**Abstract:**
Air source heat pump units have gained widespread popularity in recent years due to their dual heating and cooling capabilities, as well as ease of installation. However, these systems often operate under fluctuating conditions caused by varying air conditioning loads and environmental factors. As a result, the units frequently run under non-standard conditions, which can lead to performance issues and even compressor failures. This paper presents a detailed analysis of common compressor failure problems encountered during the actual operation of air source heat pump units, along with practical solutions aimed at improving system reliability and longevity.
**Keywords:** Heat pump; Compressor failure; System maintenance; Defrosting; Refrigerant management
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**1. Introduction**
In recent years, air source heat pump units have become increasingly popular due to their efficiency, flexibility, and ease of installation. These systems are capable of providing both heating and cooling, making them ideal for a wide range of applications. However, the performance of such units is significantly affected by external environmental conditions and the fluctuating nature of air conditioning loads. As a result, they often operate under non-standard conditions, which can lead to prolonged stress on the system components, particularly the compressor.
Compressor failures are one of the most critical issues that can occur in air source heat pumps, especially under harsh operating conditions. This paper explores the causes and symptoms of compressor failures, and proposes effective measures to prevent or mitigate such problems.
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**2. Symptoms of Compressor Failure**
Fully enclosed piston compressors are commonly used in air source heat pump units. These compressors typically fail during winter heating operations. Based on field observations and post-failure inspections, three main types of compressor failures have been identified:
**(1) Suction and Discharge Valve Rupture**
Symptoms include normal oil levels, intact bearings and crankshaft, but ruptured suction and discharge valves. This usually occurs when water enters the compressor, leading to liquid hammer.
**(2) Compressor Stalling**
This is the most common type of failure. Symptoms include black oil, worn bearing sleeves, broken connecting rods, and signs of wear on the crankshaft and bearing surfaces. The valves remain intact, indicating mechanical wear rather than valve damage.
**(3) Motor Burnout**
The insulation resistance of the motor drops to zero, while the mechanical parts (bearings, crankshaft, connecting rod) remain undamaged. This suggests electrical failure rather than mechanical wear.
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**3. Causes of Compressor Failure**
**(1) Suction and Discharge Valve Rupture**
The primary cause is water entering the compressor due to a rupture in the water-side system. This can happen in two scenarios:
- During cooling mode, if the flow switch is manually bypassed, water in the heat exchanger may freeze, causing copper tubes to crack and allowing refrigerant and water to mix.
- In winter, if the unit is not properly drained or anti-freeze measures are not taken, freezing can occur, leading to similar consequences.
**(2) Compressor Stalling**
Stalling is typically caused by poor lubrication due to refrigerant dilution or oil flooding. When the unit is idle for long periods, refrigerant can accumulate in the compressor, dissolving into the lubricating oil and reducing its effectiveness. This leads to inadequate lubrication, resulting in mechanical wear and eventual stalling.
Additionally, improper defrosting in high-humidity environments can cause excessive refrigerant backflow, leading to liquid slugging and compressor damage.
**(3) Motor Burnout**
Motor burnout is often linked to overheating, which can occur when the unit operates in cold environments with insufficient airflow for cooling. Poor defrosting control can also lead to prolonged low-load operation, causing the motor to overheat and eventually burn out. Another cause is contamination within the refrigeration system, which can lead to internal short circuits.
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**4. Improvement Measures**
To address the root causes of compressor failures, the following measures are recommended:
**(1) Implement Anti-Freeze Control**
Ensure the system has an automatic anti-freeze function that activates when ambient temperatures drop below a certain threshold. Also, provide drainage facilities to empty the water system when the unit is not in use for extended periods.
**(2) Maintain Proper Flow and Protection**
Avoid bypassing flow switches or other safety devices. Regularly monitor the temperature difference between the inlet and outlet water to detect any abnormal conditions early.
**(3) Enhance Preheating Function**
Improve the electronic control program to ensure sufficient preheating time for the compressor before startup, preventing oil dilution and mechanical damage.
**(4) Optimize Defrosting Mechanism**
Improve the defrosting method to ensure it is timely and complete. Strengthen the reliability of the control system to avoid malfunctions.
**(5) Improve System Design**
In low-temperature heating conditions, ensure proper matching of expansion valves and gas-liquid separators. Consider adding bypasses to prevent refrigerant backflow and reduce the risk of liquid slugging.
**(6) Ensure System Cleanliness**
Maintain high-quality manufacturing and installation standards to prevent contamination and moisture ingress. A clean and dry refrigeration system is essential for reliable operation.
**(7) Enhance User Awareness**
Provide training to users on proper operation, maintenance, and troubleshooting techniques to extend the life of the system and reduce the likelihood of failures.
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By implementing these improvements, the reliability and service life of air source heat pump units can be significantly enhanced, ensuring efficient and trouble-free operation under various environmental conditions.
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