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- 1. Maybe the problem was born in design — overestimating pump head, or a cooling tower that isn’t towering enough. Or it arose in installation — say, misconnected coils. Even then, a fix may create a new water mystery. Get in step with some successful detecting habits to correct the next troubled system you see. BY HOWARD MCKEW, P.E., CPE T roubleshooting building systems is a niche business that I know this works because when I designed the Massachusetts is immune to the economy or building owner’s operating State Transportation Building chilled water system over 30 years budgets. When an HVAC system doesn’t work, or doesn’t ago, I started with a system flow diagram and wrote the sequence as want to work to the owner’s expectations, the results can be I sketched out the design. disastrous. For health care institutions, a system shutdown At that time, I couldn’t ask anyone in the office, “Now, how did could interrupt an operating room procedure. For an industrial we do this the last time?” This was a first of its kind, one-million- facility, it could mean stopping product production. For an edu- sq-ft facility that could have been conventionally designed with cational facility, it could mean IAQ problems. So what are some of 2,000 tons of peak chiller capacity but instead was designed with a those problematic jobs, and how did they become problems? Here 600-ton chiller in series with two 300-ton chillers and three thermal are some of experiences from 40-plus years in the building industry, storage tanks, each with a capacity of 275,000 gal for cooling that focusing on central chilled water systems. would become thermal heating storage (no boiler or utility steam) in the heating season. HOW DO SYSTEM PROBLEMS START? I was confident the design would work. When the time came I believe most problematic systems originate in the design phase of for the system to be commissioned (before the word commission- a construction project, due to an inadequate design process. Every ing was coined), the process went smoothly as I worked with the HVAC system design should start with a single-line flow diagram in automatic temperature control (ATC) and TAB contractors. Today, the conceptual/schematic phase. computer-aided software has replaced paper and pencil for creating At this early stage of the building program, the design engineer flow diagram sketches (refer to the “Back2Basics” series for 21st- can write a sequence of operation even if the designer has not firmed century flow diagram technology), but the designer should still start up the chilled water system capacity. Having thought through the with a system flow diagram. design, sequence by sequence, the engineer is able follow the chilled I approach troubleshooting as if I were designing the chilled water flow through the system, adding control devices as the written water system way back in the design phase. It is also important to sequence is documented. note this same ATC flow diagram has a second application when 30 En gi n e e r e d S y stem s December 2009
- 2. Troubleshooting Chilled Water Sytems CHWR troubleshooting/assessing water balance. Just like when I sketched out the ATC sys- C tem flow diagrams as the design engineer, 1,000 feet (pipe & fittings) O Last coil At 3 feet/100 = 30 feet I 10 ft I would copy the diagram and re-use it L to begin plugging in the pressure drops ATC valve throughout the system to come up with my 10 ft total chilled water pump head. Figure 1 is a Chiller computer software version of this concept and a document that can be forwarded 70 ft Chiller onto the TAB firm in the construction Pumphead 20 ft phase. Armed with these two troubleshoot- ing tools, the ATC flow diagram and a TAB FIGURE 1. Budget estimating pump head. flow diagram, I’m ready to begin trouble- shooting. Open cooling tower DON’T UNDERESTIMATE THE CONSEQUENCES OF OVERESTIMATION Next to not using a system ATC flow diagram 2-ft pressure drop Tower sump in the design phase, the most common issue thru strainer water level to troubleshoot, which starts in the design phase and ends up as an inefficient pumping Condenser water pump design, is caused by designers who overesti- 10 ft elevation mate the pump head. Frequently, designers to provide 8 ft think bigger is better, but more often than } NPSH at pump Condenser inlet not, overdesign can lead to operational issues water supply and increased energy consumption. A guide to use when sizing a chilled water system is to figure the total pump head will be around 60- to 70-ft pump head. When I see pumps 8 ft NPSH heads closing in at 100 ft — or worse yet, 200 per manufacturer’s requirement ft — I’m pretty sure I will find a lot of balanc- ing valves substantially closed or maybe just one substantially closed balancing valve on FIGURE 2. Cooling tower and pump installation net positive suction head (NPSH). the pump discharge. Some engineers will tell you it is better to 0 psig oversize pump heads in VFD systems and be safe then to undersize and come up short on In vacuum Positive pressure chilled water flow. That position is fine if the design engineer is paying the extra money it takes to furnish and install a 100-hp motor (motor, starter, wiring, etc.) when a 75-hp 1/4-in. motor would do. I have had the opportunity Tubing to design some very large building HVAC sys- Typical petcock tems and re-engineer other very large systems over the years, and I have never designed a chilled water system over 125-ft pump head. At the same time, I have retro-commissioned Condenser water several chilled water systems that initially from open cooling tower ranged from 150 to 290 ft of pump head and Condenser water pump got them down under 125 ft without com- promising chilled water performance. When assessing these multiple central chilled water Typical systems, I found them to be in the range of shut-off valve 0.25 to 0.35 hp/ton, and got them to operate Typical strainer under 0.1 hp/ton. A good troubleshooting rule of thumb is 0.05 to 0.1 hp/ton. FIGURE 3. Duplex strainer with compound gauge. 32 En gi n e e r e d S y stem s December 2009
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- 4. Troubleshooting Chilled Water Sytems gauge. Monitoring the gauge reading will allow the facility person to switch from the dirty strainer to the clean strainer as the gauge begins to show signs of the condenser water system pressure going into a vacuum condition at the pump inlet. INSTALLING THE PROBLEM Troubleshooting problems don’t always originate in the design phase and manifest themselves as full-blown calamities. Experience has shown me this fourth issue occurs probably once out of every 25 chilled water coil installations: chilled water coil piped incorrectly on the job site. The results is about a 20% inherent loss in coil cool- ing capacity (Figure 4) when the chilled water supply is connected on the upstream airside of the coil and the chilled water return con- nection is piped to the downstream side of the coil. Frequently, this problem goes undetected for years because the coil was significantly oversized by the design engineer or the facil- ity person came to (wrongly) accept that the coil was originally undersized. I frequently find these misconnected coils when doing a retro-commissioning assessment, but other times they are found when troubleshooting why the cooling coil isn’t doing its job. The same issue occurs with water-to-water heat exchangers that do not have counter-flow between the condenser water and the chilled water. Again, this deficiency may surface when troubleshoot- ing the heat exchanger. On one occasion, we found the problem while retro-commissioning a primary-secondary-tertiary pumping system that was initially designed with 0.32 hp of pumping. We got the pumping energy down below 0.1 hp/ton and re-piped the water- to-water heat exchanger. You could say it was a win-win. WHERE’D ALL THE WATER GO? Occasionally, we will tune up chilled water systems by implement- ing the corrective actions discussed herein, and when we do this, a fifth problem occurs — namely, the “where is all this water going?” dilemma. What I mean by this is that when rebalancing the system, we cannot account for a percentage of the chilled water flow. When this occurs, we find we need to be extra vigilant in documenting piping supply and return connections along the chilled water dis- tribution system. In each case where the system was handling more water than we could account for via flowmeters, we found unrecorded bypass connections between the supply main and the return main. One installation had a dozen unrecorded bypasses, while a second system had ten. My only answer to these engineer-designed and unrecorded bypasses is that they were probably installed to allow flushing of segments of the pipe distribution during the construction phase and, unfortunately, remained in operation after the owner took occupancy. From a troubleshooting point of view, it is something I look for now that I would not have thought to consider in the past. I guess that is what is meant by the saying, “With age, comes experience.” A sixth troubleshooting challenge is when a second chiller plant is furnished and installed within a facility to provide additional cooling capacity because the existing chiller plant can’t handle the load. In each case, the application was a health care institution, and the need for additional air conditioning stemmed from the age of the hospital dating back to the 1960s and 1970s, when some sections of the hospital were not fully air conditioned. 36 En gi n e e r e d S y stem s December 2009
- 5. As each of these facilities received reno- vations, upgrades, and building additions, the new projects were fully air conditioned. Often, the existing chilled water plant did not have the space to accommodate future equipment expansion, so a new location was selected. The new chilled water system was designed to handle the new addition and/or renovations and was also tied into the existing chilled water plant to work in parallel. The results were less then success- ful, and so I was contracted to trouble- shoot each of these installations. Starting with a flow diagram and surveying the system distribution, the solution was the same. The cross-connection of the sup- ply of chiller plant 1 to chiller plant 2 was done incorrectly (connected to the return main by mistake). So after several years of limited chiller plant performance, a solu- tion was found. SUMMARY There are numerous installations that mirror the few addressed herein, and I Input 137 at www.esmagazine.com/instantproductinfo truly believe if troubleshooters stick to the basics of troubleshooting, solutions can be achieved. As with any quality control pro- cess, start with a flow diagram and begin with data collection. Don’t simply jump to a BREAKING NEWS ! conclusion without first analyzing the data; consider the possibilities and then focus in on the answer(s). ES NOW KLO-SHURE COUPLINGS McKew is an inte- grated project delivery facilitator for the IPD CAN SECURE 1/2" INSULATION! 1 Group, RDK Engi- neers (Andover, MA). Reach him by e-mail NO ONE ELSE at hmckew@RDKen- gineers.com. MEASURES UP. Meets 25/50 flame spread/smoke developed index Supports tubing and secures 3/8" – 1 1/2" insulation Reduced material cost Installs 60% faster – no tape, no glue Superior vapor barrier Eliminates insulation compression No Call Backs Contact us today to request your FREE product catalog, and learn how well Klo-Shure Insulation Couplings can work for your next project! 800.839.0891 www.klo-shure.com Input 75 at www.esmagazine.com/instantproductinfo w w w. esmag a zin e. co m 37 ESM05094Klo.indd 1 3/31/09 11:21:37 AM