Which pump type is right for my application?

Before comparing pump types, it is important to understand the relationship between flow and pressure.

A pump creates flow. Pressure develops when that flow encounters resistance within the cooling circuit.

This resistance can come from:

• Pipework and hoses
• Bends and fittings
• Valves
• Filters
• Heat exchangers
• Process equipment
• Differences in system design

This means a pump should not be selected from a pressure figure alone. The flow requirement and resistance of the complete system need to be considered together.

A centrifugal pump uses a rotating impeller to move fluid through the cooling circuit.

Centrifugal pumps are generally well suited to applications requiring higher flow at relatively lower pressure.

As resistance within the customer’s system increases, the flow produced by a centrifugal pump will reduce. Its actual operating point therefore depends on the characteristics of the system to which the chiller is connected.

This makes centrifugal pumps a good choice for many applications where relatively high coolant flow is required and system resistance is moderate.

A positive displacement, or PD pump, moves a defined volume of fluid through the system during each pumping cycle.

Compared with a centrifugal pump, a PD pump provides a more consistent flow as system resistance changes.

As resistance increases, the pressure within the system rises while the pump continues to deliver approximately the required flow. For this reason, PD pumps are generally suited to applications requiring a defined flow through a more restrictive cooling circuit.

ATC positive displacement pump systems incorporate a bypass to protect the pump and cooling circuit from excessive pressure.

As a general guide:

A centrifugal pump may be suitable when: 

• Higher flow is required
• The cooling circuit has relatively low or moderate resistance
• Some variation in flow as system resistance changes is acceptable

A positive displacement pump may be suitable when: 

• A more consistent flow is required
• The process has greater resistance to flow
• The application requires lower flow at potentially higher system pressures

These are general principles rather than a substitute for correctly sizing the pump for the application.

The available pump options also vary between ATC chiller models, so pump selection should form part of the overall chiller specification.

If you know the required flow and the calculated or measured pressure drop through your process at that flow, this information can be used to help select an appropriate pump.

For example, a requirement for a particular flow at a stated pressure is useful when that pressure figure represents the actual resistance of the customer’s cooling circuit at that required flow rate.

If the pressure figure has simply been copied from an existing pump or chiller nameplate, or is an estimate rather than a known system pressure drop, it may not accurately represent what the new cooling system requires.

If you are unsure, provide ATC with as much information about the process and cooling circuit as possible rather than selecting a pump based on an assumed pressure requirement.

A pump curve shows how a pump is expected to perform across a range of operating conditions.

For a centrifugal pump, it can be used to understand how the available flow changes as the resistance, or head, within the system increases.

Pump curves are useful when assessing a cooling system, but actual performance can also be influenced by factors such as process fluid, temperature, pipework, fittings, and the wider system design.

They should therefore be treated as a guide to pump performance rather than a guarantee of the exact flow that will be achieved in a particular installation.

The cooling system must always maintain an appropriate flow path.

ATC positive displacement pump systems use a spring bypass, which allows excess pump flow to recirculate internally when the pressure reaches its set level.

Centrifugal pump systems use an adjustable bypass valve.

The bypass and chiller outlet should never be completely closed. Dead-heading or excessively restricting a pump can result in excessive pressure, overheating, cavitation, reduced cooling performance, or equipment damage.

Always follow the operating instructions for your particular chiller and pump configuration.

When requesting a chiller quotation, provide as much information as possible about your cooling circuit, particularly:

• Required coolant flow rate
• Known or calculated system pressure drop at that flow
• Process fluid
• Operating fluid temperature
• Pipework or hose size and approximate length
• Significant restrictions within the circuit
• Height differences or unusual installation conditions
• Details of existing equipment where the chiller is being replaced

If you don’t know the system pressure drop, tell us rather than estimating it. Our Sales and Engineering teams can help determine what further information is needed.