What is Short Cycling?
Short cycling is the rapid turning on and off of a mechanical system, and occurs frequently in air-compressors, furnaces, chillers and AC units. It can be caused by dirty air filters, an oversized system, or the differential pressure being set too small. This rapid state-change adds unnecessary stress on the system, reducing its lifespan and efficiency. Without monitoring and automated detection, facility personnel would need to be within earshot of the equipment at the time of malfunction. In practice this can mean days, weeks and months of wasted energy and damage to the equipment before the condition is even noticed. In addition, it can mean that the air-conditioning running off the chiller, or the power-tools connected to the air-compressor will underperform.
How Does Verdigris Detect Short Cycling?
Luckily by measuring the power consumed by the mechanical equipment, it is easy to tell when a system is short cycling. A healthy compressor, Figure 1, will do work for some time, then reach its setpoint, and shut down or reduce power draw for a while.
Figure 1. Healthy Compressor A
A compressor that is short cycling, on the other hand, will do work for short bursts of time, shut down for an equally short time, only then to quickly turn back on. Figure 2 shows an example a compressor that was detected to have short cycling. The compressor stays on for 2–3 minutes, turns off for 2–3 minutes, and then turns back on, a classic example of short cycling.
Figure 2. Short Cycling Compressor B
In order to help facility personnel spot equipment that is short cycling, we created an algorithm to analyze the frequency of the power consumed by the equipment using our high-resolution data. When the fundamental frequency of the power signal is too high, it means that the periods between the on and off states are too short. A general rule-of-thumb is that if a compressor turns on and off more than 5x in an hour there is a likely short cycling occurring.
The algorithm we created reads in minutely data from each piece of equipment that is being monitored and analyzes its frequency components. To analyze the frequency content of a signal, we take many overlapping Fourier transforms, a method of converting time series data into its frequency components, of the power signal and compare its fundamental frequency with our rule of thumb of 5x an hour. Figure 3 shows the short-time Fourier transform (STFT) of compressor A, from above. The red line is the cycling threshold, 5x an hour. When there is short cycling, energy (shown in light blue) will appear above the red line. In practice we use a more mathematical determination to see if there is short cycling, but the visual below is helpful to understand what is going on behind the algorithm.
Figure 3. STFT of Healthy Compressor A
In contrast, Figure 4 shows the STFT of compressor B with short cycling, which is why we see energy above the red line.
Figure 4. STFT of Short Cycling Compressor B
So what does this mean for a facility manager or engineer?
Short cycling is often a very telling but difficult to detect symptom of a more severe mechanical issue. When short cycling goes undetected it is easy to misdiagnose the performance problems of a compressor, resulting in continual calls for maintenance while leaving the underlying fault unfixed. In the long run this could lead to complete mechanical failure.
Knowing that your system is short cycling quickly narrows down the root causes of your underperforming devices.
More immediately, short cycling will consume excess energy by lowering the efficiency of the equipment and actually increasing its overall runtime. Increased runtime at lower efficiencies can quickly add up to hundreds or thousands of dollars in wasted annual electricity spend. Catching short cycling as early as possible through automated detection will save you on both your energy bill and your maintenance bill.
Is your equipment at risk? Contact us to learn more.