By Chad Edmondson (JMP) and Norman Hall (RLD)
In Part 1 of this series we offered an example of a condenser water pumping system with 800 GPM per condenser for a total of 1600 GPM with the third condenser serving as standby. The tower elevation or lift is 10 feet, the condenser (with the 2-way valve and balance valve) has 20 feet of pressure drop, and the common piping with any safety factor has 30 feet of pressure drop.
In parallel pumping, the single pump operation is at the intersection of the single pump curve and the system curve. (See figure below). In this example we want the operation to be 800 GPM at 37.5’ but there is a problem. The intersection of the two curves is not at 800 GPM at 37.5’; it is at about 1100 GPM at 39’. When one pump shuts down and the 2-way valve closes, the flow rate will be 37% above the design flow rate of the condenser. If this exceeds the condenser maximum flow rate, the manufacturer may void the warranty on a very expensive piece of equipment.
What can we do about this?
We offer the following two solutions to this problem, each of which has its own advantages.
Solution 1 - Use flow limiting balance valves instead of manual balance valves.
This option will physically prevent the flow rate at any condenser from exceeding the design flow rate. There are two main advantages using flow limiters in this application. Since the valves replace the manual balance valves and do not require any electrical or controls, the first cost is attractive. They are pedigreed or factory set for a given flow rate and when the system pump attempts to increase the flow rate, the valve throttles to maintain flow. These valves typically have a +/- 5% accuracy.
However, there are three main disadvantages to this type of solution. First, the valves normally have a larger minimum pressure drop of 2 to 5 PSIG at design flow rate. Second, the valve selection tends to be larger than the pipe size, so in order to keep the pressure drop lower we normally select a larger valve. Since there are no pipe diameters required for accuracy, reducing flanges might be used. Third, the valves have fine openings and consideration should be given to dirt control in the open tower system.
Solution 2 - Use a variable speed drive for the pumps with simple contact closure control.
In this solution scenario, the pump starter would be replaced with a variable speed drive that has contact closure within the drive which sets the drive at a given speed. When operating with two pumps, the controls would close a set of contacts for full speed less safety factors. When operating with one pump, the controls would close a second set of contacts to operate at the lower speed needed for single pump operation. We recommend the same set of drive contacts be closed at each pump based on whether one or two chillers are on. The enable contacts in the drive would determine if the pump should operate. Having the same contacts closed in the all the drives prevents the possibility of operating two pumps in parallel at different speeds, which could dead head a pump and cause damage.
Unlike the use of flow limiters, this control solution reduces pressure drop while increasing energy savings. However, the owner does incur the cost of control contact closure and possibly added motor protection due to the use of drives.