Tuesday, 13 December 2016
Monday, 12 December 2016
Refrigeration
Refrigeration: What is a Refrigerant? Refrigerants are used as working substances in a Refrigeration systems. Fluids suitable for refrigeration purposes can be classified into primary and secondary refrigerants. Primary refrigerants are those fluids, which are used directly as working fluids, for example in vapour compression and vapour absorption refrigeration systems. These fluids provide refrigeration by undergoing a phase change process in the evaporator. Secondary refrigerants are those liquids, which are used for transporting thermal energy from one location to other. Secondary refrigerants are also known under the name brines or antifreezes
Desirable properties of a refrigerant Low boiling point (otherwise operation at high vacua becomes a necessity) Low condensing pressure (to avoid heavy machine plant scantling and reduce the leakage risk) High specific enthalpy of vaporisation ( to reduce the quatity of refrigerants in circulation and lower machine speeds, sizes etc.) Low specific volume in vapour state (reduces size and increases efficiency)
High critical temperature (temperature above which vapour cannot be condensed by isothermal compression) Non corrosive and non solvent (pure and mixed) Stable under working conditions Non flammable and non explosive No action with oil ( the fact that most refrigerants are miscible may be advantageous e.e. the removal of oil films, lowering pour points etc, provided separators are fitted Easy leak detect Non toxic cheap, easily stored and obtained
Refrigeration Working cycle: The refrigeration cycle is shown in the Figure and can be broken down into the following stages:
11. Defective Discharge valve: Indication: Continuous running of Compressor. Insufficient cooling effects. Noisy operation. High suction pressure during running. Low discharge pressure during running. Suction pressure rises faster after Compressor is shutdown. Warm cylinder head.
12. Choked Expansion valve:
Why LP Cutoff fitted?
Fitted as safety control and it protect against:
(a) Extreme compression ratio.
(b) Freezing up of Evaporator.
(c) Entrance of air and water vapor resulting from LP side leakage. Fridge compressor sump oil filling: Stop condition:
(i) Tight shut both inlet and outlet valves of compressor.
(ii) Open filling plug and fill to required level.
(iii) Air purge to be done when plant resume.
During running:
(i) Make vacuum pressure in crankcase and suck oil itself.
(ii) Ensure oil pipe immersed in oil to prevent air ingress.
What is made of fridge filter dryer?
Ans: (i) Activated Alumina (Aluminium Oxide)
(ii) Silica Gel (Thorzone) Safety Devices fitted on Fridge Compressor: Safety Head or Unloader. Bursting Disc in Compressor. LP and HP Gauges and Cutout. LO Low Pressure Cutout. Condenser cooling water Low Pressure Cutout
Charging of Refrigeration Plant:
Gas Charging of Refrigeration Plant:For gas charging, a special T piece valve block with mounted pressure gauge is provided to combine three connectors interconnecting: Vacuum pump Charging
Cylinder Charging Point Following steps are to be taken for charging gas into the reefer plant:
1. Connect gas bottle or charging cylinder, vacuum pump and charging point in the reefer system to the valve block.
Air Conditioning:
Relative Humidity: Ratio of amount of water vapour in given volume of air, to maximum amount of water vapour that can be present before precipitation occurs.
Control of temperature: Comfortable temperature range is about 22°C and RH about 60%, (usually 40 ~ 70%).
All zone temperature: Controlled by Compressor suction pressure, via solenoid valve as step controlling.
Particular zone temperature: Controlled by flap valve fitted in each zone loop. Local cabin temperature can be adjusted by volume control at delivery point of air duct controller.
Air Reducing Valve: Fitted on compressed Air Bottle outlet. Reduced compressed air is used for control of Reversing Mechanism in unidirectional gear drive engines, ship whistle, automatic controls and air motors. High pressure air enters under the valve. The spring, acting on the valve spindle, opens the valve and the air passes to thereduced pressure Compression given to the spring controls the amount of opening of the valve. If the opening increases, the higher pressure obtained on other side, acts to close down the valve to normal lift, and hence correct reduced pressure maintained. A Relief Valve is fitted on lowpressure side to prevent excessive pressure rise on reduced air system.
Where fitted Dehumidifier and its function.
Fitted at discharge side of Reducing Valve on control air line. Main function is to prevent oil and condensate water passes through control air line.
Psychrometric chart:
This chart is used for finding the relative humidity of air which has been measured using a ‘wet and dry bulb’ thermometer. This is a pair of thermometers, one of which has its bulb wrapped in a damp cloth. The drier the air,the greater the evaporation of water off the cloth and therefore the lower the reading on the ‘wet bulb’ thermometer.
Dew point: When a mixture of dry air and water vapour has a saturation temperature corresponding to the partial pressure of the water vapour it is said to be saturated. Any further reduction of temperature (at constant pressure) will result in some vapour condensing. This temperature is called the dew point, air at dew point contains all the moisture it can hold at that temperature, as the amount of water vapour varies in air then the partial pressure varies, so the dew point varies.
Desirable properties of a refrigerant Low boiling point (otherwise operation at high vacua becomes a necessity) Low condensing pressure (to avoid heavy machine plant scantling and reduce the leakage risk) High specific enthalpy of vaporisation ( to reduce the quatity of refrigerants in circulation and lower machine speeds, sizes etc.) Low specific volume in vapour state (reduces size and increases efficiency)
High critical temperature (temperature above which vapour cannot be condensed by isothermal compression) Non corrosive and non solvent (pure and mixed) Stable under working conditions Non flammable and non explosive No action with oil ( the fact that most refrigerants are miscible may be advantageous e.e. the removal of oil films, lowering pour points etc, provided separators are fitted Easy leak detect Non toxic cheap, easily stored and obtained
Refrigeration Working cycle: The refrigeration cycle is shown in the Figure and can be broken down into the following stages:
Cycle 1-2: Lowpressure liquid refrigerant in the evaporator absorbs heat from its surroundings, usually air, water or some other process liquid. During this process it changes its state from a liquid to a gas, and at the evaporator exit is slightly superheated.
Cycle 2-3: The superheated vapour enters the compressor where its pressure is raised. The temperature will also increase, because a proportion of the energy put into the compression process is transferred to the refrigerant
Cycle 3-4: The high pressure superheated gas passes from the compressor into the condenser. The initial part of the cooling process (33a) de-superheats the gas before it is then turned back into liquid (3a3b). The cooling for this process is usually achieved by using air or water. A further reduction in temperature happens in the pipe work and liquid receiver (3b – 4), so that the refrigerant liquid is subcooled as it enters the expansion device
Cycle 4-1: The high pressure subcooled liquid passes through the expansion device, which both reduces its pressure and controls the flow into the evaporator Thermostatic Expansion Valve (TEV)
The thermostatic expansion valve performs following functions:
1) Reduce the pressure of the refrigerant: The first and the foremost function of the thermostatic expansion valve is to reduce the pressure of the refrigerant from the condenser pressure to the evaporator pressure. In the condenser the refrigerant is at very high pressure. The thermostatic expansion valve has an orifice due to which the pressure of the refrigerant passing through it drops down suddenly to the level of the evaporator pressure. Due this the temperature of the refrigerant also drops down suddenly and it produces cooling effect inside the evaporator.
2) Keep the evaporator active: The thermostatic expansion valve allows the flow of the refrigerant as per the cooling load inside it. At higher load the flow of the refrigerant is increased and at the lower loads the flow is reduced. It won’t happen that the load on the evaporator is high and the flow of the refrigerant is low thereby reducing the capacity of the evaporator. The thermostatic expansion valve allows the evaporator to run as per the requirements and there won’t be any wastage of the capacity of the evaporator. The TEV constantly modulates the flow to maintain the superheat for which it has been adjusted.
3) Allow the flow of the refrigerant as per the requirements: This is another important function of the thermostatic expansion valve. It allows the flow of the refrigerant to the evaporator as per the load on it. This prevents the flooding of the liquid refrigerant to the compressor and efficient working of the evaporator and the compressor and the whole refrigeration plant.
TEV construction: Small quantity of Vapour Refrigerant is sealed in a bulb or phial, and attached to Compressor suction pipe, just coming out from Evaporator.
Other end is connected by Capillary Tube to the chamber above Flexible Bellow in valve body. The space below the Bellow is in communication with Evaporator outlet pressure(this is called Equalising Line) If no further action is taken, pressure above and below the Bellow will be equalised and hence no superheat is obtained. This is overcome by providing adjustable Bias Spring under the Bellow, and Bias Spring pressure is proportional to required superheat.
Operation: Refrigerant Liquid from Condenser enters into TEV via Dryer, it expands to Evaporation Pressure, and some flash gas is formed. Flash Gas amount varies between 25 – 35%, depending on refrigerant type,plant capacity and ambient temperature. Mixture of this expanded gases and some part of liquid, passed into Evaporator, where complete Evaporation takes place. Evaporator outlet pressure plus Spring pressure tends to close the valve, and is opposed by the pressure above the Bellow, trying to open it. This pressure above the Bellow is in relation to temperature in Compressor suction pipe. Equilibrium condition is reached, when Superheat is correct at phial attachment point. Starved condition in Evaporator will result greater Superheat, so expansion ofVapour Refrigerant in phial will tend to open the valve further, to increase the flow. Flooded condition in Evaporator will result lower Superheat, so contraction ofVapour Refrigerant in phial will tend to close the valve further, so decrease the flow. Superheat Temperature adjusted at: 3 – 6°C, by Bias Spring pressure.
Why Equalising Connection is fitted?
In some plant having large Evaporator or Multicircuit Evaporator, excessive pressure drop across Evaporator occurs, and always tend to starve the Evaporator and increase the Superheat. To counteract this, if pressure drop across Evaporator, exceeds 3 bar, anEqualising Connection must be provided at TEV. A direct connection between underside of Bellow and suction piping of Compressor, preferably between phial and Compressor.
Safety devices on Refrigeration Plant: LP cutout switch: Set at a pressure corresponding to 5°C below the lowest expected evaporating gauge reading. HP cutout switch: Set at a pressure corresponding to 5°C above the highest expected condensing gauge reading.
Lub Oil LP cutout: Oil pressure usually set at 2 bar above crankcase pressure. Condenser cooling water LP cutout. Liquid shock valve on Compressor cylinder head. Bursting disc on cylinder head, between inlet and discharge manifold. Bursting disc on Condenser, [if fitted]. Relief valve on Condenser. Master solenoid valve: To prevent liquid being entered into Compressor, when the plant is standstill, especially in Large Plant.
Refer plant survey: General examination of machinery and testing under working condition. The log examined, to ascertain successful operation during voyages. Compressor and prime mover to be openup and examined. Primary system to be leaktested to their w. p. and brine cooling coils are to behydraulically tested to 3 kg/cm². Survey is done at 1 year from the date of installation, and special periodical surveys are to be carried out at 5 years intervals. ( 1+ 5 )
CFC: Chlorofluorocarbon Due to damaging effects on OZONE layer and causing Global Warming, most CFCs are now replaced by HFCs, HFC 134a has Ozone Depletion Potential, ODP ‘0’ and Global Warming Potential, GWP ‘0.28’. Banned from 19 May 2005
Defrosting: A method of removal of frost, builtup on Evaporator coils. Defrosting should be done before snow thickness exceeds ¼”. Reasons for defrosting: Affecting heat transfer properties. Affecting air flow and circulation. Liquid back to Compressor.
1)Defrosting Systems: Water wash defrosting Hot gas defrosting Electric defrosting Manual shut down defrosting Warm brine defrosting
2)Various methods to defrost Brine System: Hot brine thawing: Best and fastest method, used powerful brine heater with separate thawing system. Watertight trays under the pipes, collected the dripping water.
3)Hot air from atmosphere: It is important that isolating doors in air trunks are perfectly tight, so as to prevent hot air going into cargo spaces.
4)By shutting off brine : Allow the snows to be melted by the heat of the air in circulation. Very slow operation and tends to throw back great deal of moisture into cargo space.
5)Direct expansion grid system: Hot gas defrosting.
6)Battery cooling system: Water spray, electrical or steam heater.
7)Brine cooling: Hot brine thawing.
Cargo Fridge Defrosting: In Battery System, hot brine passing brine heater is used. Steam is released to brine heater and brine flow is restricted by brine inlet valve, until brine temperature has risen above 0° Brine temperature of 43°C is suitable for defrosting.
Why Cold Room is defrosted and how many methods of defrosting?
Coil Room is required to defrost to gain more Heat Transfer Efficiency.
Methods of Defrosting are:
( i) Plant stopped and manual watering
(ii) Hot gas circulating
(iii) Electric Heater.
Faults in Shipboard Refrigeration Systems Undercharging of Refrigeration System
Indication: Compressor is running hot and performance of the compressor falls off due to high superheat temperature at the suction side of compressor. Suction and discharge pressure of the compressor is low. Large vapor bubbles in the liquid sight glass. Low gauge readings in the condenser. Ammeter reading for the compressor motor is lower than normal. Rise in room temperature which is to be cooled. Compressor is running for extended period of time.
Causes: Leakage of refrigerant at the shaft seal, flange couplings, valve gland etc. Expansion valve may be blocked at the strainer. Partial blockage of refrigerant at the filter or drier or evaporator may cause undercharging.
Action: Identify and rectify the leakage of refrigerant from the system. Clean the filter and drier. Charge the system with fresh refrigerant as required.
2. Overcharge of Refrigeration System Indication:
• The liquid level in the condenser is too high (high condenser gauge reading). This will reduce the available condensing surface, with corresponding increase in the saturation temperature and pressure. • High pressure switch of the refrigerant compressor activates and stops the compressor.
• The suction and the discharge pressures are high. Causes:
• It may be due to the reason that excessive refrigerant has been charged in the system.
• Air in the system may also cause over charging indication.
• It may also be due to the formation office on the regulator.
Action:
• Remove the refrigerant from the system. This is done by connecting a cylinder to the liquid line charging valve, starting the compressor, and then operating the charging valve.
• Purge the air from the system and maintain effective cooling.
• Remove ice from the regulator by using any of the defrosting methods.
3. Moisture in the System This normally comes with the ingress of air in the system. Moisture may freeze at the expansion valve, giving some of the indication of under charging. It will contribute to the corrosion in the system. It may cause lubrication problems and breakdown of the lubricating oil in the refrigerant compressor.
Action: – Renew silica gel in case of minor moisture. – collect refrigenant and remove all air and moisture by vacuum pump if the amount is huge.
4. Air in the System Indication:
• This may cause the refrigeration compressor to overheat, with a high discharge pressure
and normal condensing temperature.
• There are possibilities of small air bubbles in the liquid sight glass of the condenser.
• Condensing pressure of the refrigerant in the condenser may be high.
• If there is excessive air, it may reduce the cooling capacity of the system, making the compressor to run for the extended period of time.
• It may cause the gauge pointer of the condenser to jump indefinitely.
Causes:
• During charging, air may enter in to the system.
• If Freon12 is used air may leaks in to the suction line because the working pressure of the Freon12 refrigerant is less than the atmospheric pressure.
Action:
• Air in the system can be removed by collecting the system gas in the condenser, leaving the condenser cooling water on and venting out the air from the top of the condenser because air will not be condensed in the condenser but remains on top of the condenser above the liquid refrigerant.
• Connect the collecting cylinder to the purging line of the condenser, open the valve, and collect air in the cylinder.
• After purging the air from the system don’t forget to shut the purging valve.
• Check the level of the refrigerant in the system. If required, charge the system with fresh refrigerant.
• Restart the compressor with all safety precautions.
5. Oil in the Refrigeration System Indication:
• Temperature is not dropping in the cold rooms as normal, due to fact that oil act as insulation in the evaporator.
• It may cause excessive frost on the suction line.
• Refrigerant compressor runs for the extended period of time.
• Lubricating oil level in the compressor will drop.
• Refrigerant level will fall if oil has caused blockage.
Causes:
• This may happen if the oil separator is not working properly.
• Oil may carry over from the compressor and may not come back to the compressor due to blockage in the system.
• Defective piston rings or worn out liner of the compressor may cause the oil to carry over along with the refrigerant.
• Compressor may take high capacity current during starting.
Action:
• Check the oil separator for proper functioning.
• Check the drier for proper cleaning and if its require cleaning clean it
• It may cause excessive frost on the suction line.
• Refrigerant compressor runs for the extended period of time.
• Lubricating oil level in the compressor will drop.
• Refrigerant level will fall if oil has caused blockage.
Causes:
• This may happen if the oil separator is not working properly.
• Oil may carry over from the compressor and may not come back to the compressor due to blockage in the system.
• Defective piston rings or worn out liner of the compressor may cause the oil to carry over along with the refrigerant.
• Compressor may take high capacity current during starting.
Action:
• Check the oil separator for proper functioning.
• Check the drier for proper cleaning and if its require cleaning clean it
• Evaporator coil should be drained to remove any trace of oil.
• If there is oil in the cooling coils, increase the condenser and evaporator temperature differentials and remove excess frost on the suction pipe.
• Heat pipes with blow torch.
6. Flooding of Refrigerant in the System: This is seen as liquid getting back to the suction of the refrigerant compressor. It may be due to a faulty or incorrectly adjusted expansion valve and also due to solenoid valve leakage. It may also result from overcharging of the refrigeration system. Flooding may lead to an iced up evaporator.
7. Evaporator Coil Icing: Icing of the evaporation coils which may happen due to:
1. Cause: Too low temperature setting Action: Increase the coil temperature by adjusting TEV or it’s sensor.
2. Cause: The coil capacity is less Action: Install large capacity evaporator coils
3. Cause: Defrost is not operational Action: Check if the defrost system is functioning at regular intervals.
8. Compressor Start and Stops Frequently: If while maintaining the correct temperature of the ship’s provision room or reefer cargo, the reefer compressor is frequently cuttingin and out, then such problem needs to be sorted out immediately. The most normal causes for such operation are:
1. Cause: Wrong Setting of Cutouts: It may be because the high pressure (HP) cutout is set too high or LP cutout is set too low Action: Check and change the setting to advisable limit
2. Cause: Differential Setting Span is Small: The low pressure (LP) cut out is provided with starting and stopping pressure setting. If the setting span is too small, it will lead to frequent cutin and cutout of the compressor Action: Change the setting and increase the span between starting and stopping compressor pressures.
3. Cause:Defective Valves: If the compressor discharge valve is leaky or the line solenoid valve is not closing properly, this will lead to variation in sensor pressure and result in frequent cutin and cutout of compressor Action: Replace all the defective valves
4. Cause:Clogged Suction Filters: Compressor is provided with a filter in the suction line. If that is clogged, it will lead to frequent LP cut out Action: clean the filter.
9. Compressor Starts But Stops immediately: When the compressor in the reefer circuit starts and suddenly stops, it can be because of the following reasons:
• If there is oil in the cooling coils, increase the condenser and evaporator temperature differentials and remove excess frost on the suction pipe.
• Heat pipes with blow torch.
6. Flooding of Refrigerant in the System: This is seen as liquid getting back to the suction of the refrigerant compressor. It may be due to a faulty or incorrectly adjusted expansion valve and also due to solenoid valve leakage. It may also result from overcharging of the refrigeration system. Flooding may lead to an iced up evaporator.
7. Evaporator Coil Icing: Icing of the evaporation coils which may happen due to:
1. Cause: Too low temperature setting Action: Increase the coil temperature by adjusting TEV or it’s sensor.
2. Cause: The coil capacity is less Action: Install large capacity evaporator coils
3. Cause: Defrost is not operational Action: Check if the defrost system is functioning at regular intervals.
8. Compressor Start and Stops Frequently: If while maintaining the correct temperature of the ship’s provision room or reefer cargo, the reefer compressor is frequently cuttingin and out, then such problem needs to be sorted out immediately. The most normal causes for such operation are:
1. Cause: Wrong Setting of Cutouts: It may be because the high pressure (HP) cutout is set too high or LP cutout is set too low Action: Check and change the setting to advisable limit
2. Cause: Differential Setting Span is Small: The low pressure (LP) cut out is provided with starting and stopping pressure setting. If the setting span is too small, it will lead to frequent cutin and cutout of the compressor Action: Change the setting and increase the span between starting and stopping compressor pressures.
3. Cause:Defective Valves: If the compressor discharge valve is leaky or the line solenoid valve is not closing properly, this will lead to variation in sensor pressure and result in frequent cutin and cutout of compressor Action: Replace all the defective valves
4. Cause:Clogged Suction Filters: Compressor is provided with a filter in the suction line. If that is clogged, it will lead to frequent LP cut out Action: clean the filter.
9. Compressor Starts But Stops immediately: When the compressor in the reefer circuit starts and suddenly stops, it can be because of the following reasons:
1. Cause: Low pressure cut out gets activated Action: Ensure that all the suction line valves are in open condition, the refrigeration is properly charged and the low pressure cut out is not defective.
2. Cause: Defective oil pressure cut out Action: Check for proper functioning of oil pressure cutout and replace the defective cutout.
3. Cause: Defrosting timer is getting activated frequently Action: If the defrost timer is getting activated frequently, leading to cutout of compressor, check and repair defrost timer.
4. Cause: The lube oil level is below required level Action: This can be because of leakage of lube oil from seal or carry over of oil. Rectify the leakage and refill the oil level.
5. Cause: Foaming of oil leading to reduced oil pressure Action: Ensure no foaming takes place, renew the oil if required.
6. Cause: Motor overload cutouts are activating Action: Ensure that electrical motor trips are working properly.
10. Excessive icing up at Compressor suction: Causes: Abnormal operation of TEV. Overcharge of the system. Moisture in the system owing to dirty Dryer. Defective Suction valve:
2. Cause: Defective oil pressure cut out Action: Check for proper functioning of oil pressure cutout and replace the defective cutout.
3. Cause: Defrosting timer is getting activated frequently Action: If the defrost timer is getting activated frequently, leading to cutout of compressor, check and repair defrost timer.
4. Cause: The lube oil level is below required level Action: This can be because of leakage of lube oil from seal or carry over of oil. Rectify the leakage and refill the oil level.
5. Cause: Foaming of oil leading to reduced oil pressure Action: Ensure no foaming takes place, renew the oil if required.
6. Cause: Motor overload cutouts are activating Action: Ensure that electrical motor trips are working properly.
10. Excessive icing up at Compressor suction: Causes: Abnormal operation of TEV. Overcharge of the system. Moisture in the system owing to dirty Dryer. Defective Suction valve:
Indication: Continuous running of Compressor. Insufficient cooling effects. Noisy operation. High suction pressure.
11. Defective Discharge valve: Indication: Continuous running of Compressor. Insufficient cooling effects. Noisy operation. High suction pressure during running. Low discharge pressure during running. Suction pressure rises faster after Compressor is shutdown. Warm cylinder head.
12. Choked Expansion valve:
Causes: Due to dirt and freezeup of water present in system. Effects: Starved Evaporator High superheat temperature. Rapid Condenser pressure rise can cause stopping of Compressor,
Remedy: Clean Expansion valve and filter Renew Dehydrator.
Secondary Refrigerant: Calcium Chloride Brine ( 3 ½ lb. of Ca Cl₂ + 1 gal. of water ) with density of 1.25 is widely Sodium Dichromate or lime added to maintain pH values of 8.0 – 8.5. Sodium Chloride Brine.
Why LP Cutoff fitted?
Fitted as safety control and it protect against:
(a) Extreme compression ratio.
(b) Freezing up of Evaporator.
(c) Entrance of air and water vapor resulting from LP side leakage. Fridge compressor sump oil filling: Stop condition:
(i) Tight shut both inlet and outlet valves of compressor.
(ii) Open filling plug and fill to required level.
(iii) Air purge to be done when plant resume.
During running:
(i) Make vacuum pressure in crankcase and suck oil itself.
(ii) Ensure oil pipe immersed in oil to prevent air ingress.
What is made of fridge filter dryer?
Ans: (i) Activated Alumina (Aluminium Oxide)
(ii) Silica Gel (Thorzone) Safety Devices fitted on Fridge Compressor: Safety Head or Unloader. Bursting Disc in Compressor. LP and HP Gauges and Cutout. LO Low Pressure Cutout. Condenser cooling water Low Pressure Cutout
Charging of Refrigeration Plant:
There are two methods for charging reefer plants: Liquid charging and Gas charging. Now a day’s gas charging is preferred over liquid charging because it is more safe and simple.
Gas Charging of Refrigeration Plant:For gas charging, a special T piece valve block with mounted pressure gauge is provided to combine three connectors interconnecting: Vacuum pump Charging
Cylinder Charging Point Following steps are to be taken for charging gas into the reefer plant:
1. Connect gas bottle or charging cylinder, vacuum pump and charging point in the reefer system to the valve block.
2. The discharge of the vacuum pump is to be connected in the empty recovery bottle
3. First open the valve between vacuum pump and charging bottle located in the valve block without opening the main valve of the charging cylinder. This will remove all the air inside the pipe. Once vacuum is reached, close the valve of charge cylinder in the valve block
4. Now open the valve of the charging point pipe in the valve block and run the vacuum pump until the vacuum is reached. This will remove the trapped air from this pipe. Then shut the valve in the valve block
5. Now keep the system idle for 5 minutes to check there is no pressure drop. This will ensure there are no leakages in the system
6. Now open charging bottle pipe valve and the charging point pipe valve located in the valve block. This will set the line for charging. Ensure that the vacuum pump valve is shut
7. Now open the main valves in the charging cylinder and charging point of the reefer system
8. Do not overfill the system. Make sure the receiver has 5 % space for expansion Ensure that no refrigerant is leaked out in the environment as these effects the ozone layer in the atmosphere. Gas bottle is kept on weighing scale for measuring the amount of charged supplied to the system.
3. First open the valve between vacuum pump and charging bottle located in the valve block without opening the main valve of the charging cylinder. This will remove all the air inside the pipe. Once vacuum is reached, close the valve of charge cylinder in the valve block
4. Now open the valve of the charging point pipe in the valve block and run the vacuum pump until the vacuum is reached. This will remove the trapped air from this pipe. Then shut the valve in the valve block
5. Now keep the system idle for 5 minutes to check there is no pressure drop. This will ensure there are no leakages in the system
6. Now open charging bottle pipe valve and the charging point pipe valve located in the valve block. This will set the line for charging. Ensure that the vacuum pump valve is shut
7. Now open the main valves in the charging cylinder and charging point of the reefer system
8. Do not overfill the system. Make sure the receiver has 5 % space for expansion Ensure that no refrigerant is leaked out in the environment as these effects the ozone layer in the atmosphere. Gas bottle is kept on weighing scale for measuring the amount of charged supplied to the system.
Air Conditioning:
Relative Humidity: Ratio of amount of water vapour in given volume of air, to maximum amount of water vapour that can be present before precipitation occurs.
Control of temperature: Comfortable temperature range is about 22°C and RH about 60%, (usually 40 ~ 70%).
All zone temperature: Controlled by Compressor suction pressure, via solenoid valve as step controlling.
Thermostat, placed at some accommodation space actuates the Master Solenoid Valve of the plant, which will stop the Compressor, when preset temperature is reached. Capacity Unloader of Compressor units, does last step controlling, as required.
Particular zone temperature: Controlled by flap valve fitted in each zone loop. Local cabin temperature can be adjusted by volume control at delivery point of air duct controller.
Ozone Depletion: Ozone gas layer is a region of the atmosphere, 12 – 30 miles above Earth’s surface. This layer moderates the climate, and protects life on Earth from ultraviolet rediation Release of industrial waste and other process breakdown ozone layer and so disturb natural balance. Chlorofluorocarbons, CFCs, at ground level, rise and broken down by sunlight, whereupon chlorine reacts with and destroys ozone molecules. Single chlorine atom may destroy 10 – 100,0000 ozone molecules.
Ozone Depletion Substance (ODS):
CFC 11-1.0 Halon1211-3.0 (Used in portable extinguishers)
CFC 12-1.0 Halon1301-10.0( Used in fixed installation)
CFC 115-0.6 HCFC 22- 0.05
CFCs: Chlorofluorocarbon Refrigerant
Chlorofluorocarbon Refrigerants includes: CFC11, CFC12, CFC22, CFC 115, CFC500, CFC502, CFC 503 and CFC 504. (8 Types)
Difference between Air Cond. and Fridge: Air Cond. controls Humidity, Temperature and Flow Rate of fresh air. Fridge cools down the provisions.
CFC 11-1.0 Halon1211-3.0 (Used in portable extinguishers)
CFC 12-1.0 Halon1301-10.0( Used in fixed installation)
CFC 115-0.6 HCFC 22- 0.05
CFCs: Chlorofluorocarbon Refrigerant
Chlorofluorocarbon Refrigerants includes: CFC11, CFC12, CFC22, CFC 115, CFC500, CFC502, CFC 503 and CFC 504. (8 Types)
Difference between Air Cond. and Fridge: Air Cond. controls Humidity, Temperature and Flow Rate of fresh air. Fridge cools down the provisions.
Air Reducing Valve: Fitted on compressed Air Bottle outlet. Reduced compressed air is used for control of Reversing Mechanism in unidirectional gear drive engines, ship whistle, automatic controls and air motors. High pressure air enters under the valve. The spring, acting on the valve spindle, opens the valve and the air passes to thereduced pressure Compression given to the spring controls the amount of opening of the valve. If the opening increases, the higher pressure obtained on other side, acts to close down the valve to normal lift, and hence correct reduced pressure maintained. A Relief Valve is fitted on lowpressure side to prevent excessive pressure rise on reduced air system.
Where fitted Dehumidifier and its function.
Fitted at discharge side of Reducing Valve on control air line. Main function is to prevent oil and condensate water passes through control air line.
Psychrometric chart:
This chart is used for finding the relative humidity of air which has been measured using a ‘wet and dry bulb’ thermometer. This is a pair of thermometers, one of which has its bulb wrapped in a damp cloth. The drier the air,the greater the evaporation of water off the cloth and therefore the lower the reading on the ‘wet bulb’ thermometer.
Dew point: When a mixture of dry air and water vapour has a saturation temperature corresponding to the partial pressure of the water vapour it is said to be saturated. Any further reduction of temperature (at constant pressure) will result in some vapour condensing. This temperature is called the dew point, air at dew point contains all the moisture it can hold at that temperature, as the amount of water vapour varies in air then the partial pressure varies, so the dew point varies.
Sunday, 11 December 2016
Steering Gear System
What are the
types of telemotor system in steering gear on ships ?
1.
Hydraulic
system
2.
Electric system
What
are the types of steering system ?
§ Electro hydraulic system
a) Ram type
system (2 ram or 4 ram)
b) Vane type
system
§ All electric system
a) Ward Leonard system
b) Single
motor system
What is meant
be non follow up system in steering gear ?
§ When steering gear set to
required position, rudder is moved & when rudder reach the
required position, steering gear must be set to off position. This system
uses the three solenoid valve.
What is meant
be follow up system in steering gear ?
§ When steering gear set to
required position, rudder is moved & when rudder reaches the
set position, steering gear still remains at that position. This system
uses the hunting gear arrangement.
What is
hunting gear ?
§ It is a feed back mechanism
of steering gear which repositions the floating lever of hydraulic pump as
the tiller moves to the desire position.
What are the
safety devices for steering system ?
1.
Hunting gear
2.
Buffer spring
3.
Angle adjusting
stop (Hand over position limit switch)
4.
Double shock
valve
5.
Relief valve
6.
Tank level
alarm (oil)
7.
Over load alarm
What is the
indication of air in the steering system ?
§ Jumping pressure gauges
§ Jerky operation
§ Defective steering
What is the
effect of air in the steering system ?
§ Air being compressible
gives incorrect balance between units, time lags and irregular operation.
( which can be dangerous )
Emergency
steering gear operation?
§ In the case of Telemotor
failure, by switching the change over pin, emergency steering can be
carried out by isolating the receiver cylinder and directly controlling the
connecting rod of the main steering power unit’s pump lever.
§ The emergency rudder angel
indicator and communication system to bridge being provided at the
emergency station.
Action in
case of electrical telemotor failure ?
§ Put bridge control to
manual
§ Emergency steering gear
system is operated by (solenoid button) whether port or starboard.
§ Rudder angle indicator and
communication system between steering room and bridge must be provided.
How to
operate emergency steering gear ?
1.
Disconnect auto
pilot system.
2.
Take out
change over pin from attachment with telemotor receiver & fit to the hand
gear.
3.
Use
communication system with telephone from steering gear room to bridge.
What are
steering gear tests & maintenance ?
Control test
§ Just prior to 1 hour before
departure of vessel.
12 hour before
departure
§ Operation of main &
auxiliary steering gear.
§ Operation of remote control
system.
§ Operation of emergency
power supply.
§ Alarm test.
§ Actual rudder angle &
indicator.
§ Communication
system.(Bridge, Engine room & Steering gear room)
Every 3 months
interval
§ Emergency steering gear
drill at steering gear room to bridge with sound communication system.
Types of pump
used in hydraulic steering system ?
§ Motor driven constant speed
variable stroke delivery pumps. There are two types.
1.
Radial piston
type ( hele- shaw pump )
2.
Axial piston
type ( swash plate pump )
What are the
advantages of rotary vane type over ram type ?
§ Smaller space required
§ Low installation cost
§ Less weight
§ Smaller power required, for
the same load, because it can transmit pure torque to the rudder stock.
What are the
disadvantages of rotary vane type over ram type ?
§ Synthetic rubber backed
steel sealing strips at vane tops are not strong enough for large ship
gear.
§ Can be used for rudder
stock ratings of about 1700 KNm and less torque generated by two ram is
120 to 160 KNm and for four ram 250 to 10,000KNm.
Steering
tests required before departure ?
1.
Steering gear
should be checked at least one hour prior to departure.
2.
Telemotor
transmitter oil level to be checked
3.
Oil level of
actuating system tank should be checked and replenished if necessary.
4.
Rudder carrier
bearing and bottom sea gland checked and greased.
5.
Start pump and
check response of the gear
6.
Check abnormal
noise and heat
7.
Check load
carrying and running of the gear ( swing from port 35° to stbd 30° within
28 sec )
What are the
properties of telemotor hydraulic fluid ?
§ Good quality mineral
lubricating oil is used. Its properties are-
1.
Low pour point
(-50°C)
2.
Low viscosity (
to reduced fractional drag, but not too thin to mate gland sealing,
30 Redwood Secs at 60°C)
3.
High viscosity
index (110)
4.
High flash
point (150° C closed)
5.
Non sludge
forming
6.
Non
corrosive
7.
Good
lubricating properties
8.
Specific
gravity 0.88 at 15.5° C
What is the
purpose of buffer spring ?
§ To prevent the damages of
the control system.
§ To move the rudder in
either direction instantly when required
§ Should come to reset
immediately in the position corresponding to that shown on indicator on
the bridge.
§ Provision must be made to
protect the steering gear from damage should a heavy sea strike the
rudder.
§ The design should be
sample, the construction robust and its performance reliable at all times.
Purpose of
swivel block in steering gear ?
§ To control linear movement
of the rams to the rotary movement to the tiller arms and rudder stocks.
§ It is the one types of
electrical steering gear system. Which control the speed of DC motor from
zero to maximum in either directions.
In four rams
type steering gear system, what unit you make in service when one cylinder
damage ?
§ If one cylinder damage,
four rams steering gear can be used as two rams type steering gear.
§ First place the rudder at
mid position.
§ Isolate the circuit valves
of two cylinder inlet.
§ One cylinder is defective
and another one, but they are not in diagonal position. Open the vent
at that cylinder.
Purpose of
buffer spring ?
§ Absorb the difference
between the steering order speeds and follow up speed.
§ Absorb the movement of
steering wheel if it is mishandled when the hydraulic pump stop in.
§ Absorb the movement of the
control lever when rudder drift
§ Absorb the vibration and
shocks from the rudder.
When carry out
emergency steering system ?
§ Every voyage (UHA)
§ Once at least within 3
Months (SOLAS)
§ During Survey
What are
daily check in steering gear room ?
§ Pressure gauge of steering
pump.
§ Motor ampere on the
steering switch board & motor hand touch feeling
§ Noise and vibration.
§ Oil level in tank
§ Oil leakage in system
§ Grease in rudder carrier
bearing
§ Check the bottom seal gland
whether good or not.
1.
Every ship
shall be provided with a main steering gear and an auxiliary steering gear.
2.
The failure
of one of them will not render the other one inoperative.
3.
Relief valves
shall be fitted to any part of the hydraulic system.
4.
The main
steering gear and rudder stock shall be:
5.
(a) of
adequate strength and capable of steering the ship at maximum ahead service
speed. (b) capable of putting the rudder over from 35′ on one side to 35′
on the other side with the ship at its deepest sea going draught and
running ahead at maximum ahead service speed and, under the same
conditions, from 35′ on either side to 30′ on the other side in not more than
28 seconds. (c) So that they will not be damaged at maximum astern speed.
6.
The auxiliary
steering gear shall be: (a) of adequate strength and capable of steering
the ship at navigable speed and of being brought speedily into action in
an emergency. (b) capable of putting the redder over from 15′ on one side
to 15′ on the other side in not more than 60 seconds with the ship at its
deepest seagoing draught and running ahead at one half of the
maximum ahead service speed or 7 knots, whichever is the greater.
7.
In every
tanker, chemical tanker or gas carrier of 10,000 gross ton and upwards and in
every ships of 70,000 gross ton and upwards, the main steering gear shall
comprise two or more identical power units.
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