SOLUTIONS FOR HIGH·PERFORMANCE BUILDINGS

ebesta Blomberg was commissioned to study a humidity problem for a 400,OOO-sq-ft library at a university in the Midwest a few yea rs ago. The university shared with us the study another firm had completed for the chilled water system in the library as background information. They stated they had not implemented any of the recommen­dations in that study because they did not believe

their findin gs were accurate. The university had a small budget for th e study, so our scope

was limited to looking at three of the large AHUs serving half of the library (A HU-A , AHU-B, and AHU-C). At the time of th e proposal, we knew the library was too humid in th e summer and that the three large air handling units could not discharge less than 60°F supply air at that time of year. I knew the chilled water sys tem was the most likely culprit, so we limited our scope to a review of the air handling units, their ch illed water coils, and the pumps serving these units.

1 met with the university project manager and one of the lead facility techni cians on campus to see the chilled water system for myself. We started a tour of the facility by looking at the AHUs and taking measurements of the chilled water coi ls. 1 measured the coil face area on each coil, coun ted the fi ns/i nch, verified the

22 Engineered Systems DECEMBER 2013

header connection size, and cou nted th e number of rows and measured th eir size. 1 verified the coi ls were piped in cou nter­flow on each of the AHUs. I also collected as much information as I could on the chilled water con trol va lves.

We th en moved to the chilled water pumps that se rved these units. As it turned out, two of the three AHUs (AHU-A and AHU-B) were served by parallel pumps P-l and P-2 that were dedicated to those units. I decided to start with these pumps. The manufacturer's data tag had rusted off years earlier, so lonly recorded the motor information and manufacturer. Since it was winter, neither pump was running. My tour guide said they only run one pump and that they've tri ed running both, but it didn't seem to make any difference. The AHU (AHU-C) was served from a pump (P-S) that fed several other AHUs in the facility. The pump looked a\vfully small, and when I asked hmv many air handling units it served, my tour guide was unsure.

The documentation 1 had received prior to the trip was liter­ally a data dump of all construction drawings th ey had on th eir server and in no particular order. I had spent a day looking at their files before the trip, which did not even begin to give me a full understanding of all of th e additions and renovations the facility had gone through since it was built. Now that I was in the bui lding, it was becoming clear I would need to track down all of

Trus~ut Verify

to thi s system without com p lete verifi cat ion of wheth er

the pumps or piping could handle the addi tional loads. I then turned my attention to the pumps se rving air

handling units AHU-A a nd AHU-B. The schedule on

the drawings li sted the perFormance criteria of pumps P- I and P-2 as 960 gpm at 60 ft of head, with a 20 HP

motor operating at 1,150 rpm. I had recorded in my fi eld notes that the m otor was 20 HP a nd 1,150 rpm

so at least th at information matched. I th en con tacted m y B&G sa les representative who helped me track the pump cu rves which he had determined were obsolete. I th en compared the scheduled data aga inst the pump

curve and found the o peratin g poin t to be dead on with a 12.75 in impeller at \,]50 rpm. So far, things

were adding up.

FIGURE 1. The pump (P-5) in the photo above was connected to eleven air-handling units with a total flow demand of 963 gpm. The actUal pump flow rate was verified to be 300 gpm.

The study conducted by the previous co nsulta nt stated that the pump was se lected for 1,750 rpm and the uni ve rsity had in stalled the wrong speed motor at

some point in th e past. This issue, they stated, was the cause for the shortage of fl ow for these air hand ling units. I ca lled the manufacturer's representative and asked if it was possible for someone to in stall a 1,1 50-

rpm motor on a 1,7S0-rpm pump. He sa id it is possible but sin ce the motor fram e sizes and sha ft diameters

th e chill ed water piping in th e en tire b uild ing. I knew our scop e was limited to the three main air handlers,

b u t I couldn' t leave without kn owing what else that p ump served.

In my career, T am often faced with the dec ision of stopping with what I know or digging dee per. I am no t su re if it is my willing­ness to se rve or my inquisitive nature, but in almost every case,

I've kept digging and have never once regretted th at decision. I

ca ll ed m y project m anage r and together we m ade the decision that I should track down all of the chill ed water users on the system. My tour gu ide was up fo r it, so we sta rted following the

piping. After spen di ng more than th ree hours crawling through the basemen t and crawlspaces, we were ab le to trac k down eleven

AHUs that received chilled water from pump P-5.

THE NEXT STEP Back in the office, I star ted researching the capacities and operat­ing character isti cs of the equipment I found on site. I ca ll ed the sales representative to get the pump curves and was told that P-5

was a s tock pump, wh ich mean t they are no t typically selected fo r a certa in opera ting condition, b u t instead kept in stock fo r immediate replacem en t needs. As a result, he couldn 't tell me

what th e p ump was originally selec ted for. I had a test a nd bala nce (TAB) report from the university

whe re they had taken now and pressure read in gs on this p ump

ea rli er th at year. According to the report, the pump flow a nd pressure was 300 gpm at 185 ft of head. I reviewed the TAB data against th e pump curves and concluded tha t I had the co rre­

spondi ng pump curve. r then summed up the design flow ra tes of th e chilled water coi ls served by P-5 and came up with 963

gpm. If thi s pump was actuall y balanced to only 300 gpm , the AHUs on thi s system (includ ing AHU-C) were definitely starved of ch illed water. As it turned out, as I followed the progression of th e renovations, it was clear tha t addit ional loads were added

24 Engineered Syslems DECEMBER 2013

were different, someo ne would have had to modify the motor m ounting bracket. So while it can be done, it probably wouldn 't have been a n acc ident. The pumps didn't show any evidence of

this, so I concluded that the pumps were originally se lected at 1,150 rpm and the university d id not accidentally put the wrong speed m otor on them.

Next, r looked a t the chilled wa te r coils fo r air ha ndli ng units

A, B, and C. The original const ruction drawings had scheduled the performa nce of the chilled wa ter co il s based on a 42°P enter­

ing chill ed wa ter tempera ture. The university told me they typi ­call y su pply 44°P ch illed water from the central plant. The chill ed

water supply is then mixed with the ret u rn water to supply 46°P water to the ai r handling un its. I was also told the ch ill ed water co il s were replaced approximately 10 years ago, but the m anufac­turers order informa ti on was lost and no lon ge r availab le.

r used the data I had co ll ected o n site and coi l selec tio n soft­

ware to model the performan ce of the co il s. I then compa red th e co il performance data scheduled on the drawings with the

outpu t from the software a nd confi rmed that the original coi ls must have been replaced with new co il s tha t matched th e origi­

nally selected coi ls. This also m ea nt these co il s we re not capable of delivering 55° F supply air unless the unive rsity could g ive th em 42°P chill ed wa ter. In o rder to do this, they would have to

lower the chilled wate r supply temperature a nd stop blendin g at the bui ld ing. Knowing that th at th e university wou ld not accept either of these options, I knew these co il s were not go ing to pro ­v ide the cooling and dehumidification th e library needed.

Once I had the flow coefAcients I calculated th e pressu re drop

for each of the va lves based on the fl mv rates scheduled on the drawi ngs. In all three cases, the pressure drop of the valves far exceeded the available differential pressu re of the system. My

first thought was there must be two contro l va lves for each ai r handl ing uni t. Th e original construc tio n drawings sho wed two

TrustIiut Verify

FIGURE 2. Existing pump (P·l) wilh 1,150 RPM motor.

I ~ I

I I

del iver 55°F supply ai r with 46°F chilled water. We also recommended replacing the pumps and control va lves with ones that were appropriately sized.

The university implemented all of our solutio ns and the next summer, the d ifferences were remarkable. Th e library was cool and co mfor table for the firs t t ime in its 40-year history.

Prior to this study, th e university knew they had a problem with th is bui lding and had always wa nted to fix the lib rary, but it didn't know what to do or how much to spend. They had studies providing recom~ mendations, but the issue never got addressed because funding was always short and they weren't con fident the findings were va lid. The conclusions from our study provided the un ive rsity with a sol id p lan to fix the ch illed wate r system at the library. I st ill find it interesting that if they had on ly addressed one of the causes, that si ngle solution in and of itself would not have fixed the problem. The system would on ly func­tion correctly after all three issues were corrected.

I learned a great lesson as part of this project. My tour guide summed it up by telling me one of the most so lid truths of troubleshoot ing. His advice was both simple and wise, "Trust .. . bu t ver ify."

T absolutely love the challenging probl ems. In this case, the issues were caused by severa l fa ctors that all in teracted with each other. 1 suppose if the prob lem had lasted th is long, the answer wasn't going to be simple. Still, I am often amazed at how long these issues go unreso lved. ES

FIGURE 3. The photo above shows the 4-in chilled water valve connected to a-in piping.

Douglas Lucht, MS, P,E. has over 18 years of experience in design, troubleshooting, and fetro-com­missioning of mechanical systems. His experience comprises a wide variety of facility types with critical environments includ ing laborato­ries. museums, data centers, and libraries, Lucht also performs the role of mechanical group leader of the High Performance Build ing Sys-tems at Sebesta Blomberg, where

va lves per unit , but more recen t drawings showed the units with one va lve per un it. I only reca ll ed one va lve per un it, but of course 1 could have missed something. I called my tour gu ide at the unive rsity and asked him to go back and verify the number of control va lves on the three air handling un its. He called me a few days later confirming there was on ly o ne valve per un it.

CONCLUSIONS & CULPRITS Ult imately, we found tha t the problem had three root causes: first, the coils were not selected for the correct chilled water tempera­ture; second, the co ntrol valves were grossly unders ized; and th ird, pump P-5 could not supply enough chi ll ed water to AHU-C

Since the ex.isting cooling coi ls in the air handling units were only 10 years old, we recommended that new coils be installed in ser ies to increase the heat transfer surface and allow the coils to

26 Engineered Syslems DECEMBER 2013

his responsibilities include staffing, mentoring, and work­load management for a group of eight supporting engineers and designers. He can be reached at dlucht@sebesta.com.

-BACK TO COLLEGE -Want more articles tackling HVAC on campus? Our online archives has a faculty meeting's worth of opin­ions, insights, and experiences.

From net zero to radiant design, from chilled water retrofit features to many Case In Point entries, see the full list with a simple URL:

www.esmogozine.com/colleges