Compressed Air: Fix the Leaks
Compressed air leakage is an important opportunity to reduce energy use and improve profit. Substantial resources are often wasted on electricity used to power air compressors to “fill the leaks” throughout a facility. But, compressed air leakage rates are very difficult and expensive to measure directly in a foundry environment.
Fortunately, indirect measurement approaches can be used to determine the aggregate compressed air leakage rate at a facility to within about 20% of the true value.
Using actual baseline compressed air system measurements from a foundry, we determined several important parameters of the compressed air system used for their operations.
Static Air Leaks
Static leaks are the compressed air losses that exist regardless of whether production equipment connected to the compressed air system is in operation.
Table 1 summarizes leak rates associated with various size circular leaks. Although leak rates from more common, irregularly shaped leaks are more difficult to quantify, this table shows the relative importance of focusing repair efforts on larger leaks. Fixing a single, large leak may consume the same amount of labor and supplies as fixing a single, small leak but the larger leak may allow 100 times more air to escape compared to a smaller leak in the same compressed air system. It is important to prioritize leak repair efforts to focus first on the leaks that are perceived to allow the greatest amount of air to escape.
Static leaks exist in virtually every foundry and manufacturing operation and are often overlooked or ignored for repair. In contrast, hydraulic fluid leaks are repaired with higher priority due to the problems with fluid on the floor. The cost impacts of these different types of fluid and air leaks may not be used to determine the repair priority.
Many of these air leaks are “low-hanging fruit” and can be repaired in a very cost-effective manner.
Dynamic Air Leaks
Dynamic leaks are the compressed air losses in equipment that are only apparent when equipment connected to the compressed air system is in operation. It is difficult to separate the anticipated operational use of compressed air in a machine from the excess use of compressed air caused by leaks in the actuators and valves in the machine. These leaks are only exposed to compressed air when the machine is operating.
Depending on the age and maintenance record of the equipment, dynamic air leaks can be very significant and may be of the same order of magnitude as the static leaks. In simplest terms, dynamic air leaks result in a piece of equipment using significantly more compressed air during operations than it did when it was new.
The following example only pertains to static air leaks.
It is possible to calculate an aggregate compressed air leakage rate from measuring the time and pressure during the compressed air system discharging and charging processes.
Measurements were made during a non-production day. In addition to enabling various calculations, these measurements can serve as a baseline set of conditions for future reference to show progress in removing leaks. When a future system discharge test is performed, the time that it takes the system to change pressure from 100 psig to 90 psig could be a benchmark to rate the overall progress of efforts to remove leaks from the compressed air system. The longer it takes for the pressure to fall from 100 psig to 90 psig in future measurements compared to the current value, the more progress made in eliminating costly leaks.
This leak-down time from 100 psig to 90 psig can be used as a monthly or quarterly metric of the building’s compressed air system.
From these measurements, we calculated an approximate air leakage rate from the compressed air system including header pipes, compressed air drops, attached equipment, and the air compressor hardware. We estimated that 67% of the compressed air generated by the compressors was consumed with filling leaks during the year. This has a profound impact on the cost of running this equipment.
It should be noted that due to the complexity of compressed air systems and the details of the compressor’s electrical controller, there is not always a precise proportional energy savings between eliminating leaks and energy costs. If you start with a system that has 40% of the compressor’s capacity used to fill leaks, you may not see a 40% savings in electricity use if all leaks could be eliminated; you may only see a 30% savings, but significant savings nonetheless. It should also be noted it is extremely difficult to remove all leaks in a real-world, operating foundry. Achieving only 10% compressed air lost to leaks is a great goal.
Simplified Cost Savings Estimates
To make the calculations simple, we will make assumptions that will result in approximate values for costs and savings. Assuming the 250 hp compressor requires about 0.7kW for each hp, and the air compressor is operating at about 65% of its capacity. It needs about 2,730 kWh/day to operate. We also estimated this compressor runs about 340 days per year. Therefore, the compressor will consume about 928,200 kWh/year.
At an overall electricity rate of $0.075/kWh, the total electricity costs for one year of operation is $69,615 per year. If 67% of these costs are wasted on filling the leaks in the foundry, then up to $46,640 is wasted on electricity to fill the static air leaks.
If the compressor operates at higher set-point air pressure levels then the test range between 90 psig and 100 psig, then the leak rate and the associated costs will be significantly higher.
In this scenario we estimated that it is costing about $46,500 per year to fill the leaks. Fixing the leaks would effectively reduce your expenses and improve profits every year thereafter.
Unfortunately, air leaks are a major component of any air system. The good news is that regular maintenance can keep this cost down. Implement a regular program to find and repair leaks. Initially focus on repairing the largest leaks and as resources permit repair medium and small leaks. Track your progress by using the pressure drop test outlined in this article. Leaks are a constant battle but worth it in cost savings.
The authors of this column can be contacted at: Dr. James (Jamie) Wiczer (firstname.lastname@example.org); Robert Eppich (email@example.com); Cindy Belt (firstname.lastname@example.org); Brian Reinke (email@example.com).
Click here to see this story as it appears in the July 2018 issue of Modern Casting