aux 2 june 11 2021 Flashcards
- Explain the operation of the hydraulic system shown in figure. (10)
Fig Q1
James Erswell
- With reference to induction motor starters:
(a) state when a ST AR/DEL T A starter may be required; (2)
(b) describe the operation of a ST AR/DEL T A starter; (5)
(c) explain why the motor configuration is changed from STAR to DELTA. (3)
(a) A Star/Delta starter may be required for an induction motor when the motor is being started from standstill. This type of starter is used for starting large three-phase induction motors with a high starting current requirement. The Star/Delta starter provides a reduced voltage to the motor during starting, which reduces the starting current and thus helps to protect the motor and the power supply from damage.
(b) The operation of a Star/Delta starter involves connecting the motor windings in a star configuration during starting and then switching to a delta configuration once the motor has reached a certain speed. During starting, the voltage applied to the motor windings is reduced by connecting the windings in a star configuration, which results in a reduced starting current. Once the motor has reached a certain speed, the windings are switched to a delta configuration, which provides the motor with the full voltage and current required for normal running.
(c) The motor configuration is changed from Star to Delta because the Star configuration provides reduced voltage and current to the motor during starting, which reduces the starting current. Once the motor has reached a certain speed, the windings are switched to the Delta configuration, which provides the motor with the full voltage and current required for normal running. The change from Star to Delta configuration is necessary to allow the motor to reach its full operating efficiency, as the Star configuration only provides reduced voltage and current, which is not enough to maintain the motor’s full operating speed.
- Explain what happens to the output voltage of an a.c. generator from sudden application of a
large load to a steady state condition. (10)
When a large load is suddenly applied to an a.c. generator, the output voltage will initially drop due to the increased demand for current. This is because the generator’s internal impedance (primarily the resistance of its windings) limits the amount of current that can be supplied, and the increased load causes the current to increase, resulting in a drop in voltage.
However, as the generator continues to run with the increased load, it will eventually reach a steady state condition where the output voltage will settle at a new, lower level. This is because the generator’s voltage regulation system (which can include a voltage regulator, exciter, or other components) will adjust the generator’s output to maintain a stable voltage despite the increased load.
In steady state, the generator’s voltage will be determined by the balance between its internal impedance and the load impedance, and the generator’s voltage regulation system will continuously adjust to keep the voltage within acceptable limits.
Overall, the output voltage of an a.c. generator will experience a temporary drop when a large load is suddenly applied, but will eventually settle at a new, lower level in steady state as the generator’s voltage regulation system compensates for the increased demand.
Describe the indications of, and the remedies for, an undercharge on a refrigeration system.
Undercharging a refrigeration system can lead to several issues, often characterized by certain signs. Let’s start with the indications, and then we’ll go over the potential remedies.
Indications of Undercharge in a Refrigeration System
Inadequate Cooling: The most direct symptom of an undercharge is that the system is not providing sufficient cooling. This may be evident in higher-than-normal temperatures in the areas the system is designed to cool.
Low Pressure Readings: If you have pressure gauges installed, an undercharged system will typically show lower than normal pressure readings on both the high-side (discharge) and low-side (suction) of the system.
Ice Formation: If the refrigerant level is too low, the evaporator coil may become too cold, causing moisture from the air to freeze and form ice on the coil. This can lead to reduced air flow and even less effective cooling.
Long Run Cycles: The system may need to run for longer periods than normal to maintain the desired temperature because it isn’t cooling efficiently.
Increased Energy Usage: Inefficient cooling can lead to increased energy use, as the system has to work harder to achieve the desired temperature.
Remedies for Undercharge in a Refrigeration System
Refill the Refrigerant: The most direct solution is to refill the system with the appropriate amount of refrigerant. However, this must be done by a professional to ensure that the correct type and amount of refrigerant is used, and that it is properly installed without causing damage or leaks.
Repair Leaks: Undercharging is often the result of leaks in the system. Before simply refilling the system, it’s important to check for and repair any leaks. Otherwise, you’ll end up with the same problem again in the near future.
Check and Adjust Controls: If the system isn’t cooling properly, the issue might be with the controls rather than the refrigerant level. Check the thermostat settings and other controls to make sure they’re set correctly.
Regular Maintenance: Regular servicing of the system can prevent undercharging issues by ensuring that any potential problems are identified and addressed before they become serious.
Replacement: If the refrigeration system is old and underperforming, it might be more cost-effective in the long run to replace it with a new, more efficient model.
Remember, refrigerant handling and HVAC repairs should always be carried out by certified professionals due to the potential hazards and technical knowledge required.
Explain, with the aid of a sketch, the procedure for vapour re-charging of a refrigeration plant.
Vapor recharging of a refrigeration plant involves adding refrigerant to the system to replace what has been lost due to leaks or other causes. The process typically involves the following steps:
Preparation: Before starting the recharging process, the refrigeration system should be turned off and the pressure in the system should be relieved. This can be done by opening the discharge valve to allow the refrigerant to escape.
Connecting the recharge equipment: The recharge equipment, which typically consists of a refrigerant cylinder, a pressure gauge, and a refrigerant charging hose, should be connected to the system’s service valves. The service valves are typically located on the high-pressure and low-pressure sides of the system.
Measuring the system’s charge: Using the pressure gauge, the system’s current refrigerant charge can be measured. This information is used to determine the amount of refrigerant needed to bring the system back to its proper charge.
Adding refrigerant: The refrigerant is added to the system by opening the cylinder valve and allowing refrigerant to flow into the system. The pressure gauge is used to monitor the system’s pressure and ensure that the correct amount of refrigerant is being added.
Monitoring the system: As the refrigerant is added to the system, the pressure gauge should be used to monitor the system’s pressure. The process should be stopped when the desired pressure is reached, indicating that the proper amount of refrigerant has been added.
Checking for leaks: After recharging the system, it is important to check for leaks to ensure that the refrigerant will not escape from the system. This can be done using a refrigerant leak detector or by using a soap solution to check for bubbles at joints and connections.
Here is a sketch to illustrate the procedure for vapor recharging of a refrigeration plant:
[Sketch not available in text format]
In conclusion, vapor recharging of a refrigeration plant involves adding refrigerant to the system to replace what has been lost due to leaks or other causes. The process involves connecting the recharge equipment, measuring the system’s charge, adding refrigerant, monitoring the system, and checking for leaks.
- Describe, with the aid of a sketch, a three-stage air compressor system, suitable for compressed
air diving equipment. (SCUBA).
A three-stage air compressor system suitable for compressed air diving equipment (SCUBA) typically consists of three main components:
The first stage, which takes in atmospheric air and compresses it to an intermediate pressure. This stage is typically cooled by an aftercooler to remove moisture and prevent condensation from building up in the system.
The second stage, which further compresses the air from the first stage to a higher pressure. This stage is also cooled by an aftercooler to remove additional moisture and maintain a dry, clean air supply.
The third stage, which is the final stage of compression and raises the pressure of the air to the desired level for SCUBA diving. This stage may also have a filter to remove any remaining particles or moisture before the air is stored in a high-pressure tank.
The compressed air is then supplied to a SCUBA tank, where it is stored at high pressure and used by the diver as needed.
Here’s a basic sketch of a three-stage air compressor system for SCUBA diving:
[Insert sketch of a three-stage air compressor system here]
In this sketch, the air is drawn in from the atmosphere and enters the first stage of compression, where it is compressed and cooled. The compressed air then moves on to the second stage, where it is further compressed and cooled. Finally, the air reaches the third stage, where it is compressed to the desired level for SCUBA diving, filtered, and stored in a high-pressure tank.
- Describe the safety requirements for the electrical installation in a large locker on deck,
designated for the carriage of petrol, or vehicles with fuel in their tanks. (10)
The electrical installation in a large locker designated for the carriage of petrol or vehicles with fuel in their tanks must comply with a number of safety requirements to minimize the risk of fire or explosion. Some of the key safety requirements include:
Electrical equipment must be explosion-proof or intrinsically safe, meaning that it is designed to prevent ignition of flammable or explosive atmospheres in the event of a malfunction.
All electrical wiring must be adequately protected against physical damage and must be installed in conduit or other approved protective covering.
All electrical equipment must be properly grounded to prevent the buildup of static electricity, which could ignite fuel vapors.
Electrical equipment must be located a safe distance from fuel storage tanks and fuel dispensing areas.
Electrical equipment must be properly maintained and inspected regularly to ensure that it is functioning safely and that there are no signs of damage or wear.
All electrical equipment must be approved for use in hazardous locations and must be marked with the appropriate hazardous location rating.
Adequate ventilation must be provided in the locker to prevent the buildup of fuel vapors and to disperse any fuel vapors that may escape from storage tanks or fuel dispensing areas.
Emergency lighting must be provided in the locker to allow for safe evacuation in the event of a power failure or other emergency.
Fire suppression systems must be installed in the locker to suppress fires in the event of an ignition of fuel vapors.
All personnel working in the locker must be trained in safe electrical practices and must be aware of the potential hazards associated with fuel storage and dispensing.
These safety requirements are designed to minimize the risk of fire or explosion in a large locker designated for the carriage of petrol or vehicles with fuel in their tanks. It is important to strictly adhere to these requirements to ensure the safety of personnel and the protection of equipment and property.
- With reference to longitudinal stresses in a vessel’s hull:
(a) state the cause of the stress; (3)
(b) state the areas where the stress is a maximum; (3)
(c) describe the structure that resists the stress.
(a) The cause of longitudinal stress in a vessel’s hull is the bending moment created by the weight of the vessel and its cargo, as well as external forces such as waves and wind. These forces cause the hull to bend, creating compressive and tensile stresses along its length.
(b) The areas where the stress is a maximum are at the midship section and at the ends of the vessel, where the bending moment is the greatest.
(c) The structure that resists the longitudinal stress in a vessel’s hull is the hull girder, which is a structural member running the length of the vessel that provides stability and strength. The hull girder is made up of the keel, which runs along the bottom of the vessel, and the frames, which are transverse members running perpendicular to the keel. Together, these structural members form a box-like structure that is able to resist the bending moment and longitudinal stress in the hull. The hull girder is supported by the hull plating, which is the outer skin of the vessel that provides additional strength and resistance to the stresses. In larger vessels, additional longitudinal members may also be added to increase the strength and stability of the hull girder.
