Monday, December 5, 2011
|Figure 2 Temperature/heat energy relationships for the various states of matter|
Friday, December 2, 2011
|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 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.
Development of Refrigeration
History of Refrigeration
Wednesday, November 30, 2011
Regulations Regarding Safe Production of Drinking Water from Seawater
- 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.
- Inlet and outlet temperature of jacket cooling water
- Condenser sea water inlet and out let temperature
- Feed water level inside the shell
- Distillate level
- 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
RECIPROCATING PISTON COMPRESSOR
Thursday, January 13, 2011
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, |
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 |
|Rotation slips from frequency |
Low starting torque
|Fans, appliances, record players||Single phase AC|
|AC Induction |
|High power |
high starting torque
|Rotation slips from frequency |
Starting switch required
Stationary Power Tools
|Single phase AC|
|Universal motor||High starting torque, compact, high speed||Maintenance (brushes) |
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 |
|Stepper DC||Precision positioning |
High holding torque
|High initial cost |
Requires a controller
|Positioning in printers and floppy drives||DC|
|Brushless DC||Long lifespan |
|High initial cost |
Requires a controller
|Hard drives |
|Brushed DC||Simple speed control||Maintenance (brushes) |
Costly commutator and brushes
|Steel mills |
Paper making machines
|Direct DC or PWM|
|Pancake DC||Compact design |
Simple speed control
|Medium cost |
|Office Equip |
|Direct DC or PWM|
The power output of a rotary electric motor is:
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:
Where P is the power in watts, and F is in Newtons and v is the speed in metres per second.
To calculate a motor's efficiency, the mechanical output power is divided by the electrical input power:
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.
 Goodness factor
Professor Eric Laithwaite proposed a metric to determine the 'goodness' of an electric motor:
- 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.
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.
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.