BASIC LAWS OF THERMODYNAMIC IN HVAC

BASIC LAWS OF THERMODYNAMIC IN HVAC

In last article we came across Everything we need to know about HVAC

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Now let’s focus on what are the basic laws of thermodynamics which are been used in HVAC .

Basically in order to design a better system or design comfortable temperature we need to study the laws of thermodynamics which ultimately helps us to contribute towards best design of HVAC system.

THERMODYNAMICS DEFINITION

Thermodynamics is branch of engineering science which deals with the study of heat and its forms of energy.

Thermodynamics is basically field in which people plays with the temperature and heat to maintain desired system ,so with the hvac system.

Thermodynamics play a very important role in decorating this hvac design by calculating the space with thermodynamics derived formulas.

Before moving to thermodynamics laws we need to understand the basic thermodynamics terms

HEAT
TEMPERATURE
BRITISH THERMAL UNIT
ENTHALPY
SENSIBLE HEAT
LATENT HEAT

WHAT IS HEAT?

Heat is basically a form of energy which transfer due to change in temperature

Units: kJ and BTU

WHAT IS TEMPERATURE?

Temperature is basically a degree of hotness and coldness

Units : °F and °c

Basically friends , people get confused between this two parameter or thermodynamics terms .

HEAT AND TEMPERATURE

So lets clear this confusion, so that we can move the topic smoothly without a piece of confusion in further learning.

Basically heat and temperature are two different terms which we will understand with an example lets take a container with water and put it on cooking gas for some time, you will find the water gets heated after some times.

So heat generated by cooking gas is a energy which is been absorbed by the water through container and to measure the heated water we have term called temperature which is scale to detects it degree of hotness.

Hope we are clear with temperature and heat.

In short heat is a energy supplied to an object which is measured in terms of temperature .

Now understand the units of temperature and heat!

HEAT is basically measured in either kilo joule or British thermal unit

Lets define it !

Kilo joule (SI UNITS): it is defined as amount of heat which is need to be add to raise the temperature of 1kg of water by 1.

BTU (ENGLISH OR IMPERICAL UNITS) : it is defined as amount of heat which is need to be add to raise the temperature of 1 pound of water by 1°F.

So lets discuss this what actually it is ………..

Basically lets take the same example of container with water on cooking gas , basically this units of energy defines the amount of heat energy required to heat 1 kg or 1 pound of water by 1

or 1

. which indicated the capacity of heat energy required to obtain the desired heating temperature.

ENTHALPY

It is basically amount of heat energy content in a body in short

Now lets define it ,so basically it is defined as the sum of internal energy and the product of pressure and volume.

H = µ + PV

µ= Specific heat

p=pressure

v=volume

Unit KJ/HR OR BTU/HR

Basically in short we can defined it as rate of heat absorbed or heat release by a system or , from the system per unit time .

SENSIBLE HEAT

Sensible heat is the heat which is sensible this answer is something you will not expect but it is true. sensible heat is a heat which is sensible or measurable.

LATENT HEAT

Latent heat is the heat which is not measurable but the heat is utilized to change the phase either from solid to liquid or liquid to gases.

To understand it more properly lets take a graph of heat vs temperature.

In which heat placed is in horizontal line motion, whereas temperature placed in vertical line motion.

Now consider a solid is heated at high temperature. So the heat which is been added, increases its temperature and bring it to a temperature where is changes its phase (solid to liquid).

During phase change from solid to liquid its temperature remains constant or fix while the phase gets changed.

Basically in sensible heat the heat is detected hence it is sensible while in latent it is not detectable.

Now the question comes into mind where does the heat goes which doesn’t get detected or measure??????????????

Well the temperature is utilised to change its phase rather than changing its temperature. Basically, the amount of heat which is been added is utilised to change its phase.

Hope we are clear with it…..

Laws of thermodynamics

Basically there are certain laws which is been added in this thermodynamics after studies of great scientist and there are four law

Zeroth law of thermodynamics
First law of thermodynamics
Second laws of thermodynamics
Third laws of thermodynamics

Basically out of this we will focus on first three laws which rotates around HVAC designing.

ZEROTH LAW OF THERMODYNAMICS

James Clerk Maxwell was who put forward the law which states that when two body are in thermal equilibrium with third system ,than two system are in thermal equilibrium with one another.

Basically it says that consider there are there system A , B AND C

If B AND C have same thermal temperature with A that means B and C also have same temperature.

This question must be arriving in your mind why it is called zeroth law it could also called as First law ????

Well it was discovered after the discovery of first law hence it is named as Zeroth law.

FIRST LAW OF THERMODYNAMICS

the first law of thermodynamics is a version of the law of conservation of energy, adapted for thermodynamic processes, distinguishing two kinds of transfer of energy, as heat and as thermodynamic work, and relating them to a function of a body's state, called Internal energy.

The law of conservation of energy states that the total energy of an isolated system is constant; energy can be transformed from one form to another, but can be neither created nor destroyed.

Well in short it says that heat and work are mutually convertible

Consider a system like bicycle in which paddling is heat energy and movement of bicycle is mechanical work.

Now if the person is running that means he is doing mechanical work which is been converted into release of energy in terms of calories burn.

Hence heat energy can be converted into mechanical work and vice versa

SECOND LAW OF THERMODYNAMICS

Basically there are two statement in it

Clausius statement
Kelvin plank statement

But we only need to deal with Clausius statement

which states that it is impossible for self acting machine working in cyclic process to transfer heat from a body at a lower temperature to higher temperature without any external agency.

Before understanding this we should be clear that heat always flows from higher temperature to lower temperature.

Well now understand the law so it meant to say that there is not any machine which transfers heat from lower temperature to higher temperature.

If we want it to happen we cannot do it without any external agency which may be some electrical and mechanical machine.

Hope we are clear with it !!!!!

NOW MOVE ON TO MODES OF HEAT TRANSFER

Well now we know what is heat transfer and it flows from higher to lower temeparture.

But how does it flows ???????

Basically it flows through mediums which is as follows:

Conduction
Convection
Radiation

This is a medium through which heat transfer lets understand it in details

CONDUCTION: well it is a heat transfer process which generally takes place in solid bodies with physical movement.

Well to understand it lets take an example which we do it on daily basis that is take iron rod which have good conductivity to heat and start heating it from one end, ultimately the rod on other site too get heated after sometime.

Well how does it transfer heat??????

Basically conduction takes place in solid bodies in which the molecules are very closed to each other having strong bonds.

When heated at one side of rod the strong bonded molecules started vibrating due to heat absorption and which ultimately pass the vibration to other which generates frictional heat too .

Hence this vibrations are been transferred to every molecules which generates friction due to motion and pass the heat .

CONVECTION: It is a heat transfer process which generally takes place in liquid bodies or gases bodies due to its movement due to heat absorption.

So to understand it lets say water in container is heated on cooking gas ,so the heat is transfer through container via conduction and after the convection start where it absorbs the heat and transfer within the water throughout.

Well how does it transfer heat??????

Basically convection takes place in liquid bodies in which the molecules are very less closed to each other having less strong bonds than solid .

Heated water absorbs the heat which is closed to container and as it takes the heat its density reduces and hence it get on top or rim of the container while denser water comes close to the heated container.

RADIATION: This is the medium which doesn’t requires any medium to transfer heat . It can transfer the heat even in vacuum .

How does it transfers????

Basically this is very deep but it transfers through electromagnetic waves .

This is all about HEAT TRANSFER .................

Hope we are cleared with it this term. This terms are very important and foundation of HVAC which will ultimately help us to gain profitable design of the desired space .

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