Bus Ac: Understanding The Cooling Mechanism

how bus ac works

Bus air conditioning systems work by cooling, dehumidifying, and filtering the air within the passenger compartment of a vehicle. The process is known as the refrigeration cycle and involves several components, including the thermostat, compressor, condenser coil, evaporator, and fans. When the thermostat calls for cooling, the compressor circulates refrigerant gas through the system, which then undergoes a change of state from gas to liquid and back to gas, absorbing and releasing heat in the process. This cycle of condensation and evaporation, facilitated by various mechanical parts, is what ultimately provides cooled air to the passengers in the bus.

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The role of the compressor

The compressor is an essential component of a bus air conditioning system. It is located in the outdoor unit of the air conditioning system, at the rear or on the side of the bus. It is the workhorse of the system, operating as the go-between for the evaporator and the condenser. The evaporator is located inside the bus, while the condenser is the unit outside the bus.

The compressor moves the refrigerant between the evaporator and condenser coils, ensuring that the refrigerant changes to gas or liquid as needed. The refrigerant starts as a fluid, absorbing the heat of the air from the passenger compartment. The compressor moves the outside air into the refrigerant, which takes in the heat and converts it into colder air. The liquid refrigerant then turns into a heated vapour. The compressor packs the molecules in the gas-based refrigerant tightly together, raising both the temperature and pressure of the refrigerant. This process is essential to the cooling cycle, as it ensures the refrigerant can release the heat it has absorbed.

The compressor is responsible for powering the other components of the air conditioning system and keeping them running. It is a crucial part of the system, and if it fails, the whole system will stop working. Regular maintenance is essential to keep the compressor in good condition and prevent issues such as insufficient cooling or hard starts. Only a licensed technician should perform any maintenance or repairs on the compressor.

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The condenser coil and fan

The condenser fan complements the condenser coil by blowing air through the coil, enhancing heat dissipation. This fan can be belt-driven or electric, depending on the bus model and air conditioning system design. Together, the condenser coil and fan ensure that the refrigerant, now a hot gas, can release its heat load to the outside air, facilitating the phase change from gas to liquid.

The process begins when the refrigerant, as a high-temperature, high-pressure gas, enters the condenser coil. The condenser fan then cools the coil, drawing out the heat energy and facilitating the liquefaction of the refrigerant. This phase change is essential for the ongoing cooling process, as the liquid state enables the absorption of more heat from the bus interior.

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Evaporator assembly

The evaporator assembly is an essential component of a bus air conditioning system, and it plays a critical role in the refrigeration cycle. Typically, the evaporator is located on the roof of the bus, and its assembly is housed in SMC (Sheet Moulding Compound) housing, which is lighter and more environmentally friendly than the traditional FRP (fibre-reinforced plastic) housing. This housing protects the components and ensures good insulation performance.

The evaporator assembly works in conjunction with other parts of the air conditioning system, such as the compressor and the condenser. When the air conditioning is turned on, the compressor circulates refrigerant gas through the system. This refrigerant gas, now under high pressure, enters the condenser coil, where it comes into contact with cool air pulled through by fans. This initiates the process of condensation, where the refrigerant transforms from a gas to a liquid form.

After condensation, the liquid refrigerant passes through a filter drier to remove any moisture and impurities. It then flows through a sight glass, which allows for visual inspection, before reaching the evaporator assembly. Here, the refrigerant is metered into the evaporator coil by an expansion valve, which also reduces its pressure.

The evaporator absorbs hot air from the passenger compartment through a return air grille. This hot air is pulled through a filter, which removes particulate matter, ensuring that the air is cleaned before it comes into contact with the evaporator coil. The evaporator blower then blows the cooled and cleaned air back into the passenger compartment.

The refrigerant undergoes a significant change during this process, transitioning from a liquid to a gaseous state through evaporation. This phase change allows the refrigerant to absorb a substantial amount of heat energy, contributing to the overall cooling effect. The moisture in the warm air passing through the evaporator coil condenses and is collected and discharged outside the bus.

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Refrigerant and its circulation

The refrigerant is a gas, R134a, which circulates through the air conditioning system. It absorbs and removes heat from the passenger compartment. The compressor pumps the refrigerant under high pressure into the condenser coil. The condenser coil contains the refrigerant in the form of a hot gas.

The refrigerant then undergoes a change of state, from a gas to a liquid, through a process called condensation. During this process, the hot gas releases its heat load to the outside air, which was originally transferred from the passenger compartment into the refrigerant by the evaporator.

The refrigerant, now in the form of a cool liquid, passes through the filter drier, which removes moisture and impurities, before passing through the sight glass, which allows for a visual inspection of the refrigerant.

The cool liquid refrigerant then flows into the evaporator through an expansion valve, which controls the volume and decreases the pressure of the refrigerant as it enters the evaporator coil. The evaporator absorbs hot air from the passenger compartment through a return air grille, which includes a fresh air system that removes particulate matter without affecting the cooling process. The evaporator blower then blows the cleaned and cooled air into the passenger compartment through the evaporator coil.

The refrigerant undergoes another change of state, from a liquid to a gas, and a corresponding change in pressure from high to low, through a process called evaporation. During this process, the refrigerant absorbs a significant amount of heat energy, including the hot air from the passenger compartment, thereby achieving the cooling effect.

As the warm air passes through the evaporator coil, moisture condenses and is collected, then discharged outside the bus. The gaseous refrigerant is then suctioned back into the compressor, where it is compressed and the refrigeration cycle repeats.

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The control panel

A typical bus air conditioning control panel includes features such as temperature control, system diagnostics, and air quality monitoring. Some advanced control panels also offer touch screen capabilities and programmable settings, providing an enhanced user experience. The control panel is designed to be user-friendly, allowing for easy operation and monitoring of the air conditioning system.

One example of a bus air conditioning control panel is the Electric Bus Air-conditioning Panel Controller HKACP01, which offers a comprehensive range of functions, including automatic control, refrigeration, fresh air circulation, self-test diagnostics, power-down memory, fault alarm display, indoor and outdoor temperature monitoring, multi-stage air volume adjustment, and over-voltage automatic protection. This particular control panel is compatible with various bus brands, including Kia, Daewoo, Hyundai, and Yutong.

The construction of bus air conditioning control panels prioritises durability and reliability. They are made from robust materials to withstand daily use and varying environmental conditions, ensuring their longevity and functionality over extended periods. The control panels are designed to manage temperature and humidity levels effectively, especially in regions with high moisture and temperature levels.

Frequently asked questions

The basic structure of a bus air conditioning system includes the condenser, evaporator, compressor, magnetic clutch, alternator, fresh air device, blower, fan, return air grille, control panel, pipeline system, and electric-controlled system.

When the ON/OFF button is pressed, the compressor is activated via the electromagnetic clutch and belt system. The compressor circulates refrigerant gas through the system, which is then pumped into the condenser coil. The refrigerant undergoes a change of state from a gas to a liquid, and then back to a gas, cooling the air in the process.

The condenser coil contains the hot gas refrigerant. Fans pull cool air through the condenser coil, cooling the refrigerant and dissipating heat.

The evaporator absorbs hot air from the passenger compartment through the return air grille. The blower then pushes this air through the evaporator coil, cooling and dehumidifying it before it is released back into the bus.

The thermostat, located inside the bus, signals the need for cooling. This triggers the electromagnetic clutch on the compressor to engage and start the refrigeration cycle.

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