Research on the Technologies of the Compressor Remanufacturing Process

Abstract

This study describes the remanufacturing process scheme and key technologies of waste compressors. Firstly, the technical characteristics of compressors for home appliances such as refrigerators and air conditioners are summarised. Then, the remanufacturing process scheme is proposed with the waste refrigerator compressor as the research object. The purpose or requirements of key processes such as separation, disassembling, cleaning, detecting, remanufacturing and reassembling are also elaborated. Moreover, the corresponding remanufacturing technology or method is proposed to satisfy the structural, quality requirement, economy and environmental friendliness of products and components. The cleaning technology of the compressor motor stator and the remanufacturing technology of crankshaft is highlighted. In accordance with the structural and quality requirement of the motor stator, the cleaning method of the motor stator is proposed, and the environmental friendliness, economy and safety of several cleaning agents are analysed. The scheme of crankshaft remanufacturing is proposed on the basis of the failure form of the crankshaft and the quality requirement of remanufacturing. Moreover, the process parameters are described.

1. Introduction

Nowadays, home appliances are essential in our daily lives. An increasing number of products are being discarded or eliminated with the end of product life and the increase in people’s requirements. A compressor, as the core component of home appliances such as refrigerators and air conditioners, is a relatively high value-added product. At present, waste compressors are mainly dismantled and recycled, not only causing a huge waste of resources but also posing a serious burden to the environment. Remanufacturing is a more economical and environmentally friendly approach to disposing of waste than is recycling materials [1,2].

Remanufacturing is the process of returning a used product to at least its original performance or better than that of a newly manufactured product [3]. Compared with new products, the remanufactured products can save energy and materials and reduce waste emissions [4,5]. The remanufacturing process includes the inspection, disassembly, cleaning and testing of used products, followed by the remanufacturing of failed parts using advanced technology, and the finally reassembly into new products [6]. The concept of remanufacturing was initially formally proposed by the United States in the early 1980s [7]. After more than 20 years of development, it is widely used in military, automobile, construction machinery and other high value-added products [8,9]. The United States has the largest remanufacturing industry, which reached USD 75 billion total value in 2005, with the remanufacturing of automobile and machinery accounting for more than two-thirds. Europe, which is represented by Germany and France, mainly focuses on remanufacturing research of automobile products. It has adopted positive remanufacturing related laws and regulations, and established the remanufacturing technology research centre in Germany [10].

In automotive engines, transmission and construction machinery and other high value-added products of the remanufacturing process have made greater progress and formed a certain industrial scale. However, in the case of low value-added products, such as used appliances, the experience of automobiles and construction machinery cannot be fully borrowed due to the higher economic requirements of the remanufacturing process [10,11]. However, with the popularity of refrigerators, air conditioners and other household appliances, the number of used compressors has increased year by year, and the traditional recycling methods are no longer being adapted. In recent years, with the requirement for resource conservation and environmental protection, the scale of compressor remanufacturing has gradually expanded. Some original equipment manufacturers have established services that offer compressor reuse options [10,12]. The design of the compressor has not changed particularly dramatically over the decades, indicating that it is particularly suitable for remanufacturing. Compressor failures are associated with components, such as motors, bearings and shafts. These components are subject to more severe wear than are others. Therefore, they are the key components that require remanufacturing. This study describes the remanufacturing process from the technical characteristics of compressors for home appliances. This process is informed by the waste failure form of the products in combination with considerations of environmental friendliness, economy and production feasibility of remanufacturing; it allows for the standardised batch remanufacturing of compressors. The key technologies, such as stator cleaning and crankshaft remanufacturing, are highlighted.

The remainder of this paper is presented as follows. Section 2 introduces the technical features of compressors that should be considered in the remanufacturing process. Section 3 describes the methods of compressor remanufacturing. Section 4 describes two key processes in the compressor remanufacturing process: the motor stator cleaning technology and the crankshaft remanufacturing process. Section 5 presents the main conclusion.

2. Technical Characteristics of Compressors

A compressor is a type of pressurisation equipment that converts low-pressure gas into high-pressure gas. It can be divided into volumetric and speed types according to the working principle. Compressors for home appliances are mainly volumetric type. Volumetric compressors can be further divided into reciprocating piston, rolling rotor, scroll and screw types according to the structure form. The compressor for a refrigerator is mainly the reciprocating piston type, while that for the air conditioner is mainly the rolling rotor type.

The working principle of the reciprocating piston type is to use the reciprocating motion of the piston in the cylinder bore for gas compression. The working principle of the rolling rotor type is to use the blade and rotor for dividing the cylinder bore into suction and exhaust chambers through the crankshaft to rotate the rotor for gas compression. The two types of compressors have evident differences in terms of working principle and product structure. However, the technical features of the products have numerous similarities, as described below:

• The compressor can be divided into three parts according to different functions: shell, pump body and motor. The shell is the support and sealing mechanism, the pump body is the booster mechanism, and the motor is the driving mechanism.
• The pump body requires high machining accuracy and fit clearance. The dimensional accuracy of the pump body drive parts is usually required to be above ±4 μm, and the fit clearance is usually approximately 10–30 μm.
• The compressor has strict requirements for contaminant and moisture contents. High contaminant contents accelerate the wear of parts, which causes the compressor to jam or block. High moisture content results in the acidification and degradation of the refrigeration oil, causing carbon accumulation in parts and increased wear or damage to the insulation layer of the motor [13].
• The failure form of the compressor includes motor and mechanical failures. Motor failure consists mainly of damaged stator winding insulation or a broken circuit. Mechanical failure includes wear of moving parts of the pump body or damage to the seal device.
• Compressor refrigerants have many types, and they have significant differences in technical requirements, quality requirements and production processes.
• In the industry, the parts have a low degree of commonality. More than 90% of the parts of different compressor brands are not interchangeable. The motor or crankshaft of different models of the same compressor brand is usually not interchangeable.

3. Materials and Methods

The specific remanufacturing process scheme varies with the structure of the compressor. The remanufacturing process of the piston compressor of a refrigerator is shown in Figure 1 [14].

3.1. Inspection

Inspection is the process of classifying the recovered compressors in terms of brand type, degree of damage and refrigerant used. Inspection determines the remanufacturing value of a compressor; it facilitates the remanufacturing of compressors to achieve industrialised production. The products with low remanufacturing value and high technical difficulty should be eliminated, such as R12 refrigerant models or products with serious damage.

Table 1. Effect of refrigerant change on the main design parameters of the compressor.

Refrigerant	Lubricants	Part Strength	Exhaust Volume	Motor Parameter	Valve Strength	Mating Clearance	Security
R12	Mineral oil	–	–	–	–	–	No
R134a	Polyester oil	New Design	Unchanged	New Design	New Design	New Design	No
R600a	Mineral oil	Unchanged	Unchanged	New Design	New Design	New Design	Yes

shows the effect of refrigerant changes on the main design parameters of the compressor.

3.2. Disassembly

Disassembly is the process of breaking down a product into structurally complete or functionally independent parts. The remanufacturing disassembly follows certain technical rules and requirements. According to the value of the parts and the connection method, suitable methods and equipment should be used to preserve the core value parts or main components from damage as much as possible [15].

The structure of the refrigerator compressor is shown in Figure 2. The shell consists of upper and lower parts. They are usually made of 2.5–4.0 mm mild steel, and the movement assembly is sealed inside the shell by welding. The movement assembly consists of a motor and pump body, where the motor rotor and pump body crankshaft are assembled by interference fit, and the other parts are assembled by bolt connection or clearance fit. Therefore, cover disassembly and rotor disassembly are important for compressor disassembly.

For compressor cover disassembly, the commonly used methods are lathe turning, manual sawing, flame cutting, plasma cutting, cryogenic crushing and profile milling [16]. The profile milling method involves a simple structure, high disassembly efficiency and little damage to parts. It is the most suitable method for compressor remanufacturing cover disassembly. Its basic structure is shown in Figure 3. The cryogenic crushing method is a completely destructive dismantling technique and is inapplicable to remanufacturing dismantling. The manual sawing method is extremely inefficient for dismantling and is only suitable for very small amounts of dismantling. Flame cutting and plasma cutting produce poor section quality, which may damage the internal parts of the compressor. The lathe turning can only be used for round compressor processing.

The common methods for the rotor disassembly include the heating or pressure dismantling method. The pressure dismantling method is used to remove the crankshaft from the rotor via pressure and a push bar. It involves high efficiency, little damage to the parts and the good reusability of the parts. However, the design of the fixture has demanding requirements. The heating method uses high-frequency heating to expand the rotor bore for separation. Its energy consumption is high, and it is prone to aluminium leakage and the deformation of silicon steel or vortex loss increase phenomenon, which is not conducive to the use of rotor remanufacturing.

Compressor disassembly removes parts with low value or that are evidently damaged and cannot be remanufactured or processed, such as valves, gaskets, broken shells and seriously worn pistons. Moreover, waste oil and residual refrigerant should be collected. After the disassembly is completed, the disassembled parts should be classified and stored and should undergo antirust and antiscratch treatment.

3.3. Cleaning

Cleaning is the process of removing contaminants attached to the surface and inside the parts. The common types of contaminants are oil, rust, carbon and paint. The appropriate cleaning method should be selected according to the structure of the parts, materials, contaminant types and cleanliness requirements, and the economic, environmental protection and safety aspects should also be comprehensively considered [17]. The common cleaning methods are mainly sandblasting, shot blasting, chemical cleaning, ultrasonic cleaning, oscillation cleaning and spray cleaning.

According to the type of contaminants and cleanliness requirements of the parts, the cleaning of the parts can be classified as follows:

• Shell assembly. The contaminants of the shell are oil, paint and rust spots. The cleanliness requirement is relatively low. The rust spot and paint of the shell can be removed by shot blasting treatment. Then, the oil and dust are removed by hot water spraying. For the convenience of operation, shot blasting can be completed prior to the disassembly of the open shell. The effect of the shot blasting treatment is shown in Figure 4.

2.

Small parts of the pump body, including the piston, crankshaft and bolts. The parts with simple structure and high cleanliness requirements can be washed with ultrasonic cleaning and dried after cleaning.

3.

Large parts and the motor of the pump body. These parts have a complex structure and highest cleanliness requirements. The combination of spraying rough cleaning + ultrasonic fine cleaning can be used. The cleaning agent is R141B or hydrocarbon cleaning agent with high clean ability and volatility.

3.4. Detecting

Detecting provides standards for judging the reusability of parts. According to the different functions of the compressor parts, its detecting content and requirements are different. The main detecting items for electrical parts, such as motors, constitute the integrity of the components and electrical safety. The main detecting items for the crankshaft, crankcase and other finishing parts include requirements of the benchmark size, shape tolerance and surface quality. The detecting items for the shell and other common structural parts are the reference dimensions and appearance quality of the parts.

According to the detection results and the economics of remanufacturing, the detected parts are divided into three categories: parts that can be directly used for remanufacturing assembly, failed parts that can be remanufactured and repaired, and eliminated parts that require new product replacement [18]. The proportion of remanufacturing detecting classification of waste refrigerator compressor parts is statistically shown in

Table 2. Classification ratio of remanufacturing testing of waste refrigerator compressor parts.

Part Name	Direct Reassembly Parts	Remanufactured Repaired Parts	Need New Replacement Parts
Motor stator	50~70%	The rest	Fewer than 10%
Crankshaft and crankcase	30~50%	The rest	Fewer than 5%
Remaining fine parts	30~50%	-	The rest
Seals such as valve plates	-	-	100%
Other structural parts	95%	-	The rest

.

3.5. Remanufacturing of Failed Parts

Remanufacturing is a process of repairing or retrofitting and upgrading failed parts. Remanufacturing technology methods mainly include surface engineering, mechanical processing and part replacement methods [19,20]. Different methods have different characteristics and applicability. When determining the remanufacturing processing method of the parts, we mainly consider the technical, economic and environmental protection and ensure the applicability, durability and technical economy of the remanufactured parts.

The motor stator, crankshaft and crankcase are the three highest value parts in the refrigerator compressor, and their total value accounts for approximately 65% of the value of the compressor machine and 90% of common damaged compressor parts. We can remanufacture only three components.

The failure form of the motor includes mainly damaged coils or backward performance. Remanufacturing can use the replacement method, which is conducted by removing the stator coil and replacing it with a coil according to the original parameters or after upgrading. The failure form of the crankshaft and crankcase is mainly friction damage. Remanufacturing can use a combination of mechanical processing and surface engineering methods to repair the size, shape tolerance, surface roughness and antifriction performance of the parts.

3.6. Reassembly

The reassembly process of the entire machine includes the processes of pump body part staging and matching, movement assembly, casing welding, surface coating and performance detection. Ensuring good cleanliness of the parts, pump body clearance, motor air gap and welding air tightness of the shell is the key to product quality assurance when reassembly occurs. New refrigerant oil should be injected according to the compressor product type with corresponding specifications and dosage during detection. Choosing the correct refrigerant oil is critical to the normal operation of the compressor [21].

4. Key Technologies for Compressor Remanufacturing

The key to compressor remanufacturing is the cleaning and remanufacturing of parts. Cleaning ensures the tidiness of the compressor. The remanufacturing is the premise of matching gap optimisation. Amongst the components of refrigerator compressor, the structure of motor stator is the most complex and the most difficult to remanufacture and clean; in the meantime, the technical requirements of crankshaft are the most comprehensive [22] and the most representative.

4.1. Motor Stator Cleaning Technology

A motor stator is characterised by a complex structure, many types of materials and high requirements for cleanliness and moisture content. Many contaminants exist in the stator after compressor disassembly. Most of the dirt needs to be removed by spraying or rinsing. Then, fine cleaning is conducted using ultrasonic methods. The drying process should be subsequently performed to ensure the moisture content of the parts required. Figure 5 shows the experimental process of stator cleaning.

The cleaning agent is crucial during stator cleaning, considering the cleanliness requirements, material compatibility, environmental protection, safety and economy. The commonly used cleaning agents can be divided into three categories: water-based cleaning agents, organic solvents and hydrocarbon cleaning agents. Water-based cleaning agents have a poor cleaning effect and difficulties in waste disposal and part drying. However, the process is simple and has low equipment requirements. Organic solvent has a better cleaning effect, and waste liquid treatment is recycled by heating evaporation and condensation reuse. However, the environmental protection of the organic solvent is poor. The hydrocarbon cleaning agent has the best cleaning effect and environmental protection. However, it is flammable and contains explosive substances. Thus, it requires careful safety design of the equipment and a large investment.

Table 3. Comparison of common cleaning agent properties of the motor stator.

Cleaning Method	Cleaning Agent	Organic Contaminants (g)	Inorganic Contaminants (g)
Spray + ultrasonic cleaning	Water-based cleaning agents (pure water)	0.0042	0.0407
		0.0036	0.0418
		0.0048	0.0392
		0.0038	0.0377
Immersion + ultrasonic cleaning	Organic solvents (industrial alcohol)	0.0014	0.0297
		0.0024	0.0284
		0.0016	0.0278
		0.0017	0.0257
Immersion + ultrasonic cleaning	Hydrocarbon cleaning agent (R141B)	0.0007	0.0066
		0.0003	0.0081
		0.0013	0.0076
		0.0008	0.0089

shows the test results of the cleanliness of the parts after use of the different cleaning agents. The experimental test data show that R141B cleaning method of the stator had the best cleanliness, followed by the industrial alcohol; the worst method was that using pure water. For postcleaning treatment, because R141B has very strong volatility, the stator can be placed in a dry space after cleaning without special drying, whereas with the industrial alcohol and pure water methods, the stator needs to be dried. In addition, the test results showed that the inorganic impurities contain a small amount of metal iron filings. This problem is mainly due to the large amount of iron filings entering the motor winding when the compressor is opened and disassembled. In the disassembly of the compressor, removing only the solder part can avoid the entry of impurities into the coil caused by direct milling.

4.2. Crankshaft Remanufacturing Technology

The failure form of the compressor crankshaft is mainly friction damage, and the wear parts are the long shaft, short shaft, eccentric crank and thrust face, as shown in Figure 6. The wear of long shaft and eccentric crank is normally more serious, whereas that of the short shaft and thrust face is less so. Crankshaft remanufacturing mainly restores the shape tolerance, surface roughness and the reference size with fit requirements for the wear parts. The long shaft and eccentric crank have size fit requirements with the crankcase and connecting rod, respectively. However, the value of the connecting rod is low, and the economy of remanufacturing is poor. This fit gap can be repaired by replacing it with a new connecting rod. Therefore, the base size recovery aspect of the wear can only be for the long shaft diameter of the crankshaft for remanufacturing.

The compressor long shaft surface usually has a layer of 1.5–2.5 μm thick manganese phosphate film. Long shaft normal wear depth usually does not exceed 10 μm. Taking into account the quality requirements of the product, the experiments compared two long shaft repair methods: brushing + secondary phosphating and fine grinding + secondary phosphating.

Table 4. Comparison of the long shaft repair methods.

Index	Waste Crankshaft		Crankshaft after Brushing		Crankshaft after Fine Grinding		Secondary Phosphating		Surface Polishing
	D (mm)	C (μm)	D (mm)	C (μm)	D (mm)	C (μm)	D (mm)	C (μm)	D (mm)	C (μm)
1	18.001	3.17	17.998	3.32	Without		18.007	Without	18.004	3.01
2	18.000	4.82	17.996	4.05			08.004		18.001	3.62
3	17.997	1.57	17.992	1.46			18.001		17.999	1.42
4	17.994	5.66	17.991	5.42			17.999		17.996	4.87
5	17.998	1.64	17.994	1.52			18.003		18.000	1.55
6	18.001	7.14	17.996	5.94			18.004		18.002	5.12
7	18.000	4.96	Without		17.902	1.73	18.002		17.999	1.65
8	17.998	5.89			17.901	1.82	18.000		17.998	1.72
9	17.996	6.51			17.990	2.04	18.000		17.998	1.68
10	17.995	4.41			17.990	1.46	17.998		17.995	1.52
11	18.000	4.78			17.992	0.93	18.001		17.998	0.84
12	17.999	3.54			17.992	1.11	18.000		17.997	0.96

shows the diameter (D) and cylindricity (C) of the long shaft before and after restoration.

When the phosphate thickness exceeds 5 μm, the phosphate efficiency decreases and phosphate layer denseness is reduced. This condition is not conducive to the product’s antifriction characteristics. Therefore, the maximum repair of the crankshaft’s reference dimensions by means of secondary phosphating should preferably not exceed 4 μm on one side. The experiments showed that the remanufacturing solution of brushing + secondary phosphating does not improve the crankshaft shape tolerance significantly. It cannot completely change the impact of poor crankshaft shape tolerance on compressor noise and vibration characteristics. From the remanufacturing solution of fine grinding + secondary phosphating, the shape tolerance of the crankshaft can be improved significantly. Therefore, the remanufacturing solution for the compressor crankshaft should be fine grinding + phosphating + surface polishing. The crankshaft remanufacturing process is shown in Figure 7. The following aspects should be noted when remanufacturing.

• The fine grinding of crankshaft mainly fixes the size accuracy and shape tolerance of the wear parts of the crankshaft, namely, the long shaft and eccentric crank. The fine grinding of the long shaft is processed by the coreless grinder, and the grinding volume should be controlled below 15 μm. The fine grinding of the eccentric crank is processed by eccentric grinder, and the grinding volume must consider the requirements of the crankshaft eccentric size, which can be controlled at approximately 30–50 μm. The quality of fine grinding of the crankshaft is not only ensured by the machining accuracy of the machine tool but also by the design of the positioning tool, the size of grinding wheel, the speed of the grinding wheel and the choice of the grinding fluid selected.
• The phosphate treatment serves to increase the crankshaft antifriction properties and fix the shaft diameter size. Phosphating treatment uses a manganese phosphating agent, and the film thickness is controlled at 4–5 μm. The acidity, temperature and phosphating time of the phosphating solution during phosphating have an evident effect on the thickness and quality of the phosphating film [23]. The total acidity of the phosphating solution should include control at 35–45 points, free acidity at 4–6 points, a phosphating temperature of 85–90 °C and a phosphating time of approximately 8–10 min during crankshaft remanufacturing. The resulting phosphate film has a uniform and detailed appearance, and it has a continuous film layer and a thickness of approximately 4–5 μm.
• Surface polishing removes contaminants and larger phosphate particles from the crankshaft surface and improves the surface roughness of the crankshaft. Crankshaft polishing can be performed with a polishing machine steel brush type. Polishing treatment can only change the surface quality of the parts and not the shape tolerance of the parts.

After the remanufacturing of the waste crankshaft according to the process scheme, its shape tolerance and roughness were significantly improved. However, the long shaft diameter became slightly smaller and should be matched by part staging to achieve the fit clearance requirements. The key data comparison before and after crankshaft remanufacturing is shown in

Table 5. Comparison of key data before and after crankshaft remanufacturing.

Part Number	Long Shaft Diameter/mm		Long Shaft Cylindricity/μm		Long Shaft Roughness/Ra
	Before	After	Before	After	Before	After
1	18.000	17.992	6.96	1.79	2.41	0.18
2	17.998	17.992	7.89	2.02	4.25	0.20
3	17.996	17.991	8.51	2.52	4.80	0.20
4	17.995	17.991	6.41	1.71	3.67	0.19
5	17.999	17.992	6.54	1.37	2.52	0.20

.

5. Conclusions

This paper presents a complete process of remanufacturing waste compressors and describes in detail the cleaning technology and crankshaft remanufacturing process in compressor remanufacturing. Remanufacturing extends the life of the compressor and reduces the cost of product manufacturing and environmental pollution. It is fundamentally different from the traditional compressor repair, and it is the most economical and environmentally friendly method for processing waste compressors. Remanufactured compressors have a performance and quality not lower than those of the prototype, and the remanufacturing process satisfies standardised mass production.

Some waste compressors are technically difficult and economically inefficient to remanufacture because of their poor environmental friendliness of their refrigerant (R12) or their low refrigeration energy efficiency rating. Better remanufacturing processes and processing technologies need to be studied for these products to maximise the economic, social and environmental benefits.