What the expansion tank does

Water expands as it warms. In a closed chilled water system, the water is at its coldest while the plant runs and warms toward ambient when the plant is off. Without somewhere to go, that expansion raises system pressure until the relief valve lifts. The expansion tank absorbs it and also sets the system’s fill pressure at its connection point.

The tank connection is the point of no pressure change in the system, so connect it on the suction side of the chilled water pumps.

Step 1: Estimate the system water volume

Add up the water in pipes, chillers, coils and buffer tanks. For pipes, multiply length by water content per foot:

Take chiller evaporator and coil volumes from manufacturer data. Add 10% for anything you haven’t counted. Pipe dimensions for other schedules are in the Pipe Schedule Reference.

Step 2: Define the temperature range

Use the lowest temperature the water will reach (chilled water supply, typically 42 to 44°F / 5.5 to 7°C) and the highest it will reach with the plant off. In a plant room or in exposed pipework, that can be 95 to 110°F (35 to 43°C). Pick a realistic maximum for the site; it controls how much the water expands.

Step 3: Set the minimum and maximum pressures at the tank

Minimum pressure (P1) is the fill or pre-charge pressure. It must keep the highest point of the system positive:

P1 = Static height above the tank (ft) / 2.31 + 4 to 5 psi

Maximum pressure (P2) is the highest pressure allowed at the tank. It must stay below the relief valve setting and below the pressure rating of the weakest component at the lowest point of the system. Keep P2 at least 10% below the relief valve setting. Also check the running condition: P1 plus pump head at the pump discharge must stay below equipment ratings too.

Use absolute pressures in the formula: psia = psig + 14.7 (kPa abs = kPa g + 101.3).

Step 4: Apply the ASHRAE formula for a diaphragm tank

Vt = Vs × [(v2 / v1 − 1) − 3α × ΔT] / (1 − P1 / P2)

Where Vt is total tank volume, Vs is system volume, v1 and v2 are the specific volumes of water at the minimum and maximum temperatures, α is the linear expansion coefficient of the pipe (6.5 × 10⁻⁶ per °F for steel, 11.7 × 10⁻⁶ per °C), and ΔT is the temperature range.

The 3αΔT term accounts for the pipework itself expanding and making a little extra room. For copper or plastic pipe, use the matching coefficient.

Select a tank whose acceptance volume is at least Vs × [(v2 / v1 − 1) − 3αΔT], and whose total volume is at least Vt.

Expansion tank sizing: worked example for a 3,000 gal system

A chilled water system holds 3,000 gal (11,356 L). The tank sits at the pump suction in a basement plant room. The highest pipe is 100 ft (30.5 m) above the tank. Water ranges from 42°F (5.6°C) running to 100°F (37.8°C) with the plant off. Relief valve setting is 100 psig.

Set the tank pre-charge to 48 psig (331 kPa g) before filling the system. Run your own numbers in the Expansion Tank Calculator.

Compression (plain steel) tanks

Older systems use plain steel tanks where air and water touch. They need a much larger tank for the same system because the air cushion starts at atmospheric pressure. The ASHRAE formula changes to:

Vt = Vs × [(v2 / v1 − 1) − 3α × ΔT] / (Pa / P1 − Pa / P2)

Where Pa is atmospheric pressure (absolute). Diaphragm or bladder tanks are the normal choice in new chilled water systems.

Common mistakes

• Connecting the tank on the pump discharge, which can drop suction pressure and cause cavitation.
• Using gauge pressures in the formula instead of absolute.
• Leaving the factory pre-charge (often 12 psig) instead of setting it to the calculated fill pressure.
• Ignoring buffer tanks and large coils when estimating system volume.
• Choosing too narrow a temperature range for a plant that sits idle in hot weather.

The tank’s connection point also affects pump suction pressure, so read this together with Chilled Water Pump Head Calculation. For pipe sizing on the same system, see Chilled Water Pipe Sizing.

Frequently asked questions

Where should the expansion tank be connected?

On the suction side of the chilled water pumps, close to the pumps, so pump head adds to system pressure rather than reducing it.

How much does chilled water expand?

About 0.7% between 42°F and 100°F. Pipe expansion offsets a small part of that.

What should the tank pre-charge pressure be?

Equal to the calculated minimum fill pressure at the tank, set before the system is filled.

Can one tank serve multiple chillers?

Yes, if the chillers share one hydraulic system. Size it on the total system volume.

Why pump head matters

Pump head is the pressure the chilled water pump must add to circulate design flow through the worst circuit in the system. Undersize it and the farthest coils starve. Oversize it and the pump runs off its curve, wastes power, and control valves struggle to modulate.

This tutorial covers closed-loop chilled water systems. Open systems such as condenser water with cooling towers are covered briefly at the end.

Step 1: Confirm design flow

Flow comes from the cooling load and the design temperature difference:

Flow (GPM) = Tons × 24 / ΔT (°F)

Flow (L/s) = Load (kW) / (4.19 × ΔT (°C))

A 10°F (5.6°C) ΔT is traditional; many modern plants use 12 to 16°F (6.7 to 9°C) to cut flow and pump power. Use the Chilled Water Flow Calculator to check this quickly.

Step 2: Identify the index circuit

The index circuit is the path from the pump, through the chiller, out to the coil with the highest total resistance, and back. It is usually the farthest coil, but a nearer coil with a high-drop control valve or a large AHU coil can be worse. Check the two or three most likely candidates.

In a closed loop, static height does not count toward pump head. The water going up is balanced by the water coming down, so a 20-storey building does not need 20 storeys of head. Static height is handled by the expansion tank and the system fill pressure.

Step 3: Pipe friction loss

Add supply and return pipe lengths along the index circuit and multiply by the friction rate used for sizing:

Pipe loss = Total length × Friction rate / 100

Chilled water pipes are commonly sized for 1 to 4 ft per 100 ft (100 to 400 Pa/m). If you need to size the pipes first, see Chilled Water Pipe Sizing or use the CHW Pipe Calculator.

Step 4: Fittings

For detailed design, add equivalent lengths for each elbow, tee and reducer. For preliminary design, a fittings allowance of 30 to 50% of the straight pipe loss is a common rule. Use the higher end for plant rooms and risers packed with fittings.

Step 5: Equipment and valve pressure drops

Take these from manufacturer data at design flow. Typical ranges for preliminary work:

Only count components that sit on the index circuit. Valves on other branches don’t add to pump head.

Step 6: Add a margin

Pump head = Pipe loss + Fittings + Equipment drops + Valve drops + Margin

A 10% margin is enough. Variable speed drives are now standard, so large margins only push the pump further from its best efficiency point.

Chilled water pump head calculation: worked example for a 300 TR chiller

A 300 TR (1,055 kW) chiller serves AHUs with a 10°F ΔT. Supply and return pipe along the index circuit total 800 ft (244 m), sized at 3 ft per 100 ft.

Flow = 300 × 24 / 10 = 720 GPM (45.4 L/s)

Pump shaft power:

BHP = GPM × Head (ft) / (3960 × Pump efficiency)

BHP = 720 × 100 / (3960 × 0.75) = 24.2 HP (18.1 kW), so select a 30 HP motor.

Check your own numbers with the Pump Head Calculator and the Pump Horsepower Calculator.

Primary-secondary and variable primary systems

In a primary-secondary system, the primary pump only covers the chiller loop (evaporator, chiller-side piping, decoupler) and the secondary pump covers the distribution loop out to the coils. Calculate each separately. In a variable primary system, one set of pumps covers both, so the chiller evaporator and the index coil circuit are in the same calculation, as in the example above.

Open loops: condenser water

Condenser water systems with open cooling towers are not closed loops. Add the static lift from the tower basin water level to the tower inlet and the nozzle or distribution pressure required by the tower manufacturer. Also check NPSH available at the pump suction with the Water Pump NPSH Calculator.

Common mistakes

• Adding building height to closed-loop pump head.
• Adding pressure drops from every coil instead of only the index circuit.
• Using a dirty-strainer drop as the design value and then adding a margin on top.
• Forgetting the chiller evaporator drop in variable primary systems.

For reference drawings, see the AutoCAD HVAC Chilled Water System DWG.

Frequently asked questions

Does building height affect chilled water pump head?

Not in a closed loop. Height affects the static fill pressure and the expansion tank, not the pump head.

What is a typical chilled water pump head?

For commercial buildings, 60 to 120 ft (18 to 37 m) is common. Long campus loops can be higher.

How do I convert ft of head to kPa?

1 ft of water is about 2.99 kPa, so 100 ft is about 299 kPa.

What pump efficiency should I assume?

Use 70 to 80% for preliminary sizing, then replace it with the selected pump’s actual efficiency at the duty point.