What is a Hermetic Compressor?

A compressor is a device that is used to increase the pressure of gas from low pressure to high pressure. The compressor is like a control volume, where in and out of mass occurs along with the interaction of energy with the surroundings in the form of heat and electricity.

How to increase pressure of a gas

In a compressor, there are usually two methods to increase the pressure of a gas.

1. A compressor can raise the pressure of a gas by decreasing the volume of gas, these kinds of compressors are known as positive displacement compressors. An example of such a compressor is reciprocating compressors.
2. The compressors that can raise the pressure of a gas by first imparting kinetic energy to gas and then increasing the pressure of a gas by retarding its motion, are known as rotodynamic compressors. An example of such kind of compressor is centrifugal compressors.

A hermetic compressor falls under the category of positive displacement compressor. In hermetic compressors, the motor and the mechanical systems are enclosed in a single unit. These types of compressors are primarily used in refrigeration systems. For instance, one can find different types of compressors on the Bitzer website. Bitzer is a company that deals with the manufacturing of refrigeration and air-conditioning systems.

Types of positive displacement compressors

Reciprocating compressor

Reciprocating compressors consist of a piston, a cylinder, crank and crankshaft, a connecting rod, and a motor. The motor is connected to the crank shaft. Due to the motor rotation, the rotary motion of the crankshaft is converted to reciprocating motion in the piston. Due to the upward stroke of the piston, the volume of the gas is reduced, thereby increasing its volume.

Screw compressor

Screw compressors fall under the category of positive displacement compressors. A screw compressor makes use of a long screw inside a casing, the screw is rotated by a motor. The gas is continuously compressed as it moves along with the screw rotations. When there is a sufficient volume reduction and pressure rise, the gas moves out of the system through an outlet valve.

Parts of a refrigerating system

The vapor compression refrigeration system (VCRS) is generally used for most commercial applications including refrigerators and air conditioners. Refrigeration systems are generally open-drive and closed-drive. The VCRS is a closed-drive refrigeration system. The basic parts of a vapor compression refrigeration system are outlined below,

Compressor

The compressor that is most commonly used in the refrigeration system is the hermetic compressor. The whole unit is enclosed inside a steel shell hermetically. Inside this hermetic steel shell, there is a piston and cylinder arrangement, a motor, and necessary electronics. The piston, cylinder, and motor form the major parts of the compressor. This type of compressor is often called Hermetic Refrigeration Compressor (HRC).

Condenser

The compressor compresses the refrigerant vapor into a high-pressure, high-temperature vapor. This high-temperature vapor then moves into the condenser where it undergoes phase change by releasing latent heat and gets converted to a saturated liquid refrigerant.

Throttling valve

The high-pressure refrigerant liquid undergoes a throttling process where its temperature decreases significantly along with a loss in pressure.

Evaporator

In the evaporator, the low-temperature refrigerant gains heat from its surroundings and gets converted into a saturated refrigerant vapor. A fan increases the rate of the evaporation process and blows the cool air to the surroundings.

The refrigerant is the working fluid of a refrigerant cycle. There are many refrigerants used such as ammonia, R123, R502, and R134a.

Uses of compressors

There are several uses of a compressor, some of them are outlined below:

In aircraft engines

The compressors in aircraft engines dynamically increase the pressure of air, which is then mixed with the fuel for combustion, the combustion products are then expanded on a gas turbine that produces thrust.

Heating, ventilation, and air-conditioning (HVAC) system

HVAC systems are primarily used in commercial complexes, large buildings, and industries. They make use of large compressors for the refrigeration and air-conditioning systems.

Non-contact type manufacturing processes

In this type, compressed air along with abrasive grains/liquid is used in the machining process. Water jet machining and abrasive jet machining are such examples.

Remanufacturing applications

In remanufacturing process, existing parts are broken down and remanufactured into different parts. This remanufactured part has different functionalities. Remanufacturing also refers to repairing works. For instance, a compressor provides compressed air to the actuators that function as a pneumatic jack that can lift heavy objects, and necessary repair works can be performed.

Context and Applications

This topic is extensively taught in bachelor in science degrees. The topic is of primary focus in various engineering disciplines especially mechanical engineering and production engineering. The topic finds its application in various refrigeration and air-conditioning manufacturing companies. The concepts of these topics are majorly used in designing HVAC systems.

• Bachelors in Technology (Mechanical)
• Bachelors in Technology (Electronics)
• Masters in Technology (Electronics)
• Masters in Technology (Mechanical)

Practice Problems

1. Which kind of compressor is used in refrigeration systems?

1. Screw compressor
2. Centrifugal compressor
3. Axial compressor
4. Hermetic compressor

Correct option- d

Explanation: In refrigeration systems, hermetic compressors are majorly used. The mechanical components along with the motor are housed in a single steel shell.

2. Which of the following is a refrigerant?

1. R134a
2. Chlorofluorocarbons (CFC)
3. Ammonia
4. All of these

Correct option- d

Explanation: Ammonia, Chlorofluorocarbons (CFC), and R134a all are examples of refrigerants.

3. Which of the following is a positive displacement compressor?

1. Screw compressors
2. Reciprocating compressor
3. Axial compressor
4. Centrifugal compressor

Correct option- a

Explanation: Screw compressors are an example of a positive displacement compressor

4. Which of the following is true for Hermetic Refrigeration Compressor?

1. It is used in refrigeration systems
2. The motor is a separate part of the system
3. It make use of a screw compressor
4. None of these

Correct option- a

Explanation: Hermetic Refrigeration Compressors are used in refrigerators. The motor, along with the piston and cylinder is housed inside a steel shell.

5. Which of the following is true for VCRS?

1. It is an open-drive
2. It has an evaporator
3. It is a closed-drive
4. None of these

Correct option: Both b and c

Explanation: A VCRS is a closed-drive refrigeration cycle that has an evaporator.