Cap tube orientation

Question: I was involved in a discussion about the correct orientation of a TXV bulb capillary tube on an hvac website. According to the RSES Heat Pump Training Course, the capillary line should be above the pigtail when the bulb is placed on a horizontal line and the capillary tube should point up when the bulb is placed on a vertical line.

My understanding of this orientation is to keep the liquid refrigerant in the sensing bulb, which would make the bulb more reactive. This makes sense. However, I was quite surprised when an engineer from a well-known company posted on the same website that the capillary tube orientation makes no difference.

I checked several TXV manufacturer websites and could not find any literature that mentions any preferred direction of the capillary tube. Can you help?

-John Sweeney, plant engineer, Maverick C&P Inc.

Answer: A thermal expansion valve (TXV) is most effective when its charged, specifically in its liquid form, lies within the bulb. If the TXV bulb is in the horizontal position, we recommend that the capillary tube near the bulb is faced upward.

Likewise, if the bulb is placed on a vertical pipe, the capillary tube should be facing in the same direction. By doing this, the liquid charge is more likely to stay in its preferred area – in the bulb.

The importance of this orientation depends on the type of charge within the TXV power element. For standard charges, the power element has a large amount of charge to ensure enough is present at the area of sensing. Although the orientation mentioned above still applies, charge migration is not a common problem with most standard charges.

On the other hand, it is very important that the capillary tube is oriented in the right direction for valves with a maximum operating pressure (MOP) or air-conditioning charge. Both use a smaller amount of charge than that of the standard to achieve specific results.

The MOP charge is designed as a system protector by limiting the valves opening in high evaporating temperature situations. Also, a limited type of charge with a ballast material is used in many air-conditioning valves, which allows it to effectively adapt to constantly varying system changes.

It is very important that liquid charge migration is prevented as much as possible in these situations. Liquid migration can be significantly reduced by facing the capillary tube upward from the bulb.

Other steps can be taken to maximize TXV performance:

•  Locate the bulb in an area on the suction line where it will be least affected by liquid refrigerant and oil.

•  Do not place the bulb on a bend.

•  If mounted near a bend, place it opposite the outer wall.

•  Mount the bulb in an area on the top of the pipe between 4 o-clock and 8 o'clock.

•  Mount the bulb on a pipe surface that is smooth, clean, and continuous.

•  Avoid placing it near or across from areas of disturbance (for example, equalization tubes, process tubes, and connections).

•  Mount the bulb tightly against the suction line, creating a small indentation in the pipe (should not be easily moved by hand).

•  Make sure the bulb is aligned with the pipe.

•  Completely insulate the bulb, preventing any airflow across it.

-Bruce Busby application engineer, Danfoss Inc., Air-Conditioning and Refrigeration Division