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

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 a resistance term r divided by mvc t temperature as their deficient coefficient now this i can write supposing you know this this part

vapour 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 its looking

at slightly you know its its for the new for you if you were if you remember in case of cooling newtons 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 analogic to here the late rate of vapour transfer or mass transfer is proportional to the

vapour pressure difference so this what it is this is what it is right so we are 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 thats 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 hd ct 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 ah this this this is you know this this vapour pressure difference is this now if i want

to find out the amount of heat that will be transferred because of the vapour transfer has occurred vapour or moisture transfer is occur know it will be related to mass because m dot is the rate of mass transfer mass of moisture vapour that transfers so the latent

heat associated with 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 per unit area so latent heat transfer i have

just multiplied by latent heat and this is you know this latent heat transfer i have just multiplied by this so 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 the its related to saturation vapour pressure minus the vapour pressure you know saturation rule supposing i have moisture on my skin then this is in a saturated state of affair tw i can assume surrounding

is there is some dry bulb temperature existing and this difference is related to the vapour pressure difference saturated vapour pressure minus ah vapour pressure difference so thats how we are trying to show that t minus tw is a function of you know this wet

bulb depression as we call it we can estimate this also from vapour pressure and thats 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 ah 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 air under constant pressure i think we will we will be further clear when you discuss them somewhat later on so all that i was trying to point out here is wet bulb depression is

related to the relative humidity right wet bulb depression is related to the relative humidity ok what is dew point dew point is that temperature at which moisture will become you know supposing i reduce down go on reducing the ah temperature

of the temperature of the air now a point will come when it will become saturated so thats the dew point if you try to cool it further actually condensation will occur so thats dew point so dew point there is an empirical formula like this i think i will

not discuss this much this is an empirical formula available for dew point right so this is absolute you know this is absolute temp four zero three zero etcetera etcetera this is relative humidity minus ok so this the dew point is a i i dont think i will i will

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 of air 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 specific it of air is cp temperature absolute temperature is t then this will be the specific component because

of the dry air its temperature being higher mass into specific it into temperature difference so its for 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

vapour and this is latent heat so we assume also that all vapourization takes place at zero degree centigrade so if it has got some moisture content that moisture will come from it would have evaporated lets say as an assumption at zero degree centigrade itself and it has

got some heat content thats 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 be a

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 its actually you know ah its like this is the product of course g into t but however

we have lines so in psychrometric chart therefore lines are there for specific enthalpy the lines are the curves are therefore relative humidity you know saturation line and so on which i showed you earlier let me see if i have yeah we have i will go back again to

that diagram so thats why you find you can find out you can find out a from here you can find out so the specific enthalpy line say here ah wet bulb temperature and dry bulb temperature is same at hundred percent so if you see this

is this is let us say four degree or this ten degree and this is the wet bulb temperature is also ten degree so wet bulb temperature wet bulb temperature lines are inclined like this wet bulb temperature lines are inclined so this is ten degree wet bulb temperature

ten degree driver so this is the temperature line so wet bulb temperature lines are inclined right they are same at hundred percent vertical lines are dry bulb temperature so wherever you know when it meets that hundred percent saturation line that will corresponds

to hundred percent ten degree same amount or ten or 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 ten degree here at twenty degree the wet bulb temperature is ten dry bulb temperature is something like eighteen or something like that you know for zero relative already so they are inclined in this manner then there

are specific enthalpy line as i said specific [enthal/enthalpy] enthalpy lines there you know so the other lines are also their wet bulb temperature specific enthalpy lines this so in psychrometric chart you might have a look at that in sp forty on i think i have

given you reference and you get that it you get that in our ah 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 will get sp forty one and you

get this one straight away so you can you can click and get it there is no problem but if you look at the code anyway it will you will get it right so if you look at the code anyway you will get it all right so coming back to this so then then lets look

at ah lets look at ah relative humidity of looked into lets look into air velocity we have talked of also specific enthalpy because its a part of the psychrometric chart lets look at the air velocity next temperature we have looked into it quickly i have 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 its measured with anemometers anemometers right ah there are there are of course very common is what is called hot air hot wire anemometer hot wire anemometer so basically nothing but a

small you know this is a base and you will have a small resistance right now this will be connected in you know you got to measure the resistance ah 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 its a part of an arm of western bridge or whatever bridge it is measuring resistance measuring system now if i pass a constant current through it and air velocity changes then this temperature of this one will come down because it will

get cooled by air stream 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 am measuring the resistance change because of a constant current the change in resistance is a function of the can be correlated to the air velocity though thats you know in

a in a cross manner thats the kind of principle hot wire anemometers uses and they can be used versatile zero to thirteen meter per second so we can use them inside the room and even outside for wind velocity measurements ah but has to be study but then he would have

seen cup anemometer vane anemometer you might have seen on building top cup anemometers you know so they will have three cups right and they will rotate ok so as the wind velocity causes them to return or windmills so these are this is the one so largely one can measure

with anemometers there is something called a kata thermometer a kata thermometer also measures air velocity because if the air flows the there could be a depression in the apparent depression in the you know ah there can be apparent depression now velocity varies

from ground level to the height because in you have done ah course on loads wind load i am sure many of you would have done a course on wind load and if you have done that you would have remembered that we measure the wind velocity at ten meter height right and

ah you have ah you know k one k two k three factors multiplying for the load 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

doesnt increase further we call it gradient height so wind velocity varies with height so we measure at ten meter so ten meter is you know where your meteorological department will measure air airports stations and all those you know air bases or or ah airports

they measure because the needed deviations people so ten meter is height where it is measured but there is something called gradient height so as you go up it is ah it increases and becomes constant at a given point which you call this gradient height precipitation

and rain fall 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 ah if you come from kerala our northeast in india you have a lot of rain fall 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 imp when it impinges on your wall the moisture can penetrate through the wall so design against such kind of ah 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 d r w d r is actually is the intensity of rain fall

multiplied by rain fall intensity multiplied by the wind velocity some factor some factor multiplied by wind velocity right so w d r is wind driven rain so this is important but precipitation itself is important to classify the a location or

you know its which zone of climatic class it belongs to without going to the climatic classification right now which i will come to you a little bit later on as you can understand those who have come you know say 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 lets say mumbai where there is a lot of rain fall the temperature do not go that high but the

humidity is very high so you see when you are designing your building for 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 thats that thats why those parameters of the you know which through which you classify the climates or parameters or factors of environment thats what we are discussing one by one first we discuss temperature then relative humidity

air velocity and now just now i mentioned about precipitation and rain fall right so this is called wind driven rain the formula for wind driven rain v zero point two two this given by one ah lacy mister lacy from ihv guide institution of heating and ventilation

engineers england ah he published this you know ah this this this equation there its guideline it was available now they call it chartered institution building services engineer so whatever it is v zero is point two two two w where this is the wind velocity r is

the rain fall intensity to there is an empirical equation so that gives you the flux or rain flux again i said i will [laughter] not discuss wetting and drying that says that can be you know forty two hours lectures is not sufficient

but i wanted to introduce these parameters to you when we are looking at the subject so desirable ah air and radiant temperature relative humidity air flow air velocity control through proper choice of proper orientation envelope 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 the you know envelope now what is building envelope it is basically

there is a formal definition i will i may give you in the next class if possible ah ah formal definition is defined in some quotes actually so it is the 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 hm for day lighting and ventilation purpose natural ventilation purpose right as opposed to infiltration which is not by choice by default for example leakages through the you know gap in the window

window doesnt close doesnt seal there will be some gap so air can enter through that thats 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 etcetera and we look into in this course we look into all these issues proper glare free lighting through day lighting thats what is another aspects we should look into glare free lighting now glare i will define again later on but quickly

glare occurs when you are looking to the car headlight dont see anything its called disability glare you dont say anything but the glare can be a kind of discomfort glare also which can happen in a classroom sometime we will discuss that sometime later

we [def/design] design the illumination task illumination for writing for example if the light is not sufficient you wont be able to write right ah contrast between say this is this is blue in color this is black in color and background is white so there is a contrast

now when i am making the slides if i make it yellow you wont 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 wont see it so well so the contrast is important brightness 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 will talk about this so this is what we will cover in this particular course ah in general and just now i talked of environmental

features measurable features through which i define the environment right so i think after that we can look into thermal issues and in that context i must look into earth energy balance a part of this is available in a book written by suzhou lake kenneth berger

and all those in a right there given in the reference say cheap 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 batch you can look into that right and its available ah

written material is available maybe i will upload it sometime so in order to look at a thermal issue i should look into earth 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 hundred percents you know supposing i get hundred percent solar radiation now what happens is if you look at it some of it directly fifty percent is received 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 twenty percent by evaporation loss moisture is there if the ground level sea is there so evaporation loss and ah this is ah 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 is a terminology twenty percent long wave radiation you see

what is long wave radiation thats what it is so if i fifty 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 is a periodicity in the whole thing annual periodicity every year we receive energy from the sun right whatever you receive today three sixty five days later you are likely to receive similar kind of thing not exactly same but its likely

similar almost similar so there is a annual periodicity now this whatever energy the earth is receiving over that cycle it must be dissipating out the same otherwise temperature of the earth 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 suns radiation to come in but doesnt allow radiation

from the earth to go out therefore there may be some kind of a global warming scenario so thats where the long wave stamp terminology becomes important maybe just i will i will take talk about that in more details later on but maybe just quickly

introduce is the suns radiation belongs to wide range of wavelengths starting from ultraviolet visible range right then infrared radiation red you know red infrared radiation so if you look at all of that which we will look at it at 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 are 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 suns radiates at very high temperature million degree centigrade etcetera etcetera fusion process and so on therefore suns radiation is of that variety they are shortwave to long wave radiation

visible radiation everything well if you look at earth you know the cosmos is cool so radiation heat tests exchange always takes place between hot and the cool bodies earth 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 the earth terrestrial radiations so these are all long wave radiation you dont see that radiation but they are 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 dont see the radiation you know if you are you are close to some hot plate which is not really red hot now no color changes occurring still you can feel the heat because the infrared radiation that is occurring so

temp bodies at lower temperature radiate long wave radiation right and thats what we are talking about so terrestrial radiations are all long wave radiation for example radiation heat exchange between you know the the the the ah roof ceiling

of the of this room if it is at different temperature then my skin temperature that would be actually all long wave radiation heat transfer so its all long wave radiation heat transfer and sun gases like carbon dioxide methane etcetera at the top of the atmosphere

if they are there moisture vapour they dont allow long wave radiation to go but they do allow shortwave radiation to pass him like glass does so we will discuss about that later on and when such thing is occurring the suns radiation can come in but the earth radiation

cannot go back and thats why this ah you know this this concerned about that 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 will ah

stop here for the day and start from earth energy balance in the next class ok thank you very much