Pumps, Valves and Flow: How Chilled Water Reaches Every Coil

Part 9 of 20 in Cobler's Cooling Fundamentals course. New here? See the course page.

Turn on the kitchen tap and water comes out straight away. That's because something upstream keeps the pipes full and under pressure, and the tap only decides how much you take. A chilled water system works the same way. Pumps keep cold water moving around the building, and a valve at each cooling coil decides how much of it that coil takes.

So chilled water pumps and valves share the work between them. But first, how much water does it take to carry 900 RT of cooling? We'll use the office tower's numbers all the way through. It has three 500 RT chillers, a peak cooling load of about 900 RT, and chilled water at 6 °C supply and 12 °C return.

If your building only has split units or cassettes, like the shoplot, it has no chilled water pipes, so you can skip this part.

Flow and temperature rise

Chilled water carries heat out of the building by warming up. Each litre leaves the chiller cold, collects heat at a coil and comes back warmer. So the cooling the loop delivers depends on two things:

• Flow: how many litres of water pass each second, in litres per second (L/s).
• Temperature rise (ΔT): how many degrees each litre warms up between supply and return. In Part 8 we called this delta-T.

What links them is one property of water. Warming one litre of water by 1 °C takes about 4.19 kilojoules (kJ) of heat. That gives us a simple formula:

$$\text{Cooling (kW)} = \text{flow (L/s)} \times 4.19 \times \Delta T\ (^\circ\text{C})$$

The answer comes out in kilowatts, because a kilowatt is one kilojoule every second. Like every kW figure in this course, it's a rate, how fast heat is being carried away, not an amount per hour. To convert it to refrigeration tons, we divide by 3.517 (1 RT = 3.517 kW of cooling, from Part 5).

So let's try the office tower at its peak, when the building needs about 900 RT:

1. Convert to kW: 900 × 3.517 ≈ 3,165 kW.
2. Each litre warms from 6 °C to 12 °C, so ΔT = 6 °C. Each litre per second then carries 4.19 × 6 ≈ 25.1 kW.
3. Flow needed: 3,165 ÷ 25.1 ≈ 126 L/s.

Now suppose the water comes back at only 9 °C, so ΔT is 3 °C. Each litre now carries half as much heat, so the same 3,165 kW needs about 252 L/s, twice the water. The pumps have to move all that extra water, and they use electricity to do it. That's why a healthy temperature rise matters, and Part 17 looks at what happens when it shrinks.

The same 3,165 kW of cooling needs twice the water flow when each litre warms by 3 °C instead of 6 °C.

Pump head

Our pumps have to move about 126 L/s around the tower. But what exactly does a pump give the water? It does two things at once. It moves a quantity of water each second, the flow. And it adds pressure, so the water can get through the pipes, coils and valves. Every metre of pipe, every bend and every valve resists the water a little, and the pump's pressure is what overcomes that resistance.

Engineers usually state a pump's pressure as head, in metres. Head is the height of a column of water the pump could push up. Ten metres of head is about one bar, or about 100 kilopascals (kPa).

Now, you'd think a pump sending water to the top of a 20-storey tower must spend most of its effort lifting it. But a chilled water loop is closed, a sealed circuit that stays full of water. The water climbing the supply pipe to the top floor is balanced by the water coming down the return pipe. So the pump doesn't have to lift the water against its own weight, and its head goes into overcoming friction. The more water we push through the same pipes, the more friction there is, and it rises faster than the flow does.

So a pump and its pipework settle at a balance. As a pump delivers more flow, the head it can make falls. As more water flows through the pipes, the head they need rises. The system runs where the two meet, which is also why a pump's nameplate flow is only a design figure. The flow we actually get depends on the pipes and valves it's pushing through.

The condenser water loop to the cooling tower is different, because it's open to the air at the tower. Its pump also has to lift water from the tower basin up to the spray nozzles at the top. Part of its head is lift, as well as friction.

Two-way and three-way valves

So the pumps keep the water moving, but who decides how much each coil gets? That's the valve's job. Each cooling coil in an air handling unit or fan coil unit (Part 10 explains both) has a control valve. The building management system (BMS) opens the valve when the air leaving the coil is too warm. It closes the valve when the air is cold enough. There are two common types, and they behave very differently.

A two-way valve has one way in and one way out. As it closes, less water flows through the coil, and less water flows in the pipes overall. So when many two-way valves close on a mild morning, the whole loop needs less flow.

A three-way valve has one way in and two ways out. As it closes the coil side, it sends the rest of the water through a short bypass pipe straight into the return. So the flow in the main pipes stays about the same, whatever the coil needs.

A two-way valve reduces the flow. A three-way valve keeps the flow constant by sending unused cold water straight to the return.

That three-way valve has a side effect. The bypassed water is still at about 6 °C, because it did no cooling. It mixes into the return pipe and pulls the return temperature down, so ΔT shrinks. And as we saw in the first section, a smaller ΔT means more water has to be pumped for the same cooling. Older constant-flow systems were built with three-way valves. But plants with variable-speed pumps normally use two-way valves, so the flow falls when the building needs less cooling.

Primary, secondary and the decoupler

In Part 8 we saw that a chiller needs a steady flow through it, while the building's demand for water changes through the day. Many plants solve this with two sets of pumps, and the office tower is one of them:

• Primary pumps push a steady flow through the chillers that are running, usually one pump per chiller.
• Secondary pumps push water out to the building. They have variable speed drives (VSDs), so their flow can follow the two-way valves.
• The decoupler is a short pipe that joins the two circuits. It carries the difference between the primary flow and the secondary flow.

With two chillers running, 140 L/s passes through the chillers and 126 L/s goes to the building. The other 14 L/s of cold water flows through the decoupler.

Let's put numbers on it. Each 500 RT chiller is designed for a 6 °C rise:

1. 500 RT × 3.517 ≈ 1,759 kW per chiller.
2. 1,759 ÷ (4.19 × 6) ≈ 70 L/s per chiller.
3. With the two duty chillers running, the primary pumps move about 140 L/s.
4. From the first section, the building at 900 RT with a 6 °C rise needs about 126 L/s.
5. The difference, about 14 L/s, flows through the decoupler from the supply side to the return side.

That small flow of cold water into the return is normal. But watch what happens if the rise at the coils drops to 4 °C while the building needs 800 RT. The building then needs 800 × 3.517 ÷ (4.19 × 4) ≈ 168 L/s, which is more than the 140 L/s the chillers are passing. So warm return water fills the missing 28 L/s by flowing backwards through the decoupler into the supply.

Now the water going out to the coils is warmer than 6 °C, and rooms may start to feel warm. The plant's control logic may even start the third chiller, although 800 RT is well within the 1,000 RT that two chillers can deliver. The warning sign is the direction of the flow. A little cold water into the return is fine, but warm water flowing back into the supply means trouble. Part 17 comes back to this pattern.

Some newer plants use a single set of variable-speed pumps instead, called variable primary flow. The flow through the chillers can then change, within limits the chiller maker sets.

Slowing the pumps

The secondary pumps run on VSDs, which let them run slower when the building needs less water. A VSD changes the frequency of the supply to the pump's motor, and that changes its speed. (How Electric Motors Work and Rectifiers and Inverters in our Electricity Fundamentals course explain the motor and the drive.)

How much does slowing down save? For a pump pushing only against friction, three rules of thumb called the affinity laws describe what happens:

• Flow falls in proportion to speed.
• Head falls with the square of speed.
• Power falls with the cube of speed.

So at 80% speed, the ideal power is 0.8 × 0.8 × 0.8 ≈ 0.51, about half. And at 50% speed it's one eighth.

In a real chilled water loop, though, the saving is smaller than that. The BMS usually holds a fixed pressure difference across the coil that's hardest to reach, often the farthest one. That way, its valve can still get enough water. So the pump has to keep that minimum pressure even when the flow is low. And the cube law only holds for a system with no fixed pressure to keep.

Slowing a pump still saves far more than running it at full speed and throttling the water with a valve. And lowering that pressure setpoint when the valves are mostly closed saves more again. Part 19 covers how a BMS resets it.

Reading the cooling

Now we can come back to the formula from the start. On a real plant, it turns three readings into the cooling being delivered at any moment:

1. Flow, from a flow meter on the chilled water pipe.
2. Supply temperature, from a sensor on the pipe leaving the plant.
3. Return temperature, from a sensor on the pipe coming back.

Say the meter reads 100 L/s, and the water leaves at 6 °C and comes back at 11 °C. Then the plant is delivering 100 × 4.19 × 5 ≈ 2,095 kW, which is about 596 RT. If we divide the plant's electrical power by that figure, we get its kW per RT, the efficiency number that Part 13 explains. Part 16 covers where these instruments go, and how to trust their readings.

Into the coils

So the chilled water has now reached every coil, in the amount each one asks for. But a coil on its own doesn't cool a room. Those coils sit inside two kinds of unit that blow the cooled air into the rooms, and they're built for quite different jobs.

So what's the difference between an AHU and an FCU? That's Part 10: AHU vs FCU.

Check your understanding

1. A meter shows 60 L/s of chilled water, leaving at 6 °C and coming back at 12 °C. How much cooling is the loop carrying?

2. Why do three-way valves tend to lower the chilled water return temperature?

Recap

• Cooling carried by water = flow × 4.19 × temperature rise.
• A pump supplies flow and head. In a closed chilled water loop, that head overcomes friction.
• Two-way valves reduce the flow as they close, while three-way valves bypass cold water into the return.
• Primary-secondary plants keep a steady flow through the chillers, and the decoupler carries the difference. Warm water flowing backwards through it is a warning sign.
• Slowing a pump with a VSD saves a lot of power, but less than the cube law when a fixed pressure has to be held.

This is Part 9 of 20 in Cobler's Cooling Fundamentals course. Previous: How Does a Chilled-Water System Work? The Three Loops Explained. Next: AHU vs FCU: The Difference Between an Air Handling Unit and a Fan Coil Unit.

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