Flooded Starts in HVAC Systems & What to Do Next
It is generally a good idea for customers to consider doing some basic maintenance on their air conditioning and heating system at the beginning of every heating or cooling season. This past winter’s “Polar Vortex” has been especially tough on outdoor equipment. These ultra-low negative temperature spikes, along with the extended times below freezing, can affect the home’s air conditioning system during spring start-up. With a few quick maintenance checks and a little elbow grease, you can help to insure a trouble-free summer cooling season for your customers.
Flooded starts occur when two factors are present: 1) the compressor is off, and 2) liquid refrigerant in the system and the compressor oil are allowed to be in contact with each other. Consider the following:
- It is a good idea to inspect the crankcase heater before spring start-up, to help limit the potential for flooded starts.
- How bad will the flooded start scenario be? How long was the compressor off?
- Don’t rely on system components to protect anything outside of their design feature set.
- Know what type of compressor you are dealing with, most welded shell reciprocating (piston) or scroll compressors are refrigerant-cooled.
- Three-phase compressors require a little more attention than single phase compressors when considering problems associated with flooded starts.
Let’s go a bit more in-depth on each of these tips now:
- Crankcase Heater vs. Pump-down: If we can affect one or both of these factors, we can effectively eliminate the potential for a flooded start. In the past, a pump-down cycle has been used through various medium and low temp applications to separate the system refrigerant from the compressor oil. When a pump-down occurs, the system’s refrigerant is contained to the “high-side” of the system, typically stored in the exiting coils of the condenser or possibly a receiver. By moving all of the system’s refrigerant to the “high side,” and the oil being most commonly found in the compressor shell or body is then separated from the refrigerant and contained in the “low-side.” By keeping both the oil and refrigerant from being in contact with each other, the possibility of a flooded start is eliminated. In air conditioning systems, typically a crankcase heater is used to create this same effect. But remember that a crankcase heater can only slow the rate of migration, and in turn try to reduce the amount of migration and severity of the flooded start. Flooded starts can still happen with a crankcase heater even when properly installed. But the intent is to keep the body of the compressor warmer than the outside ambient air, so depending on how cold it is and how effective the heater is working, there could be a variations in the amount of migration when using a crankcase heater. It is a good idea to inspect the crankcase heater before spring start-up, to help limit the potential for flooded starts.
- Migration: Vapor vs. Liquid: Flooded starts are described as the compressor starting with liquid refrigerant present within the shell or body of the compressor. To understand how this liquid refrigerant becomes present in the compressor, we must first understand how refrigerant migrates. Most technicians would initially point a finger at a temperature difference needed to be present in order for migration to occur. But factors like amount of oil, amount of refrigerant, slope of line set, miscibility and time that the compressor is off, all relate to the severity of the migration as well. How bad will the flooded start scenario be? How long was the compressor off?
- Accumulators vs. Oil Separators: It is common for technicians to believe that a system with an accumulator will prevent migration and thus reduce the potential for a flooded start. Accumulators (like oil separators) are designed to trap liquids (oil or refrigerants) and meter them back at a controlled rate during the run cycle. Refrigerant migrates as a vapor and may not become trapped within the vessel of an accumulator or oil separator. The refrigerant in vapor form will travel throughout the system during the off cycle then condense with the oil in the compressor shell or body, waiting for the next start cycle. Don’t rely solely on system components to protect anything outside of their design feature set.
- Refrigerant-Cooled vs. Air-Cooled: Typically, there are two ways that compressors motors are cooled within the HVACR industry: air-cooled and refrigerant-cooled. Water-cooled is another, but is actually a subset of air-cooled, so for the purpose of this article, we will combine them into the air-cooled group. This cooling effect is how the compressor motor has heat removed; in the case of air-cooled the compressor motor heat is transferred through the shell of the compressor and distributed by use of a fan to the ambient air. These are typically no larger than three tons, have large finned shells and are found within the semi hermetic body line.Most common to modern air conditioning systems are refrigerant-cooled compressors. Refrigerant-cooled compressors remove their motor heat by way of the system refrigerant. During a normal run cycle, the system’s cool suction gas enters the compressor through the compressor suction inlet and across the compressor motor, thus cooling the motor. This refrigerant vapor is then drawn up into the compression cycle and out through the discharge port to the rest of the system by way of the condenser. During the off cycling, if vapor refrigerant is allowed to migrate back to the compressor, the vapor will condense with the oil. All HVACR systems rely on a certain amount of miscibility between the compressor oil and system refrigerant. When the compressor is off and migration occurs, this now condensed refrigerant is absorbed within the oil and starts to raise the effective oil level within the shell. The more charge in a system, the higher this oil laden refrigerant level can be. If this elevated level is allowed to approach the compression cycle, upon the next run cycle it is conceivable for a liquid oil/refrigerant mixture to enter the compression process. If severe enough, this could cause permanent damage to the compressor. These are the basics for defining a flooded start failure mode in refrigerant cooled compressors. Know what type of compressor you are dealing with, most welded shell reciprocating (piston) and scroll compressors are refrigerant-cooled.
- 1Ø vs 3Ø: Once this oil/refrigerant mixture enters into the compression cycle on start up, there is one other line of defense: how much torque does the motor have? In the case of a three phase motor, these typically have enough torque to try and compress a liquid. In our world of HVACR, liquids are non-compressible and if a severe flooded start takes place, the compressor could suffer permanent damage. In the case of a single phase compressor, they are a bit more forgiving than three phase compressors but could still sustain damage during a flooded start scenario. This is mainly due to the possibility of the motor stalling during the flooded start scenario. Three-phase compressors require a little more attention than single phase compressors when considering problems associated with flooded starts.
Wishing you a successful start-up season!
