Vol. IV: Carbon Dioxide Removal – Part III

The last two issues of “Vapors” have discussed the chemical removal of carbon dioxide by granules composed of barium hydroxide and calcium hydroxide. This issue will discuss how the canister that contains the absorbent can affect the efficiency of absorption.

Types of Canisters

There are two types of canisters defined by the movement of gas through the canister:

• One-Way Canisters: Gas flows only one way through the canister. These canisters are open on top and have a perforated bottom with a gasket or o-ring that seals both the top and bottom surfaces.
• Two-Way Canisters: Gas enters from the top, flows to the bottom, around a baffle, and back to the top. These canisters have a seal only on the top surface.

Leak Potential in One-Way Canisters

Since the one-way canister has a seal on both the top and bottom surfaces, there is a greater potential for leaks. Soda sorb granules and dust can accumulate on the lower gasket or sealing surface, preventing the canister from forming a tight seal. This is one of the most common causes of leaks in an anesthetic circuit. Care must be taken to ensure all surfaces are clean and free of soda sorb dust and granules.

Advantages of Two-Way Canisters

The two-way canister has less potential for leaks since it seals only at the top. However, there are considerations to keep in mind:

• Filling Precautions: Since there is usually a tube down the center, this tube should be covered or plugged during filling to prevent granules from spilling into it. Failure to remove the plug before attaching the canister to the machine will result in very high resistance to breathing.
• Insert Protection: Some two-way canisters with a baffle have a threaded insert to fasten the canister to the machine. This insert must be covered during filling to prevent soda sorb from entering the threads.

Filling and Overfilling Canisters

It is not necessary to fill soda sorb canisters to the very top. Overfilling can cause granules to fall off or rub against the surface of the absorber gasket when the canister is placed into position. A 1/2 to 3/4 inch space should be left at the top of the canister.

The Wall Effect

The resistance to flow of gas along the inside wall of the canister and along any baffle or return tube (in a two-way canister) is less than in the center of the absorbent mass. This is known as the “wall effect”, and it is caused by the open spaces created where the granules contact the smooth surface. These spaces exist because there are no protuberances from nearby granules to fill the gaps.

To help minimize this effect, the granules should be packed by tapping the sides of the canister. This facilitates a more uniform flow through the canister.

Channeling in Absorbent Granules

Channeling is a pattern of non-homogeneous flow through the center of the absorbent mass. If the granules are not packed uniformly, the gas will follow the path of least resistance through the granules. This forms channels that bypass most of the absorbent. This effect can also be prevented by proper packing of the granules.

The following chart summarizes the difference between the two types of canisters: