is an exponent which again depends upon
the type of the flow okay.
And its values lie between point four to one
okay. And what is delta P delta P is nothing
but that pressure different between outdoor
and indoors. So delta P is equal to P naught
minus Pi where P naught is the outside pressure
and Pi is the inside pressure and it is seen
that delta P is equal to summation of pressure
difference due to stack effect pressure difference
due to wind effect and pressure difference
due to building pressurization okay. Delta
P this is particular this is that pressure
difference build due to building pressurized.
That means if the building is pressurized
there will be a pressure difference okay.
And I have explained what do you mean by stack
effect and wind effect and last in one of
the lectures okay, stack effect occurs due
to temperature difference between the indoors
and outdoors. So these also known as chimney
effect okay. Because the temperature difference
there will be some buoyancy effect and because
the buoyancy there will be a mass transfer
between the outdoors and indoors okay. Air
enters or leaves because of this and the wind
effect when wind is blowing over the building.
That means outside this also introduces a
pressure difference okay.
So this is a pressure difference due to wind
right so the total pressure difference between
the conditioned space and the outdoors is
a summation of pressure difference due to
stack effect pressure difference due to wind
effect plus building pressurization okay.
And semi empirical methods are available for
estimating delta P stack and delta P wind
actually the estimation of delta P due to
stack effect and delta P due to wind effect
analytically can be very complicated okay
because this depends up on the construction
of the building okay. And direction of the
wind also may vary and wind is highly variable
okay. So analytical estimation of delta P
because of these two factors can be quite
complicated okay. However semi empirical methods
are available using which one can estimate
the pressure difference because of these two
effects right once you know the pressure difference
and once you know what is the type of flow
then you can calculate what is the infiltration
rate of course you also have to know what
is the area of the crack which is again difficult
okay.
So what is done in practice is for different
types of windows doors buildings etcetera.
The infiltration rates have been measured
okay, as a function of temperature difference
as a function of the wind velocity etcetera
and they are available in the form of tables
okay. So knowing the outdoor and indoor temperatures
and knowing the wind velocity one can estimate
the infiltration rate depending upon the type
of the window okay. That is what is generally
done while estimating infiltration rates.
Next in addition to all these factors if the
cooling coil has a positive by-pass factor
then some amount of ventilation air directly
enters the condition space in which case it
becomes a part of the building cooling load.
Let me explain this, what is shown here is
a, an air conditioning system right. So you
have the conditioned space here and we are
right now we are finding out what is the total
heat transfer rate to the conditioned space
okay. Sensible as well as latent and ventilation
if you if you remember I have discussed this
earlier you some amount of ventilated air
is required for indoor air quality. So generally
what is done is this outdoor air is mixed
with some amount of re-circulated air then
it is processed in the cooling coil and the
processed air is supplied to the conditioned
space okay.
But normally all cooling coils will have some
by-pass factor okay. This by-pass factor means
in general greater than zero and it will be
less than one okay. So because the by-pass
factor what happens is some amount of outdoor
air does not flow through the cooling coil
but it by-passes the cooling coil and directly
enters into the building okay. So it is not
rejecting, its heat to the cooling coil but
it is rejecting its heat to the conditioned
space in which case it becomes a part of the
cooling load of the building not a cooling
load on the coil okay. I will show the different
between the load cooling load on the coil
and cooling load on the building in the later
slide right but you have to keep in mind that
if you have a non zero by-pass factor you
must consider that while estimating the load
on the building okay.
So how do we estimate the latent and sensible
heat transfer rates because of this bypassed
ventilated air. So it is very simple the sensible
and latent loads due to the by-pass ventilation
air are obtained using the values of ventilation
rate by-pass factor and using expressions
similar to that of infiltration. All that
you have to do is you have to replace the
infiltration rate with this quantity okay.
You have to replace it with this quantity
and this quantity is nothing but the, a flow
rate of air bypassed ventilated air okay,
that is what you have to do.
In addition to this if the supply duct consists
of supply air fan with motor then power input
to the fan becomes a part of the external
sensible load on the building okay. You have
seen that normally a supply duct consists
of a fan and supply duct even though it is
insulated it is not possible to perfectly
insulate the supplied duct. So the inside
temperature, that means the temperature of
air inside the supply duct will be much less
than the temperature outside okay. So as a
result there will be some heat transfer from
the outside to the inside air through the
insulation okay. This is sensible heat transfer
in addition to that if the supplied air duct
has some leakages through which air is escaping
okay. This also a loss right and this becomes
a part of the external load on the building.
So these factors also have got to be considered
while calculating the external load on the
buildings okay. How do we consider this the
problem here is that since the power input
to the fan is not known before hand what we
normally do is we assume that the supply fan
adds about five percent of the room sensible
cooling load. Because initially we do not
know what is the power input of the fan right.
Because we have not yet selected the fan how
do we select the fan we select the fan depending
upon the flow rate. So at this moment we do
not know what is the required air flow rate.
Because we have not yet estimated the total
load on the building right but the fan power
is the part of the building load okay.
So what is normally done is we initially we
assume that the fan adds about five percent
of the total cooling load on the building
okay. So you take multiply the building cooling
load by one point zero five okay, where point
zero five takes into account the heat added
due to the fan right and you do the regular
load calculations and at the end when you
know what is the supply air flow rate then
you can select a fan and then again you can
refrain this value by taking the actual fan
power conjunction to account this is what
is generally done okay. Similarly this is
what I have mentioned similarly a safety factor
is provided to account for leakage losses
through supply duct which are part of the
external load okay. Again external leakage
losses through supplied duct this also difficult
to estimate at this point because we do not
know what is the size of the duct.
So we will see in a later lecture that the
size of the duct has to be decided at depending
upon what is the required flow rate okay.
Since we do not know the flow rate at this
point we can at find out what is the loss
though the supply duct. So what we do is we
add or we take a safety factor which will
account for heat losses through the supply
duct and which can be again defined in the
end when you have all the other values okay.
Now we have seen all the external loads. Now
let us look at internal loads how do you estimate
cooling load due to internal heat generating
sources what are the internal heat generating
sources first internal heat generation source
is the occupant himself okay. So what is the
load due to occupants as we know the load
due to occupants consist of both sensible
as well as latent components okay. Because
sensible heat transfer because of heat transfer
between the human body and the surrounding
air latent heat transfer because of evaporation
and respiration from the body okay.
So sensible load due to occupants is given
by number of people multiplied by sensible
heat gain per person multiplied by CLF okay.
Where CLF is the cooling load factor why are
we using cooling load factor here we are you
we have to use strictly speaking we have to
use cooling load factor because the heat transfer
rate sensible heat transfer rate from a human
body to the conditioned air takes place by
convection as well as radiation okay. You
also have a radiation here the moment you
have a radiation again it does not become
an instantaneous load on the building okay.
Again there is a time lag so strictly speaking
if you want to find out what is the sensible
heat transfer rate from the occupants you
have to consider the CLF for the occupants
cooling load factor just like radiation through
fenestration okay.
Next comes latent load due to occupants latent
load due to occupants is given by number of
people in the occupied space multiplied by
latent heat gain per person okay. Here you
do not use any cooling load factor because
the latent load is an instantaneous load because
the moisture is instantaneously added to the
surrounding air and it becomes an instantaneous
load. So you do not have to use any CLF value
okay. And typical values of heat gain from
occupants and CLF values are available in
the form of tables okay. Let me show a simple
table.
This table here shows what is the total heat
gain per person okay. As a function of activity
right for different activities okay. So this
is the activity this is the total heat gain
per person right seventy watts per person
hundred watts per person like that and out
of this what fraction of it is in the form
of sensible heat and what fraction is in latent
heat okay. For example if the person is sleeping
that means if the occupied space consist of
people who are sleeping then heat gain per
person is given by seventy watts and out of
this seventy watts seventy-five percent is
in the form of sensible heat and twenty-five
percent is in the form of latent heat and
if the occupants are seated okay.
Then they release about hundred watts of heat
per person and out of these sixty percent
is in the form sensible heat and forty percent
in the form of latent heat. Similarly for
different activities for example the conditioned
space has people who are walking at a rate
of three point five kilometer per hour then
they release much larger amount of heat because
of higher activities. So you find that bout
three naught five watts per person is released
and out of these only thirty-five percent
is sensible and sixty-five percent is latent
okay. Similarly for other activities for example
industrial work lot of heat is released about
three hundred to six hundred watts per person
and out of it only thirty five percent is
sensible and sixty five percent is latent
okay.
This kind of information for a wide variety
of activities are again available in air conditioning
data hand books okay. And month in year you
must keep in mind is that the fraction of
the total heat gain that is sensible that
means this factor okay. This depends very
much on the conditions of the indoors okay.
If the indoor temperature increases then the
sensible heat gain fraction decreases and
latent heat gain fraction increases okay and
vice versa. And again these kinds of information's
available what is the sensible heat gain fraction
as a function of the indoor conditions okay.
That information also available in hand books
right next comes the CLF value for occupants
CLF value for occupants are also available
in the form of tables and this depends on
the hours after the entry of the occupants
into the conditioned space the total hours
in conditioned space and the type of the building
okay.
So depending upon all these factors the CLF
values have been obtained you will appreciate
the use of CLF factor we know from common
experience that let us say that you have a
theatre okay. And lot of people is there in
the theatre and surrounding air becomes warm
okay. So even when all the people leave the
theatre still the temperature inside the occupied
space continues to increase. Because of the
time lag right this is because the fact that
the human beings release heat sensible heat
in the form of radiation as well as convection
okay. So all that radiation portion is absorbed
by the building surrounding building and it
is slowly released okay. So as a result you
feel the effect even when there are no occupants
okay.
So this factor is taken into account by the
CLF factor okay. In some of the air conditioning
load estimation methods the CLF is simply
taken as one. That means they do not consider
the radiation effect. So if you do not consider
the radiation effect and take CLF as one normally
you will be slightly over estimating the required
cooling capacity okay. Of course if you do
not have any information it is always better
to take a CLF value of one. So this is a heat
load due to occupants.
Next comes load due to lighting adds sensible
heat to the conditioned space and we know
very well that the heat added by lighting
is mainly if it is a incandescent lamp it
is in the form of radiation. So again you
have to use a cooling load factor okay. So
a cooling load factor is used account for
the time lag due to radiation from the lights
and the total heat transfer rate due to lighting
is given by this expression Q lighting is
equal to installed wattage multiplied by UF
multiplied by BF multiplied by CLF where installed
wattage is the total amount of lighting. That
means so many watts of lights inside the conditioned
space and what is UF? UF is known as a utilization
factor and the value of UF lies between zero
to one what is utilization factor utilization
factor is that at the time of load calculations
all the lights in the building may not be
on okay, only few lights may be on.
So if you take the installed wattage then
you will be over estimating the required cooling
capacity. For example if we are doing the
load calculation for day time okay. And if
it has considerable natural light then most
of the lights may be off right then there
is no point in considering the wattage of
all this lights because they are not on. So
they are not load on the building okay. So
if you want to be more accurate you must consider
the utilization factor okay. Which as I said
is lies between zero to one then comes the
ballast factor BF, BF is known as ballast
factor and the ballast factor is one point
two five for fluorescent lamps and it is one
for incandescent lamps.
That means lamps with choke you have to take
twenty-five percent higher than the rated
wattage where as for incandescent lamps which
do not have any choke the ballast factor value
is one and CLF. As I said is a cooling load
factor and cooling load factor for lights.
That is again available in tables as a function
of a type of fitting I mean what kind of light
fitting it is and the number of hours of operation.
That means how many hours lights are on and
time after the lights are switched on and
type of the building okay. So depending upon
all these factors the CLF values have been
obtained and they are available in the form
of tables okay. So using the table and using
suitable values we can find out what is the
load due to lighting of course load due to
lighting is purely sensible you do not have
any latent component okay, load due to lighting
can be considerable especially in modern buildings
which do not have any external windows okay.
So one should not neglect this one must consider
load due to lighting okay.
Next comes internal load due to equipment
and appliances and when in a conditioned space
may consist of many home appliances or several
equipments such as computers printers etcetera.
All these generate heat and this heat is added
to the air inside the conditioned space and
it has to be ultimately taken out from the
building okay. And this load can be sensible
and latent depending upon the type of the
equipment and how do we estimate the sensible
load due to appliance or equipment it is simple
it is given by Q sensible load due to appliance
or equipment is equal to rated wattage multiplied
by the utilization factor UF multiplied by
the cooling load factor. Here again you can
have a cooling load factor because some of
the sensible heat from the equipment or appliance
may be in the form of radiation.
For example if you have an oven or if you
have an electrical heater inside the conditioned
space then most of the heat may be in the
form of radiation in which case you have to
consider the cooling load factor okay. Again
as I said UF is the utilization factor and
then what is the latent load due to appliance
or equipment latent load due to appliance
or equipment is given by the rated wattage
multiplied by the utilization factor multiplied
by the latent heat fraction. That means how
much heat how much of the total heat is in
the form of latent heat okay. For example
if you have a pressure cooker okay then there
may be a lot of moisture addition to the conditioned
air okay. In which case the appliance the
adding load of latent heat to the conditioned
air. So you have to treat that separately
okay.
So one must have information about this even
though the equations for estimating internal
loads. For example for occupants for lighting
and for appliances appear to be quite similar
quite simple okay. They are simply multiplication
of wattage and you have utilization factor
etcetera. In actual case it could be quite
complicated because of the fact that the utilization
fact has been vary widely okay.
So precise knowledge about the utilization
factor is generally not available okay. Most
of the time you have to make an intelligent
guess and use in proper utilization factor.
If you are taking a utilization factor of
one and taking a cooling load factor of one.
As I said you will be over estimating the
capacity right on the other hand if you taking
two lesses two smaller value then it will
be under estimating the capacity and the system
may not be adequate okay. So this is what
brings in the difficulty and typical appliance
loads are available in the form of tables.
Let me show a typical table so this table
is taken from professor Arora's book. And
here it I have just shown the four appliances
a coffee brewer of point five gallon capacity
it rejects about two sixty-five watts of sensible
load and about sixty-five watts of latent
load and the total load of this particular
appliance is three thirty watts okay. And
a coffee warmer of point five gallon capacity
it rejects about seventy-one watts in the
form of sensible heat and twenty-seven watt
in the form of latent heat if you have a toaster
of three sixty slices per hour then the sensible
load is about fifteen hundred watts and the
latent heat load is three eighty-two watts.
And for example a food warmer it is eleven
fifty watts of sensible load per meter square
of the plate area and eleven fifty watts of
latent heat load per meter square of plate
area. So total heat radiation is two thousand
three hundred watts okay. And appliance of
loads of many other house hold and office
appliances and equipment are available in
ASHRAE hand books. In fact if you look at
ASHRAE handbooks it covers a wide variety
of all kinds of home appliances and office
equipment okay 
in addition to that if the conditioned space
is used for storing products for example if
you are doing the load calculation for a cold
storage then you will be using the conditioned
space for storing food products.
For example I am designing a cold storage
for storing potatoes okay. So potatoes are
live products so they will be adding continuously
heat to the conditioned space. So I should
be able to estimate what is the heat generation
rate because of the products stored inside
the conditioned space in this case potatoes
it could be potatoes or it could be anything
right. So I should be able to know uh what
is the amount of sensible heat generated because
of this and what is the latent heat generated
okay. Because of the product stored in the
conditioned space again lot of informations
available on the amount of heat released in
sensible as well as in latent form by a wide
variety of products which are normally stored
in cold storages okay. So you can look at
any uh air conditioning design or refrigeration
design data book.
So you find that this information is available
okay so this is very important for estimating
the loads of cold storages and other industrial
or commercial air conditioned buildings okay.
In addition to this if some process is taking
place inside the conditioned place okay. Let
us say that we are talking about air conditioning
of a chip manufacturing factory okay. So the
lot of processes will be taking place inside
the conditioned space which will be adding
heat okay, either sensible or latent or both.
So again we must have information we must
have knowledge about how much heat is being
added by all these internal processes for
estimating the loads okay. So you can see
that for load calculations a large amount
of input data is required okay, a liable accurate
input data is required only then you can have
accurate estimations of the loads okay.
Using the above equations one can estimate
the sensible total sensible load on the building
total latent load on the building and total
cooling load on the building total cooling
load on the building is nothing but some of
sensible loads and latent loads okay. So what
you mean by total sensible load on the building
sensible load due to external sources sensible
load due to internal sources. We have to add
up all these so that will give you the total
sensible cooling load on the building.
Similarly we have to add up all the latent
cooling loads on the building both internal
as well as external okay. We have seen individual
loads we have to add up now and that will
give you the total loads and when you add
up everything you get the total cooling load
on the building okay.
And from the sensible and total cooling loads
one can calculate the room sensible heat factor
RSHF for the building okay. These aspects
we have discussed in our earlier lectures
the room sensible heat factor as you know
is nothing but the ratio of the total sensible
load on the building divided by the total
load on the building okay.
So since you have got all this information
from the load calculations you can calculate
now what is the RSHF of the building okay.
And as discussed from the RSHF value and the
required indoor conditions one can draw the
RSHF line on the psychrometric chart and fix
a condition line of the supply air okay. At
this point since we know the RSHF and we also
know the inside conditions you can draw the
RSHF line and as you know the supply conditions
must lie on this RSHF line. So that it can
meet the sensible and latent loads in the
required proportion okay. So at this point
you can draw the process RSHF line okay from
this information.
Now let us look at estimation of the system
cooling capacity because ultimately we are
interested in finding out what is the required
cooling capacity of the system okay. To find
the required cooling capacity of the system
one has to consider the sensible and latent
loads due to ventilation and leakage losses
in return air ducts okay. So let me show a
picture it will be clear again this is the
picture we would have discussed earlier you
can see that this is the building and this
building is subjected to some amount of sensible
heat load and some amount of latent heat load.
And I am interested in finding out what is
the required capacity of the cooling systems.
That means what is the amount of heat that
has to be rejected in the cooling coil. That
means this okay, what is this will see a later
that this is nothing but the load on the building
plus load due to ventilated air that is flowing
through the cooling coil plus load due to
losses in the return duct okay.
So you may have return duct losses leakage
losses you may also have heat addition in
the return duct because of the presence of
return air fan okay. So this plus this has
got to be added to the building load to arrive
at the total system capacity okay. So load
on the system due to ventilated air is simply
given by this expression is almost similar
to the load due to infiltration this is equal
to this a sensible load on the coil due to
ventilated air which has flown through the
cooling coil that is equal to ventilation
rate multiplied by this factor okay, where
x is the by-pass factor so one minus x is
the fraction of this air that has flown through
the cooling coil multiplied by specific heat
of the air multiplied by temperature difference
outdoor minus indoor temperature. And this
can be written again in terms of flow rate
okay V of ventilation air. That means in terms
of meter cube per second so this is the amount
of sensible heat added to the coil because
of the ventilation. Similarly the latent heat
added to the coil because of ventilation is
given by this expression again this factor
considers the amount of air that has flown
through the coil and this the latent heat
of vaporization and this is the uh different
between the humidity ratio between the outdoors
and indoors okay. So if you know the by-pass
factor and if you know the amount of ventilation
to be provided we can easily calculate what
is the load due to ventilation right.
Next comes load on the coil due to leakage
in return air duct and due to return air fan
due to leakage to the return duct and due
to return air fan if there is any fan it is
not necessary that return air duct should
have a fan but in some cases you may have
a fan. So if you have a fan the fan and the
leakage adds to the load on the system. So
we must consider this okay, for example sensible
load due to return duct leakage is given by
Q sensible due to duct is equal to U subscript
ins into A exposed into T naught minus Ti
where U uh subscript ins is the overall heat
transfer coefficient of the return air duct
and A subscript exposed is the exposed area
of the return duct.
If the return duct is in the inside the conditioned
space then that need not be considered. Because
that will not form a load so since the return
air duct size return air fan capacity are
not known initially a safety fact is provided
to account for these. So just like supply
air duct we do not know about a supply air
duct dimensions etcetera at this moment we
have taken a safety factor similarly for return
air duct and return air fan we can take a
safety factor and these can be refined when
all the loads have been estimated and the
duct sizes are known okay.
So finally the total sensible load on the
coil is simply given by this is the total
sensible load on the coil this is the, this
is equal to sensible load on the building
plus sensible load on the coil due to ventilation
plus sensible load on the coil due to return
air duct. Similarly total latent load on the
coil is equal to total latent load on the
building plus latent load due to ventilation
plus latent load due to return air duct okay.
So finally the required cooling capacity is
nothing but total sensible load on the coil
plus total latent load on the coil.
So at this point if you add up everything
we can find out what is the required cooling
capacity. So many kilo watts or so many tonnes
and from the above one can calculate coils
sensible factor coil apparatus due point and
that is coil ADP and total supply air quantity
okay. So all these things can be calculated
at this point and these aspects we have discussed
in an earlier lecture okay. So this is in
brief is a method based on CLTD and CLF.
So normally all cooling load calculations,
for example based on CLTD and CLF method a
safety fact is always provided to account
for various uncertainties in the data used
okay. So for you finally multiply this by
a safety factor okay, to take care of the
uncertainties thus the selected system capacity
is always higher than the actual required
capacity CLTD CLF method is relatively simple
and yields reasonably accurate and economically
justifiable results in most of the cases.
So this is this method is very widely used
because of these reasons. However from economics
points of view for large buildings it is preferable
to use more accurate. But time consuming methods
such as transfer function method or building
simulation tools okay. If you want to find
out the exact cooling load or very accurately
you have to use either transfer function method
or you have to actually simulate the building
okay. For estimating the cooling loads, so
there by you can calculate the required system
capacity more accurately okay.
Next let us look at heating load calculations
as I said heating load calculations are fairly
simple and conventionally steady state conditions
are assumed and internal heat sources are
neglected. That means you do not have to bother
about solar loads or internal loads okay.
So you have to consider only external loads
due to heat transfer and due to ventilation
and procedure is similar to cooling load the
only difference is that the CLTD value everywhere
is replaced by design temperature difference
that is, that means the temperature difference
between the indoor and the outdoor that is
T subscript i minus T subscript o. Otherwise
the procedure exactly similar and you find
that if you are following this method the
calculated heating capacity is always higher
than the required okay. As a result you will
be actually spending more because the uh initial
cost will be more because of the installed
higher installed capacity than required.
But this is safer and that is why this is
conservative okay. However for buildings with
large internal heat generation and possibility
of storing solar energy it is always recommended
to use transient methods that consider thermal
capacity of the buildings and internal heat
generations.
So if you are considering the transient characteristics
of the building and if you also consider the
internal heat generation you find that the
required heating capacity will be much less
than what you get by using the steady state
methods and what you get by neglecting internal
loads. So thereby you can arrive at an economically
justified results okay. So this in brief is
the heating load calculation so let me summarize
what we have learned in this lesson.
In this lecture the following aspects are
discussed estimation of internal and external
sensible and latent cooling loads on the building
estimation of required capacity of the cooling
system and difference between cooling and
heating load calculations okay. So at this
point I stop this lecture and in the next
lecture we shall see how to select a suitable
air conditioning system.
Thank you.
