A thermal oil pump should be selected according to the heat-transfer oil, normal and maximum temperature, cold-start viscosity, required flow, total head, system pressure, materials and pump structure.

The maximum flow, maximum head or motor power shown in a product catalogue is not enough to determine whether a pump is suitable. A reliable selection requires the complete operating conditions and the pump performance curve.

What Is a Thermal Oil Pump?

A thermal oil pump circulates heat-transfer oil between a heater and process equipment. The oil carries thermal energy through a closed circulation system and returns to the heater for reheating.

Thermal oil circulation systems are used in many industrial applications, including:

• Plastic injection molding equipment
• Plastic extrusion lines
• Mold temperature controllers
• Rubber processing machinery
• Printing and laminating equipment
• Industrial rollers and presses
• Food processing equipment
• Chemical reactors
• Drying equipment
• Heat exchangers
• Solar thermal systems
• Industrial heating units

The pump must provide continuous circulation under both cold-start and normal high-temperature conditions.

Why Is Thermal Oil Pump Selection Different?

Thermal oil changes as its temperature changes. During startup, cold oil may have considerably higher viscosity. As the system heats up, the oil becomes less viscous and circulates more easily.

This change can affect:

• Pump flow
• Required motor power
• Pressure loss
• Suction performance
• Bearing lubrication
• System startup time

A pump that performs correctly with hot oil may experience excessive load or insufficient flow during a cold start if viscosity has not been considered.

1. Identify the Heat-Transfer Oil

Start by confirming the exact brand and grade of the thermal oil. If the oil specification is unavailable, provide as much information as possible about its composition and physical properties.

Important oil data include:

• Oil name and grade
• Recommended operating temperature
• Maximum bulk temperature
• Viscosity at startup temperature
• Viscosity at operating temperature
• Density or specific gravity
• Flash point
• Vapor pressure
• Compatibility with pump materials

The oil supplier’s technical data sheet is normally the best source for this information.

2. Confirm Normal and Maximum Temperature

Provide the normal continuous operating temperature and the highest possible temperature that may reach the pump.

These temperatures are not always the same as the heater setpoint. Local temperatures may change during startup, low-flow operation or abnormal system conditions.

The maximum temperature can affect:

• Pump casing materials
• Internal bearings
• Mechanical seal faces
• Elastomers and gaskets
• Magnetic coupling components
• Containment shell
• Motor heat protection
• Thermal expansion clearances

The temperature limit must apply to the complete pump assembly rather than only one component.

3. Evaluate Cold-Start Viscosity

Cold-start viscosity is one of the most important factors in thermal oil pump selection.

High viscosity may:

• Reduce the actual flow rate
• Increase the required motor power
• Increase pressure loss through pipes and equipment
• Create poor suction conditions
• Increase startup load

When possible, compare the pump requirement at two operating points:

If the oil is too viscous during startup, controlled preheating or another suitable startup procedure may be necessary. The equipment manufacturer’s instructions should always be followed.

4. Determine the Required Flow Rate

Flow rate determines how much heat-transfer oil moves through the system within a specified time. It is usually expressed in liters per minute, cubic meters per hour or gallons per minute.

The required flow may depend on:

• Heating capacity
• Oil heat capacity
• Required supply and return temperature difference
• Process heat demand
• Equipment design
• Number of circulation circuits
• Required temperature uniformity

Insufficient flow may cause uneven temperature, slow heat transfer or local overheating. Excessive flow may increase pressure loss, energy consumption and system wear.

The required flow should be provided by the heating-system designer or calculated from the heat-transfer requirement.

5. Calculate the Required Pump Head

The pump must overcome the total resistance of the circulation system. Required head is not determined only by the vertical height of the installation.

Pressure losses may occur through:

• Straight pipes
• Bends and fittings
• Valves
• Filters and strainers
• Heaters
• Heat exchangers
• Mold channels
• Reactors and process equipment
• Flow-control components

The oil viscosity at startup and normal temperature should be considered because viscosity affects pipe friction and equipment resistance.

The pump should be selected according to the required flow at the required head.

6. Read the Pump Performance Curve Correctly

A pump performance curve shows the relationship between flow and head. The required operating point should fall within a stable section of the curve.

When reviewing a pump curve:

1. Locate the required flow rate.
2. Confirm the head available at that flow.
3. Check whether the curve is based on water or the actual oil.
4. Consider the effect of oil viscosity.
5. Verify the required motor power.
6. Allow a reasonable operating margin.

Maximum flow and maximum head occur at different points on the curve and should not be treated as simultaneous values.

7. Check the Suction Conditions

Good suction conditions help prevent cavitation and unstable pump performance.

The pump inlet should receive sufficient pressure under the highest-temperature condition. As oil temperature increases, vapor conditions may change and the available suction margin may decrease.

Recommended considerations include:

• Keep the suction pipe short and direct where practical.
• Use an appropriate suction-pipe diameter.
• Avoid unnecessary bends and restrictions.
• Keep strainers clean.
• Prevent air from entering the system.
• Ensure the pump is filled with liquid before startup.
• Avoid restricting the pump inlet with an incorrectly positioned valve.

The required net positive suction head and available system conditions should be checked during detailed engineering.

8. Select Compatible Pump Materials

The pump casing and all wetted components must be compatible with the heat-transfer oil and operating temperature.

Components requiring evaluation include:

• Pump casing
• Impeller
• Shaft or internal rotor
• Bearings
• Mechanical seal faces
• Gaskets and elastomers
• Containment shell
• Magnetic coupling components

A general material description such as “stainless steel” is not always enough. The exact material grade and temperature capability should be confirmed.

9. Choose the Pump Structure

Mechanically Sealed Thermal Oil Pump

A mechanically sealed pump uses seal faces around the rotating shaft. It may be suitable when the seal materials and cooling arrangement match the thermal oil, temperature and pressure.

Mechanical seals are replaceable, but incorrect selection, dry running, excessive temperature or misalignment may lead to premature leakage.

Magnetic Drive Thermal Oil Pump

A magnetic drive pump transmits motor power through magnetic coupling without a conventional rotating shaft seal.

This design can reduce leakage risk from the shaft-seal area. It may be considered for clean thermal oil and systems where liquid containment is important.

The magnet grade, internal bearings, containment shell and internal cooling path must be suitable for the maximum temperature. Dry running and solid contamination should normally be avoided.

10. Check System Pressure

Confirm the normal pressure, maximum pressure and any pressure variation during startup or shutdown.

The system designer should also consider thermal expansion. Thermal oil expands as it heats, so the complete system must include suitable expansion and pressure-control arrangements.

The pump casing, connections, valves and related equipment must remain within their allowable pressure limits.

11. Confirm the Motor and Power Supply

The motor must provide sufficient power under the most demanding operating condition, which may occur during cold startup.

Before ordering, confirm:

• Single-phase or three-phase supply
• Voltage
• Frequency
• Required motor power
• Motor protection level
• Ambient temperature
• Indoor or outdoor installation
• Local electrical requirements

Voltage and frequency should be confirmed according to the installation country.

Information Required for Thermal Oil Pump Selection

Common Thermal Oil Pump Selection Mistakes

• Ignoring cold-start viscosity
• Providing only the heater setpoint
• Selecting only by motor power
• Comparing only maximum flow
• Ignoring the required head
• Failing to check pump materials
• Using unsuitable seal or bearing materials
• Restricting the pump inlet
• Operating the pump without liquid
• Ordering an incorrect voltage or frequency

Thermal Oil Pump Support from FENGHE

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

The pump configuration should be selected according to the oil specification, startup viscosity, normal and maximum temperature, flow, head, system pressure, connection size and power supply.

OEM and wholesale inquiries from equipment manufacturers, importers, distributors and industrial service companies are welcome.

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

Frequently Asked Questions

Can a standard water pump circulate thermal oil?

Not necessarily. The pump materials, seals, bearings, motor and hydraulic performance must be suitable for the oil, temperature and viscosity.

Why must cold-start viscosity be checked?

Cold thermal oil may be more viscous, which can increase pressure loss and motor load while reducing the actual pump flow.

Should I choose a thermal oil pump by maximum flow?

No. Select the pump according to the required flow at the required head, and consider the effect of oil viscosity on performance.

Can a magnetic drive pump circulate thermal oil?

It may be suitable for clean thermal oil when the magnets, bearings, containment shell and other components are compatible with the maximum temperature.

What temperature information should I provide?

Provide the cold-start temperature, normal continuous temperature and maximum possible temperature.

What information should I send for pump selection?

Send the oil specification, viscosity, temperature, flow, head, pressure, suction conditions, connections, voltage, frequency and application details.

Need help choosing a thermal oil pump? Please contact FENGHE with the heat-transfer oil type, normal and maximum temperature, cold-start viscosity, required flow, total head, voltage and frequency.