By Chad Edmondson
It should be clear by now that selecting a pump based on design conditions and best efficiency point (BEP) will not yield optimum system efficiency. We’ve also learned that virtually every HVAC pump system will have a minimum amount of pump head, even in the absence of demand. In between these minimum and maximum operating conditions lies the control range for our pump. Keeping all this in mind, how do we select the most efficient pump for a specific application?
The key to efficient pump selection for modern day systems is establishing a load profile. We want to select a pump that is most efficient at the heads and flows that dominate our load profile – where we will be operating most of the time.
In a perfect world the engineer will have calculated an exact load profile for the system he or she is designing. If this isn’t possible, we recommend using the Integrated Part Load Value (IPLV) load profile prescribed by AHI Standard 550/590 (Table 1).
The IPLV was developed for chillers, but can be used for chilled water pumps and boiler pumps as a means to evaluate part load pump efficiency.
It is reasonable to assume that pump flow rates will match the percent of chiller load. Obviously we want to focus on 50 to 100% chiller load because that’s where we will be operating approximately 88% of the time. We just have to make sure that the pump is capable of delivering the minimum amount or “constant head” the system requires, along with peak load.
Let’s assume we’ve picked our pump based on the IPLV load profile. We've calculated our pump efficiencies according to the IPLV recommendations and we've calculated our annual operating cost as shown in Table 2.
Notice that this calculation is based on 30 feet of constant head. Again, it is absolutely critical that we recognize that there will always be some constant head requirement. No pumping system with 2-way valves is ever 100% variable! 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 constant head and 100% of variable flow (Table 3). Notice the operating cost is much lower than it was when we had 30 feet of constant 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 IPLV as a guide for selecting pumps for today’s variable speed systems is perfectly acceptable as long as you include a reasonable amount of constant head in your calculation. If you are unsure of the exact amount of constant head, we recommend plugging in 40 percent to be on the safe side.