Technology pumps out savings
February 1, 2015 | By Mike Miller
Variable speed pumping has brought new levels of energy efficiency to the HVAC industry. While many larger buildings utilize system pumps operating on variable frequency drives (VFDs) with the guidance of pressure differential sensors, pumps that do not require external pressure sensors are an alternative. Before discussing the specifics behind this pump technology, a review of some basics behind system design, pump operation and selection is in order.
Let’s start with a 150-ton cooling system. The example shown in Figure 1 has five equal loads of 30 tons of cooling each, serviced by a chiller. Two-way control valves are used in four of the fan coils, but one of them is fitted with a three-way control valve that ensures a minimum amount of flow can remain in the system, even if all loads are satisfied.
When an engineer designs a hydronic system, he starts with the load calculations. He then selects the delta T for the system design, calculates the flow in GPM for both hot and chilled water, lays out the system, and calculates the pressure drop.
For the example in Figure 1, a 12F delta T was chosen, which equates to two gallons per minute (GPM) per ton for a total of 300 GPM. Next, we calculate the system pressure drop based on the actual piping layout and total system resistance based on length and size of distribution piping, as well as the resistance of all other components in the system, including the heating/cooling source and terminal unit, valves and fittings. In this example, it is approximately 53 feet of head.
Now that we have the design pressure drop and the design flow in GPM, we establish the design operating point.
As shown in Figure 2, a system curve data table can be generated using the formula shown. Basically, a system curve references the head pressure generated within the piping if nothing physically changes other than the flow (for example, head pressure generated in the system shown in Figure 1 with all valves wide open).
“These pumps can be used for any application, including traditional operation with changing loads.”
Some engineers may never actually draw the system curve but the system curve is implied in all designs. In most cases a system curve is generated by pump selection software. It is the interaction of the system curve with the pump curve that establishes the actual design operating point.
As valves in the system begin to close, the system curve responds to those changes as shown in Figure 3.
Most pump manufacturers test each of their constant speed pump models on a test stand in order to generate the pump’s performance data and do this for each impeller size. An example of such a setup is shown in Figure 4.
Proper testing would typically require the following components:
• Pressure gauges before and after the pump
• Flow meter
• Control valve/throttling device
• Interconnecting pipe
• Storage tanks
• Input electrical power
• Tachometer to confirm pump speed (not shown)
• Torque cell to measure HP (not shown)
• Data acquisition console (not shown)
Sample test data is shown for a constant speed pump in Figure 5. Please note that far more test points would be recorded. For clarity purposes, only six test data points are shown in this example for a pump with an eight-inch impeller. The same test is usually performed for each pump model with minimum and maximum impeller diameters, as well as three or four diameters in between.
Each constant speed pump can only operate on its pump performance curve. As the system curve changes based on added resistance in the system with zones shutting off, the flow would decrease. At this point, the pumps operating efficiency is reduced. In order to maintain highest operating efficiency, variable frequency drives (VFDs) can be added that can change the frequency provided to a pump’s motor and therefore alter the pump’s performance capacity.
Just as constant speed pump curves are generated, the same can be generated when operating the pump motors on different frequencies or speeds. As shown in Figure 6, using the same four-inch pump as was used earlier, new performance curves were generated using lower frequencies. This figure shows the performance curves overlaid with the mechanical example system curves.
Please note that pumps should not be operated below 20Hz. At that point the minimum flow threshold is reached in order to lubricate the seals. Using VFDs allows a pump performance to be matched to the system without physically changing its impeller diameter and continually maintaining the pumps most optimal operating efficiency. This results in significant energy savings.
The last remaining piece of the puzzle for this pump technology is the control curve as shown in Figure 7.
The control curve sets the direct relationship between the system curve and the pump curve for each frequency between the minimum and maximum frequency limits. This is the curve on which the VFD will self-regulate.
PAIRING PUMP AND DRIVE
Head, flow, power and speed information, as well as a resulting control curve, are programmed into the VFD. The pump and VFD are paired and operate as a unit. A typical self-sensing pump VFD has received at least 50 data points. It can now operate at any flow and head within the collected data table simply by monitoring the power consumption and frequency matched to the recorded data. Figure 8 shows the combined action that a drive can take advantage of.
It continuously adjusts the speed of the pump and the performance curve to meet the dynamics changing in parallel with that of the system. The drive can provide a digital read-out of the changing dynamics at any time. The information may also be provided to a larger building or energy management system through a drive’s building automation system (BAS).
As the demand in the system decreases, the pump rides to the left on the pump curve. The pump automatically responds by controlling the frequency provided to the motor and keeps the pump on the control curve. As the system demand increases, the opposite will occur and the frequency provided will increase, keeping with the dynamics of the system.
These pumps can be used for any application, including traditional operation with changing loads (as discussed previously), but also to provide a match for any constant volume or constant pressure applications, as highlighted in Figures 9 and 10.
While drives with this technology can be provided loose, several pump manufacturers offer these pumps with the drives directly mounted, which provides some labour savings on the installation side as well. <>
cs-Summit 2015 on September 10 in Toronto, ON.