Consulting-Specifying Engineer - August 2008 - (Page 37) for time lags and heat transfer rates provided by real-time building data greatly reduced the uncertainty as well as the number of assumptions required to provide a quick analysis. From the building’s mouth In order to have the building tell us what the load was, we logged into the control system and extracted several hours of data for one of the coldest days the facility had experienced in recent history. Initially, we thought that an assessment of the boiler gas consumption would give us the answer we were looking for. However, we also realized that it would be extremely difficult to ascertain an estimated load without knowing the efficiency of the boiler. Ideally, boiler efficiency is determined by a number of variables, including the boiler design, burner adjustment, operating temperature, and the condition of the heat transfer surfaces. Given the age and degraded condition of the boiler, however, assuming its efficiency was anywhere near the 75% figure implied by its nameplate rating would be less than sound engineering judgment. Measuring the efficiency in the field may have been possible if we had time and could get the boiler to run, but was struck as an option since the safe operating ability of the unit itself was at question. Past experience and information available from various sources told us that the efficiency of the degraded boiler could be lower than 60%. Although the gas consumption pattern was an indicator of the actual load, we did not want to base our analysis on that solitary source of data. With help from the building Fortunately, we also knew the temperature rise across the boiler. Because the hot water system served by the boiler was a constant volume system, we decided to use the pump’s rated flow rate and the logged temperature rise across the boiler to give us a real-time measurement of the actual load on the system. Because we » When I asked when he needed an answer, he said with a laugh, “by the end of the day.” the capacity of the proposed replacement. This erased any doubts about whether the new boiler would provide sufficient heating for the building. Some quick math based on the trend data revealed that if our assessment of flow was correct, the boiler efficiency was somewhere between 44% and 71%. While the efficiency of the existing boiler was probably better than 44%, it also was not likely to surpass 71%. Thus, the gas data reinforced our estimation of the load based on flow and water temperature rise across the boiler. Had the gas consumption been less than the capacity we thought we were getting from the boiler based on our estimated flow and logged temperature data, we probably would have had to reconsider our analysis. The fact that everything added up only served to strengthen our assessment of the load. were measuring the output of the boiler instead of the input, the efficiency of the boiler would not be a factor. The challenge was to determine a realistic system flow rate, which would allow us to apply the given water side load equation. Q = 500 x flow rate x (Tin- Tout) Where: Q = Load in Btuh 500 = Units conversion constant flow rate = Flow in gpm (Tin- Tout) = Temperature change across the boiler Unfortunately, determining the rated flow capacity of the pump was not easy. The original drawing with the pump schedule was missing and the nameplate on the pump had become illegible over time. Undeterred, we decided we would base our assessment on a range of flows that would be accommodated by the line size associated with the system. Specifically, the line size at the boiler and pump was 2.5 in. By using a pipe friction chart, and keeping in mind that designers tend to pick the next larger line size if friction rates are within 4 ft water column per 100 lineal ft of pipe, we determined that the flow rate in the system was probably 55 to 80 gpm. Having established a flow range for the system, we downloaded trend data from the hot water system into a spreadsheet and then added a couple of columns to perform the load calculation using the logged temperatures and the flow rates we had established. A concrete assessment As illustrated by the graph in Figure 1, the required capacity from the boiler on the coldest day in San Diego seldom went over Editor’s note This article was developed in collaboration with the Building Commissioning Assn. (BCA). As the media sponsor for the BCA Convention and Expo, ConsultingSpecifying Engineer encourages readers to attend the BCA Convention and Expo, Oct. 5-7, Long Island, N.Y. For more information about commissioning and the convention, visit www.bcxa.org. Consulting-Specifying Engineer • AUGUST 2008 37 http://www.bcxa.org http://www.bcxa.org
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