The definition of HVAC and some basic concepts wre introduced in HVAC – Understanding the Basics. In continuing your understanding of HVAC we will discuss the scientific and engineering principles used in the design of HVAC systems.  These are principles which need to be understood by anyone seeking to have a career or run a business in the HVAC industries.   

Force [Newtons, N]

In simple terms, force is defined as a push or a pull. It is the basic phenomena of Mechanical Engineering and pertains to any object that has a tendency to set a body into motion, to bring a body to rest or change the direction of any motion.  The unit of force is Newtons [N], named after Sir Isaac Newton who pioneered the study of force and motion in the 17th century.  

indiworks A force is a push or pull

A force is a push or a pull.  Source:

Pressure [Pascals] 

Pressure is the force exerted per unit area. It may be described as the measure of intensity of a force exerted on any given point on the contact surface. Whenever a force is evenly distributed over a given area the pressure at any point on the surface is the same. It can be calculated by dividing the total force exerted on a surface by the total contact area. 

Atmospheric Pressure [Pabs]

The Earth is surrounded by an envelope of air called the atmosphere, which extends upward from the surface of the earth. Air has mass and due to gravity exerts a force called weight.  The force per unit area is the pressure. This pressure exerted on the Earth’s surface is known as atmospheric pressure

Gauge Pressure [Pgauge]

Most pressure measuring instruments measure the difference between the pressure of a  fluid  and the atmospheric pressure. This is referred to as gauge pressure.

Absolute Pressure [Pabs]

Absolute pressure is the sum of gauge pressure and atmospheric pressure. 


If the pressure is lower than the atmospheric pressure, its gauge pressure is negative and  the term vacuum is applied to the magnitude of the gauge pressure when the absolute pressure is zero (i.e. there is no air present whatsoever).


Pressure is the normal force per unit area exerted on an imaginary or real plane surface in a fluid or a gas.


Density [ρ]

It is defined as the mass of a substance divided by its volume or the mass per unit volume. 

 Density(ρ) =  mass(m) ÷ volume(V). 

Specific Volume(v) is the reciprocal of density or volume per unit mass.

  v =  V/m

Specific Weight (Ws) is defined as the weight of a substance divided by its volume or the weight per unit volume.

  Ws = m/V 


If a system undergoes a displacement under the action of a force, work is said to be done. The amount of work being equal to the product of force and the component of displacement parallel to the force. If a system as a whole exerts a force on its surrounding and a displacement takes place, the work that is done either by or on the system is said to be external work.


A body is said to possess energy when it is capable of doing work. In more general terms, energy is the capacity of a body for producing an effect. 

Energy is classified as  

1.Stored Energy for example Chemical energy in fuel and Potential Energy stored in dams 

  1. Energy in Transition for example Heat and  Work.
indiworks A body is said to possess energy when it is capable of doing work.
indiworks A body is said to possess energy when it is capable of doing work.

The three main forms of energy are potential energy, kinetic energy and internal energy. The three forms  of energy are explained below. 


A body is said to possess energy when it is capable of doing work.


Potential Energy

It is the energy stored in the system due to its position in the gravitational force field. If a heavy object such as a  building stone is lifted from the ground to the roof, the energy  required to lift the stone is stored in it as potential energy. This stored potential energy  remains unchanged as long as the stone remains in its position. 

PE  = mgH   Where H = height of the object above the datum Units Joules

Kinetic Energy

If a body weighing one kg is moving with a velocity of v m/s with respect to the observer, then the kinetic energy stored in the body is given by:        K.E      =      221mv. This  energy  will  remain  stored  in  the  body  as  long  as  it  continues  in  motion  at  a  constant  velocity.  When the velocity is zero, the kinetic energy is also zero. 

Internal Energy

Molecules  possess  mass.  They  possess  motion  of  transactional  and  rotational  nature  in  liquid  and  gaseous states. Owing to the mass and motion these molecules have a large amount of kinetic energy stored  in  them.  Any  change  in  the  temperature  results  in  the  change  in  the  molecular  kinetic  energy  since molecular velocity is a function of temperature.  



Also the molecules are attracted towards each other by forces, which are very large in their solid state and  tend  to  vanish  once  they  are  in  a  perfect  gas  state.  In  the  melting  of  a  solid  or  vaporization  of  a  liquid it is necessary to overcome these forces. The energy required to bring about this change is stored in molecules as potential energy. 

The internal energy is defined as the total energy of the body – chemical, nuclear, heat, gravitational, or any other type of energy. This energy is stored within the body which is denoted by the symbol ‘μ’. It is obvious from the above definition that it is impossible to measure the absolute value of the internal energy. However, we can measure the changes occurring in the internal energy. Since thermodynamics deals  with  the  change  in  the  internal  energy  of  the  system,  it  is  important  to  know  what  causes  the  internal energy to change. The change in the internal energy can be caused either due to absorption or release of heat in the system or the work done by or on the system., or if any matter enters or leaves the system. 


Heat is one of the many forms of energy. This is evident from the fact that heat can be converted into other  forms  of  energy  and  that  other  forms  of  energy  can  be  converted  into  heat.  Heat  as  molecular  energy is universally accepted and heat as internal energy of the matter is thermodynamics.


Source: lorecentrarg



Since all other forms of energy may be converted into heat, it is considered to be energy in its lowest form. The availability of heat energy to do work depends on temperature differential. 

Heat Capacity

It  may  be  defined  as  the  energy  that  must  be  added  or  removed  from  one  kilogram  of  a  substance  to  change  its  temperature  by  one  degree  Centigrade.  In  refrigeration  technology  heat  capacity  is  used  to  determine how much heat should be removed to refrigerate various products.

Sensible heat  (QS) 

Heat which results in an increase or decrease in the temperature without it changing its phase is called sensible heat. A change in sensible heat is given by the equation when there is a change in temperature 


QS = m× CS (T2 – T1)       Note:      CS is the heat capacity at constant pressure  m = mass of the substance in kg                     (T2 – T1) = Temperature difference in °C

Latent Heat (Ql)

Latent heat is the heat at which a substance changes its phase without any increase or decrease in the temperature. It is the amount of heat required to change the state of a substance.


QL = m×Cw(w2 – w1)       Note:      Cw is the heat capacity of moisture m = mass of the substance in kg                (w2 – w1) = change in moisture content in g/kg 

Total Heat (Qt)

Total heat is the sum of sensible heat and latent heat. Heat measurements are taken above a specified datum.  These  measurements  with  water  are  at  zero  degrees  C,  since  below  this  temperature  water  is  solid. For  example:  The  sensible  heat,  latent  heat  and  total heat for steam are shown in the fig below

indiworks Total heat is the sum of sensible heat and latent heat
indiworks Total heat is the sum of sensible heat and latent heat

Source: crediblecarbon

Temperature and its measurement

Temperature  is  a  property  of  matter.  It  is  the  measure  of  intensity  of  heat  contained  in  matter  and  its  relative value. A substance is said to be hot or cold when its temperature is compared with some other reference  temperature.  A  high  temperature  indicates  a  high  level  of  heat  intensity  or  thermal  pressure  and a body is said to be hot. 


Like  other  forms  of  energy  heat  can  be  measured  because  it  has  quantity  and  intensity.  Heat  is  not  visible but manifests itself in its effects on various substances either by changing its state or by creating relative degrees of sensation when in contact with the human body.

indiworks Internal energy is the combination of potential and kinetic energy
indiworks Internal energy is the combination of potential and kinetic energy

Souce :energyeducation


Since  temperature  is  a  measure  of  heat  content,  the  temperature  can  be  measured  by  measuring  the  effects of heat on different properties of matter as follows; • Addition of heat increases the volume of the substance or pressure at constant volume. This property is used for measuring the temperature with the help of a mercury thermometer. •  With  the  increase  in  temperature,  the  resistivity  of  metals  increases  which  is  utilized  in  resistance thermometers •   If  two  junctions  made  of  two  dissimilar  metals  are  maintained  at  different  temperatures,  a  current flows in the circuit. This property is used in measuring with a thermocouple. 


When  the  temperature  of  a  substance  increases,  the  color  also  changes.  This  property  is  used for measuring the temperature in radiation pyrometers

Pressure and temperature relationship 

Water  boils  at  1000C  when  the  pressure  on  it  is  atmospheric  at  sea  level.  If  the  pressure  is  increased  above  the  atmospheric  pressure,  i.e.  in  a  deep  mine  shaft  the  boiling  point  increases  and  when  the  pressure is reduced below atmospheric, i.e. on top of a mountain, it reduces.  Boiling water does not necessarily have to be hot because if there is vacuum, water boils at a very low temperature.  The  same  is  true  when  it  comes  to  other  liquids,  such  as  various  refrigerants.  These  refrigerants have the same properties as water except their boiling point ranges are lower. This pressure temperature relationship is used in most air conditioning and refrigeration systems.

indiworks Pressure is directly proportional to temperature
indiworks Pressure is directly proportional to temperature