Consulting-Specifying Engineer - July 2008 - (Page 38)

Figure 2: Stable demand-based pressure reset loop Chilled water differential pressure (all data, left axis) 100 90 80 70 60 50 40 30 20 10 0 11:00 11:30 12:00 12:30 13:00 13:30 14:00 14:30 15:00 15:30 16:00 16:30 17:00 17:30 18:00 18:30 19:00 19:30 Source: Taylor Engineering Pump speed percent and system pressure (psi) Pump VFD speed (all data, left axis) Chilled water valve position (all data, right axis) 80 Most open valve percent 60 40 20 0 Supply air temperature reset: During occupied mode, the supply air temperature setpoint is reset from T-min (53 F) when the outdoor air temperature is 70 F and above, proportionally up to T-max when the outdoor air temperature is 60 F and below. T-max shall be reset using trim and respond logic within the range of 55 to 65 F. When the fan is off, freeze T-max at the maximum value (65 F). While the fan is proven on, every 2 min increase the setpoint by 0.2 F if there is one (adjustable) or fewer zone cooling requests. If there is more than one (adjustable) cooling request, decrease the setpoint by 0.3 F. A cooling request is generated when the cooling loop of any zone served by the system is greater than 99% until it falls to 90%. Practical experiences The sequences of operations listed in this article will all work. Through practical experience, the trim-and-respond logic offers some advantages. For one thing, it is much easier to tune; the reset loops have to act very slowly in order to function correctly. Quite often, an installer or controls technician in the field will install PID-based logic using the default “out of the box” PID parameters, which are typically much too fast for a reset loop. As a result, the loop will reset every few minutes, as if it were a reheat valve responding to VAV discharge temperature. This kind of response speed is much too fast and will make the entire building system “oscillate” as all components become unstable. The reset loop should normally work in cycles of hours, or even in a daily cycle. After all, its ultimate function is to adjust for the building demand as it varies due to weather or internal loads. An occasional peak may be present due to a particular load shift, but the reaction typically will be slow. Tuning a PID loop to respond in a matter of hours rather than minutes requires that the proportional gain (P) and integral gain (I) be set to values that are 10 to 100 times lower than typical PID loop values. Finding the correct balance of PID parameters that provide stable operation and sufficient response speed can take several days to weeks of tuning, and requires a trained operator. Figures 1 and 2 show an example of a chilled water loop with demand-based pressure reset using valve position, aimed at keeping the most open chilled water valve at 90% setpoint. As mentioned in the section on indicators, using valve position is a poor indicator, but it still works slightly better than a fixed pressure setpoint as long as there are no “rogue” zones (due to fouling or design problems). This loop required about a month to tune correctly, but ended up cutting pump energy by about 85%, as the main pump ultimately ran at an average of about 25% speed, down from the initial (fixed) speed of 50% that was selected before a demand- based loop was implemented. (Note that in this case, three-way valves were also converted to two-way valves, so that the cubic equation of power versus pump speed held true, as opposed to systems where only pressure is changed but not flow, in which case the correlation is linear.) During this month of tuning, only intermittent access to the system was required for minutes at a time to examine results from previous days or to look at performance changes over the weekend versus weekdays, and to change PID parameters slightly. For this kind of work, a Web interface is vital, since it can limit the amount of time required for such adjustments to literally 30 min, whereas visiting the jobsite might take half a day or more. In general, a trim-and-respond loop will not require as much tuning as a PIDbased reset method. In addition, the reaction speed can be set to fast for ramping up pump or fan speed (example: to react to Monday morning occupancy) while it can be set to slow for ramping down (to help stabilize a loop). In general, a PID loop does not allow tuning of this kind, as the reaction speed is inherently linked to the process signal and the set of fixed PID parameters. Seidl is a member of ASHRAE’s guideline committee GPC1.2, at present developing a guideline for commissioning existing buildings, and ASHRAE’s technical committee 7.5 on smart building systems. 38 Consulting-Specifying Engineer • JULY 2008

Table of Contents Feed for the Digital Edition of Consulting-Specifying Engineer - July 2008

Consulting-Specifying Engineer - July 2008 Contents Viewpoint Letters News M/E Roundtable 40 Under 40 Using Demand-Based Reset Strategies VFDs and Motors: Making the Right Match Grounding and Bonding Practices for Hazardous Areas Codes & Standards Case Study New Products Equipment Lifecycles Advertiser Index Green Space

Consulting-Specifying Engineer - July 2008

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