One Complaint, Three Loops: A Worked Cooling Plant Check

Part 20 of 20 in Cobler's Cooling Fundamentals course. New here? See the course page.

On a hot Thursday afternoon, the helpdesk in the office tower gets a call from level 12, saying "It's warm and sticky up here." The usual first response is to turn the thermostat down, or to blame the air handling unit (AHU) on that floor. But the air on level 12 is cooled by water from the chillers. And the chillers depend on water from the cooling towers on the roof. So the cause could be in any of those three loops, and it's worth a proper cooling plant check before anyone touches a thermostat.

Back in Part 1, we stood next to the little split unit on the shoplot wall and made a promise. We said that by the end of the course, we'd be able to stand in a 20-storey tower's plant room. And we'd be able to explain why one floor is too warm. Well, here's the floor, and here's the plant room.

So we'll follow this one complaint through the air loop, the chilled water loop and the condenser water loop. We'll use a few trend readings and the tools from the whole course. And at the end, there's a shorter version for the shoplot, which has split units and cassettes instead of a chiller plant.

The cooling plant check

Each task gives you some information and asks a few questions. So try the questions first, with a calculator and a notepad. Then open the worked answer under each task to check your working.

As a reminder, the office tower has three 500 RT chillers, two on duty and one on standby. Its chilled water is designed for 6 °C supply and 12 °C return. It also has three cooling towers on the roof and a building management system (BMS). All the readings below are made up, but they're consistent with each other.

We'll need five tools from earlier parts:

• Delta-T (ΔT) = return temperature − supply temperature. From Low Delta-T Syndrome (Part 17).
• Cooling (kW) = flow (L/s) × 4.19 × ΔT (°C). From Pumps, Valves and Flow (Part 9).
• RT = kW of cooling ÷ 3.517. From What Is a Ton of Refrigeration? (Part 5).
• kW/RT = electrical kW of the whole plant ÷ RT of cooling delivered. Lower is better. From COP, EER and kW per Ton (Part 13).
• Approach = water temperature leaving the tower − outdoor wet-bulb temperature. From How Does a Cooling Tower Work? (Part 12), where a good approach was about 3 to 4 °C.

The trend readings

The facility engineer pulls the same set of readings from the BMS for two Thursdays at 3pm. One is last week, when nobody complained, and the other is today. The relative humidity comes from a wall sensor on level 12, and the outdoor wet-bulb temperature from the weather sensor on the roof. The plant's electrical input comes from the chiller plant's sub-meter, which covers the chillers, pumps and tower fans.

The helpdesk log also shows that by 3:30pm on 17 September, levels 14 and 17 have called about warm, humid offices too.

The complaint-day readings placed on the three loops: air, chilled water and condenser water.

Task 1: the air loop

We'll start where the complaint is, with the air loop rows for AHU-12A. That's the unit whose load we added up in Part 6, and whose control loop we watched in Part 19. The comfort ranges come from the Industry Code of Practice on Indoor Air Quality 2010 (ICOP IAQ 2010) in Part 4.

1. Are the room conditions outside the ICOP IAQ 2010 comfort ranges? Those are 23 to 26 °C and 40 to 70% relative humidity.
2. Is AHU-12A trying to cool harder, or has it given up?
3. Could a dirty filter explain the complaint?
4. Why is the room humid as well as warm?

AHU-12A is working at full output, then, and still can't reach its setpoint. It looks like a victim, not the cause. The next question is whether the water reaching its coil is as cold as it should be. And the calls from levels 14 and 17 point the same way, because a problem at one AHU wouldn't usually affect three floors.

Task 2: the chilled water loop

Now let's follow the water back, with the chilled water rows.

1. What is the chilled water delta-T on each day?
2. How much cooling did the plant deliver on each day, in kW and in RT?
3. Is this low delta-T syndrome?
4. What is the most important change between the two days?

The two running chillers are rated 500 RT each, so 1,000 RT together. But on the complaint day they delivered only about 751 RT. That's less than the 834 RT of the normal day, although the building was warmer and asking for more. So something is holding the chillers back.

Task 3: the plant's efficiency

1. Calculate the plant's kW/RT on each day.
2. How much more electricity is the plant using for each RT on the complaint day?
3. How do both figures compare with the plant's design target of 0.85 kW/RT?

The plant's electrical input barely changed, from 790 to 800 kW, but its cooling output fell. So the chillers are working as hard as before and achieving less. That's what happens when a chiller has to push its heat out against hotter condenser water, which leads us to the third loop.

Task 4: the cooling towers

Now look at the condenser water rows.

1. What is the tower approach on each day?
2. Did the weather change enough to explain the difference?
3. What else in the table explains it?
4. Why does hotter condenser water hold the chillers back?

So here's the whole chain. A stopped tower fan made the condenser water hotter, and that held the chillers back. That let the chilled water warm to 8.0 °C, which left AHU-12A's coil too warm to cool and dry the air. And that made level 12 warm and humid.

Task 5: the next check

So which of these should the facility team do first?

1. Replace AHU-12A's filter.
2. Lower the level 12 room setpoint to 22 °C.
3. Ask the chiller service contractor to check the refrigerant in CH-1 and CH-2.
4. Send a technician to cooling tower CT-2 to see why its fan has stopped. The electrically competent person responsible for the plant goes too, to inspect and reset or repair the fan motor's starter.

Task 6: the fix

A failed fan motor bearing had tripped CT-2's starter, and it's repaired on Friday 18 September. On Monday 21 September at 3pm, the BMS shows:

• Outdoor wet-bulb 26.8 °C, and condenser water leaving the towers at 30.5 °C, with both tower fans running.
• Chilled water supply 6.0 °C, return 11.2 °C, flow 140 L/s. Whole plant electrical input 815 kW.
• AHU-12A supply air 14.0 °C, with its valve 75% open. Level 12 east at 24.0 °C and 59% relative humidity.

1. Calculate the approach, delta-T, cooling in RT and kW/RT.
2. Is the problem fixed?
3. Does this prove how much energy the repair saves each month?

Two items stay on the list for later. The plant's normal delta-T of about 5 °C is below its 6 °C design (Part 17). And its usual 0.94 to 0.95 kW/RT is above the 0.85 kW/RT target. Neither caused the complaint, but both deserve their own investigation.

And notice that this fault was visible in the BMS before anyone called. CT-2's fan had stopped while the BMS was still commanding it to run. So an alarm when a fan's status doesn't match its command would find the next trip before anyone calls the helpdesk.

Task 7: the shoplot

The shoplot has no chilled water and no cooling tower. Its cooling is direct expansion (DX), with six wall-mounted split units (1 to 1.5 HP class) and two ceiling cassettes (2.5 HP class). That's about 33 kW of cooling in total, or about 9 RT. On Saturday 19 September, the café on the ground floor is warm and sticky by 2pm.

The café is cooled by the two cassettes, C-1 and C-2. They're the same model and age, both set to 24 °C in cooling mode with the fan on high. Both have run without stopping since the café opened. So the owner holds a digital thermometer at each cassette's return grille, where air goes in, and at its supply louvre, where air comes out. Then the owner looks at each outdoor unit in the back lane.

1. Which cassette is moving less heat, and how can you tell without a flow meter?
2. In what order would you check things, and who should do each check?
3. Which office tower tasks don't apply here, and where will the shoplot see a saving?

Back in the plant room

So that's the promise from Part 1 kept. A floor in a 20-storey tower felt warm and sticky. We followed it from the room, through the coil, the chilled water and the chiller, to a stopped fan on the roof. And every step used something we built along the way.

The dew point explained why the room felt sticky, and AHU-12A's open valve showed it was a victim. Flow and delta-T put a number on the cooling, kW per ton showed the plant straining, and the approach pointed at the tower. Underneath it all is the idea from the little split unit on the shoplot wall. Heat only flows downhill on its own, so a machine has to push it uphill and out. So when the last step of that push can't release its heat, every loop before it feels the strain. And in the end, so does someone at a desk on level 12.

Where to go next

If one task gave you trouble, go back to the part it draws on:

• Task 1: Why Is My Office Cool but Still Humid? (Part 3) and What Is a BMS? (Part 19)
• Task 2: Pumps, Valves and Flow (Part 9) and Low Delta-T Syndrome (Part 17)
• Task 3: COP, EER and kW per Ton (Part 13)
• Task 4: How Does a Cooling Tower Work? (Part 12)
• Task 6: BEI Malaysia: Scoring Your Building's Cooling by Measuring It (Part 16)
• Task 7: Oversized Chillers, Short-Cycling and Dirty Coils (Part 18)

If all seven tasks made sense, you've finished the course. The Building Automation Fundamentals course continues from Part 19, with the BMS points, trends and alarms you used here. And there are other courses on Cobler Learn.

Check your understanding

1. A plant's chilled water flow is 120 L/s, with water leaving at 6.0 °C and coming back at 11.0 °C. The whole plant draws 680 kW. What is its kW/RT?

2. An AHU's supply air is too warm, but its valve is only 40% open. Is the problem more likely in the AHU or in the chilled water?

Recap

• Follow a complaint through the loops in order, the air, the chilled water, then the condenser water.
• At each loop, compare the readings with their setpoints and with a normal day.
• An AHU with its valve and fan at 100% that still can't reach its setpoint is a victim, not the cause.
• Delta-T and flow give the cooling in kW and RT. The plant's electrical input divided by RT gives kW/RT, and the water leaving the tower minus the wet-bulb gives the approach.
• Choose the check that goes to the cause, and work safely. Confirm the fix with fresh readings, and use weeks of data, not one afternoon, to prove a saving.
• In a DX building, compare identical units' in-and-out air temperatures. Then check filters, outdoor coils and refrigerant, in that order.

This is Part 20 of 20 in Cobler's Cooling Fundamentals course. Previous: What Is a BMS? How Building Controls Run the Cooling Plant.

Cobler builds CobiNeural, a platform that shows a facility team its building's energy, water and indoor air data as live numbers across the whole site. To see how your building performs, talk to us.