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Refrigeration Compressor of Bus Air Conditioning System Series

DATE: Nov 28th, 2023

1. What Is A Compressor?

The compressor is the power of the refrigeration cycle, it is dragged by the motor and constantly rotating, it is in addition to timely extraction of the evaporator vapor, maintaining low temperature and low pressure, but also through the compression effect of the refrigerant vapors to increase the pressure and temperature, to create the refrigerant vapors to the outside environment of the heat transfer conditions of the medium. 
That is, low-temperature and low-pressure refrigerant vapor compression to high-temperature and high-pressure state, to be able to use room temperature air or water as a cooling medium to condense the refrigerant vapor.

2. Types And Classification Of Refrigeration Compressors

Classification According To The Working Principle

Refrigeration compressors according to their working principle can be divided into: volume type and speed type

(1) volumetric compressor: mechanical methods to make the volume of closed containers smaller, so that the gas compression increases the pressure of the machine.
It has two types of structure: reciprocating piston (referred to as piston) and rotary, rotary and include (rolling piston, sliding vane, single screw, double screw, scroll).

(2) speed compressor: mechanical methods to make the flow of gas obtain a very high flow rate, and then in the expansion of the channel so that the gas flow rate is reduced, so that the kinetic energy of the gas is converted into pressure energy, to achieve the purpose of increasing the pressure of the gas, this machine is known as the speed of the compressor. Belonging to this category are centrifugal refrigeration compressors.
In this compressor work, the gas in the high-speed rotation of the impeller, not only to obtain a very high speed, and under the action of centrifugal force, along the direction of the radius of the impeller was thrown out, and then into the cross-sectional area gradually expanding the expansion of the pressure, where the gas speed is gradually decreased and the pressure is increased.

Classification According To The Working Evaporating Temperature

Single-stage refrigeration compressors can be generally according to their working range of evaporating temperature divided into high-temperature, medium-temperature, and low-temperature compressors three, but the specific evaporating temperature region is not uniform. Below is a list of well-known compressors working evaporative temperature classification range.

High-Temperature Compressors (-10 ~ 0 )°C)
Medium Temperature Refrigeration Compressors (-15 ~ 0 ) °C
Low Temperature Refrigeration Compressors (- 40 ~ -15 )°C

Classification According To The Form Of Sealing Structure

The refrigerant in the refrigeration system is not allowed to leak, which means that every part of the system that is in contact with the refrigerant should be sealed to the outside world. There are three different basic compressor types according to the type of leakage prevention and structure adopted by the refrigeration compressor.

Open type compressor
Semi-hermetic compressors
Enclosed compressor

3. The Compressors Currently Used In Passenger Cars Are Mainly: Scroll Type.

Compression parts by the dynamic scroll disk and static scroll composition. The working principle is to use the relative rotation of the dynamic and static scroll disk movement to form a continuous change in the closed volume, to achieve the purpose of gas compression.

Scroll Compressor Features:

1. super high energy efficiency: energy efficiency is 12% higher than the most advanced piston compressor on the market.
2. Outstanding reliability: With few moving parts, the axial and radial patented flexible design provides unprecedented resistance to liquid shock and tolerance of impurities.
3. Built-in motor short-circuit device: effectively protects the motor from damage caused by high temperature and high current.
4. Low noise/low exhaust pulse: noise level is more than 5 dB lower than piston compressors.
5. Simplified system design: Unique unloaded start-up design eliminates the need for start-up capacitors/relays for single-phase compressors and eliminates the need for oil heaters and gas-liquid separators in most applications.
Nearly 100% volumetric efficiency for exceptional heating capacity.

4. Scroll Compressor Common Failure Phenomenon And Cause Analysis

Failure Phenomenon

4.1. Damaged Floating Seals, High And Low-Pressure Gas Strings.

As can be seen from the structural characteristics of the scroll compressor, to provide the appropriate gas pressure in the upper part of the scroll stator, an intermediate pressure channel is opened at the appropriate intermediate compression on the scroll stator to provide intermediate pressure.
A floating seal is provided in the upper portion of the intermediate pressure chamber so that the top of the scroll is subjected to the exhaust pressure and intermediate pressure. In addition to equalizing the compressed gas pressure inside the vortex, a sealing force between the top and bottom channels is provided, which is achieved by a floating seal. The seal is made of a non-metallic material similar to rubber or plastic.
Failure phenomenon is generally manifested as the compressor motor is intact and can be energized to run, but the exhaust pressure of the unit does not increase, the suction pressure does not decrease, there is almost no pressure difference between suction and discharge, the exhaust pipe is not hot, and the suction pipe is not cool. The compressor current is very different from the rated value the compressor is idling.

4.2. Scroll Disk Damage.

Scroll disk damage In addition to the above characteristics of the floating seal damage, you can also hear the compressor inside the obvious metal impact sound, which is the scroll disk was shattered after the metal fragments hit each other or with the compressor shell impact sound.

4.3. Motor Burned.

When the power supply is turned on, the fuse is blown or the short circuit breaker trips, the compressor can not start.

4.4. The Motor Clutches The Shaft, Bearing Damage.

When the compressor power supply is turned on, you can hear a buzzing sound of the motor inside the casing, but not running, and the current rises very quickly. After a few seconds, the compressor's internal overload protection or external thermal relay protection action cut off the power supply. Sometimes the protector is too late to act, and the blocking current is quickly reached, which may directly lead to motor burnout.

5. Failure Cause Analysis And Preventive Measures

There are three main situations: First, the moment of opening a large amount of refrigerant liquid into the compressor. Second, if the evaporator water flow is not enough (evaporative load reduction), the compressor goes back to the liquid phenomenon. Third, the unit heat pump operation defrost is not good, a large number of liquid refrigerant did not evaporate into the compressor, or the four-way valve reversal moment evaporator (heat pump operation for the condenser) in the liquid into the compressor.

To solve the problem of liquid strike or liquid return, mainly from the following aspects to consider.

① Pipeline design to avoid the opening of the liquid refrigerant into the bus air conditioning compressor, which may require the system to do an excess liquid return test, especially since the charge is a relatively large refrigeration system. Increasing the gas-liquid separator in the compressor suction port is an effective way to solve this problem, especially in the heat pump unit using reverse cycle hot gas defrost.

② Before start-up, preheating the compressor oil pool for a long enough time can effectively avoid a large amount of refrigerant accumulating in the compressor lubricating oil. The prevention of liquid shock also has a certain effect.

③ Water system flow protection is indispensable, so that when the water flow is not enough to play a role in protecting the compressor, so as not to have the unit back to the liquid phenomenon or the seriousness of the evaporator freeze. Flow switch damage should be repaired or replaced promptly, do not short flow switch.

Motor winding burned with the electrical design of the protection, or by the unit operation caused by improper use. 

1. Due to the evaporator freeze cracked, the fluorine system and the water system in series, the compressor into the water caused the compressor to burn. 

2. due to various reasons [water filter dirty blockage, water pump matching unreasonable, etc.), resulting in small water system flow, and water flow protection and failure or short connection, the evaporator freezes after freezing refrigerant copper pipe.

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