Follow-up Q&A on distributive pumping

After the webcast on Distributive Pumping: Design Advantages and Lower Energy Consumption, the presenter answered additional questions from the audience.

By Kevin Anderson, Senior Technical Training Specialist, Grundfos Pumps Corp. February 9, 2015

Question: What roles do variable frequency drives (VFDs) and/or impeller trims play in your designs?

Answer: VFDs are needed in distributive pumping applications. Due to the changes in load demands, as well as the effect of other zones. In regards to impeller trims, the impellers may need to be sized larger (or larger pumps installed) if additional zones are added to the system.

Question: How do you adapt a primary-secondary (P-S) pumping system to a distributive pumping system? How do you calculate the potential savings and energy benefits between systems, and the differences in maintenance?

Answer: There are several selections that make sense. If first cost is a major driving point, P-S single pumps sized for current needs appears to be the choice. This choice makes sense because the number of VFDs and sensors is less, but flexibility for the future really suffers. Also, for future requirements, motors and VFDs need to be replaced. The use of parallel pumps for future requirements makes most sense for both distributive and P-S designs. With distributive pumping, future horsepower can be reduced by 10% and operating cost can also be reduced. Parallel variable speed drive (VSD) pumps appear to be the most cost-effective approach in this example with 250 hp present and 400 hp future requirements. Present pumps can be used. Every application can vary, so examining both methods is wise.

Question: Discuss the efficiency of small distributed pumps compared to larger end suction pumps.

Answer: The efficiency of a large flow single end suction pump will generally be better at best efficiency point (BEP) than multiple smaller pumps. However, at low flow conditions, a single smaller pump (as part of a parallel pumping solution) may be at a higher efficiency for these low flow conditions. The total load profile for the system is important for making these types of decisions.

Question: How much do you consider for the equivalent lengths of multi-function valve, control valve, and air separator? What is your reference for these items to find out their equivalent length?

Answer: For each of these components they should have the associated CV value provided by the manufacturer. Then you must calculate the friction loss based on that number. There are some software tools that can be used to do the calculations. Pumping System Assessment Tool (PSAT) software created by the Dept. of Energy is one software example.

Question: In consulting firms, design criteria for chilled water systems in the past normally included flow x 1.1 and pressure x 1.2 to account for iron barnacles and other normal Ff changes in dirty piping. All calculations are then done for clean piping. Can you comment please?

Answer: This may seem like a good idea. Is this considered an industry standard or an industry axiom?

Question: Don’t VS on primary pumps mess up the chiller efficiency, which wants a constant flow of 3 gpm/ton.

Answer: Chilled water is usually figured on 2.4 gpm/ton (based on a 10 degree delta across the chiller bundle). Condenser water is usually at 3 gpm/ton (based on a 10 degree delta and 15,000 btu/hr/ton….the added 3000 btu/hr is for the heat of the compressor added to the condenser load).

Question: This is beyond the scope of this talk, but what is your opinion of primary pumps for chillers, secondary pumps to overcome main pressure drops?

Answer: Yes, you are exactly correct. Tertiary piped systems provide the ultimate flexibility for design and operation of individual zones or buildings. Common pipe hydraulically isolate each system. At no time is the individual building dependent on its neighbor. Individual supply and or return water temperatures can be achieved and maintained. The head required to pump each building is also independent. Pumps for individual buildings are calculated as if no other building is involved. The secondary pumps are relieved from the head required to serve the individual building. The secondary pumps are selected to overcome the friction loss in the distribution piping to and from the central plant. Excess head required to pump a building due to large loads or high resistance in not imposed up or downstream of the building. Tertiary pumps are selected for the building load only.