Test: Evaporators - Mechanical Engineering MCQ

Finned evaporator:

• The finned evaporators consist of bare tubes or coils over which the total plates or fins are fastened.
• The metal fins are constructed of thin sheets of metal having good thermal conductivity.
• Since the fins greatly increase the contact surfaces for heat transfer, therefore the finned evaporators are also called extended surface evaporators.
• The finned evaporators are primarily designed for air conditioning applications where the refrigerator temperature is above 0°C.
• Because of the rapid heat transfer of the finned evaporator, it will defrost itself on the off cycle when the temperature of the coil is near 0°C.
• Moisture in the air causes rusty formation on the Aluminium fins and it can be cleared by spraying rust-removing chemicals when the unit is under service.

The correct answer is Will decrease.

• If the humidity in the atmosphere is high, the rate of evaporation is low because the air is filled with water vapor and is not able to take up more humidity. Thus, the evaporation process slows down.

In the dry evaporator, the liquid refrigerant is generally fed by an expansion valve. The valve controls the rate of flow of refrigerant in such a way that all the liquid is vaporised, and the vapour is also superheated by the time it reaches the evaporator exit.

The refrigerant at the entry to the evaporator is low pressure and low temperature liquid. The pressure is reduced in the capillary tube thus also decreasing the temperature. This low pressure, low temperature liquid is converted to low pressure vapours after absorbing heat from the evaporator.

The moisture present in the evaporator section of refrigerator is cooled and converted to the frost. This ice formed affects the heat transfer rate in the evaporator and increase the electricity consumption.

Types of Evaporator:

• Bare tube Evaporator: These are constructed of either steel pipe or copper tubing. These are the simplest type of evaporators. Steel pipe is used for large evaporators and the copper tube is used for smaller evaporators. Common shapes are zig-zag flat or spiral. These are used where the temperature is maintained below 0°C.
• Plate Surface Evaporator: These are made up of two flat sheets of metal so embossed and welded together to provide a path for refrigerant flow between two sheets. It is usually cleaned and defrosted manually. It is used in domestic refrigerators.
• Finned Tube Evaporators: Finned coil is bare tube coil upon which metal plates or fins are installed. The use of fins increases the overall surface area of the evaporator.

Evaporator is the heat absorbing part of the refrigeration system. This is also called cooling coil, freezer or chiller. These names are according to their function in the system. In the domestic refrigerator it is termed as freezer.

Refrigeration is a process of heat removal from a substance or from a space resulting in lower temperature below that of the surroundings. Major parts of the conventional refrigeration system are as followed.

Evaporator: It is the part where cooling takes place. The heat is absorbed in the evaporator. The liquid refrigerant is converted into vapour refrigerant in the evaporator.

Condenser: The function of the condenser is to remove the heat carried by the refrigerant and reduce its temperature at constant pressure.

Receiver: It is the reservoir for excess liquid refrigerant not being in the system.

A liquid line and suction line heat exchanger are used to transfer heat between liquid refrigerant leaving the condenser on the high-pressure side of the system and refrigerant vapour leaving the evaporator on the low-pressure side of the system.

This heat transfer causes following effects

• To raise the vapour temperature (Superheating) to prevent frosting or condensation on the suction line
• To evaporate any remaining liquid in the vapour stream to prevent possible compressor damage
• To sub-cool the liquid to prevent flash gas in the liquid line
• To increase the system refrigerating capacity

Types of Evaporator:

• Bare tube Evaporator: These are constructed of either steel pipe or copper tubing. These are simplest type of evaporators. Steel pipe is used for large evaporators and copper tube is used for smaller evaporators. Common shapes are zig-zag flat or spiral. These are used where temperature is maintained below 0°C.
• Plate Surface Evaporator: These are made up of two flat sheets of metal so embossed and welded together as to provide a path for refrigerant flow between two sheets. It is usually cleaned and defrosted manually. It is used in domestic refrigerators.
• Finned Tube Evaporators: Finned coil are bare tube coil upon which metal plates or fins are installed. The use of fins increases the overall surface area of evaporator.