Variable Primary Chilled Water Systems Part 3: The Basics of Variable Primary Design
/By Chad Edmondson
Now that we’ve established the real world challenges of traditional constant primary/variable secondary chilled water systems, we can begin to explore the alternative, variable primary, a singular loop system that varies flow through the chillers.
There are two basic ways to pipe the pumps and chillers in a variable primary system. The chillers and pumps can be piped so that each chiller has its own dedicated pump (Figure 1) or they can be piped with the pumps in parallel with a common header between the pumps and a common header in the chiller supply (Figure 2). If simplicity is a priority, then dedicated chiller pumps may be the way to go. But generally speaking, parallel pumps with common headers offer more flexibility.
Figure 1
Figure 2
Looking a Figure 2, you can see that we have the ability operate the system in several ways depending on the load and/or current situation. For instance, we can operate two pumps and just one chiller so that we can increase flow out into the system without having to stage on an additional chiller. In other words, we are able to slightly exceed the rated tonnage of the one chiller in order to meet load.
We also have more flexible redundancy with commonly headered pumps and chillers. If a pump goes down, the remaining pump can serve one or both chillers and still meet the required load. If a chiller goes down or needs servicing, we can compensate for the loss in capacity by increasing flow through the remaining chiller while operating both pumps. So, there are lots of operational combinations available to help us increase efficiency and provide redundancy when servicing is necessary.
Operating with Minimum and Maximum Flows
No matter what piping configuration we choose, we must make sure that we design and operate the system so that we stay within the minimum and maximum required flow of each chiller. This ensures that we maintain good heat transfer and stable operation at lower flows and avoid eroding the tubes at higher flows.
First, we want to select chillers with a pressure drop that will support (and not limit) your turndown capability. In other words, you don’t want the pressure drop through the chiller to be so high or so low that it prevents you from staying within the minimum and maximum flow range of the evaporator as the system stages pumps and chillers to meet the load. Incidentally, most manufacturers provide chillers with delta P sensors to sense flow, but other flow measuring devices can also be installed (Figure 3).
Figure 3
A bypass valve is also required to ensure that we always maintain minimum flow through the chillers, especially in situations where the system is demanding little or no flow. If the demand of the building is less than the minimum chiller flow, the bypass valve should open to allow the minimum flow. The bypass valve also ensures that there is an adequate minimum flow thru the pump if all the 2-way valves out in the system are closed, otherwise the pumps can deadhead.
Sizing and Placement of the Bypass Valve
The bypass valve should be sized so that when it is wide open it delivers the highest minimum flow that is required of any single chiller. The bypass valve differential pressure should be the same as the differential pressure setpoint out in the system so we can supply this flow. It should not be a fast opening valve like a butterfly valve because this will cause unstable flow changes during staging, which is not good for the chillers.
The bypass can go in the mechanical room or at the end of the system. However, we recommend that it be installed in the mechanical room in close proximity to the chillers. Figure 4 shows a more complex system with the bypass installed between the chillers and the system.
Figure 4
By installing the bypass between the chiller and system, we don’t have to size our entire piping system for the additional flow. This also helps minimize pump operating cost and facilitates communication between differential pressure sensors and controls.
Stay tuned for more when we talk about how variable primary chilled water pumping can help compensate for low delta T.
