Consulting-Specifying Engineer - July 2008 - (Page 36)

Figure 1: Unstable demand-based pressure reset loop Chilled water differential pressure (all data, left axis) Pump speed percent and system pressure (psi) 100 90 80 70 60 50 40 30 20 10 0 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 19:00 20:00 21:00 22:00 23:00 00:00 01:00 02:00 03:00 04:00 Source: Taylor Engineering Pump VFD speed (all data, left axis) Chilled water valve position (all data, right axis) 80 Most open valve percent 60 40 20 0 Poor demand indicators for chilled/hot water temperature reset Return water temperature provides an average of multiple coils and will not reflect insufficient heating or cooling of particular coils. Typical zone sensors and outside air temperature (see supply air temperature reset) Output from the heating or cooling PID loop: Unlike the good indicator for air temperature reset, in hydronic systems we typically lack flow sensors. Thus, a heating or cooling loop that is not satisfied may be caused by insufficient flow. As such, neither the PID loop output nor the valve position is a very good indicators for reset strategies. It now becomes clear why regulations focus on pressure-based reset in air systems while not requiring pressure resets in hydronic systems; there typically are good indicators for pressure demand in air systems, but not in hydronic systems. Similarly, for temperature resets, there are no good indicators in hydronic systems. There are some poor indicators, and they may be used, but do not have to be used if variable flow systems are employed. The intent behind this language is that the majority of energy savings will come from use of variable flow, which may be attained by either fixed pressure setpoint or variable pressure setpoint. For air systems, terminal control loop output is a good indicator of demand; however, this is not mandated in any of the regulations listed in Table 1. This is because a temperature reset range has to be chosen well to ensure energy savings. Providing very warm supply temperatures has the potential to undo cooling energy savings through increased fan energy. However, this is not inherent in the principle of the reset, and depends on the selected setpoints rather than the method itself. Sequences of operation Using the indicators of building demand defined above, we can now create sequences of operation that react in accordance with changing building demand. There are two such sets of sequences: One addresses pressure reset, and one addresses temperature reset. Each set of sequences is written in two ways, using either a PID-based reset method or a trim-and-respond based reset method. We will look at both methods in detail. PID-based reset control sequences Supply air pressure reset: The static pressure setpoint shall be reset by mapping the output of a direct acting PID loop to a setpoint range of 0.15 in. (0% PID output) to 1.5 in. (100% PID output). The input of the PID loop is the most open damper with a setpoint of 90% (adjustable). When the fan is proven off, disable the PID loop to prevent wind-up and freeze the output at zero. The loop shall be tuned to react slowly. 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. Tmax shall be reset by mapping the output of a direct acting PID loop to a setpoint range between 65 F (0% PID output) and 55 F (100% PID output). The input to the reset loop is the highest zone cooling loop output with a setpoint of 99% or greater (adjustable). When the fan is proven off, disable the PID loop to prevent wind-up and freeze the output at zero. The loop shall be tuned to react slowly. Trim-and-respond based reset control sequences Supply air pressure reset: Static pressure setpoint shall be reset using trim and respond logic within the range of 0.15 to 1.5 in. When the fan is off, freeze setpoint at the minimum value (0.15 in.). While fan is proven on, every 2 min decrease the setpoint by 0.04 in. if there is one (adjustable) or fewer pressure requests. If there is more than one (adjustable) pressure request, increase the setpoint by 0.04 in. Where variable air volume (VAV) zone damper position is known, a pressure request is generated when any VAV damper served by the system is wide open until it drops to 90% open. Where VAV zone damper position is unknown, a pressure request is made when the ratio of the zone’s actual supply airflow to supply airflow setpoint is less than 90% until it rises to 100%. 36 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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