AC 432 Refrigeration Systems

The refrigerant leaving the evaporator must be 100 percent vapor because the purpose of the evaporator is to absorb heat from the surroundings and convert the liquid refrigerant into vapor. This vapor is then compressed and condensed in the condenser to release the absorbed heat. Therefore, for the evaporator to effectively perform its function, the refrigerant leaving it should be completely in the vapor state.

In a water-cooled condenser, the leaving water temperature is normally 10°F lower than the condensing temperature of the refrigerant. This temperature difference is necessary to ensure efficient heat transfer between the refrigerant and the water. By maintaining a lower water temperature, the condenser can effectively remove heat from the refrigerant, allowing it to condense and convert back into a liquid state. This helps in maintaining the overall cooling efficiency of the system.

The automatic expansion valve is designed to regulate and maintain a constant pressure. It ensures that the pressure in the system remains consistent, preventing any fluctuations that could affect the performance or efficiency of the system. By maintaining a constant pressure, the valve helps to ensure optimal operation and stability in various applications such as refrigeration or air conditioning systems.

The correct answer is 75 because the evaporator is the part of the refrigeration system where the refrigerant absorbs heat and evaporates. As the refrigerant enters the evaporator, it is in a liquid state. Therefore, approximately 75% of the refrigerant entering the evaporator is in liquid form.

The TX valve maintains a constant superheat temperature. The superheat temperature refers to the temperature of the refrigerant vapor above its boiling point in the evaporator coil. The TX valve regulates the flow of refrigerant into the evaporator coil to ensure that the superheat temperature remains constant. This is important for the efficient operation of the refrigeration system and to prevent any damage to the compressor.

The low form of plant life that grows in condenser water is algae. Algae are simple, non-flowering plants that can grow in various aquatic environments, including condenser water. They are capable of photosynthesis and can thrive in moist and nutrient-rich conditions. The presence of algae in condenser water can cause problems such as clogging, reduced heat transfer efficiency, and corrosion. Therefore, it is important to regularly monitor and control algae growth in condenser water systems.

The temperature of the evaporator should be about 15°F colder than the box temperature. This temperature difference ensures that the evaporator can effectively absorb heat from the box and maintain the desired temperature. If the evaporator temperature is not sufficiently colder than the box temperature, the cooling process may be inefficient and the desired temperature may not be achieved. Therefore, maintaining a temperature difference of 15°F helps optimize the cooling efficiency and ensures proper temperature control.

An evaporative condenser uses air and water as the transfer medium for system heat. The air absorbs heat from the condenser coils, causing the refrigerant to condense and release heat. The water is then used to cool down the hot refrigerant and remove the heat from the system. This process allows for efficient heat transfer and helps in the cooling of the system.

If an air-cooled condenser's fan motor stops running, it will result in a decrease in airflow over the condenser coils. This decrease in airflow will cause the heat pressure to increase since the condenser coils will not be able to dissipate heat effectively. Additionally, the decrease in airflow will also lead to a decrease in subcooling, which refers to the temperature decrease of the refrigerant below its saturation point. Therefore, the correct answer is that the heat pressure will increase and the amount of subcooling will decrease.

A receiver is a device that is added to a flooded condenser type head pressure control system to accommodate the extra refrigerant. This is necessary because the flooded condenser system operates by having excess refrigerant in the condenser, which helps to maintain a constant head pressure. The receiver acts as a storage tank for the excess refrigerant, allowing it to be stored and released as needed to maintain the desired head pressure in the system. Without a receiver, the system would not be able to properly control the head pressure and may result in inefficient operation or damage to the equipment.

A coil that uses a low side float is referred to as "flooded." This means that the coil is submerged in the refrigerant, allowing for efficient heat transfer. The low side float helps maintain the desired level of refrigerant in the coil, ensuring optimal performance.

The statement is true because the changing seasons result in varying ambient temperatures, which can pose a challenge for designers of air-cooled refrigeration and air conditioning equipment. As the ambient temperature fluctuates, the equipment needs to be able to adjust and maintain the desired temperature efficiently. Designers must consider these changing conditions to ensure that the equipment functions effectively in different seasons.

As the evaporator load increases, an automatic expansion valve will close to reduce the evaporator pressure. This is because a closed valve restricts the flow of refrigerant into the evaporator, reducing the pressure and preventing excess refrigerant from entering the system. On the other hand, a thermostatic expansion valve will open to reduce the evaporator superheat. Opening the valve allows more refrigerant to flow into the evaporator, increasing the cooling capacity and reducing the superheat of the refrigerant leaving the evaporator.

This statement is true because heat transfer occurs from a higher temperature region to a lower temperature region. In the case of an evaporator, heat is transferred from the air passing over it to the refrigerant. Therefore, the refrigerant in the evaporator must be colder than the air for heat transfer to occur.

Low suction pressures, iced coils, short-cycling, and inefficient cooling can indeed result from low head pressures. When the head pressure is low, it means that there is not enough refrigerant being condensed in the condenser. This can lead to a decrease in the suction pressure, causing the coils to ice up and the system to short-cycle. Additionally, the cooling efficiency of the system will be compromised, resulting in inefficient cooling. Therefore, the statement is true.

The correct answer is "moisture, heat". An evaporator is a component in a refrigeration system that absorbs heat from the refrigerated space and removes moisture from the air. As the refrigerant evaporates, it absorbs heat from the surrounding air, thereby cooling the space. At the same time, the evaporator also helps in dehumidifying the air by condensing the moisture present in it. Therefore, the primary functions of an evaporator are to remove moisture and heat from the refrigerated space.

When the velocity of the air moving over an evaporator is slow, the "air film" refers to a layer of air that forms on the surface of the evaporator. This air film acts as an insulator, meaning that it inhibits the transfer of heat between the evaporator and the air. As a result, the rate of heat exchange is slowed down. Therefore, the statement is true.

Evaporative condensers operate more efficiently in climates where the wet bulb temperature is significantly lower than the dry bulb temperature because the condenser uses the evaporation of water to cool the refrigerant. When the wet bulb temperature is lower, the water evaporates more easily, resulting in better cooling efficiency. This means that the condenser can effectively remove heat from the refrigerant, allowing the system to function optimally.

If a low temperature evaporator is forming excess ice, all of the given options could be possible problems. A defective evaporator fan motor can cause insufficient air circulation, leading to ice formation. If the unit is not staying in defrost long enough, the ice on the evaporator may not melt completely, resulting in excess ice buildup. A defective defrost heater can also prevent proper melting of ice during the defrost cycle, leading to excess ice formation. Therefore, all of the mentioned issues could contribute to the formation of excess ice on the low temperature evaporator.

A typical evaporator operating temperature in an air-conditioning system is 40°F because this temperature allows for efficient cooling of the air. At this temperature, the evaporator coil can effectively absorb heat from the surrounding air, causing the refrigerant to evaporate and cool the air passing through the system. This temperature is also suitable for maintaining a comfortable indoor environment without causing excessive cooling or freezing of the air.

A typical superheat for an evaporator operating under normal conditions would be 8 to 12°F. Superheat is the temperature difference between the actual temperature of the refrigerant vapor and its saturation temperature at a given pressure. It is important to have a certain amount of superheat in the evaporator to ensure that the refrigerant is fully vaporized before it leaves the evaporator. A superheat of 8 to 12°F is considered normal and allows for efficient heat transfer and prevents liquid refrigerant from entering the compressor.

The capacity of an evaporator is affected by all of the above factors. The saturation pressure of the low side of the system determines the temperature at which the refrigerant evaporates, thus affecting the heat transfer process. Similarly, the saturation pressure of the high side of the system affects the condensing temperature and the overall efficiency of the system. Additionally, the amount of evaporator superheat, which is the temperature rise above the refrigerant's saturation temperature, also impacts the evaporator's capacity as it affects the heat transfer rate. Therefore, all of these factors play a role in determining the capacity of an evaporator.

The condenser in a refrigeration system performs multiple functions. Firstly, it desuperheats the vapor from the compressor, removing excess heat and reducing the temperature. Secondly, it condenses the vapor, converting it back into a liquid state. Lastly, it subcools the liquid refrigerant before it leaves the condenser, further lowering its temperature. Therefore, the correct answer is "all of the above" as the condenser performs all these tasks in a refrigeration system.

Increasing the spring pressure of a water regulating valve will result in an increase in the force exerted by the valve on the water flowing through it. This increased force will cause the valve to close more tightly, reducing the flow rate and increasing the system head pressure. Therefore, it is true that increasing the spring pressure of a water regulating valve will also increase the system head pressure.

When the quantity of refrigerant flow to an evaporator is reduced, there will be less refrigerant available to absorb heat from the surroundings. As a result, the evaporator coil will not be able to remove as much heat from the system, leading to an increase in the temperature of the refrigerant leaving the coil. This increase in temperature is known as superheat, so when the quantity of refrigerant flow is reduced, the amount of superheat in the vapor leaving the coil will rise.

By increasing the condensing surface area, more heat can be dissipated, resulting in lower condensing temperatures. Similarly, increasing the cfm (cubic feet per minute) through the condenser increases the rate of heat transfer, leading to lower temperatures. Additionally, decreasing the temperature of the air entering the condenser provides a cooler environment for heat exchange, further reducing condensing temperatures. Therefore, all of these actions can contribute to reducing the condensing temperatures in an air-cooled condensing unit.

A defrost system that pipes the compressor discharge vapor directly to the evaporator is called a hot gas defrost type. In this type of system, the hot gas from the compressor is used to heat up the evaporator coil, melting any ice or frost that has accumulated on it. This hot gas is then condensed back into a liquid state and returned to the compressor to continue the refrigeration cycle. This method is efficient and helps to quickly defrost the evaporator, allowing it to resume normal cooling operations.

The spring pressure in a thermostatic expansion valve acts as a force that pushes to close the valve. This pressure is created by the compression of the spring within the valve. The evaporator pressure, on the other hand, refers to the pressure within the evaporator coil of a refrigeration system. This pressure also contributes to the closing of the expansion valve as it creates a back pressure that helps to keep the valve closed. Therefore, both the spring pressure and the evaporator pressure work together to push the thermostatic expansion valve to close.

A stamped plate evaporator is designed in such a way that it promotes natural convection for heat transfer. It consists of multiple thin plates with small channels or passages for the refrigerant to flow through. These plates are typically arranged in a stack, allowing for a larger surface area for heat exchange. Due to this design, a fan is not required to move air across the evaporator as the natural convection currents created by the temperature difference between the refrigerant and the surrounding air are sufficient for efficient heat transfer. Therefore, the statement "A stamped plate evaporator usually does not use a fan to move air across it" is true.

Hard starting refers to difficulties in starting a machine or engine. In low ambient conditions, such as cold weather, the head pressures in the system may be low. This can result in hard starting because the system may not be able to build up the necessary pressure to start properly. Therefore, the statement that hard starting may occur in low ambient conditions causing low head pressures is true.

The pressure at the inlet of the evaporator is 25 psig because the pressure drop is subtracted from the low side pressure. In this case, the pressure drop is 10 psig, so when it is subtracted from the low side pressure of 15 psig, the result is 25 psig.

A large capacity evaporator would probably have multiple refrigerant circuits. This is because a larger evaporator requires more refrigerant to be evaporated in order to cool a larger space or handle a higher cooling load. Having multiple refrigerant circuits allows for better distribution of the refrigerant and helps to ensure efficient and effective cooling throughout the evaporator.

The condenser is operating with 15°F of subcooling. Subcooling is the temperature difference between the liquid refrigerant leaving the condenser and its saturation temperature. In this case, the condenser outlet temperature is 115°F, which is 15°F higher than the saturation temperature. Therefore, the condenser is operating with 15°F of subcooling.

In a refrigeration system, the condenser is responsible for releasing heat from the refrigerant. As the refrigerant flows through the condenser, it undergoes a phase change from a high-pressure vapor to a high-pressure liquid. During this process, the refrigerant gives up heat to the surroundings, which allows it to cool down and condense. Therefore, the correct answer is "gives up heat."

The evaporator superheat is the difference between the actual evaporator outlet temperature and the saturation temperature at the corresponding evaporator pressure. In this case, the evaporator outlet temperature is given as 34°F and the suction pressure is given as 25.4 psig. By looking up the saturation temperature of R-12 at 25.4 psig, we can find that it is 25°F. Therefore, the evaporator superheat is 34°F - 25°F = 9°F.

Mineral deposits in the water portion of a water-cooled condenser act as an insulator. This means that they hinder the transfer of heat between the water and the condenser. Insulators are substances that do not conduct heat well, so when mineral deposits build up in the condenser, they create a barrier that prevents efficient heat transfer. As a result, the condenser may become less effective in cooling the water, leading to decreased efficiency and potential operational issues.

The statement is true because it states that the desired water flow through a condenser connecting to a recirculating water system is 3 gallons per minute. This means that the condenser is designed to operate efficiently with a flow rate of 3 gallons per minute, indicating that any other flow rate may not be optimal for the system.

If the application requires a coil design to operate below 32°F, there must be provision for defrost. This is because when operating at such low temperatures, ice can build up on the evaporator coil, reducing its efficiency and potentially causing damage. Defrosting is necessary to remove the ice buildup and ensure proper functioning of the coil.

The shell and tube condenser is cleaned with a brush after removing the end caps. This is because the brush allows for effective scrubbing and removal of any debris or buildup inside the condenser tubes. Removing the end caps provides access to the tubes, making it easier to clean them thoroughly. Using a brush ensures a physical cleaning action, which can be more effective than solely relying on chemicals, compressed air, or a vacuum cleaner.

A forced draft high temperature evaporator would have its fins very close together and not need to be defrosted. This is because the close proximity of the fins allows for efficient heat transfer, preventing the formation of ice and frost. Additionally, the forced draft design ensures a continuous flow of air, further preventing the need for defrosting. Therefore, both options a and b are correct.

When the waterflow is reduced in a condenser, it means that there is less water available to remove heat from the condenser. This leads to a decrease in the overall heat transfer rate, resulting in a higher temperature difference across the condenser. Therefore, the temperature difference across the condenser will increase when the waterflow is reduced.

During the defrost cycle of a forced draft low temperature evaporator, the evaporator fan would turn off. This is because during the defrost cycle, the heat from the defrost heaters is used to melt the ice that has accumulated on the evaporator coils. If the evaporator fan remains on, it would blow the warm air away from the coils, reducing the effectiveness of the defrosting process. Therefore, turning off the fan allows the heat to be concentrated on the coils, speeding up the defrosting process.

The control of head pressure in an air-cooled condenser can be achieved through multiple methods. One way is by cycling the condenser fan, which adjusts the air flow and helps regulate the condenser temperature. Another method is by controlling dampers in the airstream through the condenser, which allows for better control of the air flow and heat transfer. Lastly, flooding the condenser with refrigerant can also help in controlling the head pressure. Therefore, all of the above options are correct ways to control the head pressure in an air-cooled condenser.

If a high temperature evaporator is forming ice, a possible problem could be that the evaporator fan is not running. The evaporator fan is responsible for circulating air over the evaporator coil, which helps to remove heat and prevent ice formation. If the fan is not running, the air flow will be restricted, causing the evaporator coil to become too cold and resulting in ice formation.

Fewer fins per inch on an evaporator means that there is more space between the fins for ice to accumulate. This allows for more ice to build up between defrost cycles because there is more surface area available for ice formation. Therefore, the statement is true.

A refrigeration system designed for the storage of flowers or candy is considered a high temperature type because it is not meant to freeze or cool the items to extremely low temperatures. Instead, it is designed to maintain a slightly cooler temperature than the surrounding environment to prevent spoilage or melting, but not cold enough to freeze the products.

Both options a and b are correct because when scale forms on the water tubes, it acts as an insulating layer, reducing the heat transfer efficiency. As a result, the outlet water temperature will increase, causing the outlet water to be 88°F instead of the usual 85°F. Additionally, the reduced heat transfer efficiency will cause the system head pressure to rise as the condenser struggles to remove heat effectively. Therefore, both a and b are possible outcomes in this scenario.

An air-cooled condenser that is operating in a climate that has four distinct seasons must have some type of head pressure control because the temperature and humidity levels can vary greatly throughout the year. Head pressure control helps to maintain the condenser's operating pressure within a desired range, ensuring efficient heat transfer and preventing the condenser from becoming overworked or damaged. By adjusting the head pressure, the condenser can effectively cool the refrigerant and maintain optimal performance regardless of the changing outdoor conditions.

The water circuit in shell and coil and coil-type tube-in-a-tube condensers must be cleaned chemically. This is because chemical cleaning is the most effective method to remove any buildup or deposits that may accumulate in the water circuit. Chemical cleaners are specifically designed to dissolve and remove scale, rust, and other contaminants, ensuring optimal performance and efficiency of the condenser.

The device used to feed refrigerant to multiple circuits in an evaporator is not called a flow circuit. The correct term for this device is a distributor. The distributor evenly distributes the refrigerant to each circuit in the evaporator, ensuring efficient cooling throughout the system.