Are you familiar with suction line P-traps?
If you work in the HVAC/R industry, you may have heard of them. But do you know when and why they should be used?
The topic of suction line P-traps can be a contentious one, with varying opinions on their necessity and placement. However, understanding their purpose and proper installation can greatly improve the efficiency and longevity of your refrigeration equipment.
In this article, we’ll dive into what suction line P-traps are, why they’re important, and when you should use them.
So, let’s get started!
When To Use A Suction Line P-trap
Suction line P-traps are an essential component of refrigeration equipment. They serve a critical function in ensuring proper oil circulation and preventing oil from being trapped in horizontal piping.
But when should you use a suction line P-trap? The answer is simple: always.
Whether you’re installing a new system or retrofitting an existing one, suction line P-traps should be included in your design. They are necessary for proper and efficient operation of the equipment, and failure to include them can lead to costly issues down the line.
It’s important to note that there is no one-size-fits-all answer when it comes to suction line P-traps. Each facility and installation will have its own considerations and concerns. That’s why it’s crucial to consult trusted resources, such as ASHRAE, for guidance and to think through the dynamics of oil return, suction gas velocity, and unnecessary trapping.
What Is A Suction Line P-trap?
A suction line P-trap is a discharge line trap that serves an important function in refrigeration systems. It is a critical piece for proper and efficient operation of the equipment. The trap is usually located in the suction line of a refrigeration compressor and is used to prevent oil from draining back to the compressor during shut-down periods.
When the compressor is running, the P-trap acts as a turbulator to bring oil droplets up the elevated discharge line. This ensures proper oil circulation when the loads are low and helps prevent oil from being trapped in the horizontal pieces of piping. During an off-period for the compressor, the trap can catch and retain any leftover oil on the walls of the piping that could otherwise potentially drain back into the compressor head.
It’s important to note that P-traps come in different sizes and depths depending on the specific installation. Each facility and installation will have its own considerations and concerns, so it’s crucial to consult trusted resources for guidance and to think through the dynamics of oil return, suction gas velocity, and unnecessary trapping.
The Importance Of Suction Line P-traps
Suction line P-traps play a crucial role in refrigeration equipment by ensuring proper oil circulation and preventing oil from being trapped in horizontal piping. When the compressor is running, the P-trap acts as a turbulator to bring oil droplets up the elevated discharge line. During an unloading period or within a multi-compressor system, the traps ensure proper oil circulation when the loads are low. They also help prevent oil from being trapped in the horizontal pieces of piping.
During an off-period for the compressor, the traps can catch and retain any leftover oil on the walls of the piping that could otherwise potentially drain back into the compressor head. Failure to include suction line P-traps in your design can lead to costly issues down the line. For example, improperly installed P-traps can trap oil, causing an upset upon start-up of the compressors. As oil “floods” back to compressor crankcase, the oil level sensor is upset due to the disruption and, as intended, the compressor fails/trips on inadequate oil level. This can cause a rise in temperature, which can lead to reduced quality and shelf life of stored products or spoilage.
To avoid these issues, it’s important to install suction line P-traps whenever the compressor is above the evaporator. The traps should be properly configured at the evaporators with a branch line properly graded back towards the compressors (1/4” per lineal ft.). If a suction line is oversized, the refrigerant velocity will be decreased, preventing the oil from moving through the suction line to the compressor’s crankcase. It’s important to use the right viscosity (thickness) of oil to prevent it from congregating in system piping.
Common Misconceptions About Suction Line P-traps
Despite their importance, there are some common misconceptions about suction line P-traps that can lead to incorrect installation or neglect.
One misconception is that suction line P-traps are only necessary when the compressor is running. In reality, they serve an important function both when the compressor is on and when it is off. During the off period for the compressor, the traps can catch and retain any leftover oil on the walls of the piping that could otherwise potentially drain back into the compressor head.
Another misconception is that suction line P-traps are only necessary in certain types of systems. However, they should be included in all refrigeration equipment designs, regardless of the system type or size.
Some may also believe that suction line P-traps are not necessary if the refrigerant line sizes are changed. However, this is not the case. Even with changes in line size, suction line P-traps should still be included to ensure proper oil circulation and prevent oil from being trapped in horizontal piping.
How To Properly Install A Suction Line P-trap
Proper installation of a suction line P-trap is key to ensuring its effectiveness. Here are some steps to follow:
1. Plan the piping system: Good piping practices are essential to guarantee a long life and minimal maintenance for refrigeration and freezer equipment. Correct line sizing is imperative to optimize the performance of the system. Undersized lines will result in high pressure drops, diminished capacity, and increased power requirements. An oversized suction line will lead to poor oil return and premature compressor changeouts. Keep the refrigeration lines as short as possible, use as few fittings as practicable, and be especially careful not to kink the lines.
2. Determine the appropriate location: The suction line P-trap should be installed at the evaporator unit discharge line, just before it rises vertically to the compressor. This location ensures that oil droplets are brought up the elevated discharge line when the compressor is running.
3. Size and depth: The size and depth of the suction line P-trap depend on the refrigerant type, compressor capacity, and piping length. Consult ASHRAE or other trusted resources for guidance on sizing and depth.
4. Proper grading: The branch line leading from the suction line P-trap should be properly graded back towards the compressors (1/4” per lineal ft.). This ensures proper oil circulation when the loads are low and prevents oil from being trapped in horizontal piping.
5. Avoid mid-riser traps: Mid-riser traps in suction lines can trap oil unnecessarily, causing an upset upon start-up of the compressors. This can lead to compressor failure and product storage problems like reduced quality and shelf life, or worse, spoilage.
Proper installation of a suction line P-trap is critical for ensuring efficient operation, longevity, and safety of refrigeration equipment. By following these steps and consulting trusted resources for guidance, you can ensure that your refrigeration system operates at its best for years to come.
Maintaining And Troubleshooting Suction Line P-traps
Once you have installed suction line P-traps in your refrigeration equipment, it’s important to maintain and troubleshoot them regularly. Here are some tips to help you keep your P-traps in good working order:
1. Check for Proper Installation: Make sure that your P-traps are installed correctly, with the trap located at the highest point in the suction line. The trap should be sized properly, never exceeding 4 times the size of the refrigerant line, and graded back towards the compressor at a rate of 1/4” per lineal ft.
2. Monitor Oil Levels: Keep an eye on your compressor’s oil level to ensure that it is not being affected by trapped oil. If you notice a drop in oil level, it may be a sign that your P-trap is not functioning properly.
3. Inspect for Accumulated Oil: Check your P-traps regularly for any accumulated oil. If you see oil in the trap, it’s a sign that the trap is functioning correctly and doing its job of catching and retaining oil.
4. Clean Your P-traps: Over time, P-traps can become clogged with debris or other materials. Regular cleaning can help prevent clogs and ensure that your traps are functioning properly.
5. Troubleshoot Issues: If you notice any issues with your refrigeration equipment, such as reduced capacity or poor performance, it may be a sign that your P-trap is not working correctly. Troubleshoot the issue by inspecting your P-trap and checking for any accumulated oil or other issues.
By following these tips and maintaining your suction line P-traps regularly, you can ensure proper and efficient operation of your refrigeration equipment and avoid costly issues down the line.