How To Size A Waterside Economizer Part 3: Cooling Tower Piping Strategies
/By Chad Edmondson
Waterside economizing relies on cold water from the cooling tower (aka condenser water) to absorb some or all of the heat from inside the building. In typical waterside economizing applications, plate & frame heat exchangers are used to transfer heat from the return chilled water to the cooling tower water.
It is a pretty simple process compared to what occurs in a chiller, where a refrigerant loop sits between the supply/return cold water and the condenser water. The refrigerant loop includes an expansion valve and compressor which are there to facilitate and maximize the heat transfer from the return cold water to the refrigerant and then from the refrigerant to the condenser water.
Here’s what you need to understand about the refrigerant cycle in a chiller before you attempt to design a waterside economizer. A chiller’s condenser water temperature can vary greatly depending on whether it is a hermetic or open chiller. This is all because of head pressure control. The thresholds vary slightly from manufacturer to manufacturer, but generally speaking hermetic chillers operate best when the entering condenser water is 75°F or above and an open chiller at 55°F or above. This is because of the direct proportional relationship between pressure and temperature. As one increases or decreases, so does the other. So if the condenser water is cold – say 55°F--it will be at a lower pressure.
This presents a problem because the condenser needs that extra head pressure to force refrigerant through the expansion valve along with oil during the refrigeration cycle. There is also newer chiller technology, such as magnetic bearings, that allow for even colder condenser water. It is VERY important to check with the chiller manufacturer regarding the minimum condenser water temperature it can handle.
Why You May Need Two Different Supply Temperatures from the Cooling Tower
This presents a dilemma when it comes to waterside economizing, where cold condenser water is a good thing. In fact, the colder the condenser water the better. If you are able to get by operating just the economizer or just the chiller(s) at one time, there wouldn’t be a problem. However, ASHRAE 90.1 requires that waterside economizing occur anytime the outdoor temperatures are 50°F dry bulb/45°F wet bulb or below. Keep it in mind that it this is required even when 50°F dry bulb/45°F wet bulb is not sufficient to cool the chilled water enough to handle the load. More often than not, the chiller(s) and the waterside economizer must operate simultaneously. The design will have to be integrated to run them together. That means the design might have to allow for two different supply temperatures from the cooling towers – one for the economizer and one for the condenser.
In most cases, one of the following approaches will work.
In the above design, notice that there is a dedicated cooling tower for the waterside economizer and one chiller, while the other two chillers are manifolded to the other two cooling towers. The waterside economizer loop is separated from the chillers, but the functionality is integrated thanks to the three-way valve. In this arrangement, the condenser water from the dedicated cooling tower will bypass its associated chiller and flow through the plate & frame heat exchanger when the system is in economizing mode. That cooling tower can be controlled to supply the coldest water possible, while the other cooling towers will supply warmer water to the two operating chillers that simultaneously share the cooling load with the economizer.
Another way to integrate economizer and chiller operation is to use one common supply and return pipe between the towers and the chillers. (See below)
This will likely require a 2-way modulating valve on the condenser water piping to maintain head pressure on the chillers. However, you should check with the chiller manufacturer to make sure the chiller will function properly with modulating condenser water flow.
