Pumps, Valves and Flow: How Chilled Water Reaches Every Coil
How chilled water pumps and valves work: why cooling equals flow times temperature rise, what pump head means, two-way vs three-way valves, the decoupler, and why variable-speed pumps save power.
Tan Kok XinCooling Fundamentals
Part 9 of 20 in Cobler's Cooling Fundamentals course. New here?See the course page.
In Part 8 we followed chilled water from the chiller to the coils and back, and ended on a question: how much water has to flow to carry the cooling? This part answers it, then explains what a pump actually provides, how valves share the water out, and why pumps on variable speed save electricity. The numbers come from the office tower: three 500 RT chillers, a peak cooling load of about 900 RT, and chilled water at 6 °C supply and 12 °C return.
A useful comparison is a tap at home. When you open it, water comes out straight away, because something upstream keeps the pipes full and under pressure, and the tap decides how much you take. In a chilled-water system, pumps keep cold water moving around the building, and a valve at each cooling coil decides how much of it that coil takes.
If your building only has split units or cassettes, like the shoplot, it has no chilled-water pipes and you can skip this part.
The cooling a loop carries is flow multiplied by temperature rise
Chilled water carries heat out of the building by warming up. Each litre leaves the chiller cold, picks up heat at a coil and returns 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.
The link between them is one property of water. Warming one litre of water by 1 °C takes about 4.19 kilojoules (kJ) of heat. That gives a simple formula:
Something in this article you want to dig into — or a situation in your own building it doesn't quite cover? Send us your question. We don't run public comments; the team replies to you directly by email.
The answer comes out in kilowatts because a kilowatt is one kilojoule every second. Like all kW figures in this course, it is a rate: how fast heat is being carried away, not an amount per hour. To turn it into refrigeration tons, divide by 3.517 (1 RT = 3.517 kW of cooling, from Part 5).
Worked example: the office tower at peak. The building needs about 900 RT of cooling on a hot afternoon.
Convert to kW: 900 × 3.517 ≈ 3,165 kW.
Each litre warms from 6 °C to 12 °C, so ΔT = 6 °C. Each litre per second therefore carries 4.19 × 6 ≈ 25.1 kW.
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 carries half as much heat, so the same 3,165 kW needs about 252 L/s, twice the water. The pumps have to move that extra water, and they use electricity to do it. This is why a healthy temperature rise matters, and Part 17 looks at what happens when it shrinks.
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The same 3,165 kW of cooling needs twice the water flow when each litre warms by 3 °C instead of 6 °C.
A pump supplies flow and pressure, called head
A pump does two things at once. It moves a quantity of water each second (the flow), and it adds pressure so that 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 roughly one bar, or about 100 kilopascals (kPa).
A chilled-water loop is closed: it is a sealed circuit that stays full of water. Water climbing the supply pipe to the top floor is balanced by water coming down the return pipe, so the pump does not have to lift the water against its own weight. In a closed loop, the pump's head is used to overcome friction. The more water you push through the same pipes, the more friction there is, and it rises faster than the flow does.
A pump and the pipework settle at a balance. As a pump delivers more flow, the head it can produce falls. As more water flows through the pipes, the head needed rises. The system runs where the two meet. This is also why a pump's nameplate flow is only a design figure: the flow you actually get depends on the pipes and valves it is pushing through.
Valves decide how much water each coil takes
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 and closes it 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. 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. The flow in the main pipes stays about the same, whatever the coil needs.
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A two-way valve reduces the flow; a three-way valve keeps the flow constant by sending unused cold water straight to the return.
The 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. From 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. Plants with variable-speed pumps normally use two-way valves, so that flow falls when the building needs less cooling.
Primary and secondary pumps, and the decoupler between them
Part 8 explained 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, with 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.
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With two chillers running, 140 L/s passes through the chillers and 126 L/s goes to the building, so 14 L/s of cold water flows through the decoupler.
Worked example. Each 500 RT chiller is designed for a 6 °C rise:
500 RT × 3.517 ≈ 1,759 kW per chiller.
1,759 ÷ (4.19 × 6) ≈ 70 L/s per chiller.
With the two duty chillers running, the primary pumps move about 140 L/s.
From the first section, the building at 900 RT with a 6 °C rise needs about 126 L/s.
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. Suppose 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. The missing 28 L/s is made up by warm return water flowing backwards through the decoupler into the supply. The water going out to the coils is now warmer than 6 °C, rooms may start to feel warm, and the plant's control logic may start the third chiller, even though 800 RT is well within the 1,000 RT that two chillers can deliver. 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 is then allowed to change, within limits the chiller maker sets.
Slowing a pump saves a lot of power, but less than the cube law suggests
A pump driven by a VSD can run slower when the building needs less water. The VSD changes the frequency of the supply to the pump's motor, which changes its speed. (How Electric Motors Work and Rectifiers and Inverters in our Electricity Fundamentals course explain the motor and the drive.)
For a pump pushing only against friction, three rules of thumb, called the affinity laws, describe what happens when it slows down:
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. At 50% speed it is one eighth.
In a real chilled-water loop the saving is smaller than that. The BMS usually holds a fixed pressure difference across the coil that is hardest to reach, often the farthest one, so that its valve can still get enough water. The pump has to keep that minimum pressure even when flow is low, and the cube law only holds for a system with no fixed pressure to maintain. 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.
On a real plant, a flow meter and two sensors give you the cooling
With the formula from the first section, three readings are enough to work out how much cooling the plant is delivering at any moment:
Flow, from a flow meter on the chilled-water pipe.
Supply temperature, from a sensor on the pipe leaving the plant.
Return temperature, from a sensor on the pipe coming back.
For example, if the meter reads 100 L/s and the water leaves at 6 °C and returns at 11 °C, the plant is delivering 100 × 4.19 × 5 ≈ 2,095 kW, which is about 596 RT. Divide the plant's electrical power by that figure and you have its kW per RT, the efficiency number that Part 13 explains. Part 16 covers where these instruments go and how to trust their readings.
Worth knowing: A small flow of cold water through the decoupler into the return is normal. The warning sign is warm return water flowing the other way, into the supply. It warms the water going out to the coils and can make the plant start a chiller it does not need.
Optional detail: The condenser-water loop to the cooling tower is 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, so part of its head is lift, not only friction.
What comes next
The chilled water has now reached the coils, in the amount each one asks for. Those coils sit inside two kinds of unit that blow the cooled air into rooms. The next part, AHU vs FCU, answers the question: what is the difference between an AHU and an FCU?
Check your understanding
A meter shows 60 L/s of chilled water, leaving at 6 °C and returning at 12 °C. How much cooling is the loop carrying? 60 × 4.19 × 6 ≈ 1,508 kW. Divided by 3.517, that is about 429 RT.
Why do three-way valves tend to lower the chilled-water return temperature? When a three-way valve closes towards its coil, it sends unused cold water around the coil into the return pipe. That cold water mixes with the warmer water coming back from the coils, so the return temperature and the temperature rise both fall, and more water has to be pumped for the same cooling.
Recap: Cooling carried by water equals 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 flow as they close, while three-way valves bypass cold water into the return. Primary-secondary plants keep 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.
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.
Tan Kok XinCooling Fundamentals
