Properties of commonly used refrigerants

Properties of Commonly Used Refrigerants:

1. Carbon dioxide:

Carbon dioxide is widely as refrigerant in mechanical systems refrigerant, marine services, hospitals etc. due to its excellent safety properties. It is odourless, non-toxic, non-flammable, non-explosive and non-corrosive.

2. Sulphur dioxide:

Sulphur dioxide was widely used as refrigerant during early 20th century. However its use has been restricted now-a-days because of its many inherent disadvantages. It is highly toxic, non-flammable, non-explosive, non-corrosive and works at low pressures

3. Ammonia:

Ammonia is one of the earliest type of refrigerants which is still widely used in many applications due to its inheritance excellent thermal properties, It is toxic in nature, flammable explosive under certain conditions, it has low specific volume¸ high refrigerating effect, low piston displacement in case of reciprocating compressors make it an ideal refrigerant for cold storage’s, ice plants, packing plants, skating rinks breweries etc.

4. Freon-11:

Freon-11 (Trichloro fluoromethane) is used under low operating pressures; it is non-toxic, non-corrosive and non-flammable. Due to low operating pressure and high displacement, it is used in systems employing centrifugal compressors. It is used for air-conditioning applications.

5. Freon-12:

Freon-12 (Dichloro difluoromethane) is non-flammable, non-toxic and non-explosive. It is highly chemically stable. If it is brought in contact with open flame or heater elements, it decomposes into highly toxic constituents. It has not only excellent safe properties but also condenses at moderate pressure under normal atmospheric conditions.

6. Cryogenic refrigerants:

Cryogenic refrigerants are those refrigerants which produce minus temperature in between range -157°C to -273°C in the refrigerated space. The cryogenic refrigerants have exceptionally low boiling point at atmospheric pressure. Some of the widely used cryogenic refrigerants are Helium, Nitrogen, Oxygen, Hydrogen.

Mechanical properties

Mechanical properties:

Mechanical properties consist of tensile strength, toughness, malleability, hardness, Ductility, stiffness, brittle ness, Elasticity, plasticity etc.

Tensile strength

Tensile strength is the ability of the material to withstand the tensile loads without breaking.

Strength:

Strength is the ability of a material to attack the applied forces without fracturing.

Toughness:

Toughness is the ability of the material to withstand bending, or it is an application of shear stresses without fracture. The rubber or many plastic materials do not scatter so they are tough.

Malleability:

It is the capacity of the substance to tolerate deformation under compression without separation or the malleable materials, to allow a useful amount of plastic deformation. This is it undergoes compressive loading before fracture occurs. Such a material is required for manipulation by the process and rolling, forging and rivet heading.

Hardness:

Hardness is the ability of the material to withstand scratching or depression by another hard body; it is an indication of the wear resistance of the material.

Ductility:

Ductility is the capacity of the substances to undergo deformation, under tension without rupture as in wire drawing.

Stiffness:

Stiffness is used to measure the ability of the material, and not to deflect under an applied load.

Brittleness

Brittleness is the property of a material that demonstrates little or no plastic deformation before break when a force is applied. Also it is typically said in the opposite manner to ductility and malleability.

Elasticity:

Elasticity is the ability of a material to deform under load and return to its original shape and size when the load is removed. If it is made from elastic material it will be the same length before and after the load is applied. All materials possess elasticity to some degree and each one of it has its own elastic limits.

Plasticity

Plasticity is the property which is strictly opposite to elasticity; the ductility and malleability are actual cases of plasticity. Plasticity is the state of a material which has been fully loaded beyond its elastic limit so as to cause the material to deform permanently. Under such conditions the material takes a permanent set and will not return to its original size and shape when the load is removed.

• What is the thing(s) that only Mechanical Engineers know?

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• We know that AC isn't just about making coldness, it can be used to make a room hot too because it is a Air conditioning. You can also use an AC as an improvised heater by turning it backwards, so it blows colder air out and hot air in but that would be far from efficient. Also, it adjusts the air velocity and the humidity.
• We know the reason behind the air conditioning system in ICU room is not just for luxury hospital but to cool the machinery also. Overheating the machinery can cause malfunctions.
• We know the reason behind why airplane windows always have round edges instead of square because sharp corner of square window is weakened by stress concentration.

• We know “why are tyres in black color ?” because of chemical compound carbon black is added in rubber which increases the strength and durability of tyres and carbon maintains the quality of tyres by protecting them from UV lights and ozone.

• We know “why are there stones alongside railway tracks ?” - The crushed stones are what are known as ballast. Their purpose is to hold the wooden cross ties in place, which in turn hold the rail in place. It is used to bear load from the rail road ties, to facilitate drainage of water & also to keep down vegetation that might interfere with the track.

• We know “why do some tuned car have very inclined tires ?” - The tilt of the wheel is know as camber angle. Tilting the wheel in that manner ( as shown in pic) is called negative camber. Mounting the wheel with a negative camber improves grip under hard cornering as it counteracts rolling.

• We also know “what does the green zone in speedometer indicate ?” - It is the zone where you will have more efficient speed where the fuel consumption is low.

• We know “why are roller bearings present in the bridge ?” - They are there to release any strain induced due to shrinkage and expansion of the bridge girder.( due to temperature, creep and shrinkage of concrete )

• We know that “It's more efficient to drive with Air conditioning on than window open above the speeds of about 55–60 kmph” because opening the window increase the air drag, and slows the car down so it needs more fuel to run.

Why IRCTC does not allow you to choose seats? Would you believe that there is a mechanical reason behind this?

Why IRCTC does not allow you to choose seats? Would you believe that there is a mechanical reason behind this?

Booking a seat in a train is far more different than booking a seat in a theatre.

Theatre is a hall, whereas train is a moving object. So safety concern is very high in trains.

Indian railways ticket booking software is designed in such a way that it will book tickets in a manner that will distribute the load evenly in a train.

Let me take an example to make things more clear : Imagine there are sleeper class coaches in a train numbered S1, S2 S3... S10, and in every coach there are 72 seats.

So when some one first books a ticket, software will assign a seat in the middle coach like S5, middle seat numbered between 30-40, and preferably lower berths (Railways first fills the lower berths than upper one so as to achieve low centre of gravity.)

And the software books seats in such a way that all coaches have uniform passenger distribution and seats are filled starting from the middle seats (36) to seats near the gates i.e 1-2 or 71-72 in order from lower berth to upper.

Railways just want to ensure a proper balance that each coach should have for equal load distribution.

That is why when you book a ticket at the last, you are always allotted an upper berth and a seat numbered around 2-3 or 70, except when you are not taking a seat of someone who has cancelled his/her seat.

What if the railways book tickets randomly ? A train is a moving object which moves around at a speed of around 100km/hr on rails.

So there are a lot of forces and mechanics acting on the train.

Just imagine if S1, S2, S3 are completely full and S5, S6 are completely empty and others are partially full. When the train takes a turn, some coaches face maximum centrifugal force and some minimum, and this creates a high chance of derailment of the train.

This is a very technical aspect, and when brakes are applied there will be different braking forces acting at each of the coaches because of the huge differences in weight of coach, so stability of train becomes an issue again.

I felt that this is a good information worth sharing, as often passengers blame the Railways citing inconvenient seats/ berths allotted to them

Differences Between Centrifugal and Axial Flow Compressors

Differences BetweenCentrifugal and Axial Flow Compressors

S.no	Centrifugal Compressors	Axial Flow Compressors
1	In centrifugal compressors air flows radially in the compressor	In Axial flow compressors air flows parallel to the axis of shaft
2	Low maintenance and running cost	High maintenance and running cost
3	Low starting torque is required	Requires high starting torque
4	Not suitable for multi staging	Suitable for multi staging
5	Suitable for low pressure ratios up to 4	Suitable for only multi staging ratio of 10
6	For given mass flow rate, it requires a larger frontal area.	For a given mass flow rate, it requires less Frontal area.
7	Isentropic efficiency is 80 to 82%	Isentropic efficiency is 86 to 88%
8	Better performance at part load	Poor performance at part load

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Comparison between centrifugal compressor and axial flow compressors can be done in aspects like air flow direction, maintenance cost, torque requirement, multi staging suitable or not, pressure ratios conditions, frontal area, isentropic efficiency and performance at part load.

Types of Brake system

Brake System Classification

Brake system is used in automobiles to slow down or stop vehicle by converting its kinetic energy into heat energy.

Classification of Brake system:

1. On the basis of mode of actuation:

1.      Foot brake (also called main brake) operated by foot pedal

2.      Hand brake – it is also called parking brake operated by hand

2. On the basis of mode of operation

1.      Air brakes

2.      Electric brakes

3.      Hydraulic brakes

4.      Mechanical brakes

5.      Vacuum brakes

3. On the Basis of Action on Front or Rear Wheels

1.      Front-wheel brakes

2.      Rear-wheel brakes

4. On the Basis of Method of Application of Braking Contact

1.      Externally – contracting brakes

2.      Internally – expanding brakes

Types of fluids

Types of fluids

Fluids are divided into five types they are:

• Ideal fluids
• Real fluids
• Newtonian fluids
• Non-Newtonian fluids
• Ideal plastic fluids

Ideal Fluids:

An ideal fluid is defined as a fluid which is in-compressible and the one that does not have viscosity. Ideal fluids are imaginary fluids. This exist some viscosity.

Real Fluids:

A real fluid is defined as a fluid which possesses viscosity. In actual state all fluids are real fluids.

Newtonian fluids:

In a real fluid the shear stress is directly proportional to the velocity gradient or shear strain, which is known as Newtonian fluids.

Non-Newtonian fluids:

In a real fluid the shear stress is not proportional to the velocity gradient or shear strain, which is known as Non- Newtonian fluids.

Ideal plastic fluids:

In a fluid the shear stress is more than the yield value. The shear stress is proportional to the rate of velocity gradient or shear strain is known as ideal plastic fluids.

Thermodynamic properties:

Fluid consists of either liquid or gases. In case of gases they are compressible fluids. The thermodynamic properties play an important key role. Due to the change in temperature and pressure the gases undergoes high variation in densities. So the relation between the absolute temperature, absolute pressure and specific volume is

= RT

 = RT

P= Absolute pressure of a gas

R = gas constant

T= Absolute temperature in kelvin

ρ = Density of a gas.

R Dimension

The gas constant R value must be depends on the particular gas.

In MKS Unit R value is  

In SI units  

Isothermal Process:

At constant pressure the change in density occurs then the process is known as isothermal process. The relation between pressure and density is:

p/ρ= constant

Adiabatic process:

The change in the density must be occurs without heat change to and fro the gases is known as Adiabatic process.

Due to friction there is no heat generation in the gases, so the relation between the density and pressure is

K value for air is 1.4

Universal gas constant:

It is also known as gas constant, ideal gas constant, molar gas constant. It is denoted by the letter R.

In SI units R value is 8314 J/kg-mole.

Compressibility and bulk modulus:

The reciprocal of bulk modules of elasticity is known as compressibility. It is defined as the ratio of compressive stress to volumetric strain.

Consider a cylinder and piston is inserted into it, when the force is applied on the piston then the pressure is increased to p+dp. Then the volume present in the cylinder is decreased to   to  .

Volumetric strain must be =

Bulk modules = K = increase in pressure / volumetric strain

Compressibility = 1/ K

For gases relation between Bulk modulus and pressure:

K=pk