Armstrong: Reduce Operating Costs by up to 30% with Parallel Pumping Technology
The design strategy you choose for hydronic pumping arrangements will affect both your installed first costs and energy costs over time. That was the message from Tony Furst, director of applications engineering for Armstrong Fluid Technology, at a recent Coffee Break webinar.
At least once a month, we invite a subject-matter HVAC expert to our webinar series to explain the most important topics in the industry. Ask your SVL sales engineer for an invitation to our upcoming sessions.
What is parallel pumping?
In contrast to hydronic systems with one operating pump and one standby pump, a parallel pumping system has two or more pumps operating simultaneously with proper staging.
In this arrangement, the flow increases but the head pressure stays the same (if the design flow is 1,000 gpm, each pump will deliver 500 gpm at the design head).
All the pumps must be identical in a parallel pumping design. When you look at your pump curves, the system curve must intersect both the single pump and parallel pump curves. If they don’t, and you reduce the number of running pumps, you risk cavitation.
The motor is sized based on the single pump operating point. If it doesn’t cross the system curve, the motor will overload. Note that we’re talking about operating pumps. If you have three or four pumps running in parallel, the single pump curves don’t need to cross the system curve because the system won’t ever run on just a single pump.
How does parallel pumping reduce operating costs?
At some point, the efficiency of a pumping system peaks and then begins to decrease. But the more pumps you run, the flatter those efficiency curves get. When you have four pumps in parallel, you can maintain peak efficiency over a much wider operating range than with a single or even two pumps in parallel.
Thus, parallel pumping is about maximizing pump efficiency with smaller horsepower motors and thus driving down operating costs.
What about redundancy? Does parallel pumping allow for duty pumps and standby pumps?
Compared to a two-pump standard design (duty/standby, 100% flow each), a two-pump parallel design (each sized for 50% flow) will satisfy the system requirements with smaller motor horsepower.
A two-pump parallel design with each sized for 75% flow might be a good idea if you think you might expand your system in the future. Three-pump and four-pump designs are also an option for further redundancy.
Redundancy is a common concern with parallel pumping. But ask yourself: do you really need a true standby pump for redundancy?
Even if one pump fails in a parallel system, the other pump will still deliver ~80% of the design flow. Perhaps for some critical applications like hospitals and data centers, that’s not acceptable. But for most applications, according to ASHRAE, coils can still deliver up to 95-98% of the design heat transfer on just 80% of the design flow rate.
The question becomes, if that is the worst-case scenario, how much redundancy does the application need? Remember that maximizing redundancy may be at odds with other design criteria like overall project budget or energy efficiency. It may also have additional first costs and annual operating costs that may not be at all necessary.
But doesn’t it cost more upfront to buy and install more pumps?
This logic doesn’t hold with vertical inline split-coupled pumps from Armstrong. Typical horizontal end-suction installation requires alignment in the field, which drives up costs. Armstrong’s pumps are aligned at the factory. They also don’t require a concrete inertia base, which is expensive and needs to be grouted to reduce vibration.
Armstrong’s vertical inline pumps are instead supported by the piping and don’t require flex connectors. Their VFDs are pre-mounted and factory-wired with the proper cabling between the motoring and the drive. Everything is assembled and tested in the factory. And because there are no bearings in the pump, only in the motor, maintenance is a breeze. You can take the motor off and not touch the wet end.
Because you eliminate all these installation requirements, the piping is a little smaller, and the motor sizes are also smaller, a parallel pumping arrangement does save on total first costs.