February 25, 2014
Precision Shaft Alignment and Energy Consumption
By Steve Matthews
It has been a long standing belief amongst maintenance and reliability professionals that electric motor energy consumption must be less when rotating machines are precision aligned vs. when misaligned. There have been several studies on this subject that have shown varied results, from 1% to as much as 10% less energy consumption for a precision aligned machine.
In the opinion of this author, energy savings specifically due to alignment is quite difficult to quantify, particularly in the testing or laboratory environment. Although it does make intuitive sense, if a machine has less stress and strain to overcome due to misalignment, power consumption should also be less. However, in the field, there are many variables in play that may affect power consumption such as motor efficiency, load, VFD’s, coupling type, proper installation, proper lubrication, state of component wear, proper design for service, etc.
Recently, I had an opportunity to actually measure the energy consumption of two identical pump sets before and after precision shaft alignment. The new Chilled Water Pumps, were each driven by 75 HP electric motors. The power consumption data was taken before and after alignment at identical speeds (CWP 1 VFD = 46.6 Hz; CWP 2 VFD = 47.3 Hz) and load (all three pumps on the Chilled Water Loop were in operation).
CWP 1 power consumption decreased by 8% (5.7kWh) and CWP 2 power consumption decreased by 2.5% (1.7kWh). In order to estimate cost savings for the reduced energy consumption, we have to make a couple of assumptions. For the sake of discussion we will assume these machines consume power at this rate for 80% of the year at an average electricity rate of 7 cents per kWh. Using these assumptions, CWP 1 cost savings is $2,796/year and CWP 2 cost savings is $834/year. The formula for cost savings used in this example is: Reduction in consumption (kWh) X 8760 hrs./year X 80% X $0.07 (kWh rate).
Full disclosure: these machines were found to be substantially misaligned. The units were shipped on a skid that was only rough aligned at the factory. Final alignment was left to precision tolerances of less than 0.5 mils/inch angularity and 2.0 mils offset. Despite having made such substantial improvement in the alignment condition, the comparison is still valid. We often see these types of machines, especially HVAC pumps, with severe misalignment. Many times this is a result of reliance on factory alignment on a pump skid, or use of a straight edge only for field alignment.
While this data does not represent a large sample size, it would appear to support the theory that precision alignment can contribute to reduced energy consumption. For the particular client site, where this data was taken, there are at least 36 similarly sized units (and many smaller units). If we apply the lower amount of the two cost savings (2.5%) to this population of machines, the potential energy cost savings for simply precision aligning 36 pumps could easily exceed $30,000. Suddenly we are talking about real money!