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Wednesday, March 19, 2008

Condenser

Since refrigerant leaves compressor in high pressure vapor form, so it requires a way to change vapor into liquid again. This is the function of condensing unit to condense vapor into liquid, so it can reuse in cooling cycle.
When cooling vapor pumped into condenser by compressor, the temperature and the pressure increase, that high temperature facilitate the effective heat transfer from condenser surface to entire room. Some of heat which is transffered to room air is a latent heat that absosrbs refrigerant in evaporator. Heat releasing into room is enough to condense cooling vapor into liquid.
There are some condenser types, i.e. air cooled with free air circulation, force air cooled with fan, indirect air cooled through thermal contact with a cabinet.
This type is usually bigger phisically than force air cooled with fan condenser, and construsted from small finned or fin that surrounded by wire. Condenser is put on back side of refrigerator and continous the thermal expansion to surrounding air room to create a convection current at pipes.
The construction of forced draft condenser type is usually made by small fin. The fan of the condenser convective to circuit, so only work during compressor work.
This type put in compartment and requires air baffles to direct air at that small fins.
Warm wall condenser is other form of static condenser. At that condenser, condenser pipe is attached directly in thermal contact with inside of refrigeration. Heat is removed from refrigeration wall to surrounding air room. The outside wall of refrigeration will feel hot if this type condenser work.



Supercharging

Supercharging meant to multiply the bunkering amounts into combustion chamber, where from the machine itself attached compressor to give the amount more airs, usually take place during intake step. The purpose of airs amount addition to piston chamber to enable the fuels amount more burning, so that will result in the effective pressure and the machine power is getting strength where machine weight only increasing but meaningless.
Supercharging provided with some forms compressor to boost up the pressure in intake channel. Machine for diesel motor at car utilized centrifugal compressor which is turned around by exhaust gas turbine.
Exhaust gas turbine alter the heat of exhaust gas and kinetic energy to rotary movement and there is no energy taken away from the machine to rotate compressor as according to pressure at rotator mechanic cover of compressor. The part of supercharging is consists of the exhaust gas turbine, bearing house and centrifugal compressor. Exhaust gas moves passing through the exhaust pipe direct to turbine side and give rotary movement at turbine impeller. Air intake flows through air filter and pipe to compressor side where cause pressure 0,2 – 1 kg/cm2 that resulted, pressure generation is depend on machine speed.Air pushed through pipe to intake channel and passes intake valve that opened into piston chamber. The shaft of impeller turbine and impeller compressor supported in bearing at bearing house and lubrication of machine lubrication system. The part of supercharging made with high precision, works above 72000 rpm and needs a good balance and bearing duty must very assure.


Saturday, March 15, 2008

Electric Defrost System

For diagnosing exactly about defrost, engineers have to understand about electric defrost system. During refrigerating cycle, evaporator collects humidity in frost form, that humidity heap must removed periodically to maintain refrigerating temperature. If the frost let continuous formed, it will close airflow in evaporator fins, so then heat transfer does not occur and freezer and food temperature will increase.
To defrost, refrigerator must be defrosted. This defrost procedure can be done automatically or by hand. If done by hand, the way is as follows.
1. Remove all frozen food from the freezer and the tray of cold storage; pack that frozen food with some lining of newspaper paper in order not to soften.

2. Direct the damper, so that the defrost water go into the tray of cold storage.

3. Turn the temperature control dial to OFF position.

4. Put a high vessel that contain hot water in freezer and then close the refrigerator door.
5. If the refrigerator overall cleaned when defrosting, food and freezer equipment and inner tray of cold storage can be removed and inside of refrigerator can be cleaned.

6. After the frost melt down, rub the freezer until clean.

7. Do not use sharp tools like knife or ice clamper to scrape the frost, use dull tool like plastic or rubber.

8. Return the tray of cold storage and damper to initial position.

9. Install temperature control dial at initial position.

10. Return the frozen food, inner equipment and food. Suggest to customer for defrosting the freezer if the frost heap reach ¼ inch.

Timed defrost with heater spool, so far the most popular automatic way to defrost refrigerator is timed defrost method that use heater spool.
This defrost system type usually consists of four main part i.e. as follows.


A. Defrost Timer

This automatic timer will active if the refrigerator enters defrost cycle. Generally, defrost timer consists of a motor timer, a gear train, a cam of I set contact, a terminal board, a wire spool, plate of prop and a container box. At some models, defrost cycle is activated by the composition of door switch, a cam at underside of trip of a counter mechanic. If the door has been opened and closed many times as much as amount that has been determined before, for example 20 times, and then defrost operation start works. If not use method above, timer defrost system will work with same pattern with timer automatic.

B. Sheath or Defrost Heater

Sheath or defrost heater, is an electric heater element that defrost (stop cooling) evaporator. Most defrost heater have the rod type that planted at fins or radiant type that placed near evaporator. That both types are easy to be changed.

C. Defrost Thermostat or Limiter

This thermostat is usually bimetal type, that function controlling the duration time of heater that given to evaporator. This thermostat installed at certain place to feel evaporator temperature, so that it can give indication if all frost has removed. Most defrost thermostat will turn off if evaporator temperature reaches 55° until 70°F, defrost heater can not be recharge until bimetal limiter switch getting cold again until 30°F.
Because of this reason, if defrost limiter switch shifted too close to defrost heater, it can reaches cut off temperature too quick, so that all evaporators defrosted. This unit will return to freezing cycle with spool that has not defrosted all. After some cycles, evaporator will be covered by ice. Air cannot flows through closed spool to cool freezer and the refrigerator.

D. Drip Through Heater

Generally, this heater is aluminum foil insulated blanket type and wrapped entire underside of drip through. This heater prevents defrost water freezing in drip through and possible to close drain and air flow to evaporator. Once again, the duration time of giving energy to this heater is determined by defrost thermostat. At model that use radiant heating element, no needs drip through heater. Radiant heating element located below evaporator and has function as evaporator heater and drip through heater.
To finish defrost operation, timer has to stop the fan motor of compressor and condenser to prevent hot air blowing to freezer area.



Thursday, March 13, 2008

Reynolds Number

Reynolds Number,
Re = (V.d1.ρ)/μ
Gives relation between density ρ, dynamic viscosity μ, and average velocity of fluid V in mass flow rates of fluid m, kg/s. From continuity equation, velocity V can be written:
v = (4m)/d­1.d1.ρ.π
By substituting V in equation Re = (V.d1.ρ)/μ, can be obtained :
Re = (4m)/d­1.μ.π




Cavitations

If in a pump installation is used high suction head or if peripheral velocity at inlet side of impeller is too high, so it is possibly happened high enough local vacuum condition at inlet side of impeller. It may cause water vaporization, so that bubbles will appear. This phenomena is called cavitations. Cavitations often happen at area where atmospheric pressure decreased, for instance at mountains area, where water boils at lower temperature than lowland area. The bubbles that occurred will flow together with water flow. If at once upon a time, the bubbles reach a high pressure region, so the bubbles will collapse and disappear. If bubbles appearance and bubbles collapse occurs continuously, pump will noise and it may cause vibration. But, worse than that, cavitations may cause erosion and impeller damage. In worst condition, finally pump can not works anymore.
To prevent that trouble, low suction head should be used, suction pipe dimension to be determined in such a manner so that loss friction that happened can be made as low as possible, suction pressure is higher than vapor pressure of water pumped temperature.


Monday, March 10, 2008

GENERATOR

Generator or referred also as a dynamo, is an appliance that change mechanic energy to electric energy. The work principle of generator is based on electro magnetic induction principality. If a wire is moved so that it cuts a magnetic field (the lines of force), hence at wire will be generated an electro motive force. Electro motive force will cause incidence of electric current that flows through the wire and generated electro motive force depend on magnetic field strength and wire speed to cut the lines of force. Increasing the density of the field or increasing the speed of the wire therefore increases the voltage.

Monday, March 3, 2008

The First Law of Thermodynamics

The First Law of Thermodynamics state that energy can't be created and can't be destroyed either, but it can be changed from one form to another form. This law can be defined as Conservation Energy Law.
This law also can be stated with simple words i.e. during interaction between system and surronding, the amount of energy which is gained by system must be equal with the amount of energy which is discharged to surronding.