Proper Motor Sizing for High Temperature Liquid Pumping with Wanner Hydra-Cell Pro^® Pumps

Hot liquid changes the motor size required for Wanner Hydra-Cell Pro^® pump applications.

What Counts as High Temperature?

Datasheets and the IOM list maximum liquid temperature by head material, then say to consult the factory from 160°F (71°C) to 250°F (121°C). The reason is not only related to elastomers or non-alloy components of the pump. See components in contact with the liquid.

Above 160°F the hydraulic end runs hotter, oil thins, and bearing life drops. That is why high temper liquid pumping limits the motor size or requires an oil cooler.

Role of the Oil

Our Hydra-Oil article covers grades and change intervals. Oil does three jobs:

1. lubricate sliding and rotating parts
2. carry heat off those parts
3. form the hydraulic pistons that move the diaphragms

What matters here is viscosity at temperature.

The hydraulic end is designed around 180°F (82°C) oil. That is the film the roller bearings require. Hotter oil thins more, reducing the oil film and thus the internal metal-to-metal contacts (bearings and piston assemblies) experience accelerated wear.

Even though a liquid temperature of 160°F is less than the hydraulic end ideal running temperature of 180°F , the finned pump body is not designed to handle the additional heat transfer from liquids > 160°F.

The hydraulic end of Hydra-Cell Pro pumps is not considered a wear item and the bearings and pistons within it can last decades if the oil is changed per the IOM manual schedule.  When operating beyond the ideal hydraulic end temperature these components may begin to wear at an accelerated rate.

Oil Film

A higher bearing load requries a thicker oil film — elastohydrodynamic lubrication (EHL). Viscosity at operating temperature and load dictate how thick that film must be.

Load is the radial or axial force on the rollers, in pounds or newtons. The bearings keep the rotating stack lined up against that force. Most of the load comes from the reciprocating hydraulic pistons.

Discharge pressure and shaft RPM raise that load. A larger motor can push more torque and RPM into the same bearings, which is why motor size is capped on hot liquid.

Two ways to keep oil film at or near its ideal thickness:

1. Cut bearing load — limit motor horsepower. That usually means a larger pump turning slower. It is a design limit, not a flow limit.
2. Add an oil cooler and hold oil at or below 180°F.

Limiting Horsepower

Maximum motor size by model and liquid temperature:

The middle column is calculated horsepower required (HPr) at each model’s highest published flow and pressure. The right column is the maximum motor you should use when the liquid is above 160°F (71°C) if not using an oil cooler.

Example

D10, “X” cam, 4 GPM at 716 PSIG, 1,800 RPM motor, liquid above 160°F.

Datasheet formula (same as the web HP calculator):

A VFD controlled system often also requires a larger motor HP due to the requirement of maintaining shaft torque at lower RPMs, so that 2.0 HPr becomes a 5.4 HPr and thus a 7.5 HP motor.

When the application is for liquid temperatures >160°F (71°C) and it’s a D10 pump, the maximum recommended horsepower is 2.1, so in our example a D10 pump with 7.5 HP motor and VFD is okay because the HPr for the application is 2.0 and the maximum acceptable HPr for D10 pumps for liquids >160°F (71°C) is 2.1 per the chart.

However, if the same VFD is later turned up to 5 GPM, HPr rises to 2.7. Even though the 7.5 HP motor with VFD can output 5 GPM,  it increased the HPr to 2.7 which is above the D10 cap of 2.1. Two choices: add an oil cooler to the D10, or increase to a pump size which can tolerate the highest possible HPr requirement.

Wanner^® air-cooled oil cooler/filters list at $5,600–$7,200 in the 2025 price guide. That is usually cheaper than tearing out a running D10.

On a new buy, price a D10 plus cooler against the next larger pump model (H25) without an oil cooler. An H25E at 1,200 RPM for 5 GPM at 716 PSIG runs at  about 288 RPM. The HPr is still 2.7 however the limit for the H25 is 5.0, so it can be used without an oil cooler.

Note: an H25 on a VFD for this duty requires a 15 HP motor to keep shaft torque and thus also a 15 HP VFD. See Calculating Horsepower Required.

So the choice is D10 + 7.5 HP motor + 7.5 HP VFD + oil cooler, or H25 + 15 HP motor + 15 HP VFD and no oil cooler. An H25 costs about twice the cost of a D10, its also physically larger, and the 15 HP motor and drive cost more than the 7.5 HP set. When you crunch the numbers using an oil cooler on the D10 is the economical path.

Without a VFD the same split holds. Belt or gearbox to the exact RPM: D10 at 2.7 HPr is over the 2.1 cap, so requires an oil cooler or H25 (still 2.7 HPr, 5 HP cap). When the VFD aspect is removed from the picture the cost increase only refelcts larger pump and maybe a bigger baseplate.

Related Articles

• Calculating horsepower required
• Hydra-Oil and oil cooling
• Components in contact with liquid
• Components of a high-pressure pump skid
• Hydra-Cell Pro^® design overview