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.
A force is a push or a pull. Source: https://studiousguy.com/direct-and-indirect-force-examples/
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.
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
- Energy in Transition for example Heat and 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.
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
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.
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.
Source: s-cool.co.uk https://www.s-cool.co.uk/a-level/physics/power-and-internal-energy/revise-it/internal-energy
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.
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.
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
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.
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.