The Case of Floodback

Sure, the compressor "tripped-out." But in The Case of Floodback, the real culprit was a system failure. Unfortunately, that doesn't let the compressor completely off the hook. System problems like floodback can lead to compressor and other damage. What follows is a line-up of the causes and effects of refrigerant floodback.

Section 1: Effects of floodback on Scroll compressors.

Refrigerant Floodback

One of the frequent causes of compressor failures is damage caused by refrigerant floodback. Refrigerant floodback occurs when refrigerant returns to the compressor before it has completely changed from liquid to gas. Refrigerant floodback is acceptable in certain heat pump designs where the amount of liquid returned to the compressor is carefully controlled with an accumulator. It is not acceptable in air-conditioning applications if the rate of liquid return is unknown and uncontrolled. No matter how much robustness is built into the valves of a reciprocating compressor or liquid handling capability of a scroll element, the bearings, thrust surfaces and scroll tips still require adequate oil film and viscosity to prevent wear. The causes of flood back may vary with system design but generally fall into these problem areas:

1. Overcharged System.
2. Inadequate airflow due to plugged filters, duct design or blower failure.
3. Incorrect or faulty refrigerant metering device.

Any of these problems may cause enough flooding that will dilute the oil to such a point that metal to metal contact and subsequent wear will occur as shown in the above pictures. A secondary consequence of floodback is a cold compressor that will increase refrigerant migration to the compressor when it is shut off. This will result in more frequent and more severe start up slugging, and a shorter compressor life.

The Copeland scroll compressor has three moving parts that are affected by refrigerant floodback. The crankshaft, the drive bushing and the orbiting scroll. Liquid Refrigerant floodback caused the galling evident on the tips of this scroll. Liquid refrigerant washes away the lubricating oil film needed to separate and cool the moving surfaces.

Continuing flood-back will further erode the scroll resulting in visible wear-down of the scroll tips.

At the center of the scroll is the discharge port. A small tip at the end of the scroll involute does not ride on the metal of the opposing scroll. This tip does not wear away like the rest of the scroll, thus indicating the amount of material that has worn away during flood back.

Section 2: Bushings and Bearings
	This drive bushing is located at the end of the crankshaft and inside the orbiting scroll drive bushing. This is a view of the loaded side of this bushing indicating wear due to insufficient lubrication caused by liquid refrigerant diluting the oil.

The drive bearing is backed by steel and bronze, which is then coated with an ablative layer that acts like a lubricant during periods when insufficient oil reaches this area.
In this example some of the bronze bearing under the ablative layer is showing, indicating poor or no lubrication.
If excessive flood-back continues the ablative layer wears away completely exposing the bronze bearing material.
This shows a severely worn out drive bearing that would result in noisy compressor operation. If the compressor is not replaced at this point the bearing will continue to wear until the hub in which it is pressed starts to wear and breaks.

Section 3: Effects of Floodback on Reciprocating Compressors
	There are more load bearing moving parts that are affected by lack of lubrication caused by refrigerant flood-back. One example is the large bore of the connecting rod that is in contact with the crankshaft journal. Here is an example of metal transfer caused by localized overheating when the oil film was washed away.

The upper portion of the connecting rod with the wrist pin hole depends upon splash or misting lubrication to keep the wrist pin and pin hole from scoring and wear. Here is an example of a pin hole that has become oval due to wear. The compressor in this case would become noisy and lose capacity since the piston can no longer be pushed up near the valve plate, leaving a lot of compressed gas in the cylinder.

Along with the pin hole wear the wrist pin will also wear. In the case of these examples the compressor may still run but would not be able to keep up with the load as the outdoor ambient nears the design point.

This is a section of crankshaft, specifically the area of the main bearing. During flood-back this area receives the least amount of oil since it is furthest from the oil pick up in the sump. When the crankshaft looks like this the main bearing is worn also.

Once the main bearing becomes severely worn the compressor can no longer maintain the small air gap between the stator and the rotor, resulting in the rotor dragging along the stator bore, eventually resulting in a grounded motor.

Section 4: Superheat To determine if a unit is operating with proper superheat the following two charts illustrate what and where to measure both temperature and pressure.