How Air Conditioning Work

Tuesday, March 10, 2009

HEAT TRANSFER

Heat is a form of energy and can be defined as energy that transmits from one body to another as the result of a temperature difference between the two bodies. All the other energy that is transferred occur as work.

Heat transfer takes place from a highes temperature to a lower temperature (from a warm body to a cold body) and never in the opposite direction. Since heat is energy, it is not destroyed or used up in any process. The rate of heat transfer is always proportional to the difference in temperature that is causing the transfer. The transfer of energy as heat occurs in three ways :
i. By conduction
ii. By convection
iii. By radiation

LATENT HEAT

Latent heat is the process whereby heat is added but there is no rise in temperature. An example is when heat is added to water while it is boiling in an open container. Once water has reached the boiling point, adding more heat only makes it boil faster, it does not raise the temperature.

SENSIBLE HEAT

When a change of temperature can be measured by thermometer and the level of heat or heat intensity has changed, it is called sensible heat.

SPECIFIC HEAT

The specific heat (c) is the amount of heat necessary to raise the temperature of 1lb of a substance to 1˚F. Every substance has a different specific heat. For example, the specific heat of water is 1Btu/lb ˚F , where as the specific heat of ice is 0.5 Btu/lb ˚F.

CALCULATING HEAT QUANTITY OF HEAT

From the definition of specific heat, it is evident that the quantity of heat energy supplied to, or given up by any given mass of material to bring about a specified temperature change can be determined from the following relationship :

Q = (m) (c) (T2 – T1)
Where Q = The quantity of heat energy in British
Thermal Units (Btu)
m = The mass in pounds
c = The specific heat in Btu per pound per degree Fahrenheit
T1 = The initial temperature in degrees Fahrenheit
T2 = The final temperature in degrees Fahrenheit, consistent with T1

CORROSION

A simple definition of corrosion is the deterioration of a material or its properties due to the reaction with the environment. Sometimes the deterioration is a weight gain; sometimes it is a weight reduction and sometimes the mechanical properties are affected. Most corrosion processes are electrochemical in nature.

TYPES OF CORROSION

Uniform

The simplest form of corrosion is a uniform attack of all surfaces that are exposed to a corrodent. It can be electrochemical in nature or simply a direct attack.


Pitting

Pitting is a local corrosion damage and caused is by the chemical nature of environment. Solutions that tend to produce pitting are brackish water, salt water, chloride bleaches, and reducing inorganic acids. Certain metals such as stainless steel are particularly prone to pitting attack.

Crevice Corrosion

Crevice corrosion is a local attack in a crevice between metal to metal surfaces or between metal to non metals surfaces. One side of the crevice must be exposed to the corrodent and the corrodent must be in the crevice. Crevice corrosion commonly occurs in poorly gasketed pipe flanges and under bolt heads and attachments immersed in liquids.

Dealloying

Dealloying is a process whereby one constituent of metal alloy is removed from the alloy, leaving an altered residual microstructure. The most common alloy susceptible to this process is yellow brass. The removal of zinc from brasses is called dezincification.

CORROSION CONTROL

The use of existing corrosion data is the first step in solving corrosion problems or preventing potential corrosion problems. To simplify the subject of corrosion control, there are three factors that have equal weight in importance : material selection , environment control and design.

STRESS, STRAIN, HOOKE’S LAW, AND MODULUS OF ELASTICITY

STRESS, STRAIN, HOOKE’S LAW, AND MODULUS OF ELASTICITY

INTRODUCTION

Structural materials used in Mechanical and Civil Engineering practice must generally have strength. What is strength ? Strength is due to the sum of forces of attraction between negatively charged electrons and positively charged protons within the material. When covalent bonds join large numbers of atoms to produce giant molecules as is the case of the carbon atoms in carbon fibre, the strength of the resultant material is great.

Constructional materials generally must be able to withstand the action of considerable forces without undergoing other than very small amounts of distortion. Materials must be capable of permanent deformation at the expense of as little energy as possible. That is, it must be malleable and ductile.

Malleability refers to the extent to which a material can undergo deformation in compression before failure occurs, whilst ductility refers to the degree of extension which takes place before failure of a material in tension. All ductile materials are malleable but malleable materials are not necessarily always ductile since a soft material may lack strength and those tear a part very easily in tension. Other mechanical properties include elasticity, hardness, toughness and also creep and fatigue properties. In each case, the property is associated with the behaviour of the material toward the application of force.

DEFINATION OF STRESS

When a force is transmitted through a solid body the body tends to undergo a change in shape. This tendency to deform is resisted by the internal resilience of the body and the body is said to be in a state of stress. Thus, a stress may be described as a mobilized internal force which resists any tendency towards
deformation. The definition to describe the force transmitted per unit area as the intensity of stress or unit stress.
Stress ‘a measurement of density of forces’ is defined as force per unit area of cross section. The Standard Imperial (SI) unit of stress is the Pascal (Pa) which is equivalent to a force of one Newton acting on an area of one square metre, i.e. N/m2 or Nm-2. Numerically it will be the same as that expressed in Mega Pascal (MPa).

All materials bodies will deform when placed in a state of stress, and as the stress is increased the deformation also increases. In such cases, when the loads causing the deformation are removed, the body returns to its original size and shape. A material or a body having this property is said to be elastic. It is also noticeable that if the stress is steadily increased, a point is sooner or later reached when, after the removal of the load, not all of the induced strain is recovered. This limiting value of stress is called the elastic limit.

STRAIN

When a force is transmitted through a solid body the body tends to be deformed. The measure of this change in shape is called strain. When a body is placed in a state of stress it undergoes strain according to the configuration of the stress applied. Thus, direct stresses cause changes in length or shearing stresses cause twisting and bearing stresses cause indentation in the bearing surface.

Strain refers to the proportional deformation produced in a material under the influence of stress. It is measured as the number of metres of deformation suffered per metres of original length and is a numerical ratio.
Strain may be either elastic or plastic. Elastic strain is reversible and disappears when the stress is removed. Strain is roughly proportional to the applied stress

HOOKE’S LAW

The relationship between the induced strain and stress causing it is found to be constant in elastic materials. Hooke’s Law defines that ‘strain is proportional to the stress causing it, providing that the limit of proportionality has not been exceeded’.



Modulus of Elasticity (Young’s Modulus),E

Young’s Modulus of Elasticity (E) is the ratio between the stress applied and the elastic strain it produces. That is, it is the stress required to produce a unit quantity of elastic

strain. It is related to the rigidity of the material. The modulus of elasticity is expressed in terms of either tensile or compressive stresses and its units are the same as those for stress.

MATERIAL PROPERTIES

MATERIAL PROPERTIES
SOLIDS, LIQUIDS, GASES AND PLASTIC (POLYMERIC)

INTRODUCTION

Since the earliest days of the evolution of mankind, the main distinguishing feature between human beings and other mammals has been the ability to use and develop materials to satisfy our human requirements. Woven cloths took the place of animal skins and manufactured goods became increasingly more sophisticated. Nowadays we use many types of materials, fashioned in many different ways to satisfy our requirements for housing, heating, furniture, clothes, transportation, entertainment, medical care, defence and all the other trappings of a modern, civilized society.

Engineers can play an important part in this conservation of material resources by a understanding of the materials used. This understanding is necessary in order to enable them to select the most appropriate materials and to use them with the greatest efficiency in minimum quantities whilst causing minimum pollution in their extraction, refinement and manufacture.

SOLIDS, LIQUIDS AND GASES

Most substances can exist as solids, liquids or gases depending upon their temperature. A notable exception is iodine which sublimes directly from the solid state into the gaseous state when heated without becoming a liquid. Most substances behave like water. Below its freezing point water is a solid (ice). Above its freezing point and below, its boiling point is in the liquid state. If its temperature is increased still further it boils and becomes vapour (steam) before turning into a gas with further heating.
The fact that a substance can exist in the solid state, the liquid state and the gaseous state is due to the fact that the atoms and molecules of substances are in a permanent state of vibration, providing the temperature is above absolute zero (-273 oC), at which temperature, all atomic and molecular movement stops.
When the temperature is low for a given substance to be in its solid state, the vibration is of small amplitude and the atoms and molecules only move to a small extent about a fixed point. When the temperature of a solid is raised to above its melting point, the atoms and molecules of the substance vibrate more violently. They no longer move about a fixed position but are free to move about within the constraints of the container holding the liquid. Finally, if the temperature is raised still further until it is above the temperature of vaporization for substances, the atoms and molecules move so freely that
they can disperse until they completely fill the vessel containing them and if they escape they continue to disperse throughout the atmosphere indefinitely.
This is due to a change in state being accompanied by the taking in or the giving out of latent heat – that is, the heat energy associated with a change of state without an accompanying change of temperature. Heat energy which causes a change of temperature without a change of state is referred to as sensible heat.

POLYMERIC (PLASTIC) MATERIALS


There is an ever-increasing number of synthetic, polymeric materials available under the popular name of plastics. This is a misnomer since polymeric materials rarely show plastic properties in their finished condition. In fact many show elastic properties. The name ‘plastics’ comes from the fact that during the moulding process by which they are shaped, they are reduced to a plastic condition by heating them to just above the temperature of boiling water.

The properties of plastic materials vary widely depending upon composition. A snooker ball is made from hard plastic such as melamine-formaldehyde and has obviously different properties from the soft plastic insulation of a flexible electric cable. For this latter application a plastic material of very different composition is used such as polymerized vinyl chloride (PVC). There are three main groups of polymeric or ‘plastic’ materials which are :-

i. Thermosetting Plastic (Thermosets)
ii. Thermoplastic
iii. Elastomers

Thermosetting Plastic (Thermosets)

This group of polymeric materials undergoes chemical change during the moulding process and can never again be softened by reheating. These materials are generally hard, rigid and rather brittle. A typical example is melamine formaldehyde used for making such articles as snooker and billiards balls, table-wear and domestic electrical fittings. The strength of thermosetting plastics can be greatly increased by reinforcing them with fibrous materials.

Thermoplastic

These become soft and can be remoulded each time they are reheated. They are not so rigid as thermosetting plastics but tend to be tougher. For example, rigid polymerized vinyl chloride (PVC) is used for rain water guttering and down-piping on buildings. Thermoplastics can also be soft. For example, the non-rigid PVC is used for the insulation of flexible cables.

Elastomers

The elastomer, or rubbers are cross-linked polymeric materials in which they are not in sufficient to make them as rigid as the thermosetting plastics, but one just sufficient to make them return to their original dimensions when the deforming load is removed. Elastomers are usually addition polymerized as thermoplastics and then vulcanised with sulphur at approximately every five-hundredth carbon atom. Increasing vulcanization is increase the stiffness and reducing the elongation properties of the materials.

Wednesday, March 4, 2009

Sanitary pipe work

Sanitary pipe work is a system of pipes installed to permit the transfer of waste water and sewage from building to foul drain. Also it provides a means of ventilation for that drain so that there can be no build up unpleasant odors or methane gas within the system which might accidentally permeate into the building. For efficient working of a disposal installation pipe work system, a number of design criteria should be fulfilled.
In the following section we will look at some of the terms related to sanitary pipe work.

DEFINITION OF TERMS

Soil Waste
This is discharged from water closets, urinals, slop sinks and similar appliances.

Soil Pipe
This pipe conveys the discharge of water closets or fixtures with similar function, with or without the discharges from others fixtures.

DIY Ductwork Installation

Typical Leak Search and Repair on Commercial A/C - Part I