QUESTION:
I just bought a new vacuum pump. When I got it to my shop, I noticed one basic difference: My old pump is set up for 1-1/2 inch plumbing and the new pump is 2-1/2 inches. I was hoping simply to replace the old pump with the new one, but now I’m wondering if there are going to be some problems. If I reduced the pump fittings to 1-1/2 inches, will it hurt anything or will it pump as if nothing is different? Or do I have to replace my plumbing with 2-1/2-inch plumbing?
Richard Zanakas
Little Rock, Ark.
ANSWER:
You have reached the worst of two worlds. Maybe not the worst, but we’ll explain what happens if you reduce the pump to 1-1/2 inches. First, remember that each pump is designed to create a certain amount of vacuum, which requires airflow. The airflow of each pump is a constant. That is to say, once you turn the pump on, it will come up to the appropriate rpms, the vanes will spin and the vacuum will be created.
What if we reduce those 2-1/2-inch fittings on the pump to 1-1/2 inches and leave the plumbing as it is? Now we have a constant airflow, but the flow is restricted, which won’t create more vacuum (same amount of rpms) but it will possibly cause the ball in the primary to rise and block the airflow completely. It is a stronger, pulling vacuum but it is not more vacuum.
INTENSITY OF FLOW
The same thing is true of the secondary. The increased speed of the air could make the ball in the secondary rise up and block airflow. So by reducing the 2-1/2-inch pump to 1-1/2 inches you run the risk of blocking all airflow. Restricting the airflow increases the intensity of the flow but not the volume of the flow. That means you’re not pumping very much and you’re not making much money.
As an example to illustrate the point, restrict a 2-1/2-inch water hose by reducing the flow to 1-1/2 inches. When you turn on the water, you are getting a more powerful spray through the restricted opening, but you aren’t getting more water. In the case of vacuum, as mentioned, the increased intensity of the vacuum can cause real problems to the overall system.
On the other hand, it is fairly costly to replace the plumbing and accessories. Primaries and secondaries of that size are expensive, not to mention the labor and the piping. But, to get maximum performance from that pump, which is what every pumper wants, this is the right way to do the job.
So blocking your airflow with the reducer fittings is expensive because you’re not getting much pumping done, and replacing the plumbing and accessories is also expensive. The basic truth is that pumps and vacuum systems work best when the size of the pump matches the size of the plumbing.
If these alternatives don’t fit your budget, return the pump and get the same inlets and outlets you had before. This takes time and money, but in the long run, may be the cheapest way to go. You’ll be able to maximize the efficiency of this pump with your existing plumbing and parts. The rule of thumb is: If you’re going to upgrade your vacuum pump and system by moving to larger ports, then increase the size of your plumbing to match.
PULLING POWER
One side issue needs to be considered before you pick up a new vacuum pump: Know and understand the strength of your vacuum pump. This is measured by the cubic feet per minute, or cfm, of airflow generated by the pump. Let’s say the pump you are replacing produces 160 cfm and has inlets and outlets of 1-1/2 inches. Assuming the pump works well with the size of vacuum tank on your truck, then get the same pump to replace the old one. If they no longer make that model, consider the size of your plumbing and the cfm of the old pump. In this case, try to get a new pump that operates around 160 cfm and has 1-1/2-inch fittings.
