By Norman Hall (RLD) and Chad Edmondson (JMP)
Although the DOE’s new pump efficiency standards raise the bar for manufacturers to build more efficient pumps, its rating system should not be looked at as a shortcut to efficient pump selection. Careful consideration of part load efficiencies applied to realistic load profiles is still the only way to ensure long term efficiency.
Even though DOE standard identifies part load efficiency ratings, these ratings are based on pumps operating for equal duration of time at various percentages of load. Engineers and plumbers know that this is not the operating norm of most pumping systems. In fact, most pumps operate at design loads a mere 1% of the time. That’s why it is essential to consider real world operating conditions when selecting pumps.
Important New Terminology!!
The Air-Conditioning, Heating, and Refrigeration Institute provides a good starting point for part load profiles by providing the AHRI Standard 550/590 for chillers. We actually covered this topic in our recent series on Modern Pump Selection. Everything contained within that series is accurate and still relevant -- except for one minor change in terminology.
The AHRI refers to the chiller efficiency rating as IPLV or Integrated Part Load Value (IPLV) load profile prescribed by AHI Standard 550/590. This rating is based on the “load” of the chillers. Understanding this is a standard that engineers are familiar with, Bell and Gossett, a Xylem Company, has introduced a new term for the pumping industry: PLEV or PART LOAD EFFICIENCY VALUE.
Similar to IPLV, this efficiency rating for pumps looks not at part load, but at part flow. PLEVV takes a weighted average of the efficiency of a variable speed pump. As the flow rate drops, the speed drops, following a control curve starting at 0 GPM at 30% of the design head in feet. (For a discussion about control head and curves visit this blog in our Modern Pump Selection Series.) PLEV weighs the efficiency of design load as shown in the chart below, with the pump operating at 100% of flow for only 1% of the time, 42% of flow for 75% of the time, and so on.
Calculating Variable Speed Operating Cost Using PLEV
Using the new “PLEV” teminology, let’s revisit the same example we used in our blog on integrated part load values (IPLV). In this example, our pump has a capacity of 2000 GPM at 95 feet. We’ve picked our pump based on the PLEV load profile. Our annual operating costs can be calculated as shown here:
Notice that this calculation is based on 30 feet of control head. Remember, it is absolutely critical that we recognize that there will always be some constant head requirement. If we erroneously assume otherwise, then the actual operating cost of our system will not reflect our predictions. In fact, we will have grossly under-estimated.
To help illustrate this point, we’ve calculated the annual operating cost for the same system as above, based on zero control head and 100% of variable flow. (Table 3) Notice the operating cost is much lower than it was when we had 30 feet of control head -- $10,057.43 vs. $14,502.21. That’s a $4,444.78 mistake that no one wants to explain to an owner.
Using the PLEV as a guide for selecting pumps for today’s variable speed systems is a good way to approximate operating cost and compare efficiencies of different pump selections over a range of operating points. You may replace the control head with the actual value you calculated. If you are unsure of the exact amount of control head, we recommend plugging in 30 percent of the total pump head for a quick analysis.