The airflow BOTH IN AND OUT of the tank, reservoir, or gearbox determines the part number to choose. Two factors create airflow, they are: temperature variations (which cause very low airflow rates), and fluid volume changes caused by the stroking of hydraulic cylinders or by tanks being filled and emptied. The airflow rating for each part number is stated on the sales literature. We also state the equivalent fluid volume change, which a customer is more likely to know. The formula for the relationship between these two measurements is: 7.5 gpm (gallons per minute) of fluid volume change creates one (1) cfm (cubic feet per minute) of airflow.
Part numbers 39100, 39101, and 39102 are rated at 35 cfm airflow. This is the amount of airflow created by 260 gpm of fluid level change. At this airflow rate or fluid level change rate the pressure drop through these breathers will be approximately 1 psi. Pressure drop is an important concern because if there is too much pressure drop created by the resistance to airflow through the breather, the sides of the tank will be subjected to stress and could fracture, creating an implosion or explosion. Even though much larger volumes of airflow can pass through the breather (creating high psi drops), we rate all our models at the cmf that creates 1 psi, which is considered safe even for very thin walled tanks.
The pressure drop rating for a group of products is based on the size of the opening in the standpipe coming out of the bottom. Since Silica Gel adds very little to the airflow resistance, all part numbers with the same standpipe size are rated at the same airflow. Therefore 39100, 39101, and 39102 can be used for the same application, however, the tallest unit, the 39102 is the most economical because it has more life (more silica gel) for the cost.
Because the mounting hole on part numbers 39131, 39132, 39133, and 39134 is smaller (½” female NPT), they are rated for 10 cfm (75 gpm).
For larger airflow requirements the 39108, which has a 2″ standpipe opening, will handle 100 cfm (750 gpm).
Posted in: Watchdog Desiccant Breather