Variable primary flow chilled water systems VPF Coffee break with DaikinWhat is a Variable Primary Flow (VPF) system, and how is it different from other chilled water pumping arrangements?

That’s the question we asked Bill Dietrich from Daikin Applied in our most recent Coffee Break. SVL’s signature webinar series features the brightest minds in the industry, presenting 35-minute demonstrations of critical HVAC knowledge.

Make sure to register for our next Coffee Break so you can stay in the loop. If you missed our presentation with Daikin, here’s a quick recap:

VPF Systems versus Primary/Secondary Flow Systems

Any chilled water pumping system aims to move enough water through the pipes at the minimum differential pressure that will satisfy all the connected loads. The challenge for mechanical engineers is determining how to design a system that accomplishes this goal with an efficient and controllable solution.

For many applications, a VPF system offers advantages over a well-known primary/secondary pumping system. Here’s a quick comparison of the two methods.

In a Primary/Secondary arrangement, a set of primary pumps delivers a constant water volume to the chillers. A secondary set of pumps delivers a variable amount of water flow through the HVAC system’s cooling coils and the rest of the secondary loop. If the primary flow is greater than the demand in the secondary loop, the excess flow is rerouted back to the primary pumps through a bypass valve (if the secondary flow requirements are greater, it’s generally a sign that it’s time to add more chiller capacity to the system).

In contrast, a VPF arrangement eliminates the need for a secondary set of pumps. Typically, the variable speed pumps are headered together on the return water side of the water-cooled chillers; the pumps send the water through the chillers and out to the building loop. The bypass valve in a VPF system is stroked open only under low load conditions, and the flow on the primary and secondary loops are matched.

That sounds more efficient, but what requirements does a VPF arrangement impose on the system?

The chillers must be able to handle a variable fluid flow through the evaporator heat exchanger. This generally works with modern chiller controls, whereas 15 years ago, the chiller controls weren’t as fast-acting, so engineers had concerns about the viability of this arrangement.

Also, a VPF system requires accurate fluid flow measurement to help the control system determine when to open the bypass valve to maintain minimum flow through the chillers.

Besides eliminating one set of pumps, what’s the critical difference between these systems?

But the crucial difference with a VPF arrangement is that it allows you to separate the system’s flow requirements from its load requirements. In a Primary/Secondary system, they are closely coupled together. VPF has more flexibility in that regard.

VPF chilled water systems are standard today. What’s driving this preference?

Lower capital costs, for one. VPF eliminates one set of pumps, motors, and bases from a Primary/Secondary arrangement. There are fewer piping and electrical connections, less labor spent in the field, and a smaller bypass line (though the VPF does require the addition of a flow meter as previously discussed).

As a bonus, VPF requires less floor space (has a smaller footprint) and offers (arguably) better reliability because there are fewer mechanical components, and any pump can serve any chiller.

Are there situations where a VPF arrangement might not be the best choice?

There are a few. If time and simplicity are of the essence, a VPF system generally takes longer to commission and start-up.VPF also works best if you have chillers of the same size and pressure drop. And as mentioned above, VPF requires a robust control system, with coordinated control of chiller isolation valves, chiller sequencing, and control of the bypass valve.

If you’re on a project that won’t have sophisticated controls or operators, a Primary/Secondary system would likely be more foolproof.

 

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