Monday, December 5, 2011

Heat

Heat is divided into two parts when name it, one is Sensible Heat and Latent Heat is the other.

A. Sensible Heat – is the heat for increasing and decreasing temperature of a matter.

B. Latent Heat – is the heat for changing the state of matter without changing its temperature. Generally, the matter changes the state into three, solid, liquid and gas according to its temperature or pressure.

C. Superheated – is vapor that has temperature higher than saturation temperature.

D.  Enthalpy – also known as heat content, it is the amount of heat and energy in a substance. It is measured in sources in terms of the change in heat accompanying a chemical reaction that take place at constant pressure.
      For system of internal energy U, pressure P and volume V.


                                  Enthalpy (H) = U + PV


E.  Entropy – a difficult content of thermodynamics, it is the measure of unavailability of a system’s energy to do work – that is, a measure of its disorder.

F.  Ton of refrigeration – a substance that by undergoing a change in phase (liquid to gas, gas to liquid) releases or absorbs a large latent heat in relation to each volume, and thus effect a considerable cooling effect. It is the working fluid, which pick sup the heat from the enclosed refrigerated space and transfers it to the surroundings.



1Pa = 1N/m2 = 9.87 ATM
1KPa = 1KN/m2



     It should be understood that there is no such thing as an ideal refrigerant, due to various application and operating conditions, different refrigerants are available for use. For instance, the heat of fusion is 79.6 Kcal/kg for water and the evaporation heat is 539 Kcal/kg at atmospheric pressure


Liquid + Latent Heat    = Vapor

Vapor – Latent Heat    = Liquid
                   ── SENSIBLE HEAT


                
HEAT     ─
                                                                                                 Add             Add
                                               ── the heat of fusion                 Heat           Heat
                                                (Congelation)                           [SOLID – LIQUID

                             LATENT __
                          HEAT
                                               ── the heat of evaporation
                                                    (Condensation) [LIQUID – GAS]
                                                                                                                                           

BASIC THERMODYNAMIC THEORY

Thermodynamic concerns the behavior of materials when they are heated or cooled. In general, when solid is heated it melts and becomes liquid boils and becomes a gas. The sequence is reversible and if heat is removed from a gas it returns to liquid form. The temperatures involved in the melting and boiling process depend on the material involved.

In changing:
From solid to liquid   :         fusion
From liquid to vapor  :         vaporization
From vapor to liquid  :         condensation
From liquid to solid   :         solidification


The Second Law of Thermodynamics

          This states that heat always flows from a hot body to a cooler one and is of fundamental significance to liquefied gas carriage. If the temperature of the sea or air is above cargo temperature, heat will flow into the cargo until the temperatures are equal. One purpose of the cargo tank insulation is to reduce the amount of the heat that leaks into the cargo.


Ideal Gas Laws

          There are many laws, which describe the behavior of gases, and the most important ones are given here. A gas that obeys them exactly is called a perfect gas. Typical cargo gases obey these laws quite closely.

Boyle’s Law

       States that at constant temperature, the volume of a given mass of gas varies inversely to its absolute pressure. If, in process, a perfect gas at constant temperature changes from initial pressure and volume, P1 and V1, to final pressure and volume, P2 and V2, then by Boyle’s Law:

                             P1V1 = P2V2

Charles’ Law

       States that the volume of a given mass of gas at constant pressure varies in proportion to its absolute temperature. If the initial and final volumes of the gas are V1 and V2 and the initial and final temperatures are T1 and T2, then Charles’ Law:

      V1             V2
     ———  =  ———
     T1              T2

The General Gas Equation

Is derived by combining the above laws and is stated as:

 P1 V1            P2 V2
————  =  ————
  T1                 T2

Or PV = mRT where m is the mass of gas and R is called the gas content which can be obtained from tables.


The gas of one-gram molecule (1 mole) occupies 22.42 at the standard state 0°C and under the standard atmospheric pressure of 760 mmHg.


 Figure 2 Temperature/heat energy relationships for the various states of matter 



Friday, December 2, 2011

Mechanical Refrigeration System

Figure A Typical Refrigeration Plant


                         Various types of refrigerating systems are used for naval shipboard refrigeration and air conditioning. The system that is used most often for refrigeration purposes is the vapor compression cycle with reciprocating compressors.
Figure B


Figure B is a simple drawing of the vapor compression refrigeration cycle. As you study this system, try to understand what happens to the refrigerant as it passes through each part of the cycle. In particular, be sure you understand why the refrigerant changes from liquid to vapor and from vapor to liquid and what happens in terms of heat because of these changes of state. We will trace the refrigerant through its entire cycle, beginning with the thermostatic expansion valve (TXV).
Liquid refrigerant enters the expansion valve, which separates the high-pressure side of the system and the low-pressure side of the system. This valve regulates the amount of refrigerant that enters the cooling coil. Be-cause of the pressure differential, as the refrigerant passes through the TXV, some of the refrigerant “flashes” to a vapor (changes state from a liquid to a gas).


From the TXV, the refrigerant passes into the cooling coils or evaporator. The boiling point of the refrigerant under the low pressure in the evaporator is usually maintained at about 20°F lower than the temperature of the space in which the cooling coil is installed. As the liquid boils and vaporizes, it absorbs latent heat of vaporization from the space being cooled. The refrigerant continues to absorb latent heat of vaporization until all the liquid has been vaporized. By the time the refrigerant leaves the cooling coils, it has not only absorbed its latent heat of vaporization but has also picked up some additional (sensible) heat. In other words, the vapor has become SUPER-HEATED. As a rule, the amount of superheat is 8° to 12°F.


The refrigerant leaves the evaporator as low-pressure superheated vapor. The remainder of the vapor compression cycle serves to carry this heat away and convert the refrigerant back into a liquid state. In this way, the refrigerant can again vaporize in the evaporator and absorb the heat. The low-pressure superheated vapor flows out of the evaporator to the compressor, which provides the mechanical force to keep the refrigerant circulating through the system. In the compressor cylinders, the refrigerant is compressed from a low-pressure, low-tem-perature vapor to a high-pressure vapor, and its temperature rises accordingly. The heated high-pressure R-12 vapor is discharged from the compressor into the condenser, which is simply a heat exchanger that uses water or air as a coolant. Here the refrigerant condenses, giving up its superheat (sensible heat) and latent heat of condensation. The cooled refrigerant, still at high pressure, is now a liquid again.


From the condenser, the refrigerant flows into a receiver, which serves as a storage place for the liquid refrigerant in the system. From the receiver, the refrigerant goes to the TXV and the cycle begins again.


This type of refrigeration system has two pressure sides. The LOW-PRESSURE SIDE extends from the TXV up to and including the intake side of the compressor cylinders. The HIGH-PRESSURE SIDE extends from the discharge valve of the compressor to the TXV.

REFRIGERATION - Development of Refrigeration

REFRIGERATION

Development of Refrigeration


The most important application of refrigeration is the preservation of food. Most foods kept at from temperature spoil rapidly. This is due to the rapid growth of bacteria. But at refrigeration temperature of about 400 F (40 ) the growth of bacteria is quite slow. Refrigeration preserves foods by keeping it cold and at this temperature it will keep much longer. Other important uses of refrigeration include air conditioning, beverage cooling, humidity control and manufacturing processes.

During the 18th century, the refrigeration industry became commercially important. Early refrigeration was obtained through the use of ice from lakes and ponds by cutting and storing in insulated storerooms during winter for summer use. The use of natural ice required the building insulated containers or iceboxes for stores, restaurants and homes. The units first appeared during the 19th century on a large scale.

Ice was first made artificially as an experiment at about 1820 until 1834 did artificial ice manufacturing became practical. An American engineer, JACOB PERKINS, Invented the apparatus, which was the forerunner of our modern compression systems. In 1855 a German engineer produced the first absorption type of refrigerating mechanism using the principles discovered by Michael Faraday in 1824. Shortly after 1890 little artificial ice was produced. During the 1890 a warm winter resulted in a shortage of natural ice, which help start the mechanical ice-making industry.

Mechanical domestic refrigeration first appeared about 1910. J.M. LARSEN produced a manually operated household machine in 1913. KELVINATOR produced the first automatic refrigerator for the American market by 1918.The first of the sealed or “hermitic” automatic refrigeration units was introduced by GENERAL ELECTRIC in 1928 naming it as MONITOR TOP.

Beginning with 1920, domestic refrigeration became one of our important industries. The ELECTROLUX, which was an automatic absorption unit, appeared in 1927.on the same year automatic refrigeration units appeared for the comfort cooling part of air conditioning. Fast freezing to preserve food for extended periods was developed about 1923. This marked the beginning of the modern frozen foods industry.

 

 




History of Refrigeration


Before the advents of mechanical refrigeration, ICE, formed by natural freezing and stored until used, was the only source of refrigeration. As ice, under atmospheric pressure, always melt at 0oC (320F), it produces refrigeration as it absorbs heat in melting. Mixtures of salt and ice produce temperature lower than 00C (320F). When ordinary salt (NaCI) and finely divided ice (snow) are brought into contact, the melting (fusion) temperature is depressed to about- 21.280C (-6.30F) and heat is absorbed at this lower temperature, while the ice melts and the salt goes into solution. Certain acids and alcohols have a similar effect in depressing the melting temperature of ice. Another refrigerating material is solid carbon dioxide (dry ice), which at atmospheric pressure sublimes at-78.0C (-109.30F) and absorbs 570.97 KJ/Kg (246 Btu/lb.) of dry ice. At the present day, the production of dry ice have been reduced for the main reason that it affects the atmospheric condition of the earth through the so called “ Global Warming” or “Green House Effect” Also it was found that modern types of refrigerants, halons and some chlorinated products causes ozone depletion which in turn destroys the earth’s protective layer or shield against ultra violet radiation off settings our very own ecological balance.

To obtain fully flexible ranges of temperature or to produce refrigeration in quantity, mechanical (artificial) means must be employed. The ton of refrigeration is the absorption of heat at the rate of 12,660 KJ/hr. (12,000 Btu/hr) or 211 KJ/min. (200 Btu/min). Historically, the ton of refrigeration represented refrigeration equivalent to one-ton weight of ice melting in 24 hours. The rating or capacity of a refrigerating machine or unit is expressed in the amount of heat absorbed or rejected per unit time (Btu/hr, KJ/hr, Kcal/min. etc.) or in tons with a statement of the temperature) or temperature range) at which the machine or units are in producing its rating. Formerly all vapor refrigeration machine were rates in terms of the tons of refrigeration they could produce, when the evaporator operated at the pressures corresponding to boiling of the refrigerant at –150C 950F) and to the condensation of the refrigerant at 300C (860F). Because of the broader present-day uses of refrigeration, as in air conditioning, quick-freezing, low-temperature, and chemical process refrigeration, the- 150C (50F), + 300C (860F) rating is inadequate and a large number of rating temperatures are used.

Temperature of –23.30C (-100F), -8.70C (200F) and 4.40C (400F) are use for the evaporator and condensation temperatures of 350C (950F), 82.2) C (1000F), 40.60C) (1050F) and 43.30C (1100F) allow for the more extreme condition met when condensing with cooling tower water or with air.

But the progress of civilization and the desire for the man to control his natural environment have led to new development in applied science as related to refrigeration and air conditioning. Today, refrigeration is essential in the production and distribution of food and for the efficient operation of industry. Because of air conditioning, people live more comfortably and healthfully, and many industrial operations are conducted more effectively.