How To Size A Waterside Economizer Part 4: Chilled Water Piping Requirements
/By Chad Edmondson
In our last blog we discussed heat exchanger piping on the cooling tower/condenser water side of a system with integrated waterside economizing to meet ASHRAE 90.1. Today, we’re going to tackle the chilled water side.
Remember, we want to pipe the waterside economizer heat exchanger so that we can run both the chiller(s) and the economizer at the same time per ASHRAE’s standard. In the past most systems were installed with the heat exchanger piped in parallel with the chillers (Figure 1). This piping arrangement allowed for only the chiller(s) or the economizer to operate at one time to independently to satisfy the demand. Under this design plan, waterside economizing can only occur during periods of low wet bulb temperatures, that is low enough to meet the entire cooling load.
Figure 1
Getting More Out of Your Economizer
So how do we get more operation out of our waterside economizer? By having the heat exchanger piped in series rather than in parallel with the chillers!
We suggest piping the waterside economizer between the return chilled water from the system and the chillers so that we can pre-cool the chilled water before it enters the chillers. This simultaneous operation of chillers and waterside economizer allows for many more hours of free-cooling and a much faster payback on the economizer equipment.
The following example shows a chiller plant with a waterside economizer providing 250 tons of cooling and the evenly loaded chillers providing 750 tons of cooling (Figure 2). We have 3000 GPM of flow in our primary loop with 2000 GPM going out to the system to deliver 1000 Tons of cooling. The return chilled water enters the heat exchanger at 57°F and leaves at 54°F. This 54°F return water mixes with 1000 GPM of 45°F water in the chiller loop to make 51°F water entering the chillers with a 6⁰F Delta T across the chillers.
Figure 2
Compare that to the exact same system and demand load, minus the waterside economizer ( Figure 3).
Figure 3
Notice that each chiller in Figure 3 is 66% loaded with a Delta T of 8°F. By comparison, the system with the waterside economizer (Figure 2) can supply this same amount of cooling by loading each chiller to only 50% which lowers the Delta T across the chillers to 6°F. In Figure 2 we have shifted a significant portion of the load to the waterside economizer and we are still within reasonable operating conditions for our chillers.
Bear in mind that if the load drops any further in our system with the economizer in series, we may have to unload one of the chillers to keep return water temperatures from getting to low. Otherwise we could have short cycling of the chillers. Look at the same example below with all three chillers at 17% load.
Figure 4
In this last example we are now at a 2°F Delta T and we are definitely in danger of short cycling. (Figure 4)
So while we want to offset as much chiller operation as we can by free cooling, we have to be mindful of the minimum load that is required on chillers!
