Q: How do you simulate differential pressure control with a variable speed pump?
A: PIPE-FLO Professional and Flow of Fluids can simulate variable speed dP control by using a Fixed dP Device at the location of the system's differential pressure transmitter. The primary/variable secondary HVAC chilled water system shown below will be used to describe the process.
The system is providing cooling water to four air handlers, sized for a design flow of 60 gpm each. The system was balanced with circuit setters and two-way control valves regulate cooling water flow to each air handler. The secondary pumps are on VFDs and modulate speed to control the differential pressure to 15 psid at the load furthest from the pumping station, circuit D.
The differential pressure transmitter (DPT) will be located at the nodes labeled dP Point 1 and 2. To set up the DPT, a Fixed dP Device will be connected to the nodes and set to 15 psid. The lines connected to the DPT are fictitious and should be sufficiently short so as not to have any effect on the system. In this model the lines were specified at a length of 0.001 ft. Circuit D is shown below with the DPT installed.
The Fixed dP Device used to simulate the DPT will circumvent the need to leave the flow unset in one circuit and avoid overcontrolled errors, which are common when using a variable speed pumping arrangement in the model. After installing the DPT and setting the VFD pumps for the system design capacity of 240 gpm, it was found that the pumps must run at about 1700 rpm to maintain 15 psid at circuit D. Once the system is set up for dP control it can be evaluated for different demands at the loads. The following steps show how to simulate different operating scenarios while maintaining the DPT setpoint with the VFD pump.
1) Create a lineup for the new operating scenario. System operation will be evalulated with all four air handlers operating at 50% of design.
2) Choose the new lineup, "50% of design" from the Lineups drop down on the toolbar.
3) Adjust the flow to 50% of design for all four handlers by selecting them and using group edit to change the setpoint to 30 gpm.
4) Adjust the secondary pumps' set flow rates to equal the total flow set at the two-way valves (4 valves x 30 gpm = 120 gpm). For this example, both secondary pumps will be set to 60 gpm, but this would also be an opportunity to assess whether it would be better to run a single pump at 120 gpm. The most critical aspect of this procedure is to ensure that the sum of flows set at the VFD pumps is equal to the sum of flows set at the two-way valves.
5) Once the two-way valves and VFD pumps are adjusted, calculate the system to display the calculated pump speed and valve positions. Notice that the differential pressure at circuit D is maintained at 15 psid and the pumps are operating at 1616 rpm.