Friday, August 26, 2011

Assumptions in Thermodynamic Cycles

                    The analysis of all thermodynamics cycles is based on the following assumptions:

1. The gas in the engine cylinder is a perfect gas, i.e. it obeys the gas laws and constant specific heats.
2. The physical constants of the gas in the engine cylinder are same as those of air at moderate temperature.
3. All the compression and expansion processes are adiabatic and they take place without any internal friction.
4. Heat is supplied by bringing a hot body in contact with the cylinder at appropriate points during the process. Similarly heat is rejected by bringing a cold body in contact with the cylinder at these points.
5. The cycle is considered to be a closed one and the same air is used again and again to repeat the cycle.
6. No chemical reaction, whatsoever, takes place in the engine cylinder.

Wednesday, August 24, 2011

Law of Equipartition of Energy

                   This Law states, "The total energy of a molecule is shared equally by the various degrees of freedom possessed by it."
                     In case of mono atomic molecules like argon and helium, the energy possessed by them is only that of transnational type, the rotational energy being negligible. We have already discussed in Art.
                              Energy of translation per molecule = 3/2*kT
                             Energy per molecule per degree freedom = 1/3*3/2kT

Degrees of Freedom

                         In order to describe completely the motion of a particle in one plane, only two quantities must be known, say its two rectangular components. Similarly, for a particle moving in space, three independent quantities must be known to describe its motion. A molecule in a rigid body can have three motions of vibration along any of three co-ordinates axis in addition to its three motions of translation. It is thus obvious that in order to completely describe the state of motion of a particle, six independent quantities must be known.
                       In general, the total number of independent quantities, which must be known for describing completely the state of motion of a body, are called its degrees of freedom.

Thursday, August 18, 2011

Assumptions in the Kinetic Theory of Gases

The kinetic theory of gases is based on the following assumptions:
1. The volume of a gas consists of a large number of minutes particles called molecules. it has been experimentally found that there are about 26.8*10(power 18) molecules in 1ml of a gas at N.T.P.
2. The molecules are mere mass points. In other words, the size of a molecules is assumed to be negligible, as compared to the distance between the molecules.
3. The gas molecules are perfectly elastic sphere and exert negligible force of attraction or repulsion on one another, or on the walls of the containing vessel. Hence in a direct impact, the molecules rebound with the same velocity after each collision.
4. The molecules are continuously colliding against each other, and with the walls of the containing vessel.Between two collisions, a molecule moves in a straight line. This distance is called the free path of the molecule.
5. The time during which a collision takes place is negligible as compared to the time required to transverse the free path, i.e collisions are instantaneous.

Wednesday, August 17, 2011

Clausius Inequality

           The Clausius inequality state that whenever a closed system undergoes a cyclic process, the cyclic integral of &Q/T is less than zero for an irreversible cyclic process and equal to zero for a reversible cyclic process. Mathematically,
                                          infinity&Q/T < 0, for an irreversible cyclic process
                                          infinity &Q/T = 0, for a reversible cyclic process
   combining the equations is written as
                                                            infinity -< 0
The Clausius inequality not only gives mathematically expression to the second law of thermodynamics, but it also gives the quantitative measure of irreversibly of the system. For example, the equation for an irreversible cyclic process may be written as,
                                                   infinity &Q/T + 1 = 0
Where 1 represents the amount by which the given cyclic process irreversible. When 1 is equal to zero, then the given cyclic process will be reversible. Moreover, a cyclic process in which infinity dQ/T is more then zero, is impossible because it violates the second law of thermodynamics.

Thursday, August 11, 2011

Units of Entropy

The unit of entropy depends upon the unit of heat employed and the absolute temperature. We know that
                                                           Change in entropy (dS) = &Q/T
       Therefore, if the heat supplied or rejected is in kJ and the temperature is in K, then the unit of entropy is kJ/K. The entropy may be expressed in so many units entropy without assigning any dimensional units. Since the entropy is expressed per unit mass of the working substance, it would be more correct to speak specific entropy. The absolute values of entropy cannot be determined, but only the change in entropy may be obtained by using equation.
     Theoretically, the entropy of a substance is zero at absolute zero temperature. Hence, in entropy calculation, some convenient datum should be selected from which measurement may be made.
        It may be noted that water 0degree C is assumed to have zero entropy, and changes in its entropy are reckoned from this temperature.

Wednesday, August 10, 2011

Available and Unavailable Heat

The heat energy of a system is considered to have the following two parts:

1.Available heat energy
2.Unavailable heat energy
  The available heat energy is that part of the heat energy which can be converted into mechanical work by ideal process which reduce the system in a state of equilibrium.
  The unavailable heat energy is that part of heat energy which can not be converted into mechanical work even by ideal process which reduce the system in a state of equilibrium.The common ideas used for unavailable heat energy, according to second Law of Thermodynamics, is the heat rejected by the system to the surrounding.
     From above, we have total energy or heat energy or heat supplied to the system,
                                                   &Q = Available heat energy + Unavailable heat energy
                                                          = A.H.E + U.H.E = Workdone + heat rejected 

Tuesday, August 9, 2011

Importance of Entropy

The maximum possible efficiency obtainable by any engine working on a reversible Carnot Cycle is given by
                                                                      n = T1-T2/T1
                                                                      T1 = Highest absolute temperature, and 
                                                                      T2 = Lowest absolute temperature.
in general, efficiency is given by
                                                     n = Maximum work obtained/Heat supplied or absorbed = &W/&Q
For one degree temperature drop, the above expression may be written as
                                                               &W = $Q/T = dS = Change in entropy 
From This expression, it can be easily understand that
1. The change in entropy represents the maximum amount of work obtainable per degree drop in temperature.
2. The change in entropy may be regarded as a measure of the rate the availability of heat for transformation into work.
3. The increase in entropy is obtained from a given quantity of heat at a low temperature.

Monday, August 8, 2011

Relation Between Heat and Entropy

Consider the heating of a working substance by a reversible process as shown whose base represents the entropy and the vertical ordinate represents the absolute temperature.
Now Consider any point A on the curve 1-2. At this point, let a small quantity of heat (&Q) be supplied to the working substance, which will increase the entropy by dS. Let the absolute temperature at this instant be T. Then according to the definition of entropy,
&Q = TdS
From Fig we see that the terms TdS represents the area under the curve during this change of entropy. 
From Equations we get
ds = &Q/T
The total change in entropy may be obtained by integration the above expression from state 1 to state 2.
then we found
TdS = dU + pdv

Wednesday, August 3, 2011

The Availability of the Clamp on Flow Meter

It is truly important to understand that there is not only one clamp on flow meter, but rather you are able to determine the type of flow meter that you need and/or want, and then you usually have the option of choosing a clamp on flow meter for that particular type of flow meter that you have chosen.
Understanding Thermal Energy
In order to understand what you would need a clamp on flow meter for to begin with, you first need to understand a bit about thermal energy; basically thermal energy is mainly distributed by fluid media to the points of consumption, and the energy manager involved in this situation would not only be interested in the total energy required, but also in that of the consumption of individual heat consumers and in the flow of the energy in the plant in general.
The reason that thermal energy relates to something such as a clamp on flow meter is due to the fact that the clamp on flow meter is a device which is typically used in this type of situation, in order for the members involved to be able to read the flow and velocity of the thermal energy that is involved in that particular situation.
The flow measurement involved in this type of situation would be based on that of the ultrasonic transit time technique, and this method would thus utilize the transmission of sound waves in the fluid itself, and sound pulses would then be sent alternatively downstream and upstream through the liquid the entire way.
Furthermore, you should also know that the transducers for coupling these sound signals through the pipe clamp from the outside onto the pipe, all the while ensuring that there is no actual disturbance to the flow itself, may be a rather expensive installation, however are incredibly important at the same time, and so they would be well worth the purchase in the end.
You can generally find these at your local hardware store, and you can find some at better prices than others, and so you will thus want to take a bit of time so that you can find the best quality and worth for your money that you possibly can; it truly will be worth it in the end, and so use the resources that are available to you as well in order to help you in this situation.

Tuesday, August 2, 2011

Thermal Profiler for Wave Soldering Process

Among the different process involved in the wave soldering process thermal profiling is one of the most important processes which can affect the wave soldering process to a great extent. Thermal profiling is one of the most important processes which should be performed very carefully so as to make the production of the printed circuit board good in quality. Thermal profiling is done during the travel of the printed circuit board through the whole process. 

Thermal profiling is the process of measuring the time against the temperature during the whole travel of the product through out the process from the beginning to the end. The collection of the data like the peak temperature, time above the liquids, immersing of the product into the wave and more also comes under the process of thermal profiling during the wave soldering process. These whole processes comprises of the thermal profiling process for the wave soldering. The data collected during the thermal profiling process helps in making the process of wave soldering even more efficient and to bring a valuable improvement in the quality and the consistency in the quality of the product. So we can say that the process of thermal profiling is an important part of the process of wave soldering and it contributes in a successful completion of the wave solder process.

The devices used for the thermal profiling is known as the “Thermal Profiler” or some people even call them as “Data Loggers” for the proves of the wave soldering. Actually the thermal profilers are basically the temperature recorders which are used for keeping the record of the temperatures during the whole process and they have the capability for traveling through the complete process of wave soldering along with the product so as to get the actual reading for the temperature that the product observes.

There are various thermal profilers or can say temperature recorders available in the market but there are some specific companies which manufactures the thermal profilers especially for the wave soldering process. These thermal profilers are used in the reflow profiling process of the wave soldering. These profilers also help in optimizing the wave soldering process. They provide pretty accurate data for the whole process which makes the optimization for the process even more affective.

General thermal profilers are equipped with the thermocouple based sensor which is attached to the surface of the PCB and then the PCB is made to travel through the thermal profiler which helps in monitoring the complete data for the wave solder process. The most common thermal profiler is the one which downloads the information while it is passing through the whole process.

Now these thermal profilers have become a must for the people involved in the business of the PCB manufacturing. It helps in optimizing the process of the wave soldering to make it even more efficient.