Understanding Primary Secondary Pumping Part 4: “What happens when...?”

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By Chris Edmondson

Now that we know what happens when we vary flows and mix temperatures in a primary secondary pumping system, let’s explore these theories in some more realistic scenarios. 

As discussed in a previous blogs, when it comes to primary secondary systems, one of three flow patterns is always in occurrence: 

  • The primary flow is equal to secondary flow

  • The primary flow is greater than secondary flow

  • The primary flow is less than secondary flow

Figure 1 shows a typical system operating so that the primary flow is equal to the secondary flow.  One of two 500-ton chillers is supplying 1000 gpm entirely to the secondary loop.  There is exactly 1000 gpm returning to the primary pumps and there is no flow or temperature mixing in the common pipe.  We have a design Delta T of 12 degrees, with a supply temperature of 44°F and a return temperature of 56°F. 

It doesn’t get any better than this.  A single chiller is spot-on, meeting the part load conditions.  We’re bringing back warm 56°F water to the chiller, flow its balanced, coils are doing what they are supposed to and all is right with the world.

Figure 1

Figure 1.jpg

But what happens if we need to supply 1500 gpm of 44°F water to the system, forcing the second chiller into operation? 

As you can see in Figure 2, we now have two chillers supplying 2000 gpm at 44 degrees; but demand is such that the secondary loop is only taking out 1500 gpm.   Where does the remaining 500 gpm go?  As we learned before (remember the Tee Law!), the additional 500 gpm will bypass the secondary loop via the common pipe and merge with the returning 1500 gpm from the secondary loop. 

This is a very typical part load scenario with both chillers loaded up to exactly 75%.  Just keep in mind that when flows merge, so do temperatures.  Even though we are bringing back 56°F warm water from our system, the return water will mix with the 500 gpm of 44°F in the common pipe, so that the final return water temperature to the chillers is 53°F (temperature mixing formula). This gives us a Delta T of 9 degrees on our chillers compared to the design Delta T of 12 degrees.  Thus, our chillers are 75% loaded.  (9°F /12°F).

Figure 2

Figure 2.jpg

Now lets look at what happens when the primary flow less than secondary flow. 

The example shown in Figure 3 shows one chiller fully loaded and supplying 1000 gpm at 44°F.  However, our system is actually taking out 1200 gpm, so we now have 200 gpm of reverse flow in the common pipe.  We are taking 200 gpm of warm 56°F return water and mixing it with 1000 gpm of 44°F supply water, which leaves us with 46°F water going to our coils.  This water may not be cold enough to satisfy the coils for dehumidification.  Thus, we could have a problem meeting demand with just one chiller on.

Figure 3

Primary Secondary Flow2.jpg

One obvious way to fix this is to start up the second chiller, bumping our flow up to 2000 gpm at 44°F.  (See Figure 4) Taking out 1200 gpm at 44°F, our coils will be fully satisfied, although we will be sending an extra 800 gpm of 44 degree water through the common pipe .  This water will mix with our return water, giving us 51°F water to the chillers.  Each chiller will be just over 50% loaded.  This isn’t a peak efficiency operational point, but at least our coils are satisfied and we are bringing back 56°F water.  

Figure 4

Figure 4.jpg

Although the water temperature entering into the chillers is far lower than ideal when it comes to operational efficiency, this should not be confused with Low Delta T Syndrome, which is indicative of a system problem.  Low Delta T is the result of what is occurring to the right of the common pipe and maybe caused by various system problems such as dirty or undersized coils, or insufficient flow balancing.  Low Delta T will prevent your chillers from loading up properly regardless of demand.