Understanding Heat Transfer, Humidity, and Air Velocity in Building Design

[Music] so continuing from what we have done what you have seen is you know rate of

flow you can be written in this manner what this is the resistance term R divided by MBC t temperature as their

diffusion coefficient now this I can write supposing you know this this part vapor pressure difference Delta PV del

PV and HD corresponds to all other terms right other than this so d mu and the distance across which it is so this is

something similar to a kind of a convective term so this is a convective now what is convective term perhaps it's

looking at slightly you know it is for the new for you if you were if you remember in case of cooling Newton's law

of cooling the rate of cooling is proportional to temperature difference something analogical to this here we

will discuss that convection later on something in a logic to here the late rate of vapour transfer or mass transfer

is proportional to the pressure difference so this sort it is this is what it is right so we're trying to get

an expression to show that t minus dry bulb temperature minus wet bulb temperature that is the wet bulb

depression is a function of relative humidity that's all we are trying to do so you see the amount of heat that will

be transferred this is the mass transfer multiplied by latent heat of evaporation mass transferred was this much mass

transferred was this much I have replaced all things by a constant H DCT remaining here so D mu and D mu per unit

length that I have done transferred into HD equivalent as if it is per unit length so this this this is you know

this this vapor pressure difference is this now if I want to find out the amount of

heat that will be transferred because of the vapor transfer has occurred pepper or moisture transfer is okano it will be

related to mass because m dot is the rate of mass transfer mass of ganja vapor that transfers so the latent heat

associated to this so if I multiply this by L I get the rate of heat transfer associated with this so I simply

multiply this by Q L is a heat flux per unit area so everything has been power unit area so latent heat transfer I have

just multiplied by latent it and this is you know this latent heat transfer I have just multiplied by this so the

amount of heat transfer is this and this will be also equals to from convective law of convection if the temperature

difference between the dry bulb temperature the wet bulb temperature temperature difference is this much

right then the amount of heat that will be transferred is given by convective heat transfer coefficient multiplied by

t TW so therefore t minus TW is HD divided by H etcetera etcetera so wet bulb depression is a function of

that it's related to saturation vapor pressure minus the vapor pressure you know saturation rule supposing I have

moisture on my scheme then this is in a saturated state of affair TW I can assume surrounding is there's some dry

bulb temperature existing and this difference is related to the vapor pressure difference saturated vapor

pressure - ah afraid of it so that's how we are trying to show that t minus TW is a function of

the you know this wet bulb depression as we call it we can estimate this also from vapor pressure and that's the

empirical relationships are possible and you know I can find out so T by it can be written as it can be written as H D

by L also related to the specificity and density of the you know this can be related to specificity and density of

the CP stands for specificity of the earth under constant pressure I think we will

be further clearer when you discuss them somewhat later on so all that I was trying to point out here is what bulk

depression is related to the relative humidity right wet ball depression is related to the relative humidity okay

what is dewpoint dewpoint is that temperature at which moisture will become you know supposing I reduce down

go on reducing the temperature of the temperature of the air now a point will come when it will become saturated so

that's the dew point if you try to cool it further actually condensation will occur so that's dew point so dew point

there is an empirical formula like this I think I'll not discuss this much this is an empirical formula available for

dewpoint right so this is absolute you know this is absolute temperature cetera et cetera this is relative humidity -

okay so this the dew point is I I don't think I'll be interested further on this at the moment now we define something

called specific enthalpy what is specific enthalpy enthalpy is the heat content of the air actually we are

talking about here and we measure we can only measure the change in enthalpy you cannot measure the enthalpy as such so

we measure the enthalpy change from a relative temperature in our case it is zero degree centigrade so if the

specificity of air is CP temperature absolute temperature is T then this will be the specific component because of the

dryer its temperature being higher mass into specific 8 into temperature difference so it's four per unit mass so

this is the amount of heat content of the air per unit mass unit kg at a temperature T and this is the moisture

content this is the specific heat of moisture vapor and this is latent heat so we assume also that all vapor ization

takes place at zero degrees n so if it has got some moisture content that moisture will come from it would

have evaporated let's say as an assumption at zero degree centigrade itself and it has got some heat content

that's what we are going to look into right so this is specific enthalpy and all these are they are in the

psychrometric chart so if you see in the psychrometric chart you know dry bulb temperature there will line for specific

enthalpy also related to dry bulb temperature and moisture content so specific enthalpy is a function of dry

bulb temperature and moisture content right so it's actually you know like this is the product of course G into T

but however we have lines so in psychrometric chart there four lines are there for specific enthalpy the lines

are the carts are there for relative humidity you know saturation line and so on which

I showed you earlier let me see if I have yeah we have I'll go back again to that diagram so that's why you find you

can find out you can find out from here you can find out so the specific enthalpy line say here wet bulb

temperature and dry bulb temperature is same at 100% so if you see this is this is let us say 4 degree or this 10 degree

and this is the wet bulb temperature is also 10 degree so wet bulb temperature what about temperature lines are

inclined like this wet bulb temperature lines are interests so this is 10 degree wet bulb temperature 10 degree timer so

this is the temperature line so wet bulb temperature lines are inclined right they are same at 100 percent

vertical lines are dry bulb temperature so wherever you know when it meets that 100 percent saturation line that will

corresponds to 100 percent n degree same amount or 10 or tilly you know whatever temperature is same wet bulb temperature

and otherwise it is inclined because for lower humidity difference between wet bulb temperature and dry bulb

temperature is high so if you look at this line let us say which is a 10 degree here at 20 degree the wet bulb

temperature is 10 dry bulb temperature is something like 18 something like that you know for zero

relative already so they're inclined in this manner then there are specific enthalpy line as I said specific intent

enthalpy lines there you know so the other lines are also their wet bulb temperature specific cousins thank you

like this so in psychrometric chart you might have a look at that in SP forty on I think

I've given you reference and you get that it you get that in our you get in website straight away most of the most

of the institution I mean IIT Delhi is definitely a member of Indian Standard institution so you in our library

website you'll get SP forty-one and you get this one straight away so you can you can click and get it there's no

problem but if you look at the code anyway it will you will get it right so if you look at the code then you will

get it alright so coming back to this so then then let's look at let's look at relative humidity of looked into it

let's look into air velocity we have talked of also specific enthalpy because it's a part of the psychrometric chart

let's look at the air velocity next temperature we have looked into it quickly I've told you how we can do the

measurement and then relative humidity I talked off and I said that if you know wet bulb temperature you can find it out

from psychrometric chart or some empirical equations are there which I just showed you but I think I solve a

problem sometimes so that it becomes clear better next parameter is air velocity now it's measured with

anemometers anemometers right there are there of course very common is what is called hot air hot wire anemometer what

way are any more meter so basically nothing but a small you know this is a base and you'll have a small resistance

right now this will be connected in you know you got to measure the resistance as you know you know resistance will

change its resistance change with temperature right so what is done is this is exposed there are one can use

two principles and it's a part of an armed of Western Bridge or whatever bridge it is measuring resistance

measuring system now if I pass a constant current through it and the velocity changes then this temperature

of this one will come down because it will get cooled by air stream now temperature will come down same current

I am trying to pass or I can maintain the same temperature by changing the current means resistance keep the

resistance constant change the current so that current is a function of the air velocity itself or if I'm measuring the

resistance change because of a constant current the change in resistance is a function of that can be correlated to

the air velocity that's me you know in a cross manner that's the kind of principle hot where enema meters uses

and they can be used versatile 0 to 13 meter per second so we can use them inside the room and even outside for

wind velocity measurements but has to be study but then he would have seen cup anemometer vane anemometer you might

have seen on building top cap anamur meters you know so they'll have three cups right and they'll rotate okay so as

the wind velocity causes them to return or windmills so these are this is the one so largely one can measure with

genom um it has there's something called a cata thermometer a cut-off thermometer also measures air velocity because if

they are flows the there could be a depression in the apparent depression in that you know there can be apparent

depression now velocity varies from ground level to the height because in Europe done course on loads who include

I'm sure many of you have done a course on wind load and if you have done that you would have remembered that we

measure the wind velocity at 10 meter height right and you have you know K 1 K 2 K 3 factors multiplying 4

now one of them is related to topography so as the topography changes this height will change the velocity near the

boundary ground boundary is zero it increases and a height beyond which it doesn't increase further we call it

gradient height so wind velocity varies with height so we measure at 10 meter so 10 meter is you know where your material

article Department will measure yep we are four stations and all those you know air bases or airports they measure

because the needed deviations people so 10 meter is height where it is measured but there's something called gradient ID

so as you go up it is it increases and becomes constant at a given point which you call this gradient I'd precipitation

and rainfall is the other feature precipitation is what it is both snow rain everything put together we call it

precipitation and driving rain index is very important in certain type of climate rain fall intensity you know we

would like to protect the building from rain as well so in certain for example if you come from Kerala our northeast in

India you have a lot of rainfall and therefore and and you know wind driven rain as we call it it would like to push

it inside into the room or space that you know so wind driven rain is important not only that it would

implement it impinges on your wall the moisture can penetrate through the wall so design against such kind of moisture

movement is wind driven rain is a factor although we will not discuss this in our class wetting and drying condensation or

moisture movement in building materials or walls or building envelop will not talk about that in this class but this

is a parameter important parameter now what is w dr w dr is actually is the intensity of rainfall multiplied by

rainfall intensity multiplied by the wind velocity some factor some factor multiplied by velocity right so w dr is

wind even so this is important but precipitation itself is important to classify that a

location or you know it's which zone of climatic class it belongs to without going to the climatic classification

right now which I'll come to you a little bit later on as you can understand those who have come you know

they say like Delhi Delhi has got a climate which in the month of May and June if you are here it would be very

very dry and very hot about forty to forty three degree centigrade sometime and if you compare that with let's say

Mumbai where there's a lot of rainfall the temperature do not go that high but the humidity is very high so you see

when you are designing your building for functional purposes thermal comfort you take got to take these aspects into

account you go to take these aspects into account all right so therefore we can classify these places according to

their climatic situations I will come to climate later on and that's that that's why those parameters of the you know

which through which you classify the climates or parameters or factors of environment that's what we are

discussing one by one first we discuss temperature the and relative humidity air velocity and now just now I

mentioned about precipitation and rainfall right so this is called wind driven rain the formula for wind driven

rain V zero point two two days given by one lacy mr. Lacey from yhv guide

institution of heating and ventilation engineers England he published this you know this this this equation there its

guideline it was available now they call it chartered institution building services engineer so whatever it is v-0

is 0.2 to 2 w where this is the wind velocity R is the rainfall intensity - there is an empirical equation so that

gives you the flux or rain flux again I said I will not discuss wetting and drying that says that can be you know 42

hours lectures is not sufficient but I wanted to introduce these parameters to you when we are looking at

the subject so desirable air and radiant temperature relative humidity air flow air velocity control through proper

choice of proper orientation and wha-la fenestration design so all these aspects you know at temperature or temperature

coming from those hot surfaces of the building relative humidity etcetera etcetera I

can actually control somewhat through proper design of that you know envelop now what is building envelope it is

basically there's a formal definition I may give you in the next class if possible formal definite definition is

defined in some quotes actually so it is that basically all everything that is in contact with your surrounding

environment you know just immediate interaction of the surrounding environment who is the

building envelope so which will include in fact the sunshades and things like that the walls and the

roof and so on so I can design that also design the fenestration now what is fenestration it is the openings which

are left by choice by design for day lighting and ventilation purpose natural ventilation purposes right as opposed to

infiltration which is not by choice by default for example leakages through the you know gap in the window hoon do

doesn't close doesn't seal there'll be some gap so air can enter through that that's infiltration so infiltration is

not by design is by default poor construction you know etcetera etcetera while this is by design fenestration is

by design right so proper orientation etcetera so then we look into in this course we look into all these issues

proper glare free-writing through day lighting that's what is another aspects we should look into glare free lighting

now there I will define again later on but quickly glare occur when you are looking to the car

headlight don't see anything it's called disability they don't say anything but the glare can be a kind of

discomfort glare also which can happen in a classroom sometime we'll discuss that sometime later we design the

illumination task illumination for writing for example if the light is not sufficient you won't be able to write

right contrast between say this is this is blue in color this is black in color and background is white so there's a

contrast now when I am making the slides if I make it yellow you won't be able to read it right

the contrast has to be there or if the background was by and large blue and I have written black maybe you don't see

it so well so the contrast is important brightness current contrast illumination design takes care of all those and noise

control privacy through various kind of zoning planning barrier design insulation and acoustic design I'll talk

about this so this is what we'll cover in this particular course in general and just now I talked of environmental

features measurable features through which I define that woman right so I think after that we can look into

thermal issues and in that context I must look into heart energy balance a part of this is available in a book

written by Suzhou Lake Innes worker and all those right there given in the reference book available

you can look into it right so also I have I have a written lecture notes on the subject typed out given two previous

years bath you can look into that right and it's available written material is available maybe I'll upload it sometime

so in order to look at a thermal issue I should look into art energy balance and you must be hearing about global warming

quite a bit of it now so this might be relevant to that you see at the top of the atmosphere we get 100 percents

you know supposing I get hundred percent solar radiation now what happens is if you look at it

some of it directly 50 percent is rusev to the ground some of it will go away reflected away straight away at the top

of the atmosphere now this is four and number four is 20% by evaporation loss Moisture is there if the ground level

sea is the saviour operation loss and this is you know this this is fifty percent comes is absorbed each one of

them will look into fifty percent is absorbed in the atmosphere itself right fifty percent is received out of this

fifty percent some goes out reflected directly and some of it will be absorbed in the atmosphere now there's a

terminology 20 percent long wave radiation you see what is long wave radiation that's what it is so if I 50

percent is received fifty percent is absorbed in the atmosphere part of it will go back straightaway and part of it

will part of it will come to the ground and get reflected right finally of course you can understand there's a

periodicity in the whole thing annual periodicity every year we receive energy from the Sun right whatever you receive

today 365 days later you are likely to receive similar kind of thing not exactly same but it's likely similar

almost similar so there is a annual periodicity now this whatever energy the art is receiving over that cycle it must

be dissipating out the same otherwise temperature of the art would have increased over this years and you know

and by the natural process and the temperature would have increased but this is not happening well what is being

said now is this because you have got a layer which allows what are called greenhouse gases which allows sun's

radiation to come in but doesn't allow radiation from God to go out therefore there may be some kind of a

global warming scenario so that's where the long wave stamp terminology becomes important

maybe just I'll talk about that in more details later on but maybe just quickly introduce is the sun's radiation belongs

to wide range of wavelengths starting from ultraviolet visible range right then infrared radiation red you know

that the infrared radiation so if you look at all of that which we'll look at it and which we look at at some time

look at some time if you look at all of them a part of it is only visible light a good lot of it is infrared region just

actually heat radiations they're you know heat radiation and longer longer wavelengths is more right now the

characteristics of the radiation that comes from a body depends upon the temperature at which it is radiating so

sun's radiates at very high temperature million degree centigrade etcetera etcetera fusion process and so on

therefore sun's radiation is of that variety they are shortwave - long wave radiation visible radiation everything

well if you look at our you know the cosmos is cool so radiation heat tests exchange always takes place between hot

and the cool bodies art itself is warm compared to the outside outer outer cosmos so it will also radiate the heat

now this quality of this radiation depends upon the temperature of that terrestrial radiations so these are all

long wave radiation you don't see that radiation but their heat radiation for example you have a hot plate here which

is not glowing but it is just hot you can still fit the fill the heat you don't see the radiation you know if you

are you are close to some hot plate which is not really red hot now color changes

still you can feel the heat because the infrared radiation that is occurring so temp bodies at lower temperature

radiates long wave radiation right and that's what we are talking about so terrestrial radiations are all longer in

radiation for example radiation heat exchange between you know the roof ceiling of the of this room if it is at

different temperature than my skin temperature that would be actually all long wave radiation heat transfer so

it's all long wave radiation heat transfer and some gases like carbon dioxide methane etc at the top of the

atmosphere if they are there moisture vapor they don't allow long wave radiation to go but they do allow

shortwave radiation to pass him like glass does so we'll discuss about that later on

and when such thing is occurring the sun's radiation can come in but the earth radiation cannot go back and

that's why this you know this this concerned about the global warming anyway but for our purpose we look into

this because this has something to do with our temperature outside climatic zone and things like that all right so I

think we'll stop here for the day and start from art energy balance in the next class okay thank you very much

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