The required circulation pump flow is determined mainly by the system heat load, liquid properties and allowable temperature difference. The required pump head is determined by the total pressure loss through pipes, valves, filters, heat exchangers and process equipment.

Flow and head must be evaluated together. A pump should be selected from its performance curve at the required duty point, rather than only by maximum flow, maximum head or motor power.

What Information Is Needed?

Before calculating pump flow and head, collect the following information:

• Required heating or cooling capacity
• Circulating liquid
• Liquid density
• Liquid specific heat capacity
• Supply and return temperature
• Pipe diameter and total length
• Number of bends and fittings
• Valves, filters and heat exchangers
• Pressure loss through the mold or process equipment
• Liquid viscosity at startup and normal operation

When exact information is unavailable, an initial estimate can be made. Final pump selection should be checked against the complete system conditions.

What Is Pump Flow Rate?

Pump flow rate is the volume of liquid circulated within a specified time. Common units include:

• Liters per minute: L/min
• Cubic meters per hour: m³/h
• Gallons per minute: GPM

The required circulation flow depends on the amount of heat that must be transferred and the allowable temperature difference between the supply and return liquid.

Basic Flow Rate Formula

For a liquid circulation system, the heat-transfer relationship can be expressed as:

Q = P ÷ (ρ × Cp × ΔT)

Where:

• Q = liquid volume flow rate
• P = heating or cooling capacity
• ρ = liquid density
• Cp = liquid specific heat capacity
• ΔT = temperature difference between supply and return

All units must be consistent. If density or specific heat changes significantly with temperature, use values corresponding to the expected operating condition.

Simplified Water Flow Calculation

For water near common industrial operating conditions, an approximate metric formula is:

Flow (m³/h) = Heating capacity (kW) × 0.86 ÷ Temperature difference (°C)

Water Flow Calculation Example

Assume:

• Heating capacity: 60 kW
• Supply and return temperature difference: 5°C

The estimated flow is:

60 × 0.86 ÷ 5 = 10.32 m³/h

The initial required water flow is therefore approximately 10.3 m³/h. A suitable design allowance may be applied according to the system requirements, but excessive oversizing should be avoided.

Thermal Oil Flow Calculation

Thermal oil normally has a lower specific heat capacity and different density from water. Its properties also change with temperature.

For thermal oil, use the actual oil data whenever possible:

Mass flow (kg/s) = Heating capacity (kW) ÷ [Specific heat (kJ/kg·K) × Temperature difference (K)]

Then convert mass flow to volume flow:

Volume flow (m³/s) = Mass flow (kg/s) ÷ Density (kg/m³)

To convert m³/s to m³/h, multiply by 3,600.

Thermal Oil Flow Calculation Example

Assume:

• Heating capacity: 60 kW
• Thermal oil specific heat: 2.1 kJ/kg·K
• Thermal oil density: 850 kg/m³
• Temperature difference: 20°C

Step 1: Calculate mass flow.

60 ÷ (2.1 × 20) = 1.43 kg/s

Step 2: Convert to volume flow.

1.43 ÷ 850 = 0.00168 m³/s

Step 3: Convert to cubic meters per hour.

0.00168 × 3,600 = approximately 6.05 m³/h

The initial calculated thermal oil flow is therefore approximately 6.1 m³/h.

This example is for explanation only. Use the actual density and specific heat values from the thermal oil supplier.

How Does Temperature Difference Affect Flow?

For the same heat load, a smaller temperature difference requires a higher circulation flow. A larger temperature difference requires a lower flow.

The correct temperature difference depends on the process, equipment and required temperature stability.

What Is Pump Head?

Pump head represents the energy required to move liquid through the system. It is commonly expressed in meters of liquid.

The required head may include:

• Pipe friction loss
• Loss through bends and fittings
• Valve pressure loss
• Filter or strainer pressure loss
• Heat exchanger pressure loss
• Heater pressure loss
• Mold or process equipment resistance
• Static height where applicable

Basic Pump Head Calculation

A simplified system-head calculation is:

Total head = Static head + Pipe friction loss + Equipment pressure loss + Design allowance

Each part should be evaluated under the required flow condition.

Static Head

Static head is the vertical height difference that the pump must overcome in an open system.

In a completely filled closed circulation loop, upward and downward elevation changes largely balance each other during normal operation. In that situation, the pump mainly overcomes friction and equipment resistance.

However, elevation still affects system filling, pressure distribution and suction conditions.

Pipe Friction Loss

Pipe friction depends on:

• Flow rate
• Pipe inside diameter
• Pipe length
• Liquid density
• Liquid viscosity
• Pipe surface condition

A small pipe diameter or high liquid viscosity can increase pressure loss significantly.

Fittings and Valve Loss

Bends, tees, reducers, valves and other fittings create additional resistance. These losses should be added to the straight-pipe friction loss.

Equipment Pressure Loss

Heat exchangers, heaters, filters, molds and production equipment may create a major part of the total system resistance.

Use manufacturer pressure-drop data at the required flow whenever available.

Converting Pressure to Pump Head

Pressure and pump head can be converted using:

Head (m) = Pressure (Pa) ÷ [Density (kg/m³) × 9.81 m/s²]

For water, 1 bar is approximately equal to 10.2 meters of water head.

For thermal oil or another liquid, the equivalent head changes according to liquid density. Therefore, the pumped liquid must be identified when converting pressure to head.

Water Pressure Conversion Example

If the total measured pressure loss is 2 bar and the liquid is water:

2 × 10.2 = approximately 20.4 meters of water head

A design allowance may then be considered according to uncertainty and expected operating changes.

Example of a Complete Duty Point

Assume a water temperature-control system has:

• Required flow: 10.3 m³/h
• Pipe and fitting loss: 6 m
• Heat exchanger loss: 5 m
• Mold and equipment loss: 7 m
• Additional allowance: 2 m

The estimated total head is:

6 + 5 + 7 + 2 = 20 m

The required pump duty point is therefore approximately:

10.3 m³/h at 20 m head

The pump should be selected from a performance curve that can provide approximately this flow and head at the same operating point.

Why Maximum Flow and Maximum Head Are Misleading

Maximum flow normally occurs when system resistance is very low. Maximum head normally occurs when flow approaches zero.

These two maximum values do not occur at the same time.

For example, a catalogue may show:

• Maximum flow: 20 m³/h
• Maximum head: 30 m

This does not mean the pump can provide 20 m³/h at 30 m head. The actual flow available at 30 m may be very low or zero. Always check the complete pump curve.

How Viscosity Changes Pump Performance

Many pump curves are tested using water. A more viscous liquid can:

• Reduce actual flow
• Reduce available head
• Reduce efficiency
• Increase required motor power

This is particularly important for thermal oil during cold startup. Pump selection should consider both startup and normal operating conditions.

Check Suction Conditions

A pump may meet the required flow and head but still operate poorly if the inlet conditions are unsuitable.

Check:

• Available inlet pressure
• Liquid level
• Suction-pipe diameter
• Suction-pipe length
• Filter condition
• Liquid temperature
• Vapor pressure
• Air entering the suction pipe

Insufficient inlet pressure may cause cavitation, noise, vibration and reduced flow.

Recommended Information for Pump Sizing

Common Calculation Mistakes

• Selecting only by motor power
• Using maximum flow as the required duty flow
• Combining maximum flow and maximum head
• Ignoring pressure loss through equipment
• Adding full elevation difference to a filled closed-loop system without checking the design
• Using water properties for thermal oil
• Ignoring cold-start viscosity
• Using inconsistent calculation units
• Adding an unnecessarily large safety margin
• Ignoring pump inlet conditions

Pump Selection Support from FENGHE

Fuzhou Fenghe Machinery Co., Ltd. supplies FENGHE circulation pumps for mold temperature controllers, hot-water systems, thermal-oil systems and related industrial equipment.

For preliminary pump selection, provide the liquid, temperature, heat load, required flow, required head, system pressure, pipe information, voltage and frequency.

Website: www.fzfenghe.com
Email: info@meccpower.com

Frequently Asked Questions

How do I calculate water circulation flow?

An approximate metric formula is flow in m³/h equals heating capacity in kW multiplied by 0.86 and divided by the water temperature difference in degrees Celsius.

How do I calculate thermal oil flow?

Use the heat load, actual oil specific heat, oil density and required temperature difference. Use oil-property data at the expected operating temperature.

Does a closed-loop system require static head?

In a completely filled closed loop, upward and downward elevation changes largely balance during circulation. The pump mainly overcomes friction and equipment resistance, although elevation still affects filling and pressure distribution.

Is one bar always equal to 10.2 meters of head?

One bar is approximately 10.2 meters of water head. The equivalent head changes for liquids with a different density.

Can I select a pump using only maximum flow?

No. The required flow and head must be located at the same operating point on the pump performance curve.

How much safety margin should I add?

The appropriate allowance depends on the accuracy of the system data and expected operating changes. Excessive oversizing should be avoided.

Need help calculating the required pump flow and head? Please contact FENGHE with the circulating liquid, heating or cooling capacity, inlet and outlet temperatures, pipe size, system layout, voltage and frequency.