We’re picking up on the thread of our last post about oversized compressed air systems, where we showed that the further away from full load a fixed speed compressor operates, the higher the energy cost is per cfm of compressed air. Energy may be the easiest cost per unit of air to recognize and measure, but it’s not the only component of cost, and it may not be the most significant cost in your operation.
Increased cycling associated with under-utilization has several negative effects on compressors, and we’ve found that for under-loaded systems, maintenance and repair costs increase as a portion of total operating cost. A review of service records showed that units with duty cycles had a significantly shorter mean time between failure (MTBF). Because compressors are usually serviced based on total run time rather than actual load time, a machine that idles a lot costs more in parts and labor per loaded (i.e. productive) hour. If you calculate the service costs based on cfm produced rather than hours of run time, you’ll find that PM and repair costs per cfm rises also.
Think of your car. Cost per mile for gas and maintenance goes down if most miles are highway miles. But highway miles are also gentler on your car (fewer starts and stops, etc.). City miles are notoriously inefficient with fuel, but they also accelerate wear on the motor, the brakes, steering and suspension.
Likewise, low-loaded compressors are more likely to show wear at an accelerated rate. Inlet valves, vent valves, and others, cycle many more times at low load. Motors starts are more frequent which can affect bearing and winding life. On direct drive units with polymer couplings, frequent cycling can reduce coupler life. Frequent starts and stops put more wear on thrust bearings in the airend.
Further, if the unit doesn’t run enough, it may not reach proper operating temperature, which results in moisture accumulation in the lubricant. This is a common cause of premature airend failures. Frequent changes in temperature can also cause metal fatigue on aluminum coolers. These conditions call for increased frequency of preventive maintenance and the likelihood of downtime for repairs.
Another downside to poorly sized systems is pressure fluctuation. Swings in pressure may result in defective products, and more sophisticated production machinery have sensors that will shut down the equipment if pressure is outside of design specifications. Depending on the cost of raw materials and value of finished product, the costs of downtime and scrap may far exceed the losses in energy efficiency and service costs.
If you are planning a compressed air system for a new plant or expansion, you may only be able to estimate your compressed air demands. So the smart money is spent splitting the estimated demand among multiple compressors and having good controls (and ample storage). Using variable output compressors as trim machines is part of a good strategy.
For existing systems, the first step is an accurate air system assessment to determine how well your system is sized and controlled. If your budget allows for replacing compressors, the ROI from lower energy consumption, lower service expenses and reduced downtime may justify replacing over-sized compressors and adding controls. In some cases, just adding one smaller machine can make the difference.
If your budget cannot accommodate new compressors, there are lower cost investments that can help mitigate over-sized compressors. Adding storage often reduces compressor cycling and can stabilize pressure. In some cases, flow controls may further improve the effect of storage. For systems with multiple compressors, adding a modern multi-unit controller will definitely help reduce starts/stops while stabilizing pressure and provide additional benefits such as remote monitoring and energy consumption information.
Downtime and scrap caused by pressure fluctuations, high service and repair costs, and high energy costs, are problems that many plants simply live with as expected costs of operating compressors. But they don’t have to be. The first step is an honest assessment of how well your compressed air system is working.