Why do compressors break?

This is the first chapter of a series of articles launched by Danfoss on the topic. The focus of the series is on the possible causes of compressor failure and the ways to treat them.


The return of liquid occurs during the operation of the equipment. It is when large volumes of refrigerant return uncontrollably to the compressor in operation through the suction line.


The amount of liquid that returns to the compressor determines the extent of the damage. We can detect this problem when there is foam on the compressor oil level sight glass.


Consequences of Coolant Return


When there is liquid inside the cylinder's lubrication channels, the lubrication of the cylinder and piston is compromised, generating wear and overheating in the cylinder, which, in turn, leads to the formation of small metallic particles contaminating the internal part of the equipment. This liquid also dilutes the lubricating oil in the

compressor housing.


As the oil becomes more diluted with the refrigerant, its lubrication capacity is impaired. When this liquid-rich oil reaches the crankshaft to lubricate the bearings, connecting rods, cylinder walls and other structures, the refrigerant in the oil begins to evaporate due to friction, forming what we call a gas flash.


This prevents the oil from properly lubricating the necessary parts and, generally, the main bearing and the connecting rods are quickly dried out and, consequently, wear out.


Evidence of aluminum waste from the connecting rod on the crankshaft


Sometimes the main bearing wears out to such an extent that it brings the rotor and stator into contact and burns, or, more frequently, the connecting rods lock on the crankshaft and the engine continues to run with normal movement, then breaking the connecting rods aluminum and eventually even the pistons.

When the liquid causes the connecting rods to lock in this way on the crankshaft, the presence of aluminum from the welded connecting rods on the bearing surface can be observed. This is due to the virtual "explosion" of coolant in the oil due to the heat generated by the friction of the bearing surfaces.


How coolant returns


The return of liquid usually occurs during the night operation period, when the thermal load is lower and the system has excess capacity. It is common for expansion valves to be oversized for the system and then, each time the compressor starts or when large product loads are introduced into the installation, the expansion valve is forced to open, and the result is an overload in the evaporator, especially if the adjusted superheat is too low. To get more ideas, check out: oil in coolant


The thermostatic expansion valves have their opening forced by the drop of the suction pressure, causing a reduction in the pressure in the diaphragm of the expansion valves. Such a pressure drop reacts faster than the load on the expansion valve bulb, leading to the opening of the valve initially. Until the charge in the bulb reacts and the pressure in the bulb begins to drop, helping to close the valve, the coolant will flow to the evaporator in an uncontrolled manner (the larger the valve, the greater the amount of liquid).


Causes of Coolant Return


1. Low charges on the evaporator;

2. Oversized equipment;

3. Distribution of products in the cold room (poor air circulation caused by lighting, pallets, shelves, etc.);

4. Failure of the evaporator fans;

5. Oil entering the evaporator;

6. Insufficient defrost in the evaporator (frozen evaporator / absence of air flow / deficient heat transfer);

7. Oversized expansion valve orifice;

8. Type of expansion valve wrong;

9. Expansion valve equalization tube restricted or blocked (capillary tube? Presence of oil? Etc);

10. Loose expansion valve clamp or bulb in the wrong position on the suction line;

11. Overheat set too low.

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