Variable Primary Chilled Water Systems Part 5: Chiller Stage Transitions
/By Chad Edmondson
So far in this series we’ve briefly discussed the ways in which chillers and pumps/pump speed can be staged to meet changing loads but we haven’t really spoken about staging control. There are a few fundamental issues that engineers and operators need to be aware of when it comes to chiller staging in a variable primary pumping system.
First, it is important to make sure the isolation valves to the chillers are set to open gradually when staging on a new chiller. Otherwise, you run the risk of freezing up the chiller or at least causing it to go off on a FAULT due to extremely low evaporator pressure.
To sketch this out, let’s say we are in the first stage of operation in our example variable primary pumping system (Figure 1).
Notice that we a have parallel layout with commonly headered pumps and chillers, giving us the option to operate the chillers and pumps in a variety of combinations. The system is demanding 1000 GPM and we are meeting the load with Chiller 1 and a single pump operating. If our building demand increases to 1100 GPM, the equivalent of 550 chiller tons, we are going to need that second chiller, right? But we can’t just go to 100% open on the isolation valve to Chiller 2, because then the flow through Chiller 1 will drop rapidly from 1100 to 550 GPM without sufficient time for the chiller to adjust its output.
Figure 1
All of a sudden Chiller 1 is operating at full 500 ton capacity at 50% flow. As a result, it is producing 34 degree chilled water and our Delta T is through the roof at 22. (Figure 2)
Figure 2
To keep this from occurring, we must make sure that we gradually open the isolation valve to Chiller 2. How gradually is a function of what a particular chiller can take. When designing a system, we recommend giving manufacturers the worst case scenario in terms of potential flow drop through the chiller and let them provide you with the necessary time lapse for the chiller(s) to adjust.
If our variable primary system has a dedicated pump per chiller (Figure 3), keep in mind that flow through Chiller 2 cannot be established until the pump speed for that chiller is high enough to produce enough pressure to overcome the Chiller 1 pump. As Pump 1 unloads from 1100 GPM to 550, eventually the pressure imposed on the check valve between Chiller 2 and Pump 2 will decrease, allowing it to fully open so that 550 GPM is going through both chillers.
Figure 3
Next up we will talk about when variable primary chilled water design should be considered.
