Aux 2 Unit 8 Q2

The basic laws of refrigeration govern the principles behind the operation of refrigeration systems. These laws, derived from the fundamental laws of thermodynamics, provide a framework for understanding how heat is transferred and manipulated to achieve cooling.

The three primary laws of refrigeration are:

1. Heat flows from a higher temperature body to a lower temperature body.

This fundamental principle states that heat energy naturally moves from a region of higher temperature to a region of lower temperature. In a refrigeration system, this principle is harnessed to transfer heat from the refrigerated space (low temperature) to the external environment (higher temperature).

2. Heat cannot be transferred from a lower temperature body to a higher temperature body without the expenditure of energy.

This law, often referred to as the Second Law of Thermodynamics, highlights the need for external work or energy input to move heat against its natural flow. In a refrigeration system, the compressor is responsible for providing this work, enabling the refrigerant to transfer heat from the cold space to the warmer surroundings.

3. A liquid absorbs heat when it changes its state from liquid to vapor (evaporation) and releases heat when it changes its state from vapor to liquid (condensation).

This law is the basis for the vapor-compression refrigeration cycle, the most common type of refrigeration system. The refrigerant undergoes phase changes (liquid to vapor in the evaporator, and vapor to liquid in the condenser) to facilitate the transfer of heat.

Additional Laws and Principles Relevant to Refrigeration:

• Boyle’s Law: The pressure of a gas is inversely proportional to its volume at a constant temperature. This law is applicable to the compression and expansion processes of the refrigerant within the system.
• Charles’s Law: The volume of a gas is directly proportional to its temperature at a constant pressure. This law also influences the behavior of the refrigerant as it undergoes temperature changes in the evaporator and condenser.
• The Ideal Gas Law: Combines Boyle’s and Charles’s laws to describe the relationship between pressure, volume, and temperature of a gas. It’s helpful for understanding the behavior of the refrigerant in the system.  
• Dalton’s Law of Partial Pressures: The total pressure of a mixture of gases is equal to the sum of the partial pressures of the individual gases. This law is relevant when dealing with refrigerant blends or the presence of non-condensable gases in the system.  

In conclusion, the basic laws of refrigeration, along with other relevant principles, provide the scientific foundation for understanding how refrigeration systems operate. By harnessing the natural flow of heat and utilizing external work to manipulate the refrigerant’s phase changes, these systems effectively transfer heat from a cold space to a warmer environment, achieving the desired cooling effect.

Location of the High Pressure Cut Out:

The high-pressure cut-out is typically located on the discharge side of the compressor, between the compressor and the condenser in a refrigeration system. It’s installed in the high-pressure line to protect the system from excessive pressure buildup that could damage components or lead to safety hazards.

Method of Resetting:

The exact method of resetting the high-pressure cut-out can vary slightly depending on the specific model and manufacturer, but generally involves the following steps:

1. Identify the Cause of the Trip:
   • The high-pressure cut-out trips when the system pressure exceeds its preset limit. It’s essential to investigate and address the root cause of the high pressure before attempting to reset the cut-out.
   • Common causes of high pressure include:
     • Clogged or dirty condenser: Restricting heat transfer and causing refrigerant pressure to build up.
     • Overcharge of refrigerant: Excessive refrigerant in the system increases pressure.
     • Non-condensable gases: Presence of air or other non-condensable gases in the system.
     • Faulty condenser fan: If the fan isn’t working, the condenser can’t dissipate heat effectively.
2. Rectify the Problem:
   • Clean or repair the condenser.
   • Remove excess refrigerant if the system is overcharged.
   • Purge non-condensable gases from the system.
   • Repair or replace the condenser fan if it’s faulty.
3. Reset the Cut-Out:
   • Once the underlying issue has been addressed and the system pressure has returned to a safe level, the high-pressure cut-out can be reset.
   • Most high-pressure cut-outs have a manual reset button or lever located on the device itself.
   • Simply press or pull the reset button/lever to reset the switch and allow the compressor to restart.
   • Some advanced systems may have automatic reset functionality, but it’s still crucial to investigate and rectify the cause of the high pressure before relying on automatic resets.

Safety Precautions:

• Electrical Isolation: Before attempting to reset the cut-out, ensure that the power supply to the compressor and associated electrical components is isolated to prevent electrical hazards.
• Qualified Personnel: Resetting the high-pressure cut-out should only be done by qualified and authorized personnel familiar with the refrigeration system and its safety procedures.
• Caution: Never attempt to bypass or tamper with the high-pressure cut-out, as this can lead to dangerous over-pressurization and serious safety risks.

Remember, the high-pressure cut-out is a vital safety device that protects the refrigeration system and personnel from potential hazards. It should only be reset after the cause of the high pressure has been identified and rectified.

Location:

The low-pressure cut-out is typically located on the suction side of the compressor, between the evaporator and the compressor in a refrigeration system. Its position allows it to monitor the pressure in the low-pressure side of the system and protect the compressor from damage caused by low suction pressure.

Method of Resetting:

The specific method of resetting the low-pressure cut-out can vary depending on the model and manufacturer, but generally involves the following:

1. Identify the Cause of the Trip:

The low-pressure cut-out trips when the system pressure drops below its preset limit. It is crucial to investigate and address the underlying cause of the low pressure before attempting to reset the cut-out. Some common causes include:

• Refrigerant Leak: Loss of refrigerant charge due to leaks in the system.
• Clogged Expansion Valve or Filter-Drier: Restricting refrigerant flow and causing low suction pressure.
• Frost or Ice Buildup on Evaporator: Impeding heat transfer and lowering the evaporator pressure.
• Malfunctioning Compressor: Internal issues with the compressor can also lead to low suction pressure.

2. Rectify the Problem:

• Locate and Repair Leaks: If a leak is detected, it must be repaired before recharging the system.
• Replace Clogged Components: If the expansion valve or filter-drier is clogged, it needs to be replaced.
• Defrost the Evaporator: If frost or ice buildup is the cause, defrost the evaporator.
• Address Compressor Issues: If the compressor is faulty, it may need to be repaired or replaced.

3. Reset the Cut-Out:

• Manual Reset: Once the underlying issue is resolved and the system pressure returns to a safe level, the low-pressure cut-out can be manually reset.
  • Most cut-outs have a reset button or lever on the device itself.
  • Press or pull the reset mechanism to restore the switch to its normal operating state.
• Automatic Reset: Some advanced systems may have automatic reset functionality. However, it’s still essential to address the root cause of the low pressure before relying on automatic resets.

Safety Precautions:

• Electrical Isolation: Isolate the power supply to the compressor and associated components before attempting to reset the cut-out to avoid electrical hazards.
• Qualified Personnel: Only qualified and authorized personnel should reset the low-pressure cut-out.
• Caution: Never bypass or tamper with the low-pressure cut-out. This safety device protects the compressor from damage and should not be overridden.

Remember: The low-pressure cut-out is a vital safety feature of a refrigeration system. It’s essential to investigate and rectify the cause of the low pressure before resetting it.