Evaluate pressure drop in vacuum systems

A simple analysis of plant vacuum systems can reduce utility expenses.

By Dan Bott

PlantServices.com

Organizations typically take great care in the selection of process vacuum pumps. They evaluate and select vacuum pumps based on factors such as performance, energy utilization, size, sound level, and price. However, the components installed between the process vacuum pumps and the production machinery they serve is vitally important to the proper functioning of the system. As it stands, one of the most neglected aspects of vacuum supply is the distribution system. The vacuum distribution system is important because of problems that pressure loss creates. Vacuum pumps must operate at an elevated level of vacuum to compensate for excessive pressure drop. This costs energy dollars and magnifies leak problems. Simple and cost effective measures alleviate these problems. These measures raise the operating efficiency of the system.

Pressure drop

What is pressure drop in a vacuum system? The idea of pressure drop in vacuum systems is similar in concept to pressure drop in plant compressed air systems. Pressure drop is the difference in operating pressure from the supply point to the use point. In compressed air systems, pressure drop is measured in psid, which means pounds per square inch differential. Pressure drop can be used as a measure of pressure drop in vacuum systems as well, but it is much more common to use inches of mercury as unit of measure. Table 1 illustrates some of the common pressure units used in vacuum and how the scales compare with one another. Note that the top of the table represents atmospheric pressure and the bottom of the table represents perfect vacuum. Also, note that there are two scales that use inches of mercury.

Finding the pressure drop in a vacuum system is a simple procedure. First, measure the vacuum at the inlet to the vacuum pump using a reasonably accurate vacuum gauge. Then, using the same gauge, measure the vacuum at the point of use or as close to the point of use as possible. It is acceptable to use two separate gauges, but use calibrated gauges or gauges that display similar values for the levels of vacuum found in the system. The difference between the vacuum levels at the pump inlet and the point of use is the total system pressure drop.

Depending on the type of system, the product manufactured, and the operating vacuum level, the average system pressure drop varies from a fraction of an inch of mercury to 15 inches of mercury. Higher pressure drop means higher operating costs.

Operating demand created by pressure drop

To illustrate the artificial demand created by pressure drop in a vacuum system, consider an average process that requires a volumetric flow of 100 actual cubic feet per minute at 20 inches of mercury vacuum. This example can be scaled up or down to fit any particular application. As system pressure drop increases, there is need for higher vacuum at the pumping system to compensate for the loss. This may not be a problem in moderate levels. A particular vacuum pump may have extra capacity available and some vacuum pump technologies actually use less brake horsepower as vacuum increases. In more severe cases, however, extra capacity will have to be added to the system to account for the greater volume of air. Additional capacity is required because, in general, as vacuum level increases, air entering the system expands in proportion to the vacuum level. The higher the vacuum, the greater the expansion. To attain the desired vacuum and overcome this expansion, more volumetric capacity is needed. At some point, the installed vacuum pump will not be able to keep up with the required expansion of air.

The amount of additional capacity required depends upon the starting vacuum level and the pressure drop. Table 2 illustrates how this phenomenon affects our example system. It shows how pressure drop can add significantly to the number of vacuum pumps required to run a production process. A pressure drop of 3 inches of mercury adds 42 percent to the required production acfm flow. It is not difficult to see that reducing system pressure drop reduces energy costs by reducing the number of vacuum pumps that must be on line. Operating fewer vacuum pumps not only saves energy dollars, it also reduces plant maintenance costs. Non-tangible benefits to fewer on-line pumps are lower noise level, less mist carryover, and lower ambient heat loads. Eliminating off-line pumps can free up usable floor space.

Leaks

Operation of a vacuum system at higher levels also affects the volumetric flow of air leaking into the system. Air entering a vacuum system through leaks adds to the production volume demand and must be treated as if it were production demand. As the vacuum level increases, the affect of leaks on the system increases as well. If our example system has a leak flow rate 6 acfm at 20 inches of mercury vacuum, then operation of the same system at 25 inches of mercury vacuum doubles the leak rate. Even though the percentage of total flow remains the same, it is still an additional load on the vacuum pumps. Also, depending upon the design characteristics of the distribution system, the increased differential pressure from operation at higher vacuum levels may open more leaks.

Reduce the pressure loss

After measuring total system pressure drop, determine which components are adding the most restriction. Review each component individually and then rank them against each other to implement a worst first repair program. To check an individual component, tap into its inlet and discharge and measure the pressure drop. Measure and record the pressure drop for each component from the vacuum pump to the point of use. Establish running logbooks for some items, like filters with replaceable elements, to determine element change-out intervals. The following list contains areas that should be checked or reviewed for each in-house vacuum system.