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The "two-thirds" Rule for Locatiing Sensors to Control Variable Flow Systems - Part 2
November 7, 2007
In my previous post, I looked at what happens to differential pressures at different points in this variable flow pumping system as the flow rates drop off.
In the example, when one of the AHUs shut down while the other remained in operation at full load, the differential pressure required at the pump dropped by about 40% relative to what was required at full load while the flow dropped by by 50%. In the energy conservation game, our goal is to exploit this potential for reduced power consumption while still meeting the requirements of the load.
Let's revisit the pump power equation.
Both head and flow appear in the numerator. Clearly, if we could control the pump in a way that reflected the reduction in both of these operating parameters, we would optimize its energy consumption.
Let's take a minute to think through what would happen in our example on a design day when one of the AHUs shuts down. For the purposes of discussion, I'm going to make a pump selection for our example using Bell and Gossett's ESP software. Entering our design flow and head (800 gpm at 44.2 ft.w.c.) generates a number of selections. I chose the one with the best efficiency, which comes at a cost premium of approximately 17% relative to the lowest cost option. The 17% price premium buys a 2 % increase in pump efficiency. In economic terms, this may or may not be justified based on the load profile. Holistically, one might argue that the price for improved efficiency is justified no matter what the load profile is since the improvement likely represents a reduction in the use of non-renewable energy with a corresponding reduction in emmissions. (Of course, I'm from Oregon and grew up in the 60's, so when I'm not busy hugging trees, I'm probably out trying to save salmon; so you have to take all of that with a grain of salt).
If AHU1 shuts down while AHU2 remains in operation at full load, AHU1's control valve closes. Eliminating one of two parallel paths by closing AHU1's control valve forces all of P1's flow through AHU2. Increasing the flow through AHU2 requires more head that was being produced by P1 with both units on line. As a result, the flow through AHU2 will not double; rather the system will shift towards a new operating point/system curve with a higher pump head than previously existed but at a reduced flow relative to what was provided with both units in operation.
Initially, the reduced system flow is still in excess of what AHU2 requires at design conditions. As a result, the control valve serving AHU2 will throttle in response to the excess capacity. Ultimately, interactions between the flow supplied by P1 and the capcity the flow produces in AHU2 will cause the control valve to throttle the flow through AHU2 to the design value of 400 gpm. The figure below illustrates our pump curve along with the design system curve associated with two units in operation and the new system curve associated with AHU2 operating alone at its design capacity.
Reducing Pump Power by Pushing the Pump Up Its Curve via a Throttling Process
Back in "the olden days", before variable speed drive technology had been perfected and made affordable we often allowed the pumps to be pushed around on their performance curves by two way valves. (By way of explanation and to provide some perspective, the "olden days is an expression my son Aaron would use when he was younger to preface a question about something in my past. Back in the "olden days", when I first priced a VFD, it was for a 40 hp motor; the price was about $50,000 and the package as about the size of 2 motor control sections) While crude by today's standards, this approach was relatively simple and could save some energy as can be seen below.
On the plus side:
The brake horsepower required at part load is reduced by about 2 bhp.
No special technology is required.
The theory of operation is simple and easy to understand.
On the minus side:
Pushing the pump up its curve moves it away from peak efficincy.
The head produced by the pump at part load is significantly above what is required , as illustrated below. The extra head is simply "chewed up" by the control valve on the load that remains inoperation as it throttles.
The above design pumping head could lift valve plugs off of their seat if care is not exercised in selecting the actuators. As a result, we may not achieve the desired reduction in flow and associated energy savings.
Plugging the desired flow and the head it would take to produce it into the pump power equation reveals that in theory, we should be able to serve the reduced load condition with about 3.3 bhp
The bottom line is that while pushing the pump up its curve provided a simple way to reduce pumping capacity and save energy, the potential exists to save even more if:
We can find a way to reduce pump flow and pump head at the same time.
We can make this shift in operating conditions while preserving the pump's efficiency.
We can control the pump head and flow in a manner that provides exactly what the loads require, no more and no less.
Variable speed drives and the 2/3 rule provide a mechanism to achieve these goals. In my next post, we'll take a look at what happens if we apply this combination of technology and technique to our example.
Posted by David Sellers on November 7, 2007 | Comments (0)
