How Does Pressure Drop Affect Pump Selection?

How Engineers Calculate Pump Requirements

Although pump manufacturers provide performance curves, engineers must first determine the hydraulic requirements of the system before selecting a specific model.

The process typically begins with two design parameters:

• Required flow rate
• Total hydraulic head

These two values determine the pump’s operating point.

Step 1 — Determine the Design Flow Rate

The required flow rate depends on:

• heating capacity,
• collector area,
• heat transfer fluid,
• design temperature difference (ΔT),
• heat pump operating conditions.

For PVT systems, flow rate should be selected to achieve efficient heat transfer while avoiding unnecessary pumping energy.

The Intertek thermal performance tests for the Solis PVT collector were conducted using a specified flow rate per unit collector area, ensuring consistent and repeatable performance measurements.

Step 2 — Calculate Total Hydraulic Resistance

After determining the target flow rate, engineers calculate pressure losses throughout the entire hydraulic circuit.

A simplified calculation includes:

Total Pressure Loss

=

PVT Collector

+

Pipes

+

Heat Exchanger

+

Valves

+

Filters

+

Accessories

Every component contributes to the final pump head requirement.

Ignoring any major component may result in an inaccurate pump selection.

Step 3 — Match the Pump Curve

Every circulation pump has a performance curve relating:

• Flow rate
• Pump head

The selected operating point should:

• satisfy the required flow,
• overcome calculated hydraulic resistance,
• remain within the pump’s efficient operating range.

Operating too close to either extreme of the pump curve should generally be avoided.

Why Independent Pressure-Drop Data Matters

Many product brochures simply state that a collector has “low pressure loss.”

From an engineering perspective, this statement has very little value.

Engineers require quantitative data that can be incorporated into hydraulic calculations.

Independent laboratory testing provides:

• standardized measurement procedures,
• repeatable test conditions,
• traceable engineering data,
• objective comparison between products.

The laboratory report for the Solis PVT collector includes both measured pressure-drop coefficients and a pressure-drop diagram rather than qualitative descriptions alone.

Measured Data vs Engineering Judgement

It is important to distinguish different types of information used during pump selection.

Measured Data

Obtained directly from laboratory testing:

• pressure-drop coefficients,
• pressure-drop curve,
• testing conditions,
• test fluid temperature.

These values originate from the independent laboratory report.

Standard Requirements

ISO 9806 specifies how hydraulic performance should be measured so that collector performance can be evaluated consistently.

The standard defines the testing methodology rather than prescribing acceptable pressure-drop limits.

Engineering Judgement

Laboratory data alone cannot determine the final pump.

Engineers must additionally evaluate:

• total pipe length,
• pipe diameter,
• collector quantity,
• manifold configuration,
• glycol concentration,
• operating temperature,
• building heating demand.

Pump selection therefore combines measured evidence with project-specific engineering calculations.

Common Specification Mistakes

Mistake 1 — Selecting a Pump Using Only Flow Rate

A pump capable of delivering the required flow under zero resistance may fail once real hydraulic losses are introduced.

Always evaluate both flow rate and pump head.

Mistake 2 — Ignoring Collector Pressure Drop

Some designers estimate only pipe friction and overlook the collector itself.

For multi-collector systems, collector pressure loss may represent a significant portion of the total hydraulic resistance.

Mistake 3 — Comparing Pumps Without Comparing Collectors

Changing to a different collector model may alter hydraulic resistance.

Whenever the collector changes, pump calculations should be reviewed rather than assuming the existing pump remains suitable.

Mistake 4 — Assuming Laboratory Water Equals Field Conditions

The laboratory measures pressure drop under defined conditions using water.

Actual systems may use glycol or brine mixtures whose higher viscosity increases hydraulic resistance.

Engineering calculations should account for the actual working fluid used in the project.

Application Connection

Brine Heat Pump Systems

Brine systems commonly circulate antifreeze solutions rather than pure water.

Compared with laboratory conditions, these fluids generally create greater hydraulic resistance.

Pressure-drop measurements provide the baseline from which engineers can adjust calculations for actual operating fluids.

Ground Source Heat Pump Systems

In hybrid systems combining PVT collectors with ground loops, the circulation pump must overcome resistance from both sources.

Accurate collector pressure-drop data improves hydraulic balancing and pump sizing.

Solar-Assisted Heat Pump Systems

Solar-assisted heat pump systems often experience changing operating conditions throughout the year.

Correct pump selection helps maintain:

• stable collector flow,
• consistent evaporator performance,
• efficient seasonal operation.

Commercial PVT Arrays

Large collector fields frequently contain dozens of collectors connected in series-parallel configurations.

Pressure-drop data becomes increasingly important because hydraulic resistance accumulates across the collector field.

Engineers use measured pressure-drop data to optimize:

• manifold sizing,
• branch balancing,
• pump head,
• energy consumption.