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.

Wednesday, November 30, 2011

How to Start a Fresh Water Generator

The modern equipment we use for this is examined in the following sections; here we will see the methods and sequence involved in start-up the various components, as well as when it is safe to run the generators. The first section deals with the regulations regarding the safe processing of drinking water from seawater.

Regulations Regarding Safe Production of Drinking Water from Seawater

Current marine regulations regarding seawater distillation using a fresh water generator stipulate;
  • The FW generator can only be started when the ship is 12 nautical miles away from the nearest coastline.
  • The engine must be running at full sea speed at start of passage as advised from the bridge.
This will ensure all the main engine temperature and pressure parameters are normal, main engine is on sea speed and not in congested waters and all maneuvering completed

Start-Up Procedures

The fresh water generator should be fired up once the above regulations and requisites have been achieved; using the guidelines listed below.
A sketch showing the various components is shown below and can be referred to when reading the guidelines.
1. Check the jacket cooling water temperature outlet from the main engine - it should be constant
2. Open both the ejector pump suction valve and the overboard discharge valves.
3. Close the vacuum breaker valve
4. Open main sea water feed inlet valve inlet and discharge valves to and from the generator condenser; (the seawater can also be supplied from a stand-by sea water pump; if this is to be used instead, open the pump main inlet and discharge valves and start the pump)
5. Check salinometer (salinity indicator) and distillate pump operation
6. Start the ejector seawater pump and maintain the pressure of 5kg/cmor higher
7. Check that the vacuum inside the shell is slowly rising as the ejector removes the air from the unit.
8. When the vacuum gauge reading reaches about 17mm of mercury, slowly open the seawater feed to heating tubes.
9. Check the seawater level inside the shell through the sight glass and adjust this water level using the feed inlet valve.
10. Once the heating coils have been covered with seawater; open the jacket water inlet valve slowly to the full open position whilst throttling back the jacket water outlet valve.
11. The effect of the jacket cooling water circulating through the heating tubes causes the shell vacuum to drop, its temperature to rise and feed water level to fall.
12. When all the above is stabilized and running normally; the seawater starts evaporating and steam can be observed rising up through the demister units to be condensed by the condenser coils.
13. Once condensed, the fresh water droplets fall downwards to be collected in the plate collector tray. This can be witnessed through the sight glass
14. When the gauge glass level on the plate collector is more than ¾ full, put the salinity indicator to 'ON' position.
15. Divert the processed water to the bilges or return it to the feed system again till the salinity level comes to the required set value of 5-10 ppm (Parts per million)
16. When the salinity level comes to set value, open the discharge valve of distillate pump to the fresh water tank through the flow meter and start the pump
17. The evaporation rate can be increased by throttling the jacket cooling water return outlet
18. Check for the tank vacuum has stabilized.
19. Observe the following gauge readings on a regular basis;
  • Inlet and outlet temperature of jacket cooling water
  • Condenser sea water inlet and out let temperature
  • Feed water level inside the shell
  • Distillate level
Notes:
  • The salinometer is designed to alarm, automatically shutting the freshwater discharge valve to the storage tank, and dumping the distillate to bilge when maximum salinity is exceeded. It is always best to check the diverter valves are all operating as these can stick due to heat/coating of salts.
  • The distillate level in gauge glass should be monitored and always maintained at half gauge glass level when the distillate pump is running
  • Remember to check the main engine jacket water cooling temperatures once the freshwater generator has settled down.

Monday, January 17, 2011

COMPRESSOR MAINTENANCE


COMPRESSOR a machine that increases the pressure of a gas or vapor (typically air), or mixture of gases and vapors. The pressure of the fluid is increased by reducing the fluid specific volume during passage of the fluid through the compressor. When compared with centrifugal or axial-flow fans on the basis of discharge pressure, compressors are generally classed as high-pressure and fans as low-pressure machines.

TYPES OF COMPRESSOR

 Compressors fall into two groups:
The first group operates on the displacement principle where is compressed by containing it in a chamber and then reducing the volume of this chamber.  This type is called piston compressor.  (Reciprocating piston compressor, rotary piston compressor.)
The second group operates on the air-flow principle: by drawing in air on side and compressing it by mass acceleration (Turbine.)


RECIPROCATING PISTON COMPRESSOR

 The reciprocating piston compressor is, at the present time, the most widely used compressor.  It can be used not only for compressing to low and medium pressures, but also to high pressure.  The pressure range extends from approx. 100 kPa (1 bar/14.5 psi) up to several thousand kPa (bar/psi).

Diaphragm Compressor

This type of compressor belongs to the piston compressor group.  The piston is separated from the suction chamber by a diaphragm, i.e. the air does not come into contact with the reciprocating parts.  Thus, air is always kept free of oil. For this reason, its use is preferred in the foodstuffs industry, pharmaceutical and the chemical industries.

ROTARY PISTON COMPRESSORS

The rotary piston compressor is a compressor with rotating pistons.  At the same time, chambers are compacted and the air in these chambers is compressed.


SLIDING VANE ROTARY COMPRESSOR

 Sliding vanes are contained in slots in the rotor and form chambers with the cylindrical wall.  When rotating, the centrifugal energy forces the vanes against the wall, and owing to the shape of the housing the chambers are increased or reduced in size. 

TWO-AXLE SCREW COMPRESSOR

Two intermeshing rotors, one having a convex profile and the other a concave profile. Displace the axial entering air to the other side 

ROOTS BLOWER

 In these compressors, the air is conveyed from one side to the other without any change in volume.  The piston edges produce sealing on the pressure side. 

FLOW COMPRESSORS
(Turbo Compressors)

 These work on the air-flow principle and are especially suitable for large delivery volumes.  Flow compressors are made as axial and radial types.
 The air state is converted in one or more turbine wheels to flow velocity.  This kinetic energy is converted to pressure energy.
AXIAL FLOW COMPRESSOR

The air is accelerated by the blades in an axial direction of flow

RADIAL-FLOW COMPRESSOR

Acceleration from chamber to chamber radially outwards, reversal of the flowing air and return to the shaft.  From there, again acceleration outwards

Thursday, January 13, 2011

Electric motor

By Wikipedia

An electric motor converts electrical energy into mechanical energy. Most electric motors operate through interacting magnetic fields and current-carrying conductors to generate force, although a few use electrostatic forces. The reverse process, producing electrical energy from mechanical energy, is done by generators such as an alternator or a dynamo. Many types of electric motors can be run as generators, and vice versa. For example a starter/generator for a gas turbine, or traction motors used on vehicles, often perform both tasks.


Electric motors are found in applications as diverse as industrial fans, blowers and pumps, machine tools, household appliances, power tools, and disk drives. They may be powered by direct current (e.g., a battery powered portable device or motor vehicle), or by alternating current from a central electrical distribution grid. The smallest motors may be found in electric wristwatches. Medium-size motors of highly standardized dimensions and characteristics provide convenient mechanical power for industrial uses. The very largest electric motors are used for propulsion of large ships, and for such purposes as pipeline compressors, with ratings in the millions of watts. Electric motors may be classified by the source of electric power, by their internal construction, by their application, or by the type of motion they give.


Categorization of electric motors

The classic division of electric motors has been that of Alternating Current (AC) types vs Direct Current (DC) types. This is more a de facto convention, rather than a rigid distinction. For example, many classic DC motors run on AC power, these motors being referred to as universal motors.

Rated output power is also used to categorize motors, those of less than 746 Watts, for example, are often referred to as fractional horsepower motors (FHP) in reference to the old imperial measurement.

The ongoing trend toward electronic control further muddles the distinction, as modern drivers have moved the commutator out of the motor shell. For this new breed of motor, driver circuits are relied upon to generate sinusoidal AC drive currents, or some approximation thereof. The two best examples are: the brushless DC motor and the stepping motor, both being poly-phase AC motors requiring external electronic control, although historically, stepping motors (such as for maritime and naval gyrocompass repeaters) were driven from DC switched by contacts.

Considering all rotating (or linear) electric motors require synchronism between a moving magnetic field and a moving current sheet for average torque production, there is a clearer distinction between an asynchronous motor and synchronous types. An asynchronous motor requires slip between the moving magnetic field and a winding set to induce current in the winding set by mutual inductance; the most ubiquitous example being the common AC induction motor which must slip to generate torque. In the synchronous types, induction (or slip) is not a requisite for magnetic field or current production (e.g. permanent magnet motors, synchronous brush-less wound-rotor doubly-fed electric machine).


Comparison of motor types

Type Advantages Disadvantages Typical Application Typical Drive
AC polyphase induction squirrel-cage Low cost, long life,
high efficiency,
large ratings available (to 1 MW or more),
large number of standardized types
Starting inrush current can be high,
speed control requires variable frequency source
Pumps, fans, blowers, conveyors, compressors Poly-phase AC, variable frequency AC
Shaded-pole motor Low cost
Long life
Rotation slips from frequency
Low starting torque
Small ratings
low efficiency
Fans, appliances, record players Single phase AC
AC Induction
(split-phase capacitor)
High power
high starting torque
Rotation slips from frequency
Starting switch required
Appliances
Stationary Power Tools
Single phase AC
Universal motor High starting torque, compact, high speed Maintenance (brushes)
lifespan
Only small ratings economic
Drill, blender, vacuum cleaner, insulation blowers Single phase AC or DC
AC Synchronous Rotation in-sync with freq - hence no slip
More expensive Industrial motors
Clocks
Audio turntables
tape drives
Poly-phase AC
Stepper DC Precision positioning
High holding torque
High initial cost
Requires a controller
Positioning in printers and floppy drives DC
Brushless DC Long lifespan
low maintenance
High efficiency
High initial cost
Requires a controller
Hard drives
CD/DVD players
electric vehicles
DC
Brushed DC Simple speed control Maintenance (brushes)
Medium lifespan
Costly commutator and brushes
Steel mills
Paper making machines
Treadmill exercisers
automotive accessories
Direct DC or PWM
Pancake DC Compact design
Simple speed control
Medium cost
Medium lifespan
Office Equip
Fans/Pumps
Direct DC or PWM

Power

The power output of a rotary electric motor is:

P = \frac {rpm \times T} {5252}

Where P is in horsepower, rpm is the shaft speed in revolutions per minute and T is the torque in foot pounds.

And for a linear motor:

P = F \times v

Where P is the power in watts, and F is in Newtons and v is the speed in metres per second.

Efficiency

To calculate a motor's efficiency, the mechanical output power is divided by the electrical input power:

\eta = \frac{P_m}{P_e},

where η is energy conversion efficiency, Pe is electrical input power, and Pm is mechanical output power.

In simplest case Pe = VI, and Pm = Tω, where V is input voltage, I is input current, T is output torque, and ω is output angular velocity. It is possible to derive analytically the point of maximum efficiency. It is typically at less than 1/2 the stall torque.

[edit] Goodness factor

Professor Eric Laithwaite proposed a metric to determine the 'goodness' of an electric motor:

G = \frac {\omega} {resistance \times reluctance} = \frac {\omega \mu \sigma A_m A_e} {l_m l_e}

Where:

G is the goodness factor (factors above 1 are likely to be efficient)
Am,Ae are the cross sections of the magnetic and electric circuit
lm,le are the lengths of the magnetic and electric circuits
μ is the permeability of the core
ω is the angular frequency the motor is driven at

From this he showed that the most efficient motors are likely to be relatively large. However, the equation only directly relates to non permanent magnet motors.


Uses

Electric motors are used in many, if not most, modern machines. Obvious uses would be in rotating machines such as fans, turbines, drills, the wheels on electric cars, locomotives and conveyor belts. Also, in many vibrating or oscillating machines, an electric motor spins an irregular figure with more area on one side of the axle than the other, causing it to appear to be moving up and down.

Electric motors are also popular in robotics. They are used to turn the wheels of vehicular robots, and servo motors are used to turn arms and legs in humanoid robots. In flying robots, along with helicopters, a motor causes a propeller or wide, flat blades to spin and create lift force, allowing vertical motion.

Electric motors are replacing hydraulic cylinders in airplanes and military equipment.[24][25]

In industrial and manufacturing businesses, electric motors are used to turn saws and blades in cutting and slicing processes, and to spin gears and mixers (the latter very common in food manufacturing). Linear motors are often used to push products into containers horizontally.

Many kitchen appliances also use electric motors. Food processors and grinders spin blades to chop and break up foods. Blenders use electric motors to mix liquids, and microwave ovens use motors to turn the tray food sits on. Toaster ovens also use electric motors to turn a conveyor to move food over heating elements.